Effect of the severe plastic deformation by ECAP on microstructure and phase transformations in Ti-15Mo alloy

Ti-15Mo alloy in a metastable beta solution treated condition was processed by equal channel angular pressing (ECAP) at an elevated temperature of 250 degrees C. The resulting microstructure is highly deformed, contains twins and shear bands, but is not ultra-fine grained. Both the initial solution treated material and the material after ECAP were subjected to ageing at 400 degrees C and 500 degrees C in order to study the effect of deformation on phase transformations, namely the a phase precipitation. The phase composition was studied by X-ray diffraction measurement; the microstructure was investigated using conventional EBSD and an advanced method of transmission Kikuchi diffraction (TKD). It was shown that the a phase precipitation is accelerated in areas with higher density of lattice defects, which provide a dense net of preferred sites for nucleation and also fast diffusion paths necessary for accelerated growth. Upon further annealing, discontinuous lamellar coarsening occurs, which had not been previously reported in metastable beta-Ti alloys. The microhardness is governed mainly by the formation of. phase particles. The fraction of omega phase increases during annealing at 400 degrees C and the specimen aged at 400 degrees C/16 h shows the highest value of microhardness of 520 HV for both ECAP and undeformed material. Upon annealing at 500 degrees C, the microhardness is significantly lower.