Microstructural evolution in adiabatic shear localization in stainless steel

Abstract

Shear bands were generated under prescribed and controlled conditions in an AISI 304L stainless steel (Fe-18%Cr-8%Ni). Hat-shaped specimens were deformed in a Hopkinson bar at strain rates of ca 104 s-1 and shear strains that could be varied between 1 and 100. Microstructural characterization was performed by electron backscattered diffraction (EBSD) with orientation imaging microscopy (OIM), and transmission electron microscopy (TEM). The shear-band thickness was ca 1–8 ❍m. This alloy with low-stacking fault energy deforms, at the imposed strain rates (outside of the shear band), by planar dislocations and stacking fault packets, twinning, and occasional martensitic phase transformations at twin-band intersections and regions of high plastic deformation. EBSD reveals gradual lattice rotations of the grains approaching the core of the band. A [110] fiber texture (with the [110] direction perpendicular to both shear direction and shear plane normal) develops both within the shear band and in the adjacent grains. The formation of this texture, under an imposed global simple shear, suggests that rotations take place concurrently with the shearing deformation. This can be explained by compatibility requirements between neighboring deforming regions. EBSD could not reveal the deformation features at large strains because their scale was below the resolution of this technique. TEM reveals a number of features that are interpreted in terms of the mechanisms of deformation and recovery/recrystallization postulated. They include the observation of grains with sizes in the nanocrystalline domain. The microstructural changes are described by an evolutionary model, leading from the initial grain size of 15 ❍m to the final submicronic (sub) grain size. Calculations are performed on the rotations of grain boundaries by grain-boundary diffusion, which is three orders of magnitude higher than bulk diffusion at the deformation temperatures. They indicate that the microstructural reorganization can take place within the deformation times of a few milliseconds. There is evidence that the unique microstructure is formed by rotational dynamic recrystallization. An amorphous region within the shear band is also observed and it is proposed that it is formed by a solid-state amorphization process; both the heating and cooling times within the band are extremely low and propitiate the retention of non-equilibrium structures.