Fatigue Fracture Mechanisms of Metal at Cryogenic Temperatures

在4K、77K和室温条件下，利用CT试样研究了亚稳奥氏体不锈钢（304L）和稳定奥氏体不锈钢（310和改性200；18Mn）在低速率下的疲劳裂纹生长机制。通过1.5 MV高压电子显微镜观察疲劳裂纹尖端周围的组织变化，并用柔度法测量裂纹闭合程度。结果表明：1)在18Mn钢中，除裂纹尖端前方外，裂纹周围存在蚀刻显示的带状区域，而在304L钢中，包括裂纹尖端区域在内的裂纹附近带状区域存在严重的蚀刻（白化）区域。2)作为塑性区的显微组织，18Mn钢中存在大量的堆积断裂带和马氏体带。304L时，马氏体进一步转变为‘马氏体’。这意味着裂纹的扩展穿透了这些变形区域。3) 304L钢在低温下的裂纹闭合程度比常温下要低。这与310钢的结果相反，310钢在77 K和室温下的闭合度几乎相同。这些结果与亚稳不锈钢在低温下相变诱发裂纹闭合的观点相反。裂纹在低温下扩展的减小可以通过流变应力的增加和与裂纹前方组织变化相关的循环加工硬化来解释。目前，对铝合金低温疲劳机理的进一步研究正在进行中。

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.