Shear Melting and High Temperature Embrittlement: Theory and Application to Machining Titanium

TL;DR

This paper presents a theory explaining how rare earth metals improve titanium alloy machinability by inducing shear melting and embrittlement, leading to easier fracture along amorphous shear bands during cutting.

Contribution

It introduces a novel mechanism of shear melting and embrittlement in titanium alloys with REMs, enhancing machinability without altering bulk properties.

Findings

REM alloying lowers shear band melting point

Shear bands facilitate localized amorphisation and softening

Improved machinability with maintained mechanical properties

Abstract

We show that alloying with rare earth metals (REMs) can dramatically improve the machineability of a range of titanium alloys, even though the REM is not incorporated in the alloy matrix. The mechanism for this is that under cutting, shear bands are formed within which the nano-precipitates of REM are shear mixed. This lowers the melting point such that the mechanism of deformation changes from dislocation mechanism to localised amorphisation and shear softening. The material then fractures along the thin, amorphous shear-band. Outside the shear band, the REM remains as precipitates. The new alloys have similar mechanical properties and biocompatibility to conventional materials.