Microstructures and corrosion behaviors of FeCoNi and CrFeCoNi equimolar alloys
Introduction
Iron, cobalt, nickel and chromium are very important transition metals which are widely used in many commercial alloys. Also, Fe, Co, Ni and Cr can be the major element of an alloys, or an additive element to improve some of its properties. For example, the addition of Cr can improve the corrosion resistance of Fe-50 wt%Ni alloy [1], and adding a little Fe enhances the corrosion resistance of Cu-30 at.%Ni alloy [2]. The coefficient of thermal expansion of Kovar alloy (Fe-Ni-Co, ASTM F-15 alloy) is close to that of borosilicate glass, and so the former has been commonly used in matched glass-to-metal sealing in microelectronic packages [3], [4]. However, 304 and 316 stainless steels are the most well-known alloys that contained the aforementioned elements.
An equimolar alloy, as described by Professor Yeh, was designed in this work. Professor Yeh introduced the concept of equimolar alloys, whose properties are not dominated by any one element [5], [6], [7]. FeCoNi equimolar alloy has a single FCC structure [8], and so contains no galvanic cell that can arise from differences between the potentials of adjacent phases [9]. Moreover, the fact that dense chromium oxide protects ferrous alloys, such as 304 stainless steel, is also well known [10]. Therefore, the microstructures of FeCoNi alloy and chromium modified FeCoNi alloy (CrFeCoNi) were investigated, as were their corrosion behaviors in 1 M sulfuric acid (H2SO4) and sodium chloride (NaCl) solutions. The results were compared with corresponding results for commercial 304 stainless steel.
Section snippets
Experimental
FeCoNi and CrFeCoNi equimolar alloys were prepared by arc melting using appropriate amounts of pure elements with purities higher than 99.9%. The alloys were made by combining the above elements and arc-melting them under a partial pressure of argon atmosphere (200 torrs). Before melting, the chamber was evacuated to at least 1 × 10−2 torr and then back-filled with argon to 200 torrs, this step was repeated 3–4 times to ensure the purity of atmosphere. The bottoms were remelted at least 4 times
Results and discussion
Fig. 1 displays OM micrographs of FeCoNi and CrFeCoNi alloys in both as-cast and as-annealed states. They all exhibit a granular structure with grain sizes of several hundred micro-meters. However, these two alloys differ in their precipitates: CrFeCoNi alloy contained many precipitates which were distributed in the grains in both as-cast and as-annealed states. Fig. 2 displays the XRD patterns of these two alloys under both as-cast and as-annealed states. The XRD patterns demonstrated that the
Conclusions
- 1.
Both FeCoNi and CrFeCoNi alloys had an FCC granular structure, with lattice constants of approximately 0.357 nm in as-cast and as-annealed states. Only a single FCC phase was observed in the FeCoNi alloy. Additionally, many HCP-structured Cr-rich precipitates were observed in the matrix of CrFeCoNi alloy.
- 2.
The corrosion resistance of FeCoNi alloy in 1 M H2SO4 solution was much better than those of CrFeCoNi alloy and 304SS. The activation energies of FeCoNi and CrFeCoNi alloys in 1 M H2SO4
Acknowledgement
The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 100-2221-E-034-009.
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