An In-Situ Study of Crack Propagation in Binary Lamellar TiAl
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An In-Situ Study of Crack Propagation in Binary Lamellar TiAl P. Wang, N. Bhate, K.S. Chan* and K.S. Kumar Division of Engineering, Brown University, Providence, RI 02912 * Southwest Research Institute, San Antonio, TX 78238
Abstract Single-colony thick compact tension specimens of binary lamellar Ti-46.5%Al were tested within a scanning electron microscope to examine the contribution of colony boundaries to crack growth resistance under monotonic loads. These specimens were obtained by machining slices from bulk material that had been heat treated to grow the colony size. Thus, the lamellae in adjacent colonies exhibit significant misorientation across the boundary. The orientation of the lamellae within a colony has been characterized in terms of two angles defined with respect to the notch orientation: an in-plane angle α and a through thickness angle β. The change in these two angles across the colony boundary quantifies the misorientation. In addition a third angle, θ, defines the colony boundary tilt to the vertical plane. These parameters were measured in several specimens and the crack growth resistance across the boundary was qualitatively and quantitatively characterized. The importance of the through-thickness angle β in providing resistance to crack growth is illustrated. Introduction Crack propagation studies have been conducted on polycrystalline and single-crystalline (PST crystals) lamellar two-phase TiAl/Ti3Al alloys with and without further alloying [1-4]. These studies have confirmed the presence of intrinsic (matrix slip and ductile phase toughening) and extrinsic (crack deflection, ductile-phase bridging, shear ligament toughening, microcrack shielding) toughening mechanisms in these alloys. Recently, Chan et al. [5] examined cracking of lamellar Ti-46.5%Al polycrystalline compact tension specimens in-situ in the SEM. They reported a low initiation K, predominantly interlamellar cracking within the colony with crack advancing by the linking up of microcracks, minimal number of microcracks ahead of the main crack, negligible resistance to crack growth within the colony, and noticeable resistance to crack growth across colony boundaries. They further showed that the magnitude of the resistance offered by the boundaries to crack growth was dependent on the lamellar misorientation (as measured on the surface) across the boundary. Frequently, crack renucleated across the boundary and advanced forward and backward to link up with the crack in the previous colony. Multiple cracking was also reported to occur within individual colonies. It is however unclear how much of this resistance as well as observed damage on the surface was a consequence of subsurface colonies and associated boundaries since the average colony size reported in [5] was 640 µm and the specimen thickness was 4.75 mm. It was therefore recognized that it would be beneficial to conduct such tests on specimens that were “single-colony thick” (but polycolony inplane) to more clearly establish the role of colony boundaries (or colony mis
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