Effect of aging on the rubber-like behavior in Cu-Zn-Al martensites

Abstract

Martensites in several shape memory alloys exhibit peculiar pseudoelastic behavior, referred to as twinning pseudoelasticity or rubber-like behavior. By loading a sample in the martensite state, internal twin boundaries, or intervariant boundaries, move so as to increase the volume fraction of variants more favorable oriented to relieve the applied stress. On unloading, those boundaries tend to return to their original positions; accordingly, the sample restores it original shape. It has been recognized that the rubber effect appears only in aged martensites. When a martensite is deformed immediately after its formation, the boundary motion is not reversible; imposed strain remains after unloading. Unlike the case of superelasticity, which is another class of pseudoelastic behavior in shape memory alloys, the rubber effects does not involve any phase transformation. Deformation is undertaken merely by the reorientation of variants of a single martensite phase. While the free energy difference between two participant phases gives rise to the restoring force in superelasticity, there is, apparently, no such counterpart in the rubber-like behavior. The original of the restoring force in the rubber effect has long been a subject of controversy. In the present study, the authors have investigated the effects of room temperature aging on themore » stress-strain behavior quantitatively in both 9R and 18R martensites of Cu-Zn-Al alloys. In addition, changes in the reverse transformation temperature by aging have been measured. Since aging is known to significantly affect the reverse transformation behavior as well, they have intended to examine a correlation between the rubber-like behavior and the stabilization of martensites.« less