Thermally activated deformation of two- and three-variant nanotwinned L10 Au–Cu–Pt

Activation volumes characteristic of room temperature compressive plastic deformation in Au- 45.2 at.% Cu- 1.7 at.% Pt, a near-AuCu alloy that orders to a fully L10 structure, are measured using the repeated stress-relaxation method. Four different ordering treatments (540 and 105s at either 250 or 400 °C) are conducted on samples prior to deformation, leading to a classical two-variant polytwin structure after ordering at 400 °C, or alternatively a three-variant highly twinned structure at 250 °C. In both structures, {110} twin boundaries only a few tens of nanometres apart separate variants and constitute the main barriers to dislocation glide. All four ordered nanotwinned microstructures have an initial effective activation volume Veff of 100–120 b3; however, the two structures differ strongly as they work harden. The two-variant polytwin structure yields a straight line on a Haasen plot, showing that work hardening is associated with an increase in slip obstacle density, most likely dislocation debris at intervariant boundaries. The three-variant structure on the other hand shows a nearly constant (slightly decreasing) activation volume. This suggests that there is little obstacle accumulation when three variants are present, likely because plastic deformation occurs by slip of superdislocation only.