Molecular Dynamics Simulation Of The Thermodynamic And Mechanical Properties Of Al-based Quasicrysatlline

Quasicrystal is a new kind of crystal owns Rotational Symmetry but lacks translational symmetry. It has attacted more and more interest nowadays due to its unique properties in physical and chemical applications. Because its incredible hardness, it's usually mixed in other alloys to get them reinforced, but a phase transtition would occur when the critical temperature is reached, so getting to comprehend and predict the thermodynamic and mechanical properities of quasicrystal is not only of great importance and benefit to the discovery and research of new quasicrystal, but also provides a guidance to the wider application of it in the modern industry. In the present thesis, the thermodynamic and mechanical properities of quasicrystal have been investigated with the modified analytic embedded-atom method (MAEAM) and molecular dynamics simulation, including:the mechanism of phase transition from quasicrystal to close-packed crystal phase in icosahedral-AlNiRh quasicrystal and the mechanical properities of quasicrystal performed by uniaxial tension in molecular dynamics simulation. The simulation result of microstructure of the single crystal block AlNiRh according to the temperature reveals that the structure goes through two remarkable change during the heating process. It remains stable when the temperature belows 600K, and there is few icosahedroris lost. When heated above 600K, the microstructure becomes disordered, and the icosahedrons get disappeared. The whole system was transformed into amorphous, with a few atoms which are BCC appears as a transition phase. Plenty of HCP and FCC atoms show up when it's over 1100K and the transition BCC phase disappears. This is consistent with the facts that metastable quasicrystal would transform into crystal phase at high temperature. The block eventually melts at 1800K. The uniaxial tension simulation of AlNiRh nanowire below 600K reveals that icosahedral quasicrystal has a smaller yield strength compared to crystal, and the stress-strain curve is divided in to three stages, which is the rising phase of the stress, the stable phase of stress and the decline phase of stress. The nanowire shows very good tensile ductility. The ico-clusters are damaged in the first stage under the action of stress, but the Ni atoms located at the center of the icosahedral substructure remain stable. The phenomenon of necking is found in the during the tensile process, and neither slidding occurs nor crystal structure is produced. The fracture is due to plastic fracture mechanism. Realistic EAM potential for AlNiCo quasicrystal is gained from the force match method. Uniaxial tension simulation of AlNiCo block is performed, and slip is found when the strain is 10%. The layered structure partly destroyed when stain reached 17%, and ratio of destroyed layers rose with the increase of the strain.