Al-Cu-Cr准晶的力学性能

用雾化法制得的Al65Cu20Cr15准晶粉末与纯铝热压合成复合材料,在热压和热处理状态下复合材料中的颗粒为尺寸在35-160μm范围的Al-Cu-Cr二十面体准晶和十次准晶及其类似相。采用维氏压痕实验技术测试估算了Al-Cu-Cr准晶颗粒的力学性能。结果表明,热压状态下的Al-Cu-Cr二十面体准晶材料的断裂韧性和抗拉强度约为0.82MPa.m1/2和715MPa,经退火处理后得到的Al-Cu-Cr十次准晶及其类似相材料的断裂韧性和抗拉强度约为1.07MPa.m1/2和879MPa,淬火处理后约为1.15MPa.m1/2和1054MPa,与Al-Cu-Cr二十面体准晶材料相比有所提高,淬火态的Al-Cu-Cr十次准晶材料的断裂韧性和抗拉强度最高。     

A mechanical spectroscopy study of the Zr69.5Cu12Ni11Al7.5 alloy

The vibrating reed technique has been used to investigate the Zr_69.5Cu₁₂Ni₁₁Al_7.5 glass forming alloy. The phase transformations of the material, as can be seen through the changes of the resonance frequency of the sample as a function of temperature, have been studied and the results have been compared with TEM and DSC measurements confirming the development, above T_g, of a metastable quasicrystalline state prior to crystallisation. Hydrogen-induced damping peaks were then observed in the different phases of the material with particular attention to the quasicrystalline state, where it seems that the microscopic reorientation mechanism responsible for the internal friction peaks should be similar to the one already known in the amorphous phase (hydrogen jumps between tetrahedral sites). Shortly after crystallisation, three damping peaks were observed, two of which are attributed to relaxation processes (Zener-type or intercrystalline Gorsky effect) taking place in the tetragonal CuZr₂ phase.     

Low temperature relaxations associated with quantum tunnelling of H in Sc and Y

Acoustic spectroscopy and electrical resistivity measurements have been employed to investigate the two relaxation processes occurring at liquid He temperature in very dilute Y–H and Sc–H alloys, and ascribed to H tunnelling systems near single or paired O traps. At very low H concentration, the peak due to the O–H tunnelling system becomes extremely narrow and is describable in terms of incoherent tunnelling with phonon assisted transitions.     

准晶增强Mg-0.6%Zr合金的力学与阻尼性能

在Mg-0.6%Zr高阻尼合金中加入质量比为5的Zn与Y元素,通过普通铸造方法向其中引入Mg-Zn-Y系准晶进行强化,并在此基础上研究准晶增强Mg-0.6%Zr合金的力学和阻尼性能。结果表明:Mg-5xZn-xY-0.6%Zr合金中生成一定含量的I-Mg3YZn6准晶相,I相的生成能大幅度提高Mg-0.6%Zr合金的力学性能;常温下Mg-5xZn-xY-0.6%Zr合金的阻尼行为可由G L位错模型解释,高温下界面阻尼机制启动,合金的阻尼值急剧升高;I相等准晶颗粒对晶界有钉扎作用,导致高温下Mg-4.5%Zn-0.9%Y-0.6%Zr等合金的阻尼性能不如Mg-0.6%Zr合金的。     

Upconversion dynamics in Er doped nanocrystalline YAlO3

We report the luminescence and upconversion spectra of nanocrystalline YAlO₃ doped with trivalent erbium at concentrations of 5.0, 1.0 and 0.1 mol.%. The powder samples were prepared using a solution combustion reaction method, and the resulting YAlO₃ nanocrystals show, under wide-angle X-ray diffraction, a size in the range 20–40 nm. Efficient green and red emissions are observed at room temperature under continuous-wave pumping at 980 nm. A weak emission can also be detected in the blue at 410 nm. The upconversion dynamics were studied measuring the decay times and the pump-power dependence of the transitions to the ⁴I_15/2 ground state starting from the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 excited states. Excited-state absorption (ESA) is found to be responsible for the higher energy (²H_11/2, ⁴S_3/2→⁴I_15/2) green transitions. On the other hand, for the ⁴F_9/2→⁴I_15/2 red transition a competing energy-transfer upconversion (ETU) mechanism is found, which accounts for the more than 100-fold increase in intensity of the red emission on passing from the lowest (0.1 mol.%) to the highest (5 mol.%) erbium concentration.     

Effects of Sn-doping on the electrical and thermal transport properties of p-type Cerium filled skutterudites

p-type Sn-doped CoSb₃-based skutterudite compounds have been prepared using melting-quenching-annealing method and spark plasma sintering technique. Sn atoms in our samples are completely soluted on Sb-site with a fixed charge state and non-magnetic feature, providing a better choice to ascertain the effect of element doping at the [Co₄Sb₁₂] framework on the electrical and thermal transport properties in p-type skutterudites. Doping Sn at the framework introduces additional ionized impurity scattering to affect the electron transport greatly. Similar electrical transport properties between Ce_0.2Co₄Sb_11.2Sn_0.8 and Co₄Sb₁₁Sn_0.6Te_0.4 suggest that Ce fillers contribute little to the valence band edge. Filling Ce into the voids and doping Sn at the framework introduce additional phonon resonant and point defect scattering mechanisms, thereby reducing lattice thermal conductivity remarkably. Moreover, our data suggest that combining these two effects is more effective to suppress lattice thermal conductivity through scattering broad range of phonons with different frequencies.     

Mechanical spectroscopy of Al-Mg alloys

The internal friction of deformed, annealed, and quenched Al-Mg alloys with Mg contents of 0–12 wt % has been studied as a function of temperature. The measurements have been performed in the range of 25–580°C at 0.3–30 Hz using a DMA Q800 TA Instruments dynamic mechanical analyzer at the maximum amplitude of deformation ?₀ = 5 × 10^?5. In annealed alloys with Mg contents up to 5 %, a relaxation IF peak with activation parameters (activation energy H ≈ 1.7–2.1 eV) has been detected. In alloys with 8–12% Mg, this relaxation peak is absent and, at a lower temperature, another IF peak appears (with an activation energy H of about 1 eV), which is interpreted as the Zener peak in terms of the combination of activation parameters. The article discusses the nature of these peaks and the influence of alloying on the relaxation mechanisms. In cold-worked alloys, an IF pseudo-peak takes place, which is related to the recrystallization of samples, and its temperature position in the alloys that contain more than 5% Mg depends not only on the degree of deformation, but also on the dissolution temperature of the β phase.     

Effects of high-pressure high-temperature treatment on the thermoelectric properties of PbTe

Changes in the thermoelectric properties of nominally undoped stoichiometric PbTe caused by high-pressure high-temperature (HPHT) treatment near 6.5 GPa and 900 °C are reported. Comparison of the electrical resistivity, thermopower, and thermal conductivity suggest that the carrier concentration and lattice defect density are decreased by HPHT treatment. Annealing the treated samples at intermediate temperatures (500–600 °C) reversed the changes. These observations are consistent with Pb vacancies being “squeezed out” by the applied pressure, and subsequently reintroduced by annealing.     

Effects of point defects on properties of B2 NiAl: A first-principles study

The basic properties, heats of formation, energies of formation, equilibrium concentration of point defects, elastic properties, and electronic structures for point defect structures of B2-NiAl crystal are detailed analyzed by the density functional theory. Compared with B2-NiAl and other B2 intermetallic compounds, purity NiAl has the better ductility and bonding strength. According to the calculated heats of formation, energies of formation, and equilibrium concentration of point defects, the Ni antisite and Ni vacancy are primary defects in B2-NiAl crystal. The calculated G/B0 and Cauchy pressure parameters C12–C44 values confirm that Ni vacancy, Ni antisite, and Al antisite can promote the brittleness of B2-NiAl, and in which Ni vacancy is the primary defect, while Al vacancy with a low concentration can improve ductility of B2-NiAl. The density of state confirms that B2-NiAl intermetallic compounds are conductors, and point defects can promote the stability of the system expect Ni antisite defect.     

Cationic doping of MgO surfaces to build corrosion protection in Mg alloys

Reactivity of MgO surfaces towards dissociative adsorption of water is crucial in determining the protective performance of oxide films formed on top of Mg alloy surfaces. Engineers and scientists have repeatedly reported that heavy metal contamination could significantly affect the oxide film stability towards water. We performed first-principles analysis on MgO surfaces adsorbed with water molecules in the presence of six different types of cationic impurities - Zn²⁺, Al³⁺, Y³⁺, Mn⁴⁺, Zr⁴⁺, and Ce⁴⁺ - to assess the effect of commonly present metal impurities in MgO films on the dissociative adsorption of water. The trend in the adsorption energies with different metal impurities shows that water is less likely to dissociate when cationic impurities with high oxidation states such as Mn⁴⁺, Zr⁴⁺, and Ce⁴⁺ are present. Charge transfer from the adsorbed species to the impurity atom is identified as the source of the trend. Increased dissociation barrier for water adsorbed on the same doped surfaces indicates that the incorporation of such impurities will effectively block the dissolution of oxide films and potentially enhance the film's resistivity to water.     

点缺陷对MOX燃料热导率的影响研究

MOX燃料作为快堆燃料重要的候选燃料类型之一,是核燃料闭式循环的关键环节。导热性能是MOX燃料在堆内服役的关键参数,将直接影响快堆的安全性。目前通常只对实测数据进行拟合得到其热导率经验公式,缺乏对不同Pu含量和氧金属比（O/M）的MOX燃料热导率的理论分析。本文基于经典声子热传导理论,通过对MOX燃料在不同Pu含量和O/M下热导率影响因素的分析,研究了不同类型点缺陷对其热导率的影响。研究结果表明,采用声子传导模型可以很好地预测不同成分MOX燃料的热导率。MOX燃料中离子替代型点缺陷造成的质量差和半径差对热导率的影响权重相对较小,而MOX燃料不同O/M比造成的氧空位型点缺陷对热导率的影响权重相对较大。本研究对于不同成分MOX燃料的热导率预测和成分设计具有重要意义。     

Snoek relaxation in a copper-precipitated alloy steel

We studied the effect of thermal embrittlement of a steel containing 1.29 at% copper on the Snoek relaxation. The hardness increased with isothermal aging at 723 K and decreased after showing a maximum. Hardness change is governed by bcc–copper clusters precipitated from the ferrite iron crystal. Using a forced-vibration torsion-pendulum method, we studied the internal-friction spectrum and recovery of the maximum relaxation strength after quenching from 723 K. We observed a broad nonsymmetrical spectrum which apparently consists of three Debye peaks. With higher hardness, the recovery rate increased. We interpreted our results in terms of Nowick’s theory of interstitial/substitutional-solute interactions. For recovery, we used the Cottrell–Bilby–Harper t^2/3 model.     

Nanostructured metals under irradiation

Particle irradiation has a pronounced influence on the mechanical and thermal properties of materials, largely due to the generation of point defects. Owing to their large free surface to volume ratio, nanostructured metals show improved irradiation resistance. This nanostructuring can negatively impact the creep resistance of these materials, but by introducing fine precipitates this effect can be balanced.     

Thermal vacancy formation in Co-based Heusler-type alloys Co2MnZ (Z = Si, Ge, Sn)

Thermal vacancy formation was studied for the Heusler-type ferromagnetic alloys Co₂MnZ (Z = Si, Ge, Sn) as a function of temperature (773–1273 K) by the density, electrical resistivity and positron annihilation measurements. The vacancy concentration increased with increase in quenching temperature and particularly, a high vacancy concentration exceeding 2% was observed in Co₂MnGe and Co₂MnSn. Estimated vacancy formation and migration energies were comparable with those for B2-type FeAl and CoGa alloys with high vacancy concentration. Further, the vacancy type and the vacancy site were examined for alloys quenched from 773 K. As a result, it was suggested that the mono-vacancies are randomly distributed over the lattice sites.     

Interstitial distribution in Fe–Al and Fe–Cr quenched and aged alloys:: Computer simulation and internal friction study

Two types of physical approaches for simulation of the Snoek-type relaxation in low and high alloyed iron are examined to explain the experimental results obtained for Fe–Al–C and Fe–C–Cr alloys. The first approach developed by Smirnov–Tomilin is to calculate all octahedral positions available for interstitial atoms with different amount of substitute atoms in the first coordination shell and to simulate the loss maximum as a sum of all partial peaks according to the above mentioned interstice positions. The second approach takes into account the all pairwise interatomic interaction between solute atoms in a few coordination shells due to their interatomic elastic and ‘chemical’ interaction according to Khachaturyan–Blanter theory. The change of activation energy of ‘diffusion under the stress’ for interstitial atoms in that case is not a linear function of substitutional concentration in solution. Both physical models (short- and long-range interatomic interaction) for the Snoek-type relaxation in quenched ternary alloys (Fe–C–Me) are examined from the viewpoint of a distance of interatomic interaction taken into account and checked using experiments. It is shown that contrary to the second approach, the first type of calculations is reasonable for relatively low alloyed solid solution only. Decomposition (Fe–Cr) and ordering (Fe–Al) change the parameters of atomic distribution in bcc solid solution and lead to the corresponding change in the Snoek relaxation parameters. The use of an adequate physical model and structure parameters allows to explain corresponding effects and, vice versa, the internal friction spectrum allows to estimate quantitatively atom redistribution in alloyed ferrite.     

Classical molecular dynamics simulation of UO2 to predict thermophysical properties

Interatomic potential parameters of UO₂, for a partially ionic potential model, were found out by a fitting method using isothermal compressibility data up to 1600 K. The inter-atomic potential used here is essentially a combined potential model comprising of Bushing–Ida and Morse-type potential to simulate UO₂ system using classical molecular dynamics. It has been shown that the deviation in the estimated lattice parameter from the experimental data as reported in literature is due to the inadequacy of the potential parameter. The calculated deviations in lattice parameter, due to the generation of point defects, with variation of temperature have been discussed. The calculated lattice parameters, thermal expansivity, isothermal compressibility of UO₂, using the current potential parameters are in close agreement with the experimental values. The model also successfully predicts Bredig transition.