The dislocation and martensite substructures of a fatigued, polycrystalline, pseudoelastic Cu-AI-Ni alloy

Abstract

Polycrystalline Cu-AI-Ni specimens, subjected to pulsating compression fatigue, while capable of pseudo-elastic deformation, nevertheless exhibit cyclic hardening and fatigue fracture, as reported inMet. Trans. A, 1977, vol. 8A, p. 955. In order to interpret this behavior, transmission electron microscopy has been used to study the microstructure. Increasing amounts of martensite and deformation substructure result from decreasing the test temperature and raising the stress level. Martensite of different morphologies has been observed and identified. Large plates ofβí, common to all samples in varying amounts, were determined to be of the 18R structure, with lattice parameters ofa = 4.382Å,b = 5.356Å, andc = 38.0Å. Samples deformed at low temperature or high stresscontain not only β₁, but two distinct forms of γ both with lattice parameters ofa = 4.41Å,b = 5.31Å, andc = 4.222Å, and of the 2H crystal structure. The largeβí and γ plates seem to be characterized by interfacial dislocations between the matrix and plates. In all samples, antiphase boundaries (APB’s) can be imaged, even in heavily dislocated areas, indicating that the deformation has not destroyed the matrix order. It is concluded that hardening results from interactions between matrix dislocations and the stress-induced martensite, as well as by collisions between groups of martensite plates having different habits.