Abstract
The deformation structure and its contribution to strain hardening of a high manganese austenitic steel were investigated after tensile deformation at 298 K, 77 K and 4 K by means of electron backscatter diffraction and transmission electron microscopy, exhibiting a strong dependence of strain hardening and deformation structure on deformation temperature. It was demonstrated that sufficient twinning indeed provides a high and stable strain hardening capacity, leading to a simultaneous increase in strength and ductility at 77 K compared with the tensile deformation at 298 K. Moreover, although the SFE of the steel is ~34.4 mJ/m2 at 4 K, sufficient twinning was not observed, indicating that the mechanical twinning is hard to activate at 4 K. However, numerous planar dislocation arrays and microbands can be observed, and these substructures may be a reason for multi-peak strain hardening behaviors at 4 K. They can also provide certain strain hardening capacity, and a relatively high total elongation of ~48% can be obtained at 4 K. In addition, it was found that the yield strength (YS) and ultimate tensile strength (UTS) linearly increases with the lowering of the deformation temperature from 298 K to 4 K, and the increment in YS and UTS was estimated to be 2.13 and 2.43 MPa per 1 K reduction, respectively.
Highlights
Total elongation (TEL) are provided in the table and inserted in Figure 1a, showing that both yield strength (YS) and ultimate tensile strength (UTS) can be greatly enhanced by lowering deformation temperature from 298 to 4 K, as a result of the high shear stress needed for dislocation gliding at a low temperature
When the deformation temperature is further lowered to 4 K, there is a small and large decrease in total elongation (TEL) compared with the 298 K
In the early deformation stage, the strain hardening rate (SHR) sharply decrease for the three deformation temperatures as a result of dynamic recovery caused by cross slipping and annihilation of dislocations as well as the formation of low energy dislocation structures [13]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. A large number of studies focus on deformation mechanisms, strain hardening, yield strength, texture, fracture and fatigue, etc. Up to now, these steels have not been widely used in the automotive industry because of their relatively high cost compared with conventional automobile steels, hydrogen-induced cracks and other problems. Luo et al [14] reported that high manganese TWIP steels can achieve high strain-hardening rates without mechanical twining at 373 K and. It is commonly accepted that the combination of strength and ductility in high manganese TWIP steel results from the high strain-hardening rates caused by mechanical twinning [1,2,15,16]. The present study will provide a better understanding of the role of mechanical twining in the enhancement of the strain hardening rate and deformation mechanism at a temperature as low as 4 K
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