Effect of High-Pressure Torsion and Annealing on the Structure, Phase Composition, and Microhardness of the Ti-18Zr-15Nb (at. %) Alloy

The Ti-18Zr-15Nb shape memory alloys are a new material for medical implants. The regularities of phase transformations during heating of this alloy in the coarse-grained quenched state and the nanostructured state after high-pressure torsion have been studied. The specimens in quenched state (Q) and HPT state were annealed at 300-550 degrees C for 0.5, 3, and 12 h. The alpha-phase formation in Ti-18Zr-15Nb alloy occurs by C-shaped kinetics with a pronounced peak near 400-450 degrees C for Q state and near 350-450 degrees C for HPT state, and stops or slows down at higher and lower annealing temperatures. The formation of a nanostructured state in the Ti-18Zr-15Nb alloy as a result of HPT suppresses the beta ->omega phase transformation during low-temperature annealing (300-350 degrees C), but activates the beta ->alpha phase transformation. In the Q-state the alpha-phase during annealing at 450-500 degrees C is formed in the form of plates with a length of tens of microns. The alpha-phase formed during annealing of nanostructured specimens has the appearance of nanosized particle-grains of predominantly equiaxed shape, distributed between the nanograins of beta-phase. The changes in microhardness during annealing of Q-specimens correlate with changes in phase composition during aging.

Automated summary

The phase transformations of Ti-18Zr-15Nb shape memory alloys during heating in different states were studied. The formation of alpha-phase occurred by C-shaped kinetics with a peak temperature near 400-450 degrees C for the quenched state and near 350-450 degrees C for the high-pressure torsion state. The nanostructured state formed as a result of high-pressure torsion suppressed the beta ->omega phase transformation during low-temperature annealing, but activated the beta ->alpha phase transformation.