Effect Of Bulk Undercooling On The Microstructure Evolution And Solidification Behavior Of Non-equilibrium Solidification Of Cu-based Alloys

Based on Cu₅₅Ni₄₅ alloy,the evolution of microstructure morphology of Cu-Ni alloy and Cu-Ni-Co alloy with undercooling was studied by increasing Cu content and adding trace Co element through molten glass purification and cyclic superheating technology.Combined with BCT model,the controlling factors of dendritic growth in undercooled alloy melts were analyzed,and the grain refinement mechanisms under different undercoolings were confirmed.Finally,the solidification behaviour of Cu-Ni and Cu-Ni-Co alloys were investigated by infrared thermometry and high-speed photography.At the same time,the effects of Co and Cu content on the microstructure,characteristic undercooling,critical undercooling,microhardness,and solidification velocity of Cu-Ni alloy were also studied.The microstructure of Cu-Ni alloy and Cu-Ni-Co alloy has the same evolution process with the increase of undercooling,and there was a grain refinement structure transformation process at low undercooling and high undercooling.The microstructure morphology of the alloy exhibits a transition from coarse dendrites to fine equiaxed grains at low undercooling,and there were a large number of high-strength textures in the refined structure,which was consistent with the characteristics of high thermal effect driving the microstructure dendrites to be remelted and broken.However,at high undercooling,the alloy exhibits a morphological transition from oriented fine dendrites to equiaxed grains,and a large number of twin structures and high proportion of high-angle grain boundaries in the refined structure were consistent with the microstructure refinement caused by recrystallization.The driving force of recrystallization was derived from the strong solidification shrinkage and thermal strain caused by the rapid solidification of the alloy melt,and the stress and strain generated by the liquid phase action on the dendrite skeleton.And it provides a strong experimental evidence for settling the debate on the refinement mechanism of alloy under different undercoolings.Combined with BCT model and metallographic analysis,the intrinsic factors of microstructure transformation of the alloy were studied.At low undercooling,solute diffusion dominates dendritic growth,but undercooled melts were limited to grow within a narrow range to form coarse dendritic morphology.The dendritic growth was gradually controlled by both solute diffusion and thermal diffusion with the undercooling increasing,and the dendrite remelting effect gradually increased.Dendrites formed during rapid solidification of undercooled melts with a lower undercoolings were remelted to form numerous crystal seeds,resulting in refined grains.In the medium undercooling range,dendrites grew in the direction of thermal diffusion and exhibited directional characteristics,resulting in stress accumulation on the dendrite skeleton.At high undercooling,the directional growth dendrites dominated by thermal diffusion underwent stress collapse,and the residual strain energy in the dendrite fragments provided a driving force for the occurrence of recrystallization in the microstructure.During rapid solidification,Cu-Ni and Cu-Ni-Co alloy melts undergone only once recalescence phenomenon.With the increase of undercooling,the recalescence degree increased approximately linearly,and a transition process of"small angle plane-sharp front-smooth arc"occured at the solidification front of the alloy melt.In addition,the solidification velocity and undercooling of the alloy satisfy a power function relationship.The increase of Cu content and the addition of Co had little effect on the microstructure transformation of Cu-Ni alloy.However,with the increase of Cu content,the critical undercooling for refining Cu-Ni alloys with high undercooling decreased,while the effect on the undercooling for refining Cu-Ni alloys with low undercooling was not significant.The addition of Co increased the characteristic undercooling of the microstructure transformation of Cu-Ni-Co alloy,but had little effect on the critical undercooling.At the same time,adding Co increased the microhardness of the alloy by about 5%,while increasing the Cu content reduced the microhardness of the alloy instead.In addition,increasing Cu and Co elements would reduce the solidification velocity of the alloy,and also weaken the recalescence effect of the alloys.