DEFECT STRUCTURE IN AL-RICH COMPOSITION REGION IN THE BETA-NIAL INTERMETALLIC COMPOUND PHASE

The defect structure in the B2 type beta-NiAl phase was investigated in the Al-rich composition region by means of powder X-ray diffractometry. In the analysis, earlier published density data were referred to for estimation of the total vacancy concentration in the alloy. As a result, the main constitutional defect was found to be the vacancy on the Ni-site (Ni-vacancy), supporting the defect structure model proposed previously by Bradley and Taylor. However, in a region of high Al concentration, some Al atoms substituted on the Ni-site, i.e. anti-structure defects of Al atoms (Al-ASD), as well as some vacancies on the Al-sites (Al-vacancies), were recognized to exist. Their amounts did not change with increasing temperature up to 1150-degrees-C. These defect types of the Al-ASD and Al-vacancy had always been excluded in usual treatments based on the triple defect model. The reliability of the defect structure determined was examined by comparison with the triple defect model. Further, thermodynamic calculations based on the Bragg-Williams approximation were performed in order to reproduce the observations.