An Electron Microscope Study of Configurational Equilibrium at Twin-Grain Boundary Intersections in FCC Metals

A transmission electron microscopy study of 304 stainless steel films has been undertaken to systematically study the interrelationships of the degrees of freedom characterizing a grain boundary. From this study a configurational theory has been developed which is useful in explaining the existence of interfacial torques at twin-grain boundary intersections. The grain boundary misorientation (Θ) is defined as the relative rotation of the <110> directions in the adjacent grains of identical (110) orientation. The two remaining degrees of freedom are represented by the tilt or inclination (θ) and the asymmetry (Φ) of the grain boundary plane. Torques arise because of a difference in grain boundary energy with a change in misorientation or tilt. 90° twin configurations (twin plane along a <112> direction) are essentially high-torque situations, as a result of the change in misorientation (ΔΘ) between the twinned grain and its neighboring grain. 35° twins (twin plane along a <110> direction) are low-torque configurations, but can exhibit high torque anomalies when there is a sufficient variation in tilt across the intersection, Δθ. Misorientation, Θ, appears to be the dominant torque producing parameter for high-torque configurations, and dominates the variations in grain boundary free energy. Also, a functional relationship between Δθ and ΔΦ observed for both high-torque occurrences. Spreads in the histograms for twin boundary-grain boundary energy ratios are due to torque terms or variations in grain boundary energy with changes in grain boundary parameters.