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.
Our research project focused on understanding the properties of a new class of TiNb alloys developed by Toyota research known as Gum Metals. These alloys display super-strength and super-elastic deformation. In addition, they display no thermal expansion, and a temperature independent Young’s modulus for a range of temperatures. Remarkably, these properties emerge fully only after substantial cold working (typically 90% cold swaging). Even more remarkably, these alloys appear to deform in a new way, suggesting that Gum Metals represent a new class of structural alloys displaying a host of super properties. Our specific goal was to apply experimental and theoretical tools to develop a detailed understanding of how these metals deform. A study of the development of Gum Metal reveals that a key design goal was to choose the composition of the alloy to be very near a structural phase transition. The primary components of Gum Metal are Ti and Nb. In its ground state, Ti assumes a hexagonal close packed structure (HCP). Nb in its ground state crystallizes in a body centered cubic structure (BCC). If we mix the two types of metals together, the ground state structure depends on the composition. For Nb rich compounds, the BCC structure is more stable, for Ti rich compounds, the HCP structure is more stable. Gum Metal was engineered to have a composition within the range for which the BCC structure is stable, but very near to the composition at which the stable structure changes to HCP. Scientific Merit Our research revealed that designing Gum Metal to be near this structural phase transition had a profound impact on the properties of the alloy, and in particular on the properties of the defects that most often enable plastic deformation: dislocations. Briefly, a dislocation is a string-like defect within a material. In fact, when viewed using transmission electron microscopy (TEM), the dislocations appear to be tiny black strings and loops of string in the image. The motion of dislocations can be quantitatively related to the plastic deformation of the material. For this reason, much metallurgical research has focused on manipulating and controlling dislocations in alloys to achieve the desired resistance to plastic (i.e. permanent, irreversible) deformation of the alloy. This manipulation often focuses on inhibiting dislocation motion to prevent plastic deformation, and indeed, much of metallurgy can be understood within this simple context. Initial TEM studies of deformation in Gum Metal revealed that there were no dislocations in heavily deformed material, but instead, that there was a large number of unusual crystal defects that became known within the research community as nanodisturbances. In our work, we showed that by choosing the composition of the alloy to be near the BCC-HCP structural phase boundary, the alloy designers were changing the properties of the dislocations. These changes were such that the dislocations themselves would no longer be easily visible within TEM images, and moreover, the dislocations would be extremely difficult to move. More specifically, the string-like appearance of dislocations within the TEM images is because the highly defective region associated with the crystal is localized at the position of the string. In Gum Metals, however, the defective region of the dislocation spreads substantially and the dislocations no longer appear as strings. This spreading of the defective region naturally accounts for the appearance of nanodisturbances within TEM images. Nanodisturbances arise from the overlap of two spread dislocations. This spreading also makes it very difficult for the dislocations to move, thus preventing the most common mechanism for plastic deformation. This spreading also accounts for the observed mechanical properties of the alloy. Another way to think of the highly defective region associated with a dislocation is to view it as melted or molten. Application of an external stress to Gum Metal increases the spreading of this defective or molten region associated with each dislocation, and if there are sufficient numbers of dislocations, these molten regions can span the crystal from one end to the other, enabling plastic deformation through sliding along these molten paths. Removal of the applied stress shrinks the molten regions, and returns shear resistance to the alloy. Broad Impact In our research we considered the potential for other alloys to display this same type of behavior. We found that other accessible structural transitions have the potential to lead to the same type of behavior. Most importantly, we have developed simple criteria to identify candidate alloys with the potential to display the same super properties as Gum Metal, criteria that can be easily be incorporated into computationally based searches for candidate alloys, thereby enabling computer aided design of a new type of structural alloy with the potential to display "super" properties. Successfully implementing this strategy has the potential for profound technological impact through the creation of improved structural materials, and all the efficiency improvements that they offer.
