Fundamental aspects of fatigue and fracture in a TiAl sheet alloy

Abstract

The fatigue mechanisms in a TiAl sheet alloy, heat treated to the lamellar and equiaxed microstructures, were studied to determine the effects of microstructure on the initiation of microcracks and their subsequent growth into large cracks. The nucleation and growth history of individual microcracks were followed. For comparison, fatigue crack growth and fracture toughness were also characterized using specimens containing a machined notch with a fatigue precrack. The results indicated that microcracks initiated at grain/colony boundaries and at slip bands. Most microcracks were arrested after nucleation, but a few grew at stress intensity ranges below the large crack threshold. The populations of nonpropagating and propagating cracks varied with life fractions. Ligaments in the wake of a fatigue crack were more severely strained than the crack-tip region of the main crack, and, as a result, they were more prone to fatigue failure. The destruction of the crack-wake ligaments is expected to result in lower fracture resistance in materials under cyclic loading than those under monotonic loading.

Microstructure Evolution of a Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y Alloy during Massive Transformation and Subsequent Annealing

“…Based on this understanding, the traces labeled by the black arrows are actually the previous grain boundaries of a high temperature α grain. After careful observation of the traces in Figure 2, one can realize that the grain sizes of the α grains during 1340 • C heat treatment are similar, i.e., approximately 200-300 µm in both images, which is in accordance with other reports [31,32]. This finding can further lead to the conclusion that the growth rate of α grains may decrease with the increase of time.…”

“…The results are shown in Figure 4. Similar to that reported in other TiAl alloys [27,32], the massive γ phase is full of various defects, including numerous stacking faults, anti-phase boundaries, and dislocations, as displayed in Figure 4a. The density of defects in some regions of massive γ is so high that it is difficult to find a local area free of defects.…”

Statistical simulation of small fatigue crack nucleation and coalescence in a lamellar TiAl alloy

“…For stresses above the fatigue limit but below the THE fatigue crack growth curves of TiAl alloys generally large crack ⌬K th , large fatigue cracks would not propagate exhibit a very steep slope. [1][2][3][4][5][6][7] This characteristic poses a but some small fatigue cracks could propagate to failure potential problem for the design and application of TiAl even though most of the small cracks nucleated may arrest. [7] components because of inadequate fatigue resistance.…”

“…[1][2][3][4][5][6][7] This characteristic poses a but some small fatigue cracks could propagate to failure potential problem for the design and application of TiAl even though most of the small cracks nucleated may arrest. [7] components because of inadequate fatigue resistance. Two Thus, it is evident that the stress states above the fatigue possible approaches can be taken to ensure a sufficient limit and ⌬K th lines in Figure 1 are regions where fatigue fatigue life in TiAl alloy components.…”

“…In such a plot, the large crack fatigue threshold is levels below the fatigue limit, they might eventually coalesce represented by a straight line with a slope of Ϫ1/2, as shown to form a larger crack and propagate, even though most were in Figure 1, which is constructed for a lamellar TiAl alloy unable to grow after nucleation. [7] Thus, a potential fatigue based on previous data. [6,7] In contrast, the fatigue limit is failure mechanism in this region is the nucleation and coalestypically represented by a horizontal line since it is indepencence of contiguous microcracks that would not propagate dent of the crack size.…”