GRAIN-BOUNDARY FACETING AND TWINNING IN COMPLEX INTERMETALLIC COMPOUNDS

Grain boundary faceting and annealing twins in a Hf-V-Nb-based C15 int ermetallic compound were studied using transmission electron microscop y. Approximately 5% of the boundaries become faceted during annealing at 1200-degrees-C and the majority of completely faceted boundaries ar e SIGMA3([110]/70.53-degrees) grain boundaries. About 65% of the facet s were found to be of the type {111}1/{111}2, with the balance being { 112}1/{112}2, {111}1/{115}2 and {001}1/{221}2. A facet in the process of transformation was also observed: SIGMA3{111}1/{115}2 --> SIGMA3{11 1}1/{111}2 + SIGMA3{112}1/{112}2. Annealing twins in the bulk were ide ntified as {111}[112] type and a mechanism for the growth of annealing twins based on the faceting reaction is proposed. In this mechanism a nnealing twins grow by the anisotropic migration of SIGMA3{112}1/{112} 2 facets via the slip of synchro-Shockley partial dislocation triplets .