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Microstructure and defects strongly affect martensitic transformations in metallic alloys. Significant progress has been made in understanding the atomic-level processes that control the role of grain boundaries and precipitates in these solid-to-solid phase transformations. Yet, the role of dislocations and their structures on martensitic transformation temperature and the resulting microstructure remains unclear. Therefore, we used large-scale molecular dynamics simulations to study the forward and reverse transformation of a martensitic material modeled after Ni63Al37 under cyclic thermal loading. The simulations reveal that dislocations in the austenite phase act as one-dimensional seeds for the martensite phase, which is present at temperatures significantly above the martensite start value. We find a reduction in the dislocation density during cyclic thermal loading, which results in the increase in martensite and austenite transition temperatures, in agreement with experiments. Importantly, we extracted a critical martensitic nuclei size for developing stable domains and found that relatively low dislocation densities are needed to grow independent martensitic variants resulting in a multi-domain structure. [ABSTRACT FROM AUTHOR]

A magnetocaloric effect (MCE) due to adiabatic change of temperature was directly measured in an Fe47.1Mn26.1Ga26.8 alloy undergoing martensitic transformation. Its values in the high-temperature region were positive, while in the temperature range below temperatures of martensitic transformation, the adiabatic change of temperature in the magnetic field was negative. The x-ray diffraction analysis revealed the presence of a Heusler L21 (B2) phase and a γ-phase in the Fe47.1Mn26.1Ga26.8 alloy above temperature of martensitic transformation. The features of field-dependent magnetization and temperature variation in MCE indicate the occurrence of ferromagnetic-to-antiferromagnetic transition in the γ-phase, which is responsible for the observed inverse magnetocaloric effect. [ABSTRACT FROM AUTHOR]

In this work, the magnetocaloric effect in Ni50Mn36In5Sb9 alloy was increased by more than 50% through directional solidification, and the magnetic entropy change increased to 36.2 J kg−1 K−1 under the field of 5 T. The calculated results of differential scanning calorimetry curves confirmed the enhanced entropy change, which also increased from 29.7 to 40.7 J kg−1 K−1. Moreover, first-principles calculations show that the surface formation energy along the L21 (220) plane is the lowest at room temperature, and it is easy to form and undergo martensitic transformation from the (220) crystal plane. Directional solidification causes the alloy to grow basically toward the (220) crystal plane, improve atomic ordering, reduce grain boundaries, and increase grain size. Thereby, the magnetic entropy change is enhanced. [ABSTRACT FROM AUTHOR]

Utilizing molecular dynamics simulation, this study aims to explore the phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) when subjected to cyclic deformation. The influences of porosity and cyclic deformation on the phase transformation of NC NiTi SMAs are examined and discussed. The simulation results show that the increase in the porosity and number of cycles leads to a decrease in both the critical phase transformation stress and peak stress whereas an increase in the residual martensite, phase boundary, and interstitial atoms; the related results can be supported by previous experiments. After cyclic deformation, the reduction in the potential energy for the entire system during the tensile phase occurs at an earlier stage, indicating that the martensitic transformation occurs earlier as the number of cycles increases. Notably, the dissipated energy demonstrates a decrease with an increasing number of cycles, and the potential energy during the austenite elastic unloading stage undergoes a transition from a decreasing to an increasing trend due to the presence of residual martensite increasing with the number of cycles. [ABSTRACT FROM AUTHOR]

Manganese stabilizes the austenite and reduces the stacking fault energy in face‐centered cubic (FCC) structures, promoting the formation of hexagonal and cubic structures. This study investigates the phase transformation and mechanical behavior of extremely low‐carbon FeCrMn austenitic stainless AISI 201 steel subjected to cold drawing in three stages, ultimately reaching a true strain of 0.93. The objective is to examine the transformation from the parent FCC structure to hexagonal close‐packed and body‐centered cubic structures as a combination of transformation‐induced plasticity and twinning‐induced plasticity phenomena. X‐ray diffraction and electron backscatter diffraction analyses are utilized to investigate phase transformations under significant plastic deformation and the crystallographic orientation relationships between phases. Additionally, tensile and hardness tests are performed to assess the impact of plastic deformation on mechanical properties. Results indicate that a true strain of 50% is optimal for balancing strength and ductility in CrMnFe austenitic stainless steel. After applying a true strain of ε1 = 26.7%, yield strength (YS) increases by 192% and ultimate tensile strength (UTS) by 35%, while elongation reduces by 41%. With a further strain of ε2 = 57.5%, YS increases by 71% and UTS by 44%, but elongation drastically decreases by 94%. Applying the final strain of ε3 = 94.0%, YS and UTS only increase by 3% and 2%, respectively, while elongation further reduced by 40%. These findings suggest that a true strain of 50% shall be considered the maximum reduction for maintaining a balance between strength and ductility in CrMnFe austenitic stainless steel. [ABSTRACT FROM AUTHOR]

This study investigates the effect of laser treatment parameters on the tensile behavior of H13 tool steel in both quenched and tempered conditions. The effects of varying laser parameters on ultimate tensile strength and elongation at fracture which is the aim of this research were explored by employing a bionic unit to apply laser treatment with two parallel lines on the steel surface. These parameters are crucial for understanding material performance under tensile stress. Statistical analysis, including an analysis of variance (ANOVA), and fractographic analysis using SEM were conducted to analyze the results. The findings reveal a significant enhancement in the ultimate tensile strength of the laser-treated samples, with values ranging between 1342.27 and 1497.25 MPa, surpassing the non-treated substrate's strength of 1247.23 MPa. However, a decrease in elongation at fracture was observed, indicating increased brittleness post-laser treatment, with elongation values notably lower than the non-treated sample's 14.71%. The hardness values within the laser-treated zone exhibited a high variance, with the highest values found in the hardened zone ranging from 65.7 to 61.4 HRC, suggesting the formation of fine carbides and lath martensite due to rapid cooling and martensitic transformation. Furthermore, the depth of the laser-transformed zone, significantly influenced by laser power, varied between 1.30 and 1.9 mm, highlighting the critical role of laser parameters in affecting material properties. The study highlights a critical balance between ductility and strength in laser-treated H13 tool steel, which is essential for optimizing the material's performance for industrial applications. [ABSTRACT FROM AUTHOR]

In the present study, the isobaric heat capacity of ultralight magnesium-lithium alloys with composition of 21, 25 and 30 at. pct Li were measured in the temperature range 185–775 K, most measurements were made for the first time. Measurements were performed by the method of differential scanning calorimetry using a DSC 404 F1 setup. The estimated uncertainty of the obtained results was 2–3 pct. The temperature dependences and the tables of recommended data on their basis were developed for scientific and practical application. For all studied Mg-Li alloys an abrupt change in the heat capacity was observed at the temperatures of about 220–260 K, which is apparently caused by the martensitic phase transformation. It was found that the specific molar heat capacity values of Mg-Li alloys containing 21–30 at. pct Li in the temperature interval of 250–685 K practically coincide with each other and can be estimated within the limits of DSC measurement uncertainties using the heat capacity temperature dependence of solid magnesium. It is possible to estimate the heat capacity of the studied alloys (with an accuracy not exceeding the measurement uncertainty) using the Neumann-Kopp rule, but in a much narrower temperature range of 250–456 K. [ABSTRACT FROM AUTHOR]

Inorganic semiconductor materials are crucial for modern technologies, but their brittleness and limited processability hinder the development of flexible, wearable, and miniaturized electronics. The recent discovery of room-temperature plasticity in some inorganic semiconductors offers a promising solution, but the deformation mechanisms remain controversial. Here, we investigate the deformation of indium selenide, a two-dimensional van der Waals semiconductor with substantial plasticity. By developing a machine-learned deep potential, we perform atomistic simulations that capture the deformation features of hexagonal indium selenide upon out-of-plane compression. Unexpectedly, we find that indium selenide plastifies through a martensitic transformation; that is, the layered hexagonal structure is converted to a tetragonal lattice with specific orientation relationship. This observation is corroborated by high-resolution experimental observations and theory. It suggests a change of paradigm, where the design of new plastically deformable inorganic semiconductors can focus on compositions and structures that facilitate phase transformations, going beyond the conventional dislocation slip. [ABSTRACT FROM AUTHOR]

The influence of different heat‐treatment conditions on the microstructure evolution and mechanical properties of cold‐rolled Fe–7.69Mn–2.76Al–0.12C (wt%) steel is investigated using various techniques, including scanning electron microscopy, transmission electron microscopy (TEM), and X‐ray diffraction. In the results, it is shown that the stability of austenite is significantly affected by different partitioning temperatures. It is found that factors such as dislocation density, temperature, and grain size collectively influence partitioning behavior. In the observations, it is revealed that when the partitioning temperature is raised from 120 to 180 °C, the dislocation density of the face centered cubic phase within the test steel decreases from 10.38 × 1016 to 7.67 × 1016 m−2. Additionally, within specimens exhibiting higher dislocation densities, carbon element diffusion is more uniform. During the experiment, the poor stability of the austenite is found to be susceptible to stress‐inducing the phase of the martensitic transformation, and the type of the martensite transformation affects the deformation process and the performance of submission behavior. In the observations under TEM, a phenomenon where variations in dislocation density within individual austenite grains may lead to differing stability across grain regions is revealed, thereby triggering localized martensitic phase transformations. [ABSTRACT FROM AUTHOR]

The surface‐wrinkled shrinkage structure of spheres is very common and also quite intriguing to explore and reveal the analogous droplet solidification kinetics in the space environment. Here, a liquid‐to‐solid phase transition experiment of floating refractory alloy droplets is designed to study the surface wrinkled shrinkage structures aboard China Space Station with a 10−5 gravity level. The experimental Ti‐Ni‐V alloy droplets undergo an extremely high overheating temperature beyond 1800 K and then achieve a supercooling of 307 K, displaying a strong thermodynamic metastability. The spherical surface deformation structures of this alloy under microgravity coupled with extreme metastable solidification are explored. Moreover, through the active control of metastable rapid solidification, a surface monophasic structure and a B2 structure capable of stress‐induced martensitic transformation are facilitated by wrinkled shrinkage dynamics under microgravity. Their findings shed further light on the surface deformation structures during the microgravity solidification process. [ABSTRACT FROM AUTHOR]

Superelastic shape memory alloys with an integration of large elastocaloric response and good cyclability are crucially demanded for the advancement of solid-state elastocaloric cooling technology. In this study, we demonstrate a giant elastocaloric effect with improved cyclic stability in a <001>A textured polycrystalline (Ni50Mn31Ti19)99B1 alloy developed through directional solidification. It is shown that large adiabatic temperature variation (|ΔTad|) values more than 15 K are obtained across the temperature range from 283 K to 373 K. In particular, a giant ΔTad up to −27.2 K is achieved by unloading from a relatively low compressive stress of 412 MPa at 303 K. Moreover, persistent |ΔTad| values exceeding 8.5 K are sustained for over 12,000 cycles, exhibiting a very low attenuation behavior with a rate of 7.5 × 10−5 K per cycle. The enhanced elastocaloric properties observed in the present alloy are ascribed to the microstructure texturing as well as the introduction of a secondary phase due to boron alloying. [ABSTRACT FROM AUTHOR]

Digital image correlation (DIC) technology can capture strain anomalies and predict crack initiation providing early warning of material failure. Herein, DIC technique is used to calculate the full‐field strain by analyzing the grayscale patterns of speckle images during the tensile process. This allowed for an analysis of the microstructure evolution of the 316LN austenitic stainless steel (SS) at cryogenic temperatures. Deformation behavior of the 316LN SS at cryogenic temperatures is further analyzed using electron backscatter diffraction technology and transmission electron microscopy. Based on the strain field obtained by the DIC technique, a comprehensive analysis of the martensite volume fraction at different strains can be conducted. The results show that the strain localization under cryogenic deformation is related to martensitic transformation, while the random distribution of slip bands aligns with local strain peak values. Notably, fracture under cryogenic deformation occurs in regions where the strain field reaches its peak, rather than at locations with the maximum strain value. [ABSTRACT FROM AUTHOR]

In this study, titanium (Ti) alloy with carbon (C) solid solution was fabricated by laser powder bed fusion (LPBF) from Ti-TiC mixture powder to investigate the effect of C solid solution on microstructure and tensile properties of LPBF Ti alloy. XRD and TEM analysis showed that part of TiC particles was decomposed from the surface during melting process of LPBF and C was solid soluted in α-Ti for Ti-(0.09~0.4 wt%) C. The solid solution of carbon changed the microstructure of the LPBF Ti alloy to fine acicular microstructure due to the martensitic phase transformation. This was also observed in Ti-0.4 wt% C, where solid solution of carbon was not confirmed, suggesting that in Ti-0.4 wt% C, C was once solid solution and precipitated as TiC after phase transformation. LPBF Ti-C alloys showed good strength-ductility, UTS and elongation at break for Ti-0.2 wt% C were 746 MPa and 26.3%, respectively. [ABSTRACT FROM AUTHOR]

40Cr10Si2Mo steel is widely utilized because of its excellent mechanical properties, with grain size being a critical factor determining subsequent phase transformation processes and material microstructure performance. This paper reports the use of high-temperature laser scanning confocal microscopy (HT-LSCM) for in situ observation experiments to systematically investigate the growth of austenite grains and the martensitic phase transformation mechanism in 40Cr10Si2Mo steel during an 1800-second isothermal hold at temperatures ranging from 900 °C to 1250 °C. A dynamic model of austenite grain growth is established to optimize the parameters of the austenitic process. The results indicate that the austenite grain size increases continuously with increasing temperature and prolonged time. The Dong model predicts grain sizes that align well with experimental values. Austenite grains grow through grain boundary migration and grain annexation, whereas the precipitation and dissolution of M(Cr, Mo)23C6 affect grain growth. With prolonged time, some grain boundaries extend into new boundaries through subgrain rotation. The fine grains at lower temperatures reduce the initial temperature of the martensite transition (Ms), and the primary martensite nucleates along the grain boundaries of the prior austenite. The secondary martensite is attached to the primary martensite nucleus at a certain angle and grows in parallel while inhibiting the phase transition of the surrounding untransformed austenite. [ABSTRACT FROM AUTHOR]

In the present study, the effect of the Nb element on the lattice parameters, phase stability and martensitic transformation behaviors of Ti-Zr-based shape memory alloys was extensively investigated using the first-principles calculation. The lattice parameters of both the β parent phase and α′ martensite phase gradually decreased with Nb content increasing. For the α″ martensite phase, the lattice constant (a) gradually increased with the increase in Nb content, whereas the lattice constants (b and c) continuously decreased due to the addition of Nb. Based on the formation energy and density of state, β→α′ martensitic transformation occurred, as the Nb content was not more than 12.5 at.%. However, the Ti-Zr-Nb shape memory alloys with a Nb content higher than 12.5 at.% possessed the β→α″ martensitic transformation. However, both the largest transformation strain and sensitivity of critical stress to temperature (dσ/dT) can be optimized by controlling 12.5 at.% Nb in the Ti-Zr-Nb shape memory alloy, which was favorable to obtaining the largest elastocaloric effect. [ABSTRACT FROM AUTHOR]

The shape memory capabilities of Heusler alloy microwires with two different contents of Fe element instead of Ga element following step-by-step ordering heat treatment were explored based on the stoichiometric ratio of Ni2MnGa. The melt-drawing technique was used to create the polycrystalline microwires, and the two microwires had Fe atomic contents of 4.7 at.% and 5.5 at.%, respectively. The field emission scanning electron microscope was used to analyze the microwire's surface condition as well as the microscopic tensile fracture morphology. Using an X-ray diffractometer, the microwires' crystal structure was identified for phase analysis. Differential scanning calorimetry was used to examine the microwires' behavior during martensitic transformation. Using a dynamic mechanical stretcher, the elongation and recovery rate of microwires' one- and two-way shape memory behavior were examined. The findings demonstrated that the microwire phase structure, martensitic transformation behavior, and shape memory capabilities all displayed good properties after the heat treatment was ordered. [ABSTRACT FROM AUTHOR]

The growing use of titanium alloys has led to the gradual replacement of traditional processing methods by laser cutting technology, making it the preferred method for processing titanium alloy plates due to its high efficiency, precision, and adaptability. In this review, the characteristics of laser cutting technology and its application in titanium alloy plate processing are summarized, outlining several aspects of the cutting process, microstructure, and mechanical properties of the material after cutting, along with simulation predictions. Previous research categorized laser-cutting input parameters into beam parameters and process parameters, with the commonly used parameters being the laser power, cutting speed, and gas pressure. Various parameter combinations can achieve different cutting qualities, and seven indices can be used to evaluate the cutting process, with the surface roughness and slit width serving as the most common indices. Different auxiliary gases have shown a significant impact on the laser cutting quality, with commonly used gases consisting of nitrogen, argon, and air. Argon-assisted cutting generally results in better surface quality. Due to the rapid temperature change, the titanium alloy microstructure will undergo a non-diffusive martensitic phase transformation during laser cutting, producing a heat-affected zone. Experimental studies and simulations of the mechanical properties have shown that the occurrence of a martensitic phase transformation increases the hardness and residual tensile stress of the material, which reduces the fatigue strength and static tensile properties. In addition, studies have found that the more streaks appear on the cut surface, the lower the fatigue strength is, with fatigue cracks arising from the stripes. Hence, the established analytical solution model and three-dimensional finite element model can effectively predict the temperature distribution and residual stress during the cutting process. This can provide a better understanding of the high residual stress characteristics of the cutting edge and the stripe formation mechanism, allowing researchers to better explore the mechanism of laser cutting. [ABSTRACT FROM AUTHOR]

After a brief review of the history of pearlite structures in carbon steels, particularly on the pearlite formation mechanism, recent experimental investigations on the pearlite substructure are presented to express a distinct point of view. The water‐quenched high‐carbon pearlite substructure is investigated in detail by means of scanning electron microscopy and transmission electron microscopy. In the experimental observation results, it is shown that the cementite layer or ferrite layer in pearlite is composed of fine grains, which cannot be simply explained by traditional nucleation and grain growth mechanisms. However, the fine grain structure can be explained by the martensitic transformation products (twinned martensite with ultrafine grains of α–Fe and twinning boundaries ω–Fe (or ω–Fe3C)) and detwinning process. Upon tempering or detwinning, recrystallization of the ultrafine grains of both crystalline phases occurs to form the initial pearlite structure, while the grain size of both phases is still fine. The twinned martensite can be treated as the precursor of pearlite structure (pearlite nucleation stage), and the detwinning process can be regarded as the growth of the pearlite structure. Thus, the pearlite reaction can be described as follows: austenite → twinned martensite → pearlite. [ABSTRACT FROM AUTHOR]

To achieve a balanced combination of high strength and high plasticity in high-strength low-alloy (HSLA) steel through a hot-rolling process, post-heat treatment is essential. The effects of post-roll air cooling and oil quenching and subsequent tempering treatment on the microstructure and mechanical properties of HSLA steels were investigated, and the relevant strengthening and toughening mechanisms were analyzed. The microstructure after hot rolling consists of fine martensite and/or bainite with a high density of internal dislocations and lattice defects. Grain boundary strengthening and dislocation strengthening are the main strengthening mechanisms. After tempering, the specimens' microstructures are dominated by tempered martensite, with fine carbides precipitated inside. The oil-quenched and tempered specimens exhibit tempering performance, with a yield strength (YS) of 1410.5 MPa, an ultimate tensile strength (UTS) of 1758.6 MPa, and an elongation of 15.02%, which realizes the optimization of the comprehensive performance of HSLA steel. [ABSTRACT FROM AUTHOR]

Underwater laser direct metal deposition (UDMD) shows great application potential in underwater high-performance repair. The susceptibility of underwater repaired components to hydrogen embrittlement (HE) is not extensively researched yet. In this study, HE in 316L stainless steel repaired by UDMD is systematically investigated and compared with in-air direct metal deposited (DMD) 316L and conventionally manufactured (CM) 316L. The experimental results show that the high cooling rates involved in UDMD contribute to a "austenite-ferrite" solidification mode. The cellular structure with a high-density of dislocations increases the tensile strength of as-built UDMD 316L. Thermal desorption spectroscopy analysis reveals that the hydrogen concentrations are similar for 316L manufactured by three processing methods, suggesting that the complex microstructural features by UDMD and DMD do not provide additional H-trapping sites. SSRT results show that the HE susceptibility is in the sequence: CM 316L (30.7%) > UDMD 316L (15.1%) > DMD 316L (9.6%). Compared with CM 316L, the stable cellular structure and higher stability towards martensitic transformation of as-built 316L are beneficial for the HE resistance. In the hydrogen-charged UDMD 316L, the hydrogen-assisted dislocation movement and mechanical twinning formation enhance the localized plasticity and induce cracking. The micron-sized oxide particles and micro-pores further contribute to the fast decohesion of interfaces in the presence of hydrogen. Those combined effects lead to the reductions in tensile strength and elongation of the hydrogen-charged UDMD 316L. • High cooling rates in UDMD lead to cellular structures with high dislocation density. • Complex microstructures in UDMD 316L do not provide additional H-trapping sites. • HE susceptibility of UDMD 316L is larger than that of in-air DMD 316L. • H-assisted dislocation movement and twinning formation induce cracking of UDMD 316L. • Micron-sized oxide particles and micro-pores with H contribute to premature failure. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) are the key components of actuators and sensors due to their shape memory effect and superelasticity. However, the thermal hysteresis associated with martensitic phase transformation limits their use in the long-duration precise control. We report a strategy that obtains SMAs with low hysteresis by constructing a composition-temperature pseudo phase diagram. This strategy is inspired by the physically parallel ferroelectric system where the hysteresis is minimized at the multiple phase coexistence point, due to the absence of energy barrier across different phases. Following this, an alloy in the phase diagram with a low hysteresis of about 5 K is synthesized. In contrast, those alloys compositionally different from the optimal one have large hysteresis. Microstructures characterization and diffraction analysis are employed to identify the multiple phase coexistence. The proposed strategy should be general and can shed light on the rational design of SMAs with low hysteresis. [ABSTRACT FROM AUTHOR]

Effect of microalloying by boron, added by two different methods, on the microstructure and martensitic transformation in as-cast and aged Ni54Fe19Ga27 alloys was studied. The increase of the aging time of the Ni54Fe19Ga27 alloy from 0.5 to 10 h at 823 K leads to the formation of a large volume fraction of the ordered γ′-phase. Microalloying with boron inhibits the precipitation of the γ(γ')-phase and prevents its ordering. The parameters of martensitic transformation are most stable with increasing aging time of a (Ni54Fe19Ga27)99.7B0.3 alloy when boron is added in the form of a NiB alloy. [ABSTRACT FROM AUTHOR]

The paper studies the alloying effect of the quenched Ni-low (Ni49.5Ti35.5Hf15)100–XNbX and Ni-rich (Ni50.3Ti34.7Hf15)100–XNbX alloys (X = 0, 15 at.% Nb) on the B2-B19' martensitic transformation, viscoelastic properties and microstructure. Alloying of Ni-low and -rich alloys with Nb, provides the β-Nb phase precipitation within the (Ti, Hf)2Ni phase. After alloying, the transformation temperature and austenite elastic modulus decrease in Ni-low alloys and increase in Ni-rich alloys, thus making them similar. [ABSTRACT FROM AUTHOR]

The two-way shape memory effect with reversible compressive strain up to 5.1% and high cyclic stability is obtained for Ni44Fe19Ga27Co10 (at.%) single crystals by stress-induced martensite aging at 398 K for 1–3 h, under 650 MPa. 100 cycles of thermal treatment at temperatures not exceeding aging temperature, do not affect the temperature of the martensite formation, and reversible strain reduces merely by 0.4%. Thermal cycling with aging overheating significantly decreases reversible strain down to 1% due to the austenite stabilization. [ABSTRACT FROM AUTHOR]

In this work emphasis was given to determine the evolution of the retained austenite phase fraction via X-ray diffractometry technique in the as-hardened AISI 440C martensitic stainless steel surface subjected to cavitation for increasing test times. Scanning electron microscopy results confirmed the preferential carbide phase removal along the prior/parent austenite grain boundaries for the first cavitation test times on the polished sample surface during the incubation period. Results suggest that the strain-induced martensitic transformation of the retained austenite would be assisted by the elastic deformation and intermittent relaxation action of the harder martensitic matrix on the austenite crystals through the interfaces between both phases. In addition, an estimation of the stacking fault energy value on the order of 15 mJ m−2 for the retained austenite phase made it possible to infer that mechanical twinning and strain-induced martensite formation mechanisms could be effectively presented in the studied case. Finally, incubation period, maximum erosion rate, and erosion resistance on the order of 7.0 h, 0.30 mg h−1, and 4.8 h μm−1, respectively, were determined for the as-hardened AISI 440C MSS samples investigated here. [ABSTRACT FROM AUTHOR]

Martensitic transformations, viewed as continuous mappings between triply periodic minimal surfaces (TPMSs), as suggested by Hyde and Andersson (Z. Kristallogr. 1986, 174, 225–236), are extended to include paths between the initial and final phases. Reversible transformations, which correspond to shape-memory materials, occur only if lattice points remain at flat points on a TPMS throughout a continuous transformation. For the shape-memory material NiTi, the density functional calculations by Hatcher et al. [Phys. Rev. B2009, 80, 144203] yield irreversible and reversible paths with and without energy barriers, respectively, in agreement with our theory. Although there are TPMSs for face-centered and body-centered cubic crystals for iron, the deformation between them is not reversible, in agreement with the non-vanishing energy barriers obtained from the density functional calculations of Zhang et al. (RSC Advances2021, 11, 3043–3048). [ABSTRACT FROM AUTHOR]

AISI 301LN metastable austenitic stainless steels (MASSs) are known to exhibit high work‐hardening rates attributed to martensite evolution during straining which results in the second‐phase strengthening mechanism. This enhances surface hardness and potentially reduces wear and rolling contact fatigue (RCF). The aim of this study is therefore to evaluate the work hardening, wear, and RCF performance of AISI 301LN metastable austenitic stainless steel as a possible alternative material for rolling and sliding applications by varying the contact conditions such as slip ratio against the standard R350HT rail steel using a twin‐disc simulator. The results show that 301LN MASS has high surface work hardening and increased hardness because of martensitic transformation, while R350HT rail steel has only slightly changed. The formation of a hard martensitic phase is also confirmed by X‐ray diffraction analysis as well as by two other techniques, indicating a secondary hardening effect. Although it shows potential for work hardening, its susceptibility to wear‐related problems may make it less suitable for rolling and sliding applications due to a cracking and spalling mechanism induced by martensite formation. The surface strains required for the observed martensite formation are calculated and found to be very high, approaching 0.4. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were conducted to explore nanoindentation-induced martensitic transformation in NiTi shape memory alloys. Three atomic compositions and two indenter sizes were considered. During loading, the B2 structure underwent martensitic transformation to the B19' structure, accompanied by atomic rearrangement in the form of bands. After unloading, a portion of the B19' phase underwent reversible transformation, while the remainder remained in the form of bands, resulting in residual strain. This effect was accentuated by the more localised deformation promoted by the smaller indenter. The influence of atomic composition was observed not to play a key role in the mechanical and structural properties, contrary to previous reports on tension and compression tests. Our findings shed light on the intricate interplay of factors influencing the mechanical performance of NiTi shape memory alloys at the atomic scale, providing crucial insights for the design and understanding of such materials. [ABSTRACT FROM AUTHOR]

The effect of Ru addition on the martensitic transformation and mechanical properties of Zr50Pd50−xRux SMAs shape memory alloys has been studied. Strain–temperature experiments, differential scanning calorimeter, and scanning electron microscope analyses were performed to validate the theoretical predictions by DFT calculation regarding the influence of the C′ parameter on martensitic stability. Despite the absence of an observed shape memory effect under the tested conditions, the findings suggest that ZrPdRu alloys hold promise for high-temperature applications. [ABSTRACT FROM AUTHOR]

Widening the martensitic hysteresis ( T Dhys ) in NiTiNb shape memory alloys (SMAs) holds potential for broadening their working temperature range while enabling the room-temperature storage. In this study, the T Dhys is divided into two parts: the thermal-induced hysteresis ( T hys ) and the deformation-induced hysteresis ( T hys ′ ). In addition to decreasing the martensitic transformation start temperature ( M S ), it is found that grain refinement is an effective method for widening both T hys and T hys ′ of the Ni47Ti44Nb9 alloy, a commercial SMA widely used for shape memory couplings. According to thermodynamic analysis, grain refinement increases the dissipation energy ( Δ E dis ) (caused by thermal friction at martensite/austenite interface), thereby widening T hys . Moreover, the Ni47Ti44Nb9 alloy with finer grains has the potential to release more elastic strain energy ( Δ E el ) after deformation, thereby exhibiting a wider T hys ′ . When deforming a large strain, the alloy with finer grains generates more dislocations which stabilize the martensitic phase, thus further widening T hys ′ . [ABSTRACT FROM AUTHOR]

Interrupted and incremental tensile tests have been carried out on 316 austenitic stainless steel at different strain rates, revealing the correlation between the strain, strain rate, and temperature rise of martensitic transformation. The FERITSCOPE FMP ferrite tester and Telops FAST M2K infrared imager measured the martensite content and surface temperature rise at different strain rates, respectively. The effect of adiabatic temperature rise and strain rate on martensitic transformation was isolated by incremental tests. Compared with the incremental tensile test at a low strain rate (0.001 s−1), it has been found that the increase in strain rate has a negative effect on martensitic transformation, and the effect of strain rate on martensitic transformation seems to be more significant than that of adiabatic temperature rise. The formation and evolution mechanism of strain-induced martensitic transformation of 316 austenitic stainless steel were studied by electron backscattered diffraction. During the process of tensile plastic deformation, the nucleation and growth of new martensite mainly occurred near the twin boundary. [ABSTRACT FROM AUTHOR]

To predict the failure of mechanical parts, it is necessary to understand the residual stress and its source, i.e., inherent strain. In this study, the distribution of three-directional residual stress components in a carburized 18NiCrMo14-6 cylindrical roller test piece with a diameter of 80mm and length of 240 mm, was measured using the extended contour method (XCM), which combines the contour method and X-ray diffraction. Thereafter, an inverse analysis was applied to the measured residual stress to reproduce the associated inherent strain distribution. First, it was shown through numerical experiments of a carburized cylindrical specimen that the distribution of the three-directional residual stress components can be accurately reproduced using XCM. Next, it was demonstrated that the distribution of the three-directional residual stress components could be obtained using general-purpose equipment by physically measuring the same type of specimen. The inherent strain distributions were evaluated. Compressive residual stress and corresponding tensile inherent strain were detected in the carburized layer. By contrast, tensile stress and inherent contraction strain were determined in the non-carburized layer just before the carburized crust. Finally, the mechanism of inherent strain generation was investigated using a thermal--elastic--plastic analysis. Possible explanations include (i) increase in transformation strain due to a change in carbon content, (ii) delay in martensitic transformation, and (iii) decrease in the martensitic transformation rate due to a decrease in the cooling rate at the core. [ABSTRACT FROM AUTHOR]

In ultra low-carbon Fe-18%Ni alloy with a lath martensitic structure, the effects of microstructure and dislocation density on the yielding and deformation behavior were investigated. The microstructure of the matrix became finer with decreasing austenite grain size but no difference was found in the yielding behavior up to 2% deformation. Dislocation density was constant independent of the microstructure and was estimated to be about 1.5 x 1015/m². The dislocations that had been introduced by martensitic transformation formed a tangled dislocation cell structure within a lath and the amount of dislocation strengthening was determined by a critical stress that was required to make the tangled dislocations bow out. As a result, it was confirmed that the 2% proof stress of the steel used can be evaluated by adding the amount of dislocation strengthening to the friction stress. [ABSTRACT FROM AUTHOR]

The present study considers the samples of an Ti-6Al-4V alloy obtained by selective laser melting with the addition of a 10% Cu-Al powder mixture. The microstructure, elemental composition and phase composition, as well as the physico-chemical properties, have been investigated by the methods of electron microscopy, X-ray phase analysis, and bending testing. The obtained samples have a relative density of 98.5 ± 0.1%. The addition of the Cu-Al powder mixture facilitates supercooling during crystallization and solidification, which allows decreasing the size and changing the shape of the initial β-Ti grains. The constant cooling rate of the alloy typical for the SLM technology has been shown to be able to prevent martensitic transformation. The formation of a structure that consists of β-Ti grains, a dispersed eutectoid mixture of α-Ti and Ti2Cu grains, and a solid solution of Al in Cu has been revealed. In the case of doping by the 10% Cu-Al mixture, the physico-mechanical properties are improved. The hardness of the samples amounts to 390 HRC, with the bending strength being 1550 ± 20 MPa and deformation of 3.5 ± 0.2%. The developed alloy can be recommended for applications in the production of parts of jet and car engines, implants for medicine, and corrosion-resistant parts for the chemical industry. [ABSTRACT FROM AUTHOR]

NbRu has a potential as a high-temperature shape-memory alloy (HTSMA) because it has a martensitic transformation temperature above 1000 °C. However, its shape-memory properties could be improved for consideration in the aerospace and automotive industry. The unsatisfactory shape-memory properties could be associated with the presence of a brittle tetragonal L10 martensitic phase. Therefore, in an attempt to modify the transformation path from B2→L10 in preference of either B2→orthorhombic or B2→monoclinic (MCL), an addition of B2 phase stabiliser, titanium (Ti), has been considered in this study to partially substitute niobium (Nb) atoms. The ab initio calculations have been conducted to investigate the effect of Ti addition on the thermodynamic, elastic, and electronic properties of the Nb50−xTixRu50 in B2 and L10 phases. The results showed that the B2 and L10 phases had comparable stability with increasing Ti content. The simulated data presented here was sufficient for the selection of suitable compositions that would allow the L10 phase to be engineered out. The said composition was identified within 15–30 at.% Ti. These compositions have a potential to be considered when designing alloys for structural application at high temperatures above 200 °C. [ABSTRACT FROM AUTHOR]

Thermomechanical coupling effects, which are almost inevitable in materials processing, are particularly notable in NiTi shape memory alloy (SMA). Understanding the dynamic mechanical properties of NiTi SMA at high temperatures and high strain rates is important to reveal its cutting mechanism. In the present work, the split Hopkinson pressure bar experiments under different temperatures and strain rates are carried out. The dynamic mechanical behaviors are discussed from both macro- and microperspectives. The results indicate that nonlinear deformation behaviors of Ni50.8Ti SMA strongly depended on strain rates and temperatures. The influence mechanisms of temperature and strain rate on the nonlinear deformation of Ni50.8Ti SMA are clarified in detail. Moreover, a constitutive model for nonlinear deformation of Ni50.8Ti SMA is established to depict the dynamic characteristics. This is helpful to study the thermomechanical effects on the Ni50.8Ti SMA during machining. [ABSTRACT FROM AUTHOR]

Surface mechanical grinding treatment (SMGT) is a promising surface strengthening technique by exerting gradient microstructure, phase transformation, and work hardening on the surface layer. However, due to the large strain and high strain rate, it is difficult to evaluate the microstructure attributes of the SMGT-processed surface. In the present work, an analytical model is developed to correlate the SMGT processing parameters to the surface microstructure, grain size, martensite content, and microhardness. Firstly, the processing force and distributions of stress and strain are identified by developing a multi-physics framework for SMGT process. Afterward, the surface grain refinement induced by dynamic recrystallization (DRX) is modeled by cellular automata (CA) method. The grain size along with the depth of cross section is predicted by Johnson–Mehl–Avrami–Kolmogorov (JMAK) DRX model. The martensite content is evaluated according to the strain-induced martensitic transformation kinetics. Ultimately, accounting for both microstructure alteration and phase transformation, the microhardness variation is predicted. To verify the results derived by analytical model, series of confirmatory experiments were carried out on 316L austenitic stainless steel. The SEM was utilized to observe the surface microstructure of the processed samples. The martensitic transformation was characterized by EBSD and XRD methods. The microhardness measurements were taken on an automatic Vickers hardness tester. It was obtained from the results that the surface microstructure, gradient grain size, phase content, and microhardness distribution derived from predictive model were consistent well with the experimental measurements. The confirmed simulation model is further utilized to understand how the process factors, i.e., ball size and penetration depth, influence the evolution of microstructure. It is found that the finer grain structure, higher martensite content, and microhardness can be obtained by adopting smaller ball and larger penetration depth. [ABSTRACT FROM AUTHOR]

The influence of a tensile stress on the thermodynamics of martensitic transformations in polycrystalline samples has been studied. The presentation of a thermodynamic framework from the principles of thermodynamics allowed the calculation of frictional work and stored elastic energy as functions of applied stress. The framework was used to analyze the martensitic transformations of a sputter-deposited Ni 46.3 Ti 51.5 Co 2.2 (at. pct) thin film, with a microstructure characterized by Guinier–Preston zones and oriented Ti 2 Ni precipitates inside grains. From experimental results of thermal cycles at different constant stresses, frictional work and stored elastic energy were calculated. The frictional work showed a constant behavior with applied stress in both B2 ↔ R and R ↔ B19' transformations in the analyzed stress range. The stored elastic energy showed a decreasing behavior with applied stress in the B2 ↔ R transformation, and a constant behavior in the R ↔ B19' transformation. It was found that the temperature interval of martensitic transformations is no longer an indicator of the stored elastic energy magnitude when a tensile stress is applied. The observed decrease of the thermal hysteresis as the applied stress increases was analyzed in terms of the stored elastic energy dissipation. Complementary, the utilization of a Clausius–Clapeyron-like equation in polycrystalline alloys has been discussed. [ABSTRACT FROM AUTHOR]

This study investigates the plastic deformation-induced fcc → hcp → bcc phase transition within nanograins in an ultra-strong gradient nanostructured surface layer on Fe45Mn35Cr10Co10 compositionally complex alloy (CCA), subjected to a single point cubic boron nitride turning process. High-resolution transmission electron microscope observations reveal a novel atomic movement mechanism of this transition following a unique Nishiyama-Wassermann orientation relationship. Unlike the previously validated Bogers-Burgers-Olson-Cohen model, which follows different orientation relationships and relies on two shear components, this study demonstrates that the phase transition in this CCA is cooperatively completed by two sets of half-partial dislocation dipoles and associated atom shuffling. Statement of Novelty A novel atomic mechanism of fcc → hcp → bcc phase transition, cooperatively completed by two sets of partial dislocation dipoles and atomic shuffling, provides potential implications for advanced structural materials. [ABSTRACT FROM AUTHOR]

Metals often lose their ductility at cryogenic temperatures owing to the decreased mobility of dislocations. TRansformation-induced plasticity (TRIP), a toughening mechanism at room temperature, can increase damage susceptibility at low temperatures, as the resultant martensite phases can become more brittle than the parent phases. Herein, we develop a high-entropy alloy (HEA) with an improved low-temperature impact-damage tolerance through a sequential plasticity mechanism. We design a trip-assisted dual-phase HEA (TADP HEA) and investigate the effects of Al addition on its mechanical properties upon deformation at different temperatures, depending on stacking fault energy (SFE). Our analysis shows that a senary (Cr20Mn6Fe34Co34Ni6)98Al2 HEA exhibits superior mechanical properties, including a 641 MPa yield strength (σy), 964 MPa ultimate tensile strength (σUTS), and 40% uniform elongation (ɛUTS) at ambient temperature (25 °C), and a 1 GPa σy, 1.5 GPa σUTS, and 36% ɛUTS at −100 °C. Notably, despite the presence of hexagonal-close packed martensite, the HEA exhibits a higher Charpy impact energy (406 J) than Cantor HEA (344 J) at −100 °C. We attribute this improvement to the sequential deformation mechanism of mechanical twinning and martensitic transformation in the HEA at −100 °C, which results in sustainable steady strain-hardening during deformation. We suggest that optimizing the sequential deformation mechanism by manipulating SFE in multi-component alloys can be an effective route for improving the damage tolerance of metals at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

The API K55 grade steel is widely utilized in seamless pipes for oil and gas exploration, especially as casing pipes for wellbores. Traditionally, this steel is processed using hot rolling followed by quenching and tempering to achieve the desired dimensional and microstructural characteristics, balancing high strength with ductility. This article introduces an alternative method to attaining the required tensile properties for API K55 grade steel by employing a biphasic microstructure (ferrite/martensite) achieved through quenching post‐intercritical austenitizing heat treatment to high‐strength‐low‐alloy steel. Thermodynamic simulations and dilatometric experiments revealed that increasing the austenitizing temperature enhances austenite formation, decreasing significantly its carbon content, which facilitates martensitic transformation and increases the Ms and Mf temperatures. A complete phase transformation mapping was presented, highlighting how the austenitizing temperature influences martensitic transformation kinetics during the quenching heat treatment. It was concluded that austenitizing at 750 °C, followed by quenching and short tempering at 650 °C, produced a biphasic microstructure with 30% ferrite and 70% martensite, providing a favorable balance between mechanical strength and ductility that meets the API K55 grade requirements, surpassing traditional methods in the industry. [ABSTRACT FROM AUTHOR]

High-manganese steel (high-Mn) is valuable for its excellent mechanical properties in cryogenic environments, making it essential to understand its deformation behavior at extremely low temperatures. The deformation behavior of high-Mn steels at extremely low temperatures depends on the stacking fault energy (SFE) that can lead to the formation of deformation twins or transform to ε-martensite or α′-martensite as the temperature decreases. In this study, submerged arc welding (SAW) was applied to fabricate thick pipes for cryogenic industry applications, but it may cause problems such as an uneven distribution of manganese (Mn) and a large weldment. To address these issues, post-weld heat treatment (PWHT) is performed to achieve a homogeneous microstructure, enhance mechanical properties, and reduce residual stress. It was found that the difference in Mn content between the dendrite and interdendritic regions was reduced after PWHT, and the SFE was calculated. At cryogenic temperatures, the SFE decreased below 20 mJ/m2, indicating the martensitic transformation region. Furthermore, an examination of the deformation behavior of welded high-Mn steels was conducted. This study revealed that the tensile deformed, as-welded specimens exhibited ε and α′-martensite transformations at cryogenic temperatures. However, the heat-treated specimens did not undergo α′-martensite transformations. Moreover, regardless of whether the specimens were subjected to Charpy impact deformation before or after heat treatment, ε and α′-martensite transformations did not occur. [ABSTRACT FROM AUTHOR]

General derivation of the well-known Ren–Otsuka relationship, 1 α d T o d x = − α β (where T o , x, α and β (> 0) are the transformation temperature and composition, as well as the composition and temperature coefficient of the critical shear constant, c′, respectively) for shape memory alloys, SMAs, is provided based on the similarity of interatomic potentials in the framework of dimensional analysis. A new dimensionless variable, t o x = T o x T m x , describing the phonon softening (where Tm is the melting point) is introduced. The dimensionless values of the heat of transformation, ΔH, and entropy, ΔS, as well as the elastic constants c′, c44, and A = c 44 c ′ are universal functions of to(x) and have the same constant values at to(0) within sub-classes of host SMAs having the same type of crystal symmetry change during martensitic transformation. The ratio of d t o d x and α has the same constant value for all members of a given sub-class, and relative increase in c′ with increasing composition should be compensated by the same decrease in to. In the generalized Ren–Otsuka relationship, the anisotropy factor A appears instead of c′, and α as well as β are the differences between the corresponding coefficients for the c44 and c′ elastic constants. The obtained linear relationship between h and to rationalizes the observed empirical linear relationships between the heat of transformation measured by differential scanning calorimetry (DSC) (QA⟶M) and the martensite start temperature, Ms. [ABSTRACT FROM AUTHOR]

The microstructure of stainless steels belonging to the martensitic and martensitic–ferritic classes was examined by orientation microscopy (EBSD) after quenching. The steels comprised 15 wt % Cr, Ni, and Nb, and were distinguished by the addition of Cu or Mo as alloying elements. Using the spectra of deviation of interfacial α/γ-boundaries from orientation relationships (OR), as well as the initial austenitic grain recovery procedure according to OR, it was found that the martensitic transformation occurring in both steels is realized according to the closest to Kurdyumov–Sachs OR, or (112)γ || (113)α; [11 ]γ || [1 0]α OR. It is demonstrated that the δ-ferrite grains present in the martensitic−ferritic grade steel prior to and after quenching are in the same OR with austenite. This appears to be a consequence of the nucleation of martensitic crystals at the δ-ferrite/austenite phase interface. It is demonstrated also that the application of orientation microscopy enables the ascertainment of the grain austenitic structure of stainless steels at elevated temperatures with an acceptable degree of precision. This is realized by analyzing the structure resulting from shear transformation. [ABSTRACT FROM AUTHOR]

The martensitic transformations in Cu–38Zn and Cu–39.5Zn (wt %) alloys with the shape memory effect have been studied using a combination of transmission and scanning electron microscopy, optical metallography, and X-ray diffraction analysis. When examining specimen upon cooling to low temperatures in the transmission electron microscope column, the features of martensite morphology and fine structure, as well as of electron microdiffraction in the alloys, have been ascetained. The structural types of martensite phases (3R) and (2H) have been identified in Cu–38Zn alloys, as well as (9R) and (2H), in Cu–39.5Zn alloys. The proposed crystallographic models of martensitic rearrangement in the alloys are based on an analysis of X-ray and electron diffraction, including diffuse electron scattering, as well as on the packing defects of the internal substructure of martensite. [ABSTRACT FROM AUTHOR]

A crystallographic theory of the polymorphic bcc → hcp transition in a zirconium single crystal has been formulated. According to the theory, a mechanism of the transition includes the main parameters of the martensitic transformation, namely, the lattice strain and the strain in an invariant lattice. The package of parallel plates that are formed in the course of the transformation is considered as a single martensite crystal. It has been established that the deformation of martensite upon this transition (with an invariant lattice) is aimed at restoring the shape of the initial section of the bcc phase (prototype), rather than at obtaining an invariant plane. The calculation shows that shape restoration in the basal plane of the hcp phase is mainly caused by the action of two systems of prismatic sliding. The sliding is concentrated in narrow layers of localized shear, which leads to the formation of a lamellar structure of the package. The shape restoration along the c axis occurs due to the pyramidal sliding. [ABSTRACT FROM AUTHOR]