Hongbao Cui

Pb plays an important role in determining the morphologies and mechanical properties of the Mg₂Si phase in Mg-2.5Si-xPb alloys. As the amount of Pb increases from 0.4 wt.% to 1 wt.%, the primary Mg₂Si phase is refined during solidification. Its morphologies transform from equiaxed-dendrite to polygonal and finally to roughly circular. The key reason for morphology evolution is the preferential adsorption of Pb atoms on Mg₂Si {100} surfaces to suppress the growth rate along the ⟨100⟩ directions, which is demonstrated by the adsorption model based on first principles. In addition, the hardness of the Mg₂Si phase decreases with the increasing solution content of Pb according to the results of the nanoindentation. With the addition of Pb at 1 wt.%, Pb content in the primary Mg₂Si phase reaches a maximum of 0.4 wt.%, and the hardness of the primary Mg₂Si phase reaches a minimum of 3.64 GPa. This reduction in hardness is attributed to the augmented ionic bond ratio resulting from the solution of Pb, which concurrently enhances the toughness of the Mg₂Si phase.

The spindle characteristic signal (forces and vibrations) at different friction stir lap welding (FSLW) parameters were studied. The result indicated that the spindle force and vibration have different trends with the change of welding parameters. For further study, the spindle dynamic performance evaluation model by means of the Technique for Order Preference by Similarity to the Ideal Solution (TOPSIS method) was established. The model was used to calculate the relative approach degree B under different welding process parameters. The correlation between the characteristic signal and the joint properties was obtained. The model was validated by mechanical performance testing and microscopic observation. The results showed that the model evaluations were consistent with the experimental results.

The joint surface of the 1060 aluminum and AZ31 magnesium alloy was prepared through friction stir lap welding (FSLW) under different welding process parameters. The joint surface was characterized three-dimensionally using a three-dimensional (3D) optical profiler, and the coordinate data were obtained. The fractal dimension of the joint surface was calculated by the box-width transformation method using a MATLAB program. Furthermore, the influence of the welding process parameters on the fractal dimension of the joint surface was studied. The response surface model was established based on the principle of central composite design (CCD), and analysis of variance (ANOVA) was carried out to test the accuracy of the response surface. The results showed that the joint surface morphology had fractal characteristics, and the fractal dimension could be used as an index to characterize the quality of the joint surface. The change of the welding process parameters had a great impact on the fractal dimension of the joint surface, the interaction between the parameters was small, and the fitting accuracy of the response surface model was high. The fractal dimension of the joint surface decreased with the increase in the welding and rotational speeds and the effect of the rotational speed was more significant.

Ni-50%Sc and Ni-51%Sc alloy were prepared with a vacuum arc smelting and water cooled copper mold suction-casting machine. The results showed that the two component alloys consisted of the primary phase B2-NiSc and lamellar (Ni2Sc+NiSc)eutectic due to the loss of Sc during melting. Two groups of alloys underwent (970 ℃, 72 h) homogenization heat treatment, and spherical or plate shape Ni2Sc particles were dispersed on the B2-NiSc matrix. With the increase of Sc content from 50% to 51%, the amount of the second phase in the alloy decreases, the microstructure becomes uniform, and the grain gradually changes from long bar to a spherical particle. According to the Jackson boundary theory, the Jackson factor α of B2-NiSc =0.5 < 2, so the interface is rough, which explains that the growth pattern of the B2-NiSc phase is a non-faceted growth. It is consistent with the dendritic growth pattern of the B2-NiSc phase, which is observed from the experiment. After a long heat treatment, the number of vacancies decreases and the microstructure became uniform. The loss rate of Sc in rapidly quenched solidification was higher than that after the heat treatment.

Using the first-principles plane-wave pseudo-potential method based on density functional theory, the effect of vacancy and anti-position defect on the mechanical and thermal properties of B2-NiSc intermetallics were discussed in detail. Several parameters, such as the shear modulus, bulk modulus, modulus of elasticity, C ₁₁–C ₁₁, the Debye temperature and Poisson's ratio, have been calculated to evaluate the effect of vacancy and anti-position defect on the hardness, ductility and thermal properties of B2-NiSc intermetallics. The results show that V_Ni, Sc_Ni, V_Sc and Ni_Sc the four point defects all make the crystal hardness decrease and improve plasticity of B2-NiSc intermetallics. The entropy, enthalpy and free energy of V_Ni, Sc_Ni, V_Sc and Ni_Sc are monotonously changed as temperature changes. From the perspective of free energy, Ni_Sc is the most stable, while Sc_Ni is the most unstable. Debye temperature of NiSc intermetallics with four different point defects shows V_Ni, Sc_Ni, V_Sc and Ni_Sc the four point defects all reduce the stability of B2-NiSc intermetallics.

A pseudo-potential plane-wave method based on first principles was used 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 found to be 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 the Ni-rich alloy, and the Ni vacancy defect is more stable than the Sc anti-position defect in the Sc-rich 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.