Phase Composition of Carbides in LM2 Molybdenum Alloy

Low-doped LM2 molybdenum alloy is obtained by the multiple electron-beam melting of Mo with the addition of 0.02 wt % of C, 0.20 wt % of Ti, 0.15 wt % of Zr, 0.30 wt % of Hf, and 0.40 wt % of Ta. After crystallization and subsequent warm and cold deformation by rolling into a sheet with a thickness of 0.17 mm, the structure and phase composition of the obtained sheets of LM2 alloy are studied using the transmission electron microscopy of thin foils, high-resolution scanning electron microscopy, color metallography, and experimental determination of the sheet density. We make a comparison of the structural states of the studied LM2 alloy both immediately after cold rolling and after the introduction of intermediate high-temperature annealing at a thickness of 2.8 mm, which corresponds to the completion of warm rolling of the alloy in a vacuum rolling mill. In the unannealed sheets of the alloy, Mo2C carbides in the rhombic and hexagonal modifications are found. Hafnium carbides with a cubic lattice are not detected. Some of the carbide inclusions in LM2 alloy that did not undergo recrystallization during high-temperature annealing are relatively large in size (more than 1 µm). They are observed on transverse metallographic cross sections after color etching, and then using transmission electron microscopy. The characteristic diffusion paths of carbon in molybdenum are estimated. The features of the binary phase-equilibrium diagram in the Mo–C system and the structural factors influencing phase equilibrium and the formation of metastable phases in this system are discussed.