Shear Melting and High Temperature Embrittlement: Theory and Application   to Machining Titanium
  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. 
Graeme J Ackland, Con Healy, Sascha Koch, Florian Brunke and Carsten   Siemers
cond-mat.mtrl-sci