Hot radial shear rolling and rotary forging of metastable beta Ti-18Zr-14Nb (at. %) alloy for bone implants: Microstructure, texture and functional properties

Abstract

In this study, a novel combination of hot radial shear rolling and rotary forging operations was applied to Ti-18Zr-14Nb shape memory alloy with the objective of forming a long-length bar stock for bone implants fabrication. Evolutions of the microstructure, texture, mechanical properties, and functional fatigue behavior of the fabricated bar stock were monitored along the technological workflow. Radial shear rolling leads to the formation of a strongly heterogeneous microstructure along the cross-section of the deformed bar stock. In the external zone of the bar stock, in the extraction direction, a dynamically recrystallized structure (d ≈ 25 μm) with the <111> fiber texture is formed. As we approach the central area, the grain size increases to d ≈ 130 μm and the texture becomes random. After the subsequent rotary forging, the alloy exhibits a dynamically polygonized substructure of β-phase with a more homogeneous across-the-section grain size distribution (d ≈ 34–43 μm) and a weak <212> texture. In this structural state, the alloy offers the following set of properties, which make it suitable for implant applications: low Young's modulus (E = 41 GPa), high value of elastic strain (e = 1.11%), and superior functional fatigue behavior with a low level of accumulated strain. In the forged condition, a calculated limit for the recovery strain for a weak <212> texture is about 5%.