Thermodynamics of point defects in ordered B2 intermetallic phases and the influence of these defects on their mechanical properties

In this manuscript, I have reviewed how the knowledge of the types and concentrations of point defects in the Hume-Rothery beta-brass type intermetallic phases (Strukturbericht designation, B2) can be used to rationalize the mechanical properties of this class of phases. These phases can be divided into two groups depending on their majority defect populations. They are anti-site defect compounds such as CuZn, AuZn, AgMg and FeCo and triple-defect compounds such as FeAl, NiAl, CoAl and PdIn. The concentrations of point defects were obtained from thermodynamic models and in the case of triple-defect phases, experimentally measured vacancy concentrations were available to verify the calculated values. However, these models can be used to calculate the concentrations of point defects at temperatures and compositions where data are not available. For the antisite defect phases, these quantities were calculated from the models and there is no experimental data available to confirm the calculated values. These defect concentrations were used successfully in rationalizing the hardening effects in a number of Hume-Rothery beta-brass type phases using solid solution strengthening theory developed in the literature. Vacancies were found to be a more potent hardener than anti-site defects. While the potency of vacancy hardening is similar to carbon in iron (interstitial defect), that of the anti-sites is closer to aluminum in copper (substitutional defect). The apparently different behavior of the hardness data in FeAl and NiAl with respect to temperature and compositional variations was resolved and can be attributed to the difference in the stabilities of these two phases. The approach used can be readily extended to other phases provided their stabilities and point defects are understood.