OBSERVATIONS AND ANALYSES OF DISLOCATIONS AND STACKING-FAULTS IN THE MASSIVE GAMMA(M) PHASE IN A QUENCHED TI-46.5 AT-PERCENT AL-ALLOY

The defect structures in the massively formed gamma (gamma(m)) grains in a Ti-46.5 at.% Al alloy, rapidly quenched from the high-temperature cc-phase field, have been studied using transmission electron tron mi croscopy (TEM). The results reveal that the defect structures are comp osed of dislocations, stacking faults and antiphase boundaries intimat ely associated with dislocations or stacking faults. Contrast analysis indicates that both 1/2 < 110] and 1/2 < 101] unit dislocations were present in gamma(m) phase, with the latter linked by highly curved non -conservative antiphase boundaries. Comparison of experimental and com puter simulated TEM images established that wide stacking faults, whic h are created by the dissociations of 1/2[101] unit dislocations, lie on {111} planes and are bound by b = 1/6[121] Shockley partial disloca tions of all possible types. In addition, antiphase boundaries are fou nd to commence or terminate on the stacking faults at the partial disl ocations with b = 1/6 < 121], but not those with b = 1/6 < 112]. Based on the observations and subsequent analyses, a model for the formatio n of these defects-involving the occurrence of an intermediate disorde red f.c.c. phase during the alpha --> gamma(m) massive transformation- is proposed. (C) 1997 Acta Metallurgica Inc.