Phase constituent of an as-cast Co–Ni–Al–W–Re–Ti alloy: correlation of DTA results with CALPHAD and map structure simulations

Abstract

Designing the chemical composition of new heat-resistant materials, including a new group of cobalt-based materials strengthened with the L12 phase with the general formula Co3(Al,X), requires the introduction of numerous alloy components, stabilizing the strengthening phase into the base composition. Typically, these are elements of high melting metals, whose main role is to stabilize the interphase boundary between the matrix and L12 precipitates and, in some way, strengthen the solid solution. These elements are characterized by high bond energy. On the other hand, the high content of this type of alloy addition increases the tendency of the alloy to release undesirable topologically compact phases, which rapidly deteriorate the mechanical properties of the alloy. These phases usually form in interdendritic areas, generating the so-called melting onset temperature, which deviates significantly from the solidus value. Therefore, the ability to predict the type and number of topologically compact phases being formed allows for the skilful design of the chemical composition of the alloy and its heat treatment, ensuring full dissolution of the mentioned phases in the matrix. This topic is the area of research in this article and concerns the Co–20Ni–9Al–7W–3Re–2Ti alloy in its immediately as-cast state. The scope of the research included simulations using the CALPHAD method and prediction of the phase composition of precipitates using two-dimensional structure maps. The obtained theoretical results were verified in microstructural tests using the STEM method and correlated with the results of DTA tests.