Dr. Takashi Saito, Toyota Central R&D Labs.,Inc., developed super elasto-plastic titanium alloy which has a lot of unique characteristics, and published this research in the 18 April 2003 issue of the journal Science.

Outline

Super elasto-plastic metal, named as GUM METAL, is an entirely new titanium alloy having low Young's modulus and high strength that cannot be obtained from conventional metallic materials (Fig.1). This alloy has super-elastic nature one digit higher in elastic deformation (2.5%) compared to general metallic materials, and super-plastic nature permitting cold working to 99.9% or more at room temperature. Furthermore, its strength can be increased to the highest in the world (2,100 MPa in tensile strength) by applying a simple heat treatment process.

Features

GUM METAL belongs to the (β-type titanium alloy having a body-centered-cubic structure and is basically expressed as Ti₃(Ta+Nb+V) + (Zr, Hf, O). Since this alloy is hard to be processed by the ordinary melting process because of excessive macro segregation making uniform structure extremely difficult, sintering is adopted for production.
While GUM METAL has considerably low Young's modulus (70 GPa) when applied with solution heat treatment, its Young's modulus drops further when cold working is applied so as to make the Hook's law as the common knowledge about metal inapplicable. In other words, its elastic coefficient is not constant and it shows non-linear elastic deformation, with Young's modulus changing greatly (60 to 20 GPa). As a result, the yield strength increases remarkably to show super-elastic nature (Fig.2). Since the alloy does not show work-hardening at all under any king of hard working, continuous deformation is possible to any desired level.

The unique characteristic of GUM METAL arises from its strange nano structure. As no dislocation or twin crystal is observed after hard cold-working, it changes to marbled structure containing fractal, layered structure involving discrete strain field and its crystal lattice is curved greatly (Fig.3). It is estimated that an unknown plastic deformation mechanism completely different from those of other metallic materials is acting.

Application

GUM METAL named after its various unique characteristics unlike other metals is a new material having infinite possibilities in practical applications. In addition to already commercialized applications including spectacles frame and precision screws, it is considered to be applicable in a wide range for automotive parts, medical equipment, sporting goods, decorative materials and aerospace industry.