Effect of Si content on microstructure, mechanical and oxidation properties of hot pressed Mo-Ti-Si alloys

Mo-Ti-Si based materials are promising candidates for high-temperature applications beyond the capability limits of nickel based superalloys. In the present investigation, two alloy compositions with different Si content namely Mo-40Ti-30Si (in at.%) (alloy 1) and Mo-40Ti-10Si (alloy 2) were prepared in the form of plates by adopting mechanical alloying and subsequent reactive hot pressing at 1600 °C. The densities of the alloys were found to be much lower than the conventional alloys used for high-temperature applications. The detailed microstructural characterization using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and electron back-scattered diffraction (EBSD) analysis indicated that alloy 1 was composed with Ti(Mo)5Si3 matrix and Ti(Mo)3Si precipitates. Alloy 2 consisted with three phase microstructure containing α-Mo(Ti)ss, β-Ti(Mo)ss and Mo(Ti)3Si. Micro-hardness values of 1400 H V and 850 H V respectively for alloy 1 and alloy 2 were noticed, however, alloy 1 showed the tendencies of micro-crack formation from the corners of the Vickers indentations. Alloy 1 showed a superior isothermal oxidation resistance at 900 °C, 1000 °C, and 1300 °C for prolonged duration due to formation of a thin duplex SiO2 (TiO2) oxide scale. However, the formation of a porous oxide layer led to evaporation of MoO3, showing drastic weight loss of alloy 2 at all three tested temperatures. Enrichment of Si was done using pack siliconizing process forming a (Mo, Ti)Si2 layer of thickness 10 μm and 30 μm on the surfaces of alloys 1 and 2 respectively at 900 °C. The silicide coated alloy 1 showed further improvement in the oxidation behaviour, and enrichment of Si on alloy 2 surface imparted a drastic improvement in the resistance against oxidation in air at 1300 °C for the prolonged exposure times of 70 h.