Effect of the High-Pressure Torsion (HPT) and Subsequent Isothermal Annealing on the Phase Transformation in Biomedical Ti15Mo Alloy
Abstract
Ti15Mo metastable beta Ti alloy was solution treated and subsequently deformed by high-pressure torsion (HPT). HPT-deformed and benchmark non-deformed solution-treated materials were annealed at 400 degrees C and 500 degrees C in order to investigate the effect of UFG microstructure on the alpha-phase precipitation.
Phase evolution was examined using laboratory X-ray diffraction (XRD) and by high-energy synchrotron X-ray diffraction (HEXRD), which provided more accurate measurements. Microstructure was observed by scanning electron microscopy (SEM) and microhardness was measured for all conditions.
HPT deformation was found to significantly enhance the alpha phase precipitation due the introduction of lattice defects such as dislocations or grain boundaries, which act as preferential nucleation sites. Moreover, in HPT-deformed material, alpha precipitates are small and equiaxed, contrary to the alpha lamellae in the non-deformed material. omega phase formation is suppressed due to massive alpha precipitation and consequent element partitioning.
Despite that, HPT-deformed material after ageing exhibits the high microhardness exceeding 450 HV.