Fracture toughness of Widmanstatten colonies of an α-β titanium alloy

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The fracture toughness behavior of individual α+β Widmanstatten colonies of the Ti-8Al-1Mo-1V alloy in sheet form was investigated. Crack extension occurs predominantly across the α-β lamellae under conditions of plane stress and on planes which are inclined to both the thickness and the width of specimens. Crack tip plasticity in both single-colony and polycrystalline material is dominated by through-thickness deformation involving slip and often twinning. In the single-colony specimens the crack tip plasticity was characterized experimentally by identifying the active slip/twinning planes and by calculating the distribution of shear stresses on the possible deformation systems using shear criteria as defined on both microscopic and macroscopic scales. As suggested by the fracture behavior of the polycrystalline specimens, the fracture plane across individual colonies is near that slip or twinning plane that experiences the largest shear stress when the shear stress is a maximum on a macroscopic scale. The plane stress fracture toughness of individual colonies depends on colony orientation and on the nature of the deformation at the crack tip. High toughness of a colony is associated with multiple slip and twinning and with the absence of low energy fracture along or near interfaces such as twin boundaries or α-β interfaces. Conversely, slip localization into coarse basal slip bands coplanar with the crack results in the extension of the crack along such bands and thus in the lowest fracture toughness observed. Such fracture behavior can be readily understood in terms of an elastic-plastic model for crack advance along slip bands coplanar with a crack. © 1980.

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Materials Science and Engineering