Locking and Unlocking of Screws and Superkinks, and the Yield Stress Anomaly in L12 Alloys

A new theory is presented to explain the yield stress anomaly for (111) slip in L1₂ alloys. It is shown that strong sessile dipole barriers are formed by the superkinks at the ends of screws which have cross-slipped from (111) to (010). These barriers stabilise the cross-slipped screws, which slip further to form Kear-Wilsdorf locks. The yield stress is controlled by the unlocking of the superkinks, which bypass the screws and Kear-Wilsdorf locks, and which generate new mobile screws which become locked again. The unlocking mechanism is thermally activated with a large athermal component. The theory accounts for the mechanical properties, including the small strain-rate sensitivity of the yield stress, and explains many of the electron microscope observations. The application of the theory to deformation of thin foils in the electron microscope is also discussed.