Microstructures and mechanical properties of (Ir,Rh) 75 Nb 15 Ni 10 alloys
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WE have attempted to design new alloys based on iridium (Ir) and rhodium (Rh). Our previous reports proposed that fcc/L12 coherent structure is a promising structure for obtaining high strength at both room and high temperatures.[1,2] The challenge now for the two-phase Ir or Rhbased alloys is to reduce their densities or cost while still retaining their desirable properties. To reduce the density and improve the properties, we replaced Ir in the Ir85Nb15 alloy with a third element, e.g., Ni, Mo, Ta, and W.[3] The results showed that the Ir75Nb15Ni10 alloy is superior to Ir85Nb15 and other binary Ir-based alloys in terms of strength and density.[4] However, the density of the Ir75Nb15Ni10 alloy is high, 18.7 g/cm3, and compressive ductility is only about 5 pct at room temperature. The Ir-based alloys are superior in strength but have higher density and lower ductility than Rh-based alloys.[5] Therefore, the question is whether a two-phase alloy in which Ir is combined with Rh would have higher strength and ductility, but lower density. In this article, we report our results on alloys with compositions (Ir,Rh)75Nb15Ni10. Starting materials were iridium of 99.99 wt pct purity, rhodium 99.97 wt pct, niobium 99.98 wt pct, and nickel 99.5 wt pct. The nominal compositions of the tested alloys are given in Table I. Specimens 6 mm in height and 3 mm in diameter were prepared by electron-discharge machine (EDM) from the ingots. These specimens were annealed at 1600 ⬚C for 72 hours in a vacuum of about 3 ⫻ 10⫺4 Pa followed by furnace cooling. Before tests, these samples were ground with 1000-grit SiC paper. Compression tests were carried out at 20 ⬚C and 1200 ⬚C in air at an initial strain rate of 3.0 ⫻ 10⫺4 s⫺1 with a Tensilon/UTM-1-50000CW (Tensilon Corp., Tokyo, Japan). Some samples were tested at 1600 ⬚C in vacuum using an Instron (Instron Corp., MA) 8560 testing machine. As a measure of room-temperature ductility, the samples were tested to failure, and compression strain was determined from load-displacement curves recorded on a strip chart. A scanning electron microscope (SEM) and a transmission electron microscope (TEM) were used to investigate the microstructures of heat-treated and deformed samples. The phase constituents in the heat-treated samples were identified by an X-ray diffractometer (XRD) and an energy-dispersive X-ray analysis (EDS) facility attached to the SEM. After compression tests, the fracture surfaces were examined by the SEM. Y.F. GU, Researcher, Y. YAMABE-MITARAI, S. NAKAZAWA, and Y. RO, Senior Researchers, and H. HARADA, Senior Researcher and Project Leader, are with the High Temperature Materials 21 Project, National Research Institute for Metals, Ibaraki 305-0047, Japan. Contact e-mail: [email protected] Manuscript submitted March 12, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
The nominal compositions of the tested alloys together with their densities and melting temperatures are given in Table I. The melting temperature decreased as Ir was increasingly replaced with Rh in the Ir75N
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