Microstructural evolution of Cu–Al alloys subjected to multi-axial compression

• Deformation mechanisms for Cu–Al alloys with different SFEs were discussed in detail.
• SFE has a significant effect on transition from twinning to de-twining.
• Migration of ITBs was studied and the motion of ITBs leads to twinning/de-twinning.
• Gentle processing can produce nano-grained materials with very low SFE (≤ 10 mJ m− 2).

Cu–Al alloys with Al contents of 1, 2, 4, and 6 wt.% were deformed using a multi-axial compression (MAC) technique to obtain ultrafine-grained (UFG) and nanocrystalline (NC) structures. The effects of stacking fault energy (SFE) on the microstructural evolution and mechanical properties were investigated. After multiple MAC passes, homogeneous and equilibrium microstructures were achieved. The grain refinement mechanism mainly consists of conventional dislocation subdivisions, twinning and shear banding. Reducing SFE promotes deformation twinning and shear banding and thus can reduce the final equilibrium grain size. After obtaining UFG/NG Cu–Al alloys (<200 nm), further deformation results in different microstructures for four Cu–Al alloys, indicating that a transition from de-twinning dominant mechanism to twinning dominant mechanism exists in UFG/NG Cu–Al alloys, occurring at a critical SFE in the range of 25 mJ m− 2 (2 wt.% Al) and 13 mJ m− 2 (4 wt.% Al). During deformation, the migration mechanism of dissociated incoherent twin boundaries (ITBs) was analyzed and the motion of ITBs was believed to be a way for twinning or de-twinning. In addition, hardness tests indicate that with increasing the strain the micro-Vickers hardness for the four alloys increased dramatically initially and then reached saturation at a relative stable value with further increasing the strain. The study suggests that gentle-MAC processing (at a low strain rate and room temperature) is a relative effective method to produce nano-grained materials (< 100 nm) with ultra-low-SFE (<10 mJ m− 2).