Martensitic Phase Research Articles

Strength‐Ductility Synergy in an Austenitic Stainless Steel Processed by Warm Rolling and Subsequent Cryogenic Treatment

The austenitic stainless steel undergo warm rolling, followed by cryogenic treatment. The warm‐rolled and cryogenically treated (WR‐CT) sample exhibits superior strength and ductility compared to the warm‐rolled (WR) sample, achieving a yield strength of 949 MPa and an elongation of ≈56.2%. The enhanced strength is attributed to the higher dislocation density, additional grain refinement, and the presence of martensite in the WR‐CT sample. The WR and WR‐CT samples exhibit similar deformation mechanisms. In the initial stage, dislocation slip predominantly governs the deformation. During the intermediate stage, the transformation‐induced plasticity (TRIP) effect dominates the deformation mechanism. In the final stage, nearly all austenite transforms into martensite, and deformation mainly occurs via dislocation slip within the martensite phase. However, in the WR‐CT sample, the TRIP effect is delayed during the stable processing stage (≈8–23%) due to the higher mechanical stability of austenite, resulting from further grain refinement and higher dislocation density. Additionally, the coordinated deformation of strain‐induced martensite with varying orientations contributes to delayed necking, achieving an optimal balance between strength and ductility.

Advanced phenomenological models guided heat treating processes for LPBF Ti-6Al-4V alloy

Ti-6Al-4V (Ti64) alloy produced by laser powder bed fusion (LPBF) technique consists of an α′-martensite phase, which is usually brittle. Hence, heat treatments are required to promote the α՛→α+β and/or α→β transformations for microstructural optimization to achieve balances between strength and ductility. To guide the selection of heat treatment parameters for the design of LPBF Ti64 microstructures exhibiting optimized strength and ductility, this work explores the suitability of a Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Lifshitz-Slyozov-Wagner (LSW) models. The work showed that the JMAK model can effectively model the heat treatment processes to predict volume fraction of constituent phases in LPBF Ti64. The transformation kinetics was predicted by an extended non-isothermal JMAK model based on differential scanning calorimetry (DSC) experimental data recorded during continuous heating. Moreover, hardness, as a mechanical property index, was measured for a set of specimens. The hardness initially decreased, then remained constant, and finally slightly increased with the increase in β phase fraction and underlying evolution of microstructure. When the β phase fraction was small < 3%, hardness decreased with the continuous coarsening of α lath during heat treatment process. The measured hardness and α lath width for the specimens followed the Hall-Petch relationship. Moreover, the α lath width and heat treatment time at a given temperature were found to undergo the LSW model. Hardness became independent on the β phase fraction of 3-8%, owing to the competition between α lath coarsening and solution strengthening. The hardness slightly increased when the β phase fraction was greater than 8%, because solid solution strengthening of the α phase played a dominant role over the coarsening of α lath width. The experimentally verified JMAK and LSW models are therefore discussed as effective tools to guide the selection of heat treatment parameters for designing the microstructure and hardness/strength of LPBF Ti64 alloy.

An efficient low-grade thermal energy dual-driven elastocaloric cooling system: Thermodynamic cycle analysis, economic and sustainability assessment

Elastocaloric cooling is regarded as the most promising emerging refrigeration technology, with heat driven elastocaloric cycles at the forefront of research. However, existing systems often have actuators with a mass of 4–20 times that of the refrigerant, resulting in bulky structures and low thermodynamic efficiency. Enhancing system compactness and efficiency has thus become a critical research challenge. This study proposes an innovative low-grade thermal energy dual-driven elastocaloric cooling system, achieving a 1:1 mass ratio (mActuator/mRefrigerant) between the actuator and refrigerant, significantly improving system compactness and coefficient of performance while effectively reducing costs. A three-dimensional numerical and geometric model was developed to analyze the thermodynamic cycle, examining the effects of material, operational, and geometric parameters on cooling performance, and a “category-wise optimization” strategy was employed to determine optimal parameter combinations. The cost and environmental impacts of the new system were evaluated and compared with vapor-compression air conditioning. Results show that the difference in transformation temperatures between austenite and martensite phases significantly impacts cooling performance. Optimal flow rates and cycling frequencies are essential to avoid performance loss. Considering the coefficient of performance and heat transfer, a 1:1 mass ratio is optimal for geometric parameters. The system achieves a maximum coefficient of performance of 0.158, exceeding the existing two schemes by more than 2 times and 7 times, with a maximum temperature rise of 13.1 K. The proposed system is economically feasible and environmentally friendly, providing valuable theoretical foundations and technical solutions for developing sustainable alternative refrigeration technologies.

Unveiling Heat Treatment Effects on Hardness and Dimensional Properties of Power Bed Fusion‐LB/M/18Ni300 Turning Tool with Transpiration Cooling Channels

This study incorporates five different transpiration cooling channels, two circular, two hexagonal, and one novel bio‐inspired blood vessel that are designed in a monolithic turning tool and produced via metal additive manufacturing. They are heat treated under direct aging (DA1 and DA2) and solution heat‐treatment (SHT1 + DA1, SHT1 + DA2, and SHT2 + DA1) conditions to study the effects of heat treatment on the hardness and dimensions of complex internal geometries as they play a significant role in cooling effectiveness. Microstructural characterization for DA revealed the formation of lath‐type martensitic phases with uniform distribution, which improves the microhardness significantly, with SHT2 + DA1 achieving the highest hardness 641.2 HV and DA2 producing 611.7 HV. From the X‐ray microcomputed tomography, various heat treatments have very insignificant deviations in the dimensions of the transpiration channels (0.05% to 1%) from the as‐printed dimensions. Further, variations in design are not associated with the heat‐treatment process and the achieved hardness. This study helps create a standard operating procedure for heat treatment for complex internal channels with minimal dimensional deviations and highest hardness.

Microstructure evolution and strain rate sensitivity of ductile Hf20Nb10Ti35Zr35 medium-entropy alloy after thermal cycling

The medium-entropy alloy Hf20Nb10Ti35Zr35 has a ductile BCC structure in the as-solution-treated state and could be age-hardening with fine precipitates. In this study, the thermal stress produced by thermal cycling was found to accelerate the precipitation of the α" and the evolution of ω → α phase of Hf20Nb10Ti35Zr35. The α''-martensite phase of Hf20Nb10Ti35Zr35 tended to grow in the (001)β plane, with the misorientation angle being concentrated around 52° after 10 thermal cycles. Moreover, the misorientation angle tended to have a bimodal distribution after 50 thermal cycles. Nanomechanical testing was performed to measure the strain rate sensitivity (SRS) at room temperature. The result shows that SRS of Hf20Nb10Ti35Zr35 changed from a small positive value of 0.09247 to a small negative one of either −0.02125 or −0.0445. This was mainly attributable to the decrease in the volume of the β phase or the increase of the α" + α composite phase. This demonstrates that thermal cycling treatment of the present alloy could induce more precipitation of the second phase for hardening and enhance uniform deformation behavior.

Investigation of microstructure, mechanical performance, and corrosion resistance of thin-walled titanium welded pipe by annealing process

Thin-wall titanium welded pipe has important applications in the fields of new energy vehicles, aerospace, and heat exchangers, the mechanical properties and corrosion resistance after high-frequency induction welding are rarely studied. To improve the comprehensive performance of thin-walled titanium welded pipe, a heat process and the performance optimization mechanism were studied in this paper. In addition, we systematically explored the influence of heat treatment conditions on the microstructure, mechanical performance, and dissolution behavior of the welded pipe through various methods, including tensile tests, immersion tests, and electrochemical tests. The results indicated that heat treatment primarily regulates the quantity and distribution of martensitic phases, acicular structures, and serrated structures in the welded joints, thereby impacting the mechanical performance of the welded pipe. By following high-temperature annealing at 800 °C for 0.5 h, with a heating rate of 5 °C/min, the acicular and serrated structures in the welded joints completely disappeared, forming an annealed equiaxed structure consistent with the base material. Subsequent furnace cooling reduced internal thermal stresses within the grains, resulting in relatively smooth grain boundaries, reduced small-sized grains, and a more uniform distribution of grain sizes, ultimately eliminating the residual stresses. Consequently, the welded pipe exhibited outstanding comprehensive performance, including ultimate tensile strength, Vickers hardness, and fracture elongation of 439.2 MPa, 178.3 HV, and 18 %, respectively. Notably, under room temperature, after immersion in a 3.5 % sodium chloride solution for 28 days, there was no significant change in the sample's mass. Therefore, the properties of thin-walled titanium welded pipe can be improved by annealing process, and it provides strong support and reliable evidence for its application in related fields.

Simultaneous enhancement of the strength and plasticity of Fe[sbnd]12Mn steel through modulating grain morphology

The relationship between grain morphology and tensile properties of Fe-12Mn steel thermo-mechanically processed by rolling annealing and re-cold rolling processes was investigated. The results indicated that the re-cold rolling deformed the mostly austenite grain morphology from equiaxed to lamellar, and the steel exhibited a yield strength of 1295 MPa, tensile strength of 1518 MPa, and elongation of 18 %. Compared to the experimental steel without re-cold rolling processes, more than 300 MPa tensile strength was improved and elongation increased from 14 % to 18 %. The steel demonstrated remarkable enhancement in mechanical properties owing to the “multistage TRIP effect”, which referred to the sequential martensitic phase transformation of austenite with varying stability as the strain increased. The change in grain morphology reduced the rate of twin formation and increased the nucleation point of the martensitic phase transition, this also stabilized the retained austenite by increasing the hard phases surrounding the retained austenite, which had multistage stability. These factors contributed to the “multistage TRIP effect”, which could be sustained to higher strains and presented regular fluctuations in work-hardening rates

An economically viable hybrid additive manufacturing process for the fabrication of hierarchically controlled porous biodegradable Fe-Mn scaffolds

The present work aims to develop a novel hybrid additive manufacturing (AM) route for fabricating biodegradable hierarchically controlled porous (HCP) Fe-Mn alloy-based scaffolds. Specifically, a systematic combination of polymer AM and loose powder sintering was used. The morphological, mechanical, magnetic and electrochemical properties of the Fe-30Mn scaffold were evaluated and compared with pure Fe. Proper bonding of particles with some random porosity was observed in the sample. XRD spectrum shows the presence of austenite (γ) and martensite (ε) phases in the sample. The prepared Fe-30Mn samples exhibited porosity = 23.08%, compressive yield stress = 127.28 MPa, modulus of elasticity = 25.9 GPa and corrosion rate = 0.75 mmpy. Moreover, the Fe30Mn sample showed weak magnetic properties, thereby making it MRI-friendly. Better properties of the prepared Fe30Mn sample as compared to pure Fe with close compatibility to the human bone were obtained. Additionally, complex structures of Fe30Mn using the developed methodology were fabricated to establish the efficacy of the process for biodegradable implant application.

Influence of solid solution time on microstructure and precipitation strengthening of novel maraging steels

Effective subsequent heat treatment is crucial for achieving the desired microstructure and excellent mechanical properties in laser-deposited high-performance maraging steel. In this paper, we systematically investigate the synergistic relationship and tuning mechanism of different solution treatment times on the microstructure-property synergy of new maraging steels fabricated using laser direct energy deposition (LDED). To determine the optimal heat treatment process, solution treatment was conducted at 840 °C for varying durations, followed by aging at 530 °C for 2 h to induce precipitation strengthening. The results indicate that after 2 h of solution treatment, the alloy exhibits optimal ductility with an elongation of 7.90 % ± 0.15 %, attributed to the refinement of the martensitic matrix and precipitated phases, along with the formation of a small amount of residual austenite. When the solution treatment time is extended to 4 h, the alloy achieves its highest tensile strength, reaching 1958 ± 24 MPa. However, the elongation decreases to 7.31 % ± 0.12 % due to the coarsening of the martensite and secondary phase particles. After 6 h of solution treatment, significant coarsening and aggregation of the martensite and Fe2Mo intermetallic compounds markedly reduce the hardness, strength, and toughness of the alloy. By adjusting the solution treatment time, the size, morphology, and distribution of the martensitic matrix, Fe2Mo, and nanoscale precipitated phases play a critical role in the strengthening and fracture processes. Therefore, optimizing the precipitation behavior of the martensitic matrix and Fe2Mo intermetallic compounds through rational solution heat treatment is key to enhancing the mechanical properties of laser-deposited new maraging steels.

Investigation of the history-dependent nonlinear micro-viscoplasticity behavior of dual-phase steel using crystal plasticity finite element method

During tensile loading–unloading cycles, steels composed of different phases may exhibit history-dependent nonlinear micro-viscoplasticity behavior. To analyze the mechanism of the evolution of the elastic behavior, in this study, loading–unloading experiments were conducted for dual-phase steels, comprising martensite and ferrite, with a thickness of 1.1 mm and a tensile strength of 980 MPa. To investigate the complex elastic behavior at the microscale, a 3D representative volume element modeling the steel microstructure was generated and the elastic response was examined using the rate-dependent crystal plasticity finite element method. The differences of the plastic deformation in the ferritic and martensitic phases were analyzed. To describe the history-dependent nonlinear micro-viscoplasticity behavior for macroscopic simulations, a micro-viscoplasticity model was developed. The developed material model successfully captured the repeated loading–unloading behavior of DP980 steel.

Achieving high strength and low yield ratio by constructing the network martensite-ferrite heterogeneous in low carbon steels

In this research, focusing on low-carbon steel, a martensite-ferrite heterogeneous structure dual-phase (MFDP) steel with a network morphology where ferrite is surrounded by martensite was obtained via cyclic annealing and subcritical quenching heat treatment processes. With the initial microstructure of ferrite and lamellar pearlite, a spherical pearlite and martensitic structure surrounding the ferrite was first obtained by applying the cyclic annealing process near the Ac1 temperature. Subsequently, the annealed structure was subjected to subcritical quenching heat treatment, thereby establishing a network-like martensite-ferrite dual-phase heterogeneous structure and named N-760 °C and N-780 °C. In comparison with the ferrite-martensite dual-phase steel where ferrite envelopes martensite, N-780 °C witnessed a marked increase in tensile strength and uniform elongation, while the yield ratio dropped by 20 %. Through cyclic loading and unloading tensile tests, it was found that the N-760 °C showed a more obvious heterogeneous deformation-induced (HDI) strengthening effect. The results from electron backscattering and transmission electron microscopy indicate that, in the N-760 °C, a small quantity of dislocations is produced in the ferrite due to the martensitic phase transformation prior to the tensile test. During the tensile process, as the strain increases, the ferrite undergoes significant deformation, and the intragranular dislocations re-arrange to form dislocation cells and deformation-induced grain boundaries (SIBs). Meanwhile, geometrically necessary dislocations (GNDs) accumulate at the ferrite/martensite interface. Therefore, the non-coordinated deformation between the mesh-like dual-phase microstructure offers additional HDI strengthening for MFDP steel.

Phase transformation mechanism and microstructure of a Y-doped TiAl gas-atomized powders

In this study, the phase transformation mechanism of a yttrium-containing β-solidified TiAl alloy (Ti-43Al-9 V-0.3Y at.%), prepared by gas atomization, was systematically investigated. X-ray diffraction, electron backscatter diffraction, scanning electron microscopy, and transmission electron microscopy were utilized to comprehensively analyze the morphology and microstructure of powders with varying sizes as well as the form and distribution of yttrium and its influence on the phase transformation of the powders. The results show that the solidification phase structure of the powders exhibits significant variations: the ultra-fine powder consists of α’ martensite and remaining β phase, while the medium-sized powder is solely composed of β0 phase. The large-sized dendritic powder comprises β0, α’ martensite and α2 phase. With an increase in powder size, there is a corresponding increase in the content of α2 phase, whereas the content of martensite initially rises and subsequently declines. Additionally, yttrium is present in the form of multiscale Y-rich precipitates (YAl2 and Y2O3) within the matrix, and the segregation degree gradually increases with increasing powder size. The primary factors contributing to the disparity in solidification structure include cooling rate and segregation defects. A faster cooling rate and a higher supercooling degree will inhibit the β → α transition, while the Y-rich precipitated phase forms a pre-existing strain zone around it, providing an effective site for martensitic nucleation. In summary, these findings offer novel insights into the mechanism of phase transformation in yttrium-containing β-solidified TiAl alloy, thereby contributing to further advancements in the theory of rapid solidification for TiAl alloys.

Exceptional strength-ductility synergy in the novel metastable FeCoCrNiVSi high-entropy alloys via tuning the grain size dependency of the transformation-induced plasticity effect

In-depth knowledge of the coupling between grain refinement and the transformation-induced plasticity (TRIP) effect in metastable alloys is a viable approach for the improvement of strength-ductility synergy, which needs systematic research with consideration of commercial austenitic stainless steels and novel high-entropy alloys (HEAs). Accordingly, in the present work, two Si-containing metastable HEAs in the Fe47Co30Cr10Ni5V8-xSix system (x = 3 and 6 at.%) were designed, and the TRIP-assisted AISI 304L stainless steel was also considered for comparison. The alloys were processed by cold rolling and annealing to obtain different grain sizes. Reducing the stacking fault energy (SFE) through adjusting chemical composition contributes to minimizing the detrimental effect of grain refinement on the ductility of TRIP alloys, while extremely low SFE must be avoided owing to the fast kinetics of deformation-induced martensitic phase transformation, which leads to the deterioration of ductility. In contrast to AISI 304L stainless steel, a strong TRIP effect was maintained upon grain refinement in the Fe47Co30Cr10Ni5V2Si6 HEA due to the remaining apparent SFE in the appropriate TRIP range. The tuned kinetics of martensitic transformation was found to be responsible for the exceptional ductility (∼65%) of Fe47Co30Cr10Ni5V2Si6 HEA at an ultrahigh tensile strength of 1250 MPa. Therefore, considering the identical trend of SFE with grain size, an appropriate initial SFE value is important for tuning the grain size dependency of the TRIP effect. Moreover, the ultrahigh strength was attributed to the high volume fraction of α΄-martensite as well as the high strength of the martensite phase due to the high Si content. Accordingly, for achieving strong-yet-ductile HEAs, a high Si content is recommended to benefit from solid solution strengthening in the martensite phase, a specially-designed chemical composition is needed for attaining a high volume fraction of α΄-martensite, and SFE should be in a desirable range to tune the kinetics of martensitic phase transformation.

Investigation of phase stability and martensitic transformation of all-d-metal Heusler alloys Fe2CrIr and Fe2CrV

The magnetism, elastic properties, phase stability and martensitic phase transition of the all-d-metal Heusler alloys Fe2CrIr and Fe2CrV are systematically investigated using first-principles calculations. Compound Fe2CrIr containing late transition metal atoms tends to have a Cu2MnAl (L21)-type structure and exhibits antiferromagnetism. Fe2CrV is relatively stable in the Hg2CuTi (XA)–type structure and exhibits ferromagnetism. Differences in mechanical stability under pressure arise from differences in phonon hybridisation. The response of Fe2CrIr to external pressure stems from a high pressure-induced martensitic transformation. On the contrary, the pressure inhibited the martensitic phase transition of Fe2CrV. The lower the value of C′, the more unstable the cubic phase. This work analyses in detail the different mechanical behaviours of Fe2CrIr and Fe2CrV under pressure and the mechanism of pressure-induced martensitic transformation, which may improve a new insight for pressure regulated martensitic phase transformation.

Black Body Cavity Apparatus for Measuring the Emissivity of Nickel-Titanium-Based Shape-Memory Alloys and Other Metals

Nickel-titanium (NiTi)-based shape-memory alloys (SMAs) have various applications in biomedicine, actuators, aerospace technologies, and elastocaloric devices, due to their shape-memory and super-elastic effects. These effects are induced in SMAs by a reversible martensitic phase transformation. This transformation can be achieved by applying stress or temperature differences. It is extremely difficult to measure the temperature of NiTi elements with contact thermometers because they disturb the phase transformation phenomenon. An alternative approach is contactless infrared thermography for which accurate emissivity data are mandatory. To determine the temperature distribution in NiTi components with an infrared camera, a newly constructed apparatus is presented to measure the emissivity of metals. For this purpose, a state-of-the-art infrared camera was employed to analyze the emissivity behavior with temperature, especially during the phase transformation. The emissivity of the NiTi samples was systematically studied by comparing them with a reference-quality black body cavity (BBC) having an emissivity better than 0.995. Calibration measurements revealed that the maximum deviation of the BBC temperature measured with the infrared camera was only 0.15% (0.45 K) from its temperature measured with the built-in contact thermometers. The apparatus was validated by measuring the emissivity of a polished aluminum sample and found to be in good agreement with literature data, particularly for temperatures above 340 K. Finally, the emissivity of two rough and one polished NiTi samples was measured covering the temperature range from 313 K to 423 K with an expanded uncertainty of about 0.02 (k=2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$k=2$$\\end{document}). The studied NiTi samples have an atomic percent composition of Ni42.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Ni}_{42.5}$$\\end{document}Ti49.9\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Ti}_{49.9}$$\\end{document}Cu7.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Cu}_{7.5}$$\\end{document}Cr0.1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Cr}_{0.1}$$\\end{document}. We observed that the emissivity of NiTi varies between 0.17 and 0.31 depending on temperature. As the temperature rises, the emissivity increases rapidly during the phase transformation, and it decreases gradually beyond the austenite finish temperature. In addition, the emissivity of NiTi depends on the microstructure of the martensite and austenite phases and the surface roughness of the samples.

Unveiling the mechanism of the carbon ordering and martensite tetragonality in Fe–C alloys studied via deep-potential molecular dynamics simulations

The martensitic transformation plays a pivotal role in the strengthening and hardening of steels, yet an accurate interatomic potential for a comprehensive description of the martensitic phase formation in Fe–C alloys is lacking. Herein, we develop a deep learning-based interatomic potential to perform molecular dynamics (MD) simulations to study the martensitic phase transformation across a range of carbon (C) concentrations. The results reveal that an increased C concentration leads to a suppression of phase boundary movement and a deceleration of the phase transformation rate. To overcome the timescale limitations inherent in MD simulations, metadynamics sampling was employed to accelerate the simulations of C diffusion. We find that C atoms tend to cluster at distances equivalent to the lattice parameter of Fe with the same sublattice occupation, leading to local lattice tetragonality. Such C-ordered structures effectively inhibit dislocation movement and enhance strength. The stress field induced by dislocations facilitates a higher degree of ordering. The formation of C-ordered structures is identified as a potentially crucial strengthening mechanism for martensitic steels. The consistency between our simulation results and reported experimental observations underscores the effectiveness of the developed DP model in simulating martensitic phase transformation in Fe–C alloys, providing detailed insights into the mechanisms underlying this process.

Micromechanical testing and microstructure analysis of as-welded and post-weld heat-treated Ti-6Al-4V alloy

The microstructural evolutions and variations in mechanical performance of electron beam welded (EBW) Ti-6Al-4V (Ti64) alloy have been investigated. The effects of heat treatment on the microstructure of welded samples have been studied after post-welding solution treatment and ageing. The martensitic phase α′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\alpha }{\\prime}$$\\end{document} has been confirmed using transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) has been used to investigate the phase and grain morphology. Results showed that the martensitic α′ phase coarsened, the size of heat-affected zone (HAZ) changed and grains in the base materials (BMs) had grown after the post-weld heat treatments (PWHT). The tensile behaviour of electron beam welded Ti64 has been investigated using in situ tensile testing monitored by optical microscopy. The deformation and failure were directly revealed during the in situ tensile process. Results showed that the EBW Ti64 samples have different failure locations after receiving different post-weld heat treatments. The relationship between the post-weld heat treatments, microstructural evolution and mechanical properties of EBW Ti64 were investigated. Thermodynamic databases were used to predict mechanical properties—including the yield strengths—of the titanium alloy for different grain sizes, representing different post-weld heat treatment operations, and these were embedded into a finite element modelling framework to simulate the tensile testing specimens to understand the mechanical fields experienced such as stresses and strains, just prior to failure.

Achieving balanced mechanical properties in Fe–0.16C plain low-carbon steel by trimodal microstructure and fine cementite

In the present study, for the first time, a trimodal microstructure (including coarse, fine, and ultrafine ferrite grains) was created in plain low-carbon steel by simple thermomechanical processing including intercritical annealing (at 770 °C, 800 °C, and 830 °C), 75% cold rolling, and finally heat treatment at 550 °C. The results showed that martensite with two different morphologies consisting of lath (low-carbon) and plate (high-carbon) were created in the microstructures of 800-75% and 830-75% sheets due to the non-homogeneous distribution of carbon atoms as a result of not using the homogenization treatment. After the final heat treatment, the samples exhibited trimodal grain size distribution of ferrite including coarse grains (larger than 5 micrometers), fine grains (between 1 and 5 micrometers), and ultrafine grains (UFGs) (smaller than 1 micrometer). The formation of ultrafine grains was ascribed to the presence of martensite with lath morphology in the 800-75% and 830-75% samples. The size of cementite particles in the 770-75%-550 sample was larger than the other two samples (800-75%-550 and 830-75%-550) owing to the presence of martensite phase only with plate morphology in 770-75%-550 steel. Unlike the 800-75%-550 sheet, the entire microstructure of the 770-75%-550 and 830-75%-550 samples had not undergone complete recrystallization and several deformed ferrite grains were still visible in the microstructures. The strength and hardness of the heat-treated sheets was larger than that of the as-received steel due to the presence of fine spherical cementite particles and fine ferrite grains in the microstructure of the heat-treated samples. Based on the obtained results, the 800-75%-550 sample revealed the best combination of strength-ductility-toughness due to trimodal microstructure along with the formation of fine spherical cementite particles. The failure mode of all heat-treated samples was ductile and there were both fine and coarse dimples in the fracture surfaces.

Depth-dependent energy absorption control of large pulsed electron beam (LPEB) irradiations for defect-free surface modification of metallic alloys

Electron beam surface finishing offers the ability to treat various materials and shapes while enhancing surface properties such as wear and corrosion resistance. However, its industrial application is limited by the formation of crater-like defects' pulse count, irradiation angle, and energy density, were optimized to achieve defect-free surfaces. By systematically investigating the synergistic effects of previously studied parameters (irradiation angle and energy density), the optimal conditions were established through strategic combination of these parameters, demonstrating enhanced surface quality and reduced defect formation. These were compared to previously optimized conditions that focused solely on increasing pulse counts. The new conditions significantly reduced the density of crater-like defects and produced smoother surfaces. This improvement is attributed to concentrated energy absorption near the surface and the creation of a high-temperature gradient, which prevents the introduction of non-metallic inclusions. Unlike conventional methods, where accumulated heat leads to phase transformations and reduces hardness and corrosion resistance, the new approach maintains a rapid cooling rate. This allows the surface to retain a fully martensitic phase even at 45 pulses, resulting in higher surface hardness. These findings highlight that the newly developed conditions not only produce defect-free surfaces but also impart enhanced mechanical properties. This approach suggests that future improvements in surface quality for electron beam-based processes can be achieved by considering adjustments to the irradiation angle and energy density.

Constitutive modeling of transformation-induced plasticity steels considering strength-differential effect

Transformation-induced plasticity (TRIP) steels undergo martensitic phase transformations due to their austenite phase. In this study, using 1-mm-thick TRIP steel at room temperature, the phase transformation behaviors under tensile and compressive modes were measured using a ferrite scope based on the detection of the magnetic volume. A strength differential (SD) effect was observed, where the tensile strength was lower than the compressive strength. The rate of tensile transformation was faster than that of compressive transformation. To account for the SD effect in finite element analysis, a martensitic kinetics-based constitutive model was developed, which was decomposed into elastic, plastic, Bain, and transformational parts. A larger transformational strain was generated in the tensile mode, and the asymmetric SD effect was captured well by the proposed model.