PLASTIC DEFORMATION BEHAVIOUR AND OPERATIVE SLIP SYSTEMS IN Ni3Nb SINGLE CRYSTALS

摘要

Temperature and orientation dependence of yield stress and operative Slip system in Ni_3Nb single crystals were examined. Four slip systems of (0l0)[l00],--(00l )[l00], {20l }(l02) and (0l0)[001] and three twinning systems of {0l l }(0l l ), {21 l } (l0 7 l3) and {012} (02l ) were activated depending on the crystal orientation and tested--temperature. Anomalous strengthening was observed for some orientations where (0l0)[l00] or {20l }(l02) slip was operative. A remarkable increase in the critical resolved shear stress (CRSS) for the (0l0)[l00] slip occurred over a wide temperature range from -- l96 to 800'C. The CRSS did not satisfy Schmid's law and the strain rate sensitivity of flow stress at a small plastic strain was negligible between room temperature and 700 deg. C where an anomalous stress increase occurred. The anomalous strengthening mechanism in Ni_3Nb crystals deformed by [l00] dislocations was discussed on the basis of anisotropy of APB energy on (0l0) and (00l) in analogy with that in Ni_3Al with the Ll2 structure.