THE OPERATION OF FRANK-READ SOURCES, YIELD STRESS REVERSIBILITY AND THE STRAIN-RATE DEPENDENCE OF THE FLOW-STRESS IN THE ANOMALOUS YIELDING REGIME OF L1(2) ALLOYS

In the anomalous yield stress regime the strain-rate dependence beta = (delta tau/delta In epsilon) of the flow stress tau is proportional to (tau - tau(y)), where tau(y) is identified with the stress required to bow out edge dislocations to a near-critical Frank-Read source configuration, at a level equal to that needed for the dynamic propagation of screw dislocations. The conditions necessary for successful Frank-Read source operation are discussed. In the microstrain region the source dislocations are predicted to form loops elongated along the screw direction. Experiments on the 'partial reversibility' of the yield stress with temperature are described; it is shown that beta is not reversible, but that the yield stress is instantaneously 'partially reversible'. This is ascribed to the instability and 'running back' of some dislocation loops in the microstrain region, on unloading. The temperature dependence of beta/tau(h) is found to reflect different obstacles at different temperatures. The temperature and strain-rate dependences of the flow stress are formulated on a model in which the critical step is the operation of Frank-Read sources.