Sn合金化MgZn_2相及Mg_2Sn相结构稳定性的第一原理研究

采用基于密度泛函理论CASTEP和DMol程序软件包,从合金形成热、结合能、热力学性能和电子结构等方面,研究Sn合金化MgZn2相及Mg2Sn相的结构稳定性,探讨Sn合金化改善ZA62镁合金抗蠕变性能的机理。结果表明:当Sn和Al分别置换ZA62镁合金中MgZn2相的Zn(Ⅰ)和Zn(II)原子时,仅Sn与Al置换的MgZn2相中Zn(Ⅰ)原子能形成稳定的MgZn2固溶体结构,而Sn在MgZn2相中的固溶量有限;与合金化形成的固溶体结构相比,其稳定性比未合金化时的弱,而析出的第二相金属间化合物Mg2Sn的结构比MgZn2的更稳定。而不同温度下热力学性能的计算结果表明:合金体系中形成了结构稳定性强的Mg2Sn,其结构稳定性在温度373～473K的范围内并不因温度的升高而消失,仍比MgZn2的高;由于ZA62镁合金体系中形成了高热稳定性的Mg2Sn相,Sn合金化有利于ZA62镁合金抗蠕变性能的提高。电子态密度和Mulliken电子占据数的分析结果表明:与MgZn2、Mg2AlZn3及Mg2SnZn3固溶体相比,热稳定性强的Mg2Sn相形成的主要原因在于Mg2Sn体系中存在强烈的离子键与共价键的共同作用。     

High-temperature stability of nano-grained B2-structured RuAl-based intermetallics by mechanical alloying

A series of nano-grained B2-structured RuAl-based intermetallics and corresponding composites are synthesized by mechanical alloying. These RuAl-based materials include stoichiometric RuAl, B2-structured ternary (Ru,Ni)Al and (Ru,Ir)Al, eutectic RuAl–Ru and particle reinforced RuAl–ZrO₂ composite. The accumulation of impurities, mainly Fe, in grain boundaries and the structural evolution during grain growth are found to be the controlling factors for the grain growth stagnation in nano-grained B2-structured binary RuAl and ternary (Ru,Ni)Al and (Ru,Ir)Al intermetallics. Ternary alloying element forming iso-structured pseudo-binary compounds, eutectic in situ and ceramic particle reinforced composites are explored to be the effective routes to enhance the stability of single-phase RuAl. The thermal stability of microstructure in various RuAl-based materials are found to decrease in a sequence from pseudo-binary RuAl–IrAl, RuAl–NiAl, eutectic RuAl–Ru composites, ceramic particle reinforced RuAl–ZrO₂ composites to binary stoichiometric RuAl.     

合金化对高铬铸铁相析出的影响

添加多元微量合金元素V、Ti、Nb和Mo到2.8C-31Cr合金中制备多元铬系合金,从合金热力学析出角度,通过计算分析Ti、V、Nb在多元体系中的存在方式,探讨添加的合金元素对高铬铸铁凝固组织中碳化物析出的影响。结果表明,Ti和Nb在高铬铸铁凝固过程中主要形成Ti C和Nb C,V主要存在于合金化合物VCr2C2和VCr Fe8中。先析出的Ti C和Nb C能充当碳化物异质形核基底,增加形核率使组织细化。但添加过量的合金元素却削弱了对碳化物的细化作用。     

The thermodynamic prediction of phase stability in multicomponent superalloys

Thermodynamic calculations, which are being performed on a daily basis in industrial laboratories such as the General Electric Corporate Research and Development Center, have proven to be of great assistance in the development of new alloys and the selection of processing conditions for existing or new alloys. This article presents an evaluation of the state-of-the-art ofthat capability for nickel-base superalloys.     

Mg17Al12相Ca合金化结构稳定性的第一原理研究

采用第一原理赝势平面波方法,计算了Mg17Al12相Ca合金化前后的能态与电子结构.计算结果显示:当Ca分别置换Mg17Al12相中Mg(Ⅰ)、Mg(Ⅱ)和Mg(Ⅲ)原子时,(Mg17-xCax)Al12相(x=0,1,4,12)的负生成热逐渐升高、结合能逐渐增大,表明Ca置换Mg17Al12相中Mg(Ⅲ)原子时其Ca合金化能力最强,合金化后形成的(Mg5Ca12)Al12相结构稳定性最高.电子态密度(DOS)分析结果表明:Ca合金化后Mg17Al12相结构稳定性升高的主要原因是由于合金化后来自Al(p)和Ca(s)的价电子使其在低能区的成键电子数增多.     

Effects of alloying elements on phase stability in Nb–Si system intermetallics materials

The effects of alloying elements, such as Ti, Cr, Al and Hf on phase stability in Nb–Si system intermetallics materials have been investigated by scanning electron microscopy (SEM), X-ray energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The binary, ternary and multicomponent alloys have been fabricated by vacuum non-consumable arc melting method. The results showed that Ti and Hf tend to stabilize Nb₃Si phase to lower temperatures, while Cr and Al promote the direct formation of Nb₅Si₃ phase. The coordinate effects of alloying elements on microstructures of Nb–Si system intermetallics materials have also been examined.     

Nickel-induced strengthening of boron-doped FeAl (B2) alloys

The effect of nickel additions on yield stress temperature dependence as well as on deformation mechanisms was investigated for Fe–Al alloys containing 40% Al. A set of different nickel containing alloys (up to 10 at.%), B-doped or B-free, was deformed in compression up to 950 °C. Subsequent dislocation structures were examined. It was shown that the addition of nickel strongly decreases the solid state solubility of boron in FeAl (B2) alloys. On the microscopic scale, nickel was confirmed to clearly decrease the dissociation width of the 〈111〉 superdislocations. From a mechanical point of view, it was shown to induce a large and linear strengthening at low temperatures (<400 °C). Owing to that specific hardening, the yield stress anomaly, usually occurring in such materials, was observed to be blurred out, but was nonetheless revealed by a zero strain rate sensitivity.     

Effect of Co additions on B2 phase stability of Ni-poor NiTi-based alloys

Based on the generalized atomic site preference model and the quasi-chemical bond approximation, the atomic distribution state free energy of the third addition in a binary alloy with the B2 lattice is derived analytically. The configurational energy coefficients are independent of the alloy composition and can be determined directly by the first principle calculation. The calculated B2 ↔ bcc transformation temperature of the Ni-poor NiTi-Co alloys changes at 200 K at.%⁻¹ by the increasing of Co concentration. The experimental results about the effect of the third Co additions on the martensitic transformation procedure of the Ni-poor NiTi-based alloys are well explained.     

High spatial resolution PEELS characterization of FeAl nanograins prepared by mechanical alloying

We investigate the nanograin “chemical” structure in a nanostructured material of possible industrial application (Fe–Al system) prepared by conventional mechanical alloying via ball milling in argon atmosphere. We restrict ourselves to the structural and nanochemical behaviour of ball-milled nanocrystalline Fe–Al powders with atomic composition Fe₃Al, corresponding to a well-known intermetallic compound of the Fe–Al system. Scanning transmission electron microscopy (STEM) equipped with a parallel detection electron energy loss spectrometer (PEELS) has provided an insight on the “chemical” structure of both nanograins and their surface at a spatial resolution of better than 1 nm. The energy loss near edge structure of the Al L loss reveals that the Al coordination is similar to a B2 compound and the oxidation of the powder during processing may play a significant role in the stabilization of the intermetallic phases. Conventional transmission electron microscopy (TEM) was used for the structural characterization of the material after the ball milling; powder X-ray diffraction (XRD) aided the investigation.     

Dynamic recrystallization and grain boundary migration in B2 FeAl

Transmission electron microscopy and optical microscopy were used to examine polycrystalline specimens of the B2-structured alloy FeAl strained under tension to fracture at elevated temperature. Strain-induced grain boundary migration was observed above 900K and dynamic recrystallization was found at 1000K and 1100K. Little evidence of dynamic recovery was evident but some networks were formed at 1100K.     

Long-range ordering in B2 FeAl: a resistometric study

Changes in the degree of order have been studied by residual electrical resistivity measurement (REST) in B2 long-range ordered Fe–44.8 at.% Al during isochronal and isothermal step-annealing temperature treatment. Restricted atomic mobility starts already below about 500 K. Assuming the vacancy concentration to remain constant over an extended temperature range, as known from the literature, the complicated variations of resistivity with temperature can be explained by two regimes of ordering: a fast local ordering due to the restricted motion of the vacancies at low temperatures and a very slow global adjustment of long-range order (LRO) within the total sample volume. Kinetic parameters are determined from the detailed process analysis of the ordering kinetics during isothermal small-step annealing. Using these parameters, the results of isochronal annealing treatment can be calculated with a very good correspondence to experiment.     

Influence of different alloying elements on the intermetallic phase seam thickness of compound forged steel-aluminum parts

In times of increasing energy costs automotive light weight construction is gaining more importance. Thus, attempts have been made to replace steel components with parts made of aluminum alloys. This, however, often involves the deterioration of important component properties and impedes material substitution in mechanically high-stressed parts like gear components. Due to this, the attempt to design load-optimized parts with hybrid structure which correspond to the requirements of structural light weight construction is made. The production of hybrid compounds by forging is a promising method to manufacture functional parts by applying resource-saving process steps. The aim of the presented research is to determine the optimal production parameters for compound forging. To characterize the component quality it is necessary to investigate the factors influencing the joining zone.     

Si合金化对C15 NbCr2 Laves相稳定性和断裂韧性影响的第一性原理研究

基于第一性原理计算方法,通过对形成焓、结合能、原子自由体积和电子结构的计算,研究了Si合金化对C15 NbCr₂Laves相稳定性和断裂韧性的影响。位点占据能表示Si原子倾向于占据Cr位点。形成焓和结合能计算表明,随着Si含量的增加,Nb₈Cr_16-xSi_x(X= 0~ 5)相的形成能力和稳定性均得到提升且与Si含量保持线性相关性。原子自由体积计算表明,Nb₈Cr_16-xSi_x相的原子自由体积较NbCr₂基体相均得到增加,其中在Si含量为8.33 at%(Nb₈Cr₁₄Si₂)时,原子自由体积取得最大值,断裂韧性达到最优。电子结构计算表明,Si合金化使得DOS曲线右移,费米能级向赝能隙峰谷靠近,稳定了NbCr₂基体相,同时所有的成键峰变得下降和展宽,削弱了Nb-Cr原子的键合强度,使得剪切变形易于进行,从而提高韧性。     

合金化对ZrMn2基Laves相贮氢合金相组成的影响

研究了Ni、V、Cr、Co、Fe、Cu和Ti等合金化元素取代ZrMn2 基Laves相贮氢合金的B侧或A侧对合金相组成的影响。结果表明 ,采用不同的元素对A侧或B侧进行部分取代 ,将引起ZrMn2 基合金相组成的变化。采用Ni取代Mn后 ,ZrMn2 合金的主相结构转变为C15型Laves相 ,表明Ni为C15相稳定元素。对Zr Mn Ni三元合金 ,V为C14相稳定元素 ,而Co、Fe、Cu则为C15相稳定元素。取代量较少时 ,Cr为C15相稳定元素 ,取代量增加时 ,C15相稳定作用减弱。Ti元素为C14相稳定元素 ,Ti对Zr的部分取代将导致合金主相结构转变为C14型Laves相。合金化元素对ZrMn2 合金的相组成的影响与元素的电子浓度和原子尺寸不同有关     

B2FeAl合金晶格动力学的嵌入原子模型计算

应用改进分析型EAM模型，系统计算了B2FeAl合金的晶格动力学性能，包括晶格常数、形成热、弹性常数、声子谱、态密度、比热、德拜温度和热膨胀系数等，并将计算结果与已有实验数据及其他理论计算结果进行了比较。总的来说，仅从合金组成元素的性质出发，不涉及合金本身性质而构建的改进分析型EAM模型成功地描述了B2FeAl合金原子间的相互作用，所得计算结果与实验值符合很好。     

Effect of boron on the thermodynamic stability of amorphous polymer-derived Si(B)CN ceramics

The reason for the higher thermal persistence of amorphous polymer-derived SiBCN ceramics (T ～ 1700–2000 °C) compared to SiCN ones (T ～ 1500 °C) has been a matter of debate for more than a decade. Despite recent experimental results which indicate a major kinetic effect of boron on the thermal persistence of the ceramics, no experimental investigation of the thermodynamic stability of the materials has been reported. In this work, we present measured energetics of a series of the amorphous ceramics with various boron contents (0–8.3 at.%) using high-temperature oxidative drop-solution calorimetry. Through measurement of the drop-solution enthalpies in molten sodium molybdate at 811 °C, the formation enthalpies of the amorphous ceramics from crystalline components (SiC, BN, Si₃N₄, C) at 25 °C were obtained and found to be between ?1.4 and ?26.6 kJ g-atom^?1. The determined enthalpy data plus the estimated positive entropy of formation values point to the thermodynamic stability of the amorphous ceramics relative to the crystalline phases, but such stabilization diminishes with increasing boron content. In contrast, the higher boron content increases the temperature of Si₃N₄ crystallization despite less favorable energetics for the amorphous phase, implying more favorable energetics for crystallization. Thus the so-called “stability” of SiBCN ceramics in terms of persistence against Si₃N₄ crystallization appears to be controlled by kinetics rather than by thermodynamic stability.     

Deformation induced transformations and grain boundary thickness in nanocrystalline B2 FeAl

Precise measurement of fundamental Bragg peak shifts during milling of nanocrystalline ordered B2 Fe₆₀Al₄₀ has allowed for the first time, the deconvolution of the 110 fundamental Bragg peak intensities of the b.c.c. disordered regions and of the ordered B2 regions from the start and before full disappearance of the latter. The evolution of the lattice parameter a₀ of the b.c.c. solid solution with milling time shows two characteristics. First a jump to higher values from the initial a₀ of the B2 phase, with a Δa₀ change of the order of 1% corresponding to a volume per atom ΔV_expansion of about 3%. Subsequently, a₀ continues to increase slowly with further milling at constant grain size D and in the absence of any B2 phase. This continuing change of a₀ with further milling up to at least 180 min is attributable to a reduction of chemical short-range order (CSRO) or the number of Al–Fe heteroatomic “bonds”. The appearance of two well-defined maxima in the hyperfine field (HF) distributions derived from the Mössbauer spectra indicated the presence of two ferromagnetic environments contributing to the broadened Mössbauer resonance sextet signal. The evolution of the second component of this Mössbauer signal scales with the grain size. Using the mean grain size D derived from X-ray peak profiles and TEM pictures together with the grain boundary thickness d_gb of 1.25 nm determined by Fultz et al. (J. appl. Phys., 1996, 7, 127) for b.c.c. Fe-based alloys, the fraction of grain boundary atoms n_gb/n_total was estimated and found to be consistent with the fraction of Fe atoms contributing to the lower HF component of the Mössbauer sextet signal. The grain boundary atom count using both methods confirms that grain boundaries in materials nanocrystallized by heavy deformation are nearly as dense as in the bulk.