Research of the crack development process in a nickel heat-resistant alloyin the process of testing

A method of direct observation of the process of low-cycle fatigue crack development during bend loading directly in the chamber of a scanning electronic microscope is described in the paper. It was found that the presence of slip bands near the apex of the crack is the determining factor of crack development in a specimen made of a nickel heat-resistant alloy with a monocrystalline structure. The fracture in the area of the crack apex was found to be of a brittle nature. In the case of crack propagation arrest before an obstacle an area of plastic deformation forms at some distance from the apex of the crack, with branches from the main crack.  Crystallographic orientation of the specimen was determined using the X-ray structure analysis and the Shmid factor was calculated for glide planes along which the fracture took place and secondary cracks were formed. The prevailing direction of the crack development was determined by slip systems with the value of Shmid factor maximum for the given conditions. It is shown that the described method of observation of the material structure and crack propagation under low-cycle fatigue makes it possible to obtain unique information about the fracture pattern and the impact of the material’s structure on its characteristics. The results can be used to determine the choice of the most favorable crystallographic orientation of the monocrystalline structure of the material of gas turbine engine blades relative to their geometry.