密排六方结构Mg-Nd-Gd系合金时效亚稳相的3D多尺度模拟研究

The high-strength light weight magnesium alloy materials are causing people’s great interest because of the demands for energy-saving, environmental protection and green manufacturing. However, the drawbacks such as low creep resistance, yield strength, ductility and oxidation resistance limit its application as a structural material. The rare earth elements such as Nd and Gd added to magnesium alloy show good effects of solid solution strengthening and precipitation strengthening, and the main strengthening phase is the meta-stable precipitation phase which is coherent to the matrix, but its corresponding precipitation sequence and mechanism is not yet clear. So, it’s significant to study the precipitation mechanism of Mg-Nd、Mg-Gd、Mg-Nd-Gd 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 metastable phases for different Mg-Nd, Mg-Gd, Mg-Nd-Gd alloys at different degree of elastic field will be calculated, and the project planned to focus on studying the early precipitation sequence, the middle metastable 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 metastable 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-Nd, Mg-Gd, Mg-Nd-Gd 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，轻质、高强镁合金因较低的抗高温蠕变性、屈服强度、延展性等，限制了其应用范围。Nd、Gd等元素的添加兼具固溶和沉淀强化作用，与基体共格的主要强化相的沉淀过程尚未明确，因此，探明Mg-Nd-Gd系合金的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(fcc)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，构建三维密排六方(hcp)晶格微观相场模型，计算Mg-Nd、Mg-Gd、Mg-Nd-Gd合金在不同程度弹性场下沉淀相的序列、析出机制、原子占位、有序-无序、簇聚、体积分数、标度律等，重点研究早期析出序列、中间亚稳现象、后期粗化规律、全程形貌演化；之后再与第一性原理热力学、力学性质计算相耦合，研究各亚稳相的热力学稳定性、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，并为稀土镁合金的强化设计提供一种多尺度研究思路。

本项目采用3D多尺度模拟研究了密排六方结构Mg-Nd-Gd系合金时效亚稳相。不同压力下β″-Mg3Gd和β′-Mg7Gd化合物的第一性原理研究表明β′-Mg7Gd在应力-应变过程中能够承受更大的压力，且在压力下两种化合物的延性可以得到改善。通过计算这两种化合物的热力学性质，可以为相场法建模提供一系列的参数（例如：吉布斯自由能，熵值，焓值，热膨胀系数等）。不同压力下β′-Mg7Gd合金德拜温度和零点能量的变化明显大于β"-Mg3Gd，且β′-Mg7Gd可以在高温吸收更多的热量。β1-Mg3Gd、β1-Mg3Nd、β'-Mg7Gd和β'-Mg7Nd合金的计算结果表明β'-Mg7Gd抗弹性、抗剪切和抗体积变形能力优于β'-Mg7Nd。这四种合金以共价键结合为主，且β1-Mg3Gd键性最强。随后采用相场法模拟了Mg-Nd-Gd系合金体系包括形核、长大和粗化在内的整个沉淀过程，阐明了合金微结构转变机制与成分及弹性能的响应关系。相分离早期阶段通过调幅分解机制形成富Gd相，随着内部成分的逐渐升高，最终形成稳定的Mg7Gd析出相。Gd含量与有序Mg7Gd结构析出速率呈正相关。弹性能的加入可以使析出相向弹性软方向生长，其形貌由球状变为椭球状或棒状。最后采用感应熔炼制备了Mg-Nd-Gd系合金并对其进行固溶和时效处理，着重分析了显微组织、力学性能及时效硬化行为，通过与模拟结果对比进行更为系统的模拟研究。