REGULARITIES IN THE MAIN-CRACK PROPAGATION AND DISLOCATION STRUCTURE EVOLUTION IN THE FRACTURE ZONE OF VT22 ALLOY AT VARIOUS FREQUENCIES OF CYCLIC LOADING

摘要

We have studied the relationship between the dislocation structure in the fracture zone and fractographic features of the main-crack propagation in a Ti-5 percent Al-5 percent V alloy tested for cyclic crack growth resistance in symmetrical tension-compression at frequencies of 140,600,3000, and 10000Hz. It is demonstrated that the prevailing types of dislocation structure are cellular over the near-threshold DELTA K range and of band-type structure for the remaining values of the stress intensity factor range. For these types of structure of the alloy studied at all loading frequencies, the characteristic micromechanism of fracture is the formation of fatigue striations. In the region of low DELTA K values, the above-mentioned types of substructure, and thus fatigue striations, are most commonly formed along certain crystallographic planes and directions. As the DELTA K values grow, the crack sensitivity to crystallographic orientation decreases. The effect of the loading frequency on the regularities and mechanisms of fatigue crack growth is governed by two main factors: the processes of plastic deformation at the crack tip during the pre-fracture period and the interaction between the crack front and the initial and formed structural and substructural elements. The appearance of the brittle-fracture elements with increasing loading frequency is due to a rather high sensitivity of the beta-phase of the loading rate.