Microstructures And Mechanical Properties Of Rapidly Solidified B2-NiSc Intermetallics

Ni-50%Sc alloy was prepared by centrifugal casting method.Volume fraction,the actual content of B2-NiSc and second phase Ni₂Sc in alloy were analyzed with an Image-Pro Plus software.The cooling rates for the solidified thin plate with thickness of 2.65mm,1.2mm,0.75mm and 0.35 mm are 1164,2570,4112 and 8811K·s^-1,respectively.Based on the phase volumetric analyzing of the microstructure with an Image-Pro Plus software,the loss of Sc element during melting was about3.01³.10 at%.Without heat treatment,the primary phase is B2-NiSc intermetallics and the microstructures between dendrite arms and dendrites are?NiSc+Ni₂Sc?_eutectic.After heat treatment,Ni₂Sc presented in the form of particles and distributed on the grain boundaries.The results show that heat treatment had a significant effect on microstructure and mechanical properties.The optimum treatment is 940?/216 h.The ultimate tensile strength and elongation of NiSc alloy reached a maximum value,130 MPa and 0.65%,respectively.Before heat treatment,the conduction of force should be"B2_primary-grain boundary₁-Ni₂Sc_eutectic-grain boundary₂-B2_eutectic-Ni₂Sc_eutectic-grain boundary₃-B2_primary".After heat treatment,the conduction of force should be"B2_primary-grain boundary₁-Ni₂Sc_eutectic-grain boundary₃-B2_primary".Based on the first-principles calculation,NiSc is ductile and Ni₂Sc is brittle.It is concluded that Ni₂Sc_eutectic and the grain boundary connected with it are the nucleation region of the crack.A pseudo-potential plane-wave method based on first principles was employed to calculate the physical parameters of B2-NiSc intermetallics and the geometrical,energetic,and electronic structures of point defects.The possible types of point defects in B2-NiSc intermetallics were analyzed and predicted by comparing the formation enthalpy and formation energy for different kinds of point defect structures.The results show that Ni vacancy defects and Ni anti-position defects are the main point defects in the B2-NiSc intermetallics,and that these point defects emerge as double vacancy defects or double anti-position defects.When the double Ni atoms at Sc sites are in the first nearest neighborhood,the point defect structures are the most stable.An analysis of the electronic structure of NiSc point defects shows that the Ni anti-position defect is more stable than the Sc vacancy defect in rich-Ni alloy and the Ni vacancy defect is more stable than the Sc anti-position defect in rich-Sc alloy.These results are consistent with the energy calculation.Through analysis of C₁₁-C₁₂,G/B and Poisson's ratio,it was found that the Ni_Sc point defect has little effect on crystal plasticity,and that the V_Ni point defect improves crystal plasticity.