Origin of rubber-like behaviour in metal alloys

Abstract

Since 1932 it has been known that a number of ordered alloys show an unusual kind of deformation behaviour^1,2,3. These alloys (including Au–Cd, Au–Cu–Zn, Cu–Zn–Al, Cu–Al–Ni)^4,5,6,7,8, after being aged for some time in a martensitic state (the low-symmetry phase of a diffusionless transformation), can be deformed like a soft and pseudo-elastic rubber (with a recoverable strain as large as a few per cent). Accompanying martensite ageing is the development of martensite stabilization⁹ (increase in the temperature of reverse transformation to the parent state), the avoidance of which is important in actuator applications of the shape-memory effect²⁹, (which these alloys also generally exhibit. The origin of this rubber-like behaviour and of the ageing effect has remained unclear^{10,11,12,13,14,15,16,17}. Here we show that this behaviour does not involve a change in the degree of long-range order, but is instead due to an atomic rearrangement within the same sublattice of the imperfectly ordered alloy during martensite ageing. This process is driven by a general tendency for the equilibrium symmetry of the short-range order configuration of lattice imperfections to conform to the symmetry of the lattice. This principle not only explains all the observed aspects of the rubber-like behaviour and the ageing effect in both ordered and disordered alloys, but may also further our understanding of some diffusion phenomena in other crystalline materials.