Mechanisms of plasticity and fracture of partially lamellar titanium aluminum

The deformation and fracture mechanisms active in the γ- and α2-phases of a binary Ti–48 at% Al intermetallic compound during tensile loading at room temperature and 800°C have been determined by scanning and transmission electron microscopy. A change of the dominant fracture mode from transgranular at room temperature to mostly intergranular at 800°C has been observed. At both room and elevated temperature the γ-phase accommodated most of the strain by activation of essentially the same modes of dislocation slip and ordered twinning, whereas a profound change in the plastic behavior of the α2-phase has been detected. At room temperature the α2-phase exhibited inhomogeneous slip of <a> dislocations, b=<a>=1/3<112̄0>, on both {11̄00} and {2̄201}, whereas <c>-component dislocations, e.g. b=<2c+a>=1/3<112̄6>, have not been observed. After straining at 800°C the α2-phase contained more uniform, denser populations of <a> dislocations, and notably also <c>-component dislocations. Climb occurred in both phases during tensile tests at 800°C. It has been proposed that the observed change in fracture mode may in part be related to the reduction in the plastic incompatibility between the two phases due to the increased level of plasticity exhibited by the α2-phase at 800°C.