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标本，在4K，77K和使用CT标本的室温下，研究了在亚稳态的奥氏体不锈钢（304L）（310和修饰200; 18MN）上，研究了低速率的Fatiguq Orack生长机制。通过1.5 mV高压电子显微镜观察到疲劳裂纹尖端周围的微观结构变化，并使用合规方法测量裂纹闭合。所获得的结果如下：1）发现带有裂纹周围露出的带区域，除了在18MN钢中的裂纹尖端前方蚀刻，而在304L中发现了靠近裂缝（包括尖端区域的裂缝）内的重蚀刻（变白）区域。 2）作为塑料区域的微观结构，在18MN钢中观察到许多堆叠断层层以及马氏体带。在304L的情况下，马氏体进一步转化为“马氏体”。这意味着裂缝生长到穿透这些转化的区域。 3）与室温相比，低温下304L钢的裂纹闭合水平较低。这与310钢的结果形成鲜明对比，在77 K和室温下，闭合启示段几乎相同。这些结果与以下论点相反：在亚稳态不锈钢中，在低温下，转化引起的裂纹闭合将是显着的。低温下裂纹生长的下降是通过在裂纹前方变化相关的流动应力和循环工作加强来解释的。目前正在对低温温度下的Al合金的FATIQUE机制进行进一步研究。