Significance of adiabatic heating on phase transformation in titanium-based alloys during severe plastic deformation.

Experiments were conducted on Ti-based alloys including a commercially pure titanium (α phase), Ti-6Al-4 V (α + β phase) and TiNi (B19'-martensitic phase) alloys in order to investigate the phase transformation during high-pressure torsion (HPT) at room temperature. An allotropic α to ω phase transformation in the commercially pure titanium occurred during HPT processing but increasing the rotation speed led to a decrease in the volume fraction of the ω-phase. This procedure facilitates the loss of the β phase in the Ti-6Al-4 V alloy and promotes a B19' to B2 (austenite) phase transformation in the TiNi alloy. The results show that the bulk temperature increment in the processed specimens was not sufficiently high to affect phase transformations. However, the calculations by assuming adiabatic conditions revealed that the local temperature increment is responsible to encourage or suppress the phase transformation in the Ti-based alloys. This fundamental understanding of phase transformations during severe plastic deformation is useful for conducting microstructure engineering of Ti-based alloys by tailoring the microstructures containing different phases and thereby producing different mechanical and functional behaviors. • Different rotation speeds were applied for the HPT processing of Ti-based alloys. • Temperature increment samples was not sufficiently high to activate any reverse phase transformations. • Stabilization of phases can be prevented at high rotation speeds due to adiabatic shearing that produces local heating. • Heat generation prevented formation of the ω-phase in Ti and promoted reverse phase transformation in TiNi at high rotation speeds. [ABSTRACT FROM AUTHOR]