Mg-Zn-Y-La合金中二元相的电子结构,力学性质和光学性质的第一性原理计算

为了研究稀土元素对镁锌合金性能的影响,利用基于第一性原理计算的平面波赝势方法,对Mg₂Y、Mg₂La和Mg₃La的结构稳定性、电子结构、力学和光学性能进行了计算和分析。形成热和结合能的计算结果表明,Mg₃La具有最强合金化能力,而Mg₂La具有最强的结构稳定性。通过电子态密度(DOS),电子占据数和差分电荷密度分析了结构的稳定机制。计算了三种结构的弹性常数,并进一步得到了体模量B,剪切模量G,杨氏模量E和泊松比ν等。计算结果表明：Mg₂Y具有最强的抵抗变形能力,Mg₃La具有最强的刚度和抵抗剪切变形能力,而Mg₂La塑性最强。进一步分析表明Mg₂Y和Mg₂La为延性相,而Mg₃La为脆性相。此外,硬度和熔点的计算结果表明,三种金属间化合物中,Mg₃La的耐磨性最好,Mg₂Y的耐热性最好。最后计算并分析了三种晶体结构的折射率,反射率,吸收系数和损失函数。     

Zr-Be二元体系金属间化合物结构、电子、力学和热力学性能的第一性原理研究

基于密度泛函理论和广义梯度近似（GGA）方法,对Zr-Be二元合金中金属间化合物ZrBe₂、ZrBe₅、ZrBe₁₃和Zr₂Be₁₇的结构、电子、力学和热力学性能进行了第一性原理计算。优化后的0 K点阵参数与已有的实验结果基本一致,证明了计算的可靠性。通过计算得到的形成焓和结合能表明,所有的金属间化合物都能在0 K自发形成,其中ZrBe₅的合金化能力最强,ZrBe₂的结构稳定性最好。随后,电子态密度（DOS）也被用于了解金属间化合物的稳定性。采用应力-应变法计算了这些金属间化合物的独立弹性常数。在此基础上,利用Voigt-Reuss-Hill近似推导出了多晶材料的体模量B、剪切模量G、杨氏模量E、泊松比ν和各向异性A等力学参数。此外,利用Pugh准则、泊松比和柯西压力对金属间化合物的延性行为进行了分析。在热力学性能方面,除了利用准调和近似（QHA）计算晶格振动能量、体模、热膨胀系数和比热随温度变化外,所有的声子色散曲线都说明了这些金属间化合物的动态稳定性。     

Al-15wt.%Mg2Si复合材料中二元相的研究

原位内生法制备Al-15wt%Mg₂Si复合材料,其中二元相成分的研究很少,本文通过XRD测试方法,定性和定量的分析材料中的相成分和相含量。利用Reflex模拟Al-Mg-Si系合金的XRD衍射图谱,并与XRD测试结果对比。利用CASTEP中基于密度泛函理论的第一性原理方法,对Al-Mg₂Si复合材料中的Mg₂Si相和Mg₁₇Al₁₂相的平衡晶格常数,热力学参数和弹性性质做了系统的研究,分析两相的稳定性和力学性能。XRD分析结果显示,Al-15wt%Mg₂Si复合材料中仅含有α-Al相和Mg₂Si相,且Mg₂Si相的质量分数为14.9wt.%。模拟与实验中,相同晶面指数的XRD标定值相差很小,实验值验证了模拟值的可靠性。模拟值证实Al-Mg₂Si复合材料中,理论上只形成α-Al相和Mg₂Si相。CASTEP计算结果表明,Al-Mg₂Si复合材料中的Mg₂Si相较Mg₁₇Al₁₂相易于形成,且稳定性较好。Mg₂Si相的弹性模量E,剪切模量G,体模量B均大于Mg₁₇Al₁₂相,Mg₂Si相的力学性能较好,但脆性较大,且塑性较差。     

Mg-8Sn系镁合金SPS制备及其组织性能研究

基于Mg-8Sn系镁合金具有较好的强度、耐腐蚀性和塑性加工性,本文采用放电等离子烧结方法(Spark plasma sintering：SPS)在530～590 ℃制备了Mg-8Sn-1Al-1Zn镁合金,对镁合金的微观组织、物相、力学性能和耐腐蚀性能进行了研究。结果表明：镁合金的显微组织由α-Mg和Mg₂Sn相组成,Mg₂Sn相在界面处产生。在制备温度为570 ℃时弯曲强度最高,可达215 MPa。当烧结温度为590 ℃时镁合金的耐腐蚀性能最好,其电化学的电极电位和腐蚀电流分别为-1.5218 V和1.9632×10_-5 A/cm₂,这主要是由于Mg₂Sn在颗粒接合边界的存在,对α-Mg起到保护作用。     

离心铸造和热轧对Mg-10Gd-3Y-1Sn合金组织和机械性能的影响

采用离心铸造及热轧工艺制备Mg-10Gd-3Y-1Sn合金,利用X射线衍射、光学显微镜、扫描电子显微镜和拉伸试验对该合金的组织和力学性能进行了研究。结果表明：离心铸造Mg-10Gd-3Y-1Sn合金由α-Mg、Mg₂₄(Gd, Y)₅、Mg₂(Sn, Y)₃Gd₂和Mg₃(Gd, Y)相组成。随着离心半径和离心转速的增大,Mg-10Gd-3Y-1Sn合金的晶粒尺寸逐渐减小,抗拉伸强度逐渐增大。在700 r/min下制备的热轧试样在室温下极限抗拉伸强度为304 MPa,在300 ℃下极限抗拉伸强度为296 MPa。Mg₂₄(Gd, Y)₅、Mg₂(Sn, Y)₃Gd₂和Mg₃(Gd, Y)相具有优异的热稳定性,因而Mg-10Gd-3Y-1Sn合金具有优异的高温抗拉伸强度。     

Mg-Al-Sn-Y合金中二元相的电子结构与弹性性质的第一原理计算

采用基于密度泛函理论的CASTEP程序包,计算了Mg-Al-Sn-Y合金中Mg_(17)Al_(12),Mg_2Sn和Al_2Y相的结构稳定性、电子结构和弹性性能等。合金形成热△H和结合能E_(coh)的计算结果表明,Al_2Y相具有最强的合金化能力与体系结构稳定性。电子结构的分析结果解释了这3种金属间化合物的结构稳定性机制和脆性本质。计算出了Mg_(17)Al_(12),Mg_2Sn和Al_2Y三相的3个独立的弹性常数,并进一步得出了体模量、剪切模量、杨氏模量、泊松比等。分析表明Mg_(17)Al_(12),Mg_2Sn和Al_2Y三相均为脆性相,其中Al_2Y最脆且最硬。     

微量Gd添加对Mg-8Zn-1Mn-3Sn合金组织转变及性能的影响

研究了微量Gd的添加对Mg-8Zn-1Mn-3Sn合金显微组织及性能的影响。结果表明,Mg-8Zn-1Mn-3Sn-xGd主要由α-Mg基体、MgZn₂、Mg₇Zn₃、Mg₂Sn相、MgSnGd相组成。MgSnGd相为高温相,在合金凝固过程中最先形成,改变了凝固过程,使晶界处半连续第二相转变为断网状。MgSnGd相与α-Mg基体存在共格位向关系,能作为异质形核核心细化合金晶粒。Mg-8Zn-1Mn-3Sn-0.5Gd合金的综合力学性能最佳,合金力学性能得到显著提高的机制为通过添加Gd元素细化晶粒组织、MgSnGd相钉扎晶界阻碍位错运动以及晶界第二相形貌转变。     

纳米Mg2Sn增强镁基复合材料组织与性能

利用纳米Sn粉高的表面活性,通过微米Mg粉与纳米Sn粉的机械合金化高效合成了含原位纳米Mg₂Sn相的复合粉末,将所得复合粉末热压烧结,获得高性能纳米Mg₂Sn增强镁基复合材料。对比研究了不同机械合金化时间对镁基复合材料组织、性能的影响,结果表明：随着机械合金化时间的延长,由纳米Mg₂Sn相组成的团簇尺寸不断减小,分布更加均匀,烧结态Mg₂Sn/Mg复合材料的各项力学性能也得到不断提高。     

海水制氢Al-Mg-Ga-Sn合金的研究

采用铸造法制备6种Al-Mg-Ga-Sn合金。研究该类合金在模拟海水(3.5%NaCl)溶液中,30℃、50℃、70℃和90℃下的产氢行为,定量分析产氢速率、产氢量、能量密度和转化率。从组织结构和电化学角度进行了定性分析。采用XRD结合SEM分析发现：Al-Mg-Ga-Sn合金主要组成相与Al/Ga比值有关,Al/Ga＝17/1合金有Alss、Mg₂Sn;Al/Ga≤14/1合金有Alss、Mg₂Sn和Ga₅Mg₂;Al-Mg-Ga-Sn合金/海水反应产物除NaCl外,30℃～70℃时产物为Alss、Mg₂Sn和Al(OH)₃;90℃时产物为Alss和AlO(OH)。室温下,Al-Mg-Ga-Sn合金在3.5%NaCl溶液中的极化电阻随Al/Ga比值增大逐渐增大;且腐蚀电流与组织结构相关。在90℃,Al/Ga=8/1时,产氢速率19.36mL/(min?g),产氢量1.156×10^₃mL/g,能量密度3.515×10^₁₀J/m^₃,转换率达97.6％。     

Mg17Al12、Al2Y及Al2Ca相稳定性与弹性性能第一原理研究

采用基于密度泛函理论的Castep和Dmol程序软件包,计算了Mg17Al12、Al2Y及Al2Ca相的结构稳定性、弹性性能与电子结构。形成热和结合能计算结果表明:Al2Y具有最强的合金化形成能力和结构稳定性;热力学性质计算结果表明:在298~573 K温度范围内,Al2Y的Gibbs自由能始终最小,其结构热稳定性最好,Al2Ca次之,Mg17Al12最差,Y和Ca合金化Mg-Al系合金形成Al2Y及Al2Ca利于提高镁合金的高温抗蠕变性能;弹性常数的计算结果表明:3种金属间化合物均为脆性相,Mg17Al12的塑性最好;采用弹性常数计算结果预测的Al2Y熔点最高,其结构热稳定性最好。态密度和Mulliken电子占据数的计算结果表明:Al2Y结构最稳定的原因,主要源于体系在Fermi能级以下区域成键电子存在强烈的共价键作用。     

Elastic constants of binary Mg compounds from first-principles calculations

Elastic constants (C_ij's) of 25 compounds in the Mg–X (X = As, Ba, Ca, Cd, Cu, Ga, Ge, La, Ni, P, Si, Sn, and Y) systems have been predicted by first-principles calculations with the generalized gradient approximation and compared with the available experimental data. Ductility and the type of bonding in these compounds are further analyzed based on their bulk modulus/shear modulus ratios (B/G), Cauchy pressures (C₁₂–C₄₄), and electronic structure calculations. It is found that MgNi₂ and MgCu₂ have very high elastic moduli. Mg compounds containing Si, Ge, Pb, Sn, and Y, based on their B/G ratios, are inferred as being brittle. A metallic bonding in MgCu₂ and a mixture of covalent/ionic bond character in Mg₂Si, as inferred from their electronic structures, further explain the corresponding mechanical properties of these compounds.     

New TiNiSi-type RMgAg and RMgPd with R=Ca, Yb, and MgZn2-type Ca(Mg,Ag)2 compounds

The phases CaMgAg, YbMgAg, CaMgPd, and YbMgPd were synthesized by melting the constituent metals in sealed tantalum crucibles and by annealing at 1023 K. All the samples were homogeneous, and the crystallographic analysis, which was performed by powder and singlecrystal techniques, shows that the four compounds are isotypic and belong to the orthorhombic TiNiSi type. Magnetic measurements showed that YbMgAg and YbMgPd behave like Pauli paramagnets, according to the divalency of Yb in both phases. Within the Ca-Mg-Ag system, the existence range of the MgZn₂-type phases in the Mg-rich CaMg_2−x Ag _x pseudobinary system goes from CaMg₂ to CaMg_1.6Ag_0.4. Another stability region of the MgZn₂ structure occurs around the Ag-rich composition (Ca_0.94Mg_0.06)(Ag_1.60Mg_0.40), where magnesium replaces both the Ca and Ag atoms.     

New TiNiSi-type RMgAg and RMgPd with R=Ca, Yb, and MgZn2-type Ca(Mg,Ag)2 compounds

The phases CaMgAg, YbMgAg, CaMgPd, and YbMgPd were synthesized by melting the constituent metals in sealed tantalum crucibles and by annealing at 1023 K. All the samples were homogeneous, and the crystallographic analysis, which was performed by powder and singlecrystal techniques, shows that the four compounds are isotypic and belong to the orthorhombic TiNiSi type. Magnetic measurements showed that YbMgAg and YbMgPd behave like Pauli paramagnets, according to the divalency of Yb in both phases. Within the Ca-Mg-Ag system, the existence range of the MgZn₂-type phases in the Mg-rich CaMg_2−x Ag _x pseudobinary system goes from CaMg₂ to CaMg_1.6Ag_0.4. Another stability region of the MgZn₂ structure occurs around the Ag-rich composition (Ca_0.94Mg_0.06)(Ag_1.60Mg_0.40), where magnesium replaces both the Ca and Ag atoms.     

Zn元素对Mg-25Sn合金组织和力学性能的影响

以Mg粉、Sn粉和Zn粉为初始原料,采用机械合金化和热压烧结的方法制备Mg-25Sn-x Zn合金。研究了Zn添加量对Mg-25Sn合金显微组织和性能的影响。结果表明:Mg-25Sn-x Zn体系的机械合金化过程中,Zn元素不参与合金化反应,但Zn的引入降低了Mg+Mg2Sn混合物的尺寸。除固溶外,烧结态Mg-25Sn-x Zn中Zn完全转变成MgZn2相。且随Zn含量的增加,MgZn2相的尺寸逐步增大,Mg晶界和Mg2Sn颗粒相周围是MgZn2相的择优分布位置。添加6%Zn(质量分数)的Mg-25Sn合金具有最优的力学性能,其显微维氏硬度为1.60GPa、屈服强度为388MPa、抗压强度为497 MPa、断裂应变7.5%。     

Structural, elastic and electronic properties of Mg(Cu1−xZnx)2 alloys calculated by first-principles

The structural, elastic and electronic properties of Mg(Cu_1−xZn_x)₂ alloys (x = 0, 0.25, 0.5,and 0.75) were investigated by means of first-principle calculations within the framework of density functional theory (DFT). The calculation results demonstrated that the partial substitution of Cu with Zn in MgCu₂ leaded to an increase of lattice constants, and the optimized structural parameters were in very good agreement with the available experimental values. From energetic point of view, it was found that with increase of Zn content the structural stability of Mg(Cu_1−xZn_x)₂ alloys decreased apparently. The single-crystal elastic constants were obtained by computing total energy as a function of strain, and then the bulk modulus B, shear modulus G, Young's modulus Y and Poisson's ratio ν of polycrystalline aggregates were derived. The calculated results showed that among the Mg(Cu_1−xZn_x)₂ alloys, MgCuZn exhibited the largest stiffness, while Mg₂Cu₃Zn showed the best ductility. Finally, the electronic density of states (DOSs) and charge density distribution were further studied and discussed.     

Theoretical investigation of typical fcc precipitates in Mg-based alloys

First-principles calculations were performed to study structural, elastic and electronic properties of typical face-centered cubic (fcc) precipitates of Mg-based alloys (Mg₃Gd, Mg₃Gd_0.5Y_0.5 and Mg₃Zn₃Y₂) within the generalized gradient approximation. The calculated results show that the substitution of part of the Gd with Y in Mg₃Gd leads to a slight decrease in the cell volume (0.35%), and the lattice parameters obtained after full relaxation of crystalline cells are in good agreement with the experimental data. The calculated negative formation enthalpies and the cohesive energies show that these typical fcc precipitates of Mg-based alloys have good alloying ability and structural stability. According to the calculated density of states of these phases, it is found that the highest structural stability of Mg₃Zn₃Y₂ is attributed to an increase in the bonding electron numbers below the Fermi level. In addition, the elastic constants C_ij of these phases were also calculated, and the bulk modulus B, shear modulus G, Young^’s modulus E, Poisson^’s ratio ν and anisotropy value A of polycrystalline materials were derived from the elastic constants. The mechanical properties are further discussed.     

Theoretical elastic stiffness,structural stability and thermal conductivity of La2T2O7 (T = Ge,Ti, Sn,Zr, Hf) pyrochlore

In order to achieve better understanding of the structural/property relationships of La₂T₂O₇ (T = Ge, Ti, Sn, Zr, Hf) pyrochlore, first-principles calculations were conducted to investigate the bonding characteristics, elastic stiffness, structural stability and minimum thermal conductivity. The results show that the relatively weak La–O bonds play a predominant role in determining the structural stability, mechanical and thermal properties of these compounds. In addition, the elastic and thermal properties are influenced when the T atom changes from Ge to Hf. When the bonding strength is enhanced by applying hydrostatic pressure, apart from c₁₁, c₁₂, and B, which normally increase at high pressures, it is found that the shear elastic moduli, c₄₄ and G, which relate to the shear deformation resistance, abnormally remain almost constant. The underlying mechanism may help to explain the damage tolerance of pyrochlore compounds. After comprehensive consideration of the elastic anisotropy, a modified David Clarke-type equation is used to calculate the minimum thermal conductivity of the studied pyrochlore materials, which display an extraordinary low thermal conductivity.     

Diffusion kinetics,mechanical properties,and crystallographic characterization of intermetallic compounds in the Mg–Zn binary system

Pure Mg was diffusion bonded to pure Zn at 315 °C for 168 h to produce equilibrium intermetallic compounds of the Mg–Zn system. All equilibrium phases at 315 °C, Mg₂₁Zn₂₅, Mg₄Zn₇, MgZn₂, Mg₂Zn₁₁, were observed to develop. Concentration profiles by electron probe microanalysis, electron diffraction patterns by transmission electron microscopy, and load–displacement curves by nano-indentation were examined to characterize the phase constituents, crystal structure, diffusion kinetics, and mechanical properties. Mg₂₁Zn₂₅ with trigonal, Mg₄Zn₇ with monoclinic, and Mg₂Zn₁₁ with cubic structures were found and their lattice parameters were reported herein. Mg₄Zn₇ and Mg₂Zn₁₁ were observed to have a range of solubility of approximately 2.4 at% and 1.6 at%, respectively. Interdiffusion in MgZn₂ occurred most rapidly, was an order of magnitude slower in Mg₄Zn₇ and Mg₂Zn₁₁, and was the slowest in Mg₂₁Zn₂₅. Composition-dependence of interdiffusion within each intermetallic phase was negligible. The intermetallic phases exhibited insignificant creep, but evidence of discontinuous yielding was observed. The average hardness and reduced moduli were similar for Mg₂₁Zn₂₅, Mg₄Zn₇, and MgZn₂ phases, ∼5 GPa and ∼90 GPa, respectively. However, the Mg₂Zn₁₁ phase had lower hardness of 3.76 GPa and higher modulus of 108.9 GPa. The mechanical properties in the characterized intermetallic phases, exclusive of Mg₂₁Zn₂₅, were strongly concentration-dependent.     

Structural variations in the ternary system HfAl2−xCux (x = 0.2–1.0)

Synthesized ternary intermetallic phases HfAl_2?xCu_x in a series with (x = 0.49, 0.88, 1.04) are characterized as Laves phase structures. X-ray diffraction revealed homogeneity within ranges 0.2 ≤ x ≤ 0.5 and 0.7 ≤ x ≤ 0.9 for structure types MgCu₂ and MgNi₂, respectively. When Cu atoms gradually replaced the Al atoms, the structure type altered in the sequence MgCu₂ → MgNi₂ → MgZn₂, and the Kagomé nets were distorted with varied bond lengths. Measurements of physical properties revealed these phases to be metallic, with resistances 4.35 (x = 0.5), 5.85 (x = 0.7), and 6.50 (x = 0.9) mΩ cm, respectively, at temperature 300 K. The coloring schemes reveal that, upon increasing the proportion of Cu atoms, the stability of these phases correlated with the arrangements of the Al and Cu atoms. Calculated electronic structures indicate that the bonding character is consistent with the experimentally observed phase width.     

Effects of phase composition on the mechanical properties and damping capacities of as-extruded Mg-Zn-Y-Zr alloys

The present work mainly investigated the microstructures, mechanical properties, and damping capacities of as-extruded Mg-Zn-Y-Zr alloys with varied phase composition. Alloys of MgZn₂, W-phases (Mg₃Y₂Zn₃), I-phases (Mg₃YZn₆), and X-phases (Mg₁₂YZn) were obtained by adjusting the Zn/Y ratio (in wt%). The crystallographic structure of the X-phase [long period stacking ordered (LPSO) phase] and the crystallographic relationship between the W-phase and the Mg matrix were determined. The strengthening effects of the phase composition on the alloys exhibited the following trend: W + LPSO > LPSO>W + I > MgZn₂. Variations in the phase composition resulted in almost consistent variations in the damping capacities of the alloys compared with their mechanical properties. The LPSO structural phase could enhance the mechanical properties and simultaneously maintain the good damping capacity of the alloys.