| Au-based solder alloys are the only choice for welding important parts at high operating temperatures on aircraft,missiles and rockets because of their good thermal conductivity,wettability,oxidation resistance and high temperature stability.The development of the electrical and aerospace industries has placed higher demands on the performance of Au-based solder alloys,but current Au-based high-temperature solder alloys still suffer from a number of problems,including difficulties in processing solder foils,low strength,and high cost.Alloying means are commonly used to improve the above mentioned defects,but a more comprehensive study of the effects of various alloying elements on the properties of Au-based solder alloys is lacking.Therefore,it is necessary to carry out further composition design and property optimization of Au-based solder alloy systems.To address the problem of low strength of Au-based solder alloys,this thesis explores the structural,mechanical,thermal,interfacial strengthening and fracture mechanism properties of Au-based solder alloys from the electron-atomic level based on first-principles calculations,and provides a basis and reference for the composition design and property development of new Au-based solder alloys.The main contents of this paper are as follows:（1）The effects of 33 alloying elements on the elastic properties of Au-based dilute solid-solution alloys were systematically investigated based on first-principles calculations,and elements with the potential to improve the strength of Au-based brazing alloys were screened.The results show that all 33 alloying elements favor the dissolution of Au,and the modulus（especially the shear modulus and Young’s modulus）of Au31M decreases with the increase of equilibrium volume.The Labusch model was used to characterize the solid solution strengthening effect of Au31M.The strengthening ability of the edge-type dislocations was more significant than that of the screw-type dislocations,and the effective elements for enhancing Au were screened in the following order of strengthening ability:La>Ce>Y>Fe>Ni.An Au-Cu alloy yield strength model was established to quantitatively describe the contributions of solid solution strengthening and fine grain strengthening,and the predicted values were in good agreement with the experimental values.The electronic structure and Mulliken population analysis revealed that La and Th elements can significantly improve the ductility of pure Au.（2）The effects of different Ni contents on the thermodynamic properties of Au-based alloys were investigated based on first-principles calculations combined with quasi-simple harmonic approximations.The disordered Au1-xNixalloy model was constructed by special quasi-simple harmonic approximation,and the disordered Au1-xNixalloy was judged to be stable using formation energy and phonon spectrum.The shear modulus and Young’s modulus increased with increasing Ni content,while B/G and Poisson’s ratio decreased gradually.The layer dislocation energy of disordered Au1-xNixalloy gradually increases with the increase of Ni concentration.Calculations of the thermal properties of disordered Au1-xNixalloys show that the total thermodynamic entropy is mainly contributed by the vibrational entropy,and the thermal expansion coefficient of disordered Au1-xNixalloys decreases slowly with the increase of Ni content at the same temperature,reflecting the fact that the crystal structure of alloys with high Ni concentration is more difficult to expand thermally when heated,the coefficient of thermal expansion of Au0.25Ni0.75alloy is most consistent with that of Ni3Al.The differential charge density analysis reveals that the plasticity of disordered Au1-xNixalloy decreases gradually with increasing Ni content due to the strong bonding formed between Ni atoms and the non-spherical distribution of Ni charge density.The physical origin of the magnetic behavior and electronic properties of disordered Au1-xNixalloys was elucidated by using the fractional density of states.（3）The strengthening mechanism of the Au/Ni3Al interface and the fracture mechanism at the interface by the alloying elements（Ni,Sn,Si and Ge）were investigated for the Au-based solder/Ni-based high-temperature alloy base material based on the first principles calculations.The Au（111）/Ni3Al（111）interfacial stacking structures at the top,bridge and center positions were constructed,and the results showed that the MT configuration had the largest adhesion work（10.624 J/m2）and the largest bond strength.The ease of segregation of alloying elements at the interface was reflected by calculating the segregation energy,and the results showed that Si and Ge atoms were easily separated from the Au surface to the interface,while Ni and Sn atoms were stably localized in the Au matrix.The interfacial bonding strength was quantified by calculating(（6）,and the results showed that the interfacial adhesion work increased by16.33%compared with the clean interface when Ni atoms were doped,and the bonding strength was significantly enhanced.The changes in chemical bonding and electronic behavior during the interfacial fracture were systematically analyzed,and the fracture of all three interfacial stacking models occurred inside the Au bulk phase,which was caused by the breakage of Au-Au（I）bonds in the Au bulk material. |
