The contradictory relationship between magnetism and ductility restricts further applications of FeCoV alloys in high-performance electrical machines. The role of the BCC-B2 transition, accompanied by vanadium (V) site occupancies, in magnetic moments and ductility has been explored using first-principles calculations. The variations in magnetism and ductility of FeCoV alloys are attributed to the coupling of the BCC-B2 transition and V occupancies. When V replaces Fe atoms in the equiatomic B2-FeCo alloy, the superior magnetism observed in B2-Fe50-cCo50Vc alloys is a consequence of the enhanced local magnetic moment of Fe and the ferrimagnetic-ferromagnetic transition in the magnetic state. Moreover, due to the preferential V occupancy in the B2 phase, the B2-Fe46Co50V4 alloy exhibits comparable ductility to the BCC-Fe50Co46V4 alloy. The results indicate that the increased brittleness in the B2 phase arises from the raised Peierls stress and the enhanced covalent component in interatomic bonding, which is caused by the strong hybridization between Fe and Co atoms. Pearson correlation analysis illustrates that valence electron concentration (VEC) and V content are significant factors in the contradictory relationship between magnetization and ductility. The theoretical results demonstrate that tuning the V content and atomic occupancies is helpful to achieve a trade-off between magnetization and ductility in B2-FeCoV alloys.

Creep rupture extrapolation is crucial for high-temperature materials served in power plants. Many analytical models can be used for creep rupture analysis, and fundamental models are also available. Machine learning is also an alternative. However, unphysical prediction curves occur readily in common machine learning algorithms, where one must manipulate the best results or ignore the less satisfactory ones. Using just high regression coefficients and low errors is not enough to obtain high accuracy of the methods. Never-theless, five soft constrained machine learning algorithms (SCMLAs), where soft con-straints, stability analysis by culling long-time or low-stress data, extrapolation from short to long times, and errors of solutions and algorithms are considered, are used for creep rupture prediction in this work. The models can generate reasonable results for fitting all data, extrapolating from short to long times, and stability analysis for Sanicro 25 after a number of tests. The errors of solutions for all the analyses are in a quite reasonable range, including extrapolation and stability analysis. The average relative standard deviation of the five SCMLAs is less than 2.5% at three times the maximum experimental creep rupture time. Creep rupture strength of the austenitic stainless steel Sanicro 25 can be predicted quantitatively by taking the average predicted stresses of the five SCMLAs. The method can also be used for other high-temperature alloys with similar creep degradation mechanisms.

Premature and unexpected creep damage is a significant concern in high-temperature engineering. Identifying outliers in creep rupture data is essential for assessing the risk of premature creep failure. This study proposes a new method to evaluate premature creep failure using log-logistic distribution fit of prediction errors and outlier positions. Fitting results for seven different alloys were obtained from extrapolation procedures using soft-constrained machine learning algorithms (SCMLAs) and constrained time-temperature parameters (TTPs) based on prior research. A comprehensive statistical analysis was conducted for all materials. The log-logistic distribution was validated as a suitable method for fitting prediction error distributions. Regression plots demonstrate effective residual balance and accurate outlier capture. The best fitting methods were identified based on the width of the distributions. Outlier positions were used to evaluate the probability of premature creep failure quantitatively. For example, a 0.5% probability of observing creep rupture strengths that are approximately 50% lower than the standardized creep stress was found for TP316H, T321H, and high Cr steels SUH616B. These findings offer valuable insights for estimating premature creep failure in materials.

Stainless steels and nickel-based superalloys are materials that have been widely used to manufacture components servicing at high temperatures. Creep strength is one of the most important properties in such conditions. High creep strength generally comes from a combination of solid solution hardening, precipitation hardening and dislocation hardening. However, some details of the mechanism of solid solution hardening are still not fully understood. The present thesis can be separated into two parts.

In the first part, fundamental creep models are used in an investigation of creep rate of nickel-based alloys. The fundamental models are based on dislocation theories without using any adjustable parameters to describe the creep data. All parameters in the models have been derived from experimental data or using computational approaches such as ab initio methods. In the models, the effects of stacking faults, strain-induced vacancies and pipe diffusion have been taken into account. 

W is a vital element to create solid solution hardening and improve creep strength of nickel-based alloys. W readily dissolves in nickel to form a solid solution and to provide a significant effect on the creep strength. Moreover, many ternary and more complex systems of nickel-based alloys are developed starting from Ni-W solid solutions. The developed models can describe the dramatic reduction of the creep rate due to W, which has not been possible in the past. The reduction has a close correlation with the stacking fault energy and drag stress.

In the second part, the exact muffin-tin orbital method combined with the coherent potential approximation has been interfaced with a quasi-harmonic Debye model to predict the elastic and other thermodynamic properties of selected metallic alloys at high temperature. The knowledge of such properties is very useful in modeling the behavior of materials servicing at high temperatures. However, few experimental studies have been focused on measurements of thermo-mechanical properties at such temperatures. Ab initio methods based on density functional theory is an alternative way to obtain information about thermo-mechanical properties. 

Therefore, in the present work, ab initio based studies of the elastic and thermodynamic properties of pure nickel, nickel-based solid solutions and Fe25Cr20NiMnNb austenitic stainless steel have been performed. Although the modeling technique cannot fully reproduce the temperature dependencies in some of the considered cases where experimental data are available, the computed values of such properties as shear moduli, thermal expansion coefficients and entropy are close to the experimentally derived values.

Rostfria stål och nickelbaserade superlegeringar används i stor utsträckning för att tillverka komponenter som utnyttjas vid höga temperaturer. Kryphållfastheten är en viktig egenskap i dessa tillämpningar. En hög kryphållfasthet år i allmänhet resultatet av en kombination av bidrag från fast lösning, partiklar och dislokationer. Vissa aspekter av lösningshärdning är inte klarlagda tidigare. Denna avhandling utgörs av två delar.

I den första delen används grundläggande krypmodeller för att undersöka kryphastigheten hos nickelbaslegeringar. De grundläggande modellerna är baserad på dislokationsteorier och inga justerbara storheter användes för att beskriva krypdata. Alla parametrar i modellen härleds från experimentella data eller med beräkningsmetoder som ab initio. I modellen beaktas effekter av staplingsfel, deformationsinducerade vakanser och diffusion längs dislokationer. I avhandlingen visas att de har stor inverkan på krypbeteendet hos nickelbaslegeringar. 

W har visats vara ett viktigt element för att skapa lösningshärdning och höja kryphållfastheten. Många ternära och mer komplexa nickelbaslegeringar är baserade Ni-W-lösningar. Modellerna i avhandlingen kan beskriva hur W dramatiskt minskar kryphastigheten bl.a. på grund av inverkan av staplingsfelsenergin.

I den andra delen görs beräkningar för egenskaper av betydelsen för kryphållfasthet med hjälp av ab initio metoder. Den exakta muffin-tenn-orbitalmetoden har kombinerats med den koherenta potentialapproximationen och en kvasi-harmonisk Debye-modell för att förutsäga elastiska och termodynamiska egenskaper vid höga temperaturer. Med hjälp av ab initio metoder kan värden för egenskaper erhållas där experimentalla data saknas. 

I avhandlingen studeras elastiska och termodynamiska egenskaper med hjälp av ab initio metoder hos ren nickel, nickelbaserade fasta lösningar och Fe25Cr20NiMnNb austenitiskt rostfritt stål. Även om modellerna inte kan reproducera temperaturberoendet i alla behandlade fall, så har värdena för skjuvmodulen, värmeutvidgningskoefficienten och entropi erhållits, som ligger nära dem från experimentella observationer.

The effects of Cr and W on high-temperature properties of Ni-based alloys are derived from first-principles calculations combined with a quasi-harmonic Debye model. The modeling procedure integrates all contributions from electronic excitations, magnetic fluctuations and lattice vibrations. The predicted lattice parameter and thermal expansion coefficient agree well with available reference values. The temperature dependence of elastic moduli for Ni-Cr alloys is described satisfactorily. Debye temperature has been calculated to show a strong volume dependence but a weak temperature dependence. A close relationship between the concentration dependence of elastic moduli and the volume dependence of Debye temperature, and thus the vibrational free energy, has been established. Besides, the softening of elastic moduli with increasing temperature is shown to be a result of thermal expansion which mainly comes from lattice vibrations. Therefore, lattice vibrations affect both temperature and concentration dependencies of elastic properties at elevated temperatures. With the help of Labusch-Nabarro model, solid solution hardening caused by Cr and W solutes in Ni has been analyzed. Moreover, the presence of solute atoms increases elastic anisotropy. The effects of Cr or W additions on the thermodynamic properties are found to be small compared to the effect of temperature.

A simple modelling method to extend first-principles electronic structure calculations to finite temperatures is presented. The method is applicable to crystalline solids exhibiting complex thermal disorder and employs quasi-harmonic models to represent the vibrational and magnetic free energy contributions. The main outcome is the Helmholtz free energy, calculated as a function of volume and temperature, from which the other related thermophysical properties (such as temperature-dependent lattice and elastic constants) can be derived. Our test calculations for Fe, Ni, Ti, and W metals in the paramagnetic state at temperatures of up to 1600 K show that the predictive capability of the quasi-harmonic modelling approach is mainly limited by the electron density functional approximation used and, in the second place, by the neglect of higher-order anharmonic effects. The developed methodology is equally applicable to disordered alloys and ordered compounds and can therefore be useful in modelling realistically complex materials.

Many metals and alloys have a stress exponent for the creep rate that is considerably higher than the value of three that is typically predicted by creep recovery models. One example is pure Ni. Creep data from Norman and Duran that are analyzed in the paper give a stress exponent of about seven in the temperature range 0.3-0.55 of the melting point. It has recently been shown that the high creep exponent of Al and Cu in the power-law breakdown regime can be explained by the presence of strain-induced vacancies. By applying a creep recovery model that does not involve adjustable parameters, it is shown that strain-induced vacancies can also explain the high-stress exponent of pure nickel.

Reliable data on the temperature dependence of thermodynamic properties of alloy phases are very useful for modeling the behavior of high-temperature materials such as nickel-based superalloys. Moreover, for predicting the mechanical properties of such alloys, additional information on the energy of lattice defects (e.g., stacking faults) at high temperatures is highly desirable, but difficult to obtain experimentally. In this study, we use first-principles calculations, in conjunction with a quasi-harmonic Debye model, to evaluate the Helmholtz free energy of paramagnetic nickel as a function of temperature and volume, taking into account the electronic, magnetic, and vibrational contributions. The thermodynamic properties of Ni, such as the equilibrium lattice parameter and elastic moduli, are derived from the free energy in the temperature range from 800 to 1600 K and compared with available experimental data. The derived temperature dependence of the lattice parameter is then used for calculating the energies of intrinsic and extrinsic stacking faults in paramagnetic Ni. The stacking fault energies have been evaluated according to three different methodologies, the axial-next-nearest-neighbor Ising (ANNNI) model, the tilted supercell approach, and the slab supercell approach. The results show that the elastic moduli and stacking fault energies of Ni decrease with increasing temperature. This "softening" effect of temperature on the mechanical properties of nickel is mainly due to thermal expansion, and partly due to magnetic free energy contribution.

Temperature-dependent properties are very useful in modeling the behavior of materials servicing at high temperature such as austenitic stainless steels. Besides, for predicting the mechanical properties of such alloys, experimental data at high temperature are very important, but scarce. In this work, we use first-principles calculations combined with a quasi-harmonic Debye model to evaluate the Helmholtz free energy of paramagnetic Fe25Cr20NiMnNb austenitic stainless steel as a function of temperature and volume. The contributions due to electronic excitations, magnetic fluctuations, and lattice vibrations are taken into account. Elastic and other thermodynamic properties of the material, such as the equilibrium lattice parameter, elastic moduli, entropy, are derived from the free energy in the temperature range from 800 to 1100 K and compared with available experimental data. The influence of Nb concentration, as well as the effects of electronic excitations and lattice vibrations, on the elastic and thermodynamic properties are discussed in detail. The elastic moduli are found to decrease with increasing temperature. This elastic softening phenomenon is mainly due to thermal expansion; the contribution of thermal electronic excitations is found to be relatively small.

Ni and Ni-W binary alloys are basis for nickel based superalloys. For most nickel based superalloys, strengthening mechanisms include both solid solution hardening and precipitation hardening. W is a vital element to create solid solution hardening and to improve the creep strength. In spite of its wide usage to strengthening of high temperature alloys, the mechanisms for solid solution hardening are not fully quantified. From the assumption that it is due to the attraction of solute atoms to dislocations and formation of Cottrell atmosphere to slow down the motion of dislocations, a fundamental model has been formulated previously. In the present paper, the model is expanded by taking the stacking fault energy and strain induced vacancies into account. Important parameters in the model are the variation of the lattice constant and the shear modulus with alloying content. Models for these variations have been formulated as a function of solute content. Another important parameter is the maximum interaction energy between the dislocations and the solutes. The model can satisfactorily predict both the large difference in creep rate between pure Ni and Ni-W alloys and the comparatively smaller differences between the three investigated Ni-2W, Ni-4W and Ni-6W alloys.