Microstructure evolution during warm working of Ti-5Al-5Mo-5V-1Cr-1Fe at 600 and 800 °C

Microstructure evolution during compression to the true height strain 0.29, 0.69, or 1.2 at 600 and 800 °C of the β-rich titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe (VT22) with an initial lamellar microstructure was established using scanning and transmission electron microscopy. It was found that microstructure evolution at both temperatures was controlled primarily by substructure evolution within the β phase. At 800 °C, extensive recovery within the β phase resulted in the formation of a stable structure comprising subgrains ∼1.5 μm in diameter. During deformation at this temperature, lamellae of the α phase fragmented via a boundary-grooving mechanism. Due to the sluggish diffusion kinetics, however, spheroidization at 800 °C was incomplete. At the lower processing temperature, recovery processes within the β phase were much slower, leading to greater refinement of the β matrix. The decomposition of the metastable β phase during warm working, gave rise to very fine α-lath precipitates, which resulted in the formation of an ultrafine microstructure with a grain size of 0.5 μm.