  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.
