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The deformation and dynamic recrystallization behavior of Waspaloy-ingot material with coarse, columnar grains was established using isothermal uniaxial and double-cone compression tests. Testing was conducted along different test directions relative to the columnar-grain microstructure at supersolvus temperatures (1066°C and 1177°C) and strain rates (0.005 and 0.1 s Ϫ1 ), which bracket typical ingot-breakdown conditions for the material. The flow behavior of axial samples (i.e., those compressed parallel to the columnar-grain direction) showed an initial strain-hardening transient followed by steady-state flow. In contrast, the stress-strain curves of samples upset transverse to the columnar grains exhibited a peak stress at low strains, whose magnitude was greater than the steady-state flow stress of the axial samples, followed by flow softening. The two distinct flow behaviors were explained on the basis of the solidification texture associated with the starting ingot structure, differences in the kinetics of dynamic recrystallization revealed in the double-cone tests, and the evolution of deformation and recrystallization textures during hot working. Dynamic recrystallization kinetics were measurably faster for the transverse samples as well as specimens oriented at ϳ45 deg to the forging direction, an effect partially rationalized based on the initial texture and its effect on the input rate of deformation work driving recrystallization. Despite these differences, the overall strains required for dynamic recrystallization were comparable to those measured previously for fine-grain (wrought) Waspaloy. However, the Avrami exponents (ϳ2 to 3) were somewhat higher than those for wrought material (ϳ1 to 2), an effect attributable to the particle-stimulated nucleation in the ingot material.

The kinetics of homogeneous and heterogeneous static recrystallization in a single-phase material were analyzed using two-dimensional (2-D) and three-dimensional (3-D) cellular automata (CA). A CA model was developed, which was then validated using the theory based on relationships developed by Johnson and Mehl, Avrami, and Kolmogorov (JMAK) for homogeneous site-saturated and constant-rate nucleation. The model was then modified for heterogeneous nucleation at grain boundaries, with either a fixed number of nuclei or a constant rate of nucleation. The fraction of boundary sites nucleated, for the case of fixed nucleation, varied from 0.006 to 0.28, resulting in Avrami exponents (k) ranging from 1.8 to 1.1 (site saturation). Site saturation with fixed nucleation produced a lamellar microstructure. The parameters of q and m, from Vandermeer's microstructural path method, were calculated and compared with theoretical values. Constant-rate nucleation at grain boundaries between newly recrystallized grains and the unrecrystallized matrix resulted in k values of Ϸ1. Simulated microstructures revealed that with a low nucleation rate, recrystallized grains formed in clusters, while a high nucleation rate resulted in a necklace microstructure, with kinetics similar to those observed in dynamic recrystallization (k ϭ 1.4).

The occurrence of cavity initiation and gross, free-surface fracture during subtransus hot pancake forging of Ti-6Al-4V with a transformed beta (colony) microstructure was established. Cavity initiation mechanisms were one of two distinct types. At temperatures approximately 75 ЊC or more below the beta transus temperature (T ␤ ), cavity initiation occurred at relatively low strains in the beta phase lying between the grain-boundary alpha phase and the lamellar colonies. By contrast, at temperatures near the transus (i.e., T Ϸ T ␤ Ϫ25 ЊC), cavity initiation occurred at much larger strains as a result of microfracture of partially-to-fully globularized alpha phase. Finite element method (FEM) modeling of the pancake forging process revealed that secondary tensile stresses had been developed in the regions that had exhibited cavitation/fracture. The FEM analyses were used to correlate both the cavity initiation and the gross free-surface fracture results to previous observations from uniaxial hot tension tests in which identical damage mechanisms had been observed. The tensile work criterion of Cockcroft and Latham (C ϩ L) gave moderately good (quantitative) correlation between the forging and uniaxial tension behaviors. An alternate comparison based on the Rice and Tracey cavity growth model gave reasonable predictions of free-surface fracture but tended to overestimate the incidence of subsurface cavity initiation.

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