Structural Characterization Of Laves Phase Evolution And Regulation Of Its Precipitation Behavior In Al-Zn-Mg-Based Alloys

Al-Zn-Mg based high strength alloys are widely used in aerospace applications due to their low density,excellent mechanical properties,and outstanding machinability.The development of modern industry has put higher demands on the comprehensive performance of Al-Zn-Mg based alloys,and the design of high-strength aluminum alloys based on material microstructure characterization has gradually become a research trend.The strengthening effect of this alloy mainly comes from the precipitation strengthening during aging,i.e.,the hindering effect of nanoprecipitates on dislocations.Therefore,a systematic study of the structural evolution of the nanoprecipitates and their growth mechanism is an important guide for the design of newly high strength alloys.In this work,the Laves structure precipitates in Al-Zn-Mg（-Cu/Y）alloy were systematically characterized,with emphasis on structural evolution and phase transformation mechanism during aging process at nanoscale.Based on the phase structural evolution,the structure characterization of Laves particles and quasicrystalline particles in the alloys at submicron-scale,as well as the regulation of the precipitation behavior after adding rare earth element Y at nanoscale,were further investigated.The main innovative results are summarized as follows:（1）Investigation on coexistence of defect structures in Laves structural nanoprecipitates.Three types of Laves structures can coexist within theη-MgZn₂ precipitates:C14,C15 and C36,and the Laves structure transformation sequence of C14→C36→C15 in this system was determined.Meanwhile,it was found that there are diverse defect structures in the MgZn₂ phase,including stacking faults,planar defects and five-fold domain structures,which have significant effects on relieving the internal stress/strain of the precipitates,and the self-accommodation mechanism among the defect structures was verified with molecular dynamics calculations.These multiple defect structures affect the growth behavior of the MgZn₂ phase,making it grow toward an equiaxed or even circular morphology instead of the conventional lath-like morphology.（2）Investigation on multiple phase transformation of Laves structural nanoprecipitates from C14 to C36 and from C14 to quasicrystal clusters.The precipitates with C14-Laves structure are extremely susceptible to phase transformation.It is found that C14 precipitates can be completely transformed into the C36 structural precipitates instead of the previously reported partial C36 stacking faults after 1048 h aging at120℃.Two possible phase transition paths from C14 to C36 were inferred based on the synchroshear mechanism.Ab initio molecular dynamics calculations were carried to verify that the energy of the C36structure can be lower than that of the C14 structure after aging,supporting the observed experimental results.It is also found that the C14Laves phase structure is very similar to the quasicrystalline structure and can also transform into quasicrystalline clusters.These investigations on various phase transformation mechanisms among Laves phases provide theoretical support for the microstructural characterization of materials containing multi-scale Laves phases.（3）Characterization of Laves and quasicrystal structural particles in submicron-scale.Submicron-scale quasicrystal particles were obtained in conventional casting Al-Zn-Mg-Cu alloys for the first time.Industrial impurity elements including Fe and Ni were found to induce the formation of quasicrystalline particles.When there is no Fe/Ni as core in the submicron-scale particles,the structure is characterized as C15-Laves phase,i.e.,face-centered cubic Mg Cu Zn phase,when Fe/Ni is included as quasicrystalline core,a stable core-shell quasicrystals with Al-Fe-Ni core and Mg-Cu-Zn shell can be formed.It was also found that the core and shell structures are completely different,with a decagonal quasicrystal structure for the core and an icosahedral quasicrystal structure for the shell.Therefore,impurity elements of industrial aluminum alloys such as Fe and Ni play an important role on the formation of Mg-Cu-Zn newly ternary quasicrystal particles,which provides a new idea for the neutralization of impurity elements in industrial aluminum alloys.（4）Investigation on the regulation of nanoscale Laves precipitates’growth.In order to regulate the defect structure of the precipitates,rare earth element Y was added in Al-Zn-Mg alloys and its influence on the precipitation behavior of the precipitates was investigated.The addition of Y element greatly refined the size of the precipitates and dynamically combined with different alloying elements during aging process to further improve the nucleation rate and precipitation rate of the precipitates,which greatly improved the mechanical properties of alloys.In addition,the mechanical behavior between the ultra-fine precipitates and dislocations was also investigated by combining in-situ and ex-situ electron microscope characterization,to provide an experimental basis for the optimization of the mechanical model.The results of this thesis update the knowledge of the evolution of the microstructure of Laves precipitates and the phase transformation process in Al-Zn-Mg based alloys,and the relationship between the Laves and quasicrystalline phases is systematically investigated from the nanoscale to the microscale for the first time.These results provide key experimental evidence for optimizing alloy composition、process and regulating alloy microstructure.Moreover,these findings also provide new perspectives and theoretical guidance for the development of new high-strength and high-toughness aluminum alloys.