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 comm...

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