Study On The Formation Mechanism Of Non-equilibrium Solidification Structure Of Deep Undercooled Binary Alloy Melt

In this paper,the solidification microstructure of Ni₉₀Cu₁₀ and Ni₇₅Cu₂₅ binary alloy systems at different undercooling(?<300K)was obtained by using the methods of molten glass coating and cyclic superheating,and the mechanism of microstructure transformation was studied.The microstructure of typical solidification was studied by optical microscopy,electron backscatter diffraction and projection electron microscopy.At the same time,the mechanism of dendrite growth and two grain refinement during solidification is predicted theoretically by using BCT model.The solid-liquid interface morphology of supercooled alloy was studied by using a high-speed camera system,and the relationship between solidification rate and degree of undercooling was calculated in detail.For the Ni₉₀Cu₁₀and Ni₇₅Cu₂₅binary single-phase alloy systems,within the obtained undercooling range,the solidification structure of the two undercooled alloys has a similar transformation process,that is,"coarse dendrites?equiaxed crystals?fine dendrites?refined equiaxed crystal",both have the phenomenon of grain refinement at small undercooling(?<65K)and large undercooling(?>200K).However,the microstructure and dislocation angles of the two grain refinements are different,which indicates that the formation mechanism of grain refinement is not consistent.Under the condition of small undercooling,we predicted the undercooled Ni₇₅Cu₂₅alloy based on the dendrite remelting mechanism proposed by Li.The maximum ratio of dendrite remelting is in the range of the undercooling value obtained in the experiment.Therefore,it is believed that dendritic remelting is the main reason for the first type of grain refinement.However,under the condition of high undercooling,the stress-induced recrystallization mechanism proposed by Liu is considered to be the reason for grain refinement.In this regard,we have also made theoretical predictions,and found that with the increase of undercooling,the accumulated stress between dendrites is also increasing.At the same time,the hardness analysis of Ni₇₅Cu₂₅ shows that with the increase of the undercooling the hardness will show a relatively large decline,indicating that due to the occurrence of recrystallization,most of the stress energy in the crystal is consumed.Regarding the grain refinement phenomenon under high undercooling,in this experiment we have done more detailed electron backscatter diffraction analysis and detection,and it is concluded that the grains have straighter grain boundaries under high undercooling conditions.The small-angle boundary is transformed into a large-angle boundary,and the presence of annealing twins can be clearly seen,indicating that obvious grain boundary migration has occurred during the grain growth process.At the same time,the transmission analysis of the samples with high undercooling degree shows that there are residual dislocations and other defects in the samples,and the existence of annealing twins,which further indicates that there is stress accumulation leading to the occurrence of recrystallization.A high-speed camera was used to take a detailed picture of the changes in the solid-liquid interface of Ni₉₀Cu₁₀ and Ni₇₅Cu₂₅ alloys under different undercooling.The solid-liquid interface solidification front morphology is also different in different undercooling ranges.Moreover,according to the real-time pictures taken,the time of the entire recalescence process is obtained,and the solidification time of the alloy under different undercooling is calculated.It is found that the solidification rate increases with the undercooling in the entire undercooling range.When it reaches a certain critical undercooling(?T*>220K),the solidification rate will suddenly increase and then remain stable.