Dislocation configurations in two-phase Ti[sbnd]Al alloys III. Mechanisms producing anomalous flow stress dependence on temperature

Abstract

Comparisons of the deformation mechanisms responsible for the flow stress dependence on temperature have been made for two alloys with atomic compositions Ti[sbnd]48Al and Ti[sbnd]47·5Al[sbnd]2·5Cr. Two different high values of flow stress have been observed, namely at −196 and 420°C. The high flow stress values measured at low temperatures have been attributed to a spontaneous blocking mechanism of the superdislocations present. However, the latter disappear at high temperatures and the anomalous strengthening effect that occurs between room temperature and 420°C has been described by the twinning activity and emission of single ½ 〈110] dislocations. An increase in twin density with increasing temperature produces increased stress concentrations at twin intersections from which a higher density of single dislocation can be emitted. An increase in the forest density produces multiplication of cutting events, leading to an increase in jog density along the dislocation line. The latter appears to be responsible for the anomalous increase in flow stress with increasing temperature. From the calculated values of activation energies below and above the peak temperature, we have confirmed that a limiting value of stress concentration is responsible for the exhaustion of the increase in forest dislocations that leads to a, maximum limiting value of the total number of jogs at the peak temperature.