Deformation mechanisms of Gum metal

Abstract

Gum metal (Ti-36Nb-2Ta-3Zr-0.3O) is a recently developed multifunctional bcc titanium (p Ti) alloy that exhibits high strength (> 1 GPa), high ductility (>10%) and high yield strain (~2.5%). In addition, this alloy possesses the invar and elinvar properties, and is highly cold workable. The encouraging mechanical properties and workability of Gum metal mean that it is a candidate material in a range of applications, from biomedical implants to aerospace and military applications. The deformation mechanisms of Gum metal have previously been reported to involve ideal shear. The rational for this suggestion is that Gum metal was designed on first principles, such that the value of the shear modulus (C') assumes a very low value. This implies that the ideal shear stress is comparable to the actual strength, such that deformation can proceed via ideal shear. Furthermore, the observation of ‘giant shear steps’ in transmission electron microscopy (TEM), whose orientation does not correspond to any bcc slip or twin systems is considered to be consistent with this hypothesis. However, the existence of a deformation mechanism involving ideal shear is against metallurgical wisdom. Many other titanium alloys of similar composition are also known to exhibit a low C. However, these alloys deform via a stress induced superelastic martensitic transformation. Therefore the aim of this work is to improve our understanding of the micromechanisms of this alloy. The single crystal elastic constants (C¡¡) of Gum metal were acquired with the aid of in- situ synchrotron X-ray diffraction (SXRD) and an Eshelby-Kroner-Kneer self consistent model. The results showed that although C is low in this alloy, the ideal shear strength (>2 GPa) is still above the material’s tensile strength, implying deformation cannot occur via ideal shear. Furthermore, analysis of the SXRD spectra during cyclic loading suggests that Gum metal undergoes a stress-induced superelastic martensitic (a") transformation. The SXRD results were complemented with TEM characterisation, which showed the presence of the a" phase, and the © phase, which exhibited a plate-like morphology. In addition, deformation twins of the type {1 12} were identified. Structures similar to the giant shear steps were observed and their formation is believed to be due to a" variants nucleating from co plates or twin boundaries. The effect of processing route and chemical composition on the deformation mechanisms and mechanical properties of Gum metal were also investigated. A more cost effective processing route involving ingot metallurgy was trialled and the mechanical properties were comparable to the alloys produced via powder metallurgy. Oxygen was found to suppress the amount of transformation strain in Gum metal (by increasing C'); and hence the majority of the observed superelastic strain was due to the low Young’s modulus and high yield strain of the p phase. However, oxygen increased the stress for permanent deformation, thus allowing more stable superelasticity. Prior deformation (extrusion or cold rolling) was found to increase the amount of transformation strain. This was considered to be a result of mechanical working providing nucleation sites, such as the co phase and twins, from which, the a" phase was able to nucleate. The amount of transformation strain could be increased through control of specimen texture. The specimens produced via the ingot metallurgy processing route, involving casting and extrusion were found to exhibit the greatest transformation strain