Fatigue Fracture Mechanisms of Metal at Cryogenic Temperatures

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The fatiguq orack growth mechanisms at low rate are studied on both metastable austenitic stainless steel (304L) and stable ones (310 and modified 200; 18Mn) at 4K, 77K and room temperature using CT specimens. The microstructure changes around the fatigue crack tips were observed through a 1.5 MV high voltage electron microscope and the crack closure was measured using compliance method. The results obtained are as follows: 1) There was found bands region revealed by etching around the crack excepting just ahead of the crack tip in 18Mn steel, whereas there found heavily etched (whitened) region within the bands region close to the crack including the tip region was found in 304L. 2) As microstructures of the plastic region, many stacking fault layers as well as martensite bands were observed in 18Mn steel. In the case of 304L, the martensite was further transformed into ' martensite. This means that the crack grows penetrating these transformed region. 3) The crack closure level in 304L steel at low temperature was lower as compared to that at room temperature. This is in contrast with the result in 310 steel, in which the closure revel was almost the same both at 77 K and room temperature. These results are contrary to the argument that the transformation induced crack closure would be significant at low temperature in metastable stainless steel. The decrease of crack growth at low temperature is explained through an increase in flow stress and cyclic work hardening associated with the structure change just ahead of crack. Further study on fatique mechanisms of Al alloys at cryogenic temperatures are being conducted at present.