Addressing retained austenite stability in advanced high strength steels

Advances in the development of new high strength steels have resulted in microstructures containing significant volume fractions of retained austenite. The transformation of retained austenite to martensite upon straining contributes towards improving the ductility. However, in order to gain from the above beneficial effect, the volume fraction, size, morphology and distribution of the retained austenite need to be controlled. In this regard, it is well known that carbon concentration in the retained austenite is responsible for its chemical stability, whereas its size and morphology determines its mechanical stability. Thus, to achieve the required mechanical properties, control of the processing parameters affecting the microstructure development is essential

Effect of holding temperature and time on ferrite formation in dual phase steel produced by strip casting

Conventional dual phase (DP) steel (0.08C-0.81Si-1.47Mn-0.03Al, wt. %) was manufactured by the laboratory simulation of strip casting. The effect of holding temperature and time on microstructure evolution was studied using a quench-deformation dilatometer. Microstructures were observed using optical and scanning electron microscopy. The results showed that the nose temperature of ferrite phase field is around 650 °C. The kinetics of ferrite formation is fast within the first 100 s of holding at this temperature, and then formation of ferrite continues at a slower rate until it reaches the fraction corresponding to that defined by the lever rule. 70~80 % ferrite was obtained after holding at 650 °C for 100~900 s. Some Widmänstatten ferrite was also observed probably because of a large prior austenite grain size and quenching after holding. In addition, austenite-to-ferrite transformation kinetics is fitted well using Johnson-Mehl-Avrami equation. The Avrami exponent for ferrite formation was approximately 1 for both 650 and 670 °C holding temperatures, which means rapid ferrite transformation. It deduces that the ferrite formation obeys a linear growth behavior, which is associated with a decrease in amount of nucleation sites

Effect of mo, nb and v on hot deformation behaviour, microstructure and hardness of microalloyed steels

Three novel low carbon microalloyed steels with various additions of Mo, Nb and V were investigated after thermomechanical processing simulations designed to obtain ferrite-bainite microstructure. With the increase in microalloying element additions from the High V-to NbV-to MoNbV-microalloyed steel, the high temperature flow stresses increased. The MoNbV and NbV steels have shown a slightly higher non-recrystallization temperature (1000°C) than the High V steel (975°C) due to the solute drag from Nb and Mo atoms and austenite precipitation of Nb-rich particles. The ambient temperature microstructures of all steels consisted predominantly of polygonal ferrite with a small amount of granular bainite. Precipitation of Nb-and Mo-containing carbonitrides (\u3e20 nm size) was observed in the MoNbV and NbV steels, whereas only coarser (~40 nm) iron carbides were present in the High V steel. Finer grain size and larger granular bainite fraction resulted in a higher hardness of MoNbV steel (293 HV) compared to the NbV (265 HV) and High V (285 HV) steels

A comparative study of a NiTi alloy subjected to uniaxial monotonic and cyclic loading-unloading in tension using digital image correlation: The grain size effect

The present digital image correlation study characterised the local axial and shear strain fields of a 56Ni-44Ti wt.% shape memory alloy with an average grain size of 100 μm, under uniaxial monotonic and cyclic loading-unloading in tension. To elucidate the grain size effect, the results were compared with a previous investigation of the same alloy with an average grain size of 10 μm. The maximum local axial strain rate signified the direction and extent of the localised transformation. The widened single inclined transformation band and multiple criss-crossing patterns assist in straightening the sample edge by releasing an in-plane moment instigated by local shear strains. Electron back-scattering diffraction analyses showed that the plastic strain within the B2 grains and the remnant B19′ variants account for the residual strains after unloading. Smaller grain sizes correspond to greater constraint from grain boundaries, higher interfacial energy and higher elastic strain energy barrier for transformation, and smaller intragranular heterogeneity of plastic deformation. This is reflected in the increases to the transformation start stress, stress level and stress-strain slope within the macroscopic stress plateau region and smaller complete transformation strain, super-elastic and residual strains upon unloading

Observation of deformation twinning and martensitic transformation during nanoindentation of a transformation-induced plasticity steel

For the first time, deformation twinning and martensitic transformation were observed in retained austenite in a low-Alloyed transformation-induced plasticity steel using nanoindentation in conjunction with electron backscattering diffraction and transmission electron microscopy. Dislocation glide, martensite formation and deformation twinning were correlated to pop-ins and deviation from linearity in the load-displacement curve. Deformation twinning was found to enhance the stability of retained austenite. This observation furthers our understanding of RA stability during straining of low-Alloyed multiphase TRIP steel

The microstructure, texture and mechanical properties of AS-ECAE Interstitial-free steel and copper

A comparison between the microstructure, texture and mechanical properties of bcc interstitial-free (IF) steel and fcc copper (Cu) for up to N = 8 passes Equal Channel Angular Extrusion (ECAE) via route BC processing was undertaken. Transmission Electron Microscopy (TEM) and Electron Back-Scattering Diffraction (EBSD) studies revealed that the deformation microstructures of both metals evolves from low-angled microbands and dislocation cells after N=2 passes towards more equiaxed, homogeneous subgrain/grain structures comprising higher-angles of misorientation after N = 8 passes. In both metals, the percentage rise in Σ3 and random boundaries are attributed to mechanisms that favour low-energy boundary configurations during ECAE. Texture evolution involves gradual changes in individual component strengths during multi-pass ECAE. The bcc and fcc textures are correlated by interchanging the Miller indices of the slip plane and slip direction between the two cubic crystal systems. The uniaxial tensile curves of both materials are representative of significant cold-working and depict higher 0.2% proof stresses, a small period of uniform elongation, necking and lastly, failure via geometrical softening. Constitutive modelling suggests that rather than a change in deformation mechanism, the preservation of ductility up to N = 8 passes is associated with an increase in the mean free path of dislocations; with slip via dislocation glide remaining as the dominant carrier of plastic strain in both metals

Experimental and Self-Consistent Modeling Study of De-twinning in a Twinning-Induced Plasticity Steel

The effect of compression-tension loading on the microstructure evolution in a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C twinning-induced plasticity steel has been investigated. Electron back-scattering diffraction was used to track a region of interest at true strains of 0 (initial), − 0.09 (after forward compression loading), and 0.04 (after reverse tension loading). All deformation twins detected after forward compression loading were found to de-twin upon subsequent reverse tension loading, likely due to the reverse glide of partial dislocations bounding the twins. The reverse loading behavior, including the twinning and de-twinning processes, was successfully simulated using a recently modified dislocation-based hardening model embedded in the visco-plastic self-consistent polycrystal framework, taking into account the dislocation accumulation/annihilation, as well as the twin barrier and back-stress effects