Si-rich Clusters Research Articles

Sn and Cu influence on Mg:Si ratios evolution in clusters based on DFT and its impact on precipitation hardening of pre-aged Al-1.0Mg-0.6Si alloys

Using atom probe tomography (APT), transmission electron microscopy (TEM), and hardness tests, the effect of Sn and Cu on the evolution of Mg:Si ratios in clusters and subsequent precipitation hardening behavior of pre-aged Al-1.0Mg-0.6Si alloys are investigated, and the underlying mechanism is revealed by density functional theory (DFT) calculation of interaction energy. It is shown that, the doping of Sn and/or Cu increases the average Mg:Si ratio in clusters. This increase is attributed to the strong interaction of Sn/Cu with Mg solutes, which enhances the binding of Mg to Si-rich clusters and results in a higher proportion of clusters with Mg:Si ratios near 1 (0.75 ∼ 1.25). Consequently, the enhanced precipitation of β" precipitates during artificial aging is observed in Sn and/or Cu doped alloys, due to the ease of clusters with Mg:Si ratios near 1 transform into β" precipitates, leading to an improved hardening response. Notably, the combined doping of Sn and Cu exhibits the strongest aggregation tendency towards Mg solutes, yielding the most pronounced strengthening effects on precipitation and hardening response during artificial aging. Furthermore, a schematic model detailing the evolution of Mg:Si ratios in clusters is proposed, based on the APT results and DFT calculation of interaction energy.

A model for the precipitate transformation of Mg–Si-rich clusters into Mg5Si6 β″ in Al–Mg–Si aluminum alloys

Abstract A model is developed that describes the kinetics of precipitate transformations in the course of natural and artificial aging of Al alloys containing Mg and Si additions. In our approach, the disordered Mg–Si-rich clusters, which form during natural aging in the highly supersaturated Al matrix, can directly transform into the monoclinic Mg5Si6 (β″), without prior dissolution of the clusters and independent nucleation of β″ in the Al matrix. The transformation rate is evaluated with classical nucleation theory (CNT), assuming that the clusters represent an infinitely large matrix phase in which the β″ precipitates can nucleate. The adapted CNT model is described, and the basic features of the precipitate transformation are discussed in a parameter study. The model can also account for the observation that, during natural aging, the parent clusters occur in a variety of Mg to Si ratios, all of which have a characteristic probability of either transforming into the β″ phase or dissolving.

Inhibiting of the negative natural aging effect in Al–Mg–Si alloys

The negative natural aging effect is commonly observed in heat-treatable reinforced Al–Mg–Si alloys. It has been observed that when the Mg/Si mass ratio in the alloy approaches 0.68 (equivalent to an atomic ratio of 0.77), which closely aligns with the stoichiometric ratios of β″, it induces a modification in the precipitation pathway. Within this alloy, clusters formed during natural aging exhibit a progression from Si-rich to Mg-rich and eventually back to Si-rich as the natural aging duration extends. Remarkably, the Si-rich clusters that form in the latter stages of natural aging exhibit stability and can attain the critical nucleation size required for β″ formation. Subsequently, an abundance of uniformly distributed nuclei emerges, contributing to the development of numerous fine β″ precipitates at the peak artificial aging temperature. Consequently, this phenomenon effectively curtails the negative effect of natural aging.

Study of natural aging behavior of Al-0.4Mg-1.0Si alloy with the addition of Sn at different solution heat treatment temperatures

In this paper, the combined effect of Sn addition and solution treatment temperature on natural aging (NA) hardness behavior and microstructure of Al-0.4Mg-1.0Si alloy were investigated by hardness test, scanning electron microscope (SEM) and scanning transmission electron microscope (STEM). The evolutions of clusters characterized by three-dimensional atom probe (3DAP) during initials stages of NA are also investigated based on the slow in formation of clusters due to Sn addition. The results shown that a Sn-rich particles with round shape retained in Al matrix after quenching treatment when the added Sn content exceeds it limit solubility, which will reduce the Mg content of the designed alloy. The suppression of NA can be enhanced with the increase in the adding Sn content and solution treatment temperature, which is strongly related to the delay of the segregation of Mg and Si to form clusters due to more trapping vacancies by Sn atoms. The addition of Sn also revealed that the segregation of Si during NA is earlier than that of Mg atoms. The Si-rich clusters are formed preferentially during initial stage of NA, and Mg–Si co-clusters are formed after long period of NA. The Mg/Si ratio and average size of Sn-containing clusters in the 0.04%Sn and 0.1%Sn alloy are obviously higher than that of Sn-free clusters regardless of the 1 day NA or 14 days NA as more Mg atoms are dragged into Sn-containing clusters due to strong interaction between Sn and Mg atoms.

Effects of Cu and pre-aging on the clustering behavior in Al-Mg-Si alloys

The clustering behaviors in two Al-Mg-Si alloys with and without Cu after different pre-aging and natural aging conditions were systematically quantified using microhardness, differential scanning calorimetry (DSC), and atom probe tomography. Cu addition slowed down the natural aging process after either solutionizing or pre-aging regardless of pre-aging temperature and time, and reduced the negative effect of post pre-aging natural aging on β” precipitation by changing the composition of the Mg and Si rich clusters. This work complements the limited literature on the effect of long natural aging, and provides insights into the impacts of alloy composition and storage time on precipitation behavior during subsequent artificial aging.

Contributions of Ni-content and irradiation temperature to the kinetic of solute cluster formation and consequences on the hardening of VVER materials

The study of solute clusters induced by neutron irradiation is fundamental to the understanding of the embrittlement phenomena of Reactor Pressure Vessel (RPV) steels. In this paper, the influence of Ni-content and irradiation temperature on these cluster formation are investigated using atom probe tomography.VVER 1000-BM (base metal) and 1000-W (weld) steels were chosen for their high Ni content of 1.2 and 1.7 at%, respectively. They were irradiated up to doses of 0.28 dpa at two temperatures (265 and 300 °C). The irradiations were performed at SCK CEN in the BR2 reactor.After irradiation, Mn, Ni and Si rich clusters were observed. A careful study of their composition indicates that these clusters actually contain iron. Moreover, the evolution of Fe cluster concentration with the dose indicates a constant supply of solutes throughout the irradiation thanks to the flux coupling. With increasing dose, both the cluster size and number density increase.The effect of temperature has to be dissociated from that of Ni content. Both of them result in an increase of the cluster number density at a given dose. The irradiation temperature has a more pronounced effect than chemical composition (mainly Ni effect). This can be explained by the fact that more matrix damage is produced at lower irradiation temperature. The influence of Ni on cluster number density is significantly higher at the lower temperature. All these observations support the hypothesis of radiation-induced segregation, even if a thermodynamic contribution to solute clustering cannot be excluded, in particular in the higher Ni steel irradiated at low temperature.In all cases, the yield strength increases proportionally to the volume fraction of radiation-induced defects (√(Vf)).

Precipitation behavior of an Al7SiMg alloy processed by laser powder bed fusion during non-isothermal and isothermal heat treatments

Laser powder bed fusion (LPBF) is drawing great interest in industry due to its ability of producing complex-shaped components with superior mechanical properties and unique microstructures. Al7SiMg alloy is one of the most used aluminum alloys in LPBF due to its easy processing window. Direct T5 aging is often applied to LPBF-processed Al7SiMg alloy to obtain better mechanical performances over T6 heat treatment. However, after T5 heat treatment, a comprehensive understanding of the precipitation sequence of the strengthening phase in LPBF-processed Al7SiMg alloy is still lacking. In the present study, a high-resolution TEM examination of the precipitation behavior of Al7SiMg alloy processed by LPBF during heating without and with isothermal holding was conducted to further understand the precipitation sequence of Si precipitates and the associated microstructure evolution. During the heating process, irregular and needle-shaped pure Si precipitates were observed which shared the same [011] zone axis with the aluminum matrix. After analyzing the phase transformation process during heating, the following precipitation sequence is proposed: Si in supersaturated solid solution → Si-rich clusters → Si-rich intermediate phases → pure Si precipitates. During isothermal holding at 160 ℃ (T5), nano-scale Si-rich clusters were observed at the peak aging condition. When the holding time extended to 24 h, larger Si-rich clusters with reduced coherence were found. Strain fields caused by these Si-rich clusters are believed to strengthen the LPBF-processed Al7SiMg alloy after T5.

Contribution of dislocation loop to radiation-hardening of RPV steels studied by STEM/EDS with surveillance test pieces

Microstructure and radiation-hardening of surveillance test pieces of reactor pressure vessel steels have been studied by STEM/EDS and Vickers micro-hardness measurements. In a low-Cu steel (0.06 wt%), the microstructures irradiated with fluence of 5.62 × 1023 n/m2 were found to include nano-size Ni–Si-rich solute clusters (SCs) and dislocation loops (DLs). Most of the DLs were sustained after post-irradiation annealing at 500 °C for 0.5 h, while the number density of SCs decreased. The stability of the DLs is suggestive of entanglement with solute atoms. On the other hand, Cu-rich SCs were observed in high-Cu steels (0.17 wt%) irradiated with fluence of 1.01 × 1023 n/m2 and 6.11 × 1023 n/m2, however, no DLs observed. The Cu content in the matrix seems to be crucial for the nucleation of the DLs, which suggests that the mobility of the vacancy coupled with Cu atom dominates the nucleation/annihilation of self-interstitial atom. The contribution of DLs and SCs are discussed on the basis of the Russell-Brown and Orowan models, respectively.

Atomic-scale clustering in a high-strength Al-Mg-Si-Cu alloy

Effects of pre-aging and natural aging on the cluster distribution and mechanical properties of an Al-Mg-Si-Cu alloy before and after artificial aging are reported . Using atom probe tomography, a difference in clustering behavior was characterized between the naturally aged and the pre-aged heat treatments. In the naturally aged then artificially aged state, the formation of Si-rich clusters that did not readily grow into Guinier-Preston zones and β’’ precipitates(the primary strengthening phase for this alloy) resulted in a strength of 216 MPa. If the alloy was pre-aged and immediately artificially aged, a high density of β’’ precipitates formed, and the yield strength increased to 289 MPa. Copper was found in all strengthening particles approximately 1 nm or larger and is deemed essential for the faster precipitation kinetics to enable growth of β’’ precipitates. In addition, pre-aging followed by an extended secondary natural aging treatment for 30 days then subsequently arfiticially aged revealed a modestly lower yield strength (282 MPa). This drop in strength is linked to the formation of more detrimental Si-rich clusters that evolve over the extended naturally aging period suggesting potential time limitations when these artifically aging conditions can achieve optimal strengthening outcomes.

Direct Observation of PFIB-Induced Clustering in Precipitation-Strengthened Al Alloys by Atom Probe Tomography.

The effect of sample preparation on a pre-aged Al–Mg–Si–Cu alloy has been evaluated using atom probe tomography. Three methods of preparation were investigated: electropolishing (control), Ga+ focused ion beam (FIB) milling, and Xe+ plasma FIB (PFIB) milling. Ga+-based FIB preparation was shown to introduce significant amount of Ga contamination throughout the reconstructed sample (≈1.3 at%), while no Xe contamination was detected in the PFIB-prepared sample. Nevertheless, a significantly higher cluster density was observed in the Xe+ PFIB-prepared sample (≈25.0 × 1023 m−3) as compared to the traditionally produced electropolished sample (≈3.2 × 1023 m−3) and the Ga+ FIB sample (≈5.6 × 1023 m−3). Hence, the absence of the ion milling species does not necessarily mean an absence of specimen preparation defects. Specifically, the FIB and PFIB-prepared samples had more Si-rich clusters as compared to electropolished samples, which is indicative of vacancy stabilization via solute clustering.

A molecular dynamics simulation of inhomogeneous silicon–germanium nucleation from supersaturated vapor mixtures

The inhomogeneous nucleation of silicon–germanium (Si–Ge) systems from supersaturated vapor mixtures was investigated using molecular dynamics simulations. Isothermal simulation runs were performed using the Tersoff potential at various supersaturations and temperatures. We focused on the inhomogeneous dynamics, nucleation rate, and critical cluster size, as well as the effect of inhomogeneity on the quantitative results. The study showed that Si atoms nucleate much faster than Ge atoms. This may lead to the inhomogeneity and final production of Si-rich critical clusters. Such inhomogeneity may also stem from the different chemical properties of Si and Ge atoms. Under the tested conditions, the nucleation rates were within 1033–1036 J/m−3 s−1. They were influenced significantly by the supersaturation and slightly by the temperature. The critical size of 2.5–4.5 atoms was heavily dependent on both the supersaturation and temperature. Our results are generally consistent with those from other nucleating systems using the same method. The inhomogeneity of the Si–Ge system has no significant effect on the nucleation rate but may contribute to smaller critical cluster sizes at low temperatures.

The nanocluster formation and vacancy behavior of step-quenched Al–Mg–Si alloy and its effect on transition to β" phase via advanced methods

The effect of step-quenching (SQ) on two/multi-step aging and the bake-hardening response in Al-0.9Mg-1.0Si (in wt.%) alloy was investigated in terms of nanocluster formation and vacancy behavior using advanced methods, including atom probe tomography (APT) and positron annihilation lifetime spectroscopy (PALS) analysis. The SQ treatment was found to be highly effective at suppressing the negative effects of natural aging during two/multi-step aging, compared with pre-aging (PA) treatment. The characteristic behaviors of nanoclusters formed by PA and SQ treatments were completely different. SQ is likely to preferentially form Si–V complexes, which are different than Cluster (1), and Si-rich clusters, as detected by the distribution of cluster compositions using APT analysis. Si–V complexes pre-formed during SQ treatment result in the formation of Mg–Si clusters with higher atomic density, inducing favorable transformation to the pre-β" phase because there are fewer vacancies inside the Mg–Si cluster. Also, the Si–V complexes reduce the fraction of Si-rich clusters commonly formed during natural aging (NA). The step-quenched Al–Mg–Si alloy with great bake-hardening response has promising potential for automotive outer panel applications.

Morphological classification of dense objects in atom probe tomography data

The technique of atom probe tomography is often used to image solute clusters and solute atom segregation to dislocation lines in structural alloys. Quantitative analysis, however, remains a common challenge. To address this gap, we combined a cluster finding algorithm, a skeleton finder algorithm, and morphological classification of dense objects to distinguish solute clusters from solute-decorated dislocation lines, both being characterized by high solute atom densities. The proposed workflow is packaged into a graphical user interface available through GitHub. We illustrate its application on a synthetic dataset containing known objects and apply it to an experimental dataset obtained from a proton-irradiated Alloy 625 that contains high densities of Si-decorated dislocations and Si-rich clusters.

Microstructure degradation of austenitic stainless steels after 45 years of operation as VVER-440 reactor internals

Specimens of reactor vessel internals (RVI) made of 18Cr–10Ni–Ti steel from the decommissioned VVER-440 unit after 45 years of operation were investigated. The aim was to assess the structure degradation degree responsible for the properties changes of RVI components. Radiation-induced structural elements of RVI baffle, former, and flux thimble specimens with the damaging dose 7.9–43.0 dpa and irradiation temperature 280–315 °C were studied using TEM, SEM, and APT methods. TEM-studies showed the negligible swelling under these conditions with a slight increase of radiation swelling due to the formation of the low-density large void population in case of higher temperature irradiation. APT-studies revealed the formation of the high-density Ni–Si-rich clusters (presumably G- and γ′- phase precursors) and an increase of its volume fraction with the temperature increase. The Orowan equation was used to assess the hardening contribution of different radiation-induced structural elements. The observed yield strength increase is insignificant under the studied conditions.

Effects of pre-aging and natural aging on the clusters, strength and hemming performance of AA6014 alloys

In this study, the effects of pre-aging and natural aging on the clusters, strength and hemming performance of AA6014 Al–Mg–Si alloys with different Mg/Si ratios were investigated through three-dimensional atom probe (3DAP) in combination with tensile test and three-point bend test. Samples are subjected to natural aging (room temperature, ~25 °C) or pre-aging (elevated temperature, 75 °C) treatments in order to generate different types of clusters. The number density, size and compositions of clusters are analyzed by 3DAP. The increased strength and deteriorated hemming performance are found in natural aged alloys, especially for Si-rich alloy. The hemmability is sensitive to the strength, and an increased strength leads to inferior hemmability. The formation of Mg1Si1 type cluster is encouraged during pre-aging treatment regardless of the alloy composition. Natural aging following directly water quenching promotes the formation of a large number density of small-sized Si-rich clusters in Si-rich alloy, whereas relatively low density of clusters with scattered Mg/Si ratios distribution in Mg-rich alloy. The concentration of the excess vacancies and the binding energy between vacancy and Mg and Si atoms plays an important role in the precipitation kinetics of cluster during different aging processes. Finally, the clusters and solid solution atoms statistics are utilized to predict the yield strength of the alloys in different aging conditions.

Clustering phenomena in quenched Al, Al–Mg, Al–Si and Al–Mg–Si alloys

Vacancy and solute atom clusters of various sizes and number densities may be formed in Al alloys during natural ageing (NA) after solutionising and quenching. We study the clustering characteristics of a series of Al–Mg–Si alloys and discuss ageing mechanisms as a function of solute content: in alloys low in Mg (i.e. <0.1 at.%), vacancies tend to aggregate during quenching and subsequently assist the formation of Si-rich clusters in the course of NA. For alloys with higher Mg contents (>0.1 at.%), clustering is initially governed by Si atoms and then by Mg as NA proceeds.

Atom Probe Tomography Interlaboratory Study on Clustering Analysis in Experimental Data Using the Maximum Separation Distance Approach.

We summarize the findings from an interlaboratory study conducted between ten international research groups and investigate the use of the commonly used maximum separation distance and local concentration thresholding methods for solute clustering quantification. The study objectives are: to bring clarity to the range of applicability of the methods; identify existing and/or needed modifications; and interpretation of past published data. Participants collected experimental data from a proton-irradiated 304 stainless steel and analyzed Cu-rich and Ni-Si rich clusters. The datasets were also analyzed by one researcher to clarify variability originating from different operators. The Cu distribution fulfills the ideal requirements of the maximum separation method (MSM), namely a dilute matrix Cu concentration and concentrated Cu clusters. This enabled a relatively tight distribution of the cluster number density among the participants. By contrast, the group analysis of the Ni-Si rich clusters by the MSM was complicated by a high Ni matrix concentration and by the presence of Si-decorated dislocations, leading to larger variability among researchers. While local concentration filtering could, in principle, tighten the results, the cluster identification step inevitably maintained a high scatter. Recommendations regarding reporting, selection of analysis method, and expected variability when interpreting published data are discussed.

Liquid structure of Al-Si alloy: A molecular dynamics simulation

Liquid structure of Al-Si alloy is a long-standing issue. Using the newly-developed Al-Si potential, we studied the liquid structure of Al-10.3 wt% Si alloy by molecular dynamics simulation for the first time. We found that Al and Si atoms tend to be well mixed and the aggregation of Si-rich clusters is weak. The results exclude the possibility of regarding the aggregation of Si-rich clusters as the origin of the hysteresis in liquid Al-Si alloy. We didn't find obvious abrupt structure change in liquid Al-10.3 wt% Si near the melting point of Al.

Natural aging behavior in pre-aged Al–Mg–Si–Cu alloys with and without Zn addition

This study investigates the clustering behavior and its correlation with the change in hardness during natural aging (NA) after pre-aging (PA) at 80 °C for 15 min in Al–Mg–Si–Cu alloys with and without Zn addition. The results show that NA after PA causes the growth of clusters in the Zn-free and Zn-added alloys, which is related to the partitioning of solute atoms from the Al matrix into clusters. Si exhibits considerably stronger partitioning than Mg during NA after PA, which leads to an increase in the number of Si-rich clusters. Cu can also be partitioned into clusters of two alloys when the NA after PA is conducted. Zn is present in the clusters of the Zn-added alloy during NA after PA. The addition of Zn increases the growth rate of clusters. In addition, the increased volume fraction of clusters explains the increased hardness of the Zn-free and Zn-added alloys during NA after PA. Compared with the Zn-free alloy, the Zn-added alloy exhibits a more significant change in volume fraction of clusters, thus achieving an enhanced age hardening response.

Effects of cluster characteristics on two-step aging behavior in Al-Mg-Si alloys with different Mg/Si ratios and natural aging periods

Age-hardening behavior during the early stages of artificial aging (AA) is associated with the clustering behavior of high-Mg and high-Si Al-Mg-Si alloys. Herein, the cluster characteristics during natural aging (NA) and the early stages of subsequent AA are obtained using atom probe tomography. Alloy composition exerts a significant impact on the internal composition of clusters after short-term NA, and clusters with a similar number of Mg and Si atoms increase after long-term NA. The cluster behavior during the early stages of AA is highly dependent on the internal chemical composition of clusters, which suggests that Mg-Si clusters with Mg/(Mg+Si) values of over 0.4 decrease, independent of cluster size. In addition, the results also suggest that Si-rich clusters with Mg/(Mg+Si) values of below 0.4 retard the formation of the β″ phase during AA. These findings are discussed in relation to the hardness change. The increase in the hardness during the early stage of AA is associated with the proportion of Si-rich and Mg-Si clusters in both high-Mg and high-Si alloys.