Deformation of microstructurally refined cast Ti46Al8Nb and Ti46Al8Ta

The response to deformation of samples of cast Ti46Al8Ta and Ti46Al8Nb alloys, which had been heat treated to produce similar refined microstructures, has been investigated; the refinement increases the ductility from about 0.2% found in cast HIPped samples to 0.5% for the Nb-containing alloy and up to 1.5% for the Ta-containing alloy. It has been shown that the tensile fracture surface of samples can show facetted regions characteristic of a brittle fracture and regions which appear ductile. Back scattered images and electron back scattered diffraction data indicates that there are large areas of similarly oriented alpha2 in the Nb-containing alloy and that the alpha2 is coarser than in the Ta-containing alloy. Examination of dislocation activity in the alpha2 and gamma phases in both alloys shows that deformation via slip and twinning within gamma is dominant and that limited slip of both c-component and a-dislocations can occur within alpha2. In some alpha2 grains, which are oriented so that slip of a-dislocations cannot relieve the applied stress, large regions, which are strongly strained elastically, can be imaged at the alpha2/gamma interfaces. In such cases, dislocation slip and twinning in gamma can appear to be transmitted across alpha2 via these elastic stresses, without any dislocation activity within the alpha2. Synchrotron observations aimed at defining the onset of plasticity in the alpha2 did not reveal any obvious difference in behaviour of the two alloys. Analysis shows that the oxygen-content of the alpha2 in the two alloys is very similar and analytical TEM shows that the site occupancy of the Nb and Ta within the alpha2 is only slightly different. These observations are discussed in terms of the tensile properties, in particular the ductilities, of these two alloys.