Deformation mechanisms in fine-grained Ti-Al alloys

We have carried out transmission electron microscopy (TEM) observations and analysis of the dislocations structures responsible for the high strength of the powder Ti-Al alloys described in the previous paper. After deformation by compression, the propagation of slip across several grains has been confirmed to occur. Although single dislocations and twinning appear to be the major deformation mechanisms, some grains appear also deformed by superdislocations. The existence of stress concentrations at boundaries between adjacent γ grains leads to emission of curved dislocations and twins, confirming that the small grain size determines the higher strength of these materials. The lack of tensile ductility in these alloys has been interpreted in terms of the low mobility of emitted dislocations, thus hindering stress relaxations and leading to microcrack formation at grain boundaries before the onset of plasticity under tension.