Abstract
The stacking fault energy y of an alloy has been reported to significantly affect the grain size d and twin nucleation size r_c during grain refinement. However, ternary relation among y, d and r_c 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 y by high-pressure torsion (HPT), and then characterized d and r_c. 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 y. The relation among y, d and r_c from experiments are consistent well with theoretical prediction from Meyers model. r_c decreases with decreasing d, and the variation is exacerbated by the decrease of y. r_c increases first and then decreases by forming a peak-shaped variation with decreasing y when d is in UFG regime, suggesting there exists an optimum stacking fault energy r_c for twin nucleation. The r_c peak becomes flat and moves to higher y value when d is in NC regime due to the enhanced geometric effect of d on r_c which weakens the role of y. Our findings reveal a comprehensive ternary relationship among y, d and r_c, and provide guidance for designing NC and UFG materials with high-density twins and good strength-ductility combination.
NSTL
Elsevier