Dissociation and core structure of 〈110〉 screw dislocations in L1₂ ordered alloys II. Effects of an applied shear stress

Abstract

The effects of an applied shear stress on [101] screw dislocations in L1₂ ordered alloys has been studied by computer simulation techniques. If the dislocation dissociates on the (111) plane into two ½[101] superpartials separated by antiphase boundary (APB), and if the superpartial cores are also spread on the (111) plane, then the Peierls stress for motion on (111) planes is low. However, when the dislocation dissociates into a ⅓[211] and a ⅓[1 12] superpartial on a (111) plane separated by superlattice intrinsic stacking fault (SISF), then the Peierls stress for motion on this plane is very high. When the dislocation dissociates on the (010) plane into two ½[101] superpartials separated by APB, motion on this plane is never observed. The Peierls stress is again high since the superpartials always move on one of the {111} planes at stresses which have to be high enough to produce APB on this plane.

The results of these calculations suggest, that there should be two classes of L1₂ ordered alloys which show different dependences of flow stress on temperature. Alloys in which dislocations dissociate on {111} planes into superpartials separated by APB will show a weak dependence of the flow stress on temperature at low temperatures, but are likely to exhibit a strong increase of the flow stress at high temperatures. When dissociation on {111} planes into superpartials separated by SISF occurs, the flow stress will increase rapidly with decreasing temperature while the increase at high temperatures is likely to be weaker than in the former case. Furthermore the results also show why [101] (010) slip occurs only at high temperatures in L1₂ ordered alloys. Since the cores of the superpartials are not coplanar with the APB on {010} planes, motion can only take place with the help of thermal activation.