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.

使用 CT 试样，研究了亚稳态奥氏体不锈钢 (304L) 和稳定奥氏体不锈钢（310 和改性 200；18Mn）在 4K、77K 和室温下的低速率疲劳裂纹生长机制。通过1.5 MV高压电子显微镜观察疲劳裂纹尖端周围的微观结构变化，并采用柔度法测量裂纹闭合情况。得到的结果如下： 1) 在 18Mn 钢中，除了裂纹尖端之外，在裂纹周围发现了腐蚀显露的带状区域，而在靠近裂纹的带状区域内发现了严重腐蚀（白化）的区域，包括在304L中发现了尖端区域。 2)作为塑性区的显微组织，在18Mn钢中观察到许多堆垛层错层和马氏体带。在304L的情况下，马氏体进一步转变为'马氏体。这意味着裂纹会扩展并穿透这些转变区域。 3) 304L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 钢的结果形成鲜明对比，310 钢的封闭性在 77 K 和室温下几乎相同。这些结果与相变诱导裂纹闭合在亚稳态不锈钢中在低温下显着的论点相反。低温下裂纹扩展的减少是通过流变应力的增加和与裂纹发生前的结构变化相关的循环加工硬化来解释的。目前正在对铝合金低温疲劳机理进行进一步研究。