Strain-induced Martensitic Transformation Research Articles

Martensitic transformation induced strength-ductility synergy in additively manufactured maraging 250 steel by thermal history engineering

Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility. Here, we report an intermittent printing strategy to tailor the microstructure and mechanical properties of maraging 250 steel via tuning the thermal history during wire-arc directed energy deposition. By introducing a dwell time between adjacent layers, the maraging 250 steel is cooled below the martensite start temperature, triggering thermally-driven martensitic transformation during the printing process. Thermal cycling during subsequent layer deposition results in the formation of reverted austenite which shows a refined microstructure and induces elemental segregation between martensite and reverted austenite. The Ni enrichment in the austenite promotes stabilization of the reverted austenite upon cooling to room temperature. The reverted austenite is metastable during deformation, leading to strain-induced martensitic transformation under loading. Specifically, a 3 min interlayer dwell time produces a maraging 250 steel with approximately 8% reverted austenite, resulting in improved work hardening via martensitic transformation induced plasticity during deformation. Meanwhile, the higher cooling rate and refined prior austenite grains lead to substantially refined martensitic grains (by approximately fivefold) together with an increased dislocation density. With 3 min interlayer dwell time, the yield strength of the printed maraging 250 steel increases from 836 MPa to 990 MPa, and the uniform elongation is doubled from 3.2% to 6.5%. This intermittent deposition strategy demonstrates the potential to tune the microstructure of maraging steels for achieving strength-ductility synergy by engineering the thermal history during additive manufacturing.

An approach for identifying the deformation-induced strain from tensile tests and x-ray diffraction measurements

ABSTRACT Non-uniform plastic deformation and phase transformation introduce residual stress (RS) in a material. The present study focuses on the influence of tensile strain on RS development in 304L austenitic stainless steel, wherein strain-induced martensite (SIM) transformation influences deformation behaviour. Room temperature tensile tests were interrupted at various tensile strains between the material yield and ultimate tensile strength at 5.21 × 10−4 and 1.3 × 10−2 s−1 strain rates. Microstructural characterisation of as-received and deformed samples by electron backscatter diffraction provides evidence of SIM formation and shear bands. Analysis of tensile flow and work hardening behaviour is discussed in correlation to the microstructural and dislocation density variations. In addition, RS in the austenite and martensite phases was measured using the x-ray diffraction method, and the RS balance that occurs between the longitudinal and transverse components and the two phases after tensile deformation is presented. It is shown that a combined analysis of RS and x-ray diffraction peak width could be utilised to find the deformation level in tensile fractured, press brake bent, and roll bent samples.

Influence of strain rate on the work hardening, strain induced martensite formation, strain partitioning, and variant selection in a medium-Mn steel

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 10−3, 1 and 103s−1. Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼0.05. Beyond the true plastic strain of ∼0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α′-martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov–Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼0.15 revealed that the samples deformed at a lower strain rate did not show any noticeable variant selection. However, in samples deformed at a higher strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strains remains predominant.

Development of TRIP phenomenon using power refining of retained austenite by niobium

AbstractThe illustrations above demonstrate that as the percentage of niobium increases, the grain size decreases, leading to an increase in the stability of transformation induced plasticity steel. This, in turn, results in improved mechanical strength. The optimal volume fraction of retained austenite for a significant transformation induced plasticity effect to occur is reported to be in the range of 10 vol. %–20 vol. %. The volume fraction of retained austenite directly determines the carbon content and grain size of the retained austenite, its two main stabilization factors. The stability of the retained austenite dictates when the strain‐induced martensite transformation (SIMT) occurs during straining of transformation induced plasticity high strength steel. Unstable retained austenite transforms almost immediately upon deformation, increasing work hardening rate and formability during the stamping process. At the appropriate stability of the retained austenite, the Strain‐Induced Martensite Transformation begins only at strain levels beyond those produced during stamping and forming, and the retained austenite is still present in the final part; it can transform into martensite in the event of a crash, providing greater crash energy absorption. The tensile strength level of micro‐alloyed transformation induced plasticity steels may exceed 1 GPa. Grain refinement mechanism is considered as the most applied mechanism used in increasing of steel strength without deterioration of ductility and toughness. Recently, it was proposed that the grain refinement could act positively on promoting the transformation induced plasticity effect of advanced high strength steel through improving the stability of retained austenite. In this research, quenching and partitioning heat treatment technique was applied to four grades of low carbon steel with different percentage of niobium. Then, the effect of niobium on grain refinement has been detected by microstructure observations. Mechanical properties and strain hardening properties of investigated steel have been determined. In addition, by using x‐ray diffraction, as well as a new electric resistance‐based sensor, it was possible to characterize the stability of the retained austenite through the plastic deformation of steel. Grain refinement by using of niobium has a great impact on promoting the stability of retained austenite against the plastic stress at the plastic deformation zone.

Microstructural evolution and dynamic recrystallization in Fe50Mn30Co10Cr10 medium entropy alloy under hot compression

This comprehensive study investigates the recrystallization behavior and deformation mechanisms of the Fe50Mn30Co10Cr10 alloy during hot compression conditions. The alloy undergoes discontinuous dynamic recrystallization, evidenced by necklace-like structures at grain boundaries, competing with the initial hexagonal close-packed (HCP) phase and γ→ε reversion. Early deformation is dominated by strain-induced martensite transformation, twinning, with slip with strain partitioning at the end of deformation. The developed Zener-Hollomon equation, validated experimentally, shows a constant activation energy of 126.96 kJ/mol. Stress-strain curves indicate that recrystallization only partially compensates for hardening effects, with no significant peaks observed. The work hardening curve exhibits significant oscillations due to encounters with pre-existing thermal HCP layers, influencing the work hardening rate. Energy efficiency maps based on temperature and strain rate identify optimal deformation conditions at temperatures ranges of 710–750 °C and strain rates of 0.0025–0.0035 s⁻1. This study provides a deeper understanding of the alloy's behavior, offering insights into the interplay between recrystallization, HCP phase formation, and twin generation. These findings contribute to the optimization of the alloy's mechanical properties for industrial applications.

Evaluation of Shear-Punched Surface Layer Damage in Three Types of High-Strength TRIP-Aided Steel

The damage properties in the shear-punched surface layer, such as the strain-hardening increment, strain-induced martensite fraction, and initiated micro-crack/void characteristics at the shear and break sections, were experimentally evaluated to relate to the stretch-flangeability in three types of low-carbon high-strength TRIP-aided steel with different matrix structures. In addition, the surface layer damage properties were related to the mean normal stress developed on shear-punching and microstructural properties. The shear-punched surface damage of these steels was experimentally confirmed to be produced under the mean normal stress of negative to 0 MPa. TRIP-aided bainitic ferrite (TBF) steel had the smallest surface layer damage, featuring a significantly suppressed micro-crack/void initiation. This was due to the fine bainitic ferrite lath matrix structure, a low strength ratio of the second phase to the matrix structure, and the high mechanical stability of the retained austenite. On the other hand, the surface layer damage of TRIP-aided annealed martensite (TAM) steel was suppressed next to TBF steel and was smaller than that of TRIP-aided polygonal ferrite (TPF) steel. The surface layer damage was also characterized by a large plastic strain, a large amount of strain-induced martensite transformation, and a relatively suppressed micro-crack/void formation, which resulted from an annealed martensite matrix and a large quantity of retained austenite. The excellent stretch-flangeability of TBF steel might be caused by the suppressed micro-crack/void formation and high crack propagation/void connection resistance. The next high stretch-flangeability of TAM steel was associated with a small-sized micro-crack/void initiation and high crack growth/void connection resistance.

In-situ synchrotron X-ray diffraction study of the effects of grain orientation on the martensitic phase transformations during tensile loading at different strain rates in metastable austenitic stainless steel

In this work, in-situ high-energy X-ray diffraction was used to analyze the effects of strain rate and austenite (γ) grain orientation on the strain-induced martensitic transformation in metastable austenitic stainless steel 301LN. The diffraction measurements were carried out at strain rates ranging from 10−3 s−1 to 1 s−1 continuously without interrupting the experiment and thus creating nearly adiabatic conditions at the highest studied strain rate. The results indicate that <100>γ fiber-oriented grains preferentially transform at the strain rate of 10−3 s−1 when the true strain is above 0.10, whereas the <111>γ fiber-oriented grains transform only at later stages of plastic deformation. The phase transformation rate of the <111>γ and <100>γ fiber-oriented grains decreases with increase in strain rate. A theoretical model based on stacking fault width as a function of external stress and temperature (stacking fault energy) was used to predict lower-bound estimates for the critical tensile stress needed to start ε-martensite and α’-martensite phase transformations. The model can predict the experimentally observed phase transformation behavior of the <111>γ fiber orientations at all strain rates but is unable to predict the decrease of phase transformation rate of <100> fiber-oriented γ grains with increase in strain rate, which could be related to change in dislocation structure.

Probing plastic deformation-related properties in static recrystallized Co–Cr–Fe–Ni–Mo alloy for biomedical applications

40Co–20Cr–16Fe–15Ni–7Mo (mass%, Co–Cr–Fe–Ni–Mo) alloys are currently used in biomedical implants such as balloon-expandable stents. Materials used as balloon-expandable stents are statically recrystallized and subsequently utilized within the body in a plastically deformed state. This study describes the microstructures and the mechanical and corrosive properties related to the plastic deformation of statically recrystallized Co–Cr–Fe–Ni–Mo alloys. The η-phase (M6X-M12X type, M: metallic elements; X: C and/or N) and M23X6-type precipitates were observed. The grain sizes of the recrystallized alloys were 6–92 μm. The mechanical properties were dependent on the grain size: the 0.2% proof strength and ultimate tensile strength increased and plastic elongation decreased with an increase in grain refinement. The stacking fault energy (SFE) of the Co–Cr–Fe–Ni–Mo alloy was determined by the width of partial dislocations, measured as 16 ± 3 mJ⋅m−2. Corresponding to this SFE, the formation of the ε-phase associated with strain-induced martensitic transformation and deformation twins were observed after plastic deformation. This suggests that the Co–Cr–Fe–Ni–Mo alloy exhibited a twinning-induced plasticity-like work-hardening mechanism. The polarization curves of the alloys in a simulated body fluid were not affected by the formation of precipitates (η-phase and M23X6-type) or plastic deformation, thereby suggesting that heat treatment does not deteriorate the corrosive property. The recrystallized alloy with a grain size of 36 μm had the ultimate tensile strength of more than 1000 MPa, 0.2% proof strength of less than 500 MPa, and plastic elongation greater than 70%. This alloy is expected to be applied in low-yield-strength-type balloon-expandable stents.

In-situ neutron diffraction study of serration-involved ultra-cryogenic deformation behavior at 15 K

This work focused on the mechanical properties and serration-involved deformation behavior of advanced alloys at 15 K. Evolution of stacking faults and ε-martensite improved the mechanical performance of CoCrNi alloys, and significant strain-induced martensite transformation of DED-SS316L led to superior strength and strain hardening. A magnitude in stress drop was governed by dislocation density, phase type, and lattice defects, irrespective of processing method. FCC {200} notably was influenced recovery behavior after stress drop, and the contribution of strain energy density by serration on tensile toughness was the greatest for HR-CoCrNi.

Experimental and numerical investigation of the yield point phenomenon and strain partitioning behavior in a dual-phase steel with lamellar structure

Severe rolling techniques show potential for enhancing the strength and ductility of metallic materials, but the resulting microstructural anisotropy affects the mechanical behavior. The study investigated the anisotropic yield point phenomenon (YPP) and strain partitioning behavior in a dual-phase steel with ultrafine lamellar structure, fabricated through severe cold- and warm-rolling procedures. Experimental results show that tensile specimens aligned parallel to the rolling direction exhibit YPP and higher ductility, while those oriented normal to the rolling direction show continuous yielding behavior and decreased ductility. Using Digital Image Correlation techniques, we studied the evolution of the strain distribution on the surface of the gauge segment, revealing that the strain within the Lüders band was almost constant during the Lüders band propagation. Strain-induced martensitic transformation (SIMT) was observed within the Lüders band and the volume fraction of martensite keeps almost constant during the Lüders band propagation. Numerically, we integrated the mechanisms of SIMT and YPP into a dislocation density-based J2 plasticity constitutive framework. Simulated tensile tests indicate a smaller Lüders strain resulted from the initial higher strain hardening rate. Additionally, SIMT suppresses local instability and therefore promotes the stable propagation of Lüders bands. Simulations verify that the lamellar orientation and SIMT significantly affect the strain hardening rate and void growth. Our findings provided a new perspective on the intrinsic mechanism of the anisotropic YPP and ductility in lamellar dual-phase steels.

Preparation and cavitation erosion resistance of nanocrystalline surface layer on 304 stainless steels

Cavitation erosion is a common damage form of hydraulic components. Constructing a cavitation-erosion-resistant layer on the surface of hydrodynamic components can effectively prolong the service life of the equipment. In this study, 304 austenitic stainless steel was machined by a diamond blade under specific processing parameters, and a nanocrystalline layer (304-Nano) was formed by the work hardening during the machining process. The machining process also allowed strain-induced martensitic transformation, creating martensite in the nanocrystalline layer. The nanocrystalline layer had an average grain size of approximately 34.7 nm, showing a Vickers hardness of 600 ± 30 Hv, which was much higher than that of the original 304 austenitic stainless steel (230 ± 14 Hv). After the 15-h cavitation erosion test, the cavitation erosion rate of the 304-Nano sample was only 19.4 % that of the original ordinary 304 stainless steel, showing that the 304-Nano sample has excellent cavitation erosion resistance. Furthermore, the in-situ SEM results showed that the 304-Nano sample could significantly inhibit crack propagation, which was probably attributed to its high density of grain boundaries and ultrahigh hardness. The excellent cavitation erosion resistance of the nanocrystalline layer is expected to be used as a protective material for anti-cavitation erosion applications.

Residual Stress Engineering for Wire Drawing of Austenitic Stainless Steel X5CrNi18-10 by Variation in Die Geometries-Effect of Drawing Speed and Process Temperature.

As a result of conventional wire-forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent forming processes such as bending operations. High tensile residual stresses typically occur in the near-surface region of the wires and can limit further application and processability of the semi-finished products. This paper presents an approach for tailoring the residual stress distribution by modifying the forming process, especially with regard to the die geometry and the influence of the drawing velocity as well as the wire temperature. The aim is to mitigate the near-surface tensile residual stresses induced by the drawing process. Preliminary studies have shown that modifications in the forming zone of the dies have a significant impact on the plastic strain and deformation direction, and the approach can be applied to effectively reduce the process-induced near-surface residual stress distributions without affecting the diameter of the product geometry. In this first approach, the process variant using three different drawing die geometries was established for the metastable austenitic stainless steel X5CrNi18-10 (1.4301) using slow (20 mm/s) and fast (2000 mm/s) drawing velocities. The residual stress depth distributions were determined by means of incremental hole drilling. Complementary X-ray stress analysis was carried out to analyze the phase-specific residual stresses since strain-induced martensitic transformations occurred close to the surface as a consequence of the shear deformation and the frictional loading. This paper describes the setup of the drawing tools as well as the results of the experimental tests.

Martensitic transformation in rolled 301 stainless steel induced by cryogenic treatment

The martensitic transformation in rolled 301 stainless steel induced by cryogenic temperature was investigated in the present work. The methods of magnetization, thermoelectric potential combined with microscopes were adopted for revealing the microstructure evolution from the global and local scale. Hardness and tensile tests were performed at room temperature to evaluate the variation of properties. The results showed that deep cryogenic treatment improved the tensile strength of 301 steel rolled with the reduction of 10%, 20% and 30% by 93.7 MPa, 71 MPa and 125.5 MPa, and the yield strength by 131 MPa, 65.2 MPa and 208.9 MPa, respectively. The improvement in tensile strength caused by deep cryogenic treatment are equivalent to that induced by the higher rolling reduction rate, which could achieve higher strength-plasticity balance of 301 ASS at relatively low deformation. Deep cryogenic treatment promoted the formation of martensitic in the rolled microstructure, which was considered to be the main reason for the strengthening. The volume fraction of martensite in 301 ASS rolled with the reduction rate of 20% and 30% was increased by 44.5% and 28.2% after cryogenic treatment, respectively. The martensitic transformation caused by deep cryogenic treatment corresponded to the principle of conventional strain-induced martensitic transformation, which nucleated in the intersections of shear bands and formed from the transition ε-martensite. Under cryogenic temperature, the decrease of stacking fault energy (SFE) and increase of lattice internal stress reduced the critical stress of shear bands formation, as a result of increasing the amount of shear bands and nucleation sites for martensitic transformation.

Further enhancement of surface mechanical properties of carburized 9310 steel by electropulsing-assisted ultrasonic surface rolling process

9310 carburized steel experiences significant wear and spalling problems when operating under high friction and heavy loads, which severely restricts its applications in the aviation industry. Therefore, it holds great significance to further enhance the surface mechanical properties of 9310 carburized steel through suitable surface strengthening technologies. The Electropulsing-Assisted Ultrasonic Surface Rolling Process (EP-USRP) has been successfully applied to carburized 9310 steel, yielding significant improvements in surface quality, hardness, friction and wear properties, and microstructure. Systematic investigations were conducted under varying EP-USRP parameters. Regarding the electric pulse parameters, it is observed that with the increase in output voltage, surface properties such as surface roughness and surface hardness tend to first optimize and then deteriorate. After applying EP-USRP with optimal parameters, the surface roughness of carburized 9310 steel was reduced to Ra 0.154 μm, a mere 5 % of its initial value. The surface hardness experienced a substantial increase, reaching 909.2 HV, representing a notable improvement of 29.9 % compared to the original sample, with a corresponding hardening depth of 1400 μm. Moreover, the wear amount of carburized 9310 steel witnessed a remarkable reduction to 11.8 % of its initial value, accompanied by a decrease in the coefficient of friction to 43.0 %. The enhancement of carburized 9310 steel through EP-USRP is attributed to two key mechanisms: deformation hardening induced by severe plastic deformation and microstructure strengthening resulting from strain-induced martensitic transformation. Among them, the severe plastic deformation mechanism is dominant, Through these mechanisms, the proportion of small-angle boundaries in carburized 9310 steel strengthened by EP-USRP increased. Additionally, the grain size of the surface layer decreased, and the content of retained austenite in the surface layer decreased, collectively contributing to significant improvements in surface properties, as well as friction and wear performance.

Systematic Study on the Microstructures of Biomedical Co–20Cr–15W–10Ni Alloys with Carbon Contents Ranging from 0 to 0.2 mass pct

Herein, the effect of carbon content on the microstructures of Co–20Cr–15W–10Ni (mass pct, CCWN) alloys was systematically studied. For this, CCWN alloys with carbon contents of 0, 0.05, 0.10, and 0.20 mass pct, i.e., 0C, 0.05C, 0.10C, and 0.20C alloys, respectively, were prepared using an induction melting furnace. The as-cast alloys were solution treated at 1523 K for 7.2 ks, followed by cold swaging and heat treatment at 1173 K–1473 K for 0.15–7.2 ks. Consequently, η-phase (M6C-M12C type, M: metallic element) precipitates were detected in the as-cast 0.10C and 0.20C alloys, whereas no precipitates were observed in the 0C and 0.05C alloys. These precipitates were dissolved via a solution treatment. After cold swaging, the ε-phase formed through a strain-induced martensitic transformation in the 0C and 0.05C alloys. Following heat treatment, a single γ-phase matrix was observed in all the alloys, and μ-phase (Co7W6-type) and η-phase precipitates were observed in the 0C and carbon-added alloys, respectively. The precipitation temperature range of the η-phase increased with increasing carbon content. The stability of the precipitates qualitatively conformed with that of the calculated phase diagram. This is the first paper that reports the microstructural changes in CCWN alloys with varying carbon contents.

Effects of Severe Plastic Deformation and Subsequent Annealing on Microstructures of a Ni50.6Ti49.4 Shape Memory Alloy

High-pressure torsion (HPT) was applied to the Ni50.6Ti49.4 (at. %) alloy ingot up to 1/4, 2, 16, 32 and 48 turns under a pressure of 6.0 GPa. The samples were examined by X-ray diffraction (XRD), transmission electron microscope (TEM) and microhardness measurement. The results indicate that martensitic transformation and formation of amorphous phase occurred during the HPT process. As the HPT turns increased, more amorphous phase formed. The fraction of amorphization was analyzed based on the X-ray results. The microhardness increased with the HPT turns, which may be related to strain-induced martensite transformation, formation of the amorphous phase, increased dislocation densities and grain refinement. Differential scanning calorimetry (DSC) test revealed that shape memory alloys can be produced by HPT and post-HPT annealing from a NiTi ingot.

The Use of Strain-Induced Martensitic Transformation Phenomena to Estimate the Surface Plastic Strain Produced by Grit Blasting on Overaged 18Ni Maraging Steel

The Use of Strain-Induced Martensitic Transformation Phenomena to Estimate the Surface Plastic Strain Produced by Grit Blasting on Overaged 18Ni Maraging Steel

Deformation Behavior and Texture Evolution of Ferrite and Austenite Phases in Lean Duplex Stainless Steel

To avoid cracks during hot rolling, a short process of direct cold rolling following solution treatment of the casting billet for preparing lean duplex stainless steel plates is proposed. The effect of cold rolling reduction on the microstructure, texture, and mechanical properties of UNS S32101 is investigated. The results show that dislocation slip is the main character in ferrite, which leads to dislocation tangles, dislocation cells, high‐density dislocation walls, and deformed microbands. However, twinning and strain‐induced martensite transformation (SIMT) occur in austenite, and the SIMT mechanism of austenite follows the classical model γ → ε → α′ and γ → α′, with the orientation relationship of Kurdjumov–Sachs ( and ) and Nishyama–Wassermann (N–W) ( and ). Meanwhile, at 50%, there is a transition from Cu texture to Brass texture in the austenite phase. At 12.5%, the yield strength is 344 MPa higher than that of the traditional hot rolling process, and the elongation remains about 35%. With the increasing cold rolling reduction, the elongation decreases while the strength rises significantly. Strengthening dislocations, fine‐tuning grains, and SIMT are the primary contributors to the improvement in strength.

Effects of Mean Normal Stress on Strain-Hardening, Strain-Induced Martensite Transformation, and Void-Formation Behaviors in High-Strength TRIP-Aided Steels

To analyze various types of cold formability in TRIP-aided polygonal ferrite (TPF), annealed martensite (TAM), and bainitic ferrite (TBF) steels, the effects of the mean normal stress on the strain-hardening, strain-induced martensite transformation, and void-formation behaviors were investigated. The strain-hardening behavior was influenced by positive mean normal stress and was hardly influenced by zero and negative mean normal stresses in all steels. Positive mean normal stress promoted the strain-induced martensitic transformation behavior, especially in TBF steel due to the high mechanical stability of the retained austenite. The void-formation behavior was also promoted by positive mean normal stress, especially in TPF steel. These behaviors were also related to the microstructural properties, such as the matrix structure, retained austenite characteristics, and second phase.

Behavior of TRIP-aided medium Mn steels investigated by in situ synchrotron X-ray diffraction experiments and microstructure-based micromechanical modelling

Medium Mn steels belong to a new generation of advanced high-strength steels whose superior mechanical properties are explained by their ultrafine-grained ferrite/austenite/martensite microstructures and a possible transformation induced plasticity (TRIP) related to the stability of retained austenite. The mechanical behaviour of a set of model medium Mn steels is investigated during tensile testing using a combination of high-energy X-ray diffraction (HEXRD) and digital image correlation (DIC) measurements. HEXRD allows for the time-resolved determination of transformation kinetics and in situ stress partitioning among the different constituting phases. DIC provides precise spatiotemporal information on the strain evolution along the gauge length, particularly at the position of the diffracting volume. These experiments served to calibrate an innovative mean field micromechanical model which accounts for the local behaviour of each phase, as well as the strain-induced martensitic transformation (SIMT) of retained austenite. The work-hardening of both austenite and ferrite is modeled using a dislocation-based size-sensitive approach which includes kinematic hardening contributions. The behaviours of fresh and strain-induced martensite are predicted using a genuine model derived from the continuous composite approach. The model for SIMT is based on a thermodynamic assessment of the stability of retained austenite. The overall model is thus sensitive to the size of the microstructure components, their local chemistry, and their respective stability.