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Fatigue Fracture Mechanisms of Metal at Cryogenic Temperatures

低温下金属的疲劳断裂机制

基本信息

批准号：
62550057
负责人：
KATAGIRI Kazumune
金额：
$ 1.22万
依托单位：
The Institute of Scientific and Industrial Research, Osaka University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1987
资助国家：
日本
起止时间：
1987 至 1988
项目状态：
已结题

项目摘要

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.

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和变质200；18Mn)在4K、77K和室温下的低速疲劳扩展机制。用1.5 MV高压电子显微镜观察了疲劳裂纹尖端周围的显微组织变化，并用柔度法测量了裂纹闭合程度。结果表明：1)在18Mn钢中，除裂纹尖端前方外，在裂纹周围有明显的腐蚀带区，而在304L钢中，在裂纹附近的带区内存在严重的腐蚀(白化)区，包括尖端区。2)作为塑性区的显微组织，18Mn钢中存在大量的层错层和马氏体带。在304L的情况下，马氏体进一步转变为‘马氏体。这意味着裂纹会穿透这些转变区域而扩展。3)304L钢的低温裂纹闭合程度低于常温。这与310钢的结果相反，310钢在77K和室温下的闭合狂欢几乎是相同的。这些结果与亚稳不锈钢在低温下相变诱发裂纹闭合显著的观点是相反的。通过流动应力的增加和与裂纹前的组织变化相关的循环加工硬化，解释了低温下裂纹扩展的减少。目前，对铝合金低温疲劳机理的进一步研究正在进行中。