Fracture behavior of binary lamellar Ti-46Al
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Crack propagation in a polycolony lamellar binary Ti-46Al alloy has been studied by experiments and computations. Fatigue precracked compact tension specimens were incrementally loaded to failure within a scanning electron microscope (SEM). The loading profile was correlated with the surface microstructure and evolution of damage on the surface. Within a colony, the crack advances along the lamellar orientation with minimum resistance, and often by the sequential linking of microcracks that occur ahead of the main crack. Colony boundaries in certain instances offer significant resistance to crack growth. In such instances, crack renucleation often occurs across boundaries, the ligament bridging the two colonies failing at a higher load subsequently; in addition, multiple cracking occurs within the colony. Extensive heat treatments were conducted to produce specimens that were single-colony thick but polycolony in-plane. Crack growth resistance across a colony boundary in such specimens clearly illustrated the importance of the through-thickness misorientation angle. Elastic and elastic-plastic plane strain calculations of crack growth were carried out on a compact tension geometry. The numerical analyses qualitatively capture several of the experimentally observed features such as repeated crack renucleation across a colony boundary, plastic deformation of residual ligaments and multiple cracking within a colony. Further, these 2D computations isolate the effect of in-plane misorientation of the lamellae across a colony boundary on crack growth resistance. However, the experimental results from the single-colony thick specimens identify the need to conduct 3D computations, that incorporate the through thickness lamellar misorientation across a colony boundary.