Observations of Slip Bands in the Study of Fatigue Fracture of High-Purity Iron

Abstract

There are two basic problems about fatigue, one is how cracks are formed, and the other is how they grow. The answers to these questions have been sought by investigators for a long time, but even now the problems remain substantially unsolved. Since the dawn of the use of electron microscopy for observing microstructural changes in metals, recent works have begun to shed light on the nature of basic physical processes involved in fatigue, and, on the other hand, a study to unravel the relationship between microscopic and macroscopic behaviors of materials has been urgently required. It is considered that if plastic replicas and optical microscope were prepared in one and the same horizon it would be possible to obtain the correlation of microscopic characteristics with macroscopic aspects of fatigue deformation.In the present paper the report is given of the examination made, to clarify the basic mechanisms of fatigue fracture in b.c.c. metals, of the thin plate specimens of high-purity iron (99.996%) subjected to alternating bending stress. Particularly the microstructural changes that occurred around the fatigue cracks were examined by optical metallography and electron microscopy, using an improved replica technique.The results obtained in this study are summarized as follows:(1) The crossing points of the two slip bands are the preferred sites for fatigue crack nucleation. In these regions the aggregation of vacancies generated by the to-and-fro slip movement could form pores in the slip bands, and these in turn could grow into microcracks.(2) There was a rather pronounced grain size effect on the nucleation and propagation of fatigue cracks. In coarse grained specimens fatigue cracks were formed preferentially in the vicinity of the grain boundaries, then spread partly along the slip bands and partly along the grain boundaries. Both the nucleation and propagation of cracks in the fine grained specimens, however, were found only in the grain boundaries.(3) Observations with an electron microscopy disclosed that large numbers of microcracks were formed in the region where the first crack was nucreated. It is confirmed that the possible coalescence of random microcracks leads to fatal cracks.(4) The microcracks are formed at the tip of the growing crack by the shear strain field which precedes the crack propagation. When microcracks are numerous enough, they in turn coalesce into the cracks.(5) Observations on the surfaces have shown that substructure develops around the fatigue crack if the grain is favorably oriented, and crack propagation occurs preferentially along the subgrain boundaries.(6) In high-purity iron, extrusions and intrusions have been observed in slip bands, and deformation bands were also formed. It is suggested that the deformations in polycrystalline materials are much complicated because of the constraints of surrounding grains.