Ternary relation among stacking fault energy, grain size and twin nucleation size in nanocrystalline and ultrafine grained CuAl alloys

The stacking fault energy γ of an alloy has been reported to significantly affect the grain size d and twin nucleation size rc during grain refinement. However, ternary relation among γ, d and rc has not been investigated comprehensively. Here we prepared nanocrystalline (NC) and ultrafine-grained (UFG) 99.99 wt% Cu, Cu-0.86 wt% Al and Cu-2.2 wt% Al alloys with different γ by high-pressure torsion (HPT), and then characterized d and rc. Transmission electron microscopy observations show that under the same experimental condition d decreases and corresponding grain refinement mechanism transforms from dislocation subdivision to twin segmentation with decreasing γ. The relation among γ, d and rc from experiments are consistent well with theoretical prediction from Meyers model. rc decreases with decreasing d, and the variation is exacerbated by the decrease of γ. rc increases first and then decreases by forming a peak-shaped variation with decreasing γ when d is in UFG regime, suggesting there exists an optimum stacking fault energy γc for twin nucleation. The rc peak becomes flat and moves to higher γ value when d is in NC regime due to the enhanced geometric effect of d on rc which weakens the role of γ. Our findings reveal a comprehensive ternary relationship among γ, d and rc, and provide guidance for designing NC and UFG materials with high-density twins and good strength-ductility combination.