【作者】 孔祥春；

【导师】 胡望宇；

【作者基本信息】 湖南大学 ， 凝聚态物理学， 2010， 硕士

【摘要】 准晶材料是只包含旋转对称性而不具有平移对称性的特殊结构,它具有独特的物理化学性质,已经引起了人们越来越多的关注。利用准晶的硬度特性将其作为合金强化相是它的重要应用,但随着温度变化准晶结构会发生与晶体相结构的可逆性转变。因此了解和预测准晶结构材料的热学与力学性能不仅对于新型准晶材料的发现和制备,而且对于现代工业中的应用都有重要的指导意义。本文应用改进的分析型嵌入原子模型(MAEAM)和分子动力学技术,研究了准晶体系的热学与力学性能,主要包括：二十面体AlNiRh准晶结构转变为密排晶体结构的相变机制以及通过单轴拉伸研究了准晶结构的力学性能。单晶二十面体AlNiRh块体微观结构随温度变化的模拟表明：体系在温度上升过程当中出现了两次结构变化,温度在600 K以下时体系保持稳定,无明显变化,结构中的二十面体只有少量的减少。加温至600 K以上体系向无序化转变,二十面体子结构消失,准晶结构转变为非晶态结构,并且在此过程体系中有过渡相的BCC原子出现。至1100 K以上,体系中出现了较多的FCC与HCP原子,过渡相消失,这与亚稳准晶会在高温下转变为晶态相的结果一致。体系在1800 K发生熔化。对AlNiRh纳米丝在600K以下温度进行单轴拉伸模拟过程表明,与传统晶体的应力应变曲线不同,二十面体准晶的屈服强度较小,且体系的应力—应变曲线明显分为应力上升、应力平稳与应力下降三个阶段,表现出良好的拉伸延性。团簇子结构在应力上升阶段受应力作用结构消失,但是团簇中心Ni原子结构维持稳定,在应变增大过程中有纳米丝断裂于颈缩位置,在拉伸过程当中没有发现滑移现象,也没有晶态原子产生,体系通过塑性断裂机制断裂。并且使用force match方法得到了AlNiCo十次准晶体系的realistic型EAM势函数,应用这种势函数对AlNiCo体系块体拉伸发现体系强度很大,在拉伸过程中有明显滑移现象出现,体系应变量17%以上出现原子层结构破坏,这种结构破坏并且随应变量增大而增大。更多还原

【Abstract】 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.更多还原