855 a Morphology

The processing route determines the a morphology, which can vary quite considerably within the P matrix. This morphology in turn strongly influences the mechanical properties [12]. Two basic processing options are available: (1) P processing, carried out completely above the P transus or in which the processing is high enough that very little a phase is present, or (2) a-P processing, carried out below the P-transus temperature in the presence of the a phase. Subsequent annealing below the P-transus temperature within about 175°C (315°F) of the transus temperature results in a distribution of primary a, which is related to the processing sequence and annealing temperature. With P processes material, a lenticular a morphology occurs, while with a-P processing (and a sufficient amount of deformation) the primary a-P becomes globular during the subsequent heat treatment (Fig. 8.4) [8, 17].

The change in morphology of a from lenticular to globular is a direct result of the prior deformation of the a. Sufficient strain energy in the a causes it to recrystallize or relax to a lower-surface-energy globular configuration. The transformation of lenticular a to globular a is a function of annealing temperature and time and the amount of working the a has received; that is, lightly worked a will remain essentially lenticular while a heavily worked a will become globular.

Strength is virtually unaffected by the shape of the primary a but other properties such as fracture toughness and elevated-temperature flow characteristics (particularly creep, superplastic forming, and diffusion bonding) are strongly influenced. High fracture toughness is associated with a having a high aspect ratio (i.e., lenticular), while lower fracture toughness values at the same strength

FIGURE 8.4 Microstructure of Ti-6Al-2Sn-4Zr-2Mo: (a) P worked followed by a-P anneal to produce lenticular a morphology, (b) a-P worked and a-P annealed to give predominantly an equiaxed a shape and (c) a-P worked followed by duplex anneal: just below the P-transus temperature [reduced volume fraction of equiaxed a compared to (b)], and significantly below the P-transus temperature (to form the lenticulary a between equiaxed regions) [8, 17].

level corresponded to a having a low aspect ratio (i.e., globular) [8, 12]. A similar trend occurs with fatigue crack growth rate. However, optimum superplastic forming and diffusion bonding is found in material with a globular microstructure; while creep performance is favored by lenticular a. Low-cycle fatigue behavior is optimized with a globular a morphology.

In P alloys, thermomechanical processing affects not only the microstructure but also the decomposition kinetics of the metastable P phase during aging. The increased dislocation density after working P alloys leads to extensive heterogeneous nucleation of the equilibrium a phase, which can suppress formation of the brittle w phase [12].

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