快冷Fe-Al合金中的原子缺陷弛豫

在多功能内耗仪上用自由衰减和强迫振动方法研究了不同Al含量淬火Fe-Al合金中的两个弛豫型内耗峰.结果显示：P1(180℃)和P2(340℃)两个内耗峰只出现在淬火样品的加热过程中，而在随后的冷却过程中不出现.P1峰是由在α和β(或γ)点阵上的空位组成的最近邻双空位偶极子在应力诱导下的重新取向产生的，其弛豫强度随Al含量非单调地变化，在大约25% Al(原子百分比，以下同)处出现最大值.Al含量较低的Fe-Al合金无     

快冷Fe-Al合金中的原子缺陷弛豫

在多功能内耗仪上用自由衰减和强迫振动方法研究了不同Al含量淬火Fe-Al合金中的两个弛豫型内耗峰.结果显示：P1(180℃)和P2(340℃)两个内耗峰只出现在淬火样品的加热过程中,而在随后的冷却过程中不出现.P1峰是由在α和β(或γ)点阵上的空位组成的最近邻双空位偶极子在应力诱导下的重新取向产生的,其弛豫强度随Al含量非单调地变化,在大约25% Al(原子百分比,以下同)处出现最大值.Al含量较低的Fe-Al合金无 关键词： 空位 弛豫 Al含量     

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2 annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 Å. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.     

Role of vacancy-type defects in magnetism of GaMnN

Role of vacancy-type(N vacancy(VN) and Ga vacancy(VGa)) defects in magnetism of GaMnN is investigated by first-principle calculation.Theoretical results show that both the VNand VGainfluence the ferromagnetic state of a system.The VNcan induce antiferromagnetic state and the VGaindirectly modify the stability of the ferromagnetic state by depopulating the Mn levels in GaMnN.The transfer of electrons between the vacancy defects and Mn ions results in converting Mn~(3+)(d~4) into Mn~(2+)(d5).The introduced VNand the ferromagnetism become stronger and then gradually weaker with Mn concentration increasing,as well as the coexistence of Mn~(3+)(d~4) and Mn~(2+)(d~5) are found in GaMnN films grown by metal–organic chemical vapor deposition.The analysis suggests that a big proportion of Mn~(3+)changing into Mn~(2+)will reduce the exchange interaction and magnetic correlation of Mn atoms and lead to the reduction of ferromagnetism of material.     

Thermal equilibrium defects in iron-based alloys

The Mössbauer spectra of ⁵⁷Fe were measured for the thermal equilibrium b.c.c. Fe_0.947V_0.053 and Fe_0.956Co_0.044 solid solutions being at temperature ranging from 300 to 1,000 K. The obtained data were analysed in terms of concentration of unoccupied sites in the 14-site surroundings of an ⁵⁷Fe Mössbauer probe in a b.c.c. sample. It turned out that the probe detects unoccupied sites in its neighbourhood when the temperature of the material studied does not exceed about 900 K. This result suggests that the Mössbauer spectroscopy “sees” the pre-vacancy effect revealed by the positron annihilation spectroscopy in the early 1960s.     

纳米铜薄膜塑性变形中空位型缺陷形核与演化的分子动力学研究

本文采用分子动力学方法模拟了纳米单晶铜薄膜在单向拉伸载荷作用下的塑性变形过程, 重点分析了空位型缺陷的形核过程和演化机理. 在模拟过程中, 采用镶嵌原子势描述原子间的相互作用. 模拟结果表明纳米铜薄膜中塑性变形起源于位错的表面形核, 而空位型缺陷的形核及演化都与晶体内部的位错运动密切相关. 空位型缺陷通常从位错割阶及层错交截处开始形核, 以单空位、层错四面体和不规则空位团等形式存在. 关键词： 纳米薄膜 塑性变形 空位 层错四面体     

Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina

2 O₃ and nanocrystalline Al₂O₃ specimens. The short-lifetime (170±20 ps), intermediate-lifetime (410±20 ps) and long-lifetime components correspond to three different kinds of defects: monovacancy-like free volumes, microvoids, and larger voids. The appearance of lifetimes in the range 1–5 ns indicates the formation of positronium. The influence of thermal annealing from 873 K to 1373 K on positron lifetime parameters was also analyzed. The components with lifetimes τ₁=170 ps and τ₂=410 ps persisted even after the grains had grown to 100 nm in size, while the long-lifetime component declined significantly when grain sizes exceeded 10 nm. The interface characteristics of polycrystalline nano-Al₂O₃ prepared by the two methods were compared by analyzing the variations of the positron-lifetime parameters with grain growth. Received: 1 April 1997/Accepted: 13 August 1997     

Free energy of quenched binary alloys and the time evolution of their structure function

An idea about the free energy of a binary alloy system under phase separation at low temperatures is discussed. Slow asymptotic decays in the structure functions for both a conserved and a non-conserved order parameter are studied.     

Adsorption on graphene with vacancy-type defects: A model approach

The influence of graphene lattice defects on the adsorption properties of graphene has been considered. The adsorption properties have been investigated in the framework of the Anderson model. The disorder of the graphene crystal lattice has been analyzed using the T-matrix approximation. It has been found that the characteristic energy levels of defects are located near the Dirac point (±1 eV), because the most significant distortions of the spectrum due to the presence of defects in the graphene crystal lattice are observed in the vicinity of this point. Analytical expressions for the density of states of disordered graphene and atoms adsorbed on it have been obtained. A numerical calculation of the charge transfer in the considered system has been carried out. The obtained values of the charge transfer are in good agreement with the results of other studies, where the charge transfer was calculated using the experimental data and the density functional theory method. In the absence of defects, the presented results are well consistent with the results obtained within the M-model of adsorption (Davydov model). An approximation for the density of states of disordered graphene and the shift function of an adsorbed atom has been proposed. This approximation allows one to obtain analytical expressions for the charge transfer, energy of adsorption, and dipole moment.     

Magnetic hysteresis in rapidly quenched rare-earth alloys

The magnetic and structural properties of rapidly quenched (Pr₈₀Ga₂₀)_100-xFe_x, RTbFe and RTbFe(Co)M alloys are examined over a wide range of chemical compositions, where R ≡ Pr, Sm, MM and M ≡ B and Si. The Ga-containing samples show relatively high coercive fields (up to 3 kOe) in the amorphous state which subsequently disappear after crystallization. On the other hand, the high coercive fields (≈5 kOe) of melt-spun RTbFe samples decrease slightly after crystallization but their magnetic moment increases substantially. Melt-spun RTbFe(Co)M samples are generally magnetically soft in the as-quenched state. Magnetic hardening is produced by annealing the samples around 750°C leading to coercive fields which could not be measured with an ordinary electromagnet (H_c #62; 23 kOe). The best properties have been obtained on a Pr₁₄Fe₇₁B₁₅ sample with a coercive field of 8 kOe and an energy product of 8.5 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 750°C. The Curie temperature of the precipitate phase is around 320°C while that of the as-quenched phase is around 160°C. Transmission electron microscope studies show a very fine precipitate structure with a precipitate size below 100 Å. The precipitate phase is believed to be highly anisotropic leading to the observed hard magnetic properties.     

Discontinuous decomposition in quenched and deformed alloys

Discontinuous decomposition in quenched and deformed samples of the alloy Cu + 4.3% Ti has been studied by optical metallography and by electron metallography (with replicas and thin foils). The structure of the discontinuous-decomposition regions changes markedly during the continuous metastable decomposition in the matrix. Cells are nucleated within grains in the deformed samples at recrystallization nuclei.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 1, pp. 108–112, January, 1969.     

Extended vacancy-type defects in silicon induced at low temperatures by electron irradiation

We have observed the formation of extended defects in silicon at low temperatures (below 25 K) by means of in-situ high-resolution transmission electron microscopy (HRTEM). The defects are distorted spheres, occasionally truncated by facets, less than 5 nm in diameter. These defects are stable up to 773 K, and they gradually shrink during annealing in the temperature range from 773 to 973 K. From the analysis of HRTEM images of the defects, we have suggested that the defects are voids formed via athermal migration of vacancies under electron irradiation.     

Magnetic properties of rapidly quenched Fe-Ni alloys

The magnetic properties of Fe-29 at.% Ni and Fe-50 at.% Ni alloys prepared by chill block melt spinning (CMBS) have been determined and compared with those of the bulk alloys. It has been found that atomic diffusion is enhanced by CMBS and that magnetic properties can change markedly even after annealing at relatively low temperatures. The increases in T_c observed are attributed to heterogeneous short range atomic ordering.     

Characterization of vacancy-type defects and phosphorus donors introduced in 6H-SiC by ion implantation

Received: 18 February 1998/Accepted: 5 June 1998     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Magnetic properties of rapidly quenched amorphous Fe-Sm alloys

The structural analysis and investigation of magnetic properties were carried out on rapidly quenched Fe_100−xSm_x (10.5 x 80) alloys. Amorphous alloys are fabricated in a wide composition range from x = 17 to 72.5. After heating the amorphous alloys up to 900 K, they transform into metastable phase I (T_c = 465 K)+ -Fe (x < 20), metastable phase II (T_c = 555 K)+-Fe (20 x 33.3) and Fe₂Sm+Sm (x40). The composition dependence of Curie temperature and magnetization is found to be similar to that of amorphous Fe-(Nd, Pr) alloys. A maximum coercive force of about 1.1 kOe at 300 K and 12 kOe at 77 K is obtained in the x = 40 alloy. The rapid decrease in coercive force with raising temperature can be explained by the wall pinning model proposed by Gaunt. The pronounced composition dependence of these magnetic properties for the amorphous Fe-Sm alloys can be considered to be caused by the change in the short-range atomic order with Sm concentration on the basis of the results of X-ray structural analysis.     

Anomalous recoil free fraction in rapidly quenched Y-Fe-Cu alloys

We report the results of X-ray diffraction, atomic density, differential scanning calorimetry, and Mössbauer Effect measurements in rapidly quenched, heat treated, and as-cast Y_1?xFe_x and Y_{0, 90}Cu_1?zFe_z alloys. Unusually high values of the recoil free fraction (f) are observed for the rapidly quenched partially crystalline samples. From our measurements we conclude that these samples are composed of an amorphous phase and a Y(Fe, Cu) interstitial solid solution. We estimate that the interstitial solid solubility of Fe and Cu in hcp Y is greater than 1.5 at .%, and less than 3 at .%.     

Positive muon spin depolarization in quenched aluminium dilute alloys

Aluminum dilute alloys (Al-0.047 at% Mg, Al-0.052 at% Cu and Al-0.051 at% Si) were quenched from 600 and 450C. The positive muon behavior in the quenched alloys has been studied. Three peaks (Peak I, II and III) are found. Peak I is due to the trapping and detrapping of positive muons at single impurity atoms. The muon spin depolarization rate at the trap in Peak I was found to be 0.36sec^–1. Peak II is connected to impurity clusters. Peak III is the trapping of muons at quenched-in vacancies. The depolarization rate at the trapping site was found to be 0.2sec^–1. The activation energy of the trapping rate was found to be 25 meV. As the quenched-in vacancies annealed out, the trapping rates decreased to near zero.