Title: Shear-Induced Bcc-to-Plastic Flow Transition Prior to Melting

Speaker: Christine J. Wu, Lawrence Livermore National Laboratory

Date/Time: Tuesday, November 3, 2009, 9:00 am ­ 10:00 am        

Location: Building 897, Room 1010

Brief Abstract: Despite dramatic advancements in experimental methods, a remarkable controversy remains for the melting of metals at high pressures. Drastically different melting lines are often obtained from the two major high-pressure techniques, laser-heated diamond-anvil cell (DAC) and shock-wave experiments. For example, DAC melting for tantalum is thousands of degrees lower than shock melting at high pressures. Using molecular dynamics simulations[1], we discovered that under shear, bcc tantalum undergoes an unusual structural transition at elevated temperatures prior to melting.
This shear-induced transition produces a unique viscous flow with the rheological characteristics of Bingham plastics (e.g. hot tar). We have identified the underlying physics origin for this transition as the unusually flat energy landscape near bcc structure, leading to significant thermal softening under shear. This mechanism is not specific to tantalum and is expected to hold more generally for other metals that exhibit the melting discrepancy.

1. C. J. Wu, P. A. Soderlind, J. N. Glosli and J. E. Klepeis, ³Shear-Induced Anisotropic Plastic Flow From BCC Tantalum Prior to Melting². Nature Mater.
v. 8, pp. 223-228 (2009).

CSRI POC: Aidan Thompson, 505-844-9702