Additive manufacturing (AM) features repeated thermal cycles due to the track- and layer-wise fabrication process. However, the unique thermal cycling often encourages the precipitation of detrimental phases, such as the isothermal ω phase in metastable β titanium alloys, which cause severe embrittlement. This study aims to address ω phase embrittlement in Ti−13.5Mo (wt%) metastable β titanium alloy fabricated by laser powder bed fusion (L-PBF) through Sn additions. It is shown that 5.0 wt% Sn microparticles can be reliably in-situ alloyed with Ti−13.5Mo by L-PBF to effectively inhibit the formation of the commensurate isothermal ω phase in the binary Ti−13.5Mo alloy. Detailed microstructural characterizations and simulations of the precipitation kinetics reveal that both Ti−13.5Mo with and without Sn exhibit densely populated ω phase throughout the microstructures. However, the Sn addition retards development of the final commensurate form of isothermal ω phase, thereby mitigating its embrittling effects. As a result, Ti−13.5Mo+5Sn fabricated by L-PBF exbibits a good balance of strength and ductility which outperforms those of similar alloys produced by conventional manufacturing routines. Since the Ti−Mo binary system forms the basis of important multicomponent titanium alloys, the finding in this work is expected to be applicable beyond the binary alloy considered here and provides a framework for the design of β titanium alloys for AM that are resistant to ω phase embrittlement.

Coarse columnar grains and heterogeneously distributed phases commonly form in metallic alloys produced by three-dimensional (3D) printing and are often considered undesirable because they can impart nonuniform and inferior mechanical properties. We demonstrate a design strategy to unlock consistent and enhanced properties directly from 3D printing. Using Ti−5Al−5Mo−5V−3Cr as a model alloy, we show that adding molybdenum (Mo) nanoparticles promotes grain refinement during solidification and suppresses the formation of phase heterogeneities during solid-state thermal cycling. The microstructural change because of the bifunctional additive results in uniform mechanical properties and simultaneous enhancement of both strength and ductility. We demonstrate how this alloy can be modified by a single component to address unfavorable microstructures, providing a pathway to achieve desirable mechanical characteristics directly from 3D printing.

Additive manufacturing (AM) creates digitally designed parts by successive addition of material. However, owing to intrinsic thermal cycling, metallic parts produced by AM almost inevitably suffer from spatially dependent heterogeneities in phases and mechanical properties, which may cause unpredictable service failures. Here, we demonstrate a synergistic alloy design approach to overcome this issue in titanium alloys manufactured by laser powder bed fusion. The key to our approach is in-situ alloying of Ti−6Al−4V (in weight per cent) with combined additions of pure titanium powders and iron oxide (Fe2O3) nanoparticles. This not only enables in-situ elimination of phase heterogeneity through diluting V concentration whilst introducing small amounts of Fe, but also compensates for the strength loss via oxygen solute strengthening. Our alloys achieve spatially uniform microstructures and mechanical properties which are superior to those of Ti−6Al−4V. This study may help to guide the design of other alloys, which not only overcomes the challenge inherent to the AM processes, but also takes advantage of the alloy design opportunities offered by AM.

Precipitation hardening is one of most effective strengthening mechanisms in steels, and much research has been performed in the past. To evaluate the contribution of precipitates, the quantitative features of precipitates including mean size and particle size distribution etc., are vital and needed. However, the predictive modeling of precipitation is still a challenge so far, especially on a quantitative level. Thus, in the present work, precipitation of carbides after tempering of martensitic FeCr-C alloys, consisting of hierarchically arranged substructures within the prioraustenite grains, namely packets and blocks of individual laths, up to 1000h has been investigated. Experimental measurements using electron microscopy and modeling using a Langer-Schwartz theory with the Kampmann-Wagner -Numerical (KWN) method have been conducted. The importance of a proper definition of the initial microstructure for predictive modeling is discussed, in terms of the comparison of calculated and experimental results.

Precipitation hardening is one of the most important strengthening mechanisms in metallic materials, and thus, controlling precipitation is often critical in optimizing mechanical performance. Also other performance requirements such as functional and degradation properties are critically depending on precipitation. Control of precipitation in metallic materials is, thus, vital, and the approach presently in the limelight for this purpose is an integrated approach of theory, computations and experimental characterization. An empirical understanding is essential to build physical models upon and, furthermore, quantitative experimental data is needed to build databases and to calibrate the models. The most versatile tool for precipitation characterization is the transmission electron microscope (TEM). The TEM has sufficient resolving power to image even the finest precipitates, and with TEM-based microanalysis, overall quantitative data such as particle size distribution, volume fraction and number density of particles can be gathered. Moreover, details of precipitate structure, morphology and chemistry, can be revealed. TEM-based postmortem and in situ analysis of precipitation has made significant progress over the last decade, largely stimulated by the widespread application of aberration corrected microscopes and accompanying novel analytics. The purpose of this report is to review these recent developments in precipitation analysis methodology, including sample preparation. Application examples are provided for precipitation analysis in metals, and future prospects are discussed.

Austenitizing temperature is one decisive factor for the mechanical properties of medium carbon martensitic stainless steels (MCMSSs). In the present work, the effects of austenitizing temperature (1000, 1020, 1040 and 1060 degrees C) on the microstructure and mechanical properties of MCMSSs containing metastable retained austenite (RA) were investigated by means of electron microscopy, X-ray diffraction (XRD), as well as tensile and impact toughness tests. Results suggest that the microstructure including an area fraction of undissolved M23C6, carbon and chromium content in matrix, prior austenite grain size (PAGS), fraction and composition of RA in studied MCMSSs varies with employed austenitizing temperature. By optimizing austenitizing temperature (1060 degrees C for 40 min) and tempering (250 degrees C for 30 min) heat treatments, the MCMSS demonstrates excellent mechanical properties with the ultimate tensile strength of 1740 +/- 8 MPa, a yield strength of 1237 +/- 19 MPa, total elongation (ductility) of 10.3 +/- 0.7% and impact toughness of 94.6 +/- 8.0 Jcm(-2) at room temperature. The increased ductility of alloys is mainly attributed to the RA with a suitable stability via a transformation-induced plasticity (TRIP) effect, and a matrix containing reduced carbon and chromium content. However, the impact toughness of MCMSSs largely depends on M23C6 carbides.

In the present work, a novel medium carbon martensitic stainless steel (MCMSS) with an excellent combination of strength, ductility, and impact toughness was designed on the basis of quenching-tempering and partitioning (Q–T&P) technology. Q–T&P is an identical heat treatment with a standard quenching and tempering (Q–T) process but has the same role with quenching and partitioning (Q&P) on microstructure control, i.e., promoting carbon-rich retained austenite via inhibiting carbide precipitation. Results show that, without compromise on strength, the total elongation and room temperature impact toughness, i.e., 9.6 pct and 90 J cm−2, of the proposed alloy (23Cr13MnSi) increase by 14 and 110 pct, respectively, as compared to those of the commercial AISI 420. The significant improvement of ductility and impact toughness in the proposed alloy is mainly a result of the gradual transformation induced plasticity (TRIP) effects, which are caused by carbon-rich retained austenite with heterogeneous stability and carbide-free martensite formed in the Q–T&P process

Athermal martensite transformation in duplex fcc+hcp Co-based entropic alloys during continuous cooling was investigated in-situ. The real time observation was carried out using high temperature confocal laser scanning microscopy (HT-CLSM). This technique enables the detection of the athermal fcc to hcp transformation in entropic alloys, which is not sensitively detected by conventional thermomechanical methods e.g. dilatometer. The martensite fraction increases with increasing martensite starting temperature, and vice versa. Meanwhile, the martensite starting temperature decreases with the increasing grain size. In addition, the morphology and nucleation sites for martensite formation is discussed. This is the first time the that HT-CLSM technique is utilized in the field of entropic alloys. This in-situ observation technique coupled with thermodynamic calculations may help in the design of entropic alloys through the tailoring of the desired microstructure.

The coarsening of cementite in a martensitic Fe–1C–1Cr (wt-%) alloy upon tempering at 700°C is investigated. When considering that the main location of cementite is at grain boundaries, classical coarsening theory can accurately predict the mean size evolution, while the predicted size distribution evolution disagrees with the experimentally observed log-normal distribution maintained throughout the whole tempering (5000 h). We conclude that classical theory of coarsening, as given by Lifshitz–Slyozov–Wagner and included in the Langer–Schwartz Kampmann–Wagner numerical approach for modelling precipitation reactions, is not fully adequate to simulate coarsening of cementite for tempering in practice.

Martensitic microstructures with engineered precipitation of nano-scale carbides formed during tempering are key in the development of, for example, wear-resistant steels. These steels often experience multiple carbide precipitation with evolving compositions and where the metastable phases transition to more stable carbide phases. In the case of low alloy CrMoV steels, the incomplete understanding of the complex precipitation evolution during tempering is preventing their further optimization. Therefore, in the present work we perform an in-depth experimental investigation of the precipitation of carbides in an Fe-0.32 C-1.4 Cr-0.8 Mo-0.14 V-1.1 Si-0.8 Mn-0.7 Ni (wt.%) martensitic steel tempered at 550 degrees C by transmission electron microscopy and atom probe tomography. The experimental data is compared to thermodynamic calculations and these are subsequently used to expose further potential improvements to the alloying strategy.