本文選題：相穩(wěn)定性 + 層錯能　； 參考：《重慶大學(xué)》2016年博士論文

【摘要】：隨著現(xiàn)代工業(yè)生產(chǎn)的飛速發(fā)展,高性能材料日益成為人們關(guān)注的熱點。其中金屬間化合物具有其它普通金屬或合金所不具備的長程有序的超點陣結(jié)構(gòu),原子間表現(xiàn)出獨特的金屬共價雙重鍵合特征,宏觀上表現(xiàn)出低密度,較高的硬度、熔點,耐腐蝕性強(qiáng),良好的抗蠕變、抗氧化能力以及優(yōu)異的高溫服役能力,被廣泛地應(yīng)用于航空航天、汽車船舶等諸多領(lǐng)域,有望成為最具發(fā)展前景的高性能的高溫結(jié)構(gòu)材料。然而,其中最大的不足之處就是它的低溫脆性,這極大的限制了其廣泛應(yīng)用。從晶體結(jié)構(gòu)來看造成金屬間化合物脆性的主要原因是由于其較低的對稱性從而缺少足夠多的獨立的滑移體系。實驗上表明通過合金化手段來穩(wěn)定高對稱的亞穩(wěn)相從而提供較多的可開動的滑移體系以滿足多晶體延性條件的von Mises定則有望提升材料的塑性。然而這些機(jī)理并不十分明了,尤其關(guān)于微觀電子機(jī)制的研究還比較匱乏,因此本文將主要通過第一性原理計算和實驗方法相結(jié)合的手段研究合金化元素對幾種具有代表性的金屬間化合物材料性能的影響。本文主要基于第一性原理方法研究了MoSi2、TiAl、TiAl3以及Al3Zr這四種比較具有代表性的的金屬間化合物在不同類型原子摻雜情況下的相穩(wěn)定性、力學(xué)性能,結(jié)合相應(yīng)的實驗觀測并從彈性性質(zhì)、層錯以及原子成鍵角度解釋了材料的脆塑性機(jī)制,同時給出了的硬度、熔點等相關(guān)性質(zhì)。研究結(jié)果對改善金屬間化合物的塑性以及為高性能結(jié)構(gòu)材料的設(shè)計研發(fā)提供了理論參考與指導(dǎo)。本文的主要研究內(nèi)容如下:(1)MoSi2是由Mo、Si原子組成,表現(xiàn)出特有的金屬和陶瓷雙重屬性,具有高熔點、適中的密度以及極好的高溫穩(wěn)定性,然而低溫脆性是一個嚴(yán)重的缺陷。本文基于第一性原理研究了第三元素Al、Mg、Ge摻雜情況下MoSi2的相穩(wěn)定性及力學(xué)性質(zhì)。計算結(jié)果表明Al、Mg元素能夠有效地穩(wěn)定C40亞穩(wěn)相,并且當(dāng)Al、Mg元素的濃度分別達(dá)到7 at.%、6 at.%時MoSi2將會發(fā)生結(jié)構(gòu)相變從C11b相轉(zhuǎn)變?yōu)镃40相。相應(yīng)的材料的延展性也得到了很好的改善,這與Dasgupta等人的實驗結(jié)果符合的很好。此外研究還表明Al、Mg元素的加入能夠降低材料的硬度,相應(yīng)的熔點也有略微的降低,而Ge摻雜并沒有引起相結(jié)構(gòu)的轉(zhuǎn)變因而材料的各項性能并沒有發(fā)生較大的變化。此外還計算了Nb以及Al-Nb、Mg-Nb、Ge-Nb共摻雜情況下對材料性能的影響,計算結(jié)果表明只有Mg-Nb共摻雜的情況下,材料的力學(xué)性能才有所改善。以上表明,相結(jié)構(gòu)的轉(zhuǎn)變是提升材料力學(xué)性能的關(guān)鍵因素。最后從電子層次上研究了摻雜元素對原子鍵合作用的影響并對相結(jié)構(gòu)穩(wěn)定性以及上述力學(xué)性能做了分析。結(jié)果表明摻雜元素的存在能夠有效地削弱MoSi2中Mo-Si之間的共價相互作用,相應(yīng)的塑性得以改善。(2)作為優(yōu)良的輕質(zhì)高溫結(jié)構(gòu)材料的重要候選者之一,tial金屬間化合物表現(xiàn)出其它傳統(tǒng)金屬或合金所不具備的獨特的優(yōu)良性質(zhì)。本文基于第一性原計算了過渡金屬元素w、mo以及稀土元素sc、yb摻雜情況下tial的相穩(wěn)定性以及力學(xué)性質(zhì)。計算的選擇替位表明稀土元素sc、yb最有可能占據(jù)ti位置,而過渡金屬元素w和mo在l10相中傾向于占據(jù)ti的位置,在b2相中則更傾向于占據(jù)al的位置。計算的生成焓表明過渡金屬w和mo的摻入能夠有效地增強(qiáng)高對稱的亞穩(wěn)相b2相的穩(wěn)定性,當(dāng)w和mo的濃度分別達(dá)到10.5at.%和11.50at.%時,b2相將取代l10相成為穩(wěn)定相�；趐ugh判據(jù)分析了摻雜原子對tial脆塑性行為的影響。脆塑性相圖表明由于b2相的產(chǎn)生使得tial由本征脆性材料轉(zhuǎn)變?yōu)楸菊魉苄圆牧�。此�?計算結(jié)果還表明低濃度的稀土元素sc、yb雖能夠提高材料的延展性但效果很有限。這表明塑性改善的本質(zhì)原因是由于相結(jié)構(gòu)的轉(zhuǎn)變,這和實驗中觀測到的結(jié)果符合的很好。相應(yīng)的合金化元素對材料的硬度、各向異性等力學(xué)性質(zhì)的影響也進(jìn)行了詳細(xì)的研究。電子結(jié)構(gòu)表明tial金屬間化合物脆性的根本原因是由于ti、al原子之間較強(qiáng)的方向共價鍵存在,而塑性的改善是由于w、mo、sc、yb的摻入極大地削弱了ti、al之間的共價鍵,相應(yīng)的使得ti、al之間的金屬間相互作用得到增強(qiáng)宏觀上表現(xiàn)出延展性的極大提高。(3)具有長程有序的d022結(jié)構(gòu)的tial3由于其極低的密度,良好的耐腐蝕性以及抗氧化性,優(yōu)異的力學(xué)性能及高溫穩(wěn)定性,成為一類具有很好發(fā)展前景的輕質(zhì)高溫結(jié)構(gòu)材料。但其室溫脆性限制了材料的廣泛應(yīng)用。本文基于第一性原理密度泛函理論計算了ds區(qū)過渡金屬cu、zn、ag摻雜情況下tial3的相穩(wěn)定性及力學(xué)行為。生成焓表明在cu、zn、ag取代tial3中的al位置時能夠有效地穩(wěn)定高對稱性的立方l12相從而提供足夠多的有效的可開動的滑移體系。pugh比值、cauchy壓強(qiáng)以及泊松比表明l12相的產(chǎn)生使得材料的塑性得到改善。層錯能計算表明l12相穩(wěn)定性增強(qiáng)的原因是由于cu、zn、ag的摻入極大地促進(jìn)了d022相中110{001}滑移系的開動,從而引起結(jié)構(gòu)的轉(zhuǎn)變。此外,計算的l12相的層錯能表明塑性提升的原因很大程度上歸因于l12相中提供了有效的可開動的110{111}滑移系�；趃riffith斷裂理論并結(jié)合計算的解理能以及層錯能引入了關(guān)于脆塑性行為的斷裂韌性以及rice、zct判據(jù)。結(jié)果表明造成d022相tial3脆性的根本原因是由于在外加應(yīng)變下微裂紋極易在材料中萌生。最后電子態(tài)密度表明d022相脆性的原因是由于該相中al的3p電子與ti的3d電子的強(qiáng)烈的方向共價鍵的存在。而l12相的產(chǎn)生能夠使共價鍵得到抑制,主要是由于cu、zn、ag的摻入削弱了al的3p電子與ti的3d電子的共價相互作用,同時增強(qiáng)了ti原子和合金化原子的d-d相互作用,從而使得tial3中原子之間的鍵合力趨于平衡材料的塑性得到了提升。(4)研究了長程有序結(jié)構(gòu)的Al3Zr金屬間化合物。通過實驗手段我們用Al-35wt.%Cu、Al-4wt.%Zr以及高純度的Al制備了直徑為12mm長為110mm的Al-Cu-Zr三元合金鑄件。熱處理后通過透射電鏡觀察微組織結(jié)構(gòu)。結(jié)果表明Al基底中出現(xiàn)L12相Al3Zr沉淀顆粒。能譜分析(EDX)表明其中有Cu的出現(xiàn),經(jīng)確認(rèn)這些顆粒成份為Al2.5Cu0.5Zr,這表明一部分Cu原子取代Al原子的位置生成了穩(wěn)定相的Al3Zr顆粒。第一性原理計算的生成焓表明Cu確實是容易取代Al位置,理論和實驗符合的很好。此外高分辨率電子顯微鏡(HRTEM)發(fā)現(xiàn)沿著Al基底Al001晶帶軸出現(xiàn)L12相的Al-Cu-Zr顆粒并且在平行于該相的(001)面出現(xiàn)110(001)層錯。根據(jù)理論計算發(fā)現(xiàn)Cu的摻入會很大程度的降低D023相的110(001)層錯能;相反L12相110(001)層錯能卻會由于Cu的摻入大大的提升。實驗和理論相結(jié)合表明Cu的摻入能夠有效地增強(qiáng)L12相得穩(wěn)定性。通過計算的彈性性質(zhì)并基于Pugh判據(jù)以及泊松比分析了材料的力學(xué)性能,計算表明相比D023相L12相具有較好的塑性。此外,計算結(jié)果還表明過渡金屬元素Zn和Ag也具有和Cu相似的效果�；贕riffith脆性斷裂理論計算了斷裂韌性因子IcK并結(jié)合Rice以及ZCT判據(jù)對相變前后的脆塑性行為進(jìn)行了分析,不過斷裂理論給出的結(jié)論和彈性理論給出的結(jié)論有一定的矛盾。這些矛盾可能是Griffith脆性斷裂理論過于粗糙,因為真實材料中還會存在其它較為復(fù)雜位錯的影響。因此簡單的采用脆性斷裂或者不穩(wěn)定層錯能來定義材料的脆塑性行為還不夠精確,有必要結(jié)合電子結(jié)構(gòu)分析材料的力學(xué)性能。最后計算的電子態(tài)密度表明材料的脆性本質(zhì)是由于Al、Zr原子之間強(qiáng)的方向共價性結(jié)合,而Cu、Zn、Ag的摻入能夠很大程度上削弱Al-3p與Zr-4d的相互作用使得原子之間的鍵合力趨于平衡是提升材料塑性的本質(zhì)因素。
[Abstract]:With the rapid development of modern industrial production, high performance materials have become a hot spot of concern. Among them, intermetallic compounds have long range ordered superlattice structures that other ordinary metals or alloys do not possess. The unique metal covalent double bonding characteristics are shown between the atoms, and the macroscopically show low density, high hardness, and melting. With its strong corrosion resistance, good resistance to creep, antioxidation and excellent high temperature service, it is widely used in aerospace, automobile and ships and many other fields, and it is expected to become the most promising high performance high temperature structural material. However, the biggest disadvantage is its low temperature brittleness, which greatly restricts it. The main reason for the brittleness of intermetallic compounds from crystal structure is due to their low symmetry and the lack of enough independent slip systems. Experimental results show that the stability of highly symmetric metastable phase by means of alloying is provided to provide more open sliding systems to satisfy the ductility of polycrystal. The von Mises rule is expected to improve the plasticity of the material. However, these mechanisms are not very clear, especially the research on microelectronic mechanisms is still scarce. Therefore, this paper will study the properties of several representative intermetallic compounds by the combination of the first principle calculation and the experimental method. In this paper, based on the first principle method, the phase stability and mechanical properties of four representative intermetallic compounds with different types of atoms in MoSi2, TiAl, TiAl3 and Al3Zr are studied, and the material brittleness is explained from the elastic properties, the stacking faults and the atomic bonding angles. The research results provide theoretical reference and guidance for the improvement of the plasticity of intermetallic compounds and the design and development of high performance structural materials. The main contents of this paper are as follows: (1) MoSi2 is composed of Mo, Si atoms, showing a unique dual properties of metal and ceramics. There is a high melting point, moderate density and excellent high temperature stability, however, low temperature brittleness is a serious defect. Based on the first principle, the phase stability and mechanical properties of the third elements Al, Mg, and Ge doping are studied. The results show that Al, Mg elements can effectively stabilize the C40 metastable phase, and when Al, Mg element concentration is strong. The structure phase transition from C11b phase to C40 phase will occur at the degree of 7 at.% and 6 at.%, and the ductility of the corresponding materials is well improved. This is in good agreement with the experimental results of Dasgupta et al. In addition, the addition of Al and Mg elements can reduce the hardness of the material, and the corresponding melting point also decreases slightly. The Ge doping does not cause the phase structure transformation and the properties of the materials have not changed greatly. In addition, the effects of Nb and Al-Nb, Mg-Nb and Ge-Nb co doping on the properties of the materials are also calculated. The results show that the mechanical properties of the materials are improved only when the Mg-Nb co doping is codoped. The transformation is the key factor to improve the mechanical properties of the materials. Finally, the effects of doping elements on the cooperation of atomic bonds are studied at the electronic level, and the structural stability and the mechanical properties are analyzed. The results show that the existence of doped elements can effectively weaken the covalent interaction between Mo-Si in MoSi2 and the corresponding plasticity. (2) as one of the important candidates for excellent lightweight and high temperature structural materials, TiAl intermetallic compounds exhibit unique excellent properties that other traditional metals or alloys do not possess. Based on the first analysis, the phase stability and mechanics of the transition metal elements, W, Mo, and rare earth element SC, and Yb doped TiAl are calculated. The selected substitutions show that the rare earth element SC, Yb is most likely to occupy the Ti position, while the transition metal elements W and Mo tend to occupy the Ti position in the L10 phase, and are more inclined to occupy the Al position in the B2 phase. The calculation enthalpy of the formation of the transition metals W and Mo can effectively enhance the stability of the highly symmetric metastable phase B2 phase. When the concentration of W and Mo reached 10.5at.% and 11.50at.%, the B2 phase would replace the L10 phase as a stable phase. Based on the Pugh criterion, the influence of the doping atom on the brittle plastic behavior of TiAl was analyzed. The brittle plastic phase diagram showed that the TiAl from the intrinsic brittle material was converted to the intrinsic plastic due to the formation of the B2 phase. Furthermore, the calculation also showed that the low concentration of the TiAl was low. The rare earth element SC, Yb can improve the ductility of the material but the effect is very limited. This indicates that the essential reason for the plastic improvement is the phase structure transformation, which is in good agreement with the results observed in the experiment. The influence of the alloying elements on the hardness and anisotropy of the material is also studied in detail. The basic reason for the brittleness of TiAl intermetallic compounds is that the strong direction covalent bond exists between Ti and Al atoms, and the plastic improvement is due to the incorporation of W, Mo, SC, Yb, which greatly weaken the covalent bond between Ti and Al, and the corresponding intermetallics between Ti and Al can increase the maximum extension of the ductility between the Ti and al. (3) the TiAl3 with long range ordered D022 structure, due to its very low density, good corrosion resistance, antioxidation, excellent mechanical properties and high temperature stability, has become a kind of lightweight high temperature structure material with good prospects, but its room temperature brittleness restricts the wide application of material. This paper is based on the first principle density. The functional theory has calculated the phase stability and mechanical behavior of TiAl3 in the transition metal Cu, Zn, and Ag in the DS region. The enthalpy of formation indicates that the cubic L12 phase of high symmetry can be effectively stabilized when Cu, Zn, Ag take place of Al in TiAl3, thus providing enough effective and open sliding system.Pugh ratio, pressure and Poisson's ratio table. The formation of the L12 phase makes the plasticity of the material improved. The calculation of the stacking fault energy shows that the reason for the enhancement of the stability of the L12 phase is that the incorporation of Cu, Zn, and Ag greatly promotes the opening of the 110{001} slip system in the D022 phase, thus causing the transformation of the structure. In addition, the stacking fault energy of the calculated L12 phase indicates that the reason for the plastic lifting is largely attributable to L1. In the 2 phase, an effective 110{111} slip system is provided. Based on the Griffith fracture theory and the calculation of the cleavage energy and the stacking fault energy, the fracture toughness and the rice, ZCT criterion of the brittle plastic behavior are introduced. The result shows that the root cause of the D022 TiAl3 brittleness is that the micro crack is easily germinated in the material under the external strain. Finally, the electronic density of States indicates that the reason for the D022 phase brittleness is due to the existence of the strong directional covalent bond of the Al 3P electron and the 3D electron of the Ti. The formation of the L12 phase can inhibit the covalent bond, mainly because the incorporation of Cu, Zn, and Ag weakens the covalent interaction between 3P electrons of Al and the Ti electron. The D-D interaction between the atoms and the alloying atoms makes the bonding force between the atoms in the TiAl3 tend to balance the plasticity of the material. (4) the Al3Zr intermetallic compound of the long range ordered structure is studied. By means of experimental means, we use Al-35wt.%Cu, Al-4wt.%Zr and high purity Al to prepare Al-Cu-Zr with a diameter of 12mm as 110mm. The microstructure was observed by transmission electron microscope after heat treatment. The results showed that the L12 phase Al3Zr precipitated particles were found in the Al substrate. The energy spectrum analysis (EDX) showed that there was the emergence of Cu, and it was confirmed that these particles were Al2.5Cu0.5Zr, which indicated that a part of the Cu atoms replaced the Al atoms in the Al3Zr particles of the stable phase. The enthalpy of formation of the calculation of the principle of sexual principle indicates that Cu is really easy to replace the Al position. The theory and the experiment agree well. In addition, the high resolution electron microscope (HRTEM) found the Al-Cu-Zr particles in the L12 phase along the Al001 crystal band of the Al base and appeared 110 (001) layer faults in the (001) plane parallel to the phase. According to the theoretical calculation, the incorporation of Cu is very large. The 110 (001) layer of the D023 phase is reduced to a degree of degree, while the 110 (001) fault energy of the L12 phase will be greatly enhanced by the incorporation of Cu. The combination of the experiment and the theory shows that the incorporation of Cu can effectively enhance the stability of the L12 phase. The mechanical properties of the materials are analyzed by the elastic properties calculated and based on the Pugh criterion and Poisson's ratio. The L12 phase is better than the D023 phase. In addition, the calculation results also show that the transition metal elements Zn and Ag have the same effect as Cu. Based on the Griffith brittle fracture theory, the fracture toughness factor IcK is calculated and the brittle plastic behavior before and after the phase transition is analyzed with Rice and ZCT criterion, but the conclusion and projectile given by the fracture theory are given. There are some contradictions in the conclusions given by the theory of sex. These contradictions may be that the Griffith brittle fracture theory is too rough, because there will be other more complex dislocation in the real material. So it is not accurate to use brittle fracture or unstable layer fault energy to define the brittle plastic behavior of the material. It is necessary to combine the electrons. Structural analysis of the mechanical properties of the material. The final calculation of the electronic density of States indicates that the brittle nature of the material is due to the strong direction covalent bond between Al and Zr atoms, while the incorporation of Cu, Zn, and Ag can greatly weaken the interaction between Al-3p and Zr-4d, making the bond force between atoms balance the essential factor of improving the plasticity of the material.
【學(xué)位授予單位】：重慶大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TG146

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