Dislocation Cores in Gum Metal Daryl C. Chrzan, University of California-Berkeley, DMR 0706554

1. Dislocation Cores in Gum MetalDaryl C. Chrzan, University of California-Berkeley, DMR 0706554 Gum metal is a Ti-Nb based alloy that displays a number of remarkable properties including a very large elastic limit, significant ductility and high strength. Interestingly, post-deformation examination of the microstructure reveals few, if any, dislocations. Instead, one observes giant faults and, using high resolution electron microscopy, local atomic scale distortions termed nanodisturbances. Dislocation core structures for Ti-V alloys viewed along the line direction (left, using standard displacement map) and at glancing incidence (right) revealing a nanodisturbance in the Ti80V20 alloy not present in Ti25V75. In order to understand these phenomena, we have conducted a detailed study of dislocation cores in gum-metal-like Ti-V alloys using first principles electronic structure total energy methods. We found that for a normal alloy, Ti25V75, the dislocations were compact. However, for Ti80V20, the dislocations were spread significantly. We explained this spreading using elasticity theory and demonstrated that the interaction between dislocation cores gives rise to defects that bear a striking resemblance to the nanodisturbances observed experimentally. D. C. Chrzan, M. P. Sherburne, Y. Hanlumyuang, and J. W. Morris, Jr., submitted for publication.

2. Gum Metal Research Fosters International CollaborationDaryl C. Chrzan, University of California-Berkeley, DMR 0706554 The elemental constituents of Gum Metal make the alloy quite expensive. Consequently, researchers at Toyota Central Research and Development Laboratory have been searching for other materials that might display gum-metal-like properties. To date, they have identified one candidate, a Ni-Fe-Co alloy which is also expensive (due to the inclusion of Co). In general, the search for alternative alloys is made more difficult by the lack of understanding as to what makes Gum Metal behave so differently from other alloys. Maps of grain orientation obtained for swaged Gum Metal. The panel on the left shows that the grains all share a common <110> direction. The image on the right shows that the grain size perpendicular to the <110> direction is small. This small grain size inhibits the mechanically induced phase transformation. Accordingly, we have been working with Toyota to understand just what it is that makes a Gum Metal unique. We have made a number of important contributions. The figure presented here is from a paper authored jointly with researchers at Toyota in which we outline how the microstructure of Gum Metal inhibits a mechanical phase transformation that would otherwise degrade Gum Metal mechanical properties. J. W. Morris, Jr. et al., Acta Materialia 58, 3271 (2010).