Transformation-induced Plasticity Effect Research Articles

Synergistic improvement of strength and ductility by high magnetic field assisted intercritical annealing in lightweight medium Mn steel

The microstructures and mechanical properties of lightweight medium Mn steels were studied by high magnetic field (HMF) assisted intercritical annealing process (HMF-IA). The results show that the volume fraction of retained austenite increased and reached the maximum value of 56.2 % when the magnetic flux density (MFD) was 10 T at first and then decreased with the increase of MFD. The application of 10 T HMF promoted the diffusion ability of carbon atoms into austenite by magnetic stress-driven dislocation movement and increased the volume fraction and grain size of austenite. HMF also played a critical role in dislocation movement and element partitioning during the IA process. Due to the wide range of bimodal size distribution of austenite after the HMF-IA process, stronger transformation induced plasticity effect and twin induced plasticity effect happened in the process of plastic deformation. For the sample annealed under 10 T, comparing with the sample annealed without HMF, the product of strength and elongation value increased from 45.7 GPa·% to 55.1 GPa·%. Additionally, it also kept a persistent high strain hardening. The present study provides a new understanding of the role of HMF in adjusting the relationship between microstructure and mechanical properties during the annealing process and opens up a new path for designing industrial medium Mn steel with low cost, high performance and simple process.

Remarkable contribution of stress-induced martensitic transformation to strain-hardening behavior in Ti−Mo-based metastable β-titanium alloys

The role of stress-induced β → α″ martensitic transformation in contributing to the exceptional strain-hardening behavior of a Ti-11Mo (wt. %) model alloy was studied. The results reveal that the α″-martensite plates, undergoing relatively high transformation strains upon formation, act as effective barriers against dislocation motion. As deformation proceeds, the progressive generation of these plates dynamically reduces the dislocation mean free path (dynamic Hall-Petch effect), while simultaneously encouraging the accumulation of dislocations. These dual effects substantially enhance the flow stress with increasing strain, thereby resulting in remarkable strain-hardenability, a crucial factor for maintaining excellent ductility. This study presents innovative experimental evidence elucidating the mechanism of the transformation-induced plasticity effect in metastable β-titanium alloys, while also offering insights for designing novel alloys with superior strain-hardenability and ductility.

Research on deformation behaviors at different temperatures of lean duplex stainless steel S32101 produced by direct cold rolling process

In this paper, the mechanical properties, work hardening characteristics and deformation mechanism of lean duplex stainless steel (LDSS) S32101 produced by direct cold rolling were studied under different temperature conditions. The yield strength (YS) and tensile strength (TS) of S32101 decreased with the increase of deformation temperature, while the elongation increased first and then decreased. The TS of S32101 at −70 °C was 1052.6 MPa, the elongation remained 57.2%, and the product of strength and elongation (PSE) was as high as 60.2 GPa·%, which was mainly due to the synergistic strengthening of transformation induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects in austenite at cryogenic temperatures. The efficiency of grain refinement at cryogenic temperature was improved by TRIP effect and deformation twins (DTs), indicating that it had the potential to be applied at low temperature (−70 ∼ −30°C). Moreover, the high density nanotwins and the strong interaction between the dislocation and nanotwin bundles also contributed to strain hardening at low temperatures and thus better comprehensive mechanical properties could be achieved.

Understanding plasticity in multiphase quenching & partitioning steels: Insights from crystal plasticity with stress state-dependent martensitic transformation

This study develops a novel crystal plasticity (CP) model incorporating deformation-induced martensitic transformation (DIMT) and transformation-induced plasticity (TRIP) effect to predict the complex interplay between microstructural evolution and mechanical behavior in a third-generation advanced high-strength steel QP980. This model introduces phenomenological theory of martensite crystallography (PTMC) based TRIP theory and DIMT kinetics grounded on nucleation-controlled phenomenon. Notably, the DIMT model is improved by utilizing a geometric approach for calculating shear band intersections. A virtual multiphase representative volume element (RVE) based on the Voronoi tessellation is generated for the QP980 steel, which comprises ferrite, martensite, and retained austenite (RA). The study highlights how phase transformation affects mechanical properties, notably the strengthening from transformed martensite and the mechanical alterations in RA due to the TRIP effect. The DIMT kinetics dependent on stress states such as uniaxial tension (UT), uniaxial compression (UC), plane strain tension (PST), and equi-biaxial tension (EBT) are analyzed using the developed model. The role of microstructural surroundings on martensitic transformation is also examined. Furthermore, analysis under biaxial loading angles using the model reveals an asymmetric yield surface, with more pronounced changes in yield stress in the tensile region due to accelerated transformation behaviors, as opposed to the more gradual transformations in the compressive region. This study provides valuable insights into the deformation mechanisms of the third-generation advanced high-strength steels including relationship between plastic anisotropy, transformation kinetics, and microstructural evolution.

Synergistic improvement of strength and ductility by high magnetic field assisted intercritical annealing in lightweight medium Mn steel

The microstructures and mechanical properties of lightweight medium Mn steels were studied by high magnetic field (HMF) assisted intercritical annealing process (HMF-IA). The results show that the volume fraction of retained austenite increased and reached the maximum value of 56.2% when the magnetic flux density (MFD) was 10 T at first and then decreased with the increase of MFD. The application of 10 T HMF promoted the diffusion ability of carbon atoms into austenite by magnetic stress-driven dislocation movement and increased the volume fraction and grain size of austenite. HMF also played a critical role in dislocation movement and element partitioning during the IA process. Due to the wide range of bimodal size distribution of austenite after the HMF-IA process, stronger transformation induced plasticity effect and twin induced plasticity effect happened in the process of plastic deformation. For the sample annealed under 10 T, comparing with the sample annealed without HMF, the product of strength and elongation value increased from 45.7 GPa·% to 55.1 GPa·%. Additionally, it also kept a persistent high strain hardening. The present study provides a new understanding of the role of HMF in adjusting the relationship between microstructure and mechanical properties during the annealing process and opens up a new path for designing industrial medium Mn steel with low cost, high performance and simple process.

Enhanced yield stress and reduced elastic modulus of metastable β Ti–Nb alloys deformed by equal channel angular pressing attributed to the Synergistic effect of deformation mechanisms

This work aims to obtain Ti–Nb alloys with a high yield stress, good plasticity, and low elastic modulus. To this end, metastable β Ti–Nb alloys were subjected to multipass equal channel angular pressing (ECAP) deformation at room temperature, and a thorough investigation of their microstructure evolution, deformation mechanisms, and strengthening/toughening mechanisms was conducted through X-ray diffraction, transmission electron microscopy, electron backscatter diffraction, and tensile tests. In the initial deformation stages, multiple deformation mechanisms are observed, including dislocation slip, {332} <113> twinning, stress-induced martensite (SIM) α″-phase, {112} <111> twinning, and stress-induced ω-phase. However, as the strain accumulates, the ω-phase disappears, the volume fraction of the SIM α″-phase gradually decreases, and the dominant deformation mechanism changes to dislocation slip and {332} <113> twinning. The ECAP deformation results in a complex-network microstructure and the precipitation of the nanoscale SIM α″-phase, which leads to the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects. As a consequence, the Ti–Nb alloy simultaneously exhibits a high yield stress and a good plasticity. Furthermore, the high-density dislocations and interfaces generated by the ECAP deformation reduce the stability of the β-phase, and the introduced nanopores cause a decrease in density, resulting in a decrease in the elastic modulus of the alloy.

Overcoming strength-ductility trade-off in Si-containing transformation-induced plasticity high-entropy alloys via metastability engineering

Benefiting from the transformation-induced plasticity effect has been recognized as a new approach to develop high-performance Si-containing high-entropy alloys with outstanding room-temperature strength-ductility balance. However, closely controlling the kinetics of α΄-martensite formation is essential to further boost ductility in addition to ultrahigh strength. In the present work, this aim was fulfilled by manipulating the chemical composition of the promising FeCoCrVMnSi system with the replacement of Mn with Ni and adjusting the Si content to fine-tune the stacking fault energy and the driving force of austenite to martensite transformation. The experimental results were obtained by EBSD, TEM, XRD, and tensile testing, and the results were supported by thermodynamic calculations, work-hardening analysis, and assessment of α΄-martensite formation kinetics. Accordingly, the single-phase Fe47Co30Cr10Ni5V8-xSix system was designed. In this system, the Fe47Co30Cr10Ni5V2Si6 alloy exhibited exceptional tensile strength (∼1.1 GPa) and total elongation (72%), outperforming the competitive dual-phase Fe46Co30Cr10Mn5V2Si7 alloy. High strength was attributed to the high solid-solution strengthening effect as well as the volume fraction of α΄-martensite, while its ductility benefited from fine-tuned kinetics of martensitic phase transformation.

Designing the heterogeneous microstructure via a direct warm rolling process to synergistically enhance the strength-ductility in a medium Mn steel

This work introduced a facile method for fabricating heterostructures in a medium Mn steel via a direct warm rolling process. The heavy warm rolling promoted the transformation of metastable austenite (∼12.2 vol%) into hard domain α’ martensite, which formed a heterogeneous microstructure with the untransformed γ (soft domain). After 600 °C re-annealing, the hard domain α’ re-reversed to α + γ, and the heterostructure disappeared. Compared to the homogeneous structure, the heterostructure exhibited the synergistic enhancement of strength-ductility (yield strength (YS): 782 → 1113 MPa; ultimate tensile strength (UTS): 1099 → 1304 MPa; total elongation (TEL): 20.8 % → 21.7 %). Quantitative analysis showed that the heterogeneous deformation induced (HDI) strengthening (∼261 MPa) was the primary contributor to the increased ΔYS (782 → 1113 MPa) value, rather than the traditional strengthening mechanisms. Transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effect were the main deformation mechanisms of both homogeneous/heterogeneous structures. However, compared to the homogeneous microstructure, the heterogeneous structure possessed the more pronounced TRIP and HDI hardening effects as well as L-C locks, the cooperative effect of these three factors further improved the strain hardening ability to break through the strength-ductility trade-off dilemma.

Cyclic quenching treatment doubles the Charpy V-notch impact energy of a 2.3 GPa maraging steel

A cyclic quenching treatment (CQT) succeeded in turning a 2.3 GPa maraging steel with a Charpy impact energy of 9 J into a new grade with the same strength but a Charpy impact energy of 20 J upon 4 cyclic treatments. The improvement of mechanical properties is attributed to the refinement and increased chemical heterogeneity of the martensitic substructure, rather than the refinement of prior austenite grain (PAG), as well as the Transformation-Induced Plasticity (TRIP) effect facilitated by small austenite grains. The role of local segregation of Ni during CQT in the formation of Ni-rich austenite grains, Ni-rich martensite laths and Ni-poor martensite laths, was investigated and verified by DICTRA simulations. This study highlights the important influence of Ni partitioning behavior during CQT, providing insights into microstructural evolution and mechanical properties.

Heterogeneous Multi-Phase Grains Improving the Strength-Ductility Balance in Warm-Rolled Fe-18Mn-3Ti Steel.

The thermal properties, microstructure, and mechanical properties of Fe-18Mn-3Ti (wt%) were investigated, focusing on the effects of different heat-treatment processes. Results revealed that the 450 °C warm-rolling sample (450 WR) exhibited promising mechanical properties. Specifically, this sample displayed a yield strength of 988 MPa, an ultimate tensile strength of 1052 MPa, and total elongation of 15.49%. Consequently, a favorable strength-ductility balance was achieved. The strain-hardening ability surpassed that of the cold rolling sample (CR). Microstructure analysis indicated the simultaneous occurrence of dynamic equilibrium between grain deformation and re-crystallization because of the co-influence of thermal and strain in the warm rolling process. This desirable mechanical property was attributed to the presence of a multi-phase (α-martensite, austenite, and ε-martensite) and heterogeneous microstructure. The improvement of ultimate tensile strength was based on grain refinement, grain co-deformation, and the transformation-induced plasticity (TRIP) effect in the early stage of plastic deformation (stage Ⅰ). The improvement of ultimate elongation (TEL) was ascribed to the TRIP effect in the middle stage of plastic deformation (stage Ⅱ).

Enhancing the adiabatic shear resistance of Ti-Zr-Nb-Al high entropy alloy via metastability engineering

Dual-phase Ti-Zr based high entropy alloys are lightweight structural materials with high specific strength, which are promising for the application in warheads and munitions. However, they usually experience adiabatic shear under impact, resulting in early failure. Here, we demonstrate an effective method to enhance the adiabatic shear resistance, benefiting from the transformation-induced plasticity (TRIP) effect. The composition of Ti-Zr-Nb-Al is designed based on a combination of lattice distortion enthalpy, mixing enthalpy, and Bo¯-Md¯ criteria, to enable a metastable solution structure. The alloy is directly water quenched from 900 ℃ to maximally keep the metastable BCC phase (MA), while an annealed alloy with the BCC+HCP dual-phase structure (PA) is also fabricated for comparison. The metastable MA alloy exhibits a doubled fracture strain and 79 % enhancement of impact energy compared with the dual-phase PA alloy, while its dynamic ultimate strength remains almost unchanged. Microstructural analyses at different dynamic stain levels reveal a BCC-α" phase transformation in MA alloy, which delays the generation of phase boundaries, avoids the premature stress concentration, and thereby delays the occurrence of adiabatic shear.

The effect of double intercritical annealing on micromechanical behavior of a medium Mn transformation induced plasticity steel investigated by in situ high energy X-ray diffraction

The micromechanical behavior of a medium Mn transformation induced plasticity (TRIP) steel processed by intercritical annealing (IA) and double intercritical annealing (DIA) has been investigated by using in situ high energy X-ray diffraction during uniaxial tensile deformation. The results show that the ultimate tensile strength (UTS) and total elongation are 777.1 MPa and 39.6 % for IA and 953.6 MPa and 59.1 % for DIA, and that the product of UTS and total elongation is 30.8 GPa·% for IA and 56.4 GPa·% for DIA, respectively. Discontinuous phase transformation from austenite to martensite leads to Portevin-Le Châtelier (PLC) band propagation for both IA and DIA. The pronounced TRIP effect takes place in DIA step by step during deformation and contributes to higher strain hardening, which results in almost doubling the product of UTS and total elongation. The stepwise phase transformation highly corresponds to the lattice strain evolution of ferrite and austenite, indicating that load partitioning occurs during the deformation. The change of grain structure generated by different heat-treatments correlate with the load partitioning, thereby inducing large discrepancy in micromechanical behavior. The present investigation provides a deeper fundamental understanding of the effect of double inter-critical annealing on micromechanical behavior and effective engineering strategies for improving the product of UTS and total elongation for the medium Mn TRIP steels.

Unveiling the roles of initial phase constituents and phase metastability in hydrogen embrittlement of TRIP‐assisted VCrCoFeNi medium‐entropy alloys

Medium-entropy alloys (MEAs) that exhibit transformation-induced plasticity (TRIP) from face-centered cubic (FCC) to body-centered cubic (BCC) are considered promising for liquid hydrogen environments due to their remarkable cryogenic strength. Nonetheless, studies on hydrogen embrittlement (HE) in BCC-TRIP MEAs have not been conducted, although the TRIP effect and consequent BCC martensite usually deteriorate HE susceptibility. In these alloys, initial as-quenched martensite alters hydrogen diffusion and trap behavior, and deformation-induced martensitic transformation (DIMT) provides preferred crack propagation sites, which critically affects HE susceptibility. Therefore, this study aims to investigate the HE behavior of BCC-TRIP MEAs by designing four V10Cr10Co30Fe50–xNix (x = 0, 1, 2, and 3 at%) MEAs, adjusting both the initial phase constituent and phase metastability. A decreased Ni content leads to a reduced fraction and mechanical stability of FCC, which in turn increases HE susceptibility, as determined through electrochemical hydrogen pre-charging and slow-strain rate tests The permeation test and thermal desorption analysis reveal that the hydrogen diffusivity and content are affected by initial BCC fraction, interconnectivity of BCC, and refined FCC. As these initial phase constituents differ between the alloys with FCC- and BCC- dominant initial phase, microstructural factors affecting HE are unveiled discretely among these alloy groups by correlation of hydrogen-induced crack behavior with hydrogen diffusion and trap behavior. In alloys with an FCC-dominant initial phase, the initial BCC fraction and DIMT initiation rate emerge as critical factors, rather than the extent of DIMT. For BCC-dominant alloys, the primary contributor is an increase in the initial BCC fraction, rather than the extent or rate of DIMT. The unraveled roles of microstructural factors provide insights into designing HE-resistant BCC-TRIP MEAs.

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.

Superior combination of strength and ductility in Fe–10Mn-0.6C steel trigged by austenite reversion transformation

Austenite reversion transformation process was employed on Fe–10Mn-0.6C steel to adjust the content and stability of austenite and the transformation-induced plasticity effect was effectively improved. With increasing annealing temperatures from 550 °C to 590 °C and 620 °C, the content and proportion of lath-like austenite significantly increased. Austenite exhibited different stability after annealing at different temperatures, mainly related to the distribution of Mn and C. The higher the local Mn and C concentration, the more stable austenite, vice versa. After annealing at 620 °C for 6 h, the highest austenite content of 61 % was achieved, and half of that had a lath-like morphology. The steel had a superior combination of ultimate tensile strength of 1500 MPa and elongation of 59 %, resulting from the enhanced transformation-induced plasticity effect. The key reason was the alternating distributions of austenite and ferrite laths, accompanied with the formation of nanotwins in the austenite with high stability.

Hybrid Analytical-Numerical Modeling of Surface Geometry Evolution and Deposition Integrity in a Multi-Track Laser-Directed Energy Deposition Process

Abstract Modeling multitrack laser-directed energy deposition (LDED) is different from single-track deposition. There is a temporal variation in the deposition geometry and integrity in a multitrack deposition, which is not well understood. This article employs an analytical model for power attenuation and powder catchment in the melt pool in conjunction with a robust fully coupled metallurgical-thermomechanical finite element (FE) model iteratively to simulate the multitrack deposition. The novel hybrid analytical–numerical approach incorporates the effect of preexisting tracks on melt pool formation, powder catchment, geometry evolution, dilution, residual stress, and defect generation. CPM 9V steel powder was deposited on the H13 tool steel substrate for validating the model. The deposition height is found to be a function of the track sequence but reaches a steady-state height after a finite number of tracks. The height variation determines the waviness of the deposited surface and, therefore, the effective layer height. The inter-track spacing (I) plays a vital role in steady-state height evolution. A larger value of I facilitates faster convergence to the steady-state height but increases the surface waviness. The FE model incorporates the effects of differential thermal contraction, volume dilation, and transformation-induced plasticity. It predicts the deposition geometry and integrity as a function of inter-track spacing and powder feed rate. The insufficient remelting of the substrate or the preceding track can induce defects. A method to predict and mitigate these defects has also been presented in this article.

Microstructure evolution of V-containing low carbon ultra-fine bainitic steel during medium-temperature tempering and strengthening-toughening mechanism

In the present study, the microstructure evolution and the corresponding mechanical properties of V-containing low carbon ultra-fine bainitic steel during tempering were studied. The results show that during the initial stages of tempering, part of retained austenite in V-containing ultra-fine bainite steel transforms into bainite. The stability of the retained austenite improves when the tempering period is increased to 5 h. Simultaneously, nano-sized VC precipitates are precipitated in the microstructure. On the one hand, these phases can give full play to the transformation-induced plasticity (TRIP) effect by modulating the carbon content of the retained austenite. On the other hand, they can also reduce the lattice distortion in the matrix and combine it with the lower dislocation density, thereby improving the plasticity of the specimen. At the same time, the fine precipitated phase can also effectively improve the toughness of the cleavage crack region in the transition zone, preventing the micro-cracks from generating at the interface of the precipitated phase and then extending to the adjacent matrix to cause fracture, so that the uniform elongation, total elongation, impact toughness at room temperature and impact toughness at low temperature of the 5 h specimen are as high as 8.8 ± 0.2 %、16.2 ± 0.7 %、113 ± 4 J/cm2 and 76 ± 2 J/cm2, respectively. Furthermore, the primary contribution of the strengthening mechanism in the microstructure has shifted from fine grain strengthening and dislocation strengthening to precipitation strengthening, and part of the retained austenite undergoes a martensitic transformation during cooling, resulting in the highest yield strength of 1086 MPa for the 5 h specimen. The tensile strength is effectively maintained at 1402 MPa by increasing the tempering period, which has good tempering resistance, to achieve the comprehensive performance optimization of bainitic steel.

Segregation-dependent mechanical stability of reverted austenite and resultant tensile properties of Co-free maraging steel fabricated by wire-arc directed energy deposition

The segregation-dependent mechanical stability of reverted austenite (RA) and resultant tensile properties was investigated on Co-free maraging steel fabricated by wire-arc directed energy deposition (DED). The results showed that the segregation in the interdendritic regions enabled the formation of gradient reverted austenite in Co-free maraging steel fabricated by wire-arc DED when subjected to aging heat treatment. The content and mechanical stability of reverted austenite and resultant tensile properties were strongly dependent on the degree and compositional gradient of segregation in the interdendritic regions as well as the post heat treatment process. The reverted austenite with compositional gradient inherited from the segregation, transformed into martensite successively from the edge to the interior within individual reverted austenite grain at different strain levels. Besides, the variations in the degree and compositional gradient of segregation in the interdendritic regions widened the range the mechanical stability of the reverted austenite and thereby activating martensite transformation over a broad strain regime. Annealing process prior to aging could modify the degree and compositional gradient of segregation in the interdendritic regions and accordingly tune the content and the range of mechanical stabilities of reverted austenite grains, thus attaining a gradual and consistent transformation induced plasticity effect over whole deformation regime and thereby increasing ductility without sacrificing strength of Co-free maraging steel fabricated by wire-arc DED.

Modulating hierarchical microstructural components to enhance strength-ductility synergy of a high-entropy alloy containing brittle sigma phase

Although brittle intermetallic phases are usually unwanted in the design of strong and ductile alloys, high strength and good ductility can be achievable in high-entropy alloys (HEAs) containing brittle phases. Here, we present an investigation on modulating the hierarchical microstructural components in a prototype metastable HEA (Fe30Cr25Co25Mn10Ni10, at. %) containing sigma (σ) phase to optimize the mechanical properties. An excellent combination of tensile strength (999.3 MPa) and ductility (57.7 %) is achieved in alloy sample containing recrystallized grains of various sizes, which is mainly attributed to the continuous transformation-induced plasticity (TRIP) effect and the back stress effect caused by the pile-up of geometrically necessary dislocations at various interfaces in the hierarchical microstructures. The decrease in fraction of hard domains (σ particles, non-recrystallized and small-sized grains) can lead to the decrease of tensile strength due to the weakened back stress effect. However, an excessively high fraction of hard domains (particularly with small-sized grains and σ particles) suppresses the TRIP effect, facilitating crack propagation and reducing ductility. The back stress effect can be rationally controlled through tuning the fraction and size of small-sized grains (accompanied with fine σ particles) by thermo-mechanical treatments, which can also enable the activation of TRIP effect in grains of various sizes to render high strain-hardening rate and good strength-ductility synergy. This provides a strategy for achieving an optimum combination of strength and ductility for metastable HEAs containing fine brittle phases via alleviating the embrittling effect of brittle phases and retaining the precipitation strengthening effect.

Effect of Si on the deformation behavior of retained austenite and annealed martensite in medium Mn steels

The present study systematically investigated the effect of Si additions from 0 to 4 wt % on the deformation mechanism of a Fe-0.2C-6.8Mn (wt. %) medium Mn steel. The duplex microstructure consisting of film-like retained austenite (RA) and lath-shaped annealed martensite (AM) was produced for different Si alloyed steels by annealing treatments. They have similar volume fraction of RA. The effect of Si content on the dynamic strain aging (DSA) of RA and the serration type of tensile curves, as well as the fine structure evolution within AM during deformation was studied in detail. For the different Si alloyed steels, their tensile curves all exhibit flow serrations, accompanied with the propagation of the Portevine-Le Châtelier (PLC) band caused by DSA in RA. The serration type was sensitive to Si content. For the high Si steel, only type B serrations appear. This is attributed to the high Si content in RA, promoting aged dislocations by C atoms due to the repulsive interaction between C and Si atoms. For the low Si and Si free steels, type A serrations occur after type B serrations disappear at a certain strain. The mechanical stability and Mn/Si contents of RA play an important role in the PLC band propagation. The Si content has also an important effect on the evolution of dislocation configuration within AM during deformation. In the low and Si free steels, the dislocation multiplication and tangles, as well as the formation of dislocation walls, dividing up the laths appear in the AM. While in the high Si steel, it is found that the dislocation cells appear within the AM due to Si addition promoting the formation of screw dislocations by neutron diffraction technique, and the cross slip of screw dislocations as a necessary mechanism for dislocation cell formation. The formation of dislocation configuration with low energy is believed to be another essential factor improving the mechanical properties, besides the transformation-induced plasticity effect. This work paves a new pathway towards tailoring dislocation configuration for designing a new generation advanced high-strength steel.