Effect of low and high temperature ECAP modes on the microstructure, mechanical properties and functional fatigue behavior of Ti-Zr-Nb alloy for biomedical applications

A Ti-18Zr-15Nb (at%) shape memory alloy was subjected to a high-temperature thermomechanical treatment (HTMT) combining an equal channel angular pressing (ECAP) at 500 degrees C for n = 4-8 passes and a short-time post-deformation annealing (PDA) at 600 degrees C. The phase composition, microstructure, texture, mechanical and functional properties were studied. The functional fatigue behavior observed in this study was compared to that resulted from the reference low-temperature thermomechanical treatment (LTMT) by ECAP at 200 degrees C for 3 passes + PDA (600 degrees C for 5 min). The ECAP at 500 degrees C (n = 4) led to the formation of a highly deformed, dynamically polygonised substructure of beta-phase with a crystallographic texture close to the [101] direction. In this state, the alloy exhibited an excellent combination of the static functional and mechanical properties: a relatively high strength (UTS = 670 MPa), a sufficient ductility (delta = 13.3 %), a low Young's modulus (E < 40 GPa), and a high superelastic recovery strain (epsilon(se)(r)(max) = 3.1 %). An increase in the number of passes during ECAP to n = 8 led to a greater substructural hardening of the material, grain/subgrain refinement, and the release of alpha-phase. This significantly increased the alloy strength (UTS = 897 MPa), but reduced its ductility (delta = 5.9 %) and suppressed martensitic transformation. The alloy after both the HTMT (ECAP at 500 degrees C, n = 4) and TMT (ECAP at 200 degrees C, n = 3) processes exhibited an equally excellent functional fatigue resistance accompanied by a superior superelastic behavior with small accumulated strains. However, the HTMT process appears to be more technologically advanced, as it eliminates the need for PDA and reduces the risks of specimen cracking during processing.