Effects of stacking fault energy on deformation induced grain boundary relaxation in nanograined Cu alloys

Deformation induced grain boundary (GB) relaxation with resultant enhancements in thermal stability was observed in various metals with grain sizes below a critical size. In this study, effects of stacking fault energy (SFE) on the GB relaxation and thermal stability are investigated in several Cu-Ni and Cu-Al alloys with gradient nanograined samples prepared using the surface mechanical grinding treatment. For each alloy, thermal stability drops with a decreasing grain size from submicrometers to about 70 nm. However, two distinct grain size dependences of thermal stability were observed below 70 nm. For Cu-10Ni and Cu-5Ni alloys with higher SFEs than Cu, thermal stability increases for smaller grains, similar to that in pure Cu. For Cu-10Al and Cu-5Al with lower SFEs than Cu, as grain sizes decrease the thermal stability elevates firstly and then drops, exhibiting a stability peak at a certain size. The observed thermal stability in the Cu alloys below 70 nm can be attributed to the GB relaxation induced by plastic deformation dominated by partial dislocation activities. The different behaviors of GB relaxation in these Cu alloys demonstrated its obvious dependence on SFE, which determined the governing deformation mechanisms and hence the degree of relaxation of GBs.