TEM examination shows the variety of deformed substructure and grain size with the annealing temperature, as shown in Fig. 2(a) to (e). The ultrafine-grains with a high density of dislocations were produced in the as-rolled samples (Fig.2(a)). The less well-defined grain boundaries and high density of dislocation tangles were the predominant features in the samples subjected to anneal below 400oC, as shown in Fig.2(b) and (c). High density of dislocation cells were formed at 300oC (Fig. 2(b)). Grain boundaries became distinguishable at 400oC (Fig.2(c)). When the annealing temperature was higher than 500oC, well-defined grain boundaries were observed. Most dislocations in the interior of grains disappeared, as shown in Fig.2 (d) and (e). The diameter of grains in the sample annealed at 400oC is larger than that annealed at 300oC. Grain obviously grew and grain size significantly increased as the annealing temperature increased to 500oC and 600oC. The effect of annealing temperature on grain sizes of UFG Ti are summarized in table 1. There is not an obvious increase of grain size under 400oC. It indicates that the deformed microstructure remained stable under 400oC. However, grains apparently grow when the annealing temperature increased to 500oC and 600oC. In other words, microstructural stability was lost over 500oC. Figure 2 . TEM micrographs in UFG pure titanium annealed at different temperatures: (a) as-repeat rolled; (b) 300oC; (c) 400oC; (d) 500oC; (e) 600oC. (a) (c) (b) (e) (d)
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