Phase distribution and solute-partitioning in pesting-resistant Mo-Si-Ti alloys
Further efficiency improvement in high temperature applications is facing tough challenges due to the limited temperature capability (1100 °C) and considerable density (8.5 g/cm3) of commercially applied Ni-based superalloys. Alloy development in the ternary Mo-Si-Ti system revealed promising characteristics for potential alternative high temperature materials, since solidus temperatures of 1700 to 2000 °C and reduced densities of 6 to 7 g/cm3 are attained. While numerous studies focus on the development of oxidation protective coatings for Mo-Si based alloys in order to prevent the typical catastrophic oxidation behaviour called ‘pesting’ (formation of volatile MoO3) at temperatures below 1000 °C, this study aims to understand the intrinsic good oxidation behaviour recently observed in novel Mo-Si-Ti alloys. A eutectic alloy Mo-20Si-52.8Ti (at%) being composed of a fine-scaled lamellar microstructure of Mo solid solution MoSS and Ti5Si3 was found to possess an outstanding oxidation behaviour at 800 °C which was attributed to the formation of a thin mixed SiO2-TiO2 oxide scale. Moreover, Ti-rich eutectic-eutectoid alloys comprising lamellar MoSS + Ti5Si3 regions (eutectic) and lamellar MoSS + Mo5Si3 regions (eutectoid), respectively, were found to be pesting-resistant as well. In the course of this study, the following possible reasons or requirements for suppression of pesting will be assessed: (i) fine microstructural length-scale achieved through eutectic and eutectoid reactions, (ii) volume fraction and distribution of the eutectic regions and (iii) chemical composition of MoSS and the M5Si3 silicides. Thereby, the eutectic alloy is artificially coarsened and compared to the as-cast variant and the oxidation behaviour of eutectic-eutectoid Mo-Si-Ti alloy series with systematic varying Ti content (34 to 43 at%) is analysed at 800 °C.