Creep of Cu-Bi Alloys with High Bi Content Near and Above Melting Temperature of Bi
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INTRODUCTION
TWO-PHASE metallic alloys formed of constituents with little mutual solubility and chemical reaction products are interesting engineering materials as they allow for fabrication of metallic materials following the principles of composites. Consequently, metals with significantly different mechanical properties can be alloyed together in any proportion to achieve an optimum combination of properties. However, the possibility of finding such a set of metals is highly limited because the majority of metals either chemically react with each other (thereby forming intermetallic compounds) or form a solid solution. Due to such interactions, the distinct identity of the original constituents is lost, in due course of time, in an alloy. Interestingly, Cu-Bi is one of the material systems that allows mixing of its constituent
SHOBHIT P. SINGH, DIPALI SONAWANE, and PRAVEEN KUMAR are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: [email protected] Manuscript submitted July 6, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
elements (i.e., Cu and Bi) in any proportion to produce a metallic ‘‘composite’’ alloy.[1,2] Cu and Bi have excellent mutual wetting behavior, 0 deg dihedral angle, limited solubility (e.g., only ~ 0.003 at. pct of Cu in Bi at 800 C), and moreover, they do not form any intermetallic compounds.*[3,4] Therefore, the microstructure *Although a metastable intermetallic superconducting Cu-Bi compound, Cu11Bi7, was discovered recently; it is stable at high pressures and under special conditions only.[5] Hence, it is not observed under ordinary test conditions or applications.
of Cu-Bi alloys remain stable over a range of temperatures, including temperatures above the melting temperature of Bi, over long periods of time. Because of the above unique combination of properties, Cu-Bi alloys can be used for thermal surge protection and energy storage, wherein the heat energy is stored in the form of latent heat energy required to melt Bi, the low melting phase (LMP).[2] Since it is warranted to use high-volume fractions of the phase changing material (i.e., LMP in the case of Cu-Bi) for increasing the overall energy storage capacity of the material, it is reasonable to fabricate Cu-Bi alloys with a high volume fraction of Bi, such as > 20 vol pct.[2] For realization of a suitable thermal surge and energy storage material relying on the principle of liquefaction of LMP, it is critical to evaluate the effect of liquefaction of LMP on the mechanical behavior of the material.
Studies focusing on the effect of liquefaction of LMP on the mechanical response of metallic alloys are limited and, often, conflicting. Compression creep of a-brass, with 3 at. pct of Pb, was conducted above and below the melting point of Pb; however, the effect of the liquid phase on creep response was not observed.[6] Herein, the authors concluded that the grain boundary area wetted by the liquid phase must be substantially high for affecting the creep behavior of t
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