Recrystallization in cast 45-2-2 XD™ titanium aluminide during hot isostatic pressing

XD™ TiAl alloys are well known to have the advantage of a homogeneous microstructure when compared with monolithic TiAl-based alloys. In this research, however, an inhomogeneous microstructure consisting of a fully lamellar structure in the edge region and a duplex structure of equiaxed grains and lamellar colonies in the central area was discovered in as-HIPed Ti–45Al–2Mn–2Nb+0.8 vol.% TiB2 bar. Electron probe microanalysis and optical/transmission electron microscopy of HIPed material, optical microscopy of cast material indicated the formation mechanism of the duplex structure in the central area was recrystallization due to the strain energy stored by the collapse of cast porosity during HIP. Crack initiation at the interface between regions of a duplex and a fully lamellar structure in high cycle fatigue testing suggested that the duplex structure might have a detrimental effect on the mechanical properties of the material.