Observations and analyses of dislocations and stacking faults in the massive γm phase in a quenched Ti–46.5 at.% Al alloy

Abstract

The defect structures in the massively formed gamma (γm) grains in a Ti–46.5 at.% Al alloy, rapidly quenched from the high-temperature α-phase field, have been studied using transmission electron microscopy (TEM). The results reveal that the defect structures are composed of dislocations, stacking faults and antiphase boundaries intimately associated with dislocations or stacking faults. Contrast analysis indicates that both 1/2<110] and 1/2<101] unit dislocations were present in γm phase, with the latter linked by highly curved non-conservative antiphase boundaries. Comparison of experimental and computer simulated TEM images established that wide stacking faults, which 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 dislocations of all possible types. In addition, antiphase boundaries are found to commence or terminate on the stacking faults at the partial dislocations with b=1/6<121], but not those with b=1/6<112]. Based on the observations and subsequent analyses, a model for the formation of these defects—involving the occurrence of an intermediate disordered f.c.c. phase during the α→γm massive transformation—is proposed.