The role of dispersed particles in strengthening and fracture mechanisms in a Mo-ZrC alloy processed by mechanical alloy
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RODUCTION
MOLYBDENUM (Mo) exhibits a high melting point (2890 K), a low thermal-expansion coefficient, a high thermal conductivity, and good fabricability. Therefore, Mo is widely used for applications at high temperatures. However, coarse-grained Mo, which is fully recrystallized, generally shows low ductility and poor creep resistance. To improve the poor mechanical properties, high-ductility and highly creep-resistant Mo alloys have been developed by doping Mo with a small amount of Al, K, Si,[1–5] or rare-earth (RE) elements.[6–9] The origin of the high ductility and high creep resistance in the doped Mo alloys is related to the elongated grain microstructure. Doping with Al, K, Si, or RE elements serves to control the grain morphology. Recently, Kurishita et al.[10,11] have developed a hightoughness TiC-dispersed Mo by mechanical alloying (MA). The high toughness of the Mo-TiC alloy is a result of the carbide particles strengthening grain boundaries due to good coherency with adjacent matrix grains.[10,11] In addition, it has been reported[12] that a ZrC-dispersed Mo, which was processed by MA, exhibited a lower ductile-to-brittle transition temperature (DBTT) than a pure Mo. More recently, it has been shown[13] that a high strength of more than 1000 MPa at room temperature and superplastic-like behavior at 1970 K were attained in a Mo-ZrC alloy processed by MA. In the present investigation, the tensile properties of a ZrCdispersed Mo, which is processed by spark plasma sintering (SPS) with mechanically alloyed powder, are investigated at room temperature and at elevated temperatures of 1170 T. TAKIDA, Researcher, is with the R & D Division, Tokyo Tungsten Co., Ltd., Toyama, 931-8543, Japan. M. MABUCHI, Senior Researcher, and N. NAKAMURA, Research Manager, are with the National Industrial Research Institute of Nagoya, Nagoya 462-8510, Japan. T. IGARASHI, formerly Director, R & D Division, Tokyo Tungsten Co., Ltd., is Assistant General Manager with Sumitomo Electric Industries, Ltd., Osaka 554-0024, Japan. Y. DOI, Senior Researcher, and T. NAGAE, Researcher, are with the Toyama Industrial Technology Center, Tokaoka 933-0981, Japan. Manuscript submitted February 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
to 1970 K. Based on the results, the roles of dispersed particles in strengthening and fracture mechanisms in the Mo-ZrC alloy are described. II. EXPERIMENTAL PROCEDURES A mixed powder of pure Mo, with an average particle size of 4.1 m, and ZrC, with an average particle size of 2.0 m, was prepared. The mixed powder was subjected to MA by ball milling with a planetary ball mill machine. The MA was carried out for 108 ks, with vessel rotation of 230 rpm, in an argon atmosphere. The mechanically alloyed– treated powder was put into a graphite electrode mold. The mold was set in a SPS machine. The mechanically alloyed powder was uni-axially pressed by a graphite electrode rod under 74 MPa in vacuum at 10 Pa. It was heated, at the rate of 2.0 K/s, from room temperature to 2070 K, and then it was held for
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