On the chemical and microstructural requirements for the pesting-resistance of Mo-Si-Ti alloys

In recent publications [Schliephake et al. in Intermetallics 104 (2019) 133-142 and Obert, Kauffmann & Heilmaier in Acta Materialia, 184 (2020) 132-142], an unexpected pesting-stability of fully eutectic and specific eutectic-eutectoid Mo-Si-Ti alloys was found. Several potential reasons were proposed: microstructural length scale being typically very fine due to the eutectic and eutectoid reactions, the phase distribution including the fraction of the eutectic and eutectoid regions, as well as the chemical composition of the individual phases. In the present study, we prove the Ti content to be decisive for the pesting-resistance in air at 800 °C by investigating the microstructure and oxidation behaviour of Mo-Si-Ti alloys with systematically varying nominal Ti content. A critical Ti content of 43 at% was identified. Due to the phase equilibrium, this corresponds to a local Ti content in Mo solid solution of 35 at%. Other microstructural properties such as (i) lamellar size within eutectic and eutectoid regions and (ii) volume fraction of the eutectic and eutectoid regions were demonstrated to have an insignificant influence on the pesting-resistance of the alloys. ... mehrRather, the assessment of the oxidation behaviour of the monolithic phase MoSS, which was identified to be crucial for the oxidation behaviour of the Mo-Si-Ti alloys, confirmed an improvement in oxidation behaviour with increasing Ti content.