Welds in materials intended for service at temperatures above the creep activation temperature often develop damage before the base metal. The weld is a discontinuity in the material and stresses and strains often accumulate in the weld. Knowledge of the properties of the weld is essential to the safe operation of the component containing the weld. The work in this thesis has been aimed at the study of welds in service at high temperatures: The work is divided into two main chapters. The first chapter deals with welds in stainlesssteels and dissimilar metal welds and includes three papers, and the second chapter dealswith welds in copper intended for nuclear waste disposal, also including three papers. Common to both parts is that the temperature is high enough for most of the damage in the welds to result from creep.

In the first part the role of the weld microstructure on the creep crack propagation properties has been studied. Experiments using compact tension specimens have been performed on service exposed, low alloyed heat resistant steels. The results show good correlation with the crack tip parameter, C*, during steady state creep crack growth. The test methodology has also been reviewed and sensitive test parameters have been identified. The results from the creep crack propagation tests on service exposed material has been modeled using uniaxial creep data on both new and ex-service material. The development of the weld microstructure in a dissimilar metal weld between two heat resistant steels has also been investigated. A weld was made between one ferritic and one martensitic steel and the development of the microstructure during welding and post-weldheat treatments has been studied. The results show that the carbon depleted zone that develops near the weld metal in the lower alloyed steel depends on the formation and dissolution of the M23C6-carbide. Variations of the weld parameters and the post-weld heat treatment affect the size and shape of this zone. The process has been successfully modeled by computer simulation.

The second part focuses on oxygen free copper intended for nuclear waste disposal containers. The containers are made with an inner core of cast nodular iron and an outer core of copper for corrosion protection. The copper shell has to be welded and two weld methods has been tested, electron beam welding and friction stir welding. Creep specimens taken from both weld types have been tested as have base metal specimens. The technical specifications of the waste canisters demand that the creep ductility of both the copper shell and the welds has to be as high as possible. The creep test results show that base material doped with at least 30 ppm phosphorus has high creep ductility, and friction stir welds made from this material has almost as high creep strength and creep ductility. Copper without phosphorus does not exhibit the same ductility. The creep properties evaluated from testing has been modeled and extrapolated for the intended purpose

Uniaxial creep tests have been performed at 175 °C to study the influence of phosphorus, sulphur and grain size on the creep properties of oxygen free copper. Copper with no phosphorous content and copper with 2000 μm grain size showed lower creep strength and ductility than the reference material which contained 58 ppm phosphorous and had 350 μm average grain size. Phosphorous content of 29 and 106 ppm showed no difference in relation to the reference material, and neither did grain sizes of 100 and 800 μm average grain size. 6 or 12 ppm sulphur did not affect the creep properties at all. The main creep rupture mechanisms were found to be cavitation and microcracking at the grain boundaries. The observed influence of P on creep is consistent to previously published models both with respect to creep rate and creep ductility.

The microstructural features required to optimize both the strength and ductility of copper are investigated by examining the as-cast pure Cu and Cu-(1.0e3.0)Fe-0.5Co and Cu-1.5Fe-0.1Sn (wt %) alloys. Uniaxial tensile tests show that (Fe, Co)- or (Fe, Sn)-doping improves both the strength and ductility of pure copper. The microstructure evolution with Fe, Co, or Sn doping is characterized by using optical and scanning and transmission electron microscopies. The effects of Fe, Co, and Sn doping on the microstructure clearly show that (i) iron-rich nanoparticles are dispersed inside the grains. The spherical nanoparticles grow in size with increasing Fe content, and when the Fe content exceeds 2.0 wt %, the particles transition into a petal-like morphology. (ii) The microstructure of the alloys (grain size and morphology) is notably influenced by the Fe and Co contents, and the grain size is reduced from an average of 603 mu m in pure Cu to an average of 26 mm in the Cu-3.0Fe-0.5Co alloy. (iii) The addition of 1.5wt % Fe and 0.1wt % Sn dramatically reduces the grain size to an average of 42 mu m, and this reduction is correlated with the appearance of smaller spherical iron-rich nanoparticles. The evolution mechanisms of the iron-rich nanoparticles and grain structure under the alloying effect are discussed.

The microstructure evolution in the as-cast pure Cu and Cu-(1.0–3.0)Fe-0.5Co and Cu-1.5Fe-0.1Sn (wt. %) alloys was characterised in the previous work. Herein, the plastic deformation characteristics were examined by uniaxial tensile tests at room temperature. Along with the microstructure evolution, the yield strength increased with increasing Fe content and reached a peak value at 1.5 wt % Fe, but thereafter decreased with the further addition of Fe in the Cu–Fe–Co alloys. Nevertheless, the tensile strength and elongation synchronously improve with increasing Fe content. In particular, the Cu-1.5Fe-0.1Sn alloy achieved the optimal strength–ductility combination. In terms of the strengthening mechanism, the (Fe, Co)- or (Fe, Sn)-doped copper encouraged impediment, trapping, and storage of dislocations by the iron-rich nanoparticles and grain boundaries, which enhanced the strength and sustained the work hardening and elongation. The evolution of mechanical properties under an alloying effect was quantitatively described by the strengthening models. The results indicate that the optimum balance between strength and ductility was achieved by designing a microstructure containing fine grains, intragranular smaller spherical nanoparticles, and a minor solute element with higher misfit and higher growth restriction effect. The necessities for engineering a microstructure to achieve simultaneously strong and ductile bulk metals were discussed.

This thesis is meant to serve as part of a competence platform for future product development projects at Sandvik Coromant AB, Solid Round Tools Department, Västberga, Sweden. The project objective is to gain generic knowledge of the wear mechanisms that restrict tool lifetime when drilling austenitic stainless steel. Thus, identifying if the weakest link of the tool is located within the coating, the coating adherence or in the strength of the substrate. A theoretical review of the work-piece and tool materials has been conducted as a background, along with definition of tool geometry and process parameters. Furthermore, the review includes chemical and process design effect on mechanical properties of the austenitic stainless steel, TiAlN coatings and cemented carbide substrates. Additionally, the basic principles of the wear mechanisms and wear types that are specific to drilling have been reviewed. During the experimental procedures both solid and exchangeable tip drills from cemented carbide with multilayered PVD TiAlN coatings were tested. Two series of tests were conducted, the first series aimed to identify wear type dependency on cutting speed, focusing on wear of the tool margin. The second test series was performed to map the wear progression depending on distance. Analyses including identification the main wear mechanism, quantification the amount of wear, identify wear location on the tool, crack investigation and WDS analysis of chemical wear. Adhesive coating wear was found on the tool margin at an early stage. The adhesive wear rapidly progressed into a stable intermediate stage. Leaving the substrate exposed and more susceptible to other wear types resulting in crack and oxide layer formation. 

Materials intended for disposal of spent nuclear fuel require a particular combination of physical and chemical properties. The driving forces and mechanisms underlying the material’s behavior must be scientifically understood in order to enable modeling at the relevant time- and length-scales. The processes that determine the mechanical behavior of copper canisters and iron inserts, as well as the evolution of their mechanical properties, are strongly dependent on the properties of various defects in the bulk copper and iron alloys.

The first part of the present thesis deals with precipitation in the cast iron insert. A nodular cast iron insert will be used as the inner container of the spent nuclear fuel. Precipitation is investigated by computing effective interaction energies for point defect pairs (solute–solute and vacancy–solute) in bcc iron using first-principles calculations. The main considered impurities in the iron matrix include 3sp (Si, P, S) and 3d (Cr, Mn, Ni, Cu) solute elements. By computing interaction energies possibility of formation of different second phase particles such as late blooming phases (LBPs) in the cast iron insert is evaluated.

The second part is devoted to the fundamentals of dislocations and their role in plastic deformation of metals. Deformation of single-crystal copper under high strain rates is simulated by employing dislocation dynamics (DD) method to examine the effect of strain rate on mechanical properties as well as dislocation microstructure development.

Creep deformation of copper canister at low temperatures is studied. The copper canister will be used in the long-term storage of spent nuclear fuel as the outer shell of the waste package to provide corrosion protection. A glide rate is derived based on the assumption that at low temperatures it is controlled by the climb rate of jogs on the dislocations. Using DD simulation creep deformation of copper at low temperatures is modeled by taking glide but not climb into account. Moreover, effective stresses acting on dislocations are computed using the data extracted from DD simulations.

Copper canister will be used in Scandinavia for final storage of spent nuclear fuel. The copper will be exposed to temperatures of up to 100 degrees C. The creep mechanism at near ambient temperatures has been assumed to be glide of dislocations, but this has never been verified for copper or other materials. In particular, no feasible mechanism for glide based static recovery has been proposed. To attack this classical problem, a glide mobility based on the assumption that it is controlled by the climb of the jogs on the dislocations is derived and shown that it is in agreement with observations. With dislocation dynamics (DD) simulations taking glide but not climb into account, it is demonstrated that creep based on glide alone can reach a quasi-stationary condition. This verifies that static recovery can occur just by glide. The DD simulations also show that the internal stress during creep in the loading direction is almost identical to the applied stress also directly after a load drop, which resolves further classical issues.

First-principles-based thermodynamic modeling of cubic alpha and beta phases of Mn represent a challenge due to their structural complexity and the necessity of simultaneous treatment of several types of disorder (electronic, magnetic, and vibrational) that have very different characteristic time scales. Here we employ mean-field theoretical models to describe the different types of disorder and then we connect each layer of theory to the others using the adiabatic principle of separating faster and slower degrees of freedom. The slowest (vibrational) degrees of freedom are treated using the Moruzzi, Janak, and Schwarz formalism [Phys. Rev. B 37, 790 (1988)] of the Debye-Gruneisen model parametrized based on the first-principles calculated equation of state which includes the free-energy contributions due to the fast (electronic and magnetic) degrees of freedom via the Fermi-Dirac distribution function and a mean-field theory of transverse spin fluctuations. The magnetic contribution due to transverse spin fluctuations has been computed self-consistently within the disordered local moment picture of the paramagnetic state. The obtained results for thermodynamic properties such as lattice parameter, linear thermal expansion coefficient, and heat capacity of both phases show a good agreement with available experimental data. We also tested the assumption about the nature (localized versus delocalized) of magnetic moment on site IV in alpha-Mn and site I in beta-Mn on the thermodynamic properties of these two phases. Similar to the findings of experimental studies, we conclude that magnetic moment on site IV in alpha-Mn is not of a localized character. However, a similar analysis suggests that the magnetic moment of site I in beta-Mn should be treated as localized.

Seven different thermal barrier coatings (TBC) intended for coating the inside of an exhaust manifold to reduce its material temperature were studied. They comprised five plasma-sprayed (mullite, forsterite, La2Zr2O7, 8YSZ, and nanostructured 8YSZ) and two sol-gel composite (one sprayed and one dipped) coatings, which were examined for their thermal insulation properties and oxidation and spallation resistance. Thermal cyclic tests in air and in exhaust gas in a diesel test engine showed that thermal expansion mismatch between substrate and TBC was most crucial for TBC lifetime. Moreover, thermal modeling indicated that it is possible to reduce the material temperature by 50 °C, which is important for improving the fatigue life of exhaust manifolds. This reduction can be obtained with a 0.2 mm thick TBC with thermal conductivity close to 0.1 W/m K, or a 3–6 mm thick TBC with thermal conductivity 1.5–3 W/m K. 

Solid state Friction stir welding (FSW) is of major interest in the welding of aluminium since it improves the joint properties. Many applications where Al-alloys are used are subject to varying load conditions, making fatigue failure a critical issue. In the scope of this thesis, the fatigue performance of friction stir welded AlMgSi-alloy 6082 has been investigated. Static and dynamic properties of different joint configurations and welds produced with varying process parameters have been determined. Microstructures of fractured surfaces have been studied to evaluate the effect of weld discontinuities on fatigue. The mechanical strength of the friction stir welds was set in relation to that of conventional fusion welds, and that of other FS welded Al-alloys.

The friction stir process produced aluminium butt welds with high and consistent fatigue strengths, which exceeded the strengths of similar fusion welded samples. A smooth weld geometry showed to be of great importance for the fatigue performance, favouring the friction stir welds. Welding speed in a tested range of 0.35-1.4 m/min had only a modest influence on the properties of the friction stir welds; properties were not deteriorating at the highest speed. The softening of the alloy around the weldline was modelled. A fair description of the hardness profiles across the weld was obtained. At a low and high welding speed a full and partial softening respectively was predicted, indicating that full softening is not required to obtain a flawless weld.

In case of friction stir overlap welds, tool design is even more important than in butt welding to secure weld quality. A broad tool shoulder with a concave pin end gave the best performance. In particular, the minimal influence on the sheet interface when welding with such a tool was beneficial for the fatigue strength. The stress distribution in overlap and T-type test specimens has been modelled. The stress intensity factors were determined. The corresponding crack propagation rates were in fair accordance with the experimental results. It was found that a simplified approach, developed to estimate ∆K for overlap spot welds, could be used also for friction stir overlap joints.

Friction stir welding (FSW) is currently used for many applications involving lap or T-joints, e.g. hermetically closed boxes such as cooling elements and heat exchangers. The frequent pressure changes in these make them susceptible to fatigue. The fatigue characterization of lap joints involves a combination of shear and bending. Forces applied to the ends of lap joints result in non-axial stresses in the connection area. FSW lap joints of Al-Mg-Si alloy 6082 in the artificially aged condition T6 were studied. A pin (probe) based on the Triflute (TM) concept was used with two modifications to the pin, the pin end being either convex or concave. Tool shoulders of 15 and 18 turn respectively were utilized, producing four different weld series. Fracture was initiated in the highly stressed area where the weld cuts through the interface between the two sheets. The cracks typically propagated through the weld in the upper sheet (tool side). The broadest tool shoulder with a concave end of pin design gave the best fatigue performance. This was due to an improved flow path provided by the hollowed out end of the pin; allowing material flow around the pin which resulted in minimal hooking of the sheet interface adjacent to the weld nugget. Additionally heat energy was supplied by the increased contact area. The stress intensity factor Delta K was determined. It was found that a simplified approach, developed to estimate Delta K for overlap spot welds, could be applied to friction stir overlap joints. The corresponding crack propagation rates were in fair accordance with the experimental results.

The fatigue crack propagation characteristics of a friction stir welded AI-Mg-Si alloy, 6082, have been investigated. The electrical potential drop method was used for measurements. A low and a high load ratio (R) level were tested. At low load ratio (R=0.1) and a low stress intensity Delta K the propagation rate in the weld was higher than in the parent material by a factor of 3 to 5. However, the propagation rates were approaching each other close to fracture. At high load ratio (R=0.8) the propagation rate was similar in the parent material and weld. The weld crack growth rate was about the same at low and high R (except close to fracture), while the parent material growth rate increased at high R. Paris law was used to describe the measured crack propagation rates in the weld. In the case of the parent material, showing an R-dependence, Forman's law was used.

The elastic and inelastic properties were investigated for an air plasma-sprayed bondcoat (Ni-23Co-17Cr-12Al-0.5Y) at temperatures up to 800 degrees C. The yield strength was evaluated by using a miniaturised disc-bending test while the stress-strain behaviour and elastic modulus were determined through spherical indentation. The material was tested in both as-coated and heat-treated conditions (1500 h, 1000 degrees C). From the disc-bending tests it was observed that both conditions were macroscopically brittle both at low and high temperatures, with slow and stable crack propagation in the inter-splat region. Between room temperature and 500 degrees C the yield strength is constant and yield occurs by inter-splat shearing, but above this temperature the yield strength decreases due to yielding of the splats. This behaviour was confirmed by the fractographical examination that showed an inter-splat fracture surface at room temperature with small amount of plastic features, but at 800 degrees C a more dimpled and ruptured fracture appeared. Small difference in yield strength was found between the two conditions, and they had similar temperature dependence. However, their indentation stress-strain response was different, where the heat-treated condition had the largest resistance against inelastic deformation. The value of the elastic modulus was for the as-coated condition 137 GPa at 25 degrees C and 127 GPa at 800 degrees C, and for the heat-treated condition 226 GPa at 25 degrees C and 192 GPa at 800 degrees C, the difference between the two conditions being a result of internal oxidation.

With rapid economic developments and rising living standards, the demand for electricity all over the world is greatly increased. Due to high fuel costs, the steam boilers with higher steam temperature and pressure are needed to decrease the cost of power generation throughout the world extensively. In recent years, human awareness of the gradual strengthening of environmental protection increases, therefore to reduce the CO₂ emissions the power generation efficiency needs to be improved. The development of high temperature materials with improved creep rupture strength and oxidation resistance is critically needed. Materials for these demanding conditions are austenitic stainless steels such as 310, 310NbN and Sanicro 25.

Fundamental models have been developed for the precipitation of coarse particles during long time ageing of austenitic stainless steels and the influence of the particles on the mechanical properties. The models have been verified by ageing experiments. The austenitic stainless steel 310 was aged for up to 5000 h at 800 ºC. The precipitation models could satisfactorily describe the influence of ageing time on the radii and the volume fractions of particles. Models for the influence of the coarse precipitates on the tensile properties and the toughness were developed and reproduce the measured mechanical properties without the use of any fitting parameters. These developed models were utilised to investigate the influence of bands on ductility and toughness at room temperature. Up to 10 % σ-phase was observed to precipitate, which has a pronounced influence of the mechanical properties. Thermodynamic analysis demonstrated that the amount of precipitates due to ageing can significantly be reduced if the nitrogen or the carbon content is increased.

Microstructure investigations of austenitic stainless steel 310NbN and Sanicro 25 were carried out by light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The austenitic stainless steel 310NbN was aged for up to 10000 h at 650 and 750 ºC. The austenitic stainless steel Sanicro 25 was also aged for up to 10000 h at 650 and 700 ºC. Phase fractions and mean radii evolution of precipitates were calculated and compared to the experimental results. Size distributions of the precipitates in these steels were determined. Models for the different contributions to the creep strength have been applied: i) a recovery creep model for the dislocation hardening; ii) a climb controlled model for the precipitation hardening; iii) solid solution hardening from Cottrell clouds of solutes around the dislocations, and iv) A modified Dobes model for the effective stress. The total contributions can describe the experimental creep strength satisfactorily without the use of adjustable parameters.

During service at elevated temperatures extensive formation of particles can take place that can have a dramatic influence on mechanical properties. Precipitation of σ-phase and M23C6-carbides have been studied both experimentally and with thermodynamic modelling for 25Cr20Ni austenitic stainless steels (AISI 310) at 800 °C for up to 5000 h. Previous work has demonstrated that the modelling could describe the nucleation and growth satisfactory. After long term ageing the particles form bands in the microstructure. In the present paper the influence of these bands on ductility and toughness at room temperature is analysed. For this purpose previously developed models for ductility and toughness are utilised. Model values for banded and non-banded microstructures have been generated for casts of 310 in fine and coarse grained conditions with 0.04 and 0.12%N. The model values show that in the coarse grained condition, no reduction in ductility and toughness can be expected in the banded microstructure. In the fine grained condition a modest reduction is predicted.

During service at elevated temperatures extensive formation of particles can take place that can have a dramatic influence on mechanical properties. Precipitation of sigma-phase and M23C6-carbides have been studied both experimentally and with thermodynamic modelling for 25Cr20Ni austenitic stainless steels (AISI 310) at 800 degrees C for up to 5000 h. Previous work has demonstrated that the modelling could describe the nucleation and growth satisfactory. After long term ageing the particles form bands in the microstructure. In the present paper the influence of these bands on ductility and toughness at room temperature is analysed. For this purpose previously developed models for ductility and toughness are utilised. Model values for banded and non-banded microstructures have been generated for casts of 310 in fine and coarse grained conditions with 0.04 and 0.12%N The model values show that in the coarse grained condition, no reduction in ductility and toughness can be expected in the banded microstructure. In the fine grained condition a modest reduction is predicted.

Extensive precipitation can occur in high temperature materials during service. This is known to significantly affect the mechanical properties. To be able to study this phenomenon, particle formation in the austenitic stainless steel 310 has been quantified. In particular, sigma-phase and M23C6-carbides have been analysed. Specimens have been aged up to 5000 h at 800 degrees C. The precipitation of the two phases has been modelled. The basis of the model is diffusion-controlled growth. sigma-phase shows a spherical growth and an M23C6 planar growth. The soft impingement of the diffusion zones with respect to both chromium and carbon is taken into account. The influence on mechanical properties is analysed in a parallel paper.

Metal Oxide Semiconductor (MOS) capacitor devices comprised of monolayer iron oxide-coated as well as non-coated polycrystalline Pt deposited on oxidized silicon carbide substrates have been fabricated and their usefulness as realistic model systems in catalyst studies development was evaluated. The CO oxidation characteristics of both iron oxide- and non-coated Pt catalysts were investigated using mass spectrometry, monitoring the carbon dioxide production rate for different combinations of carbon monoxide (CO) and oxygen concentrations at various temperatures. Additionally, the output capacitance of the MOS model catalysts was recorded for each individual CO oxidation activity. A low-temperature shift in CO oxidation characteristics for the monolayer-coated compared to the non-coated Pt catalysts was observed, similar to that previously reported for monolayer iron oxide grown on single-crystalline Pt substrates. A strong correlation between the output capacitance of the MOS structures and the CO oxidation characteristics was found for both monolayer- and non-coated model catalysts. Furthermore, the devices exhibit retained MOS electrical output and CO oxidation characteristics as well as an unaffected catalyst surface composition, as confirmed by photoelectron spectroscopy, even after 200 h of continuous model catalyst operation. In addition to the implications on practical applicability of monolayer iron oxide coating on widely used polycrystalline Pt films in real-world catalysts and sensors, the findings also point to new possibilities regarding the use of MOS model systems for in situ characterization, high throughput screening, and tailoring of e.g. catalyst- and fuel-cell-electrode materials for specific applications.

The effect of sub-monolayer silicon on the oxidation of Al(111) and Al(100) surfaces was investigated using X-ray Photoelectron Spectroscopy (XPS) and density functional theory (DFT) calculations. On both surfaces the adatom site is preferred over substituting Si into the Al-lattice; on Al(100) the four fold hollow site is vastly favored whereas on Al(111) bridge and hollow sites are almost equal in energy. Upon O 2 exposure, Si is not oxidized but buried at the metal/oxide interface under the growing aluminum oxide. On Al(111), Si has a catalytic effect on both the initial oxidation by aiding in creating a higher local oxygen coverage in the early stages of oxidation and, in particular, at higher oxide coverages by facilitating lifting Al from the metal into the oxide. The final oxide, as measured from the Al2p intensity, is 25–30% thicker with Si than without. This observation is valid for both 0.1 monolayer (ML) and 0.3 ML Si coverage. On Al(100), on the other hand, at 0.16 ML Si coverage, the initial oxidation is faster than for the bare surface due to Si island edges being active in the oxide growth. At 0.5 ML Si coverage the oxidation is slower, as the islands coalesce and he amount of edges reduces. Upon oxide formation the effect of Si vanishes as it is overgrown by Al 2 O 3 , and the oxide thickness is only 6% higher than on bare Al(100), for both Si coverages studied. Our findings indicate that, in addition to a vanishing oxygen adsorption energy and Mott potential, a detailed picture of atom exchange and transport at the metal/oxide interface has to be taken into account to explain the limiting oxide thickness.

Vacancy-solute interactions play a crucial role in diffusion-controlled processes, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. Detailed knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on the density functional theory and employing the locally self-consistent Green's function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Sc-Cu), and 4d (Y-Ag) elements. Unrelaxed interactions within the first three coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a strong effect on the vacancy-solute interactions. Implications of the obtained results for interpreting the effects of vacancy trapping and enhanced impurity diffusion are discussed.

The methods of quantum-mechanical simulation have been used to study alloys of bcc iron with 3d transition metals in the ferromagnetic and paramagnetic states. It has been shown that the main factor that determines the solubility of the 3d elements is their electronic structure. The energy of the solution, mixing, and effective interatomic interactions vary regularly depending on the position of the element in the Periodic Table and on the magnetic state of the matrix. In some cases, depending on the magnetic state, changes in these quantities lead to the violation of the Hume-Rothery rules that determine the solubility of substitutional elements in alloys. The results obtained help us to understand the microscopic mechanisms that determine the solubility of alloying elements and their effect on the phase stability and structural state of steels.

Basic creep cavitation models have been used to predict brittle rupture of austenitic stainless steels. It involves the grain boundary sliding models, which is the basis of the creep cavitation models, the recently developed cavity formation models and the constrained cavity growth models. The individual creep cavitation models are verified with experimental observations. Brittle rupture due to creep cavitation that appears as intergranular failure is found to be dominant at high temperatures and long creep exposure times.

Two models are presented for grain boundary sliding (GBS) displacement during creep. GBS is considered as crucial for the formation of creep cavities. In the first model, the shear sliding model, GBS is accommodated by grains freely sliding along the boundaries in a power-law creeping material. The GBS rate is proportional to the grain size. In the second model, the shear crack model, the sliding boundaries are represented by shear cracks. The GBS rate is controlled by particles in the boundaries. In both models, the GBS displacement rate is proportional to the creep strain rate. Both models are consistent with existing experimental observations for GBS during creep of austenitic stainless steels. For cavity nucleation at particles, Harris’ model (1965) for the relationship between GBS and a critical particle size has been analysed and found to be in agreement with observations.

Creep, low cycle fatigue (LCF) and creep fatigue tests have been conducted for modified HR3C (25Cr20NiNbN) at high temperatures ranging of 650-750 degrees C. Both LCF and creep fatigue test results could be described with the Coffin-Manson relationship. The number of cycles to failure in the creep fatigue tests was more than one order of magnitude lower compared with LCF. The effect of the total hold time in tension (the total creep time) was compared to creep rupture data. The creep fatigue results were in reasonable agreement with the creep tests. The short creep fatigue lives may be due to the low creep ductility which was found in the creep tests. Fractography showed that the rupture mode was intergranular. Cavities were observed at grain boundaries due to the fracture of the primary Z phase particles in both LCF and creep fatigue tests. In comparison to Sanicro 25, the modified HR3C showed better LCF properties.

Plasticity in crystalline solids is controlled by the microscopic line defects known as “dislocations”. Decisive role of dislocations in crystal plasticity in addition to fundamentals of plastic deformation are presented in the current thesis work. Moreover, major features of numerical modeling method “Discrete Dislocation Dynamics (DDD)” technique are described to elucidate a powerful computational method used in simulation of crystal plasticity.

First part of the work is focused on the investigation of strain rate effect on the dynamic deformation of crystalline solids. Single crystal copper is chosen as a model crystal and discrete dislocation dynamics method is used to perform numerical uniaxial tensile test on the single crystal at various high strain rates. Twenty four straight dislocations of mixed character are randomly distributed inside a model crystal with an edge length of 1 µm subjected to periodic boundary conditions. Loading of the model crystal with the considered initial dislocation microstructure at constant strain rates ranging from 10³ to 10⁵s¹ leads to a significant strain rate sensitivity of the plastic flow. In addition to the flow stress, microstructure evolution of the sample crystal demonstrates a considerable strain rate dependency. Furthermore, strain rate affects the strain induce microstructure heterogeneity such that more heterogeneous microstructure emerges as strain rate increases.

Anisotropic characteristic of plasticity in single crystals is investigated in the second part of the study. Copper single crystal is selected to perform numerical tensile tests on the model crystal along two different loading directions of [001] and [111] at two high strain rates. Effect of loading orientation on the macroscopic behavior along with microstructure evolution of the model crystal is examined using DDD method. Investigation of dynamic response of single crystal to the mechanical loading demonstrates a substantial effect of loading orientation on the flow stress. Furthermore, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Likewise, strain induced microstructure heterogeneity displays the effect of loading direction such that more heterogeneous microstructure evolve as single crystal is loaded along [111] direction. Formation of slip bands and consequently localization of plastic deformation are detected as model crystal is loaded along both directions.

Dynamic deformation of single crystal copper at high strain rates ranging from 10³ to 10⁵ s^-1 is modeled using three dimensional discrete dislocation dynamics method. Numerical uniaxial tensile test is performed on a model crystal along [0 0 1] orientation to examine the resulting macroscopic behavior along with microstructure evolution at high strain rates. Twenty-four straight dislocations of mixed character are randomly distributed inside a model crystal with an edge length of 1  subjected to period boundary conditions. In the simulated single crystal with the considered initial dislocation microstructure, plastic flow demonstrates a significant strain rate dependency at imposed strain rates. Rate sensitivity of flow stress observed at strain rates >> 10³ s^-1 agrees well with the reported experimental studies on copper single crystal.  Furthermore, strain rate considerably affects the microstructure evolution of the sample crystal as a result of influence of strain rate on dislocations generation and interactions. Formation of heterogeneous microstructure is observed at all imposed strain rates. We find that heterogeneity of microstructure escalates as strain rate increases.

Uniaxial tensile loading of copper single crystal along [001] and [111] orientations is modeled at two high strain rates of 10⁵ and 10⁶ s^-1. Discrete dislocation dynamics method is used to study the anisotropic characteristic of plastic deformation in the model crystal. Furthermore, strain rate sensitivity of the flow stress in copper crystal is examined. Investigation of mechanical response of single crystal to the external loading demonstrates a substantial effect of loading orientation on the plastic flow. We find that at both imposed strain rates flow stress increases significantly when tensile load is applied along [111] crystallographic axis. Similarly, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Moreover, plastic flow behavior exhibits a profound strain rate sensitivity at both loading orientations which agrees well with experimental observations regarding strain rate dependency of flow stress in copper single crystal as strain rate exceeds 10³ s^-1. At both applied strain rates dislocations evolve into a heterogeneous microstructure and highest heterogeneity is observed as model crystal is loaded along [111] direction at strain rate of    10⁶ s^-1. Formation of slip bands and consequently localization of plastic deformation are detected for all considered cases. However, at the higher strain rate of 10⁶ s^-1, slip band formation is more pronounced for both loading orientations.

Friction stir processing (FSP) of Al 5059 is studied in which subsequent heat treatment is conducted to investigate its effects on grain size and hardness. It was found that mainly elongated and rhomboidal morphologies of Al-6(Mn,Fe) particles are present in the alloy both before and after FSP, where the rhomboidal particles are more effective in pinning grain boundaries during heat treatment. The stir zone reached a temperature of 705 K (432 A degrees C), and ThermoCalc modeling confirmed that the Al-6(Mn,Fe) particles will remain stable at this temperature. Negligible grain growth was observed during FSP of the Al 5059 due to low grain boundary mobility resulting from slow diffusion associated with a high Mg content in the alloy. During heat treatment at 448 K (175 A degrees C) grain growth could be correlated with time using a particle-controlled grain growth model. Microhardness values indicate that Hall-Petch behavior occurs in the processed alloy, while dislocation density and particle dispersion play a minor role in strengthening.

A number of carbon-rich (containing up to 47 wt% C) and lime-rich (containing up to 96 wt% of CaO-compounds) waste products from the pulp and paper industries can be used in iron and steel industry as fuels and slag formers for various metallurgical processes such as blast furnaces (BF), cupola furnaces (CF), argon oxygen decarburization (AOD) converters and electric arc furnaces (EAF). In most cases, these wastes consist of different size powders. In order to facilitate loading, transportation and charging of these powder wastes, briquetting is required. In this study, a pulverized AOD slag was tested as a binder component for briquetting of CaO-containing wastes (such as mesa, lime mud and fly ash) from pulp and paper industries. Moreover, mechanical testing of the possibilities for loading, transportation and unloading operations were done, specifically drop test trials were done for briquettes with different chemical compositions and treatments such as heating and storage. The results showed that an addition of 10-20% of AOD slag as a binder component followed by heat-treatment at 850 °C significantly improved the mechanical properties of the CaO-containing briquettes. An application of these briquettes will significantly reduce the consumption of natural resources (such as nature lime) in the metallurgical processes. Moreover, it can reduce the landfill area of wastes from pulp and paper industries, which is important from an environmental point-of-view.

In an attempt to map the residual stress distributions after friction stir welding of copper canisters, a three-dimensional thermo-mechanical model has been formulated by coupling heat transfer and elastoplasticity analyses. The transient temperature field around the tool is simulated by a moving heat source. The simulation shows that the residual stress distribution in a thick-wall copper canister is sensitive to the circumferential angle and asymmetrical to the weld line. Both tensile and compressive stresses emerge along the weld line and its vicinity. The maximum tensile stress appears in the circumferential direction on the outer surface. The maximum tensile stress, whether it is predicted by the finite element method or measured by the hole-drilling technique and the X-ray diffraction method, does not exceed 50 MPa in general.

On the basis of dislocation climb and mobility, a steady-state non-Newtonian flow law is derived for two metallic and one intermetallic material,namely electrolytic tough pitch copper, phosphorus alloyed pure copper, and Fe24AlMo iron aluminide. The purpose is to develop a flow lawapplicable to the finite element simulation for hydrodynamic flow of incompressible metals. The model can accurately represent the experimentalflow stress values and the viscosity of the materials. The mathematical form of the model is similar to that of the free-volume approach, which isused for liquids and amorphous metals. The study indicates that in the temperature and strain-rate regimes that are appropriate to the hot-workingprocesses, the Cohen–Grest model, which is essentially related to the total thermal expansion of fluid, can phenomenologically be extended to thecrystalline solid-state materials for the depiction of viscosity data.

The structure and binding energy of vacancy-impurity complexes in copper are studied using first-principles calculations based on density functional theory. A single vacancy is found to be able to trap up to six hydrogen atoms which tend to be situated inside the vacancy at off-center positions (related to the octahedral interstitial positions of the ideal fcc lattice). The binding energy of an H atom dissolved in the Cu lattice (octahedral interstitial position) to a vacancy is calculated to be about 0.24 eV, practically independent of the number of H atoms already trapped by the vacancy, up to the saturation with 6 hydrogens. For an oxygen impurity in Cu, a monovacancy is shown to be a deep trap (with a binding energy of 0.95 eV). The position of a trapped O atom inside a vacancy is off-center, almost a half-way from the nearest octahedral interstitial to the vacancy center. Such a vacancy-O cluster is shown to be a deep trap for dissolved hydrogen (the calculated binding energy is 1.23 eV). The trapping results in the formation of an OH-group, where the H atom is situated near the vacancy center, and the O atom is displaced from the center along a 〈100〉 direction towards a nearby octahedral interstitial position. Further hydrogenation of the monovacancy-OH cluster is calculated to be energetically unfavourable. McNabb-Forster's equations are generalised to describe the competition between a deep hydrogen trap and a shallow one. It is demonstrated that the deep trap is almost fully filled, which explains why some of hydrogen is strongly bound and cannot be removed without vacuum treatment at elevated temperatures.

By the use of first-principles calculations based on density functional theory, lattice misfit parameters for alloying elements in the austenitic stainless steel 23Cr25NiWCuCo have been derived. These lattice misfit parameters have been applied to determine the solid solution hardening of the elements W, Nb, and Cu in the steel. The model for solid solution hardening is based on work by Hirth and Lothe, where solutes are creating Cottrell clouds around the dislocations and slow down their motion. The model is also verified by comparison to creep tests for Ni-20%Cr and Ni-20%Cr-6W, where W is almost completely in solid solution and no other strengthening mechanism than solid solution hardening should be active. The contribution from the interstitial elements C and N to the solid solution hardening is found to be negligibly small for the studied steel.

We apply density functional perturbation theory together with experimental studies in order to investigate the structure and physical properties of possible stable and metastable copper(I) compounds with oxygen and hydrogen. Copper(I) hydride, CuH, is found to be a metastable phase which decomposes at ambient conditions and exhibiting a semiconducting gap in the electronic spectrum. The calculated structure and phonon spectra are found to be in good agreement with experimental data. The phonon spectra of a novel metastable phase, copper(I) hydroxide, are also determined.

New important applications of copper metal, e.g., in the areas of hydrogen production, fuel cell operation, and spent nuclear fuel disposal, require accurate knowledge of the physical and chemical properties of stable and metastable copper compounds. Among the copper(I) compounds with oxygen and hydrogen, cuprous oxide Cu(2)O is the only one stable and the best studied. Other such compounds are less known (CuH) or totally unknown (CuOH) due to their instability relative to the oxide. Here we combine quantum-mechanical calculations with experimental studies to search for possible compounds of monovalent copper. Cuprous hydride (CuH) and cuprous hydroxide (CuOH) are proved to exist in solid form. We establish the chemical and physical properties of these compounds, thereby filling the existing gaps in our understanding of hydrogen- and oxygen-related phenomena in Cu metal.

Destructive testing of creep exposed components is a powerful tool for evaluation of remaining lifetime of high temperature pipe systems. The most common destructive evaluation method used today is uniaxial creep testing. Uniaxial creep tests can produce accurate creep curves but the test method has some drawbacks such as costliness and long testing times. It also demands large sample material outtake which often involve weld repair.

Impression creep (IC) testing is a relatively new alternative test method for evaluating primary and secondary creep rates. The scope of this work is to evaluate the benefits and drawbacks of IC testing over uniaxial creep testing in order to determine its usefulness as a test method.

A literature survey was carried out over the area creep testing of high temperature pipe systems, with particular focus on impression creep testing. The result of the literature survey clearly showed several benefits with impression creep testing. An IC test series was performed in order to determine the secondary creep rate of a service exposed 10CrMo9-10 high temperature pipe steel. The IC tests were performed by VTT in Finland, using the same test parameter and sample material as in previous projects where the creep properties of the test material were determined by uniaxial creep testing.

The result of the predicted secondary creep rate obtained from the IC tests was compared with the secondary creep rates measured during the uniaxial tests. The IC tests results did not align satisfactory with the results from the uniaxial creep tests, which would have been expected. The reason for this may be due to sources of error during impression creep testing, since very small displacements due to creep have to be measured with high precision during the tests. Further testing of the impression creep test method is recommended as a result of this work, in order to evaluate the method.

The study of defects and disorder in condensed matter remains a central subject of materials science. Newly emerging experimental and theoretical techniques promote our understanding in this field, and reveal many interesting phenomena in which the atomic picture plays a crucial role. In this thesis we present a study on the fundamental and applied aspects of defects and disorder in industrial materials.

We consider the basic aspects of defective and disordered crystals, and discuss the structural, electronic, thermodynamic and mechanical properties of such materials. In particular, we have systematically investigated the defects in copper metal based on ab initio calculations. The point defects, point defects interactions, stacking faults, and the grain boundaries have been studied. Efforts are made to relate the atomistic information to the macroscopic mechanical behavior of copper metal possessing defects. The stackingfault energy of copper is found to be sensitive to the change of temperature and the presence of point defects. The atomic size effect of phosphorous is more evident for the change of the stacking-fault energy of copper among the 3sp impurities. While the change of the work of separation of grain boundary is found to follow the pattern of the chemical effect. When the chemical effect dominates, the impurity enhances the cohesion strength of grain boundary, and vice versa. The study well explains the various influence of the defects on the macroscopic mechanical properties of copper, including the anomalous behaviour of phosphorous in copper.

The structure and properties of monovalent copper compounds with oxygen and/or hydrogen were also explored. The ground-state cuprice–CuOH(s) was identified using a combined theoretical-experimental effort. The structure determined with DFT was validated by comparison with the X-ray diffraction data obtained from the synthesized material. The ground-state structure of CuOH(s) has a layered structure that is stabilized by antiferroelectric cation ordering which, in turn, is caused by collective electrostatic interactions. The electronic and thermodynamic properties of the cationordered CuOH(s) are intimately linked to the bonding topology in this material, which is composed of one-dimensional (folded and interlocked chains) and two-dimensional (layers) structural units. The solid CuOH is an indirect band gap semiconductor, while the band gap varies between 2.73 eV and 3.03 eV due to cation disorder. The hydrogen in CuOH has little effect on the ionic nature of the Cu–O bonding relative to that in Cu2O, but lowers the energy levels of the occupied states by giving a covalent character to the O–H bond. The competing structures of copper hydride were also investigated. Structure–property relationships were analyzed on this series of materials to gain fundamental understanding of their behaviour.

Defects and disorder are also important for understanding the structure γ-alumina. Our calculations have confirmed that the most stable structure of γ-alumina is the defective spinel phase with disordered cation vacancies. The hydrogenated spinel phase is also dynamically stable, but thermodynamically unstable with respect to the defective spinel phase and H2O, as well as relative to the defective spinel phase and Boehmite (γ- AlO(OH)). This is in spite of the high entropy content of hydrogenated γ-alumina. Our calculations and analysis allow us to conclude that the hydrogenated spinel structure is only a metastable phase that forms during the decomposition of Boehmite above 753 K. However, dehydration of the metastable phase into the ground state is expected to be a slow process due to the low diffusion rate of H, which leaves hydrogen as a locked-in impurity in γ-alumina under conditions of normal temperature and pressure.

The electronic structure and chemical bonding of Cu(I) compounds with O and/or H are investigated using ab initio calculations based on density functional theory. A hybrid functional PBE0 is employed, which accurately reproduces an experimental band gap of cuprite Cu2O. Cuprous hydroxide CuOH (cuprice) is found to be an indirect band gap semiconductor. Depending on the bond network configuration of CuOH, its band gap is found to vary between 2.73 eV and 3.03 eV. The presence of hydrogen in CuOH has little effect on the character of Cu-O bonds, as compared to Cu2O, but lowers the energy levels of the occupied states upon O- H bond formation. The bonding charge density and electron localization function calculations reveal that a closed-shell Cu-Cu interaction takes place in Cu2O and CuOH between the neighbouring Cu cations belonging to different bond networks. Besides, three structures of cuprous hydride CuH are investigated. We find that the halite structure of CuH can be stabilized at high pressure (above 32 GPa) while wurtzite is the most stable structure of CuH at ambient pressure. The H-H interaction contributes to the dynamical stabilization of the halite structure. The wurtzite and sphalerite structures of CuH are predicted to be semiconducting with small band gaps, while the halite structure is calculated to be metallic.

The interactions of stacking faults and point defects in oxygen-free phosphorus-containing copper are investigated using ab initio methods. Although monovacancies can act as traps for H impurities or OH groups, the calculations show that two vacancies only weakly bind with each other and this interaction terminates at the third nearest-neighbor distance. An interstitial P tends to form a Cu-P dumbbell-like cluster around the lattice site and can readily combine with a vacancy to become a substitutional impurity. It is also found that the intrinsic stacking-fault energy of copper strongly depends on the temperature as well as on the presences of point defects. The intrinsic stacking-fault energy varies between 20 and 77 mJ/m2 depending on the presence of point defects in the faulted region. These point defects are also found to affect the unstable stacking-fault energy, but they always increase the twinning tendency of copper. Among them, the substitutional P is found to have the strongest effects, decreasing the intrinsic stacking-fault energy and increasing the twinnability.

The broad range of applications of copper, including areas such as electronics, fuel cells, and spent nuclear fuel disposal, require accurate description of the physical and chemical properties of copper compounds. Within some of these applications, cuprous hydroxide is a compound whose relevance has been recently discovered. Its existence in the solid-state form was recently reported. Experimental determination of its physical-chemical properties is challenging due to its instability and poop crystallinity. Within the framework of density functional theory calculations (DFT), we investigated the nature of bonding, electronic spectra, and optical properties of the cuprous oxide and cuprous hydroxide. It is found that the hybrid functional PBEO can accurately describe the electronic structure and optical properties of these two copper(I) compounds. The calculated properties of cuprous oxide are in good agreement with the experimental data and other theoretical results. The structure of cuprous hydroxide can be deduced from that of cuprous oxide by substituting half Cu∗ in Cu2O lattice with protons. Compared to CU2O, the presence of hydrogen in CuOH has little effect on the ionic nature of Cu-O bonding, but lowers the energy levels of the occupied states. Thus, CuOH is calculated to have a wider indirect band gap of 2.73 eV compared with the Cu2O band gap of 2.17 eV.

We performed a density functional theory (DFT) investigation of the mechanisms of oxide growth at Al(100), Al(110) and Al(111) up to 1 monolayer (ML) coverage of O-atoms with 0.125 ML increments. We found that the surface binding site preferences of O-atoms are largely affected by the presence of neighboring O-atoms. Based on this we constructed two oxide growth models: the formation of clusters that evolve to stripes with increasing coverage and the formation of a more homogeneous distribution of O-atoms. While the former model is characterized by a lower symmetry of distribution of O-atoms at the surfaces, the latter corresponds to higher symmetries. We found that the prevalence of each oxide growth mode depends on the coverage of O-atoms and that this dependency is different for each surface. For Al(100) and Al(110), up to coverages of 1 ML the oxide grows preferably via the formation of clusters that evolve to stripes with increasing coverage, while for Al(111) the stripes and clusters are the preferred growth mode for coverages up to 0.375 ML, beyond which the homogeneous growth mode is energetically favored. The calculated Al-O pair distribution functions show that the formation of clusters and stripes leads to shorter Al-O bond lengths when compared to the homogeneous growth. The oxides formed at Al(100) and Al(110) have Al-O bond lengths and geometries typical of the shorter bonds of -alumina while at Al(111) the bond lengths are typical of -alumina and -alumina. These results suggest that for low coverages, the oxides formed at Al(100) and Al(110) are resemblant of defective -alumina while the oxide formed at Al(111) is similar to less disordered -alumina and -alumina. For Al(111), the small energy difference between the growth of clusters and stripes and homogeneous growth does not exclude the coexistence of both growth modes; this could lead to the formation of a defective or amorphous oxide.

Using density functional theory (DFT) with the PBE0 density functional we investigated the role of surface dopants in the molecular and dissociative adsorption of O-2 onto Al clusters of types Al-50, Al50Alad, Al50X and Al49X, where X represents a dopant atom of the following elements Si, Mg, Cu, Sc, Zr, and Ti. Each dopant atom was placed on the Al(111) surface as an adatom or as a substitutional atom, in the last case replacing a surface Al atom. We found that for the same dopant geometry, the closer is the ionization energy of the dopant element to that of elemental Al, the more exothermic is the dissociative adsorption of O-2 and the stronger are the bonds between the resulting O atoms and the surface. Additionally we show that the Mulliken concept of electronegativity can be applied in the prediction of the dissociative adsorption energy of O-2 on the doped surfaces. The Mulliken modified second-stage electronegativity of the dopant atom is proportional to the exothermicity of the dissociative adsorption of O-2. For the same dopant element in an adatom position the dissociation of O-2 is more exothermic when compared to the case where the dopant occupies a substitutional position. These observations are discussed in view of the overlap population densities of states (OPDOS) computed as the overlap between the electronic states of the adsorbate O atoms and the clusters. It is shown that a more covalent character in the bonding between the Al surface and the dopant atom causes a more exothermic dissociation of O-2 and stronger bonding with the O atoms when compared to a more ionic character in the bonding between the dopant and the Al surface. The extent of the adsorption site reconstruction is dopant atom dependent and is an important parameter for determining the mode of adsorption, adsorption energy and electronic structure of the product of O-2 adsorption. The PBE0 functional could predict the existence of the O-2 molecular adsorption product for many of the cases investigated here.

We investigated the performance of the density functional theory (DFT) functionals PBE, PBE0, M06, and M06-L for describing the molecular and dissociative adsorption of O2 onto pure and doped Al(111) surfaces. Adsorption of O2 was studied at the perfect Al(111) surface and compared with the case where an additional Al atom was present as an adatom. Additionally, we studied how these functionals perform when different dopants are present at the Al(111) surface in two distinct geometries: as an adatom or as a substitutional atom replacing an Al atom. The performance of the different functionals is greatly affected by the surface geometry. The inclusion of Hartree-Fock exchange in the functional leads to slight differences in adsorption energies for molecular adsorption of O2. These differences become very pronounced for dissociative adsorption, with the hybrids PBE0 and M06 predicting more exergonic adsorption than PBE and M06-L. Furthermore, PBE0 and M06 predicted trends in adsorption energies for defective and perfect surfaces which are in line with the experimental knowledge of the effects of surface defects in adsorption energies. The predictions of the non-hybrids PBE and M06-L point in the opposite direction. The analysis of the contributions of the van der Waals (vdW) forces to the adsorption energies reveals that the PBE and PBE0 functionals have similar difficulties in describing vdW interactions for molecular adsorption of O2 while the M06 functional can give a description of these forces with an accuracy which is at least similar to that of the correction of the D3 type.