Mechanical properties and fracture behaviour of nanostructured and ultrafine structured TiAl Alloys synthesised by mechanical milling of powders and hot isostatic pressing

Bulk nanostructured (grain sizes in the range of 50-200nm) and ultrafine structured (grain sizes in the range of 100-500nm) γ-TiAl based alloys with compositions Ti-47Al (in at%) and Ti–45Al–2Cr–2Nb–1B–0.5Ta (in at%), respectively, have been produced using a combination of high energy mechanical milling of mixtures of elemental powders and hot isostatic pressing at 800 and 1000oC respectively, and the microstructures of the samples have been characterised. At room temperature, the HIPed samples fractured prematurely at tensile stresses in the range of 200-300MPa and showed no ductility, very likely due to the relative high oxygen content (0.6wt%) in the samples and very low tolerance of TiAl based alloys on dissolved oxygen. At 800oC, the HIPed samples showed a yield strength in the range of 55-70MPa, a tensile strength in the range of 60-80MPa, a large amount of elongation to fracturing around 100% and clear strain softening. Examination of the fractured tensile test specimens at room temperature and 800oC showed that the level of the consolidation was fairly high, but the HIPed samples do contain a small fraction of interparticle boundaries with weak atomic bonding. The fracture of the HIPed samples in tensile testing at room temperature and 800oC, respectively, is predominately intergranular, and the large amount of plastic deformation prior to fracture at 800oC is achieved mainly through grain boundary sliding in conjunction with dislocation gliding, in agreement with the deformation mechanisms of nanostructured and ultrafine structured alloys generally agreed by researchers