Regularities of main-crack propagation and evolution of dislocation structure in the fracture zone of a VT22 alloy at various cyclic loading frequencies

摘要

A relation between the dislocation structure in the fracture zone and fractographic features of main-crack propagation was studied for a Ti-5%Al-5%V alloy upon cyclic crack-propagation resistance tests with symmetrical tension-compression loading with frequencies of 140 Hz, 600 Hz, 3 kHz, and 10 kHz. Honeycomb and band-type dislocation structures were demonstrated to prevail in the near-threshold region of the ΔK values and in the remaining region of the stress-intensity factor, respectively. The structure this type features the fracture micromechanism by forming fatigue striations, characteristic of the alloy studied. Over the range of low values of ΔK, substructures of the indicated types and thus fatigue striations are most often formed along certain crystallographic planes and directions. With an increase in the ΔK values crystallographic sensitivity of a crack decreases. The influence of loading frequency on the regularities and mechanisms of fatigue crack growth is determined by two basic factors, namely, the processes of plastic deformation at the crack tip during the preparation of a material to fracture and interaction of the crack front with the initial and formed structural and substructural elements. The occurrence of elements of brittle fracture when increasing loading frequency is explained here by an increased sensitivity of the fl-phase to loading rate.