A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 1 6 (a) be inhomogeneous due to the presence of neighboring hard colonies, the shape of the lamellae per se, and other such factors. Thus, it is quite common to see microstructural features such as kinked lamellae, lamellae that have rotated to Fig. 2 — Microstructure evolution during thermomechanical processing of α/β titanium alloys: (a) as-cast ingot; (b) after recrystallization; and (c) after spheroidization of the lamellar αmicrostructure via α/β hot working[6]. (b) (c) as vacuum arc or electroslag melting/ remelting, electron beam melting, or plasma cold hearth melting to control interstitial content and remove high-density/low-density inclusions. However, slow cooling following solidification leads to coarse, columnar grains that are multiple millimeters in length and diameter (Fig. 2a)[6]. A number of hot working and heat treatment steps are subsequently performed (principally in the high temperature, single phase bcc β field) to obtain billet or slab with recrystallized, equiaxed β grains, typically with a size of ~2-3 mm in diameter (Fig. 2b). Following β recrystallization, the relatively slow cooling associated with the thermal inertia of large-section workpieces results in the decomposition of each β grain into a microstructure comprising a number of ~0.5-to-1.5-mm- diameter colonies of hcp α lamellae (Fig. 2b), each of which has its own crystallographic orientation relative to a specified set of reference coordinates such as the radial and axial directions of the cylindrical billet. Due to a Burgers orientation relationship (BOR) between the high temperature β phase and low temperature α phase, the number of possible orientations of the α colonies (also called α variants) within a given β grain cannot exceed 12, and typically lies in the range of 3-10. Each colony of α lamellae formed during cooling from the β field can be thought of as a nascent MTR, or a region in which all of the α phase has the same (or nearly the same within a specified tolerance limit) crystallographic orientation. The objective of subsequent hot working steps performed in the α/β phase field is therefore twofold: (1) break down each colony to develop a uniform, fine, equiaxed structure of globular α particles within the β matrix (Fig. 2c), and (2) randomize the orientation of each α particle relative to its neighbors to minimize (or eliminate) the extent of MTRs. Accomplishing these objectives can be quite difficult due the plastic anisotropy of the hcp α phase. The anisotropy translates to a sizeable difference (of the order of three times) between the material flow stress when deformation is imposed along the c-axis (i.e., the normal to the closepacked planes of the hcp Ti crystal) versus perpendicular to it, and this results in very inhomogeneous deformation. “Soft” colonies (having c-axes oblique to the forging direction) undergo large deformation, and “hard” colonies (with c-axes parallel or nearly parallel to the forging direction) suffer relatively small strains (Fig. 3)[7]. Depending on the hot working tem- perature, the difference in local stresses and strains may lead to generation of deleterious cavities between the harder and softer colonies (Fig. 4)[8-12]. Equally important, the hard oriented, less deformed colonies may retain their nature as relatively equiaxed microtextured features. By contrast, deformation within the softer colonies can be relatively large overall, but it tends to Fig. 3 — EBSD compression axis, inverse pole figure map for a region in a Ti-6Al-4V pancake forging illustrating the variation in deformation among colonies with hard orientations (red) and soft orientations (other colors). Courtesy of T.R. Bieler.
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