Origin of rubber-like behaviour in metal alloys

Since 1932 it has been known that a number of ordered alloys show an unusual kind of deformation behaviour(1-3). These alloys (including Au-Cd, Au-Cu-Zn, Cu-Zn-Al, Cu-Al-Ni)(4-8), 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 stabilization(9) (increase in the temperature of reverse transformation to the parent state), the avoidance of which is important in actuator applications of the shape-memory effect(29), (which these alloys also generally exhibit. The origin of this rubber-like behaviour and of the ageing effect has remained unclear(10-17). 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.