Study On The Mechanism Of Non-equilibrium Solidification Microstructure Transformation Of Bulk Deeply Undercooled Ni-Cu-Co Ternary Alloy Melt

Using the molten glass purification combined with cyclic superheating technology,the microstructure evolution of Ni-Cu-Co alloys under different undercooling was studied by adding Co element on the basis of Ni-Cu system.The solidified dendrite growth and grain refinement mechanisms of the alloy in different undercooling ranges were systematically explained from the perspective of theory and experiment.At the same time,the effects of the increasing Co content on the microhardness and non-equilibrium solidification of Ni-Cu-Co alloys were studied by using high-speed camera,infrared detection and microstructure characterization,including solidification velocity,recalescence effect and refined critical undercooling.There are two refinement microstructure transformations in the undercooling range of Ni-Cu-Co alloys.Under small undercooling,it shows the transformation from coarse dendrite to equiaxed grain and there are many structures with similar orientation This is consistent with the fragmentation and refinement driven by high thermal effect,so it conforms to the dendrite remelting mechanism.Under high undercooling,it shows the transformation from directional fine dendrite to equiaxed grain with smaller grain size.The microstructure contains a high proportion of high angle GBs and TBs,so it conforms to the recrystallization mechanism,and the recrystallization driving force comes the stress and plastic strain accumulated by the primary solid in the recalescence after the critical undercooling?T*.The solidified dendrite growth in the undercooling range was studied by combining BCT theory with the solidification velocity under high-speed camera.At small undercooling,the solute diffusion has a strong control effect and the dendrite tip radius is large.With the increase of the undercooling,the increasing solidification velocity has improved solute trapping effect,and the increasing thermal diffusion has weakened the control effect of solute diffusion,and the dendrite tip radius has decreased rapidly.This can significantly reduce the solidification segregation of the melt,and make the solidification gradually evolve from rough interface to smooth interface of uniform migration.Meanwhile,the evolution law of“coarse dendrite?directional coarse dendrite?directional fine dendrite”appeared in the microstructure.With the increasing Co content,the solidification velocity and recalescence effect in undercooling range gradually improve,which can significantly affect the refined structure transformation and microhardness of the undercooled alloy.Firstly,the dendrite remelting fraction in rapid solidification is positively correlated with the maximum recalescence temperature T_R.The improvement of recalescence effect is accompanied by the increase of temperature T_R,which can significantly enhance the remelting effect at the same undercooling,so that the critical undercooling?T₁ gradually decreases,?T₂ increases gradually.Secondly,the improvement of solidification velocity and recalescence effect can significantly increase the thermal strain and solidification shrinkage in recalescence,thus inducing more liquid flow to interact with primary dendrite and accumulate greater stress.This results the structure tend to occur the grain refinement and critical undercooling?T*decreases gradually.At the same time,the improvement of solidification velocity can significantly aggravate the spontaneous deformation effect induced by stress,and increase the Low angle GBs in the microstructure and decrease the grain size obviously.When Co content reaches 6%,the hardness of the alloy in the whole undercooling range increases by more than 100%compared with the as-cast.