and 600oC respectively for 20 minutes in the VTHK-350L vacuum furnace under 10-3 Pa. The heating rate was 8-10oC/min. The samples of 30mm×30mm were cut along rolling direction of plates for microstructural examination and micro-hardness measurement. TEM foils were thinned to about 70-80μm by mechanical grinding, and then electronically thinned with a twin jet electropolisher at 25V and -30oC. TEM examinations were carried on with a JEOL JSM-200CX TEM at 160KV. Texture was determined using an X’Pert PRO XRD equipped with a Cu-Kα ray source. Micro-hardness was measured in Tukon 2100B tester with a load of 20g holden for 10 seconds. 3. Results and discussion 3.1 Microstructure evolution of UFG Ti at different annealing temperatures The optical microstructure in UFG pure Ti samples subjected to anneal at different temperatures is shown in Fig. 1. The as-rolled deformed microstructure remained (Fig.1(a)) and grain boundaries became obscure when the annealing temperature was below 400oC, as shown in Fig. 1(b) and (c). However, as the annealing temperature increased to above 500oC, equiaxed grains were observed, as shown in Fig. 1(d) and (e). Furthermore, the grain size increased with the annealing temperature. The average grain size increased to 3μm and 7μm, respectively, when the annealing temperature increased to 500oC and 600oC. Figure 1. Optical microstructure in UFG Ti samples 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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