Sigmoidal creep of Ni3(Al, Ta)

Incremental creep tests have been used to explore the time-dependent plastic behavior of single-slip oriented Ni₃(Al, Ta) at low temperatures in the anomalous flow regime. For selected incremental creep experiments at 20 and 100 °C, it was discovered that Ni₃Al exhibited sigmoidal creep, where there is a significant time delay before the plastic strain rate accelerates to a maximum value during a creep experiment. Several of the factors that affect the sigmoidal creep response have been identified. The origin of sigmoidal creep is accounted for using a simple model of work hardening in Ni₃Al, where the acceleration of the creep rate is a direct result of the annihilation of the existing dislocation substructure.     

Influence of pre-existing dislocations on the pop-in phenomenon during nanoindentation in MgO

In order to study the influence of pre-existing dislocations on the pop-in phenomenon in magnesium oxide, an original experimental protocol, based on low-rate cleavage, has been developed to introduce a controlled dislocation density in MgO single crystals. Nanoindentation tests have been performed using a spherical indenter and the pop-in load has been measured as a function of the pre-existing dislocation density. The dislocation structures have been then characterized individually by chemical etching and atomic force microscopy observations. A double etching method has allowed distinguishing the pre-existing dislocation behaviour in the indenter stress field and the dislocation nucleated below the indenter during the pop-in. These experiments show that the pre-existing dislocations lower the pop-in load and promote the dislocation nucleation below the contact area. However, the analysis of the dislocation structures nucleated during the pop-in shows no direct relation between the pre-existing dislocations and those nucleated during the pop-in.     

Two-phase intermetallic alloy Ni3(Al, Si):microstructure and mechanical properties

Yield stress in compression (0.2% flow stress) from ambient temperature up to 800 °C has been studied on Ni₃(Al, Si) alloy with the atomic composition Ni₇₈Al₁₁Si₁₁. When annealed at 1000 °C, the alloy has a pure L1₂ (γ′) ordered structure. After subsequent annealing at 750 °C, the disordered solid solution of Al and Si in Ni (face centred cubic, γ) precipitates in fine coherent particles. Calorimetry helps to describe the various phase transformations necessary to obtain the last microstucture. Solute addition of Si, which replaces Al atoms, increases the 0.2% flow stress of Ni₃Al in the fully γ′ microstructure. The γ precipitation shifts the peak stress towards higher temperatures and stresses.     

Reaction synthesis of Ni/Ni3Al multilayer composites using Ni and Al foils: High-temperature tensile properties and deformation behaviour

The effects of temperature on the tensile properties and deformation behaviour of Ni/Ni₃Al multilayer composites have been systematically investigated. With increasing tensile test temperature from room temperature to 600 °C, the ultimate tensile strength decreased. The ‘abnormal’ strengthening of Ni₃Al gave rise to a reduction in the capability for cooperative deformation between the Ni and the Ni₃Al layers at elevated temperatures. As a result, the ultimate tensile strength of the multilayer composites decreased at elevated temperatures. A mixture of transgranular cleavage and intergranular failure at relatively low temperatures, and an intergranular failure at elevated temperatures were observed in the fracture surfaces of the Ni₃Al layers in the multilayer composites. The splitting of the coarse precipitates along the {0 0 1} planes at 800 °C resulted from the differences in solubility of Al in Ni between room temperature and 800 °C.     

Hot deformation behaviour and grain refinement of intermetallic compound Ni3Al

The paper describes the preparation of Ni₃Al-based intermetallic compounds and investigations of their structures in the ascast, worked, and annealed states. The performed experimental works approved a possibility of hot working of the highly brittle polycrystalline intermetallic compound Ni₃Al with large grain cast structure. The facility of originating the recrystallization seems to be surprising in such a material. On the other hand, the growth of recrystallization nuclei is strongly retarded. The case is further complicated by the fact of heterogeneous deformation in the microvolume. Microalloying with boron or zirconium influenced the original structure parameters as well as the optimum deformation temperature. Simultaneously the probability of a transgranular cleavage fracture increased. For the stoichiometric Ni₃Al intermetallic compound the optimum deformation temperature 1150 °C has been assessed. At temperature 1250 °C, the alloy Ni₃Al + 0,1 at. % B manifested the best plasticity. The special type of rotary working combined with friction heating gave the best results from point of view of obtaining a fully recrystallized region.     

Evaluation of Ni–Ni3Al(5 wt.%)–Al(3 wt.%) as an anode electrode for molten carbonate fuel cell. Part II: wetting ability and performance in unit cell operation

In part I of this paper, the sintering and creep resistances of the five kinds of anode electrodes were compared and those of Ni–Ni₃Al(5)–Al(3) were even better than any other electrodes. In part II of this paper, the wetting abilities of the same five kinds of anode electrodes to the electrolyte and their performance in unit cell operation were investigated. Their contact angles, which indicate the wetting ability, were within the range between 77 and 84°. The contact angles of Al- and/or Ni₃Al-added electrodes such as Ni–Al(5), Ni–Ni₃Al(7) and Ni–Ni₃Al(5)–Al(3) were relatively lower than those of Cr-added electrodes. Although there was no evidence that the effect of Al and/or Ni₃Al addition to pure Ni could enhance the number of pores or improve their structure for more wetting ability, it could be clearly known that the component of Al and/or Ni₃Al in anode electrode could make the electrode be wetted by the electrolyte very well.

In unit cell operation, the electric resistance of Ni–Al(5) and Ni–Ni₃Al(5)–Al(3) were relatively lower than those of any other electrodes. After 120 h operation of the unit cell, the cell performance and the endurance of Ni–Ni₃Al(5)–Al(3) were even better than those of any other electrodes. And also its thickness shrinkage and porosity changes after unit cell operation were the least in five kinds of electrodes. The performance of Ni–Ni₃Al(5)–Al(3) as anode seems to be caused by the synergy effect between the strengthening characteristics of Ni₃Al and electric conductivity of Al.     

纳米压痕过程的多尺度模拟与Rice-Thomson位错模型分析

采用多尺度准连续介质法（简称QC方法）对单晶Ag薄膜纳米压痕过程进行模拟,研究压头宽度对纳米压痕过程中接触应力分布、位错形核临界载荷以及纳米硬度的影响,并用Rice-Thomson位错模型（简称R-T位错模型）进行分析。结果表明,纳米压痕获得的载荷-位移曲线呈现出的不连续性与位错之间的协同作用密切相关;压头尺寸对纳米压痕过程中接触应力分布、位错形核临界载荷以及纳米硬度具有明显的影响：随着压头宽度的增加,法向和切向接触应力以及纳米硬度值递减,呈现出明显的压头尺寸效应;而压头下方薄膜内位错形核临界载荷却递增,且与压头半宽度的平方根成正比。模拟结果与相应实验结果以及R-T位错模型计算结果吻合     

Understanding pop-in phenomena in FeNi3 nanoindentation

In this work, the spherical nanoindentation of FeNi₃ crystal was simulated by molecular dynamics (MD) for (0 1 0), (1 1 0) and (1 1 1) surfaces to study the origin of pop-in phenomena at the atomistic level. Simulated results demonstrate that the critical load, indentation modulus as well as dislocation nucleation processes are strongly dependent upon crystallographic orientations. The pop-in behaviors observed in nanoindentation are associated with dislocation reactions. The first pop-in event indicates the homogeneous nucleation of the 1/6<1 1 2>-type partial dislocations. Activated partial dislocations can transform into dislocation locks through complex dislocation reactions.     

Mechanical properties of polycrystalline copper and single-crystal LiF initial components for composite system Cu/LiF

The paper deals with an investigation of mechanical properties and deformation features of polycrystalline copper and single-crystal LiF under dynamic nano/microindentation. It is shown that the values of hardness and Young’s modulus depend on the magnitude of the applied load (P_max): when the load is increased, H and E decrease. General regularities of the indenter penetration process in a wide range of loads are revealed: the appearance of a “pop-in” effect at the initial stage of the loading process, the formation of more pop-in steps with the growth in load, and the formation of pileups around the indentations. Such a nature of deformation is the result of sequential activation of different dislocation mechanisms with indenter deepening. Along with a great similarity in the specificity of deformation, some differences are noted at the unloading stage. The results serve to compare the mechanical properties of Cu and LiF individual components with similar parameters of the “coating/substrate” composite systems (CS Cu/LiF) produced on their basis.     

High temperature thermal expansion characteristics of Ni3Al alloys

The thermal expansion of Ni₃Al alloys with and without ternary additions have been investigated with the aid of a dilatometer. The Ni₃Al alloys were studied over the temperature range 25–1000 °C. The coefficient of thermal expansion of all the aluminides studied in this investigation varies linearly with the temperature. The coefficient of thermal expansion of Ni₃Al is found to show an increase with the decrease in Al content from stoichiometric composition. B and Zr additions decrease the value of Ni₃Al alloys at room temperature while Hf and Ti additions do not alter it significantly.     

Al薄膜纳米压痕过程的多尺度模拟

采用多尺度准连续介质法分别模拟无缺陷和具有初始缺陷两种状态下,单晶Al薄膜纳米压痕初始塑性变形过程,得到载荷-位移响应曲线和应变能-位移变化曲线.研究了初始缺陷对纳米压痕过程中位错形核与发射、Peierls应力以及位错发射.临界载荷的影响.结果表明,在整个纳米压痕过程中出现了多次位错形核与发射现象,初始缺陷对第1和第3对位错的形核与发射影响较小,而对第2对位错的形核与发射具有明显的推迟作用,并伴随有裂纹扩展现象;由于初始缺陷引起薄膜材料内部严重的晶格畸变,导致系统应变能和位错运动的Peierls应力增加;裂纹扩展前,发射第2对位错需要的临界载荷增加,裂纹失稳后,位错发射需要的临界载荷下降.模拟获得的纳米硬度和Peierls应力与实验结果吻合.     

Recrystallization and grain growth in Ni3Al with and without boron

The recrystallization and grain growth of boron-free and boron-doped (500 ppm by weight) Ni₃Al polycrystals were investigated in the temperature range 700–1000°C. The levels of deformation were 17% reduction in length in compression for both alloys and 40% rolling reduction applied only to the boron-doped alloy. For alloys with or without boron, the Kolmogorov-Johnson-Mehl-Avrami relationship holds, with the Avrami exponent near unity, and the recrystallization rate is very slow. The activation energies for recrystallization are respectively 480 and 492 kJ/mol for boron-free and boron-doped to 40%. The grain growth kinetics obey a power law with the time exponent ranging from 0·2 to 0·4. The boron effect at 500 ppm was found to be small on both recrystallization and grain growth in Ni₃Al.     

Investigation of Strain Rate Effect by Three-Dimensional Discrete Dislocation Dynamics for fcc Single Crystal During Compression ProcessEI北大核心CSCD

Microelectromechanical systems (MEMS) have become increasingly prevalent in engineering applications. In these MEMS, a lot of micro-components, such as thin films, nanowires, micro-beams and micropillars, are utilized. The characteristic geometrical size of those components is at the same scale as that of grain, the mechanical behavior of crystal materials exhibits significant size effect and discontinuous deformation. In addition, those MEMS are often subjected to high strain rate at work, such collision and impact loading. The coupling deformation characteristics of small scale crystals and high strain rate makes their mechanical behavior more complicated. Accordingly, investigation of the effect of the strain rate on crystal materials at micron scale is significant for both the academia and industry. In this work, a plastic deformation model of fcc crystal under axial compression was developed based on three-dimensional discrete dislocation dynamics (3D-DDD), which considered the influence of externally applied stress, interaction force between dislocation segments, dislocation line tension and image force from free surface on dislocation movement during the process of plastic deformation. It was applied to simulate the plastic deformation process of a Ni single crystal micropillar during compression under different loading strain rates. 3D-DDD and theoretical analysis are carried out to extensively investigate the effect of strain rate on flow stress and deformation mechanisms during plastic deformation process of crystal materials. The results show that the flow stress and the dislocation density increased with the loading strain rate. In the case of low strain rate, the flow stress was dominated by the activation stress of FreakRead (FR) source in plastic deformation. With the increase of strain rate, the contribution of activation stress of FR source to the flow stress decreases and the effective stress gradually dominated the flow stress. Under high strain rate loading, with the increase of the initial FR source, the dislocation density also increased at the same strain correspondingly, which makes it easier to meet the requirement of the loading strain rate, so the flow stress is smaller. In addition, under the low strain rate loading, a few activated FR sources can meet the requirement of the plastic deformation, a single slip deformation come up as a result. While, as the loading strain rate increases, more and more activated FR sources would be needed to coordinate the plastic deformation, the deformation mechanisms of the single crystal micropillar transformed from single slip to multiple slip.     

Effect of Y203 on microstructure and oxidation of γ-Ni＋γ′-Ni3Al coatings transformed from electrodeposited Ni-Al films at 1 000℃

The electrodeposited Y2O3-dispersed γ-Ni＋γ-Ni3Al coatings on Ni substrates were developed by the conversion of electrodeposited Ni-Al-Y2O3 films with dispersed AI microparticles in Ni matrix into Ni3Al by vacuum annealing at 800 ℃ for 3 h. For comparison, Y2O3-free γ-Ni＋γ＇-Ni3Al coatings with a similar AI content were also prepared by vacuum annealing the electrodeposited microparticle-dispersed composite coatings of Ni-AI under the same condition. SEM and TEM characterizations show that the electrodeposited Y2O3-dispersed γ＋γ＇ coatings exhibit finer grains, a more homogeneous distribution of γ＇, and a narrowed γ＇ phase spacing compared with the electrodeposited Y2O3-free γ-γ＇ coatings. The oxidation at 1 000 ~C shows that the addition of Y2O3 significantly improves the oxidation resistance of the electrodeposited γ＋γ＇coatings. The effect of Y2O3 particles on the microstructure and oxidation behavior of the electrodeposited γ＋y＇ coatings was discussed in detail.     

The oxidation of nanocrystalline Ni3Al fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline Ni₃Al was fabricated through mechanical alloying of elemental powders and spark plasma sintering. The nanocrystalline Ni₃Al has a nearly full density after being sintered at 1223 K for 10 min under a pressure of 65 MPa. Isothermal and cyclic oxidations of nanocrystalline Ni₃Al were tested at 1173–1373 K with intervals of 100 K. The results indicate that nanocrystalline Ni₃Al exhibits excellent isothermal and cyclical oxidation resistance. The oxide scales consist primarily of dense and continuous -Al₂O₃. The grain refinement is beneficial for improving the oxidation resistance of Ni₃Al by providing more nucleation centers for the Al₂O₃ formation, promoting the selective formation of Al₂O₃ and improving the adhesion of oxide scales to the matrix.     

等离子喷涂氮化硼纳米片增强Ni3Al复合涂层的摩擦学性能

采用等离子喷涂技术在钛合金表面制备出六方氮化硼纳米片(BNNP)增强Ni₃Al复合涂层。结果表明,涂层物相主要为原位生成的Ni₃Al和少量Al₂O₃。相比于Ni₃Al涂层,BNNP优异的力学性能和自润滑性能赋予复合涂层良好的减摩耐磨性能,其耐磨性能提高了约1.5倍。Ni₃Al涂层的主要磨损机制是脆性断裂和三体磨粒磨损,BNNP/Ni₃Al复合涂层的主要磨损机制则转变为轻微的磨粒磨损,且磨损表面BNNP润滑转移膜的形成有益于抑制对磨偶件的接触损伤。     

Evaluation of Ni–Ni3Al(5 wt.%)–Al(3 wt.%) as an anode electrode for molten carbonate fuel cell: Part I: Creep and sintering resistance

To develop an anode electrode of molten carbonate fuel cell (MCFC) that has higher creep and sintering resistance and more stable reactivity than the traditional anode electrode, the five kinds of nickel anode electrodes such as Ni–Al(5), Ni–Cr(10), Ni–Ni₃Al(7), Ni–Ni₃Al(5)–Cr(5) and Ni–Ni₃Al(5)–Al(3) (number in parenthesis means the weight percent of its component) were prepared and their performance were investigated under the same conditions of MCFC operation. In part I of our study, their relative creep and sintering resistance were compared.

The Ni–Ni₃Al(5)–Al(3) anode electrode was the most resistant against the sintering and its porosity was kept over 60% even at 1000 °C. And its porosity decrease and thickness shrinkage by creep were the least among the five kinds of anode electrodes. Thus, the effects of the aluminum and nickel–aluminum intermetallic compound (Ni₃Al) addition to nickel as the dispersant strengthening and the solid solution strengthening agents were confirmed by comparative tests for sintering and creep resistance of five kinds of anode electrodes.

Besides these results, we could also know that the creep of anode electrode for MCFC mostly proceeded within 60 h from the start of operation irrespective of the kinds of anode electrodes. And in part II of paper, for more information about the wetting ability and cell performances of these five kinds of anode electrodes, the measurement of wetting ability and unit cell operations were carried out.     

Grain boundary self-diffusion of Ni in pure and boron-doped Ni3Al

Grain boundary (gb) self-diffusion in pure Ni-rich Ni₃Al was measured between 882 and 1374 K using the radiotracer ⁶³Ni, a serial sectioning technique and sensitive liquid scintillation counting. The results of the gb diffusivity P = δD_gb (δ : gb width, D_gb : gb diffusion coefficient) can be represented by the Arrhenius parameters P₀ = 3.27 · 10¹³and Q_gb = 168 kJ/mol. Additionally gb diffusion was investigated in boron-doped (0.24 at%) Ni-rich Ni₃Al in the range from 882 to 1352 K yielding P₀ = 1.24 · 10⁻¹² m³/s and Q_gb = 187 kJ/mol. The increase in the activation enthalpy Q_gb and the decrease of P upon boron-doping is explained by the segregation of B in Ni₃Al gbs, which may lead to an increase in the vacancy formation enthalpy and to a blocking of energetically favourable diffusion paths in the gbs. For comparison gb self-diffusion in pure Ni was remeasured yielding Q_gb = 112 kJ/mol. Ordering of the lattice and the preservation of ordering up to the gb planes, as predicted in Ni₃Al, therefore has a pronounced decelerating influence on gb diffusion, stronger than on bulk diffusion. Applying the semi-empirical relation of Borisov et al. (Phys. Met. Metallogr., 17 (1964) 80) gb energies γ_gb were determined for arbitrary high angle gbs in pure and B-doped Ni₃Al, resulting in 915 and 870 mJ/m², respectively, at 1100 K.     

金属间化合物Ni3Al薄膜制备工艺

采用直流磁控共溅射法,通过一步法、两步法工艺在衬底为SiO₂基体上制备了厚度为500 nm的Ni₃Al薄膜,X射线衍射(XRD)和透射电镜(TEM)等测试表明,一步法工艺制备的Ni₃Al薄膜为(111)取向的多晶型晶体结构金属间化合物优于两步法工艺。采用光学显微镜观察Ni-Al合金薄膜和金属间化合物Ni₃Al薄膜经高温氧化后的形貌,结果表明,Ni₃Al金属间化合物薄膜的高温抗氧化性能明显优于Ni-Al合金薄膜。     

Effect of Trace O Element on High-temperature Wettability Between Ni_3Al Melt and Y_2O_3 Ceramic

Effect of trace O element on the high-temperature wettability between Ni_3Al scrap melt and Y_2O_3 ceramic at 1873 K is investigated. With the increase in O content in Ni_3Al scrap melt from 6 to 21 ppm, the equilibrium contact angle decreases from 93.3° to 88.9°. The initial surface tension and adhesive work of molten Ni_3Al scrap drop from 3153.5 to 807.4 mN m~(-1) and from 2974.6 to 882.2 mN m~(-1), respectively. The average spreading rate increases from 0.03° s~(-1) to 0.17° s~(-1). The whole wetting processes are divided into four periods. The main driving force for spreading in period(1) is the adsorption and diffusion of active atoms around the interface. Trace O content in Ni_3Al scrap has a significant impact on interfacial reactions which mostly take place in the second stage, and can accelerate the spreading process on Y_2O_3 substrate. The reaction products during wetting process are Y_3Al_5O_(12), YAlO_3 and Al_2O_3.