Fe对NiAl力学性能影响的第一原理计算

采用第一原理赝势平面波方法,基于虚拟晶体势函数近似(VCA),计算Fe合金化(浓度x<3.0%,原子分数,下同)时完整与缺陷B2-NiAl晶体的弹性性质,并采用弹性常数C44、Cauchy压力参数(C12-C44)、杨氏模量E、剪切模量G及其与体模量B0的比值G/B0等,表征和评判Fe合金化浓度x对NiAl金属间化合物延性与硬度的影响。结果表明:无论是无缺陷的理想NiAl晶体,还是含Ni空位或Ni反位的NiAl缺陷晶体,x<0.6%的Fe合金化均可使其硬度大幅提高。Fe合金化浓度低于0.5%时,虽然完整NiAl晶体的延性变差,但含Ni空位的缺陷NiAl晶体的延性却可明显改善,并以x=0.2%～0.4%时韧化效果最好。Ni空位或Ni反位降低B2-NiAl晶体的本征延性。实验中0.20%～0.25%的Fe合金化对NiAl晶体延性的改善很可能源于Fe原子与NiAl晶体中Ni空位间的关联与协同作用。     

微量P对NiAl力学性能影响的理论研究

采用第一原理赝势平面波方法和基于虚拟晶体势函数近似(VCA),计算了P微合金化(浓度x<0.2at%)时完整与缺陷B2-NiAl晶体的弹性性质,并采用弹性常数C44、Cauchy压力参数(C12-C44)、杨氏模量E、剪切模量G及其与体模量B0的比值G/B0等,表征和评判了P微合金化浓度x对NiAl金属间化合物延性与硬度的影响。结果表明:无论是理想NiAl晶体,还是含Ni空位或Ni反位的NiAl缺陷晶体,P合金化不能有效的影响其硬度的变化趋势;随着P合金化浓度的稳步增加,理想NiAl晶体的Cauchy压力参数和G/B0比值的变化再现了0.07at%的P可以提高NiAl的室温压缩塑性的实验结果,而缺陷NiAl晶体的Cauchy压力参数和G/B0比值的变化比较好的解释了实验上出现0.03at%的P提高NiAl室温压缩塑性可能是因为Ni反位协同占据Al原子位的P相互作用的缘故。     

3d过渡金属在NiAl中占位的第一原理计算

采用第一原理赝势平面波方法研究了NiAl-X(X为3d过渡金属)合金体系的几何与电子结构.通过合金形成能的计算与分析发现:合金化元素的外层价电子数对其在B2-NiAl中的占位有非常明显的影响,低价电子数的前过渡金属Sc、Ti、V与后过渡金属Zn主要占据Al原子位,具有未满d壳层高价电子数的Mn、Fe、Co则主要占据Ni原子位,而含有半满或满d壳层的Cr与Cu,则既可占据Ni原子位也可占据Al原子位,但倾向于占据Ni原子位.随着合金化元素外层价电子数的增加,3d过渡金属优先占据Ni原子位的趋势增大,至Mn时达到最大,然后随着价电子数的进一步增加,这种趋势逐渐减小.通过对这些合金化元素价电子态密度图的变化,说明3d过渡金属在B2-NiAl中的占位优先趋势.     

NiAl力学性质合金化效应的第一原理计算

采用第一原理赝势平面波方法,计算几种合金化元素X(X=Cr、Mn、Fe、Co和Cu)与不同Fe含量(0,3.125%,4.167%,6.25%,摩尔分数)(NiFe)Al超胞的几何与电子结构,并采用如下几个力学参数:弹性常数C44、Cauchy压力参数(C12-C44)、弹性模量E和剪切模量G及比值G/B0等,表征和评判了合金化元素X与不同Fe含量对NiAl金属间化合物延性与硬度的影响。结果表明:高浓度(6.25%)合金化虽可提高NiAl晶体的硬度,但却导致NiAl延展性降低,合金化后NiAl硬度增加的次序为:(Ni7Mn)Al8>(Ni7Co)Al8>(Ni7Fe)Al8>(Ni7Cr)Al8>(Ni7Cu)Al8>NiAl,其延性降低次序则与硬度增加次序相反;随着Fe含量的升高,NiAl晶体的硬度增加,但其使硬度增加的上限约为4%,而随着Fe含量的降低,NiAl晶体的延性逐步增大,当Fe含量低到一定程度时,可改善NiAl晶体的本征脆性。     

α2-Ti-25Al-xNb合金力学性质的第一原理计算

采用第一原理赝势平面波方法计算了D0_19结构的α_2-Ti-25Al-xNb(x=O—12,原子分数,%)晶体的弹性模量(B, G和E)和抗拉强度(σ_b),并利用Cauchy压力(c_(12)-C_(44))与G/B比值表征和评判了不同浓度Nb合金化时α_2一Ti-25Al- xNb合金的韧脆化倾向.结果表明:在x=2—12时,α_2-Ti-25Al-xNb晶体的抗拉强度(σ_b)与σ_2相合金的弹性模量(B, E和G)随x增加而增大;在x=0—6时,α_2-Ti-25Al-xNb合金脆性有一定改善,且x值越大韧化效果越好;但在x=7—9时,相对于α_2-Ti_3Al,合金脆性不但没有得到弱化,反而随x增加而加剧;随后,当x进一步增大时,合金脆性又随x增加再次得到改善,至x=12时,α_2-Ti-25Al-xNb合金的韧化效果最好.通过电子态密度(DOS)和投影电子态密度(PDOS)等电子结构的分析,初步解释了Nb的这种强化与韧化作用.     

Calculation Of Interdiffusion Coefficients Of Ni And Alatoms In Β--Nial Phase As Aluminide Coatings

通过纯Ni和K465合金表面渗Al涂层中Al原子浓度分布曲线的分析，考虑到K465合金复杂的化学成分和显微组织，基于一定的假设，推导出Wagner修正式，并分别计算了850，950和1050℃纯Ni和K465合金表面渗Al涂层中β-NiAl相Ni，Al互扩散系数。计算结果表明：K465合金表面渗Al层中β-NiAl相Ni，Al互扩散系数明显小于纯Ni表面渗Al涂层中β-NiAl相Ni，Al互扩散系数；与化学计量比β-NiAl相相似，K465合金表面渗Al中β-NiAl相Ni，Al互扩散系数与Al原子浓度有很大的相关性。简要分析了合金元素和析出相对渗Al涂层中β-NiAl相Al，Ni互扩散系数的影响。     

Co,Cr和Ni对Fe3Al金属间化合物相平衡的影响

利用集团变分法，计算了Fe-Al-Co,Fe-Al-Cr,Fe-Al-Ni三元合金700K等温截面的Fe3Al相区，三个三元合金等温截面的计算结果显示，Fe-Al-Ni相图中的Fe3Al相区最小，Fe-Al-Cr相图中的Fe3Al相区最大；根据等温截面的几何性质，可以将Co和Ni看成是一类原子，而Cr原子属于另一类，利用Monte Carlo方法分析了Co,Cr和Ni对Fe3Al金属间化合物原子组态的影响，从不同原子在不同亚点阵上的占位情况和原子组态图都可以看出，Cr原子倾向于占据β亚点阵；Co和Ni原子倾向于占据α亚点阵，也有一部分Co和Ni原子倾向于与Al原子形成B2结构晶粒，分布于DO3晶粒中，这将加剧晶体中缺陷的形成，从Monte Carlo法得到的这些结果，也可以看出Co和Ni属于一类原子，而Cr属于另一类。     

Re与Ru合金化对Ni/Ni3Al相界电子结构影响的第一原理研究

采用第一原理赝势平面波方法研究了Re与Ru合金化前γ-Ni/γ'-Ni3Al相界的电子与能态结构.断裂功计算结果显示:Re置换γ-Ni相区中的Ni或Ru置换γ'-Ni3Al相区中的Al,均可提高Ni/Ni3Al相界的断裂强度;Re与Ru在相界区复合合金化时,当Re与Ru分别占据共格(002)γ/γ'原子层邻近(001)γ原子层上的Ni原子位与(001)γ'原子层上的Al原子位时,γ-Ni/γ'-Ni3Al相界的断裂强度可进一步提高,若其中的Ru置换γ'-Ni3Al相区内层Al,刚复合合金化Ni/Ni3Al相界的断裂强度不仅没有提高,反而比Ru单独合金化时Ni/Ni3Al相界的断裂强度还低.电子态密度与电子密度分布图的分析表明:Re与Ru合金化对γ-Ni/γ'-Ni3Al相界断裂强度的影响可归因于Re和Ru与其最近邻Ni原子间强烈的电子相互作用引起的相界区域层间原子成键相互作用的改变.     

Cu合金化Mg2Ni氢化物能量与电子结构的赝势平面波计算

采用基于密度泛函理论的第一原理赝势平面波方法,计算了Cu合金化前后Mg2Ni相及其氢化物的能量与电子结构.负合金形成热的计算结果表明:Cu合金化Mg2Ni形成Mg2Ni(Ⅱ)"1-xCu(x=1/3)的相结构稳定性最高,两个Cu原子最易占据Ni(Ⅱ)的(0,0.5,0.166 67)与(0.5,0,0.5)位置;进一步对其氢化物的解氢反应热进行计算,发现Cu合金化后,氢化物体系解氢反应热与合金化前相比,明显降低,表明Cu合金化Mg2Ni氢化物的解氢能力增强;电子态密度(DOS)与电子密度的计算结果发现:Mg2Ni(Ⅱ)" 1-xCux(x=1/3)相结构最稳定的主要原因在于:Cu合金化后氢化物在费米能级处的成键电子数N(EF)与其它结构相比最少;而Cu合金化Mg2Ni氢化物解氢能力增强的主要原因在于:Cu合金化削弱了氢化物中Mg-Ni和Ni-H间的成键作用以及相应原子在低能级区成键电子数的减少.     

B掺杂对NiTiNb合金相结构稳定性的模拟研究

采用第一原理计算方法,研究了NiTi(B)和NiTi(Nb,B)晶体的形成热焓、结合能、态密度及其电子密度差分。结果表明,在NiTi相基体中,掺杂元素B占4Ti2Ni的形成能力比其在4Ni2Ti中的形成能力更强。对于NiTi(Nb)固溶相,B在相同基体中占据最近邻位比其占据次级邻位更稳定、形成能力更强。态密度和电子密度差分计算表明,B原子与Ti原子、Nb原子形成非常强的共价键,而其与Ni之间成键强度较弱,导致B周围的Ti原子活性降低,从而起到提高Ni/Ti摩尔比,降低NiTiNb合金形状记忆效应的作用。     

Cr、Mo对DO3-Fe3Al力学性能和电子结构影响的理论研究

采用基于密度泛函理论的第一性原理方法,研究了Cr、Mo在DO₃-Fe₃Al 金属间化合物中的占位、以及Cr、Mo对Fe₃Al力学性能和电子结构的影响。研究显示：Cr、Mo在Fe₃Al中优先替代Al原子;Cr、Mo的添加均提升了Fe₃Al的体模量、剪切模量、弹性模量以及塑性,其中Mo对Fe₃Al的模量以及塑性的提升效果更明显。电子结构和电荷密度分析表明：Cr、Mo对Fe₃Al力学性能提高的主要原因是Cr、Mo的s,p和d轨道电子参与了Fe₃Al的电子杂化,Cr、Mo增加了Fe₃Al的成键峰数量,且增加了Fe₃Al中原子间的重叠电子数,此外Cr、Mo减弱了Fe₃Al中Fe-Fe间电子云的方向性。     

Synergistic effect of co-alloying elements on site preferences and elastic properties of Ni3Al: A first-principles study

The site preferences of co-alloying elements (Mo–Ta, Mo–Re, Mo–Cr) in Ni₃Al are studied using first-principles calculations, and the effects of these alloying elements on the elastic properties of Ni₃Al are evaluated by elastic property calculations. The results show that the Mo–Ta, Mo–Re and Mo–Cr atom pairs all prefer Al–Al sites and the spatial neighbor relation of substitution sites almost has no influence on the site preference results. Furthermore, the Young's modulus of Ni₃Al increases much higher by substituting Al–Al sites with co-alloying atoms, among which Mo–Re has the best strengthening effect. The enhanced chemical bondings between alloying atoms and their neighbor host atoms are considered to be the main strengthening mechanism of the alloying elements in Ni₃Al.     

Molecular dynamics study of reaction pathways in an Al-coated Ni nanoparticle

Using molecular dynamics simulation in combination with an embedded atom method potential we analyze the alloying reaction in an Al-coated Ni nanoparticle with equi-atomic fractions and a diameter of about 9.5 nm. The first stage of the alloying reaction is controlled by interdiffusion between the f.c.c. Al-shell and f.c.c. Ni-core. Then, the large driving force for further alloying of Ni and Al promotes solid state amorphization of a supersaturated metastable f.c.c. Ni–Al solid solution in the vicinity of the interface region and, eventually, the whole shell of the nanoparticle. It is shown that there are at least two further possible pathways of the reaction. The first pathway occurs through crystallization of the amorphous shell into an Al-rich B2-NiAl phase before the complete dissolution of the f.c.c. Ni-core followed by interdiffusion between the Al-rich B2-NiAl shell and the f.c.c. Ni-core. A prediction is made that under certain conditions, interdiffusion between an Al-rich B2-NiAl shell and an f.c.c. Ni-core in such a nanoparticle may result in the formation of a hollow B2-NiAl nanoparticle. The second pathway leads to the complete dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell. In order for this scenario to be realised the rate of temperature increase at the stage of dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell should be higher than in the previous case. We also demonstrate some possibilities to control the self-heating rate at the stage of dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell, which is important for a switching of the reaction pathways in a desired direction.     

Influence of Alloying Elements on the Mechanical Properties of PtAl<Subscript>2</Subscript> from First-Principles Calculations

Although PtAl₂ is a promising high-temperature alloy, the improvement of its strength is still a big challenge. To solve this problem, we apply first-principles calculations to study the influence of alloying elements on the structural stability, elastic properties and brittle-or-ductile behavior of PtAl₂. The results show that alloying elements prefer to occupy the Al site in comparison to the Pt site. Importantly, the calculated bulk modulus of doped PtAl₂ is much larger than that of the parent PtAl₂ due to the formation of TM-Pt and TM-Al bonds. In addition, alloying elements effectively improve the ductility of PtAl₂. Finally, our work can provide new information to improve the mechanical properties of Pt-Al high-temperature materials.     

Microstructural changes of aluminized Alloy 617 during high-temperature aging

In this study, aluminized Alloy 617 was prepared by Al-pack cementation of high temperature high Al activity process. The microstructure evolution and microstructural changes of aluminide coating were investigated after Al-pack cementation and high-temperature aging. The aluminide coating was composed of Ni-aluminide layers, such as δ-Ni₂Al₃, β-NiAl, Cr₂Al, Al_3 + xMo_1 − x, and inter-diffusion zone by pack cementation. After high-temperature aging, the aluminide coating was transformed from the δ-Ni₂Al₃ to the β-NiAl because of outward Ni diffusion from substrate. The Cr₂Al and the Al_3 + xMo_1 − x were dissolved during aging. On the other hand, the α-(Cr, Mo) particles were precipitated during aging due to the low solubility of alloying elements in the β-NiAl. The β-NiAl newly formed by the outward Ni diffusion during aging and resulted in the formation of the inter-diffusion zone. The inter-diffusion zone consisted of β-NiAl, Ni₃(Al, Ti), Cr-rich M₂₃C₆ carbide, and sigma phases.     

过渡元素改善B4C/Al材料界面润湿性的机理研究

B₄C/Al复合材料是目前最理想的中子吸收材料,但工业上常用的液态搅拌法制备过程中存在着界面润湿性差的问题。本文结合实验及第一性原理的方法,通过研究Al(111)/AlB₂(0001)和Al(111)/TiB₂(0001)界面的结构来分析工业上添加过渡元素Ti对B₄C/Al界面润湿性的改善机制。通过计算发现,Al(111)/TiB₂(0001)界面相对Al(111)/AlB₂(0001)界面具有更高的粘附功值,说明其界面结合更强。进一步对比Ti掺杂二硼化物和AlB₂的偏态密度结构,发现Ti掺杂体具有较低的反键态,表明Ti-3d和B-2p轨道电子杂化后,在B、Ti原子间形成了较强的化学键,从而促进了Al(111)/TiB₂(0001)界面处的强结合作用,提高了Al(111)/TiB₂(0001)界面粘附功,故而改善了B₄C/Al界面的润湿性。根据同样的理论依据,V掺杂体也具有较低的反键态,V和B之间的强结合效果或许能够改善B₄C/Al界面的润湿性,成为又一理想的溶体改性掺杂元素。     

Ti 元素对B4C/Al复合材料力学性能的改善作用研究

B₄C/Al复合材料是目前最理想的中子吸收材料,广泛用于乏燃料储存。本文利用液态搅拌法制备B₄C/Al复合材料,通过添加Ti元素,探讨了界面反应对材料的界面结构和力学性能的影响。研究发现,Ti元素通过参与界面反应,改变了界面结构,在B₄C颗粒表面形成了紧密结合的纳米TiB₂界面层;Ti的添加消除了界面微裂纹、微孔、分离等缺陷。随着界面反应程度的加强,材料强度提高,尤其反应脱落的纳米TiB₂颗粒作为原位第二强化相进一步增强基体。B₄C/Al复合材料断裂过程表现为韧窝延性断裂;TiB₂界面层增强了B₄C颗粒与基体的结合,断裂行为从B₄C-Al界面脱落转变为B₄C颗粒断裂;但过渡的界面反应会形成微韧窝,引起材料延伸率下降。     

An experimental study of bonding and crystal structure modifications in MoSi2 and MoSi2+xAl (x=10 to 40 at% Al) via Auger parameter shifts and charge transfer calculations

The alloying behaviours of as-cast MoSi₂ and MoSi₂+xAl alloys have been studied using high-energy XPS with Cr K_β radiation. The charge transfer occurring upon alloying was calculated using the variations in the Auger parameters of Mo, Si and Al between alloyed and unalloyed conditions and the linear potential model of Thomas and Weightman and the non-linear potential model of Cole, Gregory and Weightman. In MoSi₂ there was a significant increase in the Auger parameter of Si, while the shift in the Auger parameter of Mo was negligible. The charge transfer towards the Si atoms was close to zero and is smaller compared to theoretical calculations. It is concluded that the atomic bonding between Mo and Si is of a covalent p–d character. In MoSi₂+xAl alloys, similar observations were made for Mo and Si, while the Auger parameter of Al was reduced. Donation of electronic charge by Al atoms is possible; covalent bonds of Al with Mo are formed. The plasmon loss structures of the Si 1s and Al 1s peaks showed reduced intensity in the alloys relative to the pure metals. This was attributed to more strongly bound valence electrons. The opposite was the case for the Mo 2p_3/2 peak. The substitution of Si by Al atoms is confirmed, in agreement with previous studies.     

Changes of an Outer β-NiAl and Inner α-Cr Coating on Ni–40 at% Cr Alloy during Oxidation at 1373K in Air

Formation mechanisms of a coating with a duplex layer, outer β-NiAl(Cr) and inner α-Cr(Ni) layer structure on a Ni–40.2 at% Cr alloy were proposed and change in the coating structure was investigated during high temperature oxidation. The Ni–40.2 at% Cr alloy was electro-plated with about 12μm Ni followed by a high Al activity pack cementation at 1073K to form a coated layer with an outer δ-Ni₂Al₃ and an inner layer containing Al more than 70at% which grew with an inward diffusion of Al. The coated Ni–40.2at% Cr alloy was oxidized at 1373K in air for up to 2592ks. It was found that at the initial stage of oxidation the as-coated layer structure changed to a two-layer, outer β-NiAl(Cr) and inner α-Cr(Ni), structure. Al contents in the α-Cr(Ni) layer was less than 0.3at%. With long term oxidation an intermediate γ-Ni(Cr, Al) layer formed between the outer and inner layers, whereas the inner α-Cr(Ni) layer became thinner and then disappeared after the 2592ks oxidation at 1373K. Coating processes and changes in the coating structure during high temperature oxidation were discussed based on diffusion and composition paths plotted on a Ni–Cr–Al phase diagram