Mg-Sn-Si合金凝固过程中 Mg2(SixSn1-x)相的球化与韧化机理

It is the key to effectively modify coarse and dendritic Mg2Si to form a new similar intermetallic phase which has the same structure as Mg2Si but with a granular shape and ductile property for magnesium alloys strengthened with Mg2Si. Therefore, in this proposal silicon and tin which are in the same group are selected as mainly alloying elements to form an Mg-Sn-Si alloy system. The Mg-Sn-Si alloy system will then be modified during solidification by a combination of trace elements that consist of bismuth, antimony, strontium and yttrium, in order to change the growing interface conditions of the new Mg2Si similar intermetallic phase, melting entropies and faceted growing modes. During solidification tin can replace some part of silicon and a new granular ductile Mg2(SixSn1-x) intermetallics which has the same reciprocal CaF2 structure, very close lattice parameters and competitive high temperature properties as Mg2Si can form. In this proposal, solidification phase diagram will be calculated with THERMO-CALC software, microstructures and phase transformation of the system will be studied with optical microscope and DSC, defect characteristics of the granular and ductile Mg2(SixSn1-x) phase extracted using 3%CH3COOH solution from matrix will be highly analyzed with HRSEM, composition distribution of Mg2(SixSn1-x) phase will be scanned with EDS. The comprehensive results obtained from above research and from First Principle calculation (FP) (physical properties, point defects, micro-defects and dislocation energy) will be used to reveal nucleation procedure, defect modes, and the granular growing mechanism of the Mg2(SixSn1-x). The hardness distribution will also be tested with a nano-hardness tester, superfine morphology of Mg2(SixSn1-x) and interfacial combination condition will observe with HRTEM. These results will be used to study the strengthing and ductile mechanisms of the granular Mg2(SixSn1-x) itself and Mg-Sn-Si alloy alike in company with the results obtained from FP calculation.

Mg-Sn-Si合金经微量（BiSbSrY）复合元素变质以改变强化相凝固前沿温度和溶质场、熔化熵和平面生长模式，粗大硬脆Mg2Si枝晶转变以球形Mg2Si、Mg2Sn和韧塑Mg2(SixSn1-x)金属间化合物析出。研究形成球形韧塑Mg2Si类似相的球化和韧塑化机理，对于高温镁合金具有重要意义。在THERMO-CALC凝固计算基础上，结合OM、DSC和SEM/EDS分析研究Mg2(SixSn1-x)形核、生长前沿溶质场，进而揭示其生长模式和形貌。用第一性原理（FP）计算其基态、点缺陷、电子和位错核结构，结合HRSEM观察Mg2(SixSn1-x)萃取物形貌，提出其球形生长机理；用纳米硬度分析球状Mg2(SixSn1-x)相硬度规律，HRTEM分析球状相和基体界面结合性质，结合FP物性参数和位错核计算结果，揭示Mg2(SixSn1-x)和材料的强韧化机理。

研究形成球形韧塑Mg2Si类似相的球化和韧塑化机理对于高温镁合金具有重要意义。本项目对Mg-Sn-Si合金经微量（BiSbSrY）复合元素变质改变Mg2Si相凝固前沿温度和溶质场和平面生长模式，粗大硬脆Mg2Si枝晶转变以球形Mg2(SixSn1-x)韧塑金属间化合物；利用OM、DSC和SEM/EDS分析研究Mg2(SixSn1-x)形核、生长模式和形貌，用第一性原理计算晶体和界面结构和力学性质，提出其球形生长机理；HRTEM分析球状相和基体界面结合性质，揭示Mg2(SixSn1-x)和材料的强韧化机理。结果发现，Mg2(SixSn1-x)相的晶格常数介于Mg2Si和Mg2Sn之间，Sn原子数量增多，Mg2(SixSn1-x)晶体共价键减少，金属性增强，脆性减弱，硬度降低，塑性增大。Sn取代Si原子使Mg2(SixSn1-x)晶体中对生长形态起重要作用的晶面发生改变，在Mg2(SixSn1-x)晶体更容易形成多面体而容易球化。Mg2(SixSn1-x)/Mg界面过渡区域有两个Mg2Sn纳米带层组成，Mg2Sn/Mg2Si和Mg2Sn/Mg均具有低的界面能和高的界面强度，可以将提高Mg2(SixSn1-x)/Mg的强韧性。经（Bi、Sb、Sr和Y）元素变质处理后，元素易在其生长表面形成吸附薄层，阻碍晶体继续长大，因此，变质后的合金组织相比于Mg-Sn-Si合金更加细小和均匀。对合金的高温蠕变性能进行研究，Mg2(SixSn1-x)相可以提高合金的高温性能，结合组织分析和硬化指数计算，合金的蠕变机制为位错滑移机制。