Development of High Strength and High Corrosion Resistant Mo Composite Material under Severe Environment

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For pure Mo and Mo-0.5 mass% alloy nitrided at 1073K to 1373K in NH_3 gas, the microstructural observations of molybdenum nitride layer were carried out through an optical and a transmission electron microscope and X-ray diffraction analysis. Corrosion behavior of the nitrided specimens was also examined. A surface nitriding layer consisted mainly of two Mo nitrides : an outer one of γ-Mo_2N and an inner one of β-Mo_2N.A great number of{011}<011>-type micro-twins were observed in the β-Mo_2N layer whereas the microstructure of the γ-Mo_2N layer is almost free from lattice defects. The nitrided specimens exhibited excellent corrosion resistance even in 75wt% sulfuric acid solution at the boiling temperature of 460.8K compared with unnitrided specimens. Its corrosion rate decreased with decreasing nitriding time because of the suppression of cracking by decreasing thickness of the surface nitriding layer. The corrosion rate of the specimen nitrided at 1123K for 24h was less than 0.001mm/y in boiling 75% sulfuric acid. In a nitrided Mo-0.5Ti alloy an internal nitriding layer with remarkably high hardness of Hv〜800 was formed beneath the surface nitriding layer. The high hardness is considered to be ascribed from fine dispersion of extremely small titanium nitride particles in the Mo matrix. The hardness in the internal nitriding layer gradually decreased as the distance from the interface increased.