MICROCRACK PROPAGATION IN Ti-6Al-4V ALLOYS

摘要

In this study, microcrack propagation in Ti-6Al-4V was examined in regions just above and below the fatigue limit. Two different Ti-6Al-4V alloys with nominally similar chemical compositions were examined: the first alloy was thermo-mechanically treated in the laboratory to develop a bi-modal and a fully lamellar microstructure and the second alloy is from the High Cycle Fatigue MURI program (forging #197) containing a duplex microstructure. Microcracks were nucleated in smooth specimens of each microstructure in the LCF regime during tests conducted at R = -1 (R =σ_(min)/σ_(max)) so that internal stresses could be minimized. These samples were then tested in the HCF regime to determine crack propagation rates for R values of -1, 0.1, and 0.5. In addition, conventional fatigue crack propagation curves for macrocracks using CT-specimens were measured. Comparisons in microcrack growth behavior were evaluated and discussed for the bi-modal and fully lamellar microstructures, for bi-modal and forging #197 microstructures, and for forging #197 examining the influence of R = 0.1 and 0.5. Much of the observed behavior appeared to be explained by differences in crack front roughness.