Mg-Zn系合金沉淀过程含GP区非平衡相演化的多尺度耦合模拟

Mg-RE alloys are costly and limited in wide application; Mg-Zn alloys are cheaper and have high strength due to the aging solution and some precipitates. The potential precipitates can be coherent, semi-coherent and non coherent to its matrix, and the precipitates can be GP zones and some non equilibrium meta-stable phases or structures. Yet the corresponding precipitation sequence and mechanism is not yet clear. So, it’s significant to study the precipitation mechanism of Mg-Zn, Mg-Zn-Cu and Mg-Zn-Cu-Zr alloys under elastic field. Here, the Micro Phase Field Method-MPFM for fcc alloys system based on atom-phase-microstructure scale will be mainly employed, and the 3-Dimensional (3D) MPFM for hcp Magnesium alloys will be developed, some required parameters will be acquired by First Principles Method-FPM based on electronic-atomic scale. The potential precipitates, occupied sites, order-disorder, clustering, volume fraction, scaling law etc. of meta-stable phases for different Mg-Zn, Mg-Zn-Cu and Mg-Zn-Cu-Zr alloys at different degree of elastic field will be calculated, and the project planned to focus on studying the early precipitation sequence, the middle meta-stable phenomenon, the later coarsening law and the whole course morphology evolution of precipitates. Then the results will be compared with thermodynamic and mechanical properties of first-principles calculation to study the macroscopic properties of each meta-stable phase, such as thermodynamic stabilities, strength and plasticities, and will be verified through experiments. The project can help to provide a method for component and strengthening design of Mg-Zn, Mg-Zn-Cu and Mg-Zn-Cu-Zr alloys and extend the application of phase field simulation, and we expect to match a kind of multi-scale dynamic process description approaching the fact well without given model and route for the first-principles calculation.

当前，稀土镁合金因成本昂贵限制了其广泛应用。低成本Mg-Zn系合金兼具固溶和沉淀强化作用，在高强耐热方面极具潜力；其含GP区的与基体共格、半共格或非共格的主要强化相的沉淀过程尚未明确，探明其沉淀机制具有重要意义。本项目采用基于相-组织尺度的面心立方微观离散格点相场法，从基于电子-原子尺度的第一性原理计算中获取参数，构建三维含GP区的密排六方晶格微观相场模型，计算Mg-Zn、Mg-Zn-Cu、Mg-Zn-Cu-Zr合金在不同工艺下非平衡态的GP区、沉淀相序列、析出机制、原子占位、有序-无序、簇聚、体积分数、标度律等，重点研究早期序列、非平衡相、后期粗化、全程形貌演化；同时与第一性原理热力学、力学性质计算相耦合，研究各非平衡相的热力学稳定性、强度、塑性等宏观性能，并进行实验验证。本项目可拓展相场、第一性原理方法的适用范围，并为低成本高性能镁合金设计提供一种多尺度研究思路。

由于Mg-Zn系合金是一种可产生明显时效硬化效应的低成本、高性能镁合金，因此目前正朝着耐热、耐蚀、阻燃、高强、高韧及低成本等方向发展。但Mg-Zn系合金易产生显微缩松倾向且过渡相的析出过程、数量、位向、相结构等对Mg-Zn系合金的时效强化效应有很大影响。.在第一性原理计算的基础上，对反位点缺陷对MgZn2和MgCu2的力学和热力学性质的影响进行了理论研究。结果表明，Mg在Zn或Cu部位的反位缺陷具有增强作用，但表现出较脆的行为。而Mg亚晶格上的Cu反位缺陷相具有延性。通过弹性常数和多晶弹性模量的计算发现Zn-Zr金属间化合物相比MgZn2是有效的强化相。最小导热系数计算表明Mg-Zn-Zr合金的热导率将明显改善锌原子从α-Mg中沉淀出来进而形成Zn-Zr金属间化合物。.基于第一性原理得出的参数，采用相场法建模。通过研究Mg-Zn合金发现Mg-Zn合金在时效后期形成稳定的有序MgZn2结构。在Mg-Zn-Cu三元合金体系中，适当提高Cu含量能有利于加快有序MgZn2结构的析出。Mg-Zn-Cu-Cr四元合金中弹性能的加入使得沉淀相增多且沿弹性软方向发展。.以Mg-8Zn-0.5Cu合金作为研究对象发现铸态Mg-Zn-Cu合金中Mg-Zn、Mg-Cu和Cu-Zn共晶相连续分布在晶界处。且Mg-8Zn-0.5Cu合金含有α-Mg，MgZnCu和MgZn2相，这与相场法得出的结果相匹配。经过时效处理，合金力学性能得到显著提升。.本项目实施会为Mg-Zn系合金的成分设计和强化设计提供指导，并提高相场模拟的可靠性。同时通过模拟与实验相结合的方法可改变了以前实验设计方法成本高、周期长的缺点。