Twin Plane Re-entrant Edge Research Articles

Effect of Al-Sr-Y intermediate alloy on microstructure and mechanical properties of A356 alloy

In this experiment, an intermediate alloy (Al-3Sr-8Y) that can refine and modify A356 alloy simultaneously was developed, and the synergistic effect of Sr and Y on the microstructure and mechanical properties of A356 alloy was investigated. The results show that when the content of Al-3Sr-8Y intermediate alloy reaches 0.3 wt%, the morphology of α-Al dendrites is significantly refined and the secondary dendrite arm spacing (SDAS) was significantly reduced. Moreover, the morphology of eutectic Si transforms from acicular to small fibrous, and the average area and aspect ratio of eutectic Si decrease to 0.81 μm2, and 2.01. This change is caused by the twin plane re-entrant edge (TPRE) poisoning mechanism, where the addition of Al-3Sr-8Y can poison the intrinsic growth position of the Si phase, reduce the growth rate of the Si phase, promote isotropic growth, and achieve a highly branched morphology of the Si phase to form a fibrous structure. Due to the excellent synergistic effect of Sr and Y, the tensile strength and elongation of the alloy reached the maximum values of 303.5 MPa and 9.5% when 0.3 wt% Al-3Sr-8Y was added after heat treatment, which is an increase of 18.2% and 86.3% compared with the untreated A356 alloy, respectively.

A new synergy to overcome the strength-ductility trade-off dilemma in Al–Si–Cu alloy by micro-nano-particle complex clusters

The effects of the novel micro-nano-particle clusters (nano-AlN and submicron-TiC particles doped with B, referred as TiCB) on the microstructure and mechanical properties of the Al–9Si–3Cu alloy are investigated herein. Comprehensive analysis revealed that the nano-AlN particles resulted in a significant modification of eutectic Si by causing the poisoning of twin plane re-entrant edge growth of Si and restricting the growth of Si crystals. Correspondingly, the morphology of Si transformed from flaky to fibrous. Furthermore, the synergy of areas with and without particle clusters was emphasized to address their contributions to the simultaneous improvement of the strength and ductility. When inoculating the micro-nano-particle clusters into the Al–9Si–3Cu alloy, the yield strength, ultimate tensile strength, and elongation were 138 MPa, 253 MPa, and 5.8%, respectively, corresponding to increases of 23.2%, 23.4%, and 123.1% compared with those of the uninoculated Al–9Si–3Cu base alloy (112 MPa, 205 MPa, and 2.6%, respectively). After T6 heat treatment, the spheroidal Si and θ′ phases on the Al matrix further improved the strength of the Al–9Si–3Cu alloy. This work may provide a promising strategy to design and fabricate Al–Si–Cu alloy with high strength and ductility.

Modification mechanism and tensile property of Al-9Si-0.4Mg-0.1Cu alloy

In this work, eutectic Si modification grade formula was established and effect of Sr element on modification level and mechanical properties of Al-9Si-0.4Mg-0.1Cu alloy was studied. Thermal analysis, SEM, EPMA and TEM were applied to analyze cooling curves, microstructure and Si twinning of the alloys. The results showed that precision and accuracy of eutectic Si modification grade formula were significantly improved and quantitative evaluation modification level of eutectic Si could be achieved. Modification level of eutectic Si increased first and then decreased with Sr content increasing. Though twin plane re-entrant edge (TPRE) mechanism and impurity induced twinning (IIT) mechanism were effective under certain conditions, they cannot fully explain the modification mechanism of eutectic Si in Al-9Si-0.4Mg-0.1Cu alloy. Sr element caused constitutional undercooling at the front of solid-liquid interface, promoted the formation of fine Al 2 Si 2 Sr phases and nano-precipitates, resulting in the changes of size and morphology of eutectic Si. With the improvement of eutectic Si modification level, the UTS and El of the alloys increased. While, adding excessive Sr led to the formation of coarse Al 2 Si 2 Sr, which was detrimental to the improvement of modification level and mechanical properties of alloy. • Modification level function suitable for quantitatively evaluate modification grade of eutectic Si was established. • Fine Al 2 Si 2 Sr phases and nano-precipitates change the size and morphology of eutectic Si. • Coarse Al 2 Si 2 Sr reduces the modification level of eutectic Si and the mechanical properties of alloy.

Singly-twinned growth of Si crystals upon chemical modification

During crystallization a continuum of patterns could emerge due to the interplay of growth kinetics, material or solution chemistry, and crystallographic defects. The coherent twin boundary is widely known to catalyze growth in pristine crystals including polycrystalline Si. Much remains unknown about the impact of changing the chemical environment of the crystallization process through the deliberate addition of trace metallic species---termed chemical modification. Pristine Si has been reported to grow through the classical model of two parallel twin planes acting in concert to enable steady-state propagation of the solid-liquid interfaces. Here, we achieve a vision on the growth process via in situ synchrotron x-ray microtomography and further corroborated by ex situ crystallographic investigation. We find that steady-state growth is impossible in chemically modified alloys that consist of trace (0.1 wt.%) Sr. This is because the Sr modifier poisons the concave re-entrant grooves, thereby deactivating the advantage of the twin-plane re-entrant edge mechanism and leading to a singly-twinned interface. This study may serve as a proxy to chemically modified crystallization pathways of eutectic Si in Al-Si alloys and, more broadly, as a framework for the crystallization-mediated synthesis of materials.

In-situ X-radiographic study of nucleation and growth behaviour of primary silicon particles during solidification of a hypereutectic Al-Si alloy

Abstract Real-time observation of structure evolution in a hypereutectic Al-Si(-Cu) alloy under near-isothermal melt solidification condition was conducted by using in-situ micro-focus X-radiography. The nucleation, growth rate and morphological development of primary Si particles (PSPs) were studied at different cooling rates. It is found that an increase of cooling rate increases nucleation rate, reduces the crystal growth and extends the nucleation temperature range of primary Si particles, which all help to refine primary Si particles. The minimum nucleation undercooling also increases with increasing cooling rate. Based on the experimental findings, it is proposed that Si atom clusters act as the nuclei of Si crystals. In addition, cooling rate does not alter the branching growth mechanism of PSPs but only shorten the branch length and reduce the development of side plates. Higher cooling rate may increase the peak growth velocity of PSPs due to the higher nucleation undercooling. Further, electron backscatter diffraction (EBSD) characterization of the post solidified in-situ sample confirms that twinning in primary Si occurs in the nucleation stage or during the growth process, and the growth of twined branches and plates follows the twin plane re-entrant edge (TPRE) mechanism. Twinning during growth may facilitate the branching.

Microcharacterization and mechanical performance of an Al-50Si alloy prepared using the sub-rapid solidification technique

In this reported work, the hypereutectic Al-50Si alloys with a wide mushy zone of 465 °C were successfully fabricated using twin roll casting technique. The experimental results showed that the overall hardness of the roll-cast alloy increased, as a result of the decrease in the size of Si crystal and the increase in the relative solubility of Si in the α-Al. However, the EBSD results showed that the primary Si crystals in the twin-roll cast alloy grew according to twin plane re-entrant edge (TPRE) mechanism.

Modification of the silicon phase and mechanical properties in Al-40Zn-6Si alloy with Eu addition

The influence of Eu additions upon the silicon phase and mechanical properties of Al-40Zn-6Si alloys have been investigated via scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The primary Si was discovered to be remarkably refined and modified to nodular with 0.5% Eu addition. Meanwhile, the plate-like eutectic Si was modified to a fibrous structure. The effect of Eu on nodular primary Si was clarified from five aspects: (a) the adsorption of single Eu atomic column at the twin boundary, inducing stacking faults and parallel twins, (b) the adsorption of single Eu atomic column and triangle-like Eu-rich clusters at the junction of Si twins, inducing multiple twins, (c) the adsorption of Eu linear distribution along the twin boundary, inducing poisoning of twin plane re-entrant edge (TPRE), (d) the adsorption of Eu linear distribution at special grain boundary ∑9 twin, reducing the lattice distortion and grain boundary energy, (e) the solute entrainment within Si. These results prove that the widely accepted impurity induced twinning (IIT) and poisoning of TPRE mechanisms are valid for nodular primary Si. By addition of 0.5%Eu, the ultimate tensile strength and elongation were enhanced by 31% and 358% respectively.

Comparative study on the microstructures and hardness of the AlSi10.6CuMg alloy produced by sand casting and high pressure die casting

ABSTRACT This study compares the microstructures and hardness of an AlSi10.6CuMg alloy obtained by sand casting and high pressure die casting (HPDC). The results show that the morphology of HPDC dendrites was not uniformly distributed as in sand casting. HPDC refines the morphology of intermetallics and produces a modification of eutectic silicon. Chinese-script is the dominant iron intermetallic in sand casting, while primary polyhedral and secondary proeutectic particles are dominant in HPDC. The results of the X-ray diffraction show the presence of high residual stresses in the HPDC. Deep etching for HPDC samples reveals θ-Al2Cu phases in very thin compact dendrites. No evidence of twin plane re-entrant edge assistance was found in the growth of primary silicon. The primary silicon plate-like becomes unstable in HPDC, resulting in smooth and rounded outer surfaces. Hardness of HPDC alloy was about 11% higher than that of sand casting alloy.

Review on the Mechanisms of Modification Al-Si alloys

The present paper investigates some mechanisms of modification the eutectic of aluminum-Silicon (Al-Si) alloys. These alloys have been many applications in general industries especially in military, aerospace and automobiles. These alloys in some cases need to modify their properties In particular the wear resistance. This can be overcome by modification the morphology of Si-eutectic. Many mechanisms were suggested in this field. The most common mechanisms are chemical modification and quenching modification. The chemical modification achieved by adding some rare earth elements like sodium (Na), strontium (Sr), and antimony (Sb). Also can be refined the structure of these alloys by using master alloys such as Al-Ti, Al-Ti-B and Al-B. According to restricted growth theory, the impurity induced twinning which reduce the growth. According to the restricted nucleation, the twin plane re-entrant edge (TPRE) Poisoning, which stopped the twin plane. The addition of sodium (Na) in the range of 0.005 to 0.01% modifies the eutectic Si but it has high vapourity and it is difficult to determine its resulting level. Strontium is added in a range of 0.02 to 0.04% but it is ability to oxidation is high as contact with atmosphere.

Synchrotron radiation micro-beam analysis of the effect of strontium on primary silicon in Zn–27Al–3Si alloy

The influence of Sr on two- and three-dimensional morphology of primary Si in Zn–27Al–3Si alloy was investigated. Distribution and crystalline structure of Sr were analyzed by using synchrotron radiation micro-beam. The results show that Sr could modify polygonal primary Si to nearly standard spherical shape. Modified primary Si enriched with Sr, which partially evolves as Al2Si2Sr, while partially presents as independent atoms. Sr promotes high-density paralleled and intersected {111}Si twins during the growth of primary Si. These observations thus provide experiment evidence to support the modification mechanisms based on impurity-induced twinning and poisoning of twin plane reentrant edge.

Effect of Eu on the silicon phase in Al-40Zn-5Si alloys

A series of Al-40Zn-5Si alloys with different additions of Eu were prepared by permanent mold casting. The nucleation phenomenon incorporating the modification of silicon phase was investigated by using the synchrotron radiation imaging technology, scanning electron microscopy, electron probe microanalysis and high resolution high-angle annular dark-field scanning transmission electron microscopy. It was found that the primary phase was α-Al dendrites and the pre-eutectic Si particles were precipitated continuously at the front of the solid/liquid interface in Al-40Zn-5Si alloys. The nodular pre-eutectic Si particles were observed when the Eu addition was 0.3%. Meanwhile, a flake-to-fibrous transition of eutectic Si was also observed. The needle-like eutectic Si crystals were seen to shoot out from the vicinity of primary α-Al dendrites into melt with high velocities leading the irregular eutectic interface in the unmodified Al-40Zn-5Si alloy. However, the eutectic grains with smooth interfaces nucleated mainly near primary or secondary α-Al branches in 0.3% Eu-modified Al-40Zn-5Si alloy. The eutectic grains began to nucleate on the nodular pre-eutectic Si particles when the α-Al surface reached so close to them. The formation of nodular pre-eutectic Si particles was attributed to the adsorption of Eu atoms along the <112> growth direction of Si and at the intersection of Si twins. This is fully consistent with the well-known poisoning of twin plane re-entrant edge (TPRE) and the impurity induced twinning (IIT) modification mechanisms proposed for interpreting the modification of eutectic Si.

Crystallization kinetics in Si-1 at%Sn during rapid solidification in undercooled melt

In order to elucidate the cause of the morphological transition of crystals growing in an undercooled melt of semiconducting materials, we carried out the containerless solidification of undoped Si and Si-1at%Sn using a CO2 laser-equipped electromagnetic levitator (EML). The crystallization of these materials was successfully achieved under controlled undercooling. The relation between the shape of growing crystals and the degree of undercooling in Si-1at%Sn was similar to that in undoped Si; that is, plate-like needle crystals were observed at low undercooling, whereas at medium and high undercooling the shape of growing crystals changed to massive dendrites. The grain-size of as-solidified samples of Si-1at%Sn was remarkably small compared with that of undoped Si. The surface morphologies of samples solidified by dropping the melt onto a chill plate of mirror-polished silicon consisted of typical twin-related <110> dendrites. On the other hand, samples that were dropped from the undercooled state consisted of twin-free <100> dendrites. The nucleation rate of two-dimensional nuclei calculated on the basis of two mechanisms, which are the twin-plane re-entrant edge mechanism and the twin-free mechanism, suggested that the morphological transition to twin-free <100> dendrites from twin-related <110> dendrites occurs when the degree of undercooling becomes larger than the critical value. These results indicate that the cause of the morphological transition of Si growing in the undercooled melt is not the roughening transition of the crystal–melt interface but the transition of the nucleation kinetics to the twin-free mechanism from the twin-related mechanism.

Phase-field modeling of twin-related faceted dendrite growth of silicon

We investigated the growth of a twin-related silicon dendrite through a novel phase-field model. The correctness of the model for an equilibrium twined crystal was examined first before we modeled the faceted dendrite growth. The simulated morphologies of 〈112〉 and 〈110〉 faceted dendrites were consistent with experimental observations. The growth orientation of the simulated dendrite depended on the growth rates at the ridges and the re-entrant corners. We further extended the Twin-Plane-Reentrant-Edge (TPRE) mechanism and then proposed a new growth model for faceted dendrites based on our simulations. The undercooling and twin spacing could affect the growth rates at the ridges and the re-entrant corners, which were also explained by the proposed model.

Effects of trace elements (Y and Ca) on the eutectic Ge in Al–Ge based alloys

Effects of trace elements (0.2Y and 0.2Ca (wt%) on the eutectic Ge in high purity Al–20Ge (wt%) alloys were investigated by multi-scale microstructure characterization techniques. Particularly, the distribution of trace elements (Y and Ca) within the eutectic Ge and/or at the interface between eutectic Ge and eutectic Al was investigated by atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging and atom probe tomography (APT). The combined investigations indicate Al–Y and Al–Ca co-segregations. Such co-segregations change significant morphology and growth of the eutectic Ge. In addition, large Al2Ge2Y and Al2Ge2Ca phases were also measured. The modification of eutectic Ge is discussed in terms of previously postulated modification mechanisms: twin plane re-entrant edge growth mechanism, impurity-induced twinning, and growth restriction of eutectic Ge.

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.

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.

Modification of eutectic Si in Al–Si alloys with Eu addition

Al–5wt.% Si-based alloys with 0.05wt.% Eu addition were produced by controlled sand-casting and melt-spinning, respectively. The modification of eutectic Si caused by 0.05wt.% Eu addition was investigated by thermal analysis, differential scanning calorimetry and multi-scale microstructure characterization techniques. In the case of controlled sand-casting, 0.05wt.% Eu addition was found to modify the eutectic Si into a fibrous morphology. Multiply twinned Si particles were observed within eutectic Si. Furthermore, the Al2Si2Eu phase was also observed both in the vicinity of eutectic Si and within eutectic Si, which was believed to hinder Si growth. In the case of melt-spun samples, nanometer-sized Al2Si2Eu phases were observed. However, after controlled cooling in a differential scanning calorimeter, multiply twinned Si particles were observed. The formation of multiply twinned Si particles was attributed to the adsorption of Eu atoms along the 〈112〉Si growth direction of Si and at the intersection of two {111}Si facets. Twinning was observed in both fundamentally different casting processes: controlled sand-casting and melt-spinning. This is fully consistent with the well-known poisoning of the twin plane re-entrant edge and the impurity-induced twinning modification mechanisms.

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.

Revisiting the Twin Plane Re-entrant Edge Growth Mechanism at an Atomic Scale by Electron Microscopy

We conducted an extensive electron microscopy study on surface and defect structures of boron suboxide/suboxycarbide platelets by examining them under various imaging conditions, e.g., side-view and top-view perspectives. It was determined that a twin plane re-entrant edge mechanism was responsible for the growth process at an atomic scale. Moreover, this thorough investigation provided an opportunity to resolve several critical issues regarding this otherwise well-known growth mechanism for metallic nanostructures. In this study, the platelets contained multiple {001} twin lamellae parallel to basal planes and their side faces were mainly enclosed by {101} facets. Vertical growth was heterogeneously nucleated of {001}-type growth twins that were confined at the corners. In lateral growth, nucleation sites were greatly extended to twin re-entrant edges around all side faces. No secondary growth twins were introduced during lateral growth because {101}-type side faces were not twin planes. This work clearly establishes that surface structures of twinned platelets determine nucleation behaviors on basal/side faces and thus control the final morphology, which is relevant to the shape control of metal nanoplates.