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

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 are supplanted by amorphization in a highly localized nanoscale band, which allows for 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.

Figure 1

Dynamical phase transition. Snapshots from molecular dynamics simulation at 1500 K and 55% strain at a rate of $2.5\times {10}^8$, showing shear localization and amorphization imaged by AtomEye software [34]. (a) Slicing through the system identifies atoms by centrosymmetry parameter; blue indicates atoms close to bcc configurations. (b) 3D projection of only those atoms identified as noncrystalline. The separation between the amorphous localized shear band and the crystalline region is clear, with the “hole” indicating a nanocrystal “floating” in the amorphous zone. (c) 3D projection showing only those atoms with fewer than 10 of their original 14 neighbors, showing that shear and diffusion have been largely confined to the amorphous layer.

Figure 2

Chips from conventional turning experiments. From left to right, chips of standard Ti6Al4V, and with addition of 0.9 wt% La and 0.9 wt% Er. Chip analyses show that only the La-containing material exhibits sufficient shear-band softening to fracture the chip. The effect of La on larger scale chip morphology is self-evident.

Figure 3

Imaging the localized shear band. (Left panel) Optical microscopy image of a Ti6Al4V segmented chip produced in orthogonal high-speed cutting at $v_c=10\mathrm{m}/\mathrm{s}$; $a_p=0.1\mathrm{mm}$. The average $\alpha$ grain size in the segments is $20\mu \mathrm{m}$. (Right panel) Detail from position 1 showing that the shear band consists of equiaxed, nanocrystalline $\alpha$ grains. The average grain size is about 10 nm. The shear band can be clearly distinguished from the segment. The SAD (inset) shows that a metastable $\beta$ phase is not present. Supplemental Material [8], Fig. 6 shows the same behavior in a $\beta$-Ti alloy.