Fatigue Fracture Mechanisms of Metal at Cryogenic Temperatures

本文用CT试样研究了亚稳奥氏体不锈钢（304 L）和稳定奥氏体不锈钢（310和改进的200; 18 Mn）在4K、77 K和室温下的低速率疲劳裂纹扩展机制。用1.5MV高压电子显微镜观察了疲劳裂纹尖端的微观组织变化，并用柔度法测量了裂纹的闭合情况。结果表明：（1）在18 Mn钢中，除裂尖前方外，在裂纹周围均发现了腐蚀条带区，而在304 L钢中，在包括裂尖在内的裂纹附近的腐蚀条带区内发现了严重腐蚀（白化）区。2)18 Mn钢塑性区的显微组织为层错层和马氏体带。在304 L的情况下，马氏体进一步转变为“马氏体”。这意味着裂纹穿透这些转化的区域生长。3)304 L钢在低温下的裂纹闭合水平低于室温下的裂纹闭合水平。这与310钢的结果相反，310钢在77 K和室温下的闭合速度几乎相同。这些结果与亚稳不锈钢在低温下相变诱导裂纹闭合的观点相反。在低温下裂纹扩展的减少解释通过增加流动应力和循环加工硬化与结构变化的裂纹前。目前，对铝合金低温疲劳机理的研究还在进行中。

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.