MECHANICAL-PROPERTIES AND MICROSTRUCTURAL EVOLUTION OF A COBALT-BASE ALLOY COATING

Several isothermal mechanical tests have been performed on a cobalt su peralloy to determine the parameters of a viscoplastic behavior law in the range (20-700-degrees-C). These parameters have been identified b y means of a systematic numerical procedure (a computer program for si mulation and identification). The microstructural evolution during the mechanical tests has been examined in relation to the evolution of th e internal variables of the viscoplastic model. It was found that the material softens at room temperature, mainly as a result of an isotrop ic component decrease. However, at high temperatures (500-700-degrees- C), the material hardens, mostly owing to a kinematic component increa se. The deformation mode, i.e. the martensitic transformation, has bee n identified for all the temperatures studied. At high temperatures ca rbide precipitation on stacking faults and deformation bands has been observed. Complementary tests have shown that, at high temperatures, t he further addition of carbide precipitates dissolves by dislocation g lide and a diffusion mechanism. At room temperature, after heat treatm ent at 700-degrees-C, carbides precipitated on deformation bands induc ed the formation of ''hard'' zones. These microstructural observations have been correlated with the important part of the kinematic term in the stress increase.