Melting behaviour and homogeneity range of B2 CoAl and updated thermodynamic description of the Al–Co system

The central part of the Al–Co phase diagram is dominated by the high-melting B2 CoAl phase. Although this phase is well-known since long and has been intensively investigated in the literature, neither its melting behaviour nor its homogeneity range is well established. Therefore, a series of Al–Co alloys in the composition range 30–60 at.% Co were produced and analysed by differential thermal analysis, scanning electron microscopy, and electron-probe microanalysis. The composition dependence of the melting temperatures was determined and from the analysis of heat-treated, two-phase CoAl + Al5Co2 samples, the Al-rich phase boundary of the CoAl phase and the Co-rich boundary of the Al5Co2 phase were established. The melting temperatures of CoAl were found to be significantly higher than previously reported in the literature; for the stoichiometric compound a value of 1673 °C was obtained. On the basis of these new results and further relevant literature data, the complete Al–Co phase diagram was finally thermodynamically re-assessed applying the CALPHAD technique. The optimized thermodynamic parameters not only give a good description of the new experimental data, but also fit very well to most of the literature data.