本文選題：β′相 + LPSO相�。� 參考：《北京工業(yè)大學(xué)》2016年碩士論文

【摘要】：析出強(qiáng)化是提高鎂合金性能的一種重要手段。在Mg-RE二元合金體系中,β'相是提高合金強(qiáng)度的最重要析出強(qiáng)化相。最近本課題組研究發(fā)現(xiàn),在β'相析出過程中,與其結(jié)構(gòu)密切相關(guān)的一種變體(β_t'相)也可以在某些區(qū)域伴生析出。因此,為了揭示這種β_t'相的形成條件,有必要了解與之對應(yīng)的兩相(β'相與β_t'相)之間形成能的差異。而在Mg-RE-X三元合金體系(X代表Al及過渡族合金元素)中,長周期結(jié)構(gòu)相則成為另一種備受關(guān)注的重要強(qiáng)化相。盡管針對長周期結(jié)構(gòu)的形成及結(jié)構(gòu)特征已有大量的研究開展,但其具體的原子排列方式至今仍沒有最終定論。例如,Yokobayashi等人通過HAADF實(shí)驗(yàn)觀察手段提出了Mg-Gd-Al體系中長周期結(jié)構(gòu)的原子排列模型,但其中Al原子位置還僅僅是推測。因此,分析長周期結(jié)構(gòu)中各種可能出現(xiàn)的原子排列方式、了解不同合金原子的占位特點(diǎn),這對于揭示長周期結(jié)構(gòu)形成條件和結(jié)構(gòu)本質(zhì)具有重要意義。本研究利用基于密度泛函理論的第一性原理計算方法開展了兩方面的工作。一方面是對二元Mg-Gd體系中β'相及其變體(β_t'相)在不同應(yīng)力條件下的吉布斯自由能進(jìn)行了計算分析,另一方面是對三元Mg稀土合金中長周期結(jié)構(gòu)的原子排列方式進(jìn)行了分析研究。取得的主要結(jié)果概要如下:在三向應(yīng)力狀態(tài)下,應(yīng)力從拉伸5GPa到壓縮4GPa區(qū)間變化時,壓縮狀態(tài)的Mg-Gd合金中β'相比β_t'相更穩(wěn)定。但在c軸單向壓縮應(yīng)力狀態(tài)下,應(yīng)力大小為1GPa時的小范圍內(nèi)出現(xiàn)了β_t'相穩(wěn)定存在的情況,這清楚地表明β_t'相的形成會受到應(yīng)力狀態(tài)的影響。此外,通過p-V的狀態(tài)方程擬合計算了β_t'及β'相的體積模量,結(jié)果表明前者的體積模量略高于后者。關(guān)于Mg-RE-X體系長周期結(jié)構(gòu)中合金原子排列的計算結(jié)果表明,在考慮繼承層錯最高對稱性的情況下確定的稀土原子(RE)占據(jù)的最穩(wěn)定位置與實(shí)驗(yàn)觀察到的RE原子位置完全相同,而X原子的最穩(wěn)定占位則根據(jù)其種類及含量的不同出現(xiàn)兩種可能,對應(yīng)的原子構(gòu)成比分別為Mg_xRE_8X_6及Mg_xRE_8X_(12)。當(dāng)X為Zn原子時,Mg-RE-X體系的長周期結(jié)構(gòu)具有Mg_xRE8Zn6型擇優(yōu)結(jié)構(gòu),而當(dāng)X為Al原子時則具有Mg_xRE8Al12型擇優(yōu)結(jié)構(gòu)。在Mg-Y-Al體系中,Mg_xY8Al6與Mg_xY8Al12兩個擇優(yōu)結(jié)構(gòu)的選擇與合金元素的含量相關(guān)。另一方面,通過計算典型X元素對長周期結(jié)構(gòu)穩(wěn)定性的影響,揭示了長周期結(jié)構(gòu)形成元素選取經(jīng)驗(yàn)規(guī)則所對應(yīng)的結(jié)構(gòu)變化實(shí)質(zhì)。其中Fe和Zr作為形成元素時,所形成的長周期結(jié)構(gòu)處于高能態(tài),不利于長周期結(jié)構(gòu)的形成。Ag元素可以形成長周期結(jié)構(gòu),但形成的長周期結(jié)構(gòu)的穩(wěn)定性相對較差。Ni元素可以形成相對穩(wěn)定的長周期結(jié)構(gòu)。此外,為了提高計算效率,在本研究中還提出了用4N層錯單元模型代替6N堆垛單元模型的簡化計算模型方法,利用這種簡化計算模型可以更加快速地獲得有關(guān)長周期結(jié)構(gòu)的關(guān)鍵信息。
[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.
【學(xué)位授予單位】：北京工業(yè)大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2016
【分類號】：TG146.22

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1 宋洋;關(guān)于高強(qiáng)度稀土鎂合金中強(qiáng)化相形成及原子排列的模擬研究[D];北京工業(yè)大學(xué);2016年

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本文編號：1993597