Shape memory in Cu-based alloys: phenomenological behavior at the mesoscale level and interaction of martensitic transformation with structural defects in Cu-Zn-Al

The martensitic transformation in copper-based alloys is thermoelastic, tha t is, the transformation progresses following the undercooling below the eq uilibrium temperature. In addition. a hysteresis is observed because of irr eversible processes taking place during the transformation-retransformation path. These phenomena depend on the complexity of the problem and the rela ted metastable phases being time dependent. In this report the most complex situation (temperature induced and stress free) is characterized via calor imetric and acoustic-emission measurements. Reduction in complexity is thus necessary if intrinsic phenomena are to be separated and quantified. Speci al experimental equipment with appropriate resolution is briefly outlined. The single-interface beta-18R transformation was chosen and then the comple xity was increased progressively to characterize the intrinsic phenomena. E ven in the single-interface transformation, an intrinsic thermoelasticity i s found, which was ascribed to the interaction of growing martensite with t he existing dislocations. In addition, the narrowest hysteresis width was m easured. Nucleation and single-interface friction are distinguished. Disloc ations show a paradoxical behavior in the martensitic transformation. Class ically there is a perturbative component, but in samples without dislocatio ns, breakdown of shape memory can be observed. The following rise in comple xity relates the behavior of several martensite plates of the same type. Th e shape of the hysteresis cycles in stress-induced transformations can be v ery well described and simulated by using the elementary parameters measure d in single-interface experiments. The experimental analysis shows the metastability is relatively important: it was found that the diffusional processes are important near room tempera ture or above. Several time-dependent contributions to the M-s and hysteres is cycles are introduced. Time constants were measured and predictable rule s were established. Avoiding a stochastic interpretation, suitable algorith ms to compute the time behavior of the M-s or the hysteresis cycles and int ernal loops were developed. Since the defects play a decisive role on the m artensitic transformation characteristics, the crystallographic and energet ic changes of the dislocations when embedded in the parent or martensitic p hases were analyzed quantitatively. The interaction of the martensitic tran sformation with precipitates and the evolution of hysteresis width with cyc ling and precipitate size is studied. Finally, the two-shape memory effect is analyzed in terms of the interactions of the martensite with dislocation s and other defects. (C) 1999 Elsevier Science Ltd. All rights reserved.