Study On Microstructures And Solidification Rate Of Highly Undercooled Ni-Cu Alloys

In this paper,Ni₈₀Cu₂₀ and Ni₉₅Cu₅ alloys were undercooled up to 300K by situ melting and arc melting combined with purifying glass and cyclic superheating.The microstructures evolution and transformation mechanism of Ni-Cu alloys were studied.And the dendrite growth in the undercooled melt was predicted by BCT model.The recrystallization mechanism of grain refinement under high undercoolings was verified by quenching and annealing.High undercooling solidification process of Ni₈₀Cu₂₀ and Ni₉₅Cu₅ alloys was photographed by means of purified glass and cyclic superheating combined with high-speed camera,and solidification rate was calculated.The microstructures of Ni₈₀Cu₂₀ and Ni₉₅Cu₅ alloys underwent four transformations within the measured undercooling ranges.The critical undercooling ranges for grain refinement of Ni₈₀Cu₂₀ alloys were 50K<?T<90K and?T>165K,respectively.The critical undercooling ranges for grain refinement of Ni₉₅Cu₅ alloys were 20K<?T<50K and?T>160K,respectively.The relationship between superheating and undercooling of Ni₉₅Cu₅alloys during rapid solidification calculated by Li's model showed that grain refinement under low undercoolings was due to dendrite remelting.The relationship between solid fraction and undercooling during high undercooling solidification calculated by Liu's model showed that grain refinement under high undercoolings was due to stress-induced recrystallization.The recrystallization mechanism of grain refinement under high undercooling was verified by quenching and annealing.Ni₈₀Cu₂₀ alloys quenched before recalescence,the rapid solidification structures could be preserved as far as possible.At the same time,after annealing,the equiaxed grains in the microstructures of quenched alloys increased obviously,and annealing twins appeared.In addition,the hardness of the annealed alloy decreased,the deformation region and sub-structure region decreased,the grain boundaries flattened,which indicated that recrystallization occurred.The appearance of annealing twins and sub-grains,the abrupt decrease of the hardness of Ni₉₅Cu₅ alloys in high undercooling ranges also indicated the occurrence of recrystallization.The solidification rate of Ni₈₀Cu₂₀ and Ni₉₅Cu₅ alloys increased with the increase of undercooling,and the plateau stage?within the range of moderate undercooling?and sudden change stage?critical undercooling?appeared in the graph of the relationship between the solidification rate and undercoolings.With the increase of solute concentration,the range of abrupt change of solidification rate increased.Within the undercooling range,the relationship between the calculated solidification rate and the undercooling satisfied the power function.Ni₈₀Cu₂₀alloyssatisfiedV=3.2681×10^-4?T^2.09182,Ni₉₅Cu₅alloyssatisfied V=2.52648×10^-4?T^2.14493.The variation of solute partition coefficient k with solidification rate of Ni-Cu alloys was calculated.At same solute concentration,k increased with the increase of solidification rate.When the solidification rate increased to the solute diffusion rate V_D in liquid phase,k increased to 1,and complete solute trapping occurred.In addition,with the increase of solute concentration,the solute trapping was more obvious at the same solidification rate.