Si-rich Clusters Research Articles

Formulation of initial artificial age-hardening response in an Al-Mg-Si alloy based on the cluster classification using a high-detection-efficiency atom probe

Cluster classification according to the composition based on the atomic density inside clusters in an Al-Mg-Si alloy is originally suggested using a high-detection-efficiency atom probe. The high detection efficiency is better capable for detection of clusters and has a major impact on measured number densities. It is shown that Si-rich clusters with Mg/(Mg+Si) value below 0.4 have smaller atomic density inside clusters, suggesting high vacancy concentration inside the Si-rich clusters. It is revealed that the initial artificial age-hardening response related to the transition behavior from a cluster to the β″ phase at 443 K after aging with various conditions can be determined by the relative volume fractions of the Si-rich clusters and Mg-Si clusters. The relative proportion of the Si-rich clusters can be estimated from the initial artificial age-hardening response in various Al-Mg-Si alloys using the proposed formula.

Effects of natural aging after pre-aging on clustering and bake-hardening behavior in an Al–Mg–Si alloy

Atom probe tomography (APT) analysis was used to characterize the clustered state after pre-aging at 90°C (PA) and subsequent natural aging (NA) in an Al–0.62Mg–0.93Si (mass%) alloy. It was revealed that larger Si-rich clusters increase in number after prolonged NA. The long-term NA after PA is believed to promote the preferential aggregation of solute Si toward clusters formed during PA rather than the independent formation of Si-based clusters. The prolonged NA inhibits the hardness increase during bake-hardening (BH) treatment at 170°C. The deterioration of the BH response appears to be due to the increase of the Si-rich clusters.

Cluster evolution mechanisms during aging in Al–Mg–Si alloys

Using Atom Probe Tomography (APT) and Phase Field Crystal (PFC) modelling, the early-stage precipitation phenomenon is investigated for naturally- and artificially-aged Al–Mg–Si alloys of two different Mg/Si ratios, i.e. 1 and 2. It is shown that, regardless of alloy composition and aging history, the earliest precipitates appear as finely-dispersed Si-rich clusters which gradually undergo a simultaneous coarsening and Mg-enrichment. In addition, the energetic factors for the instability of natural aging clusters are analysed. The energy analysis also suggests that the initial Si-enrichment of the earliest precipitates is due to the affinity of locally-strained areas for higher Si concentrations when achieving a meta-stable equilibrium with the strain-free surrounding matrix. The subsequent Mg-enrichment is shown to be energetically necessary for an evolving precipitate to survive and grow. The alloy with smaller Mg/Si ratio shows a delay in the onset of Mg-enrichment during natural aging, which is attributed to the higher average Si content, as well as the slow kinetics of diffusion at natural aging temperatures. It is shown that this alloy exhibits a smaller nucleation barrier and critical nucleus size as well. We suggest that the above described mechanisms govern the evolution of early-stage clustering in the common range of age-hardenable Al–Mg–Si alloy compositions.

Formation and reversion of clusters during natural aging and subsequent artificial aging in an Al–Mg–Si alloy

The continuous changes in number density, size distribution and chemical composition of clusters during natural aging (NA), and subsequent artificial aging (AA) at 443K, in an Al–0.62Mg–0.93Si (mass%) alloy have been evaluated using atom probe tomography. The data show almost no change in the average size and Mg/Si ratio (at% ratio) of clusters during NA. The highest hardness and most-dense clusters, following NA, and the largest drop in hardness, during AA for 1.2ks, are observed in the material with the longest NA period. In contrast with the published literature, which suggests that only small clusters revert into solution, the number density of clusters decreased, with no dependence on the cluster size, during AA for 1.2ks, resulting in a slight decrease in the average radius of clusters. Things to be emphasized are that the number density of clusters with the Mg/Si ratio below 0.4 stays pretty much the same during AA, whereas other clusters decrease during the first 1.2ks and then increase again during the prolonged AA for up to 3.6ks. This work is the first to reveal that the typical Si-rich clusters can neither be dissolved nor grow further, although the size is small. In other words, it is believed that the typical Si-rich clusters can lead to the retardation of the hardness increase during AA called as negative effect of NA.

Evaluation of Solute Clusters Associated with Bake-Hardening Response in Isothermal Aged Al-Mg-Si Alloys Using a Three-Dimensional Atom Probe

Temporal changes in the number density, size distribution, and chemical composition of clusters formed during natural aging at room temperature and pre-aging at 363 K (90 °C) in an Al-0.62Mg-0.93Si (mass pct) alloy were evaluated using atom probe tomography. More than 10 million atoms were examined in the cluster analysis, in which about 1000 clusters were obtained for each material after various aging treatments. The statistically proven records show that both number density and the average radius of clusters in pre-aged materials are larger than in naturally aged materials. It was revealed that the fraction of clusters with a low Mg/Si ratio after natural aging for a short time is higher than with other aging treatments, regardless of cluster size. This indicates that Si-rich clusters form more easily after short-period natural aging, and that Mg atoms can diffuse into the clusters or possibly form another type of Mg-Si cluster after prolonged natural aging. The formation of large clusters with a uniform Mg/Si ratio is encouraged by pre-aging. It can be concluded that an increase of small clusters with various Mg/Si ratios does not promote the bake-hardening (BH) response, whereas large clusters with a uniform Mg/Si ratio play an important role in hardening during the BH treatment at 443 K (170 °C).

Effects of Pre-Aging and Natural Aging on Bake Hardening Behavior in Al-Mg-Si Alloys

The effects of pre-aging and natural aging on the bake hardening behavior of Al-0.62Mg-0.93Si (mass%) alloy with multi-step aging process were investigated by means of Vickers hardness test, tensile test, differential scanning calorimetry analysis (DSC) and transmission electron microscopy (TEM). The characteristics of nanoclusters (nano scale solute atom clusters) formed during pre-aging and natural aging were also investigated using the three dimensional atom probe (3DAP) analysis. The results revealed the occurrence of natural age hardening and that the bake hardening response was decreased after the extended natural aging even though the pre-aging was conducted before natural aging. Since the 3DAP results exhibited the Si-rich clusters were newly formed during extended natural aging, it was assumed that the Si-rich clusters caused the natural age hardening and the reduced bake hardening response corresponding to Cluster(1). The decrease of the bake hardening response was markedly higher in the later stage of bake hardening than in the early stage. The size of the β’’ precipitates were reduced with increasing the natural aging time. Exothermic peaks of Peak 2 and Peak 2’ were observed in the DSC curves for the alloys pre-aged at 363K. Peak 2’ became larger with the natural aging time. This is well understood by the following model. The transition from Cluster(2) to the β’’ phase occurs preferentially at the early stage of the bake hardening. Then the growth of the β’’ phase is inhibited by the presence of Cluster(1) at the later stage of bake hardening. The combined formation of Cluster(1) and Cluster(2) by the multi-step aging essentially affects the bake hardening response and the β’’ precipitates in the Al-Mg-Si alloys.

Quantitative atom probe tomography characterization of microstructures in a proton irradiated 304 stainless steel

Irradiation of 304 stainless steels induces complex microstructural changes such as solute clustering, precipitation, and segregation to dislocations, which have been best characterized by atom probe tomography. However, reliably and reproducibly quantifying these localized chemical changes can be challenging. To this end, an approach for quantitative cluster and dislocation analysis of the atom probe tomography data is proposed. The method is applied to the quantification of Cu clusters, Ni–Si rich clusters and Si, Ni and P segregation to dislocations that are observed in a 304 stainless steel that was proton irradiated at 360°C to 10dpa.

Aluminum ordering and clustering in Al-rich synthetic phlogopite: The influence of fluorine investigated by 29Si CPMAS NMR spectroscopy

The influence of fluorine on cationic and anionic ordering in the mica mineral phlogopite has been investigated using 29Si, 1H, and 19F MAS as well as {1H}/{19F} → 29Si CPMAS and CP-depolarization NMR spectroscopies. It can be shown that the mere presence of fluorine achieves a tremendous loss of capability to incorporate aluminum into the phlogopite structure. Fluorine is usually located in Mg-rich octahedral and Si-rich tetrahedral clusters of the phlogopite structure while hydroxyl groups are located in Al-rich octahedral and tetrahedral clusters as derived from {1H}/{19F} → 29Si CPMAS NMR spectroscopies. The ordering effect in these two basic structural clusters can also be proven by a smaller 29Si linewidth in the {19F} → 29Si CPMAS NMR experiments compared to the usual 29Si MAS NMR experiment showing a stronger ordering of Si environments near the two different anion types fluorine and hydroxyl. Intensities of the {1H}/{19F} → 29Si CPMAS NMR signals as function of the contact-time show a deviation from the classical I-S model and can be attributed to the I - I *- S model. Time constants like the proton/fluorine spin diffusion time ( T df ), the spin-spin relaxation time ( T 2), the λ parameter (λ), and the proton/fluorine spin-lattice time in the rotating frame ( T 1ρ) were extracted to give information about the local structure.

First-principles study on mixed SimNn (m + n = 2–9) clusters

The structures and stabilities of the mixed SimNn (m+n=2–9) clusters have been investigated systematically by using Amsterdam Density Functional (ADF) program with TZ2P basis set in conjunction with self-consistent field (SCF). In the silicon–nitrogen binary cluster system, their structures transfer gradually from the three-dimensional structures of Si-rich clusters into the like-planar or like-linear structures of N-rich clusters. Near m=n stoichiometry, the lowest energy structures have higher stability. They usually feature a strong alternation of Si and N atoms. In addition, some N dimers and one small isomer can be observed in N-rich clusters. The binding energy between the N dimers and the small isomer is less than 0.02eV. The most stable structures for pure Nn clusters with odd atom number also contain one N trimer besides some N dimers. A series of meta-stable like-zigzag linear structures in the pure Nn clusters have been also studied. The investigation on magnetism for the mixed clusters indicates that only SiN2 cluster presents the magnetic moment of 2.0μB.

Magnetic properties of a highly neutron-irradiated nuclear reactor pressure vessel steel

We report results of minor B– H loop measurements on a highly neutron-irradiated A533B-type reactor pressure vessel steel. A minor-loop coefficient, which is a sensitive indicator of internal stress, changes with neutron fluence, but depends on relative orientation to the rolling direction in the low fluence regime. At a higher fluence of ∼10 × 10 23 m −2, on the other hand, an anomalous increase of the coefficient was detected irrespective of the orientation. The results were interpreted as due to competing irradiation mechanisms of the formation of Cu-rich precipitates, recovery process, and the formation of late-blooming Mn–Ni–Si-rich clusters.

A theoretical study of the dependence of the ASxSi6−x cluster structures and properties on composition

The effect of the composition ratio between arsenic and silicon atoms on the structures and properties of AsxSi6−x (x = 0–6) have been systematically investigated using the density functional theory at the B3LYP/6-311+G* level. The AsxSi6−x clusters prefer substitutional rather than attaching structures; the Si-rich clusters favor Si6-like structures, whereas the As-rich clusters prefer As6-like structures. The As atoms locating at the framework may explain the difficulty of removal of arsenic impurities from polycrystalline silicon. In general, the average binding energies gradually decrease, implying the AsxSi6−x clusters become increasingly unstable as x increases. Both the HOMO-LUMO gaps and the As-dissociation energies present a strong even–odd alternation, implying alternating chemical stability, with the even x members being more stable than the odd ones. The dissociation energies of an As atom from AsxSi6−x are: 3.07, 2.84, 1.84, 2.52, 1.86, and 2.85 eV, for x = 1–6, respectively, and 3.80, 3.08, 2.64, 3.01, 2.93, 3.16 eV for Si (x = 0–5). These dissociation energy results should provide a useful reference for further experimental investigations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Effects of alloying elements on radiation hardening based on loop formation of electron-irradiated light water reactor pressure vessel model steels

Electron irradiations using a high voltage electron microscope were conducted on several reactor pressure vessel model alloys in order to investigate the effects of alloying elements on the formation and development of defect clusters. In addition, the effects of alloying elements on yield stress change after irradiation were considered, comparing the mean size and number density of dislocation loops with the irradiation-induced hardening. High Cu alloys formed Cu and Mn–Ni–Si rich clusters, and these are important in determining the yield stress increase. High Ni alloys formed a high density of small dislocation loops and probably Mn–Ni–Si rich cluster, which have the effect of increasing the yield stress. High P enhanced radiation-induced segregation on grain boundary, helping prevent dislocation movement.

Understanding silicon-rich phase precipitation under irradiation in austenitic stainless steels

Atom probe samples have been Fe + ion irradiated at different doses (from 0.5 to 10 dpa) and different temperatures (between 300 and 400 °C) in order to understand the mechanism of formation, under irradiation, of Si-rich phases in austenitic stainless steels. Atom probe results show the presence of Si-enriched clusters which can also be enriched in Ni and depleted in Cr. Number densities of solute clusters can be linked to number densities of dislocation loops already observed by transmission electron microscopy in a previous work. This suggests that solute clusters are formed by heterogeneous precipitation on dislocation loops. Furthermore, the evolution of the composition of solute clusters as a function of the irradiation temperature is consistent with a radiation-induced mechanism. Results are also compared with previous results obtained after neutron irradiation at lower dose rate (in term of dpa s −1). The comparison is, here again, consistent with the radiation-induced mechanism. Thus, Si-rich clusters may be formed by radiation-induced segregation to dislocation loops. Results also show that Si is probably dragged to sinks via the interstitial mechanism.

Effects of chemical composition and dose on microstructure evolution and hardening of neutron-irradiated reactor pressure vessel steels

The correlation of microstructure evolution and hardening was studied in two kinds of A533B-1 steel with high and low levels of Cu irradiated in a range of dose from 0.32 to 9.9 × 10 19 n cm −2 ( E > 1 MeV) under a high flux of about 1.7 × 10 13 n cm −2 s −1 using three-dimensional local electrode atom probe (3DAP), positron annihilation (PA) techniques, and Vickers microhardness. The early rapid hardening was found to be caused by mainly matrix defects such as mono- or di-vacancies ( V 1 − V 2) and/or dislocations indicated by the PA analysis. The 3DAP analysis showed that dense dispersion of dilute Cu rich clusters and lean distribution of Mn–Ni–Si rich clusters, which were identified to possess the same dislocation-pinning effect by applying a Russell and Brown model, were responsible for large and small hardening in high- and low-Cu steels irradiated above 0.59 × 10 19 n cm 2, respectively.

First-principles study on mixed Si 10−nN n ( n=0−10) clusters

The mixed Si 10− n N n ( n=0−10) clusters have been investigated systematically using Amsterdam Density Functional (ADF) program with TZ2P basis set in conjunction with self-consistent field (SCF). For the silicon–nitrogen binary cluster system, Si-rich clusters favor three-dimensional structures. The nitrogen atoms separate from each other, if possible. Near the n=5 stoichiometry, the lowest energy structures are highly stable like-planar structures. They feature a strong alternation of Si and N atoms. For N-rich clusters, the forming N dimers are observed in the most stable structures. But, the N dimers are easy to be removed from the N-rich clusters. The remaining parts become linear or planar structures.

Structures and electronic properties of SimN8-m(0 < m < 8) clusters: a density functional theory study

The geometries, electronic structures and related properties of SimN8-m(0 < m < 8) clusters are studied using density functional theory (DFT) with hybrid functional B3LYP. The calculated results reveal several trends. For any stoichiometric clusters, the lowest energy isomers with an alteration of N and Si atoms are favourable in energy if the numbers of Si and N atoms are large enough to form … Si–N–Si–N… alternative chains. The bond lengths of single Si–N bonds are very close to the corresponding values of the bulk and other Si–N clusters. The geometries for N-rich and Si4N4 clusters are planar structures, but three-dimensional structures are favourable in energy for Si-rich clusters. With the increase of m, the isotropic polarizability and average polarizability increase, the total binding energies generally decrease, the HOMO-LUMO gap and vertical ionization potential oscillate with increasing number of valence electrons, and their values with even valence electrons are larger than those with odd valence electrons. The atomic charges, IR and Raman properties are also reported.

Geometries and electronic properties of [formula omitted] clusters

The equilibrium geometries and electronic properties of Na m Si n ( m + n ⩽ 7 ) clusters have been studied by using density functional theory at the B3LYP/6-311+G( d) level. The ground state structures of Na m Si n clusters can be obtained by adding one Na atom to Na m - 1 Si n clusters, and the Na atom locates on the site which maximally tends to form Na Si bonds. The Si-rich clusters are more stable than Na-rich clusters with the same number of atoms. The Si Si bond is stronger than the Na Si bond, and the latter is stronger than the Na Na bond. The vertical ionization potentials of Na m Si n clusters obtained by B3LYP are in good agreement with the experimental values available. For such a certain cluster size ( m + n = 4 , 5 , 6 , 7 ) , the energy gaps of the most stable Na m Si n clusters show odd–even oscillation with changing m, the clusters with odd Na atoms exhibit stronger chemical reactivity than those with even Na atoms.

A first-principles study of oxidation pattern in magic Si 7 cluster

Oxidation pattern of the magic cluster Si 7 was studied by first-principles calculations of Si 7O n ( n = 1–14) clusters. The lowest-energy structures of the Si-rich clusters consist of a pure Si fragment and an oxide fragment. The oxidation is found to extend from one edge throughout the whole cluster as the number of the oxygen atoms increases. Fragmentation energy analysis shows that the Si-rich clusters can often dissociate into a small pure Si n ( n = 2–6) cluster and a silicon oxide fragment. The O-rich clusters tend to separate into a SiO molecule and a silicon oxide fragment, except for Si 7O 14 which can easily lose an oxygen molecule in the fragmentation. The ionic clusters Si 7 O n ± ( n = 1–14) were also studied.

Oxidation Pattern of Small Silicon Oxide Clusters: Structures and Stability of Si6On (n = 1−12)

We have performed systematic ab initio calculations to study the structures and stability of Si(6)O(n)() clusters (n = 1-12) in order to understand the oxidation process in silicon systems. Our calculation results show that oxidation pattern of the small silicon cluster, with continuous addition of O atoms, extends from one side to the entire Si cluster. Si atoms are found to be separated from the pure Si cluster one-by-one by insertion of oxygen into the Si-O bonds. From fragmentation energy analyses, it is found that the Si-rich clusters usually dissociate into a smaller pure Si clusters (Si(5), Si(4), Si(3), or Si(2)), plus oxide fragments such as SiO, Si(2)O(2), Si(3)O(3), Si(3)O(4), and Si(4)O(5). We have also studied the structures of the ionic Si(6)O(n)(+/-) (n = 1-12) clusters and found that most of ionic clusters have different lowest-energy structures in comparison with the neutral clusters. Our calculation results suggest that transformation Si(6)O(n)+(a) + O --> Si(6)O(n+1)+(a) should be easier.

Ａｌ‐Ｍｇ‐Ｓｉ系合金におけるクラスタ形成と二段時効挙動

The cluster formation and two–step aging behavior of Al–1.4%Mg2Si and Al–1.4%Mg2Si–0.3%Si alloys were investigated using low-temperature adiabatic calorimetry, hardness measurement, electrical resistivity measurement and transmission electron microscopy. The specific heat changes in the alloys aged at low temperatures clearly demonstrated the existence of two exothermic peaks at temperatures between 273 and 343 K and higher than 343 K. These exothermic peaks were attributed to the formation of two types of pre-precipitates; i.e. Si-rich clusters and GP zones. The formation of these pre-precipitates was also detected by the electrical resistivity and hardness changes. The complicated two-step aging behavior of the present Al–Mg–Si alloys were well understood by taking into account the stability at higher temperatures and the effectiveness on the nucleation sites for the β″ phase of Si-rich clusters and GP zones during the final aging. The Si-rich clusters give a negative effect on the two-step aging phenomena is proposed. As the both pre-precipitates are formed competitively, such heat treatments as to give priority to the GP zone formation are effective to control the negative effect of the two-step aging and to increase age-hardening of Al–Mg–Si alloys.