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Ultrafine grained austenitic stainless steel, with two different types of grain size distributions, was studied for tensile deformation behavior. Tensile deformed specimens were analyzed by electron backscatter diffraction using scanning electron microscope. It was found from this study that uni-modal grained stainless steel (SS) having a larger fraction of submicron grains exhibited early fracture, which is attributed to the development of extensive strain localization. On the other hand, the microstructure of SS having bimodal grain size distribution showed a good combination of strength and ductility. EBSD analysis of the deformed region of these two samples revealed the presence of a distinct transition zone between undeformed or less deformed and extensively sheared regions. Multiple micro shear bands were found to be associated with the transition zone of unimodal type microstructure. The micro shear bands seen in the transition zone of unimodal SS led to the development of strain-induced martensite (SIM), which, in turn, is helpful in delaying the strain localization. However, in bimodal grained SS, the larger fraction of micron size grains undergoes a shape change, with a rotation towards [112] orientation, which results in the formation of a larger fraction of SIM having [112] orientation. The propensity for development of high SIM was found to prevent strain localization in bimodal grained SS. [ABSTRACT FROM AUTHOR]

The present study deals with the grain refinement of a high nitrogen duplex stainless steel through cold rolling up to 80% and subsequent reversion annealing in the range of 750 °C to 1100 °C at various duration times. The microstructural observations showed that the nano/ultrafine-grained (NG/UFG) austenite with the grain size of 400 nm was obtained by annealing at 900 °C for 30 s due to ά-martensite to austenite complete reverse transformation. The NG/UFG duplex steel indicated a superior mechanical properties consist of 1 GPa yield strength (YS) and 40% elongation to fracture. The transmission electron microscopy study of the tensile deformed NG/UFG material indicated that the nature of extraordinary plasticity in NG/UFG material is because of the dense planar defects, specifically, the twin/twin-like induced plasticity effect in austenite, accompanied with strain accommodation of coarse grain ferrite phase. [ABSTRACT FROM AUTHOR]

The coupled temperature-strain rate sensitivity (SRS) of Ti-29Nb-13Ta-4.6Zr β-type Ti-alloy has been studied. The martensitic transformation is identified as the main reason of negative rate sensitivity during deformation at room temperature and 100°C. In addition dynamic strain aging (DSA) is in charge of further decrease in SRS values at temperatures higher than 100°C. The SRS values start increasing beyond 200°C due to dissipating the DSA effects. This finally reaches the positive domain due to domination of dislocation slip. [ABSTRACT FROM AUTHOR]

Abstract: This rapid communication presents a novel approach for controlling the material property evolution for austenitic stainless steels during incremental sheet forming (ISF), offering a route to affect the strength and ductility of the parts produced. The method relies on the modelling and control of the formation of strain induced martensite. [Copyright &y& Elsevier]

Abstract: A near-Ni-free, Mn–N bearing duplex stainless steel (D-SS) that shows transformation induced plasticity was developed. The present D-SS exhibited an excellent strength–ductility combination over 1000MPa tensile strength and 50% elongation. An analysis of the element partitioning during annealing revealed that the stacking fault energy of austenite was low enough for a strain induced martensite (SIM) transformation to occur. The strain hardening rate began to increase at ∼10% strain with the same manner of SIM fraction. The TEM and EBSD analyses showed that not only the ɛ martensite band intersections but the austenite grain boundaries acted as the SIM nucleation sites. The SIM transformation was saturated because of the austenite grain refinement and the corresponding austenite stabilization. The austenite grain refinement was caused by the mutual impingement of growing SIM and as a result by the engulfment of remaining austenite by SIM. The deformation behavior of the present D-SS was characterized by analyzing the kernel average misorientation (KAM) of the constituent phases with strain. The KAM distribution of austenite, ferrite and SIM exhibited different characteristics. The average KAM of austenite and ferrite increased as the strain increased, but its increasing rate of austenite was higher than that of ferrite. These KAM characteristics were discussed along with the dislocation glide modes of austenite and ferrite. By contrast, the average KAM of SIM was insensitive to strain and higher than that of the other two phases. [Copyright &y& Elsevier]

Abstract: Near Ni-free duplex stainless steels (D-SS) containing the different N contents of 0.1–0.35wt.% were tensile-tested. Microstructure of D-SSs consisted of a contiguous band structure of coarse ferrite and fine austenite. The austenite fraction increased almost linearly as the N content increased. All D-SSs exhibited transformation induced plasticity (TRIP), resulting in an enhancement of both strength and ductility, especially over 1GPa and 60% for the 0.35 N D-SS. Strain induced martensite (SIM) that causes TRIP nucleated at not only in-grain ɛ martensite band intersections but austenite grain boundaries (AGBs) and annealing twin boundaries (TBs). Growth of SIM at AGBs and TBs was found to be faster than that of in-grain SIM. The apparent SIM fraction increased as the N content increased (correspondingly, the austenite fraction), leading to a better strain hardening. The overall strain hardening was dominated by the amount of accumulated SIM and the strain hardening of the remaining untransformed austenite, not the SIM transformation rate. Analysis using the SIM fraction normalized to the initial austenite fraction revealed that the SIM transformation was delayed as the N content increased due to the austenite stabilization increase. Also, it was found that regardless of the austenite fraction, there is a critical SIM fraction showing the transition of the flow mechanism of remaining untransformed austenite. [Copyright &y& Elsevier]

Abstract: The effects of nitrogen addition on the strain induced martensitic transformation (SIMT) behavior of duplex stainless steel (D-STS) with the low nickel content were examined in a wide range of strain by means of cold rolling. Nitrogen of 0.1, 0.2 and 0.3wt.% was added into Fe–20Cr–5Mn–0.2Ni D-STS (in wt.%) and cold rolling was conducted up to the effective strain of ∼1.45 after annealing at 1000°C for 30min. In the as-annealed state, the austenite fraction increased with increasing the N content. Regardless of the N content, the ferrite grain size was coarser than that of austenite. The stacking fault energy of austenite of the present D-STSs inferred by the element partitioning analysis was low enough so that SIM transformation is available. Accordingly, during cold rolling, SIMT occurred in austenite with a sequential manner of austenite→ ɛ martensite→ α′ martensite with increasing strain. By contrast, the deformed microstructure of ferrite was dominated by dislocation cells. The SIM fraction, which was normalized with reference to the initial austenite fraction in order to eliminate its effect, increased with increasing the N content. Along with the present results, the factors influencing the SIMT kinetics in the present D-STSs with the different N content were discussed. [Copyright &y& Elsevier]

Type 304 stainless steel is a metastable austenitic stainless steel that undergoes strain induced transformation from austenite to α ′ martensite during deformation at room temperature. Role of prior deformation, by rolling at room temperature and at 200 °C (a temperature at which the strain induced martensitic transformation does not occur), on the subsequent room temperature tensile deformation behaviour of the austenite phase is investigated. The changes in dislocation density of the parent phase and the volume fraction of the martensite formed due to deformation were estimated by X-ray diffraction line profile analysis of data obtained from Beam Line - 12 of the Indus 2 synchrotron. The contrast caused by dislocations in the austenite phase has been addressed using the modified Williamson-Hall method. The character of dislocations changes from screw to edge above a critical equivalent strain of ~0.14. The influence of martensite formation on the increase in dislocation density in the austenite phase is confirmed from XRDLPA. An alternative approach to determine dislocation density is proposed which is independent of elastic constants of the material and is found to correlate well with the values determined by the Williamson-Smallman approach. The results obtained from XRDLPA are corroborated with EBSD analysis. It is also seen that the composite strength of the steel, which has undergone different levels of deformation, is related to the changes in dislocation density of austenite and volume fraction of martensite based on Taylor's equation enabling correlation of structure-property. • Deformation induced change in dislocation density (ρ) and character are studied by XRDLPA. • Strengthening due to α′ martensite and ρ in austenite are related through rule of mixtures. • SIM contributes to strengthening by introducing extra dislocations into γ phase. • Results from EBSD analysis corroborate the findings of XRDLPA. • Alternative approach to determine ρ from different forms of mWH equation is proposed. [ABSTRACT FROM AUTHOR]