Development of α-phase morphologies during low temperature isothermal heat treatment of a Ti–5Al–5Mo–5V–3Cr alloy

Development of different morphologies of the α-phase and their influence on the mechanical properties during low temperature isothermal aging of the near-β titanium alloy Ti–5Al–5Mo–5V–3Cr (Ti-5553) have been studied. A wide variety of different morphologies of the α-phase resulted from variations of the isothermal temperature and/or time. While after aging at 673 K or higher, a Widmanstätten star-shape was the dominant morphology of the α-phase, aging at lower temperatures resulted in the formation of triangular-shaped precipitate morphology. The first α-phase nucleated after 10 min isothermal holding time at 673 K, and the volume fraction increased to more than 80% after 8 h of holding time. This increase in the volume fraction of the α-phase caused an increase in the micro-hardness of the bulk material. X-ray diffraction and microstructural analyses provided evidence of ω-assisted nucleation of the α-phase. It is proposed that this mechanism by which the α-phase nucleates on pre-existing ω-phase particles account for the formation of the very large number of small α-particles that precipitated in the β-matrix.