ON THE TWINNING INITIATION CRITERION IN CU-AL ALPHA-SINGLE CRYSTALS .2. CORRELATION BETWEEN THE MICROSTRUCTURE CHARACTERISTICS AND THE TWINNING INITIATION

The influence of a prestrain on the subsequent plastic response of fc.c. single crystals is investigated both experimentally and numerically in terms of slip and dislocation microstructures evolutions, and in relation to the twinning initiation process. Proposals available from the literature on twinning initiation criterion are considered and the more relevant expression with respect to the here reported observations and analyses is discussed. It has been shown in the first part that, as long as plastic straining operates by slip, the numerical modelling used for the simulations gives predictions in qualitative agreement with the observed crystallographic and microstructural characteristics of the performed experiments on Cu-Al alpha samples. It mainly comes out from this second part that: (1) The dislocation microstructure built up during the pretwinning slip stage in tension is strongly dependent on the previous compression amplitude, and so are the twinning initiation conditions. (2) Local stress effects ahead of dislocations piled up on strong obstacles, as proposed in recent twin nucleation mechanisms, are more likely to justify the prestrain dependancy of the critical twinning stress and the preferred twinning mode experimentally obtained than the simple estimate of the macroscopic shear stresses on the twinning systems.