854 Microstructural Development

In addition to chemistry, the mechanical properties of titanium alloys are strongly influenced by the microstructure [12]. In turn the microstructure is critically dependent on the processing, particularly whether this is carried out above or below the P-transus temperature, the temperature below which the a phase is stable. In general terms two microstructural features are important in commercially important terminal alloys: (a) the P-grain size and shape and (b) the morphology of the a phase within the P grains. Similar features strongly influence the properties of the intermetallics, but a discussion of these features is beyond the scope of this chapter, the interested reader is referred to [13-16].

P Grains Control of the P-grain size is dependent on two factors: recrystal-lization (when this occurs because of sufficient working) and subsequent grain growth [12]. A number of techniques have been developed for recrystallization of the P grains in a and a-P alloys by working followed by high P-field annealing.

The metastable P alloys require careful thermomechanical processing to achieve the required final microstructure. This controlled processing involves, first, the worked or recrystallized condition and, then, if recrystallized, the grain size. Under most melting conditions, the structure that occurs in an ingot ranges from small equiaxed P grains at the surface, to elongated columnar grains, to large equiaxed grains at the center of the ingot. Recently, it was shown that there is a supratransus "processing window" through which the alloy can be taken to result in a final fine equiaxed P-grain structure [12]. This processing window is relatively wide for the leaner (in P-stabilizer content) alloys and for heavy amounts of deformation.

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