Modification of Mo–Si alloy microstructure by small additions of Zr

Molybdenum and its alloys are potential materials for high-temperature applications. However, molybdenum is susceptible to embrittlement because of oxygen segregation at the grain boundaries. In order to alleviate the embrittlement small amounts of zirconium were alloyed to a solid solution of Mo–1.5Si alloy. Two Mo-based alloys, namely Mo–1.5Si and Mo–1.5Si–1Zr, were investigated by the complementary high-resolution methods transmission electron microscopy and atom probe tomography. The Mo–1.5Si alloy shows a polycrystalline structure with two silicon-rich intermetallic phases Mo5Si3 and Mo3Si located at the grain boundaries and within the grains. In addition, small clusters with up to 10 at% Si were found within the molybdenum solid solution. Addition of a small amount of zirconium to Mo–1.5Si leads to the formation of two intermetallic phases Mo2Zr and MoZr2, which are located at the grain boundaries as well as within the interior of the grain. Transmission electron microscopy shows that small spherical Mo–Zr-rich precipitates (<10 nm) decorate the grain boundaries. The stoichiometry of the small precipitates was identified as Mo2Zr by atom probe tomography. No Si-enriched small precipitates were detected in the Mo–1.5Si–1Zr alloy. It is concluded that the presence of zirconium hinders their formation.

Research highlights
► Intermetallic Mo3Si and Mo5Si3 phases investigated by atom probe tomography.
► Clusters enriched in Si (up to 10 at%) within the Mo solid solution phases were detected by 3D-AP.
► Si clusters in the Mo solid solution accounts for the high strength of the alloy.
► Mo2Zr and MoZr2 were measured by TEM/EDX.
► Small spherical precipitates <10 nm were identified as Mo2Zr by 3D-AP and were found both at grain boundaries and within grains.