A dislocation model which reproduces some of the plastic properties of L1 2 alloy single crystals

Abstract

There has been considerable success in explaining the orientation dependence of the flow stress of single crystals of alloys having the Li{sub 2} structure. Less attention has been paid to the dependence of this flow stress on strain rate and temperature. This paper assumes that a screw dislocation can have either a locked or an unlocked configuration, the locked configuration having the lower energy. Screw dislocations can reach the locked configuration only by a process of thermal activation. It follows that at room temperature, which is a low temperature within the temperature range of interest, screw dislocations, which must be generated in an unlocked configuration, should lie predominantly in unlocked configurations in {l brace}111{r brace} planes rather than in locked configurations in {l brace}100{r brace} planes. However, recent evidence shows that observations of screw dislocations in relaxed samples always reveal {l brace}100{r brace} as the APB plane, at least when {l brace}100{r brace} has been previously stressed. It follows that the models quoted cannot be accepted without modification. An alternative approach has been proposed a model of the behaviour of prismatic slip in beryllium, which, like slip on close-packed planes in Li{sub 2} alloys, shows an anomalous temperature dependence. Themore » analogy is not close, because the easy slip plane in beryllium is the basal slip plane, and prismatic slip is imposed only by a special orientation of the tensile axis, while in Ll{sub 2} the predominant slip plane is the easy slip plane.« less