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b5 e_

½5

where cb is the boundary energy, M is the boundary mobility, L is the mean slip distance for dislocation, G is the shear modulus, b is Burger’s vector, and K is a constant. The critical dislocation density for DRX increases with the increase of strain rate and is consistent with the inhibited DRX with the increasing strain rate as the strain rate is below 1 s1. While the DRX is also promoted by increasing the strain rate as the strain rate is above 1 s1, this can be attributed to the accelerated pile-up of dislocations. As a result, the qm is much easier to reach at high strain rate condition. Furthermore, a higher strain rate will result in the increase of dislocation density, which in turn will

Fig. 12—Schematic diagram for the effect of strain rate on DRX evolution by dislocation density variation. (The blue line represents the accelerating effect of increasing strain rate on DRX fraction, the red line represents the inhibiting effect of increasing strain rate on DRX fraction, and the green line represents the synthetic effect of strain rate on DRX fraction).

dramatically reduce the recrystallization temperature.[30] There is a competition between the raised critical dislocation density for DRX by high strain rate and the positive effect of high strain rate for DRX. As illustrated in Figure 12, the effects of the two factors are controlled by strain rate and a threshold exists in the competition. On one hand, the increase of strain rate can lead to the increase of critical dislocation density for the occurrence of DRX, which will definitely result in the inhibition of DRX as shown by the red line. On the METALLURGICAL AND MATERIALS TRANSACTIONS A

other hand, the increasing strain rate can facilitate the accumulation of dislocation, providing necessary local orientation gradient for the occurrence of DRX and resulting in the promotion of DRX.[13,35] The accelerating effect of strain rate for DRX is shown by the blue line in Figure 12. The evolution of DRX fraction with strain rate is the competition result of the two factors, leading to the fact that DRX fraction decreases first and then increases with the increasing strain rate as illustrated by the green line. Combined with the experimental results, the threshold is 1 s1 in this study. C. Multiple DRX Nucleation Sites Generally, the two major DRX nucleation mechanisms are discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), which can be affected by strain rate.[13,18] The previous work[1] has confirmed that the dominant DRX mechanism of alloy 617B is DDRX accompanied by CDRX as an assistant nucleation mechanism. Nucleation usually takes place in areas with high strain gradient and high dislocation density.[55,56] It has been widely accepted that grain boundary and triple junctions are the preferential sites for DRX nucleation[55,57,58]; however, it has been found in the investigation of Cu-Si bicrystal that such preference became weaker with the increase of strain rate.[46] Nucleation at grain boundary is induced