Melting behaviour and homogeneity range of B2 CoAl and updated thermodynamic description of the Al–Co system
2013
Abstract The central part of the Al–Co phase diagram is dominated by the high-melting B 2 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 + Al 5 Co 2 samples, the Al-rich phase boundary of the CoAl phase and the Co-rich boundary of the Al 5 Co 2 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.
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