The role of quenched‐in vacancies for the decomposition of aluminium alloys

The very early stages of decomposition during room temperature storage, i.e. just a few minutes after quenching, are investigated by positron annihilation lifetime spectroscopy for both an AlMgSi alloy and an AlCuMg alloy. It turns out that by freezing the decomposition kinetics during measurements we can detect vacancy–solute atom pairs. The formation of larger solute clusters with structural vacancies is seen by an increase of the mean positron lifetime in the course of storage at room temperature (RT). Earlier findings concerning aging at RT were unable to discover this effect. The detected changes are interpreted in terms of cluster formation. Thus we show that positron annihilation spectroscopy (PAS) is one of the very few methods to access early stages of decomposition in metallic alloys. Moreover, the lower limit of the concentration of quenched‐in vacancy‐like defects is calculated to be at least 2 × 10^–5 per atom. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Early stage ageing effects and shallow positron traps in Al–Mg–Si alloys

Room temperature ageing, so-called natural ageing, of Al–Mg–Si alloys has a subtle but striking influence on the mechanical properties achievable by subsequent ageing at more elevated temperatures. Though strongly debated, different clustering processes are generally accepted to give rise to this effect. Using temperature-dependent positron lifetime measurements of naturally aged Al–Mg–Si alloys, it is shown that in the early stages of ageing, small clusters of alloying atoms without embedded vacancies take part in the decomposition process. These clusters serve as shallow positron traps with a binding energy of about 55(10) meV, grow in the course of natural ageing and transform to deep positron traps with binding energies well above thermal energies. Thus, results of positron annihilation spectroscopy techniques need to be interpreted carefully with respect to the microstructure of age-hardenable Al alloys. Moreover, it is shown that a simple approach to bind positron states using a three-dimensional potential well and (bulk) positron affinities cannot explain the present findings.     

Early precipitation stages of aluminum alloys—The role of quenched-in vacancies

We present preliminary results on the structure of nano-sized particles (clusters of solute atoms or Guinier-Preston-Bagaryatsky (GPB) zones) in aluminum alloys. Hindering the motion of dislocations, these GPB zones (precipitates) are responsible for the strength of AlCuMg-alloys - used e.g. as AA2024 - for the outer shell plates in aircraft industry. We will discuss the role of quenched-in vacancies for the formation and growth of the precipitates. Using positron annihilation (Doppler broadening together with the recently developed tool of high-momentum analysis (HMA)) and X-ray absorption spectroscopy measurements, we are able to characterize the local atomic environment in the vicinity of vacancies and selected elements (Cu) forming the precipitates. The interpretation will be based on a comparison to numerical calculations of positron lifetimes and momentum distributions as well as simulated X-ray absorption spectra. Often reliable numerical calculations of experimental quantities are possible only if relaxed atomic positions - calculated employing an ab-initio code like SIESTA - are used as input.     

Positron annihilation spectroscopy and specific heat study of Neon ion irradiated MgB2

Specific heat studies under magnetic field and positron annihilation spectroscopy were carried out on 160 MeV Ne ion irradiated polycrystalline MgB₂ samples. There is an unusual decrease in positron lifetime in the irradiated sample which may be due to neon ion implantation. This was also indicated by change in cell volume. Coincidence Doppler Broadening Spectra of Mg, B, irradiated and unirradiated MgB₂ show that positrons primarily annihilate in boron sublattice in the unirradiated sample whereas there is some similarity of the spectrum of the irradiated sample with that of Mg. There is Mg deficiency in the unirradiated sample whereas predominantly boron vacancies exist in Ne ion irradiated MgB₂ sample. Specific heat measurements show that there is a small increase in electronic part of the specific heat and electron-phonon coupling constant.     

Microstructural characterisation of a commercial Al–Cu–Mg alloy combining transmission electron microscopy and positron annihilation spectroscopy

Transmission electron microscopy and positron annihilation spectroscopy techniques were used to characterise the microstructure of the 2024 Al–Cu–Mg commercial alloy artificially aged with and without pre-deformation. In non-deformed samples, a very dense dispersion of small, needle-shaped particles, with mean size in the order of the nanometres, was observed homogeneously distributed in the matrix, together with a coarse distribution of S-phase precipitates. In pre-deformed samples, the needle-shape particles were not seen, only a high density of S-phase precipitates nucleated on dislocations. The needle-shaped particles were identified as solute-rich Guinier–Preston–Bagaryatski (GPB) zones by combining coincidence Doppler broadening positron annihilation measurements with TEM imaging. The relationship between the microstructure and measurements of hardness and positron lifetime evolution during artificial ageing is also discussed.     

非晶态Mg_(70)Zn_(30)合金结构因子的预峰

利用X射线衍射仪对非晶态Mg70 Zn30 合金的结构进行了研究 ,获得了强度曲线、结构因子、双体分布函数和原子间最近邻距离 .结果表明 ,Mg70 Zn30 合金在小Q区间存在强烈的预峰现象 .根据预峰的特性 ,提出了Mg70 Zn30 熔体的结构模型 ,即Mg原子位于中心 ,8个Zn原子位于顶角所形成的简单立方结构模型 .该模型以共享顶点的方式相连接 ,能够满足预峰对Mg—Mg原子间距离的要求 .Mg70 Zn30 非晶合金中预峰的产生是相邻原子团中心Mg原子之间相互关联的表现     

Multiscale characterization of dislocation processes in Al 5754

Multiscale characterization was performed on an Al–Mg alloy, Al 5754 O-temper, including in situ mechanical deformation in both the scanning electron microscope and the transmission electron microscope. Scanning electron microscopy characterization showed corresponding inhomogeneity in the dislocation and Mg distribution, with higher levels of Mg correlating with elevated levels of dislocation density. At the nanoscale, in situ transmission electron microscopy straining experiments showed that dislocation propagation through the Al matrix is characterized by frequent interactions with obstacles smaller than the imaging resolution that resulted in the formation of dislocation debris in the form of dislocation loops. Post-mortem chemical characterization and comparison to dislocation loop behaviour in an Al–Cr alloy suggests that these obstacles are small Mg clusters. Previous theoretical work and indirect experimental evidence have suggested that these Mg nanoclusters are important factors contributing to strain instabilities in Al–Mg alloys. This study provides direct experimental characterization of the interaction of glissile dislocations with these nanoclusters and the stress needed for dislocations to overcome them.     

Short-to-medium-range order in Mg65Cu25Y10 metallic glass

The atomic configurations of liquid and glassy Mg₆₅Cu₂₅Y₁₀ alloy have been simulated in the temperature range of 300 K to 2000 K via ab initio molecular dynamics. The variations of pair correlation function (PCF), structure factor (SF), coordination number (CN) and bond pairs with the temperature for this alloy are characterized. It has been shown that the atoms are near densely packed and icosahedral type of short-range order (SRO) is predominant in the glass state. Icosahedral medium range order (MRO) can be formed by vertex or intercross connection of icosahedral SROs. In this work, an icosahedral MRO which is composed of 55 atoms has been found. It has been also clarified that Mg and Cu occupy the centre or vertex, and Y atoms only occupy the vertex of the icosahedron in this glassy alloy. It is believed that these findings have implication for understanding the glass forming mechanism of magnesium based metallic glasses.     

The growth of Au/Pd/alumina/Cu-Al system studied by SRPES

An ultra-thin alumina layer grown on Cu-9at.%Al (1 1 1) surface was studied using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). By deconvolving SRPES spectra of the Al 2p doublet, four components belonging to metallic as well as oxide phases were recognized. Pd-Au alloy formation was confirmed by SRPES measurement during Pd and Au deposition. The study of the system's thermal stability reveals diffusion of Pd and Au atoms through the alumina layer. While Au atoms start to diffuse under the alumina layer at 670 K, Pd atoms are forming Pd-Al surface alloy at this temperature. The diffusion of Pd atoms through alumina occurs when sample was heated over 770 K. Alumina layer was stable even after heating the sample at 870 K, but its structure was corrupted probably due to the diffusion of metal atoms.     

Characterisation of atomic-scale defects in ODS ferritic alloys by positron annihilation

The vacancy-type defects and their local chemical environment in different ODS alloys produced in the USA (14YWT), China (K5) and Russia (ODS EP-450) are studied. The Angular Correlation of Annihilation Radiation (ACAR), which is one of the positron annihilation spectroscopy method, was used. It was shown that in all alloys, except 14YWT, the dominant type of positron traps are vacancy-like defects, localised in matrix or associated with dislocations and/or interfaces of the incoherent particles. In the case of 14YWT alloy, which contains Y–Ti–O nanoclusters of a high density, the positrons confine and annihilate at O-vacancy pairs or complexes within nanoclusters. It is testified by enhanced electron density in annihilation sites and neighbourhood of Ti and Y atoms. These results, obtained by the ACAR method, indicate that the vacancies play an essential role in the formation of nanoclusters in ODS 14YWT alloy as it was theoretical predicted by first-principle calculations.     

Adsorption behavior and mechanism of multiple Mg atoms on the surface of AlNi compound at Mg alloy/steel interface

The interface between Mg alloy and the intermetallic compounds (IMCs) is the weak point during the welding of Mg alloy and steel. The models of AlNi surface under different number of absorbed magnesium atoms were established. The absorption energy and electronic properties of the Nth (N represents the number of Mg atoms, which may be 1, 2, 3, and 4 in this paper) Mg atom on the AlNi surface were calculated using first-principles calculations. The results show that the previously absorbed Mg atoms will promote the absorption of the latter Mg atom when the distance between them is equal or less than 2.89 Å. The absorption energy of the Mg atoms on the AlNi surface is related to the electronic exchange between different atoms. The results will provide a new perspective for understanding the atomic scale mechanism of the bonding between the Mg atom and AlNi IMCs.     

利用正电子湮没技术研究钾掺杂钨合金中的缺陷

钨合金中钾的掺杂会引入大量的缺陷,如尺寸几十纳米的钾泡、高密度的位错以及微米量级的晶粒带来的晶界等,这些缺陷的浓度和分布直接影响合金的服役性能.本文运用正电子湮没谱学方法研究钾掺杂钨合金中的缺陷信息,首先模拟计算了合金中各种缺陷的正电子湮没寿命,发现钾的嵌入对空位团、位错、晶界等缺陷的寿命影响很小;然后测量了不同钾含量掺杂钨合金样品的正电子湮没寿命谱,建立三态捕获模型,发现样品中有高的位错密度和低的空位团簇浓度,验证了钾对位错的钉扎作用,阐述了在钾泡形成初期是钾元素与空位团簇结合并逐渐长大的过程;最后使用慢正电子多普勒展宽谱技术表征了样品中缺陷随深度的均匀分布和大量存在,通过扩散长度的比较肯定了钾泡、晶界等缺陷的存在.     

Band-gap bowing and p-type doping of (Zn,Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N_O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.     

Vacancy–solute interactions during ageing of a cold-worked Mg–RE-based alloy

The vacancy–solute interactions during artificial ageing at 250^○C of cold worked samples of a commercial magnesium alloy WE54 (Mg–RE based) were studied by coincidence Doppler broadening of positron annihilation radiation and positron annihilation lifetime spectroscopy. The results show that, in the as-cold-worked state, the vacancies are associated with dislocations that are generated by the cold work and that, after artificial ageing at 250^○C, the vacancies are associated with solute elements and help the formation of precipitate precursors. This mechanism accelerates the formation of hardening precipitates without any apparent changes in the precipitation sequence and in the products of the decomposition of the supersaturated solid solution. The present study demonstrates that the stronger hardening response achieved in the cold-worked samples originates from the presence of a higher concentration of vacancies that is introduced by the cold work and is retained in the first few minutes of ageing.     

镁合金电子结构与腐蚀特性研究

采用递归法计算了镁合金电子结构.研究发现Mg的态密度在晶内与表面接近,当表面有氧或氢氧时态密度形状改变很大,因此Mg在晶内、表面性质是接近的,但当合金表面渗透氧或氢时,合金性质有明显变化.Al,Y,La三种元素在晶体表面的掺杂原子镶嵌能均低于各自在晶内的掺杂原子镶嵌能,Al, Y, La从晶内向晶体表面扩散、并在合金表面偏聚.Al-O, Y-O, La-O, Mg-O及Mg-O-H间的亲和能均为负数,这些原子间存在亲和力,可以在合金中相互作用形成化合物.由于Mg-O-H间的亲和能远低于Mg-O的亲和能,因此Mg(OH)₂比MgO更稳定.氧化初期氧与Mg, Al, Y, La等生成氧化物,当合金与腐蚀介质接触时,MgO与腐蚀介质中的水发生反应生成Mg(OH)₂.Al₂O₃,（Y,La）₂O₃及Mg(OH)₂能对合金起到保护作用,提高合金的耐腐蚀性能. 关键词： 电子结构 Mg合金 腐蚀特性     

Atomic fraction around defects associated with nanoparticles in AlCuMg alloys

A positron annihilation spectroscopy analysis method to obtain a quantitative determination of the chemical composition around defects inside nanoparticles is presented here. This methodology is applied to AlCuMg alloys to study the rapid hardening phenomena associated with solute-vacancy aggregation. Coincidence Doppler Broadening (CDB) and lifetime spectroscopy measurements of reference samples of pure elements with and without defects were analyzed to give quantitative information of the average chemical environment around vacancies, i.e. the atomic fraction of the first neighbors of these defects, in the alloys studied. The accuracy and reproducibility of the methodology is confirmed not only by good fits to the experimental data but, in most cases, by the consistency between the mean lifetime values predicted, using the CDB estimation, and the mean lifetime values independently measured. Discrepancies in the methodology are expected when there is poor CDB contrast between elements, i.e. having similar electronic structure (for example, Al and Mg). The criterion for establishing the statistical accuracy of the separation of elements in these special cases is discussed. The methodology can be applied not only to study homogeneous materials as metallic alloys, but also to study the depth profile in thin films.     

AA 2037新型连铸铝合金热轧板退火的正电子湮没研究

应用正电子湮没等技术对AA 2037连铸铝合金热轧板不同温度退火下空位-溶质相互作用及沉淀相进行了研究， 研究表明：室温时效形成的主要是空位-铜复合体以及空位-镁-铜复合体；温度为200℃退火时由于过渡相的形成正电子平均寿命出现峰值，符合多普勒展宽谱的商谱中观察到锰信号的存在，表明形成了空位-镁-锰复合体或过渡相中可能存在锰；温度高于250℃时，随着过渡相变粗、溶解，锰信号消失，而铜信号增强，在350℃后铜信号达到饱和，温度为450℃左右时，稳定相形成. 关键词： 正电子湮没技术 空位-溶质复合体 沉淀 退火     

Refinement of the structure of hydrogen–vacancy complexes in titanium by the Rietveld method

The VT1-0 titanium alloy (phase α-Ti) with various hydrogen and hydrogen-vacancy concentrations has been studied. The stability of the 32-atom Ti–nV–mH supercell (n is the number of the V vacancies, and m is the number of hydrogen atoms H) with varying numbers of vacancies and hydrogen atoms has been calculated from the first principles. The structural state of the α-Ti phase has been identified by the Rietveld method based on the calculations of the supercell stability and the data on the defect concentration obtained using positron spectroscopy. The complete structural information on the considered states of the α-Ti phase (the lattice parameters, spatial distribution of titanium and hydrogen atoms and vacancies) has been obtained.     

Positron annihilation probing of crystallization effects in TAS-235 glass affected by Ga additions

Crystallization effects in Te₂₀As₃₀Se₅₀ glass known also as TAS-235 affected by Ga additions to Ga₂Te₂₀As₂₈Se₅₀ and Ga₅Te₂₀As₂₅Se₅₀ compositions are probed with positron annihilation spectroscopy in the measuring modes exploring positron lifetimes and Doppler broadening of annihilation line. Occurring of cubic-phase Ga₂Se₃ droplets with character nanoscale sizes in partially-crystallized Ga₂Te₂₀As₂₈Se₅₀ alloy is shown to be associated with agglomeration of intrinsic free-volume voids, this process being enhanced over microcrystalline scale in Ga₅Te₂₀As₂₅Se₅₀ alloy. Crystallization changes in the void structure of TAS-235 glass are considered in terms of free-volume evolution under the same principal chemical environment responsible for positron trapping in amorphous and partially crystallized substances.     

Electronic properties of the amorphous FeCuNbSiB alloy

Variations in the electronic properties of amorphous Fe-Cu-Nb-Si-B alloys during crystallization have been studied by methods of positron annihilation spectroscopy. The measurements of the positron lifetime in samples and the angular distribution of annihilation photons have demonstrated substantial variations during crystallization. There is a correlation of the results with the performed measurements of the thermopower. The experimental results allow the assumption that degenerate electron gas is not formed in the amorphous alloys under study, and localized electrons participate mainly in the conduction. It can be suggested that the main role in formation of the amorphous alloy structure is played by the covalent bond. The spectrum of momentum distribution of the amorphous alloy differs little from the pure iron spectrum. Irradiation induces vacancy-copper complexes in the amorphous alloy.