Effect of W addition on microstructure and mechanical properties of Ni base dual two-phase intermetallic alloys
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Effect of W addition on microstructure and mechanical properties of Ni base dual twophase intermetallic alloys Daisuke Edatsugi, Yasuyuki Kaneno, Hiroshi Numakura, and Takayuki Takasugi Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan ABSTRACT The effect of W addition on microstructure and mechanical properties of Ni3Al (L12) and Ni3V (D022) two-phase intermetallic alloys has been investigated. W was added to the base alloy composition, Ni75Al10V12Nb3 (at. %) in place of either Ni, Al or V. The W-added alloy ingots were heat-treated in vacuum at 1575 K for 5 h. The majority of W-added alloys showed a dual two-phase microstructures while the alloy in which 3 at. % W substituted for Ni exhibited the dual two-phase microstructure containing W solid solution dispersions. Vickers hardness was significantly enhanced by W addition, which is primarily due to solid-solution strengthening. INTRODUCTION Ni-based superalloys with microstructures consisting of Ni solid-solution (A1) and Ni3Al (L12) phases have been widely used as high temperature structural materials. To further improve the mechanical properties of these alloys at high temperature, addition of the refractory elements, such as W, Re, Ta, Mo, Nb and Ru, has been investigated [1-6]. However, the decrease in strength at high temperatures still occurs because the Ni solid-solution rapidly softens at high temperature. Ni3X-type intermetallic compounds categorized as the geometrically close-packed (GCP) structure exhibit high phase and microstructural stability up to their melting point, which is due to low atomic diffusivity [7]. These intermetallic compounds generally have good hightemperature properties such as high strength and high oxidization resistance. However, they also have serious drawbacks, such as poor ductility at room temperature. Therefore, studies of multiphase intermetallic alloys aiming to overcome these drawbacks have been conducted by the present authors, and Ni-based two-phase intermetallic alloys have been developed [8-10]. The Ni base dual two-phase intermetallic alloys are composed of Ni3Al (L12) and Ni3V (D022). The microstructure of these alloys consists of primary Ni3Al precipitates and Ni solid solution at high temperature and the Ni solid solution transforms to Ni3Al and Ni3V by eutectoid reaction at low temperature. These alloys display high crystallographic coherency among the constituent phases, and high microstructural stability [9-12]. By the present authors’ group, the addition of a fourth element such as Nb, Ti, Re and Ta [13, 14] to the alloy has been investigated: Nb, Ti and Ta enhance the strength (hardness) of the alloy by solid-solution hardening, while Re enhances the hardness by precipitation hardening. It has been reported that the addition of W improved the hardness of the Ni base superalloy [1]. However, no detailed study has been carried out for the Ni base dual two-phase intermetallic alloy. In this study, W was added to the base alloy by three methods: W
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