LOW-CYCLE FATIGUE RESISTANCE OF NI-CR ALL OY WITH PROTECTIVE COATINGS

Low cycle fatigue and cyclic plasticity of Ni-Cr alloy and the effect of two types of protective coatings were experimentally investigated. Tubular specimens of Ni-17%Cr alloy coated on the outer surface by a 9 5 mum metallic coating (Co-Cr-Al-Y) or 80 mum metallic coating and 20 mum ceramic coating (ZrO2) were subjected to elastoplastic cyclic stra ining at room temperature. Fractured specimens were observed in scanni ng electron microscope. The analysis of fracture surface and secondary cracks led to the following picture of the fatigue failure. Main fati gue crack initiates at the interface of Ni-Cr alloy and metallic coati ng and is not affected by many long shallow cracks which were initiate d in the protective coatings earlier. The main crack links up with the nearest crack in the coating. The proposed mechanism is in agreement with the minor effect of the protected coatings on the fatigue life of Ni-Cr alloy.