A stability of oxide layers formed in LBE on HCM12A to external loading

Stability of oxide layers associated under applied external load was investigated for oxide layers formed on the inner surface of a steel pipe in flowing LBE with respect to fracture behavior, fracture mechanism and nano-hardness. Ring specimens obtained from the pipe were compressed by a compression test machine, and a fracture of the inner oxide layers was observed. Cracks crossed the oxide layers occurred from the interface between the oxide layers and the base metal to the surface of oxide layers and delamination of oxide layers occurred at the Fe-Cr spinel layer in flatted part of compressed ring specimen. In the compressed part of the ring specimens, all oxide layers were spalled off from the base metal. Engineering strains in the base metal near oxide layers were determined from the deformation of micro-indentation sizes. A tensile strain component to the circumferential direction was responsible for the cracking. It appears that the tensile strain component in the radial direction was responsible for the delamination of the oxide layers. Nano-hardness and Young's modulus of each oxide layers were measured by a nano-indentation method for the evaluation of the fracture behavior. Young's modulus of Fe-Cr spinel layer is lower than that of Fe3O4 layer by 10% with the same hardness in both layers, which yields excess strain to the spinel layer rather than to the Fe3O4 layer and consequently causes cracking.