Shear melting and high temperature embrittlement: theory and application to machining titanium

TL;DR

This paper investigates a shear-induced phase transition in titanium alloys at high temperature, revealing a shear melting mechanism that causes embrittlement and can be exploited to improve machinability.

Contribution

It introduces a theoretical model of shear melting in titanium alloys and demonstrates how adding low-melting point elements triggers this transition to enhance machining.

Findings

Shear melting occurs in titanium alloys under high shear and temperature.

Adding rare earth metals lowers the shear band melting point.

Shear melting leads to localized fracture along shear bands.

Abstract

We describe a dynamical phase transition occurring within a shear band at high temperature and under extremely high shear rates. With increasing temperature, dislocation deformation and grain boundary sliding is supplanted by amorphization in a highly localized nanoscale band, which allows massive strain and fracture. The mechanism is similar to shear melting and leads to liquid metal embrittlement at high temperature. From simulation, we find that the necessary conditions are, lack of dislocation slip systems, low thermal conduction and temperature near the melting point. The first two are exhibited by bcc titanium alloys, and we show that the final one can be achieved experimentally by adding low-melting point elements: specifically we use insoluble rare earth metals (REMs). Under high shear, the REM becomes mixed with the titanium, lowering the melting point within the shear band and triggering the shear-melting transition. This in turn generates heat which remains localized in the shear band due to poor heat conduction. The material fractures along the shear band. We show how to utilize this transition in the creation of new titanium-based alloys with improved machinability.