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 r elation to the twinning initiation process. Proposals available from t he literature on twinning initiation criterion are considered and the more relevant expression with respect to the here reported observation s and analyses is discussed. It has been shown in the first part that, as long as plastic straining operates by slip, the numerical modellin g 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 bui lt up during the pretwinning slip stage in tension is strongly depende nt on the previous compression amplitude, and so are the twinning init iation conditions. (2) Local stress effects ahead of dislocations pile d up on strong obstacles, as proposed in recent twin nucleation mechan isms, are more likely to justify the prestrain dependancy of the criti cal twinning stress and the preferred twinning mode experimentally obt ained than the simple estimate of the macroscopic shear stresses on th e twinning systems.