S-816 Cobalt-based Alloy Hot Deformation Characteristics

Solid oxide fuel cell (SOFC) is the third generation fuel cell. It has many advantages such as high efficiency, no pollution, solid-state structure and a wide range of adaptability of various fuel gases and so on. A solid oxide fuel cell now developed needs structure sealing materials. The material works under high temperature at 800℃, while takes an important role of being elastic support to the battery. Due to the high temperature, long working hours, it is difficult for the ordinary elastic alloy to work effectively at such high temperature. So a new type of flexible materials need to be found to meet the needs of solid oxide fuel cell structure. S-816 alloy is a traditional high temperature elasticity of cobalt-based alloys. It has high strength and elasticity at 800℃. It is often used to make high temperature elastic components, and it is a potential SOFC elastic material. This paper selected S-816 cobalt-based alloy as the research object, then researched the hot deformation behaviors, and its antioxidant properties, microstructure. It accumulated first-hand information for the alloy used as solid oxide fuel cell sealing materials. Getting the following results:S-816 cobalt base alloy was tested by thermal simulation, and obtained the flow curves at various strain rates for different temperatures, and the peak stress and peak strain were determined in different deformation conditions.The hot deformation activation energy of the S-816 cobalt-based alloy was 518kJ/mol from 900℃to 1200℃for the strain rate of 0.1s-1, 1s-1 and 10s-1. The hot deformation equation were showed as follow:The dynamic organization of state diagram of S-816 Co-based alloy was established in the different deformation conditions, and obtained the relationship between dynamic recrystallization critical stress and Z parameter: mc=18.65lnZ-743.2The average oxidation rate of S-816 cobalt-based alloys is 0.0031 g/(m2·-h), and it is on totally oxidation resistant level. There are two kinds of alloy surface oxide forms, one is the spherical particles; the other is irregular edges small particles with corners, both containing Cr, Mn, Fe oxides.The oxide layers of S-816 cobalt-based alloy at 800℃were consisted of both the Cr oxide with structure similar to Fe3O4 and Fe2O3. The excellent oxidation resistance of tested alloys mainly because the dense oxide (FeCr2) O4 with Cr in it which is similar to Fe3O4 structure which accounted for most of the oxide film. The oxide layer was slightly loose after 200h oxidation. The oxide layer was continuous and dense after 500h oxidation with nospalling phenomenon.The major precipitates of S-816 cobalt-based alloy after aging contained M23C6 and NbC. NbC phase was precipitated in grain boundary and inner grain. There were two forms exist. One was large spherical or irregular-shaped particles. The other is the smaller particles, and they precipitated during solid solution and aging. M23C6 was precipitated mainly in grain boundary. It was irregular-shaped small particles, and the size was between 100nm and 500nm.M23C6 was precipitated easier after aging. It was the main carbide strengthening phase. The small particles NbC precipitating in grain boundary and inner grain also play a role in promoting the strengthening of the alloy.