The effect of temperature on pseudoelastic cycling of CuZnAl single crystals

Abstract

Fatigue, due to pseudoelastic transformations, has been studied by several researchers during the last few years. A transition between an ordered bcc phase ({beta}) and an 18R structure is found in single crystals of Cu-Zn-Al of 3/a = 1.48. It consists in the formation of martensite (M), when a stress is applied on the {beta} phase, and in the retransformation to the original structure after unloading, giving rise to pseudoelastic behavior. Repetition of this process induces changes in the material, which finally leads to fracture. Understanding the evolution of materials during cycling should be the first step in controlling fatigue in these materials. Research shows that temperature, during fatigue experiments, plays an important role in material behavior. For example, the number of cycles to reach fracture is clearly higher at liquid nitrogen temperature if compared with fatigue at room temperature. In order to explain this, the evolution of the material during cycling has to be considered. Several aspects should be analyzed: (a) the evolution of the stress-deformation curves ({sigma}-{var_epsilon}); (b) the introduction of bulk defects; and (c) the formation of surface defects. Previous work shows that surface defects, with an intrusion-extrusion morphology, form after cycling at room temperature. These defects,more » which accommodate in bands, parallel to the habit plane of the transformation (habit plane defects), join to form microcracks, which are believed to be the main cause for fracture. On the other hand, the intrusion-extrusion defects are absent at liquid nitrogen temperatures, giving rise to a longer life of the material. So far, no information is available on the effect at intermediate temperatures on the evolution of {sigma}-{var_epsilon} curves as well as on bulk defects. The purpose of this work is to report a further advance in these studies for temperatures ranging from {minus}196 C to 50 C.« less