Twin-induced plasticity of an ECAP-processed TWIP steel

The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.Postprint (published version

Stress-strain response and microstructural evolution of a FeMnCAl TWIP steel during tension-compression tests

© 2016. The stress-strain response of a Fe-17.5Mn-0.7C-2Al TWIP steel during cyclic loading has been investigated by means of tension-compression tests within the strain limits of ±2%, ±5% and ±10%. In addition, the microstructural evolution during the ±5% cyclic test has also been studied. The difference between the forward and reverse stress for each pre-strain has been analyzed at 0.2% offset strain and at the strains in which forward and reverse curves were parallel in order to study the Bauschinger effect (BE) and permanent softening, respectively. The evolution of the BE with pre-strain for this steel is similar to other FeMnC TWIP steels, that is, increasing values of BE are obtained as the pre-strain increases. However, its absolute values are half those reported in the literature on other FeMnC steels. This diminution of the BE is related to the lower activity of mechanical twinning in FeMnCAl TWIP steels at the pre-strains herein investigated, which promotes less polarized stresses in the matrix due to the lower dislocation storage capacity.Regarding permanent softening, the evolution is similar to that of the BE and the same analysis can be applied. During reverse compression, a slight increase of twin thickness and twin spacing with respect to the first tensile stage took place. This fact might be linked to the lower flow stress observed in the permanent softening period during reverse straining.Peer ReviewedPostprint (author's final draft

Hole Expansion Simulations of TWIP Steel Sheet Sample

In this work, the stretch flangeability of a TWIP steel sheet sample was investigated both experimentally and numerically through the hole expansion test. Uniaxial tension and disk compression tests were performed to characterize the flow behavior and plastic anisotropy for the TWIP steel sheet sample. The punch load-stroke curve, hole diameter and specimen surface strain distribution near the hole was measured. Then finite element simulations of the hole expansion test were carried out using the finite element code ABAQUS with three yield criteria: von Mises, Hill 1948 and Yld2000-2d. The predicted and experimental results were compared in terms of the final hole radii and the strain distribution.open111Nsciescopu

Modeling the deformation textures and microstructural evolutions of a Fe–Mn–C TWIP steel during tensile and shear testing

The high manganese austenitic steels with low stacking fault energy (SFE) present outstanding mechanical properties due to the occurrence of two strain mechanisms: dislocation glide and twinning. Both mechanisms are anisotropic. In this paper, we analyzed the effect of monotonous loading path on the texture, the deformation twinning and the stress–strain response of polycrystalline high Mn TWIP steel. Experimental data were compared to predicted results obtained by two polycrystalline models. These two models are based on the same single crystal constitutive equations but differ from the homogenization scheme. The good agreement between experiments and calculations suggest that the texture plays a key role in twinning activity and kinetics with regard to the intergranular stress heterogeneities. Rolling direction simple shear induces single twinning while rolling and transverse direction uniaxial tensions induce multi-twinning leading to lower twin volume fractions due to twin–twin interactions

Study of internal stresses in a TWIP steel analyzing transient and permanent softening during reverse shear tests.

Recent Bauschinger-type tests conducted on a twinning-induced plasticity (TWIP) steel highlights the important contribution of internal stresses to work hardening [1]. Along this line we present Bauschinger experiments in a Fe-22Mn wt.%-0.6C wt.% TWIP steel. The mechanical behaviour upon load reversal shows transient and permanent softening effects. Determination of the internal stress from the magnitude of the permanent softening yields a contribution to work hardening of the order of 20%. Analysis of the transient softening, during strain reversal, indicates that internal stress are consistent with reported data on high carbon spheroidized steels.Acknowledgements The authors would like to acknowledge the financial support by the German Research Foundation within the framework of the SFB 761 ‘‘steel ab initio’’ and the CICYT grant MAT2009-14452 awarded by the Spanish Ministry of Science and Innovation.Peer reviewe

The formability prediction of twinning-induced plasticity steels

The proposal of this work is to predict and analyse the formability of twinning-induced plasticity steels through the Marciniak-Kuczinsky approach with emphasis on the solutions for improving the prediction results. The selected constitutive equations involve the Yld2000-2d plane stress yield function and the Swift strain-hardening power law. To understand the formability of the TWIP steel and the factors influencing it, a sensitive study on the effect of the mechanical properties of the TWIP steel on the Marciniak-Kuczinsky (MK) theory concept and the predicted forming limits is performed.publishe

Effect of temperature on microstructure and deformation mechanism of Fe-30Mn-3Si-4Al TWIP steel at strain rate of 700 s-1

As twinning-induced plasticity (TWIP) steel is one potential material for shaped charge liner due to the combination of high strength and high plasticity, deformation mechanism at high strain rate and high temperature is required to study. Compression experiments of Fe-30Mn-3Si-4Al TWIP steel were conducted using a Gleeble-1500 thermal simulation machine and a split-Hopkinson pressure bar (SHPB) between 298 and 1073 K at strain rates of 10-3 and 700 s-1, respectively. Microstructures were observed using optical microscopy (OM) and transmission electron microscopy (TEM). Results show that flow stress and densities of deformation twins and dislocations decrease with increasing deformation temperature at strain rates of 10-3 and 700 s-1. The stack fault energy (SFE) values (Γ) of Fe-30Mn-3Si-4Al TWIP steel at different temperatures were calculated using thermodynamic data. Based on corresponding microstructures, it can be inferred that at 700 s-1, twinning is the main deformation mechanism at 298-573 K for 30 mJ/m2≤Γ≤63 mJ/m, while dislocation gliding is the main deformation mechanism above 1073 K for Γ≥ 145 mJ/m2. In addition, with increasing strain rate from 10-3 to 700 s-1, the SFE range of twinning is enlarged and the SEF value of twinning becomes higher