PLASTIC-DEFORMATION BEHAVIOR AND SUBSTRUCTURE IN COTI SINGLE-CRYSTALSFATIGUED AT ROOM-TEMPERATURE

Cyclic deformation and deformation substructure of CoTi single crystal s were examined focusing on operative slip systems and cyclic hardenin g. Fatigue tests were performed in a tension/compression mode under a fixed amplitude of total strain (Delta epsilon=+/-0.1-+/-0.3%) in air at room temperature. CoTi deformed by {110}[001] slips at various stra in amplitudes. At Delta epsilon=+/-0.1% primary (110)[001] slip was do minant and the stress amplitude gradually increased with number of cyc les until it reached a saturated value. As secondary slips were more a ctivated with increasing Delta epsilon, cyclic hardening was more acce lerated and the fatigue life was shortened. Highly piled-up dislocatio ns composed of primary and secondary dislocations formed strong residu al stress fields, resulting in crack initiation and propagation. A lar ge number of dipoles and loops which also contributed to cyclic harden ing were produced during the to-and-fro motion of [001] screw dislocat ions.