Deviations of Microcrack during Propagation in Thin Films of Austenitic Steel and Accompanying Accommodative Processes

Abstract

The work deals with the transmission electron microscopy (TEM) study of thin films of chromium-nickel Х18Н10 steel. The films were prepared from bulk samples after low cycle fatigue (LCF) tests. Focus was made on the processes accompanying propagation of small microcracks. Particularly, the microstructure changes near the crack tip were analyzed in terms of accommodation processes taking place during crack propagation, such as formation of slip bands, twins etc. The authors conducted crystallographic analysis of the defects formed during crack propagation in correlation with the reasons of their initiation and homogenous length of the slip bands. Thus, the reasons of microcrack deviation from the initial direction were determined. The research has shown that the most convenient microstructure variables in the austenitic crystals of polycrystalline sample, affecting the microcrack deviation, are microstructure, crystallography and the homogenous length of slip bands.

Study of Fracture Mechanisms at Cyclic Fatigue of Austenitic Steels Used in Nuclear Reactors

“…This is the evidence of the fact that the slip bands at initial stage of deformation are formed on large microstructure elements. At subsequent deformations slip bands in smaller elements come into action as well [9]. As it was pointed out above, deformation in the plastic zone flows heterogeneously.…”

“…Therefore, the crystallography of the material plays an important role in fracture process and propagation of micro-and mesocracks. Namely, crystallography of the material determines the location and nucleation process of microcracks [8,9]. On the scale of the entire microstructure, the changing crystallographic orientations of grains, naturally, can create inhomogeneous stress fields.…”

“…Naturally, during the growth of the microcrack, the possibility of its encounter with another slip bands is not excluded, depending on the level of the accumulated energy. The microcrack will either stop growing along the initial direction or propagate along the direction of the encountered slip band (direction of propagation), or, finally, upon the intersection with the encountered (intersecting) slip band, it initiates a new microcrack into this band, while itself will continue growing along the initial direction of propagation (Figures 6-13) [8,9]. …”

“…The thing is that this process also can depend on factors such as the size of inclusions and the magnitude of elastic stresses before the tip of the microcrack, etc. [9].…”

TEM Study of Microstructure Changes, Formation and Distribution of Slip Bands in Austenitic Steels after Low-Cycle Fatigue (LCF) Deformation - II

“…During the low cycle fatigue (LCF) deformation slip bands appear on the polished surface of the samples. The authors of [1][2][3][4][5][6][7][8] have shown that at LCF of FCC structures deformation is realized via slip through almost all the 12 slip systems of {111} <110> type. According to [2,6] a transversal glide of dislocations takes place in the slip bands, and the authors of [3][4][5]7] propose that microcracks form exceptionally along {111} planes.…”