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The microstructures and hardness property of dual two-phase intermetallic alloys that are composed of various kind of volume fractions of geometrically closed packed (GCP) Ni 3 Al(L1 2 ) and Ni 3 V(D0 22 ) phases was studied. Higher volume fraction of primary Ni 3 Al precipitates was observed in the Ti and Nb added alloys when keeping Al content the same. Also, the microstructures in the eutectoid (channel) region consisting of Ni 3 Al+Ni 3 V were sensitive to alloying addition. The hardness of dual two-phase intermetallic alloys was basically explained by mixture rule in hardness between primary Ni 3 Al precipitates and eutectoid region. Nb and Ti addition raised hardness of dual two-phase intermetallic alloys by solid solution hardening in the constituent phases. This hardening was more significant in Nb addition than in Ti addition. In addition to hardness owing to the mixture rule, additional hardening arising from interfacial area between primary Ni 3 Al precipitates and eutectoid region was found. With increasing Ni 3 Al/channel (eutectoid) interfacial area, the additional hardening increased. As temperature increases, the additional hardening monotonously decreased for the base and Nb added alloys but little decreased for the Ti added alloys.

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