Characterization of the prior particle boundaries in a powder metallurgy Ti_2AlNb alloy

Ti₂AlNb-based alloy powder metallurgy (PM) compacts were prepared via hot isostatic pressing (HIP) under relatively low temperature (920 and 980 °C) and at certain pressure (130 MPa). The microstructure, composition and orientation of B2, α₂ and O phases in the compacts were characterized and analyzed with an aim to investigate the effect of unsuitable HIPping parameters on the appearance of prior particle boundary (PPB), which seriously affects the mechanical properties of the alloy. The results show that more α₂ phase is the characteristics of the PPB in Ti₂AlNb-based alloy when HIPped at relatively low temperature. Increasing HIPping temperature to the upper part of the two-phase region can effectively inhibit the formation of PPB. Electron backscatter diffraction measurements show the specific orientation relationship between phases, which helps us understand the origin of α₂ and O phase and the corresponding transformation path. The HIPping at a higher temperature can weaken the micro-texture intensity of the α₂ and O phase due to the increase of misorientation in B2 phase. The α₂ phase at cell wall keeps the Burgers orientation relationship (BOR) with the grain on one side, and does not satisfy the BOR with the other. It is found that some O phase variants inside the cell HIPped at 980 °C can only maintain α₂-O OR with α₂ owing to the α₂→O phase transformation forming the O phase, while these O variants deviate from B2-O OR with B2 phase.