Study On The Microstructures And Properties Of Rapidly Solidified Ni-Sc Intermetallics

In recent years,intermetallic compounds are considered to be the new materials with the important application value in the field of aviation materials and high temperature structural materials,and it gets more and more attention and favors by the material industry.NiSc is the intermetallic compounds with B₂ structure.There is no research on the undercooled B₂ organization evolution and the correlation between organization and toughness.And it lacks awareness of B₂ rapid solidification mechanism,the relationship between the organization evolution and toughness.And there is even no research on toughness B₂ compound non-equilibrium solidification research work.Based on the experimental study and theoretical analysis,it explores the B₂ preparation mechanism under the condition of different undercooling,and analyzes its organizational evolution.On the basis of microstructural research,the intrinsic properties of B₂-NiSc are revealed after analyzing and testing the mechanical properties of the material,and calculating its physical properties through first-principles.Studies show that the probability of obtaining homogeneous single-phase B₂-NiSc is very low under the condition of quench rapid solidification,and it is found that eutectic mixture of?Ni₂Sc+NiSc?is dispersed at grain boundary or between dendritic arms due to the loss of Sc element during melting.With the increase of solute content in alloy the microstructure is mainly composed of single-phase B₂-NiSc,but there are still traces of the second Ni₂Sc phase.The suction casting size or thickness of the thin plate specimens has a great influence on the cooling rate and the microstructure of Ni-Sc alloy.The smaller the suction casting size or thickness of thin plate specimens,the faster the cooling rate,the finer the microstructure and the fewer the eutectic mixture.After quenching and rapid solidification,the microstructure of the rapidly solidified structure disappears due to the solid state diffusion and the formation of the new grain.The rate of solidification has a significant effect on the heat treatment organization.The smaller the diameter of the alloy or the thickness of the thin plate specimens,the greater the cooling rate and the more obvious granulation.After the heat treatment to the thin plate specimens thickness of 2.50 mm of Ni-50%Sc alloy with 940?/3¹5 d at different times,it is found that with the extension of thermal treatment time,the Ni₂Sc phase dispersed in the matrix becomes more diffuse and the amount increases gradually.The Ni₂Sc phase volume fraction decreases with the increasing of heat treatment time and then increases.The tensile strength and hardness increase with the increasing of heat treatment time and then decrease.The optimum properties can be achieved with heat treatment temperature 940?and holding 9 d.And the ultimate tensile strength and hardness of NiSc alloy reach the maximum value of 131.85MPa and 63.26HRC,respectively.According to the interface theory of Jackson,the B₂-NiSc intermetallic grows in dendrite characteristics.The calculation result is consistent with the microstructure observation.Ni-50%Sc alloy can realize undercooling of?T=170k processed with high vacuum sealed crucible combining with molten glass purification method.This undercooling can change its growth model,but cannot realize completely solute intercept and therefore cannot suppress the precipitation of second-phase.When the melt undercooled to?T=110K,the microstructure consisted of B₂-NiSc and?Ni₂Sc+NiSc?_eutectic.When the melt undercooled to 170K,the microstructure consisted of granular grains.However the unidirectional growth feature can be observed.Therefore it is clear that?T=170k is corresponding to the critical supercooling of the secondary granulating?T₂^*as in the one in solid solution alloys.The dispersed second phase in the form of divorced eutectic was find in the deep undercooling rapid solidified Ni-50%Sc alloy.Because Sc is highly active,single phase B₂-NiSc intermetallic compounds are unable to prepare by using undercooling method in our experiment.The solidified microstructure is B₂-NiSc+?Ni₂Sc+NiSc?_eutectic.The alloy prepared under 170K undercooling has good mechanical working property.After rolling working,the properties along the rolling direction are about 70%of that in the perpendicular direction.It can be reasonabley deductived that the grain boundary and second-phase on the grain boundary have effective affect on the performance of the material.During compress,specimen's deformation is mainly done by the movement and rotation of grain boundary and the slip within the grain.A large number of intracrystalline slip band and sliding steps show that NiSc with B₂ structure has the very good plasticity.According to the BCT model calculation,in obtaining deep undercooling?T 170k and during the growth process of the intermetallic compounds,kinetics undercooling is the most controlling one,and the diffusion of the solute is the second main factor to control the growth,reveals the intermetallic compounds undercooling growth with the main difference between solid solution alloys.With the increasing of undercooling,the dendrite tip radius?R?decreases sharply.37.2K??T₁^*?is the negative unidirectional critical undercooling of Ni-46.76%Sc.When the undercooling??T?is between 58.8K and 63K,B₂-NiSc dendrite grows in accordance with Ni-50%Sc.If the undercooling is larger than 63K,during the growth process of dendrite,B₂ intermetallic compounds will deviate from the strict linear composition,and form intermetallic compound of V_Ni and V_Sc vacancy and Ni_Sc anti-site.Under extremely cold conditions,according to the results of first-principles calculation,the formation of Ni vacancy and the formation of double Ni vacancy is the easiest and stable.When Ni vacancy is formed,redundant Ni atoms can be accumulated to other positions or form anti-site atoms,finally form Sc-poor intermetallic compounds.Moreover,the formation of double Ni_Sc can also reduce the energy of the system,through the Ni part of the Sc to form Sc-poor intermetallic compounds.We used first-principles to calculate the physical properties.It is found that intermetallic compounds of Ni-Sc,NiSc,Ni₂Sc and Ni₅Sc are all stable.Among them,Ni₅Sc is most easily generated during solidification,while NiSc is the most difficult one to generate.Ni₅Sc is with the highest hardness,and Ni₂Sc the lowest.NiSc and Ni₅Sc is ductile,and Ni₂Sc is brittle.Ni₅Sc has the best plasticity and Ni₂Sc the worst.The contribution of Ni and Sc to the state density is mainly the d-orbit of Ni and Sc,followed by the s-orbit of Ni and the p-orbit of Sc.The point defects of B₂-NiSc intermetallic compounds are Ni vacancy defect and Ni anti-site defect,and that take the form of double vacancy defects and double anti-site defects.Ni-Ni double vacancy defects and double anti-site defects are the best stability when Ni-Ni is the first nearest neighbor.In the deviation from NiSc ideal stoichiometry,B₂-NiSc point defect is mainly Ni vacancy defect and Ni anti-site defect.As for Ni-rich alloy,the state of Ni anti-site defect is more stable than Sc vacancy defect;while for Sc-rich alloy,the state of Ni vacancy defect is more stable than Sc anti-site defect,which is in conformity with the result of energy calculation.V_Ni,Sc_Ni and Ni_Sc have a great influence on the hardness of B₂-NiSc,while V_Sc has little effect on the hardness of B₂-NiSc.Both Sc_Ni and V_Nii defects improve the plasticity of B₂-NiSc intermetallic compounds,while V_Sc and Ni_Scc defects reduce the plasticity of B₂-NiSc intermetallic compounds.The fracture crack of Ni-50%Sc alloy was analyzed.It was found that the fracture occurred first on the grain boundary and then extended along the grain boundary or between the dendrites.There are a large number of slip bands in the grains,which indicates that the B₂ grains have a certain degree of deformation and slip,in other words,they have good plasticity,and the emergence of the second-phase,reduces the plasticity of the material.Multiple cleavage fracture surfaces can be clearly observed in the tensile fracture,which belong to brittle fracture.First-principles calculation shows that NiSc is ductile while Ni₂Sc is brittle.The theoretical results are in good agreement with the experimental results.The fracture crack occurs at the grain boundaries of the“B₂-NiSc-grain boundary-Ni₂Sc”first,and then extends along the grain boundary,and then causes intragranular plastic deformation,the work hardening,and the initiation,propagation and fracture in the B₂-NiSc.