关于高强度稀土镁合金中强化相形成及原子排列的模拟研究

发布时间：2018-06-08 00:47

本文选题：β′相 + LPSO相　；参考：《北京工业大学》2016年硕士论文

【摘要】：析出强化是提高镁合金性能的一种重要手段。在Mg-RE二元合金体系中,β'相是提高合金强度的最重要析出强化相。最近本课题组研究发现,在β'相析出过程中,与其结构密切相关的一种变体(β_t'相)也可以在某些区域伴生析出。因此,为了揭示这种β_t'相的形成条件,有必要了解与之对应的两相(β'相与β_t'相)之间形成能的差异。而在Mg-RE-X三元合金体系(X代表Al及过渡族合金元素)中,长周期结构相则成为另一种备受关注的重要强化相。尽管针对长周期结构的形成及结构特征已有大量的研究开展,但其具体的原子排列方式至今仍没有最终定论。例如,Yokobayashi等人通过HAADF实验观察手段提出了Mg-Gd-Al体系中长周期结构的原子排列模型,但其中Al原子位置还仅仅是推测。因此,分析长周期结构中各种可能出现的原子排列方式、了解不同合金原子的占位特点,这对于揭示长周期结构形成条件和结构本质具有重要意义。本研究利用基于密度泛函理论的第一性原理计算方法开展了两方面的工作。一方面是对二元Mg-Gd体系中β'相及其变体(β_t'相)在不同应力条件下的吉布斯自由能进行了计算分析,另一方面是对三元Mg稀土合金中长周期结构的原子排列方式进行了分析研究。取得的主要结果概要如下:在三向应力状态下,应力从拉伸5GPa到压缩4GPa区间变化时,压缩状态的Mg-Gd合金中β'相比β_t'相更稳定。但在c轴单向压缩应力状态下,应力大小为1GPa时的小范围内出现了β_t'相稳定存在的情况,这清楚地表明β_t'相的形成会受到应力状态的影响。此外,通过p-V的状态方程拟合计算了β_t'及β'相的体积模量,结果表明前者的体积模量略高于后者。关于Mg-RE-X体系长周期结构中合金原子排列的计算结果表明,在考虑继承层错最高对称性的情况下确定的稀土原子(RE)占据的最稳定位置与实验观察到的RE原子位置完全相同,而X原子的最稳定占位则根据其种类及含量的不同出现两种可能,对应的原子构成比分别为Mg_xRE_8X_6及Mg_xRE_8X_(12)。当X为Zn原子时,Mg-RE-X体系的长周期结构具有Mg_xRE8Zn6型择优结构,而当X为Al原子时则具有Mg_xRE8Al12型择优结构。在Mg-Y-Al体系中,Mg_xY8Al6与Mg_xY8Al12两个择优结构的选择与合金元素的含量相关。另一方面,通过计算典型X元素对长周期结构稳定性的影响,揭示了长周期结构形成元素选取经验规则所对应的结构变化实质。其中Fe和Zr作为形成元素时,所形成的长周期结构处于高能态,不利于长周期结构的形成。Ag元素可以形成长周期结构,但形成的长周期结构的稳定性相对较差。Ni元素可以形成相对稳定的长周期结构。此外,为了提高计算效率,在本研究中还提出了用4N层错单元模型代替6N堆垛单元模型的简化计算模型方法,利用这种简化计算模型可以更加快速地获得有关长周期结构的关键信息。
[Abstract]:Precipitation strengthening is an important means to improve the properties of magnesium alloys. In Mg-RE binary alloy system, 尾 'phase is the most important precipitated strengthening phase to improve the strength of the alloy. Recently, our research found that a variant (尾 -T 'phase) closely related to the structure of 尾' phase can also be precipitated in some regions in the process of 尾 'phase precipitation. Therefore, in order to reveal the formation conditions of 尾 -T 'phase, it is necessary to understand the difference of formation energy between the corresponding two phases (尾' phase and 尾 -T 'phase). In the Mg-RE-X ternary alloy system, in which X represents Al and transition alloy elements, the long-period phase becomes another important strengthening phase. Although a great deal of research has been done on the formation and structural characteristics of long-period structures, its specific atomic arrangement has not yet been finalized. For example, Yokobayashi et al proposed an atomic arrangement model of long-period structure in Mg-Gd-Al system by HAADF method, but the Al atom position is only speculated. Therefore, it is of great significance to analyze various possible atomic arrangements in long-period structures and to understand the occupation characteristics of different alloy atoms, which is of great significance to reveal the formation conditions and structural nature of long-period structures. In this study, the first principle method based on density functional theory is used to carry out two aspects of work. On the one hand, the Gibbs free energies of 尾 'phase and its variants in binary Mg-Gd system under different stress conditions were calculated and analyzed. On the other hand, the atomic arrangement of long-period structure in ternary mg rare earth alloy was studied. The main results are summarized as follows: when the stress varies from 5 GPA to 4 GPA, the 尾 'phase of Mg-Gd alloy in compression state is more stable than that of 尾 s T' phase in the triaxial stress state. However, under the uniaxial compression stress of c axis, the 尾 s T 'phase exists stably in a small range when the stress is 1 GPA, which clearly indicates that the formation of 尾 T' phase will be affected by the stress state. In addition, the volume modulus of 尾 s t 'and 尾' phase is calculated by fitting the p-V equation of state. The results show that the volume modulus of the former is slightly higher than that of the latter. The calculated results on the arrangement of the alloy atoms in the Mg-RE-X system show that the most stable positions of the rare earth atoms determined with the consideration of the highest symmetry of inherited stacking faults are exactly the same as the positions of the RE atoms observed in the experiments. However, the most stable occupation of X atoms has two possibilities according to their species and contents. The corresponding atomic composition ratio is MgxRES _ 8X _ 6 and mg _ xRES _ 8X _ T _ 6 respectively. The Mg-RE-X system has MgxRE8Zn6 preferential structure when X is Zn atom, and MgxRE8Al12 preferential structure when X is Al atom. In Mg-Y-Al system, the selection of the preferred structure of mg _ XY _ 8Al _ 6 and MgS _ XY _ 8AL _ 12 is related to the content of alloy elements. On the other hand, by calculating the influence of typical X elements on the stability of long-period structures, the structural changes corresponding to the empirical rules for the selection of long-period structural formation elements are revealed. When Fe and Zr are used as forming elements, the long-period structure formed is in the high-energy state, which is not conducive to the formation of long-period structure. However, the stability of the formed long-period structure is relatively poor .Ni element can form a relatively stable long-period structure. In addition, in order to improve the efficiency of calculation, a simplified method is proposed to replace the 6N stacking element model with 4N stacking element model. Using this simplified computational model, the key information about long period structures can be obtained more quickly.
【学位授予单位】：北京工业大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG146.22

【相似文献】