Nonequilibrium Solidification, Grain Refinements, and Recrystallization of Deeply Undercooled Ni-20 At. Pct Cu Alloys: E
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IN 1959, Walker first found the grain refinements occurring in the rapid solidification of deeply undercooled nickel melt. He found that the grain size would abruptly refine when the initial undercooling DT prior to nucleation exceeded a critical value DT*. In addition, he found that the decrease of the grain size was also accompanied by the emission of sound waves. Thus, he inferred that the copious homogeneous nucleation induced by the pressure pulse generated from the collapse of shrinkage cavities could result in the grain refinement,[2] which was supported by the studies of Horvay[3] and Jackson et al.[4] Powell[5] also found the existence of a critical undercooling DT* when he investigated the microstructural evolution of Ag-, Cu-, and Ni-based alloys as a function of undercooling. However, he suggested that the recrystallization process [1]
XIAOLONG XU is with the College of Materials Science and Engineering, North University of China, Taiyuan, Shanxi 030051, P.R. China, and also with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P.R. China. Contact email: [email protected] HUA HOU and YUHONG ZHAO are with the College of Materials Science and Engineering, North University of China. FENG LIU is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University. Manuscript submitted February 11, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
during recalescence and postrecalescence periods would be responsible for the grain refinement.[5] Herlach[6] measured the dendrite growth velocity in many undercooled melts, and he found that DT* was correlated to a critical crystal growth velocity, which was about 20 m s1 and was the solute diffusion velocity VD in the undercooled melts. In recent years, grain refinement occurring at undercoolings much lower than DT* has attracted much attention.[7–9] Karma[10] has proposed a physical mechanism, which suggests that the breakup of dendrites due to remelting brings about the grain refinement both at low and high undercooling regimes. However, this model only considered the remelting due to liquid/solid interface tension, ignoring the effect of the chemical superheating[11] during recalescence, which may also play a much more important role in the dendrite remelting and have a stronger influence on the final grain morphology. This study investigates the remelting mechanism and stress-induced recrystallization mechanism of the two kinds of grain refinement events. Thus, other mechanisms are ignored. The stress-induced recrystallization mechanism can explain the grain refinement occurring at high undercooling regimes.[7–9] For a deeply undercooled alloy melt, partial melt will transform into solid during the recalescence, i.e., a period under the nonequilibrium conditions, where the stress-induced dendrite distortion and breakup could occur.[9] The residual melt will also subsequently change into solid during the postrecalescence, i.e., a period under quasi-equilibrium conditions.
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