The Effect of Nb Addition on Phase Equilibria in the Ni-Rich Ni-Al-V Ternary System
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The Effect of Nb Addition on Phase Equilibria in the Ni-Rich Ni-Al-V Ternary System Eiki Hayashi1, Satoru Kobayashi2, Yasuyuki Kaneno1, Takayuki Takasugi1, 2 1 Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho Naka-ku, Sakai, Osaka 599-8531, JAPAN 2 Osaka Center for Industrial Materials Research, Institute for Materials Research, Tohoku University, 1-1 Gakuen-cho Naka-ku, Sakai, Osaka 599-8531, JAPAN ABSTRACT The effect of Nb addition on phase equilibria among Ni (A1), Ni3Al (L12) and Ni3V (D022) phases was investigated in the Ni-rich Ni-Al-V ternary system. The addition of Nb to the Ni-Al-V ternary system shifts the three-phase coexisting region of A1 + L12 + D022 towards the Ni-rich side at 950 ºC. Nb is partitioned into the D022 phase more strongly than the A1 and L12 phases. These results suggest that the addition of Nb stabilizes the D022 phase against the A1 and L12 phases in the systems. The alloying element raises the temperature of a eutectoid-like reaction (A1ĺL12+D022) by ~30 ºC in the vertical section of Ni-Al-V ternary system at 75 at. % Ni. INTRODUCTION The Ni base dual two-phase (Ni3Al(L12) and Ni3V(D022)) intermetallic alloys have attractive high-temperature properties such as strength, creep and corrosion resistance [1-3]. Since the microstructures of the alloys are formed along a eutectoid-type reaction pathway (Ni(A1) ĺ L12 + D022), the reaction temperature (TE) is a decisive factor for their high temperature mechanical properties; the disordered A1 phase is formed when the alloys are heated above TE, and hence the two-phase microstructures and the strength are lost. It is therefore important to understand the effect of alloying elements on TE to control the high temperature mechanical properties. Recently, we have investigated the effect of Ti addition on phase equilibria among A1, L12 and D022 phases in the Ni-Al-V ternary system and found that Ti addition stabilized the L12 phase rather than the D022 and A1 phases [4, 5]. In this study, the effect of Nb addition on phase equilibria among the A1, L12 and D022 phases was investigated at 950 ºC, just below the TE in the Ni-Al-V ternary system. The vertical sections of phase diagram at 75 at. % Ni in the Ni-Al-V ternary system and the Ni-Al-V-Nb quaternary system were also determined. EXPERIMENTAL PROCEDURES The nominal compositions of alloys studied are listed in Table I. Hereafter, the alloys will be referred to as Base and 2.5Nb alloys. The compositions of Base and 2.5Nb alloys were selected to have the three phases of A1/L12/D022 at 950 °C in the alloys, according to our previous study [4]. The alloys were prepared by arc melting in an argon atmosphere, a homogenization treatment at 1300 °C for 5 h and a heat treatment at 950 °C for 2000 h, followed by water quenching. The microstructures of heat-treated samples were observed with scanning electron microscopy (SEM) and phases present in the samples were identified with transmission electron
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microscopy (TEM). Chemical compositions
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