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 7 that have rotated to lie along the primary metal flow direction with little or no spheroidization or orientation randomization may form especially long and deleterious MTRs (Fig. 5)[13]. HEAT TREATMENT CONSIDERATIONS During static heat treatment following hot working, remnant stored work (dislocation substructure) may promote static spheroidization without additional changes in the misorient- ation of the α particles in a prior colony and thus result in little mitigation of MTR characteristics. The severity of MTRs associated with spheroidized (but not randomized) α particles can be exacerbated by the tendency of surrounding secondary α plates with a similar orientation to form in the β matrix during cooling following hot working or final heat treatment[3-5]. Such a tendency can be mitigated somewhat by imposing a high cooling rate following hot working or heat treatment to develop multiple α variants within the β grains surrounding each α particle[14]. A second heat treatment step at a lower temperature may then be applied to coarsen the secondary α produced in the first step[15]. It is worth noting that such a condition can only occur if the β phase does not recrystallize dynamically during deformation or subsequent static heat treatment and hence retains a long range common orientation (denoted as “β microtexture” in Fig. 1). Begley et al.[16] hypothesized that this could occur under two conditions: (1) α and β phases co-rotate during deformation such that the orientation relationship is preserved, or (2) where very little deformation occurs such that there is insufficient strain to cause recrystallization. Moreover, extended recovery processes that occur during near but sub-transus annealing can result in substantial changes to the β phase texture[17]. A great need exists for additional research in this area because β microtexture is believed to be a key contributor to dwell fatigue, and it has also been implicated in abnormal grain growth in β annealed structural titanium forgings. lie along the direction of primarily metal flow, and regions of very high strain near β grain boundaries. High local strains within the soft colonies lead to the development of dislocation walls within individual α lamellae, which promote spheroidization during deformation (i.e., dynamically), especially if a multiplicity of slip systems has been activated. Further, the combination of high local strains, evolution of high angle boundaries (associated with dislocation walls), and strain gradients may serve to rotate individual portions of long lamellae to different degrees and thus result in both spheroidization and randomization of the orientation of the resulting α particles, thereby eliminating MTRlike features, at least locally. On the other hand, lamellae within a given prior colony (or several adjacent colonies) Fig. 4 — EBSD inverse pole figure maps for Ti-6Al-4V samples with an initial colony—a microstructure in which cavities developed during hot deformation at 1089 K (815°C) via (a) uniaxial tension[9], (b) pancake forging[10], or (c) torsion testing[11]. Hexagons indicate the orientations of hard and soft colonies adjacent to some of the cavities. Fig. 5 — EBSD radial direction, inverse pole figure maps illustrating (red) microtextured regions in a 209-mm-diameter Ti-6242 billet at various radial locations[13]. (a) (b) (c)
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