Ultrafine grain recrystallisation of a metastable-β Ti-alloy via conventional thermomechanical processing

Alexander Edwards ,Ivan Guillot ,Philippe Vermaut ,Frédéric Prima

doi:10.1051/matecconf/202032112031

Abstract

Alloys with ultrafine grains (UFG) offer high strength potentially combined with ductility. Until now, producing ultrafine grains in ingot alloys has required either severe plastic deformation techniques or flash annealing, neither of which are scalable to bulk alloy production. In this work, we formed submicronic grains in the metastable β titanium alloy Ti-20Nb-6Zr (at%), using conventional cold rolling and annealing at 823K in a conventional furnace. The cold rolling (298K) transformed the β structure mostly to α” martensite, but if the rolling temperature was raised to 453K, martensite formation was supressed, and no grain refinement occurred during the subsequent similar annealing treatment. Therefore, we attribute the formation of ultrafine grains to a mechanism involving stress-induced martensite and its reverse transformation.

Highlights

Most strengthening approaches for improving the strength of metals do so at the expense of ductility
The ultrafine grains (UFG) grow from a high density of recrystallization nuclei, and this is promoted by a high density of defect-energy in the lattice
We are aware of only two examples of Stress-Induced Martensite and Reverse Transformation (SIMRT) attaining UFGs using only conventional mechanical working: Ma and Lee et al, with a metastable austenitic steel alloy [6,7] and Cai et al with a metastable β titanium alloy (Ti77.47V14.93Al6.38Sn1.21) [8,9]

Summary

Introduction

Most strengthening approaches for improving the strength of metals do so at the expense of ductility. We are aware of only two examples of SIMRT attaining UFGs using only conventional mechanical working (cold rolling): Ma and Lee et al, with a metastable austenitic steel alloy [6,7] and Cai et al with a metastable β titanium alloy (Ti77.47V14.93Al6.38Sn1.21) [8,9] In both cases, the authors identified SIMRT as the principle cause of the observed UFG refinement. The work presented here attempts to understand the different thermomechanical and microstructural processes involved in SIMRT which induce UFGs by studying a metastable-β titanium alloy Ti-20Nb-6Zr (at%), hereafter called TNZ This alloy’s superelastic properties and biocompatibility makes it interesting for biomedical applications [11,12], but it has all the necessary characteristics for the SIMRT process: its Ms is 246K and α” martensite is induced by applied stress at room temperature [12]. We propose here a unique approach to isolate the role of SIMRT by comparing cold rolling, which produces stress-induced martensite (SIM), to rolling at a slightly higher temperature (453K: so-called “warm rolling”) which suppresses SIM

Experimental Procedures

Results and Discussion

Conclusions