Experimental and Theoretical Investigations of Gum Metal

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

TECHNICAL SUMMARY: Gum metals are Ti-Nb based alloys that display remarkable mechanical properties, including super-elasticity, super-strength, large ductility, zero coefficient of thermal expansion and elastic constants that remain constant over a range of temperatures. Surprisingly, plasticity in these materials appears to be mediated neither by dislocation motion nor by phase transformation. Instead, these materials appear to deform at their ideal strength. If so, these alloys represent a new type of structural alloy. Their unusual mechanical behavior is linked to a strong elastic anisotropy that develops as the composition of the alloy is driven towards a phase transformation. This anisotropy allows for easy dislocation pinning, the extreme spreading of the dislocation cores, and plays a role in the suppression of phase transformations that might otherwise weaken the alloys. The project involves two primary goals. First it aims to understand experimentally the precise mechanism governing plasticity in these unusual alloys. This understanding will be developed using a combination of nanoindentation and transmission electron microscopy experiments. Second, the project will develop theoretical tools and ideas capable of explaining the observed deformation behavior. Initial theoretical studies will focus on the structure of dislocations within Gum Metals paying particular attention to the implications of spread dislocation cores and dislocation core overlap. The understanding so obtained will be used to identify new alloys systems with the potential to deform similarly to Gum Metals. NON-TECHNICAL SUMMARY: The ideal strength of a material is the largest possible load the material can withstand before becoming permanently deformed. Typically, the observed strength of a material is 1/100th or less of its ideal strength, largely because defects known as dislocations are able to move throughout the crystal at stresses well below the ideal strength. This understanding lies at the heart of modern metallurgy, and guides the development of most structural alloys. Gum Metal is a recently discovered Ti-Nb based alloy with a long list of remarkable mechanical properties. The most striking characteristic is that Gum Metal appears to deform at ideal strength via a unique dislocation-free deformation mechanism. The goal of this research is to identify and understand the deformation mechanism(s) that gives rise to the remarkable properties of Gum Metal, and to use this understanding to develop the metallurgical principles that govern Gum Metal behavior. These principles will then be used to search for new materials that display mechanical properties similar to Gum Metal. The identification of novel metallurgical principles has the potential to enable the development of a new class of structural materials. Such materials would impact a broad range of technologies including transportation and energy generation. Students working towards their doctoral degrees will conduct much of the research. Course materials geared toward introducing high school physics students to the discipline of materials science and engineering will be developed and exemplified by the role of structural materials in the sport of skateboarding---a sport popular with teenagers.