Abstract
An HfC-doped molybdenum (Mo-Hf-C; MHC) alloy was prepared via a powder metallurgy process, including dry direct doping followed by ball-milling, cold-isotactic-pressing, and vacuum sintering. An oxidation comparison experiment was conducted, and the oxidation and volatilization behaviors were analyzed using the mass change, volatile generation rate, and morphology transformation. The results show that relatively uniform powder morphology can be obtained by the direct doping of carbide and high-energy ball milling. The oxidation of the MHC alloy at a lower temperature was characterized by the oxygen-absorption and a slight weight gain, while at a higher temperature and longer holding time, it was characterized by the mass volatile weight loss. A significant weight change appeared at 800 °C for 30 min with a weight loss rate of 4.8%. Surface oxidation products developed horizontally from ridged oxides at lower temperature stages to a flaky oxide layer at higher temperatures. The peeling of the oxide layer was the result of interfacial pore development, which led to exposure of the alloy matrix and further oxidation. Based on the oxidation and volatilization characteristics of HfC-doped MHC alloys, we conclude that the oxidation and volatilization of the MHC alloy conformed to the general law; however, the significant weight loss temperature, weight loss rate, volatilization temperature, and volatilization rate were improved compared with pure molybdenum and traditional molybdenum alloys, thus, indicating that the precipitation of the second phase HfC particles at the grain boundaries and within the grains can inhibit the oxidation and volatilization of matrix elements to a certain extent.
Highlights
The metal molybdenum can exist stably; when heated to about 300 ◦ C in air, it begins to oxidize, and a cyan oxide film forms; at higher than 600 ◦ C, a tightly adhered dark green oxide layer is formed; and, at even higher temperatures, the oxidation is accelerated, and the formed MoO3 with high vapor pressure causes a large amount of volatilization causing serious weight loss of the metal molybdenum [1,2]
Mo powder and powderChemical formed by mixing process areand the property key raw materials for the production of mixed reactions, structure, changes for the production of Chemical reactions, structure, and property changes are induced by mechanical energy provided by high-energy ball milling process
Alloytime showed a similar trend to thatweight of pureincrease oxygen absorption, indicating that the alloy can be applied to working condition of is, the weight loss rate increased with the prolongation of oxidation time or the increase for a long time or 800 ◦ C forHowever, a short time destructive oxidation corrosion compared of oxidation temperature
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. It can be seen that, due to the doping of different elements and the preparation process, the oxidation and high-temperature volatilization weight loss behavior of certain alloy will be different from that of pure molybdenum and other types of molybdenum alloys. FCC HfC, which has the highest melting point and optimal thermodynamic stability among all carbides, was adopted to strengthen the molybdenum matrix by dry-directly introducing the powder metallurgy process to prepare an MHC (Mo-Hf-C) alloy. The oxidation corrosion mechanism and second phase anti-corrosion mechanism were analyzed, providing a theoretical and practical basis for improving the anti-oxidation ability of molybdenum alloys and their performance at high temperatures for the further development and application of MHC alloys
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