Pseudoelasticity and the strain memory effect in Cu-Zn-Sn

An investigation of pseudoelasticity and the strain memory effect was carried out on the β’-bcc phase of alloy of composition Cu-33Zn-3.4Sn. Pseudoelasticity was found to occur by a stress-induced, martensite transformation. Maximum pseudoelasticity occurred at the Af temperature and was ~8% for single crystal specimens and ~4% for polycrystalline specimens. Calculations indicated that the large strains were due to a transformation from a bcc to a fct martensite structure. The strain memory effect was studied by deforming specimens below Af and then heating above Af. At temperatures between Ms and Af, the deformation is accommodated by the stress-induced martensite formation. At temperatures below Mf, deformation of the martensite takes place and it is suggested that there is a change in the martensite structure with an increase in the amount of orthorhombic martensite present. The pseudoelastic and strain memory effects have very similar origins and over a wide temperature range from well below Mf to well above As the combined pseudoelastic and strain memory recovery is essentially 100%.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat

Deformation behaviour of Cu-Zn-Si alloys close to their martensitic translormation temperature

Variations in the pseudoelastic and shape memory effects are described for large-grained specimens of Cu-Zn-Si alloys on altering 1. (a) the amount of strain, and 2. (b) the temperature. The progressively decreasing pseudoelastic recovery as the strain is increased is attributed to grain boundary deformation and interaction between different martensite variants. Yield drops were found in the stressstrain curves, but not always the two clearly defined stages reported by Pops. When the temperature was varied, complete pseudoelasticity was not found with temperatures close to Ms, nor with temperatures much higher than Ms, but it was found at intermediate temperatures. The sum of shape memory recovery plus pseudoelastic recovery was close to 100% in general, and the stress for a given strain always increased with temperature. There is evidence that pre-martensitic crystallographic changes occur near Ms, and these are thought to be involved in the shape memory behaviour, the pseudoelasticity being essentially due to stress induced martensite nucleation. The latter is more difficult at higher temperatures and the stress may eventually cause more general yielding instead. Successive cycles of deformation and recovery (strain and temperature constant) increased pseudoelastic recovery, thought to be due to the sweeping out of obstacles; and a two-way memory effect is also reported