The high temperature phase diagrams for zirconium-molybdenum and hafnium-molybdenum

Abstract

The high temperature regions of the Zr−Mo and Hf−Mo binary phase diagrams have been constructured from temperature-composition data obtained by gravimetric and pyrometric methods. The liquidus curves were obtained directly from the measurements of saturation solubilities of molybdenum (single crystal) in liquid Zr and Hf. The solubility results are supported by electron microprobe analyses which identify the formation of thin (∼10 μm) layers of nearly stoichiometric compounds ZrMo2 and HfMo2 on the surface of the single crystal molybdenum below the respective peritectic temperatures 1918±5 and 2206±5°C. These thin layers and the negligible diffusion zones of Zr and Hf in single crystal molybdenum do not significantly affect the measured solubilities. The diffusion coefficient of Hf in Mo-single crystal at 2080°C is ∼5×10−12 m2 s−1. The melting, solidus, liquidus, eutectic and peritectic temperatures were directly measured by pyrometrically observing the partial or complete destruction of “black-body” conditions inside an effusion cell with the appearance of a liquid phase that forms a highly reflecting mirror. The melting points of Zr and Hf metals, 1860±3 and 2228±3°C, respectively, are in good agreement with previously assessed values. The respective eutectic temperatures peratures and compositions 1551±2°C, 29.0±0.5 at. pct Mo and 1896±3°C, 40.5 at. pct Mo, are considerably more precise and only in fair agreement with previously measured or estimated values. The liquidus composition at the peritectic temperature for the Zr−Mo binary is precisely fixed at 54.0±1.0 at. pct Mo and that for the Hf−Mo binary is 61 ±3 at. pct Mo. The thermodynamic activities of molybdenum in the liquid Zr−Mo alloy indicate positive deviations from Raoult's Law.