Martensitic Phase Transformation in Au–Cu–Zn Alloys Studied by <SUP>63</SUP>Cu NMR

Abstract

In order to obtain much information about the microscopic mechanism of martensitic transformations, 63 Cu nuclear magnetic resonance (NMR) and relaxation measurements have been performed at temperatures above and below the transformations in Au-Cu-Zn alloys with different compositions and transformation temperatures. The 63 Cu NMR spectra taken from the parent phase with a L2 1 type structure of the alloys comprising less and more Au contents can be decomposed into three and two components, respectively. It implies that the majority of Cu atoms occupy the corner sites, referring to the fundamental bcc lattice, and the minority of Cu atoms the body-centered sites with different Knight shifts. The component spectra reveal different distribution of the Cu atom, changing with Cu content. Although the NMR spectra taken from the martensite phase are too broad to decompose, we can obtain the average Knight shift with respect to the Cu atom distribution. The temperature dependence of the 63 Cu spin-lattice relaxation time follows the Korringa-like behavior in different ways by the alloys. The data on the relaxation rate are discussed in terms of the Knight shift value and the enhancement of electron-electron correlation. The transformation temperature has no clear relation with the NMR parameters.