Modelling the flow stress anomaly in γ-TiAl II. The local pinning-unzipping model: Statistical analysis and consequences

Abstract

Based on experiments carried out on γ-TiAl polycrystalline samples and presented in part I of this work, a statistical model is proposed in order to account for stress anomalies observed in this material in the intermediate-temperature range. This statistical analysis is based on a particular mechanism in which dislocations are successively pinned on localized intrinsic obstacles and unzipped by lateral motion of cusps. Owing to the increase in the pinning-point density with increasing temperature, the probability of operation of the unzipping mechanism is shown to depart from unity above a critical temperature. The resulting strong exhaustion mechanism of mobile dislocations naturally yields a stress anomaly. The model also accounts for some other characteristic features of TiAl in this temperature range, such as the high work-hardening rate and the very small strain rate sensitivity.