TRANSIENT-BEHAVIOR AFTER STRESS INCREASE DURING CREEP OF ALPHA(2)-TI3AL INTERMETALLICS IN THE LOW-STRESS REGION

Sudden stress-increase tests were carried out during creep of the alph a(2)-phase materials of Ti-Al system containing 27.5 and 34.0 mol%Al a t 1150 K and 1250 K under 126 MPa which is well located within the low stress region. The range of stress increase Delta sigma was from 3 MP a to 18 MPa. The type of transient creep curves is essentially the sig moidal one. The instantaneous increase in creep rate Delta epsilon is proportional to Delta sigma. These results indicate that dislocations move viscously in near-steady-state creep in these materials. The appa rent mobility obtained from Delta epsilon, i.e. (Delta epsilon/(D) ove r tilde)/Delta sigma, is independent of temperature and composition, w here (D) over tilde is the interdiffusion coefficient, which is almost independent of the composition in the alpha(2) phase. In the near-sto ichiometric material, however, the type of transient creep curves chan ges from the sigmoidal one to the normal one, when the stress increase exceeds a critical value. The value of Delta epsilon becomes to be im proportional to Delta sigma under these high stresses. These results s uggest that dislocations can move jerkily and the multiplication can o ccur under these conditions.