Effect of Re addition on microstructure and mechanical properties of Ni base dual-two phase intermetallic alloys
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Effect of Re addition on microstructure and mechanical properties of Ni base dual-two phase intermetallic alloys S. Ishii1, T. Moronaga2, Y. Kaneno1, H. Tsuda1 and T. Takasugi1 1 Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan 2 Kobelco Research Institute. Inc, Kobe 651-0073, Japan ABSTRACT The effect of Re addition on microstructure and hardness of the Ni3Al (L12) and Ni3V (D022) dual two-phase intermetallic alloys was investigated as functions of alloying (substituting) method of Re and aging condition (temperature and time). Re was added to the base alloy composition by three methods: Re was substituted for Ni, Al and V, respectively. The Re-added alloys were solution-treated at 1553 K and then aged at lower temperatures of 1123 K-1248 K. Apparent age hardening occurred in the alloy where Re was substituted for Ni while no age hardening was observed in the alloys where Re was substituted for Al or V. In the case of the latter two alloys, the hardness was unchanged or reduced with a progression of aging time. These results were discussed in terms of phase separation and ordering in the channel region, and hardening due to Re-rich phase precipitation. INTRODUCTION Ni-based superalloys are composed of Ni solid solution (A1) and Ni3Al (L12) whose crystal structure is geometrically close packed (GCP) structure. They are widely used as high temperature structural materials. The mechanical properties of the Ni-based superalloys are known to be improved by addition of refractory elements, such as W, Re, Ta, Mo, Nb, and Ru [1-6]. However, the strength of the Ni-base superalloys decreases significantly at high temperature due to softening of the Ni solid solution phase. Ni3X-type intermetallic compounds categorized as the geometrically close packed (GCP) structure exhibit high phase and microstructural stability up to their melting point due to low atomic diffusivity [7]. These intermetallic compounds generally have good high-temperature properties such as high strength and good oxidization resistance. However, monolithic intermetallic polycrystals have a serious drawback, i.e. poor tensile ductility at room temperature. To overcome this drawback, the present authors have developed an Ni-based two-phase intermetallic alloys [8-10]. The Ni-based dual two-phase intermetallic alloys are composed of two intermetallic compounds of Ni3Al (L12) and Ni3V (D022 ). The microstructures of these alloys consist of primary Ni3Al precipitates and Ni solid solution at high temperature (above about 1273 K) and the Ni solid solution is transformed to Ni3Al and Ni3V by eutectoid reaction at low temperature (below about 1273 K). These alloys display a high coherence among the constituent phases, and a high microstructural stability [11,12,14,15]. Hence, the dual two-phase intermetallic alloys have better mechanical properties (for example, higher high-temperature tensile and creep strength) than many conventional superalloys [13,15,16]. It was reported by present authors’ gro
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