A Fracture Crystallography and Anisotropy of Propagation of Microcracks Nucleated in Stainless Austenitic Steels after LCF

Abstract

The tested materials were structural austenitic CrNiNb steels with FCC structure. The working parts of standard cylindric samples were given square cross sections. After testing at different amplitudes of deformation, from the standard cylindrical samples washers were cut for TEM examinations. The slices for TEM were cut after rupture of cylindrical specimens' perpendicular to the fatigue load axis. Foils were prepared by standard method of electro polishing. TEM studies were performed with Zeiss EM-10CR microscope operating at 100kV. For the analysis of the obtained micrographs and diffraction patterns stereographic and trace analysis were conducted.After low-cycle fatigue tests, in the TEM samples, microcracks are formed in the places of stress concentration (caused by redistribution of stresses accumulated during fatigue tests). The nucleated microcracks are of different sizes. Here focus is made on microcracks with the lengths comparable with austenite grain diameter and less. Trajectories of microcracks in the samples of austenitic steels were studied by TEM methods after low-cycle fatigue tests. It is revealed that the direction of microcrack propagation changes at austenite grain boundaries as well as at substructure elements within the grain. At the same time, the microcrack remains parallel with one of the slip bands. Based on stereographic and trace analyses crystallographic planes of fracture and directions of microcrack propagation directions were determined. It is shown that the fracture plane is of {111} type, and propagation direction of <110> type. Analysis of the experimental data allowed making a conclusion regarding the reasons of microcrack propagation trajectory deviation within grain, and about crystallographic character of microcrack propagation direction along <110>.

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

“…The trace analysis and microdiffraction studies showed that the direction of propagation of microcracks is <110>. This directly indicates that the microcracks are initiated either in the slip bands or (on) along [8].…”

“…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]. …”

Introductory Chapter: Why Austenitic Stainless Steels are Continuously Interesting for Science?

“…The microcrack propagation process and its correlation with the steel microstructure are also quite disputable. Despite numerous experimental results on alteration of microcrack propagation direction, there is no convincing hypothesis explaining the real reasons for the process [19,20]. …”

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.…”