in UFG pure Ti remains a good thermal stability below 400oC. 3.2 Decrease in hardness during annealing Hardness was measured in the UFG samples annealed at different temperatures, as shown in Fig 5. The hardness almost remained a constant below 300oC. However, the hardness dramatically decreased when the annealing temperature increased to above 400oC, especially 500oC. The hardness of sample annealed at 600oC is close to that of coarse-grained titanium. This means that the stability of mechanical properties could be kept in the UFG pure Ti below 400oC. Figure 5. Effect of annealing temperature on micro-hardness in UFG pure titanium The decrease of hardness in the UFG Ti could be attributed to the following two reasons. When the annealing temperature is below 500oC, the decrease of dislocation density is a predominant factor. While, as the annealing temperature increase to above 500oC, recrystallization and grain growth predominately contributed to the decrease of hardness. 4. Conclusions 1) The average grain size of UFG pure titanium slightly increased when the annealing temperature is lower than 400oC. The microstructure remains stable under 400oC. While the grain size dramatically increased as annealing temperature increased to 500oC or higher. Recrystallization and grain growth took place in ultrafine-grained Ti as annealing temperature is above 500 oC. 2) The rolling texture orientation of UFG pure Ti is {0002} and it disappears when the sample is annealed at 500oC and 600oC. 3) The hardness of UFG Ti slightly decreased when the annealing temperature was lower than
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