ＳＥＭ内クリープ疲労試験による微視的損傷過程の連続観察

Abstract

Creep-fatigue damage occurs gradually in high temperature components and structures. It is necessary to clarify the micro-damage evolution mechanism under creep-fatigue conditions for the quantitative evaluation of creep-fatigue damage in local portions of the components. In order to examine the relation between damage evolution and change of microstructure, such as creep cavities and surface microcracks, strain controlled creep-fatigue tests were performed and interrupted at several damage levels. The creep-fatigue tests were also performed under a low stress level by the high temperature fatigue machine combined with a scanning electron microscope (SEM), and the microcrack initiation and growth behavior were continuously observed by the SEM during the tests.It was found that even though many cavities were initiated and grew at the internal grain boundaries of the specimens during the strain controlled creep-fatigue tests, the failure life was governed by the propagation of surface cracks. On the other hand, microcracks of about one grain size order initiated mainly on the grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the microcracks was slow and at random by the influence of microstructure. The microcracks gradually opened to the loading direction with increasing number of cycles and coalescening occurred to grow. From the experiments, the creep-fatigue damage extension mechanism was divided into the Fc type, where the main cracks on the surface propagated through damaged grain boundaries, and Cf type, where damage evolution was characterized by opening and coalescening of the microcracks.