The effects of HIP processing on microstructure and phase relations in α2-base titanium aluminides

Abstract

Cast α2-base alloy (Ti–25Al–10Nb–3V–1Mo, at.%) underwent hot isostatic pressing (HIP) at temperatures ranging from 925 to 1125 °C and pressures of 100–270 MPa. Effects of temperature and pressure during HIP, on microstructure and volume fraction of the phases present, were investigated. Microstructural characterization was carried out using optical microscopy and transmission electron microscopy. The phases present were identified using selected area and convergent beam electron diffraction. Volume fraction of the phases was determined using X-ray diffraction. Optimum refinement in the microstructure was obtained following HIP at 1020 °C/262 MPa/6 h. The volume fraction of the B2 phase increased from 12% in the as-cast condition to 45% following HIP at 982 °C/262 MPa/6 h. Experiments on forged Ti–25Al–10Nb–3V–1Mo alloy demonstrated that changes in volume fraction of the phases owing to HIP was applicable for cast as well as forged material. Sharp decrease in the volume fraction of the B2 phase was observed when as-HIP materials were aged at elevated temperature. Possible mechanisms of these changes in the phase stability and microstructure are discussed.