Athermal Martensitic Transformation Research Articles

ЗАСТОСУВАННЯ МАТЕМАТИЧНОГО МОДЕЛЮВАННЯ ПРИ АНАЛІЗІ ДИЛАТОГРАМ МАРТЕНСИТНОГО ПЕРЕТВОРЕННЯ У СТАЛЯХ

The paper provides practical experience of applying mathematical modeling in the analysis of the kinetics of athermal martensitic transformation in low-alloy structural steel 30HGSA (Fe-0.30С-0.86Si-1.02Mn-0.84Сr, % wt.) using the dilatogram of the real quenching process. The change in the amount of martensite formed in the studied steel as a function of temperature during continuous quenching cooling is shown. The used technique for determining the phase ratio provides in situ correlation with the microstructural mechanisms implemented in the studied material during heat treatment. The obtained results will be used to develop parameters of heat treatment based on the principle of quenching and partitioning (Q&P) to improve the complex of properties of 30HGSA steel due to the creation of a multiphase structure.

Investigating the role of the austenitizing temperature and cooling rate on the martensitic transformation kinetics in a SAE 9254 spring steel

The effect of austenitizing temperature (ranging from 850 to 1050 °C) and cooling rates on the phase transformations were investigated, particularly when a two cooling steps protocol was adopted. It was possible to observe that the heat treatment parameters play a major role on the martensitic transformation kinetics a possible occurrence of bainitic transformation. The results indicate that a microstructure composed by small austenite grains and the formation of bainite prior the athermal martensitic transformation significantly contribute to increase the martensitic transformation rate. The Koistinen-Marburger model was employed to analyze the non-isothermal kinetics, revealing an increase in the KM and k parameters due to an increase in austenite grain size and the presence of bainite in the microstructure. The results herein demonstrate that the optimization of the heat treatment parameters is crucial to the proper control of the phases that will be present in the microstructure of the alloy, as well as the volume fraction of martensite. Thus, the accurate control of the heat treatment process is a promising approach to enhance the properties of SAE 9254 spring steel, which finds extensive use in the automotive industry.

An integral transformation model for the combined calculation of key martensitic transformation temperatures and martensite fraction

Understanding the process of martensitic transformation and accurately predicting the fraction of retained austenite and martensite after athermal martensitic transformation are of great significance for optimizing the mechanical properties of various advanced high strength steels. This paper proposes an integral transformation model for the description of both the martensitic transformation temperature and its volume fraction considering both chemical driving force and non-chemical free energy. The proposed model was validated in both incomplete and complete martensitic transformation systems with different austenite grain sizes. Compared with the conventional Koistinen and Marburger model, the integral transformation model can predict the Ms and Mf temperatures as well as the transformation fraction in an integrated manner and depict the relationship between martensite fraction and temperature as a more realistic S-shaped curve. This can be attributed to the fact that the integral transformation model is developed from a generic energy viewpoint and takes into account not only the chemical driving force but also the significant non-chemical free energy contribution.

Martensite transformation kinetics and microstructural evolution during quench and partitioning process of high aluminium medium carbon steel

The quench and partition process was studied in a high aluminium (Al) medium carbon (C) steel by varying the quench temperature which is one of the process parameters The first part of the paper deals with modelling athermal martensite transformation using an empirical approach. This model is an improvement on Koiestein and Marburger's model, and it better represents the martensitic transformation. The second part of the paper deals with microstructure evolution in high Al steel when subjected to the quench and partition process. The maximum retained austenite (RA) obtained was 13% at a quenching temperature of 260°C. The addition of Al promoted the formation of bainite during the partition stage.

Dilatometric Analysis and Kinetics Research of Martensitic Transformation under a Temperature Gradient and Stress

Based on material constitutive models and the classic Koistinen–Marburger (KM) kinetics model, a new dilatometric analysis model was developed to extract the kinetics curve of martensitic transformation under a temperature gradient and stress from the measured dilatometric data and to determine the transformation parameters. The proposed dilatometric analysis model is generally for athermal martensitic transformation, relying only on the average atom volume of martensite and austenite. Furthermore, through theoretical calculations, the proposed model also provided a more accurate method for obtaining the martensite start temperature, which is different from the traditional method. According to the dilatometric analysis results for the martensitic transformation of a type of high-strength low-alloy steel, and the thermodynamic basis of martensitic transformation, a refined kinetics model was developed that successfully predicted the martensitic transformation kinetics curves under different stresses, taking into account the physical significance of the transformation parameter α and the driving force of stress for martensitic transformation.

Effect of Nb microalloying on microstructure evolution and mechanical properties in low carbon medium manganese steel

The effect of Nb microalloying on microstructure evolution and mechanical properties of 0.02C–7Mn steel was investigated. The results showed that the microstructure was fully quenched martensite after subjected to reheating and quenching (RQ) process. The microstructure consisted of tempered α-martensite, reversed austenite (RA), and athermal ε-martensite after tempered at 620 °C, while the microstructure contained tempered α-martensite, RA, athermal ε-martensite, and athermal α-martensite after tempered at 650 °C. The thermal and mechanical stability of both RA and athermal ε-martensite decreased after the 0.05% Nb was added to the 0.02C–7Mn steel, and also decreased as the intercritical tempering (IT) temperature increased, which was mainly because the content of solid solution C in RA decreased. The calculated stacking fault energies (SFEs) were all negative, which spontaneously induced athermal martensitic transformation. Multiphase microstructure resulted in the remarkable mechanical property evolution. After tempered at 620 °C, the optical mechanical properties of 7Mn-0.05Nb steel were obtained: yield strength of 720.0 MPa, tensile strength of 819.0 MPa, total elongation of 26.0% and −40 °C impact energy of 266 J, while the yield strength of 675.0 MPa, tensile strength of 776.5 MPa, the total elongation of 29.5% and the −40 °C impact energy of 274 J appeared in 7Mn–0Nb steel. As the IT temperature increased to 650 °C, the yield strength of both the steels sharply decreased (475.5 MPa for 650 °C–0Nb steel and 460.5 MPa for 650 °C-0.05Nb steel), and the tensile strength of both the steels increased about 56 MPa, while the −40 °C impact energy decreased by 40 and 60 J, respectively, showing that the −40 °C impact energy was not very sensitive to the mechanical stability of RA and athermal ε-martensite. The mean size of NbC particles was mainly about 5–7 nm, which significantly improved the precipitation strengthening, but there was just little damage to low-temperature toughness.

The effect of Ni depletion on athermal martensitic transformation in 304 austenitic stainless steel

Chemical and microstructural analyses were performed on Ni depletion zones in 304 austenitic stainless steel sheets after annealing and cooling with three different rates (0.2, 18.6, 160.0 °C/s). The analysis was focused on the characteristics and the origin of the retained δ-ferrite and α’-martensite phases in the austenite matrix. Simultaneous Ni depletion and Cr enrichment belong to the retained δ-ferrite due to incomplete decomposition after the solidification process. Athermal α’-martensite was observed to grow within Ni-depleted zones with no Cr enrichment. The fraction of α’-martensite increases as the cooling rate increases. EPMA analysis reveals that the 1 wt% reduction of Ni contents to the bulk chemical composition promotes the thermally-activated martensitic transformation. Such athermal martensite tended to grow within twinning boundaries holding the Nishiyama-Wassermann orientation relationship with the austenite matrix confirmed with the electron diffraction patterns in TEM and the misorientation analysis in EBSD.

Synergy effect of multi-strengthening mechanisms in FeMnCoCrN HEA at cryogenic temperature

High-entropy alloys (HEAs) have attracted great research interest owing to their good combination of high strength and ductility at both room and cryogenic temperatures. However, expensive raw materials are always added to overcome the strength-ductility trade-off at low temperatures, leading to an increased production cost for the cryogenically used alloys. In this work, a series of nitrogen-doped FeMnCoCr HEAs have been processed by homogenization annealing, cold rolling and recrystallization annealing followed by water quenching. The microstructural evolution and mechanical properties of the alloys are studied systematically. The Fe49Mn30Co10Cr10N1 alloy shows excellent mechanical properties at both 293 K and 77 K. Particularly, the yield and ultimate tensile strength of 1078 and 1630 MPa are achieved at the cryogenic temperature, respectively, while a satisfactory uniform elongation of 33.5 % is maintained. The ultrahigh yield strength results from the microstructure refinement caused by the activation of athermal martensitic transformation and mechanical twinning that occur in the elastic regime together with the increased lattice friction due to the cryogenic environment. In the plastic regime, the dynamic Hall-Petch effect caused by twinning, martensitic transformation, and reverse transformation together with the high barrier to dislocation motion jointly contribute to the ultrahigh tensile strength. Simultaneously, the transformation induced plasticity (TRIP) and the twinning induced plasticity (TWIP) effects jointly contribute to the ductility. The design strategy for attaining superior mechanical properties at low temperatures, i.e. by adjusting stacking fault energy in the interstitial metastable HEAs, guides the development of high-performance and low-cost alloys for cryogenic applications.

Austenite reversion through subzero transformation and tempering of a boron-doped strong and ductile medium-Mn lightweight steel

Effects of subzero treatment and B doping on austenite reversion are investigated in quenched and tempered Fe–9Mn–5Al-0.3C and Fe–9Mn–5Al-0.3C-0.005B (wt.%) lightweight steels. In the as-quenched condition, the steel microstructure consist of a triplex structure of austenite, ferrite, and martensite. B doping leads to a reduction in the prior austenite grain size by grain boundary segregation and precipitation of boro-carbides, which increases the stability of austenite against the athermal martensitic transformation. After tempering at 200 °C for 2 h, nano-lath reverted austenite is formed by the C-enrichment instead of the carbide precipitation owing to the high Al content. This reversion effect is promoted further by the subzero treatment at −196 °C for 0.5 h prior to tempering, which enables the remaining austenite in the as-quenched state to transform and, thus, provides additional sites for austenite reversion. In addition, the subzero treatment and B doping result in the synergistic effect of the delay of crack initiation through the transformation of retained austenite in contact with ferrite and the improvement of bonding strength. Thus, the B-doped steel subjected to quenching, subzero treatment, and tempering exhibits a very high yield strength of approximately 1 GPa, the tensile strength of over 1.3 GPa, and an excellent elongation of 29.4%, which outperform the tensile properties of conventional austenitic or (austenite + ferrite) duplex lightweight steels.

In-situ real time observation of martensite transformation in duplex fcc+hcp cobalt based entropic alloys

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

Microstructure evolution and mechanical properties improvement of selective laser melted Co-Cr biomedical alloys during subsequent heat treatments

Abstract Selective laser melting (SLM) technology has garnered considerable attention as an advanced manufacturing method that produces biomedical Co-Cr devices specifically developed for dental restoration applications. However, SLM technology inevitably causes unstable microstructures and excessive residual stress due to rapid heating and cooling, which negatively affects their mechanical properties. Also, information on the phase transformation and precipitation behavior during subsequent heat treatments is limited. In this work, SLM-manufactured Co-Cr specimens were initially subjected to stress-relieving annealing (SRA) at various temperatures (850, 950, 1050, and 1150 °C) and then porcelain firing (PF) treatments, and the microstructure evolution and mechanical properties improvement during subsequent heat treatments was investigated in detail. Scanning electron microscopy (SEM) combined with energy-dispersive x-ray spectroscopy (EDS), x-ray diffractometer, transmission electron microscopy, and Image Pro Plus 6.0 (IPP) software were used to analyze alloy microstructures, whereas tensile tests were performed to evaluate corresponding performance. Results showed that after SRA and/or PF treatments, a large number of nanoscale precipitates were observed, and acicular or lamellar e phase appeared. For SRA-treated specimens, as annealing temperatures increased, the volume fraction of γ phase decreased, especially after annealing at 950 °C, the γ phase was completely transformed into the e phase. In contrast, for (SRA + PF)-treated specimens, the volume fraction of γ phase first decreased and then increased with the increase of annealing temperatures. IPP analysis found that the amount, size, and morphology of the precipitates varied with the change in annealing temperature. During annealing, isothermal martensite transformation (γ →e) dominated due to the long duration between 600 and 900 °C, while the amount of athermal martensite transformation (γ →e) gradually increased when porcelain firing. Changes in the above-mentioned microstructures affected corresponding mechanical properties. From 850 °C to 1050 °C, tensile strength and elongation continue to improve. At 1150 °C, both tensile strength and 0.2% yield strength decreased, but the elongation was further improved. The variation in hardness (HV10) with temperature was the same as that of the volume fraction of e phase, that is, the higher the volume fraction of e phase, the higher the hardness. SEM/EDX analysis of tensile fracture surfaces revealed a quasi-cleavage fracture. In conclusion, 1150°C-SRA and PF treatments are an effective method for homogenizing microstructure and improving mechanical properties.

Austenitizing Temperature and Cooling Rate Effects on the Martensitic Transformation in a Microalloyed-Steel

Abstract The effects of the austenitizing temperature and the cooling rate upon the kinetic of athermal martensitic transformation in a microalloyed steel were evaluated. Considering the studied steel, the knowledge about these effects on the martensitic transformation has a great relevance for naval manufacturers and steel researchers. In this study, computational simulation was performed aiming to evaluate the phase’s stability. Specimens were submitted to quenching simulations in a dilatometer, considering four different austenitizing temperatures and four cooling rates. The results shown that the austenite chemical composition was not significantly affected by the austenitizing temperatures. Both the austenitic grain size and the cooling rate affected the martensitic transformation kinetics. The larger the austenitic grain size, the higher the Ms. The austenitic grain growth promoted a decrease in the required chemical energy to compensate the free energy increase associated with the lattice strain and the creation of new interfaces, leading to a lower austenite undercooling. An extrinsic effect of the cooling rate on the Ms was observed. For lower cooling rates, the carbide precipitation modified que austenite chemical composition, changing its stability and increasing Ms. A predictability equation, correlating the MS with the austenite grain size and the steel cooling rate, was proposed.

Joint contribution of transformation and twinning to the high strength-ductility combination of a FeMnCoCr high entropy alloy at cryogenic temperatures

The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications.

ДИФУЗІЯ У ЗАЛІЗІ ТА ЙОГО СПЛАВАХ ПІСЛЯ НАСИЧЕННЯ ПОВЕРХНІ ІОНАМИ АРГОНУ ТА КРИПТОНУ ПРИ ВІДПАЛАХ ТА ІМПУЛЬСНИХ НАВАНТАЖЕННЯХ

The effect of the preliminary introduction of inert gases (argon and krypton) into iron and its alloys in a glow discharge plasma on diffusion (mass transfer) under various external influences was studied using methods based on the use of radioactive isotopes and xrays. The studies were carried out by methods of x-ray diffraction analysis, layered radiometric analysis of residual integral activity, macro-, micro-, electron-microscopic and activation autoradiography. Isotopes of iron 55Fe, nickel 63Ni, carbon 14C, argon 41Ar and krypton 85Kr were used. The following external effects were applied to the metal (alloy) in the initial state and after saturation with an inert gas: galvanic coating, isothermal annealing, athermal martensitic transformation with explosive kinetics and a large volume effect (~ 3%), ultrasonic impact treatment, compression due to deformation when the load falls, magnetic pulse deformation, etc. It is shown that the presence of a pre-introduced inert in iron and alloys based on it leads to a decrease in the mobility of atoms during annealing, low-temperature pulsed loading in a wide range of deformation rates, and further ion bombardment in a glow discharge. No difference was found in the effect of the type of inert gas — argon or krypton — on the change in atomic mobility, surface distribution, and the shape of the concentration profile. The barrier effect of the introduced gas is associated with structural-phase changes in the surface layers by the occurrence of gas-filled pores and a subtraction solid solution in the surface layers, which transforms into a substitutional solid solution away from the surface. Additional studies have shown that the barrier effect of the introduced inert gas on diffusion during subsequent exposures is observed not only on iron, but also on copper and aluminum. This effect is especially pronounced during the subsequent ion bombardment of copper and aluminum saturated with one or another inert gas in a krypton or argon medium as compared to the subsequent pulsed compression or deformation under martensitic transformations. This suggests the universal nature of the discovered regularity. It should be noted according to autoradiographic studies that the presence of an inert gas, which slows down the transfer of matter during subsequent treatments, does not change the nature of the distribution of atoms. There is diffusion over the grain volume with some depletion of the surface layer, and the concentration profiles are described by an exponential dependence on the square of the penetration depth. Therefore, the mechanism of volume diffusion is preserved over the entire range of temperatures and strain rates, and also does not depend on the type of external influence applied. This is true for native atoms and atoms forming solid solutions of various types: substitution (nickel), interstitial (carbon), subtraction (argon and krypton).

Laser Powder Bed Fusion: tailoring the microstructure of alloys for biomedical applications

Abstract Additive manufacturing (AM) is particularly attractive for biomedical applications, where complex geometries and a high degree of individualization are required. Laser powder bed fusion (LPBF) is an AM technology exploiting the action of a solid-state laser to locally melt a metal powder according to a computer aided design (CAD) model. In the present study, the EOS Cobalt Chrome SP2 (Co-Cr-Mo-W) and Ti64 (Ti6Al4V) powders were sintered by the system equipped with a Yb fiber laser. During LPBF, the Co-Cr-Mo-W metal powder undergoes total melting followed by rapid cooling, giving rise to athermal martensitic phase transformation from the high-temperature γ (fcc) phase to the low-temperature e (hcp) phase. This produces an intricate network of thin e-lamellae inside the γ phase matrix. After the firing cycle this structure is maintained, and a massive presence of coarse precipitates is also observed. Owing to the rapid cooling taking place during LPBF, in Ti6Al4V sintered samples only the acicular martensitic α’ phase is present. The firing cycle induces the β phase formation at the α plate boundaries and this microstructure leads to reduced values of strength, with respect to those of the as-sintered samples. The highlighted behaviors show that by tuning the post-production heat treatments it is possible to tailor the microstructure and the mechanical properties.

Densification behavior, microstructure evolution, and mechanical performances of selective laser melted Ti-5Al-2.5Sn α titanium alloy: Effect of laser energy input

Selective laser melting (SLM) technology was employed for the additive manufacturing of Ti-5Al-2.5Sn α titanium alloy in this work and the influences of laser energy input on densification behavior, microstructure evolution and mechanical performances of the as-deposited products were investigated. It was found that the relatively high volumetric energy densities (VEDs) of 167–417 J/mm3 resulted in large quantities of keyhole-induced spherical pores whereas the relatively low VEDs of 51–83 J/mm3 led to notable lack-of-fusion defects. At the proper VED of 125 J/mm3, highly densified products with internal porosity of merely 0.05% were obtained. At the relatively high VEDs of 167–417 J/mm3, as-built samples presented duplex microstructures composed of acicular α′ martensites and bulk αM massive grains. In contrast, overwhelmingly martensitic microstructures were formed at lower VEDs of 51–125 J/mm3. The martensitic microstructure exhibited a unique hierarchical feature, which could be explained using the theory of athermal martensitic transformation kinetics. With the decrease of VED from 417 J/mm3, the average size of α′ needles increased first, then came to its plateau at the VEDs of 167–208 J/mm3, and decreased again at the VEDs of 51–167 J/mm3. At the VEDs of 167–417 J/mm3, the formation of spherical pores and bulk αM grains resulted in relatively poor tensile properties. At the VEDs of 51–83 J/mm3, the formation of lack-of-fusion defects masked the enhanced fine-grain strengthening effect provided by the much refined α′ needles and the decrease in the fraction of bulk αM, thus also leading to very limited tensile properties. At the proper VED of 125 J/mm3, as-built tensile properties reached to their peak with ultimate tensile strength of 1173 MPa, yield strength of 1061 MPa, and elongation of 7.7%. This can be attributed to the high densification degree, the almost complete vanish of bulk αM grains, and the moderate size of α′ needles.

Effect of Magnetic state of Austenite on Martensitic transformation in Fe–Ni Alloys in High and Zero Magnetic Fields

The effect of high pulsed magnetic fields on the martensitic γ → α transformation in Fe–Ni and Fe–Ni–Mn alloys characterized by dual kinetics has been analyzed. It has been shown that the higher the iron content and the lower the martensitic transformation temperature, the more substantial the effect of pulsed magnetic field on the athermal martensitic transformation (MT) in the Fe–Ni alloys. The possibility of athermal martensite nucleation at magnetic inhomogenities of austenite, which is characterized by disordered magnetic moments (antiferromagnetic contribution) within a local area that assumes coherent conjugation at the interface, has been discussed. It has been suggested that athermal MT in Invar Fe–Ni alloys characterized by dual kinetics occurs in a pulsed magnetic field via a first-order magnetic phase transformation.

Comparing the deformation-induced martensitic transformation with the athermal martensitic transformation in Fe-Cr-Ni alloys

The microstructure of martensite formed athermally or via deformation in Fe-Cr-Ni alloys with different austenite (γ) stability has been investigated using microscopy. Two different types of microstructures, viz. blocky and banded structure, are observed after athermal and deformation-induced martensitic transformation (AMT and DIMT, respectively). The blocky structure form during AMT or DIMT if the stability of γ is low. In both cases, there is a significant chemical driving force for the transformation from γ to α′-martensite (α′), and if it is not hindered by e.g. planar defects it can grow uninhibited into a blocky morphology without the necessity to nucleate new crystallographic variants to accommodate the transformation strains. On the other hand, the banded structure is due to the formation of ε-martensite (ε) during AMT, or the wider concept shear bands in the case of DIMT. The shear bands, and in particular ε, lower the nucleation barrier for α′ that forms within individual shear bands if the stability of γ is low. Neighbouring α′ units predominantly have a twin-related orientation relationship to accommodate the transformation strains. With increasing γ stability during DIMT, variant selection becomes pronounced with preferred formation of variants favorable oriented with respect to the applied stress/strain field. The formation of α′ at individual shear bands is also rare, since no ε is present and instead α′ forms at the intersection of shear bands for more stable γ. In conclusion, AMT and DIMT for low γ stability lead to similar microstructures, whereas the DIMT microstructure for high γ stability is distinct.

The effect of alloying element Zn on magnetic properties of martensite phase in FeNiMn alloy

The effect of alloying element Zn on the characteristics of thermal induced martensite and magnetic properties of martensite phase were investigated with scanning electron microscopy (SEM) and Mössbauer spectroscopy characterization techniques. SEM observations revealed that the athermal martensitic transformation occurred with lath martensite morphology for different amount of Zn element. On the other hand Mössbauer spectroscopy showed that magnetic characteristic of martensite phase remarkably altered by increasing the amount of Zn element. The increased amount of Zn also causes the disappearance of the magnetization of the martensite phase.

On the Prediction of α-Martensite Temperatures in Medium Manganese Steels

A new composition-based method for calculating the α-martensite start temperature in medium manganese steel is presented and uses a regular solution model to accurately calculate the chemical driving force for α-martensite formation, $$ \Delta G_{\text{Chem}}^{\gamma \to \alpha } $$ . In addition, a compositional relationship for the strain energy contribution during martensitic transformation was developed using measured Young’s moduli (E) reported in literature and measured values for steels produced during this investigation. An empirical relationship was developed to calculate Young’s modulus using alloy composition and was used where dilatometry literature did not report Young’s moduli. A comparison of the $$ \Delta G_{\text{Chem}}^{\gamma \to \alpha } $$ normalized by dividing by the product of Young’s modulus, unconstrained lattice misfit squared (δ 2), and molar volume (Ω) with respect to the measured α-martensite start temperatures, $$ M_{\text{S}}^{\alpha } $$ , produced a single linear relationship for 42 alloys exhibiting either lath or plate martensite. A temperature-dependent strain energy term was then formulated as $$ \Delta G_{\text{str}}^{\gamma \to \alpha } \left( {{\text{J}}/{\text{mol}}} \right) = E\varOmega \delta^{2} (14.8 - 0.013T) $$ , which opposed the chemical driving force for α-martensite formation. $$ M_{\text{S}}^{\alpha } $$ was determined at a temperature where $$ \Delta G_{\text{Chem}}^{\gamma \to \alpha } + \Delta G_{\text{str}}^{\gamma \to \alpha } = 0 $$ . The proposed $$ M_{\text{S}}^{\alpha } $$ model shows an extended temperature range of prediction from 170 K to 820 K (−103 °C to 547 °C). The model is then shown to corroborate alloy chemistries that exhibit two-stage athermal martensitic transformations and two-stage TRIP behavior in three previously reported medium manganese steels. In addition, the model can be used to predict the retained γ-austenite in twelve alloys, containing e-martensite, using the difference between the calculated $$ M_{\text{S}}^{\varepsilon } $$ and $$ M_{\text{S}}^{\alpha } $$ .