Uniform Fine Grain Structure Results

GRAM GROWTH RETARDED BYLOW STRAIN OF RECOVEREO REGIONS. MIXEO GRAIN STRUCTURE RESULTS'

* RECRYSTALLIZATION MAY BE DYNAMIC AT THE LOW TEMPERATURE SECA&E OF THE EXTREMELY HIGH LOCALIZED STRAINS, IS

EITHER METADYNAMIC OR STATIC AT INTERMEDIATE AND HIGH TEMPERATURES, t IN THE HIGH TEMPERATURE CASE THE LOCATION OF THE FINE GRAINS DOES NOT NECESSARILY COINCIOE WITH THE LOCATION OF THE ORIGINAL FINE GRAINS.

FIGURE 8.3 Restoration process by which strain in titanium grains is reduced during annealing treatment [12]:

However, it is much more constricted for the rich p alloys and for lighter amounts of deformation, making control of the P grains much more difficult in these richer alloys. The mechanism by which the restoration to a low strain condition occurs is suggested schematically in Fig. 8.3 [12]. In general, a fine P-grain structure is promoted by working below the P-transus temperature and then heating through the transus.

Recrystallization follows the typical sigmoidal behavior, which is a function of temperature and prior deformation. The rate of grain boundary migration decreases inversely with annealing time, indicating a concurrent recovery process obeying second-order kinetics.

Grain growth follows the relationship

D1/n - Dj/n = At, where D is the grain size after annealing at temperature for a time t, D0 is the apparent initial grain size at t = 0, and n and A are constants. Generally, the kinetics of grain growth are not influenced by the prior grain size or amount of deformation, unless both recovered and recrystallized grains are measured, in which case a critical growth phenomenon occurs at low deformation levels (7-12%) [12].

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