In situ investigation on plastic deformation behaviors in austenite-ferrite heterostructured stainless steel

Heterostructured (HS) materials have attracted extensive attention due to their superior mechanical properties. However, there are still many fundamental issues to be solved, like the microstructural evolution process of the hetero-zones during deformation. Here we reported a HS 316L stainless steel generated by exploiting the chemical heterogeneity strategy via thermomechanical processing. The plastic deformation behaviors of the hetero-zones were investigated using three-point bending tests and interrupted tensile tests characterized by electron channeling contrast imaging (ECCI) combined with electron backscattered diffraction (EBSD) techniques. These in situ observations demonstrate the evolution of hetero deformation-induced (HDI) stress field near the austenite/ferrite interfaces in terms of geometrically necessary dislocation (GND) pileups, which can modulate the plastic deformation behaviors of both austenite and ferrite phases. At the early deformation regime, the imposed plastic deformation can be well maintained by the dense and homogeneous activation of slip systems in austenite phases and the stress concentration near austenite/ferrite interfaces can be alleviated by the gradual activation of plastic deformation in the ferrite phases. At the large deformation stage, a progressive twinning-induced plasticity (TWIP) effect caused by the chemical heterogeneity can provide a stable work hardening ability in a broad strain range, which can significantly enhance the ductility. At the late stage of deformation, strain-induced α′-martensite is also triggered, which provides an additional work-hardening ability. The strategy of micro-tuning deformation mechanisms by chemical heterogeneity is versatile and can be applied to many alloys.