Fabrication, microstructure and properties of a multifunctional titanium alloy

Abstract

Multi-functional titanium alloys, termed Gum Metal™, are a suite of β-phase Ti alloys first developed by Japanese Toyota Central Laboratory in 2003. They exhibit many interesting properties including a low rate of work-hardening and superplasticity during cold deformation, non-linear elasticity, ultra-high strength, ultra-low modulus, and Elinvar and Invar effects. Furthermore, a new plastic deformation mechanism not involving major dislocation activity was proposed to explain the behaviour of these alloys. The aim of this thesis was to reproduce the alloy of the original inventors using the same processing route (powder sintering/hot forging/solution treatment/cold swaging) and to investigate the relationship between microstructural development during thermomechanical processing and properties. X-ray diffraction showed that the forged/solution treated alloy was -phase with transmission electron microscopy (TEM) revealing a very small amount of -phase, which increased during cold swaging. After cold swaging to 96% area reduction, TEM revealed the existence of deformation twins, dislocations, nanodisturbances and lattice bending, with electron backscatter diffraction (EBSD) showing the grains to be highly elongated in the swaging direction, fragmented and distorted. Swaging also generated a strong <110> fibre texture, even after moderate strains. The foregoing TEM and EBSD analysis provides substantial evidence that dislocations are present in the alloy after both forging and cold swaging. The major support of dislocations dominating plastic deformation of the Ti alloy during swaging is via the generation of this strong fibre texture which is a feature of all cold drawn/swaged body centred cubic (BCC) metals and alloys. Hence, while several deformation modes appear to be operative in the alloy, the strong <110> fibre texture provides the strongest evidence of dislocations dominating the deformation process. Furthermore, the curly or so-called “Van Gogh sky structures” generated in the transverse section of the as-swaged alloy, as observed in other BCC metals after cold drawing/swaging, is consistent with the “marble-like” structures first reported in cold-swaged Gum Metal. In terms of properties during tensile straining, the as-swaged Ti alloy exhibited non-linear elasticity, low elastic modulus, high elastic limit, high elongation to failure and high yield strength and tensile strength. The coefficient of thermal expansion of the alloy was also low. These results are largely comparable to the original research on Gum Metal™ and indicate that plastic deformation involving dislocation activity during cold swaging is not detrimental to the generation of the unique set of properties in these alloys.