Martensitic Transformation Research Articles

The influence of intercritical temperature, ferrite-austenite morphology, and orientation relationship on martensitic transformation in dual phase steels

Two different morphologies of martensite in dual-phase (DP) steel were obtained through two processing routes. In the first method, the steel was water quenched from a fully austenitic state, followed by intercritical annealing (IA). In the second method, the steel was water quenched, cold rolled, and then subjected to intercritical annealing (C-IA). For IA and C-IA steels, the intercritical temperatures varied from 735oC to 765oC to obtain different fractions of ferrite. To understand the effect of ferrite content, morphology, and the orientation of ferrite/austenite on martensitic transformation, a combination of scanning electron microscopy, electron backscattered diffraction, and dilatometric tests were conducted. It was observed that layered martensite presented in IA samples, and blocky martensite appeared only at prior austenite boundaries at 765oC. In C-IA samples, equiaxed martensite grains were present, which become coarser by elevating the temperature to 765oC. In IA samples, martensitic transformation was much faster than in C-IA samples; the difference in martensitic transformation rate is particularly significant at lower annealing temperatures. Carbon enrichment of the partial austenite had a converse effect on the martensitic transformation at 745°C in both samples. The highest transformation rate was recorded in IA samples at 745°C among all IA and C-IA samples. The ferrite/martensite in IA samples maintained Kurdjumov–Sachs orientation relationship (K-S OR) with prior austenite and selected the same variant in the same block. Whereas, in polygonal samples, ferrite/martensite had an irrational OR with the prior austenite, no variant selection appeared in ferrite and martensite grains. Martensite nucleation accelerated on the ferrite surface when it had a K-S OR with the matrix grain, forming a low-energy interface with the ferrite. In contrast, ferrite surfaces irrationally oriented with the matrix grain did not promote martensite nucleation. This was likely due to the large misorientation and higher carbon build-up near the boundary during intercritical holding. The lower activation energy for martensitic nucleation in IA samples compared to C-IA samples resulted in an accelerated rate of martensite transformation in IA samples.

Comprehensive mapping of mechanical properties and martensitic transformation in Ti–Al–Mo standard ternary system: Integrating literature data and fabricating novel high-Al-content superelastic alloys

Titanium alloys are increasingly used as structural and functional materials owing to their exceptional properties. This study aimed to develop β-Ti shape memory alloys by creating a compositional map of the phase constitution and mechanical properties of the Ti–Al–Mo ternary system at room temperature, particularly near the αʺ+β phase region. Data for 95 alloy compositions were collected from the literature, and the composition region of Ti–(7–14) mol% Al–(3–7) mol% Mo, which has rarely been reported, was experimentally investigated. The phase constitution of Ti–Al–Mo alloys primarily depends on the Mo concentration; however, at Al concentrations of 10mol% or higher, the β phase region expands. The yield stress was found to be minimal around 6mol% Mo on the low-Al side, and this minimum shifted to lower Mo concentrations as the Al content increased, corresponding to the β/(αʺ+β) boundary. The ultimate tensile strength increased marginally in low-Mo compositions because of the twinning-induced plasticity effect, with the optimal composition near Ti–11mol% Al–4mol% Mo. Superelasticity was observed in high-Al regions, particularly in the Ti–11mol% Al–6mol% Mo alloy, which exhibited a superelastic recovery strain of 3.2% and a total shape recovery strain of 6%. These findings highlight the potential for developing new room temperature superelastic alloys in the Ti–(10–14) mol% [Al]eq–(5–6) mol% [Mo]eq composition region.

Effect of surface nanocrystallization induced by mechanical grinding treatment on stress corrosion cracking behavior of 316 L austenitic stainless steel in boiling MgCl2 solution

The effect of surface nanocrystallization prepared by surface mechanical grinding treatment (SMGT) on stress corrosion cracking (SCC) behavior of 316 L austenitic stainless steel was studied in boiling MgCl2 solutions at 155 °C and 130 °C, respectively. The refined-grain structures of different morphologies produced by three different SMGT penetration depths were employed to assess how different microstructures and corrosive environments affected the SCC behavior of nanocrystallized surfaces. Results showed that in the 155 °C boiling MgCl2 solution, the refinement of grains resulted in an increase in the critical stress for SCC initiation and progressively enhanced the ability to inhibit crack propagation with the SMGT penetration depth increasing from 20 μm to 60 μm. In the 130 °C MgCl2 solution, the grain refinement still contributed to resisting crack propagation, but the threshold stress for SCC initiation on the surface with the deepest SMGT penetration depth of 60 μm was lower than that in the 155 °C MgCl2 solution. This behavior was attributed to the significant martensitic phase transformation formed in SMGT with penetration depth of 60 μm, resulting in the mechanism of SCC transforming from anodic dissolution in MgCl2 solution at 155 °C to hydrogen embrittlement at 130 °C.

Elaboration and characterization of Cu–Zn–Al shape memory alloys

AbstractThe study examines Cu–Zn–Al shape memory alloys, vital in aeronautics and automotive sectors. It aims to characterize their thermo-mechanical transformations induced by composition and heat treatments, focusing on how these impact mechanical properties, especially grain size refinement. Analysis covers transformation temperatures, micro-hardness, induced transformations, and mechanical tests. Results show thermoplastic martensitic transformations, with micro-hardness aiding in identifying characteristic points. The study's novelty lies in understanding how grain size refinement affects these transformations and the role of micro-hardness in precise characterization.

Modeling martensitic transformation temperatures in Zirconia–Ceria solid solutions using machine learning interatomic potentials

Abstract Shape memory ceramics (SMCs), while exhibiting high strength, sizeable recoverable strain, and substantial energy damping, tend to shatter under load and have low reversibility. Recent developments in SMCs have shown significant promise in enhancing the reversibility of the shape memory phase transformation by tuning the lattice parameters and transformation temperatures through alloying. While first-principles methods, such as density functional theory (DFT), can predict the lattice parameters and enthalpy at zero Kelvin, calculating the transformation temperature from free energy at high temperatures is impractical. Empirical potentials can calculate transformation temperatures efficiently for large system sizes but lack compositional transferability. In this work, we develop a model to predict transformation temperatures and lattice parameters for the Zirconia–Ceria solid solutions. We construct a machine learning inter-atomic potential (MLIAP) using an initial dataset of DFT simulations, which is then iteratively expanded using active learning. We utilize reversible scaling to compute the free energy as a function of composition and temperature, from which the transformation temperatures are determined. These transformation temperatures match experimental trends and accurately predict the phase boundary. Finally, we compare other relevant design parameters (e.g. transformation volume change) to demonstrate the applicability of MLIAPs in designing SMCs.

Mn vaporization control and stress-induced martensite transformation in high-manganese steel welds under different laser keyhole conditions

Vaporization modeling of high-Mn steel during laser welding, along with the effects of Mn vaporization on the tensile properties at room and cryogenic temperatures, were investigated for various laser defocusing (LD) conditions. The LD10 exhibited a full penetration keyhole, while LD20 showed a partial penetration keyhole. The vaporization modeling based on computational fluid dynamics (CFD) analysis confirmed that a significant Mn flux occurred in LD20, which corresponded with the most substantial loss of Mn observed in the partially penetrated weld. The room temperature tensile testing showed a slight increase in the yield strength and a minor reduction in tensile elongation for LD20, which could be attributed to the stress-induced ε-martensitic transformation. In cryogenic tensile testing, LD20 exhibited a higher yield strength with more martensitic transformation, including some stress-induced α′-martensitic transformation observed. Through the vaporisztion modeling, significant Mn vaporization in LD20 was found to decrease the stacking fault energy, which enhanced stress-induced martensite transformation and yielding stress during tensile deformation at 110 and 298 K.

Cryogenic deformation strengthening mechanisms in FeMnSiNiAl high-entropy alloys

The mechanical properties and deformation mechanisms of a newly developed Co-free FeMnSiNiAl high entropy alloy (HEA) at room and cryogenic temperatures were systematically investigated. The initial tensile deformation at room temperature was dominated by dislocation slipping, with modest strengthening from the Transformation-Induced Plasticity (TRIP) effect due to the deformation-induced FCC → HCP martensitic transformation. Subsequently, the TRIP effect was markedly enhanced during the middle and later stages of deformation, leading to an excellent combination of yield strength (σy, 315.1 MPa), ultimate tensile strength (σu, 773.4 MPa), and fracture elongations (εf, 78.3%). The strengthening by the TRIP effect was significantly enhanced at cryogenic temperatures as a result of enhanced FCC → HCP martensitic transformation. This resulted in a synergetic improvement in strength and ductility at 223 K, with σy of 363.6 MPa, σu of 832.1 MPa, and εf of 87.2%. The enhanced ductility at 223 K was linked to the FCC → HCP → BCC sequential martensitic transformation during the middle and later stages of deformation, which acted as an additional way to accommodate plastic strain and delay strain localization. However, the rapid FCC → HCP transformation at the early stage of deformation at 173 K and 77 K impeded the FCC → HCP → BCC sequential martensitic transformation during subsequent deformation stages, thus remarkably enhancing strength but reducing ductility. Our findings provide new insights into the design and development of TRIP-assisted single-phase FCC HEAs for cryogenic applications.

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.

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.

From deformation twinning to α'' martensitic transformation in deforming Ti-12Mo alloy with increasing grain size

From deformation twinning to α'' martensitic transformation in deforming Ti-12Mo alloy with increasing grain size

Large and reversible elastocaloric effect induced by low stress in a Ga-doped Ni-Mn-Ti alloy

The Solid-state cooling based on caloric effects is considered a potential alternative to conventional refrigeration technology, which uses ozone-depleting gases. Several shape memory alloys have attracted attention for solid-state cooling since they present a high reversibility of the caloric effect, which depends mainly on thermal hysteresis and sensitivity to the applied field. In the present work, a study substitution of Mn by Ga in the Ni50Mn34Ti16 alloy led to diminished thermal hysteresis in the martensitic transformation by 6 K. The elastocaloric effect, thermal and microstructure properties of a polycrystalline Ni50Mn32Ti16Ga2 alloy have been characterized. The elastocaloric effect was obtained indirectly from the length change as a function of temperature at constant stress. An isothermal entropy change (ΔSISO) of 23.0 J kg−1 K−1 during heating and 22.0 J kg−1 K−1 during cooling was observed for applied stress of 160 MPa. In addition, the ΔSISO is reversible for a temperature span between 287 and 319 K, reaching a maximum of 20.5 J kg−1 K−1 at 299 K. The thermal hysteresis changed slightly while the applied stress increased up to 160 MPa since the sensitivity of the martensitic transformation temperatures to stress was 0.150 K M/MPa during cooling and 0.160 K/MPa during heating. The X-ray diffraction analysis revealed a mixture of B2-type cubic austenite, 5 M modulated martensite, and a second intermetallic phase identified as Ni3Ti. All these results were obtained around room temperature.

Simultaneous Determination of Transformation Plasticity Coefficients of Bainite and Martensite Transformations by Oil Quenching of Steel Sheet for Machine Structural Use and Accurate Prediction of Distortion by Oil Quenching

Simultaneous Determination of Transformation Plasticity Coefficients of Bainite and Martensite Transformations by Oil Quenching of Steel Sheet for Machine Structural Use and Accurate Prediction of Distortion by Oil Quenching

Mechanical behaviour of low-, medium- and high-entropy Ti-Hf-Zr-Ni-Cu-Co shape memory alloys

The mechanical behaviour was studied in twelve senary Ti-HfXZrX-Ni-CuXCoX alloys with various configuration entropies, which were regulated by the X value - concentration of doping elements (Hf, Zr, Cu, and Co) that varied from 1 at% to 17 at% and the concentration of the Ni group (Ni+Cu+Co) that varied from 49 to 51 at%. The stress vs strain curves were obtained at three temperatures (-100oC, 22oC and 100oC) at which the alloys were in different states (martensite, austenite, or a mixture of both). It was found that the senary Ti-Hf-Zr-Ni-Cu-Co alloys could undergo deformation via martensite reorientation, stress-induced martensitic transformation, and dislocation slip. The activation of the mechanisms depends on the deformation temperature compared to the temperatures of the martensitic transformations, as in the binary NiTi alloys. An increase in the X value from 1 to 10 at% increased the yield limit for dislocation slip in the austenite phase, while a further rise in the X value by 17 at% decreased this limit. An increase in the X value decreased the strain up to failure. The larger the [Ni] concentration or deformation temperature, the more pronounced the reduction in strain up to failure. An increase in X value from 1 to 5 at% significantly changed the fracture mechanisms from ductile to brittle due to the suppression of the dislocation movement.

Design and control the selection of martensitic variant to simultaneously improve strength and toughness of low-carbon martensitic steel

In this paper, the microstructure evolution and mechanical properties of low carbon steels during direct quenched and rolling followed by water-cooled processes were studied. Two experimental steels, which were directly quenched at 900 °C (Q900) and 1000 °C (Q1000), were compared with steels that were water-cooled after rolling at the same temperatures (R900 and R1000). Microstructural analyses using EBSD and TEM revealed that rolling reduced the size of prior austenite grains (PAGs), resulting in an average width of 3.6 μm, which influenced grain boundary distributions and variant selection. The best combination of strength, ductility and toughness was obtained in R900 steel, including tensile (with the yield strength of 1304 MPa, the total elongation of 22.95 %), Charpy impact (with the impact energy at 20 °C is 182 J), and fracture toughness evaluations (with the J1c is 326.28 KJ/m2), this demonstrates that R900 steel exhibited significantly enhanced strength and ductility compared to Q900 steel. Moreover, EBSD analysis of crack propagation paths highlighted the role of high-angle grain boundaries (HAGBs) in enhancing fracture toughness by deflecting cracks. These findings underscore the critical role of PAGs size in tailoring microstructures to achieve superior mechanical properties in low carbon martensitic steels, offering insights for advanced material design and application in demanding structural and industrial contexts.Keyworks.low-carbon martensitic steel; strength and toughness; martensitic transformation; ductile-to-brittle transition phenomenon; selection of martensitic variants.

The effect of cryogenic treatment on a high Co bearing DIN 1.2888 steel used as die in hot forging process

Abstract Hot work steels used as dies in metal forming processes must endure harsh conditions, typically achieved through a martensitic structure. However, retained austenite can persist after martensitic transformation, negatively impacting mechanical properties. Cryogenic treatment (CT) refines martensite, reduces retained austenite, and increases fine carbide density, thus enhancing performance. DIN 1.2888, a hot work tool steel with high cobalt content (~10%) and low carbon content (~0.2%), is less studied concerning the effects of CT. The objective of this study was to investigate how cryogenic treatment affects the mechanical and microstructural properties of DIN 1.2888 steel. Samples underwent conventional heat treatment (HT) and CT at -100, -140, and -180°C for 6 hours, followed by double tempering. Various analytical methods, including X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and Vickers Microhardness tests, were used to examine the samples. Wear mechanisms were evaluated through pin-on-disc tests under different loads, and impact toughness was measured both at room temperature and the working temperature of dies (350°C). Results indicated that lowering the cryogenic treatment temperature resulted in a slight increase in hardness values from 507HV for the heat-treated sample to 529HV for the sample treated at -180°C. Additionally, the impact toughness improved significantly, with values rising from 12.35J for the heat-treated sample to 23.44J at 350°C for the cryogenically treated sample at -180°C. Wear rates also decreased by approximately 50%, particularly at higher cryogenic treatment temperatures. The improvement in wear resistance was attributed to finer carbide precipitation and a more homogeneous distribution of carbides. These findings suggest that deep cryogenic treatment has a positive effect on DIN 1.2888 steel by decreasing retained austenite and increasing fine carbide density, leading to enhanced mechanical properties and extending the lifespan of the steel in die applications.

Strain-induced martensitic transformations in tailored microstructures by L-PBF: in situ characterization via advanced neutron imaging

Strain-induced martensitic transformations in tailored microstructures by L-PBF: in situ characterization via advanced neutron imaging

Effect of grain size on precipitation and microstrain of nanocrystalline NiTi alloys

The effect of aging treatment on the microstructure, phase transformation behavior and mechanical properties of nanocrystalline NiTi alloy was studied. The nanocrystalline NiTi alloys with different grain sizes were acquired by cold drawing followed by annealing at 350–500 °C for 10 min. The annealed samples were aged at 250–400 °C for 48 h. The Ti3Ni4 precipitates were found in aged nanocrystalline NiTi alloys. In the sample with smaller average nanograin size, the precipitates were found at the edge of grain boundaries and little lattice strain was shown in R phase matrix. In the sample with larger average grain size, the precipitates were found in the nanograins and exhibited a coherent interface with the matrix. The nanocrystalline R phase NiTi matrix exhibited a significant compressive stress at the end of the coherent precipitate. The coherent precipitates in sample aged at 250 °C after annealing at 500 °C suppress the stress-induced R → B19′ phase transformation and increased the upper plateau stress. The precipitation in sample aged at 250 °C after annealing at 350 °C unable to suppress the martensitic transformation effectively.

Effect of cyclic loading on microstructure and plastic deformation in heat-treated Co–28Cr–6Mo alloy fabricated via laser powder bed fusion: an in situ synchrotron X-ray diffraction study.

We present a systematic microstructure-oriented plasticity investigation of an additively manufactured cobalt-based alloy aiming to relate microstructural changes to the fatigue resistance. A load-controlled cyclic test in the low-cycle fatigue regime was utilized to study mechanical response and microstructural changes in the alloy after reverse phase transformation heat treatment. Deformation-induced FCC → HCP phase transformation was followed using in situ synchrotron X-ray diffraction combined with ex situ electron backscatter diffraction and scanning electron microscopy. Scanning transmission electron microscopy provided experimental evidence of the presence of precipitates in the solution annealed sample. It was found, that a stepwise increase in plastic deformation is attributed to nucleation and growth of different martensitic crystallographic variants. These insights into plasticity and microstructural changes suggest that the reverse phase transformation heat treatment is an effective approach for improving the fatigue resistance of low stacking fault energy alloys, that are susceptible to deformation-induced martensitic transformation.

Enhancement on mechanical properties via phase separation induced by Cu-added high-entropy alloy fillers in metastable ferrous medium-entropy alloy welds

This study evaluated the weldability of metastable ferrous medium-entropy alloys using high-entropy alloy fillers with various Cu contents and elucidated the strengthening mechanisms at room and cryogenic temperatures by focusing on the formation of a Cu-rich phase (FCC2) in the weld metal (WM). The WM with higher Cu content exhibited more dual face-centered cubic (FCC) phases with dendrite-shaped compositional heterogeneity due to phase separation driven by higher mixing enthalpy. The FCC2 induced greater lattice distortion, resulting in higher nano-hardness and stronger solid-solution hardening. In addition, the increased presence of FCC2 in the WM promoted grain refinement and the formation of additional grain boundaries within individual grains. After cryogenic deformation, the base metal and heat-affected zone in transverse welds underwent martensitic transformation from metastable FCC (transformation-induced plasticity), while the diluted WMs exhibited twinning-induced plasticity because of their higher stacking fault energy (SFE) compared to the base metal. The combination of these deformation mechanisms enhanced the cryogenic tensile properties without compromising ductility. Furthermore, the cryogenic tensile properties of weld with higher Cu content were further improved due to the increased formation of deformation twins in the WM. The consumption of solute Cu to form more Cu-rich FCC2 resulted in compositional variations in the matrix (FCC1), leading to a further reduction in the SFE of FCC1 and the activation of additional twin formation. Additionally, FCC2 with a relatively higher SFE refined secondary twins as well as contributed to further dislocation accumulation. This study suggests that the formation of Cu-rich phase in the WM can enhance mechanical properties at both room and cryogenic temperatures and provides a valuable approach for the future development of welding materials to improve the mechanical properties of FCC-based welded structures.

Hardening mechanism and thermal-solid coupling model of laminar plasma surface hardening of 65 Mn steel

In the present work, the laminar plasma surface hardening method is employed to enhance the service life of metal components fabricated from 65 Mn steel. The mechanical and wear behaviors of the laminar plasma surface hardened 65 Mn steel were analyzed. The martensite transition transformation of the temperature of the laminar plasma-hardened 65 ferrite Mn steel was determined by a thermal-solid coupling model. Based on the orthogonal experimental results, the optimal hardening parameters were confirmed. The scanning velocity, quenching distance and arc current are 130 mm/min, 50 mm and 120 A, respectively. The pearlites and ferrites are transformed into martensites in the hardened zone, while the ratio of martensite in the heat-affected zone decreases with the increase in the hardening depth. Compared to the untreated 65 Mn steel, the average hardness increases from 220 HV0.2 to 920 HV0.2 in the hardened zone and the corresponding absorbed power increases from 118.7 J to 175.5 J. At the same time, the average coefficient of friction (COF) decreases from 0.763 to 0.546, and the wear rate decreases from 5.39×10−6 mm3/(N·m) to 2.95×10−6 mm3/(N·m), indicating that the wear resistance of 65 Mn steel could be significantly improved by using laminar surface hardening. With the same hardening parameters, the depth and width of the hardened zone predicted by the thermal-solid coupling model are 1.85 mm and 11.20 mm, respectively, which are in accordance with the experimental results; depth is 1.83 mm and width is 11.15 mm. In addition, the predicted hardness distributions of the simulation model are in accordance with the experimental results. These results indicate that the simulation model could effectively predict the microstructure characteristics of 65 Mn steel.