Modelling structural changes of a molten Cu55 cluster on cooling by molecular dynamics

应用分子动力学研究了一个熔融Cu55团簇冷却过程中变为二十面体的结构变化. 根据原子密度分布函数和对分布函数随温度变化的分析表明, 在降温过程中由于原子之间连续地交换位置, 团簇结构发生了三个阶 段的变化: 首先形成三壳层结构, 继之初步形成四壳层结构, 并在团簇内部开始形成由13个原子组成的二十面 体结构, 最终形成原子分布于4个壳层内具有二十面体结构的Cu$_{55}$团簇. 模拟中由原子密度分布函数确定了不同壳层的原子位置.     

Using cluster dynamics to model electrical resistivity measurements in precipitating AlSc alloys

Electrical resistivity evolution during precipitation in AlSc alloys is modeled using cluster dynamics. This mesoscopic modeling has already been shown to correctly predict the time evolution of the precipitate size distribution. In this work, we show that it leads too to resistivity predictions in quantitative agreement with experimental data. We only assume that all clusters contribute to the resistivity and that each cluster contribution is proportional to its area. One interesting result is that the resistivity excess observed during coarsening mainly arises from large clusters and not really from the solid solution. As a consequence, one cannot assume that resistivity asymptotic behavior obeys a simple power law as predicted by LSW theory for the solid solution supersaturation. This forbids any derivation of the precipitate interface free energy or of the solute diffusion coefficient from resistivity experimental data in a phase-separating system like AlSc supersaturated alloys.     

Grain-boundary segregation in binary alloys

Results of experimental studies of grain-boundary segregation in binary alloys have been considered and analyzed. The model of grain-boundary segregation and an isotherm that allows for the possibility of the formation of complexes in grain boundaries have been suggested.     

Extending cluster dynamics to concentrated and disordered alloys: The linear-chain case

Cluster dynamics is often used in the multi-scale procedure to link atomistic and macroscopic approaches, in particular when modelling precipitation processes. To extend its use to concentrated alloys, it is necessary to take into account accurately the exclusion zones and their overlap. By investigating the one-dimensional case, one obtains an exact formula for these zones, and its generalization to higher dimensions is proposed. By integrating cluster fragmentation/coagulation processes into cluster dynamics equations, a perfect agreement between atomistic simulations (Kinetic Monte Carlo) and cluster dynamics is reached on the whole range of concentrations for kinetics which govern microstructure in one dimension.     

Graph theory and binary alloys passivated by nickel

The passivity of a nickel binary alloy is considered in terms of a network of -Ni-O-Ni- bridges in the oxide film, where Ni is the component of the binary alloy which produces passivity. The structure of the oxide is represented by a mathematical graph, and graph theory is used to calculate the connectivity of the oxide, given by the product of the number of edges in the graph and the Randic index. A stochastic calculation is employed to insert ions of the second metal into the oxide film so as to disrupt the connectivity of the -Ni-O-Ni- network. This disruption occurs at a critical ionic concentration of the oxide film. Mathematical relationships are developed for the introduction of a general ion B⁺ⁿ into the oxide film, and critical ionic compositions are calculated for oxide films on the nickel binary alloys. The notation B refers to any metal B which produces B⁺ⁿ ions in the oxide film, where +n is the oxidation number of the ion. The results of this analysis for Fe-Ni and Cu-Ni binary alloys are in good agreement with experimental results.     

Non-classical homogeneous precipitation mediated by compositional fluctuations in titanium alloys

This paper presents experimental evidence of homogeneous precipitation of the α-phase within the β matrix of a titanium alloy, and then accounts for this phase transformation by a new, non-classical mechanism involving compositional fluctuations, based on the pseudo-spinodal concept [1]. This mechanism involves local compositional fluctuations of small amplitude which, when of a certain magnitude, can favor thermodynamically certain regions of the β matrix to transform congruently to the α-phase but with compositions far from equilibrium. Subsequently, as measured experimentally using the tomographical atom probe, continuous diffusional partitioning between the parent β- and product α-phases during isothermal annealing drives their compositions towards equilibrium. For a given alloy composition, the decomposition mechanism is strongly temperature dependent, which would be expected for homogeneous precipitation via the compositional fluctuation-mediated mechanism but not necessarily for one based on classical nucleation theory. The applicability of this mechanism to phase transformations in general is noted.     

Twinned dendrite growth in binary aluminum alloys

The formation of twinned dendrites (feathery grains) in binary Al–Zn, Al–Mg, Al–Cu and Al–Ni alloys has been studied in specimens directionally solidified under identical thermal conditions, i.e. G ≈ 100 K cm⁻¹, v ≈ 1 mm s⁻¹, and with slight natural convection in the melt. The influence of the solute element nature and content has been found to be of less importance than previously reported since feathery grains were formed in all four alloys, regardless whether the alloying elements are hexagonal close packed (Zn and Mg) or face-centered cubic with a high (Ni) or low (Cu) stacking fault energy. A detailed analysis confirmed that twinned dendrites grow along 〈1 1 0〉 directions in all four cases, with a complex branch morphology made of up to six to nine arms. Surprisingly, at high Zn or Mg compositions for which regular dendrites grow along 〈1 1 0〉 instead of 〈1 0 0〉, [Gonzales F, Rappaz M. Metall Trans A 2006; 37: 2797. [1]] no twinned dendrites could be formed. In terms of both the growth kinetics advantage of twinned dendrites over regular ones and the associated tip shape, some experimental evidence seems to contradict the doublon conjecture suggested by Henry [Henry S. PhD thesis, Ecole Polytechnique Fédéral de Lausanne, 1999. [21]], at least for the solute compositions studied in the present work.     

Point defects in binary Laves phase alloys

Point defects in the binary C15 NbCr₂ and NbCo₂, and C14 NbFe₂ systems on both sides of stoichiometry were studied by bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000°C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were observed in NbCr₂ Laves phase alloys after quenching from 1400°C. However, there are essentially no thermal vacancies in NbFe₂ alloys after quenching from 1300°C. Anti-site hardening was found on both sides of stoichiometry for all the three Laves phase systems studied. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.     

Grain refinement of Mg-Al binary alloys inoculated by in-situ oxidation

Utilizing oxide inclusion to induce heterogeneous nucleation event is an available method to achieve grain refinement. In this study, Mg-Al binary alloys were refined by inoculation of in-situ oxidation process. Results show that MgO and MgAl₂O₄ phases are primary oxide products for Mg-xAl alloys inoculated by in-situ oxidation. For pure Mg and Mg-1Al alloy, MgO is the only oxide product. MgAl₂O₄ is another oxide product for Mg-xAl alloy as Al content increases to 3 wt.%. For Mg-3Al alloy, average grain size significantly decreases from 1135 to 237 μm, with a high grain refining ratio of 79.1%. Both MgO and MgAl₂O₄ possess nucleating potency for α-Mg grain. MgAl₂O₄ exhibits a higher nucleating potency due to the lower misfit with α-Mg. The grain refinement of Mg-xAl alloys inoculated by in-situ oxidation process is attributed to heterogeneous nucleation events of α-Mg grains on MgO or MgAl₂O₄ particles.     

Optimization of conductivity and strength in Cu-Ni-Si alloys by suppressing discontinuous precipitation

Cu-Ni-Si alloys with and without Ti were solution treated at 980 °C for 1 h and cooled by air cooling and water quenching, after which the specimens were aged at 500 °C. The two alloys showed different aging characteristics with different cooling rates during the aging process. The conductivity of all alloys increased during aging; for alloys that were water quenched, hardness increased at the early stage of aging and then drastically decreased. The air-cooled Cu-Ni-Si alloy without Ti also experienced an increase in hardness, which then decreased during aging, but the air-cooled Cu-Ni-Si alloy with Ti did not show a drastic decrease in hardness during prolonged aging. A combination of yield strength and conductivity of 820 MPa and 42% IACS, respectively, was achieved in the air-cooled Cu-Ni-Si-Ti alloy after solution treatment.     

Nanophases of binary and multicomponent alloys

I present a theory which explains the appearance of an amorphous-like, disordered phase in nanoparticles of binary alloys. The theory claims that this phase represents a transition state between two bulk phases of the binary system. While the transition state is completely unstable in the conditions of an open system, where exchange of the species between the particle and its surrounding is not limited, it may become stabilized in the closed system where the species exchange is prohibited. I derive the material-parameters criterion, which is the condition for the stabilization. The transition state represents a shallow minimum of the molar Helmholtz free energy as a function of the order parameter and is not significant in a large system because the global optimum there is delivered by the heterogeneous mixture of the two bulk phases connected by the common-tangent construction. However, in a particle of the size below the critical, the transition state becomes the global optimizer because of the prohibitively large energy “cost” of the phase separating interface. Thus the theory explains the effect as due to the free energy of the phase separating interface, not the energy of the free surface of the particle. The theory sheds light on the structure of the critical nuclei in the process of nucleation of one phase from another one. New experiments to verify the theory are suggested.     

The oxidation of binary iron-manganese alloys

Oxide scales formed on iron-manganese alloys are generally considered to be similar to those formed on the parent metals. Metallographic examination is used in conjunction with EMPA, XRD, and X-ray imaging to reveal the presence of the additional phases Mn₂O₃ and Mn₃O₄ in the scales formed on binary alloys containing up to 40% manganese at 700 and 800°C in 200 Torr oxygen. A mechanism is proposed to explain the apparent change in growth from iron-rich to manganese-rich scales during oxidation of Fe-40Mn at 800°C.     

Some properties of binary vanadium alloys

Conclusions Increasing the concentration of the alloying element in binary alloys of vanadium with titanium, chromium, tin, and aluminum at concentrations of 0–25 wt. % leads to an increase of hardness and electrical resistivity, particularly for alloys with tin and aluminum. The strength will be highest for alloys of vanadium with tin.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 62–63, July, 1971.     

Numerical simulation of precipitation in multicomponent Ni-base alloys

A comprehensive particle size distribution model has been developed for the simulation of γ′ precipitation in multicomponent Ni alloys. Nucleation, growth and coarsening of the precipitates are described by a particle size distribution. The growth rate of each precipitate class is calculated with a multi-component diffusion model formulated for non-diagonal matrices of diffusion coefficients. The model is fully coupled with CALPHAD calculations of the thermodynamic equilibrium at the interface, including a direct treatment of the effect of curvature through modification of the Gibbs free energy. An optimization strategy was developed to minimize the computational cost. The model was used to simulate ageing heat treatment at 600 °C of Ni–7.56 at.% Al–8.56 at.% Cr, which was studied experimentally by Booth-Morrison and others (Booth-Morrison C, Weninger J, Sudbrack CK, Mao Z, Noebe RD, Seidman DN. Acta Mater 2008;56:3422; Mao Z, Booth-Morrison C, Sudbrack CK, Martin G, Seidman DN. Acta Mater 2012;60:1871). The comparisons showed that the precipitation stages of γ′ precipitates are correctly captured by the numerical model. It was shown that non-diagonal diffusion coefficients substantially influence the selection of the operating tie-line and the overall transformation kinetics. With non-diagonal diffusion matrices, complex phenomena such as uphill diffusion of Cr due to the Al gradients were evidenced and explained.     

Thermal stability data of amorphous binary alloys

Data on the thermal stability of amorphous (glassy) binary alloys are compiled. Where available, alloy preparation method, crystallization temperature and method of measurement of crystallization temperature are tabulated. Other relevant aspects such as crystallized phases and compositions, glass transition temperature and analytical parameter variations also are given when data are available.     

Microstructure and properties of Mg-Al binary alloys

O ver the past decade, the large-scale development and application of magnesium alloys have been greatly stimulated by the demand for lightweight and environment- friendly materials in the automotive industry [1-4]. Most commercial casting magnesium alloys are mainly Mg-Al alloys, such as AZ91D, AM60B, AM50A, AE42, and AS21 alloys. Since AZ91D, AM60B and AM50A alloys contain relatively high Al content they offer a good combination of mechanical properties, corrosion resistance and ca…     

Al-Er-Zr合金的时效析出过程

利用显微硬度、TEM、HREM等测试方法对Al-Er-Zr合金的时效析出过程及析出相Al3(Er1-xZrx)的粗化行为进行研究。结果表明:二元合金Al-0.04Er在375℃时效5 min后出现硬度峰(约为40.3HV),随后迅速下降出现过时效;添加Zr能够显著提高其热稳定性,三元Al-Er-Zr合金中过时效现象显著滞后于Al-Er二元合金。在三元合金Al-Er-Zr中,随着Zr含量的增加,合金在长时间时效后,由于Er、Zr的协同析出而出现第二个更高的时效峰值,约为53.5HV,明显高于Al-Er二元合金的硬度。Al3(Er1-xZrx)粒子在高温粗化过程中逐渐长大,其平均直径d与退火时间t的关系符合LSW理论中的关系式。