The deformation of “Gum Metal” in nanoindentation

“Gum Metal” describes a newly developed set of alloys with nominal composition Ti–24(Nb + V + Ta)–(Zr,Hf)–O. In the cold-worked condition these alloys have exceptional elastic elongation and high-strength; the available evidence suggests that they do not yield until the applied stress approaches the ideal strength of the alloy, and then deform by mechanisms that do not involve conventional crystal dislocations. The present paper reports research on the nanoindentation of this material in both the cold-worked and annealed conditions. Nanoindentation tests were conducted in situ in a transmission electron microscope (TEM) stage that allows the deformation process to be observed in real time, and ex situ in a Hysitron nanoindenter, with samples subsequently extracted for high-resolution TEM study. The results reveal unusual deformation patterns beneath the nanoindenter that are, to our knowledge, unique to this material. In the cold-worked alloy deformation is confined to the immediate neighborhood of the indentation, with no evidence of dislocation, twin or fault propagation into the bulk. The deformed volume is highly inhomogeneous; the deformation is accomplished by a series of incremental rotations that are ordinarily resolved into discrete nanodomains. The annealed material deforms in a similar way within the nanoindentation pit, but dislocations emanate from the pit boundary. These are pinned by microstructural barriers only a few nanometers apart, a condition that recent theory suggests is necessary for the material to achieve ideal strength.