Strain reversal in simple shear extrusion (SSE) processing: Microstructure investigations and mechanical properties

Simple shear extrusion (SSE) is one of the severe plastic deformations which are based on the direct extrusion of the samples. By passing the material through the deformation channel, gradually increased shear strain is imposed to the material. The direction of the shear is reversed at the middle of the channel after the maximum distortion angle is achieved. Therefore, the change in the direction of the shear affects the microstructure and texture of the sample in a unique way. Hence, in the present search role of shear reversal on microstructure and mechanical properties of pure copper after multi passes and during a single pass of the simple shear extrusion (SSE) process was investigated. For SSE processing an appropriate die with a linear die profile was designed and constructed. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to evaluate the microstructure of the deformed samples. Strain reversal during the process results in a slight reduction in dislocation density, the hardness and mean disorientation angle of the samples, and an increase in the grain size. Strain softening happens as a result of the reduction in the dislocation density and strain induced grain growth. Also, a decrease in hardness and yield strength and an increase in the uniform elongation are observed after softening. Two stages for softening were observed. First, at the middle of the deformation channel in each passes of SSE and second after eight passes of SSE. The mechanism for the latter is the stress coupled grain-boundary migration while the former happens as a result of shear reversal.