Configurational equilibrium at twin-grain boundary intersections in F.C.C metals and alloys, and the measurement of relative interfacial torque

Abstract

By considering the misorientation of directions between certain unique crystallographic orientations at twin-grain boundary intersections in thin metal and alloy foils, a configurational theory has been developed which is applicable in explaining the origin of intrinsic interfacial torque. This theory is shown to be applicable with an average confidence of 80% in the analysis of twin-boundary-grain boundary intersection systems using electron transmission and selected-area electron diffraction microscopy techniques. Defining intrinsic interfacial torques to arise primarily because of a change in grain boundary free energy with a variation in misorientation, Θ, low-torque configurations are described as those where the relative interfacial free energy free ratios which arise at the interface between grains A and B, γ_tb/γ^AB, and between the twin of grain A, TA, and grain B, γ_tb/γ^T _A ^B, are essentially equal. High-torque configurations are those where geometrical ratios related to the interfacial energy ratios differ by more than 10% for specific crystallographic situations which promote a significant change in the misorientation across the twin and intersecting grain interfaces. The mean ratios of net effective torque to grain boundary free energy, ∼M/γgb, for high-torque configurations observed in thin foils of nickel, 304 stainless steel, Inconel 600, and Cu-5 at. % Al were measured as 0·0188, 0·0082, 0·0073 and 0·0091 respectively. The configurational theory has also been extended to double twin-grain boundary systems and the high temperature equilibration of grain-corner twins.