Structural Evolution Of Ni-Sc Intermetallic Compound Under Supercooling Conditions

It is the key to prepare polycrystalline intermetallics with a single-phase for their essential performance studies.Therefore,Ni-50at%Sc,Ni-51at%Sc and Ni-51.92at%Sc alloy were prepared with a vacuum arc smelting and water cooled copper mold suction-casting machine.The microstructure evolution of the rapidly solidified alloy mainly consisted of B2-NiSc phase was analyzed by way of an optical metalloscope and a scanning electron microscope.XRD was used to identify phase microstructural components,and further more the composition of each phase was analyzed with EDS.Microstructure evolution mechanism of the alloy by rapidly solidified was analyzed by BCT dendrite growth model.The cooling rates for the solidified specimens with diameters of 2,5 and 8 mm were 3112,497.9 and 194.5 K·s-1,respectively.It is found that Ro = 3/2 mm is a critical dimension which corresponds to an abrupt change in solidification rate.It is also found that eutectic mixture structure of(Ni2Sc+NiSc)eutectic was dispersed at grain boundary or between dendritic arms due to the loss of Sc element during melting.While R > Ro(corresponding to the specimens with diameters of 5 and 8 mm),solidification structure consisted with coarse B2-NiScdendrite and(Ni2Sc+NiSc)eutectic.While R < Ro(corresponding to the specimens with diameter of 2 mm),solidification structure consisted of fine globular B2-NiScdendrite and relatively small amounts of(Ni2Sc+NiSc)eutectic.Microstructures underwent 970°C×72 h homogenization heat treatment of the rapid solidified specimens were analyzed.Spherical Ni2 Sc particles were dispersed on the B2-NiSc matrix for the specimen with 2 mm diameter,but the second phase Ni2 Sc was in plate shape for the specimens with 5 and 8 mm diameters.Therefore,it could be concluded that B2-NiSc intermetallics without any second phase was difficult to achieve under the condition of sub-rapid solidification.According to the interface theory of Jackson,the value of Jackson factor of B2-NiSc intermetallics was ? = 0.5 < 2,so the phase interface was rough.This was the reason alloys were dendrite growth and non-faceted phases.Based on the phase volumetric analyzing of the microstructure with an Image-Pro Plus software,the loss ofSc element during melting before and after homogenization heat treatment were about3.20~3.71at% and 2.56~2.92at%.It was performed by adopting crystal defect model with cavity and the first principle calculation based on density function theory.The reason was that the Ni vacancy defects were easier to form,and the number of Ni vacancy defects in alloys was larger than Sc.A lot of vacancy defects were produced at high temperatures and preserved under supercooling conditions,then vacancy defects decreased due to the diffusion of atoms after homogenization heat treatment.The results of BCT dendrite growth model showed that thermal undercooling replaced constitutional undercooling and acts as the main contribution in the dendrite growth with the increasing of undercooling.Supercooling degree within the scope of the experiment,constitutional undercooling still acts as the main contribution.With the rapid cooling ability enhancement,the granulation mechanism of the granular grains was owing to the dendrite breaking-up and recrystallization.