In situ observation of solidification velocity and refined structure transformation in nonequilibrium solidification of highly undercooled and single-phase alloys

In this work, the relationship between the critical undercooling ΔT∗ and nonequilibrium solidification velocity of single-phase alloys as well as the refined structure transformation mechanism have been fully investigated. The results show that the greater the solidification velocity of the alloy, the more solid phase can be produced at recalescence, and thus the greater stress can be accumulated in the structure, which makes grain refinement more likely to happen and the critical undercooling ΔT∗ smaller. There are higher misorientation, high-proportion ∑3 twin boundaries and high-angle GBs in refined structures at high undercooling, which indicates that recrystallization has happened in microstructures. With the increase of ΔT, the stress and plastic strains accumulated in recalescence increased rapidly. However, when the ΔT increased to exceed the critical undercooling ΔT∗, although stress was still increasing. The strain degree and hardness decreased suddenly. This fully indicates that when ΔT≥ΔT∗, the stress and plastic strains accumulated in recalescence were vanished by the system as the driving force during post-recalescence period, and the recrystallization happened in the microstructures. The results fully indicate that the stress-induced recrystallization has caused the refined structure transformation at high undercooling.