Growth Of Eutectic Si Research Articles

Microscopic mechanism of mechanical property improvement of the casted Al-7Si-0.4Mg-0.3Y alloy

This paper investigates the effect of different casting conditions on the microstructure of Al-7Si-0.4 Mg alloys with 0.3 wt.% rare earth Y addition and the effect of T6 heat treatment on their mechanical properties. The experimental studies showed that the addition of 0.3 wt.% rare earth Y at 30 min or 1200 k resulted in the optimum microstructure and mechanical properties of the alloys, and the addition of Y led to the formation of Y-containing nanoparticles on eutectic Si, which were re-solidified by T6 heat treatment and precipitated a more stable orthorhombic cubic structure of Al2Si2Y compounds at the grain boundaries. According to the growth kinetic relationship, the addition of Y replaces the nucleation center of the original eutectic Si, and the preferential precipitation of the Y-containing phase destroys the heterogeneous nucleation conditions of the original eutectic Si, which leads to the growth of eutectic Si towards the original growth direction close to 60°, and generates more heterogeneous twins at the interface and gradually builds up, which has an inhibitory effect on the growth of eutectic Si. The fracture morphology and energy spectra show that the precipitated Y-containing coexist in these precipitates will lead to local stress concentration resulting in microcracks and ultimately fracture, which will change the mechanical properties of the alloy significantly.

Effect of melt purification treatment on the solidified microstructure of hypereutectic Al-Si alloys

In this paper, hypereutectic Al-Si alloys were purified by a composite sodium salt, and the solidified microstructure of hypereutectic Al-Si alloys before and after melt purification was studied by OM, SEM/EDS, XRD, and TEM. The results showed that the Fe content decreased from 0.73 wt% to 0.23 wt% after purification with the Na salt. This also prevented an increase in heterogeneous nucleation. The Si content remained almost unchanged, which excluded the possibility of removing Si from the molten Na salt. According to DSC analysis, Na salt purification increased the undercooling degree of the melt, so that the hypereutectic Al-Si alloy melt was deeply undercooled to the pseudo eutectic region, and a nearly-perfect pseudo eutectic structure was obtained. Moreover, it was confirmed that NaAlSi changed the growth of eutectic Si, thereby changing its microstructure. At the same time, nanoscale spherical precipitates formed by trace Fe in the alloy could significantly promote the nucleation of eutectic Si.

Al-Si nano clusters and short-range orders induced fibrous modified eutectic Si

Fine-grained structural material with the same composition was used to modify the eutectic Si in present study. The modification mechanism was investigated by SEM, TEM, HRTEM and APT analysis. AlSi nano clusters and short-range orders form at the solid/liquid interface through eutectic Si growth, which can induce plenty of crystal defects such as stacking faults and twins and consequently modify the eutectic Si.

Microstructural evolution and strengthening mechanisms of a novel Al–11Si–3Cu alloy microalloyed with minor contents of Sr and Sc

Alloying technology usually improves the mechanical properties of alloys; however, the high cost of alloying elements limits their wider applicability to conventional cast aluminium alloys. In this work, a novel high-strength, ductile, and low-cost Al–11Si–3Cu alloy microalloyed with minor Sr + Sc was developed and shown to improve the ultimate tensile strength and elongation relative to the majority of developed Al–Si-(Cu) alloys to date. Atom probe tomography (APT) reconstruction showed that Sc atoms existed separately instead of being enriched with Al and Sr atoms in eutectic Si, which indicated that the growth of eutectic Si in different crystallographic directions was inhibited. Many interactive stacking fault planes were formed in the novel alloy, which, as a series of barriers, effectively prevented dislocations from passing through, thereby providing a significant boost to the alloy strength. Moreover, a large number of Al3Sc and AlSi2Sc2 nano-precipitates exerted positive effects on the alloy strength by effectively slowing the dislocation movement. In addition, the strengthening mechanisms of the novel high-strength ductility Al–11Si–3Cu alloy are discussed.

Accelerating pore nucleation and eutectic Si growth kinetics by increasing Cu and Sc for Al-Si-Mg alloys: In-situ observation

Abstract The mechanical performance of Al-Si-Mg cast alloys is highly dependent on the pore size and morphology of eutectic Si, which is especially critical for high fatigue life components. Many studies have shown the linear relationship between them. However, it is unknown how the beneficial alloying elements for grain refinements and precipitation strengthening will have an impact on those critical properties. In this study, we used synchrotron X-ray to quantify the kinetics of eutectic Si growth for three alloys. These eutectic Si nucleate and grow rapidly at the initial stage of growth, and then the morphology tends to remain unchanged. Surprisingly, the Sc addition which refines the grains and results in fibrous morphology instead of highly faceted Si particles. The addition of precipitation strengthening element Cu can significantly increase the hydrogen content and increase the number of porosity by an order of magnitude. Therefore, in situ observation and X-ray tomography in this study enabled us to understand the key roles of Cu and Sc and control the kinetic process of both eutectic Si and defects for better mechanical performance.

Effects of Zn on Microstructure Modification and Mechanical Properties Improvement of Al-Si-Cu-Mg Alloys

In this research, the influence of Zn on the microstructure and mechanical properties of Al-Si-Cu-Mg alloys under different solidification rates was investigated. Differential scanning calorimetry was used to detect the nucleation and growth of eutectic Si, while scanning electron microscopy was used to analyze Fe-rich phases. Two growth models were deduced to explain the modification of the eutectic Si and Fe-rich phases upon the addition of Zn. The results show that an addition of 0.6 wt pct Zn modified the eutectic Si from flake like and fibrous to micron- and submicron-granular structures, depending on the different secondary dendrite arm spacings of ~ 33 μm and ~ 16 μm, respectively. For the Fe-rich phases under steel mold casting conditions, β-Al5FeSi was modified into π-Al8FeMg3Si6 upon adding 0.6 wt pct Zn. Under sand mold casting conditions, the 0.6 wt pct Zn addition only refined the size of the β-Al5FeSi. The Zn addition significantly improved the ultimate tensile strength and elongation. The excellent modification effect of Zn on the eutectic Si and Fe-rich phases was attributed to the fact that the reaction of the Zn-Si eutectic on the {111}Si planes forced the Si atoms from these phases into the eutectic droplets.

Synchronous improvement of electrical and mechanical performance of A356 alloy reinforced by boron coupling nano-AlNp

High strength and excellent electrical conductivity of Al alloys are highly desired in the field of power transmission. However, these two factors are usually contradictory in metallic materials. A novel boron coupling nano-AlN particles (about 0.6 wt% nano-AlNp and 0.06 wt% B) reinforced A356 alloy is proposed in this paper, which obtains excellent electrical conductivity (σ: 46.2% IACS), high ultimate tensile strength (UTS: 300 MPa), superior yield strength (YS: 240 MPa) and good elongation (EL: 7.5%) synchronously. Contrasting with the general A356 alloy modified by Sr, the σ, UTS, YS, and EL increased by about 8.2%, 9.1%, 4.3% and 66.7% respectively. This strategy is mainly based on boron treatment and nano-AlNp inhibiting the lapped growth of eutectic Si to ensure high conductivity, and the modification of Si, the refinement of the SDAS of α-Al grains and the relative homogeneous distribution of the individual or small flocculent nano-AlNp in the matrix and at the Al/Si interface to ensure high mechanical properties.

Solidification Rate and the Partial Modification of 319 Aluminum Alloy with Strontium

Trace strontium (Sr) additions to Al–Si alloys can transform coarse, acicular eutectic Si particles into a fine, fibrous morphology. Much research has focused on the production of well-modified structures, given their superior material properties. However, the partial modification of Al–Si alloys has received minimal attention, with little data available on the role of solidification rate. In this study, the partial modification of industrial 319 Al alloy was investigated by producing castings with Sr contents up to 300 ppm and total solidification rates between 0.14 and 5.89 °C s−1. Eutectic Si growth temperatures ($$ T_{\text{G}} $$) and median particle morphologies were characterized using thermal analysis and quantitative image analysis. This revealed synergistic modification effects between Sr and solidification rate on either feature. At intermediate Sr contents, a gradual Si transformation occurred only at slow solidification rates. Yet, at 3.03 °C s−1 and higher, the transformation was drastic with no appreciable partial modification range. Furthermore, an unusual distribution of fine Al2Cu particles was observed in the eutectic regions of the Sr-containing alloys. Their presence was also influenced by the combination of Sr and solidification rate. The knowledge and alloy-specific $$ T_{\text{G}} $$ and modification data presented will enable effective alloy processing and property control of 319 Al components.

A new modification effect of eutectic Si in selective laser melted AlSi10Mg

ABSTRACTA distinct modification of eutectic Si was observed in the selective laser melted AlSi10Mg. The eutectic Si particles were modified substantially and demonstrated a nanoscale size, a fibrous morphology with obvious ‘necking effect’. The modification mechanism and the influence of heat treatment on eutectic Si growth were explored. It was suspected that Si experienced the necking effect under a tensile environment due to the large temperature gradient and α-Al erosion during the SLM process. Upon heat treatment, the Si particles were spheroidised and distributed homogeneously. Under the synergistic action of Si crystal fragmentation and the continuous precipitation of saturated Si from the Al matrix, the coarsened and spheroidised eutectic Si were distributed homogeneously in the Al matrix.

Utilizing a novel modifier to realize multi-refinement and optimized heat treatment of A356 alloy

Al-5Sr-8Ce master alloy was fabricated as a novel modifier, and its effect on the microstructure and tensile properties of A356 alloy was systematically investigated. The solution treatment time was also optimized based on the modification efficiency of Al-5Sr-8Ce. Results showed that Al-5Sr-8Ce mainly comprised α-Al, Al4Sr, and Al11Ce3 phases. With increasing addition of Al-5Sr-8Ce to 0.4 wt%, both the grain size and secondary dendrite arm space (SDAS) were significantly refined, while the eutectic Si morphology was modified from plate-like to small granular and rod-like morphologies. Grain growth inhibition caused by Ce segregation and the formation of Ce containing intermetallic compounds along the grain boundary was mainly responsible for the grain refinement. The excellent modification effect was attributed to the enhanced Sr modification and the inhibited growth of eutectic Si under the action of Ce. The multi-refinement of microstructure by Al-5Sr-8Ce improved the tensile properties of alloy, in particular a significant increase in elongation. However, the excess addition of Al-5Sr-8Ce (>0.4 wt%) increased the formation of coarse Ce containing intermetallic compounds, deteriorating the ultimate strength and elongation of the modified alloy. Based on the excellent Si modification by adding 0.4 wt% Al-5Sr-8Ce, the solution treatment time was successfully shortened to 90 min.

The Formation of Nanoparticles and Their Competitive Interaction with Twins during Eutectic Si Growth

In order to investigate the competitive interaction between nanoparticles and twin, the eutectic Si microstructures in Al–10Si (wt. %) base alloys with exclusive and combined addition of Sr and Sb are characterized by combined TEM and atom probe tomography (APT). The chemical short range order in Sb–Sb and Sb–Sr pairs is revealed by ab initio molecular dynamics simulation, which promotes the formation of clusters and nanoparticles. The coexistence of nanoparticles and twins is observed in Sb containing alloys, with a negative correlation in the corresponding number density, owing to the competitive stacking of precursors and individual atoms at the solid–liquid interface. Large size particles around 70 nm with a uniform distribution of Sr atoms are formed in Al–10Si–0.35Sb–0.015Sr (wt. %) alloys, due to the precursor aggregation and homogeneous nucleation in the droplets that nucleation are depressed. A model for the formation of nanoparticles and their interaction with twins is proposed.

Synergistic effects of Gd and Zr on grain refinement and eutectic Si modification of Al-Si cast alloy

The effects of Gd and/or Zr additions on the grain refinement and eutectic Si modification of A356 cast alloys have been systematically investigated. The results showed that a separate addition of Gd or Zr had almost no effect on eutectic Si modification, while except for an appreciable grain refinement, a combined addition of Gd and Zr significantly refined the eutectic Si and changed its morphology from plate-like to fibrous. As compared with Gd, Zr and Sr separate modification, the Gd and Zr combined addition produced fewer twins but more nanosized precipitates within eutectic Si. The excellent modification effect was attributed to the increase of constitutional undercooling and nanosized precipitates inhibited the growth of eutectic Si during solidification. The Gd and Zr combined addition obviously improved the mechanical properties of A356 cast alloy due to grain refinement and eutectic Si modification. Optimal tensile properties were achieved in the 0.4Gd+0.5Zr modified alloy. A high amount of Zr addition resulted in the formation of coarse Al5Si2Zr3 phase and the decrease of ductility.

Determination of Optimum Sr Level for Eutectic Si Modification in Al–Si Cast Alloys Using Thermal analysis and Tensile Properties

This study presents a correlation between the depression in eutectic growth temperature as a result of Sr modification and the tensile properties of Al–Si cast alloys. In order to study the role that Sr exerts on the solidification behavior, modification and mechanical properties, controlled solidification experiments including thermal analysis were performed. Using three mold materials for different cooling rates, tensile testing was conducted on Al–Si alloys with various Sr levels (~35–500 ppm). The gradient solidification technique was used to produce directionally solidified tensile test specimens containing low levels of defects. The depression in eutectic Si growth temperature was correlated with the Sr additions and the tensile properties (elongation to failure and tensile strength).

Cluster formation at the Si/liquid interface in Sr and Na modified Al–Si alloys

Atom probe tomography was used to compare Na and Sr modified Al–Si hypoeutectic alloys. Both Na and Sr promote the formation of nanometre-sized clusters in the Si eutectic phase. Compositional analyses of the clusters show an Al:Sr ratio of 2.92±0.46 and an Al:Na ratio of 1.07±0.23. It is proposed that SrAl2Si2 and NaAlSi clusters are formed at the Si/liquid interface and take part in the modification process by altering the eutectic Si growth.

Modeling of aluminum-silicon irregular eutectic growth by cellular automaton model

Due to the extensive application of Al-Si alloys in the automotive and aerospace industries as structural components, an understanding of their microstructural formation, such as dendrite and (Al+Si) eutectic, is of great importance to control the desirable microstructure, so as to modify the performance of castings. Since previous major themes of microstructural simulation are dendrite and regular eutectic growth, few efforts have been paid to simulate the irregular eutectic growth. Therefore, a multiphase cellular automaton (CA) model is developed and applied to simulate the time-dependent Al-Si irregular eutectic growth. Prior to model establishment, related experiments were carried out to investigate the influence of cooling rate and Sr modification on the growth of eutectic Si. This CA model incorporates several aspects, including growth algorithms and nucleation criterion, to achieve the competitive and cooperative growth mechanism for nonfaceted-faceted Al-Si irregular eutectic. The growth kinetics considers thermal undercooling, constitutional undercooling, and curvature undercooling, as well as the anisotropic characteristic of eutectic Si growth. The capturing rule takes into account the effects of modification on the silicon growth behaviors. The simulated results indicate that for unmodified alloy, the higher eutectic undercooling results in the higher eutectic growth velocity, and a more refined eutectic microstructure as well as narrower eutectic lamellar spacing. For modified alloy, the eutectic silicon tends to be obvious fibrous morphology and the morphology of eutectic Si is determined by both chemical modifier and cooling rate. The predicted microstructure of AI-7Si alloy under different solidification conditions shows that this proposed model can successfully reproduce both dendrite and eutectic microstructures.

The roles of Eu during the growth of eutectic Si in Al-Si alloys.

Controlling the growth of eutectic Si and thereby modifying the eutectic Si from flake-like to fibrous is a key factor in improving the properties of Al-Si alloys. To date, it is generally accepted that the impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as poisoning of the TPRE mechanism are valid under certain conditions. However, IIT, TPRE or poisoning of the TPRE mechanism cannot be used to interpret all observations. Here, we report an atomic-scale experimental and theoretical investigation on the roles of Eu during the growth of eutectic Si in Al-Si alloys. Both experimental and theoretical investigations reveal three different roles: (i) the adsorption at the intersection of Si facets, inducing IIT mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE mechanism or poisoning of the TPRE mechanism, and (iii) the segregation ahead of the growing Si twins, inducing a solute entrainment within eutectic Si. This investigation not only demonstrates a direct experimental support to the well-accepted poisoning of the TPRE and IIT mechanisms, but also provides a full picture about the roles of Eu atoms during the growth of eutectic Si, including the solute entrainment within eutectic Si.

Twin-controlled growth of eutectic Si in unmodified and Sr-modified Al–12.7%Si alloys investigated by SEM/EBSD

The growth characteristics of eutectic Si in unmodified and Sr-modified Al–12.7%Si alloys were investigated by microstructure-correlated crystallographic analyses. For the unmodified alloys, the formation of repeated single-orientation twin variants enables rapid growth of eutectic Si according to the twin plane re-entrant edge (TPRE) mechanism. Microscopically, Si crystals are plate-like elongated in one 〈110〉 direction that is not in accordance with the 〈112〉 growth assumed by the TPRE model. The 〈110〉 growth direction is realized by paired 〈112〉 zigzag growth on parallel twinning planes, leading to alternate disappearance and creation of 141° re-entrants. As each twinning plane is associated with three re-entrants, Si crystals may extend in three co-planar 〈110〉 directions and cause the formation of equilateral plates. With the formation of α-Al around eutectic Si, the number of re-entrants is reduced. The planar isotropic growth of eutectic Si becomes anisotropic, leading to the formation of long plates. The reduction of the number of re-entrants also accounts for the width and thickness changes over the length of Si plates. This complex growth mode results in Si crystals exposing only their low-energy {111} planes to the melt. For the Sr-modified alloys, substantial changes appear in the eutectic Si morphology, attributable to the restricted TPRE growth and the impurity induced twinning (IIT) growth. The former enhances lateral growth by forming new twins with parallel twinning planes, while the latter leads to isotropic growth by forming differently oriented twins.

Microstructural evolution in semisolid forging of A356 alloy

A semisolid forging study using A356 alloy was carried out to investigate the evolution of microstructures affected by the solid fraction, the forging pressure and the addition of Sr. The semisolid slurry for forging was made by an electromagnetic stirring method. Microstructures were evaluated at two typical positions of the semisolid forged specimens: one is the region, where direct forging pressure is applied, and the other is the region, where the slurry was squeezed and extruded indirectly. Microstructural characteristics, such as the morphology of the primary Al particles and the eutectic Si, were changed with the positions observed. As the solid fraction increased, the morphology of the primary Al particles was transformed from rosette-like to globular. As the forging pressure increased, the size of eutectic Si became fine. An increase of the forging pressure is speculated to improve the interfacial heat transfer coefficient between the semisolid slurry and the mould wall, leading to an increase of the cooling rate on the growth of eutectic Si. The addition of Sr is effective in modifying the morphology for eutectic Si, especially when the forging pressure was low.

Nucleation and Growth of Eutectic Si in Al-Si Alloys with Na Addition

Al-5 wt pct Si-based alloys with Na additions (19 and 160 ppm) have been produced by controlled sand casting and melt spinning. Entrained droplet technique and differential scanning calorimetry were employed to investigate the nucleation behavior of eutectic Si. High-resolution transmission electron microscopy and atom probe tomography were used to investigate the distribution of Na atoms within eutectic Si and at the interfaces between eutectic Si and eutectic Al. It was found that (i) only 19 ppm Na addition results into a high undercooling (49 K (49 °C)) of the entrained eutectic droplet. However, further increasing Na addition up to 160 ppm exerts no positive effect on the nucleation of eutectic Si, instead a decreased undercooling (29 K (29 °C)) was observed. (ii) Na addition suppresses the growth of eutectic Si due to the Na segregation at the interface between eutectic Si and eutectic Al, and (iii) Na addition promotes significant multiple Si twins, which can be attributed to the proposed adsorption of Na atoms at the intersection of Si twins and along the 〈112〉Si growth direction of Si. The present investigation demonstrates, for the first time, a direct observation on the distribution of Na atoms within eutectic Si and thereby provides strong experimental supports to the well-accepted impurity-induced twinning growth mechanism and poisoning of the twin plane re-entrant edge growth mechanism.

Solute adsorption and entrapment during eutectic Si growth in A–Si-based alloys

The solute adsorption and/or segregation as well as the solute entrapment of Sr, Na and Yb atoms during eutectic Si growth in a series of high-purity Al–5wt.% Si alloys was investigated by multi-scale microstructure characterization techniques, including high-resolution transmission electron microscopy and atomic-resolution scanning transmission electron microscopy. The adsorption of Sr atoms was directly observed along the 〈112〉Si growth direction of Si and/or at the intersection of multiple Si twins, which can be used to interpret the poisoning of the twin plane re-entrant edge and impurity induced twinning modification mechanisms, respectively. In contrast, Yb shows a different mechanism compared to the adsorption of Sr atoms. No significant Yb-rich cluster was observed at the intersection of Si twins. However, considerable Yb-rich segregation lines were observed along the 〈112〉Si direction, which can be attributed to the solute entrapment caused by a few Si twins through the natural twin plane re-entrant edge and growth mechanism. Active poisoning of the twin plane re-entrant edge and impurity induced twinning growth mechanisms cannot be observed due to the absence of Yb atoms within eutectic Si. Furthermore, the solute entrapment of modifying elements (X, Sr or Yb) together with Al and Si was proposed to interpret the formation of Al2Si2X phases or X-rich clusters within eutectic Si. Such types of Al2Si2X phases or X-rich clusters were further proposed to be an “artefact” caused by the solute entrapment during eutectic Si growth, rather than an active factor affecting the modification. The observed solute adsorption and entrapment can be used to interpret the different observations in the cases of different modifying elements, including impurity effects and so-called “quenching modification”, thereby elucidating the modification of eutectic Si in Al–Si alloys.