MODELING THE FLOW-STRESS ANOMALY IN GAMMA-TIAL .2. THE LOCAL PINNING-UNZIPPING MODEL - STATISTICAL-ANALYSIS AND CONSEQUENCES

Based on experiments carried out on gamma-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.