High Pressure Sintering And Characterization Of Intermetallics In Al-Cr System

Quasicrystal is a special crystal that does not have translational symmetry but has a quasi-periodic arrangement.The structure of quasicrystals has always been a difficult point in crystallographic research.The study of the structure of quasicrystal approximate phases has contributed to the analysis of the structure of quasicrystals.In the dissertation,the Al-Cr system quasicrystalline approximate phases have been investigated.Some complex metallic compounds in the Al-Cr system were synthesized by the high-pressure sintering method,and the crystal structures of these compounds were analyzed and refined by single crystal X-ray diffraction method.From the topological point of view,the intrinsic building units of several quasicrystalline approximate phases in Al-Cr system are analyzed,and the structural relationship between these quasicrystalline approximate phases has been systematically studied.Firstly,a third low-temperature?₂'-Al₈Cr₅ structure was found in a sample with an Al:Cr atomic ratio of 11:4 through high-pressure sintering.Its unit cell parameters are:a=7.8777?,c=7.8777?,and?=109.566°,and the space group is R3m:R.There are Al5/Cr5,Al6/Cr6,Al8/Cr8 co-occupied positions in the structure,and the occupancy factor of Al5,Al6 and Al8 is 0.772,0.5 and 0.958,respectively.The crystal structure of?₂'-Al₈Cr₅ is the same type as the brass phase Cu₅Zn₈,and the basic characteristics of its brass structure are characterized by topological analysis.Secondly,the high-temperature metastable?'-Al₁₁Cr₂ phase in the form of twins was successfully prepared using high-pressure sintering technology.The unit cell parameters are:a=17.7519(11)?,b=30.4850(11)?,c=17.7526(8)?,?=17.7526(8)°,and the space group is C2/c.After topological analysis,the complex crystal structure?'-Al₁₁Cr₂ with 616atoms in the unit cell can be represented by a variety of"nanoclusters"with typical characteristics,which clearly and intuitively characterizes the basic building units and characteristics of the structure.Finally,the four structures of?(?')-Al₁₁Cr₂,?-Al₄Cr,?-Al₄Cr,?-Al₄₅Cr₇ are analyzed using the"nano-cluster"topology analysis method by the Topos Pro software.The Mackay type or pseudo-Mackay type clusters and the 1@12@46 type clusters of the four structures are compared,and the structural relationship between them is analyzed,which lays the foundation for the subsequent structural analysis of related quasicrystals.