Structural and Mechanical Properties of Ti⁻Co Alloys Treated by High Pressure Torsion.

Boris Straumal,Anna Korneva,Robert Chulist,Askar Kilmametov,Mikhail Karpov,Alena Gornakova,Paweł Zięba,Lidia Lityńska-Dobrzyńska

doi:10.3390/ma12030426

Abstract

The microstructure and properties of titanium-based alloys can be tailored using severe plastic deformation. The structure and microhardness of Ti–4 wt.% Co alloy have been studied after preliminary annealing and following high pressure torsion (HPT). The Ti–4 wt.% Co alloy has been annealed at 400, 500, and 600 °C, i.e., below the temperature of eutectoid transformation in the Ti–4 wt.% Co system. The amount of Co dissolved in α-Ti increased with increasing annealing temperature. HPT led to the transformation of α-Ti in ω-Ti. After HPT, the amount of ω-phase in the sample annealed at 400 °C was about 8085%, i.e., higher than in pure titanium (about 40%). However, with increasing temperature of pre-annealing, the portion of ω-phase decreased (60–65% at 500 °C and about 5% at 600 °C). The microhardness of all investigated samples increased with increasing temperature of pre-annealing.

Highlights

Titanium and its alloys possess low density, high strength, as well as high corrosion resistance in the broad temperature interval
The amount of Ti2 Co phase slightly decreased with increasing annealing temperature
High pressure torsion leads to the phase transformations in the studied Ti–4 wt.% Co alloy

Summary

Introduction

Titanium and its alloys possess low density, high strength, as well as high corrosion resistance in the broad temperature interval. SPD drives bulk and grain-boundary phase transformations [29,30,31,32,33] In titanium these are the transitions between the low-temperature α-phase, high-temperature β-phase, and high-pressure ω-phase [34,35,36,37,38,39]. The high-pressure ω-phase appears in Ti-based alloys during HPT and retains after pressure release [26,27,40,41]. The α-to-ω transformation in Ti-based alloys encounter more troubles than β-to-ω [26,43] It is because the orientation relationship between α and ω phases is less favorable [26,34,35,36,37,38,39,44]. We annealed the Ti–4 wt.% Co alloy for extremely long durations below eutectoid temperature in order to produce the α-Ti solid solution with a different (and equilibrium) concentration of cobalt, as well as a different amount of possible coarse Ti2 Co precipitates

Experimental

Results and Discussion

Conclusions