Substrate For Heterogeneous Nucleation Research Articles

Enhanced nucleation at Al(111)/Ti3AlC2(0001) interfaces: The role of doping in adhesion and interfacial stability

This study investigates the structure of the Al(111)/Ti3AlC2(0001) interface and elucidates the mechanism of heterogeneous nucleation of Ti3AlC2 particles within aluminum-based composite materials. A comprehensive examination of adhesion work, interfacial energy, and electronic structure is conducted for pristine and doped Al(111)/Ti3AlC2(0001) interfaces. The findings reveal that the C(TiC)-terminated hexagonal close-packed (HCP) stacking configuration at the Al(111)/Ti3AlC2(0001) interface exhibits the highest adhesion work and the lowest interfacial energy. This is attributed to the prominent covalent bonding between Al-3p and C-2p orbitals, indicating robust interfacial bonding strength and stability. Consequently, the results substantiate the potential of Ti3AlC2 particles as suitable substrates for heterogeneous nucleation of α-Al grains, effectively enhancing the strength and ductility of aluminum-based composites. Notably, doping the interface layer with specific elements significantly influences the adhesion work and interfacial energy. Introducing Ti and Mn at the interface substantially enhances adhesion work and concurrently reduces the interfacial energy of the C(TiC)-terminated HCP stacking Al(111)/Ti3AlC2(0001) interface, actively promoting nucleation of the aluminum matrix. However, incorporating Mg, Cu, and Zn at the interface decreases the adhesion work, unfavorable for the nucleation of Ti3AlC2 on the aluminum matrix. The binding energy at the doped Al(111)/Ti3AlC2(0001) interface follows the hierarchy: Mn > Ti > Mg > Cu > Zn. These findings provide valuable insights into the distinctive characteristics of Al(111)/Ti3AlC2(0001) interfaces and the underlying nucleation processes, holding substantial promise for the development of advanced aluminum-based composites.

Nature of Oxides in Al–Mg Alloys

AbstractActing as substrates for heterogeneous nucleation, native oxides in Al–Mg alloys have shown their potential for grain refinement. However, the limited knowledge about the nature of the oxides in Al–Mg alloys impedes the widespread application as native grain refiners. The aim of this work is to comprehensively investigate the native oxides in Al–Mg alloys through electron microscopy. Our results show that the predominant inclusions in Al–Mg alloys are oxides in three types of oxide films at the micrometer scales: young films, old films and oxide skins. All oxide films consist of discrete oxide particles of three types in nanometer scale depending on the Mg contents: γAl2O3 (< 0.4 wt.%), MgAl2O4 (0.08–3.5 wt.%) and MgO (> 2 wt.%). Specifically, MgAl2O4 particles have sizes ranging from a few tens to a few hundreds nanometer and possess an elementary shape of octahedron faceted by {111} planes. In Al–Mg alloys, the native oxides have a lognormal size distribution, with the average mean size fluctuating in accordance with the oxide configurations as Mg content varies. The agglomerating feature causes inhomogeneous sampling, and dual-peak lognormal curves are found for low-Mg-content alloys (0.08/0.4%), which could be eliminated by increasing the Mg content (2.0/3.5%) or by using the high-shear melt conditioning (HSMC) technology. Understanding the native oxides in Al–Mg alloys shall provide instructions on their application in grain refinement.

Effect of trace rare earth La on microstructure and properties of Al-7%Si-0.6%Fe alloy

<sec>Al-Si alloys have been widely used in electronic information, communication, and other fields because of their high specific strength, excellent castability and good thermal conductivity. In recent years, with the rapid development of 5G communication technology, electronic communication equipment is gradually developing towards high integration and lightweight. The power of related equipment is higher and higher, which puts forward higher requirements for thermal conductivity and mechanical properties of materials.</sec><sec>Si can improve the fluidity and strength of the Al-Si alloy, but a large amount of Si will aggravate the lattice distortion and increases amount of eutectic Si. This will reduce the plasticity of the alloy, increase the electron scattering and reduce the thermal conductivity. In order to improve the mechanical properties and thermal conductivity of Al-Si alloys, chemical inoculation is generally used. Sr is usually used as modifier and Al-B serves as grain refiner. However, the simultaneous addition of Sr and B into Al-Si alloy results in “poisoning” phenomenon, it becomes impossible to refine <i>α</i>-Al grains and modify eutectic Si simultaneously.</sec><sec>In recent years, rare earth La has attracted more and more attention in improving the properties of aluminum alloys. However, previous studies mainly focused on the effects of La addition, consequently, the research on the effects of combined addition of La, Sr, B on the microstructure and properties of Al-7%Si-0.6%Fe alloy is lacking. In this work, solidification experiments are performed to investigate the effects of combined addition of La, Sr, B on the microstructure and properties of Al-7%Si-0.6%Fe alloy. The results show that the addition of trace rare earth La can effectively eliminate the poisoning effect of Sr and B, and enhance the modification effect of eutectic Si. Besides, the addition of La can promote the formation of <i>α</i>-Al heterogeneous nucleation substrate LaB<sub>6</sub> and La can be used as a surfactant to reduce the undercooling of <i>α</i>-Al nucleation, thus it refines <i>α</i>-Al grains. The thermal conductivity of the alloy is significantly improved when the addition of La ranges from 0.02% to 0.06%; with the further increase of La addition, LaAlSi intermetallic compounds are formed in the alloy, leading the thermal conductivity of the alloy to decrease.</sec>

First-principles investigation on the tensile strength, interfacial stability and electronic structure of the heterogeneous nucleation Al/VB2 interfaces

First principles calculations based on the density functional theory were performed on the interfacial properties of Al(111)/VB2(001) and Al(100)/VB2(100) interfaces including interfacial stability, bonding features and tensile strength. Fourteen different interfaces with different VB2 terminations (V-terminated and B-terminated) were studied. The convergence test shows that 7-layered Al(111) and 9-layered Al(001), 9-layered VB2(001) and 13-layered VB2 (100) are thick enough to build Al/VB2 interface. The (001)-V-hcp and (100)-V-cen site interface have the largest work of adhesion (Wad) among the Al(111)/VB2(001) and Al(100)/VB2(100) interfaces. And under V-rich condition, there interfacial energies are − 0.44 J/m2 and − 0.45 J/m2 which are much lower than 0.15 J/m2, indicating it is an effective heterogeneous nucleation substrate for α-Al. The main bonding characteristics is metallic for the (001)-V-hcp interface, while it is covalent with metallic for the (100)-V-cen interface, making it has the largest Wad. The calculated tensile strength of (001)-V-hcp site interface is about 7.36Gpa when the strain is 12 %. While it is 6.27Gpa for the (100)-V-cen site interface at 10 %. Finally, the VB2 particles are proved to be an effective heterogeneous nucleation substrate for α-Al as observed from experiment.

Segregation of Yttrium at the Mg/MgO interface in an Mg-0.5Y Alloy

Interfacial segregation of selected elements can be exploited to manipulate the potency of solid substrates for heterogeneous nucleation, thus controlling the solidification process. As the native inclusions in Mg alloys, MgO acts as the nucleating substrate, but it has rarely been studied in terms of its interactions with alloying elements. In this work, investigations of yttrium (Y) segregation at interfaces between native MgO particles and Mg in an Mg-0.5Y alloy were carried out by state-of-the-art aberration-corrected scanning transmission electron microscopy (STEM) and associated spectroscopy. Experimental results show that native MgO particles in Mg-0.5Y possess two typical morphologies: truncated octahedron primarily faceted by {111}MgO and minorly by {100}MgO, and cubic shape with unique {100}MgO facets. Y atoms are found to segregate at both Mg/{111}MgO and Mg/{100}MgO interfaces, leading to the formation of two different 2-dimensional compounds (2DCs). The 2DC at the Mg/{111}MgO interface is identified as two atomic layers of a face-centered cubic Y2O3 phase in terms of crystal structure and chemistry, whilst it is an Mg(Y)-O monolayer at the Mg/{100}MgO interface, coherently matching with the terminating {100}MgO plane. Discussion is focused on the mechanisms underlying the formation of the 2DCs, their effects on the nucleation potency of MgO particles, and grain refinement. This work sheds light on how heterogeneous nucleation can be manipulated by altering the nucleation potency of a substrate through deliberately promoting elemental segregation of carefully chosen element(s).

Study on the Heterogeneous Nucleation Mechanism of SiCp/AZ91 Magnesium Matrix Composites under Pulse Current

SiCp/AZ91D magnesium matrix composites with 30% SiCp were successfully prepared by pulsed current melting in this work. Then, the influences of the pulse current on the microstructure, phase composition, and heterogeneous nucleation of the experimental materials were analyzed in detail. The results show that the grain size of both the solidification matrix structure and SiC reinforcement are refined by pulse current treatment, and the refining effect is gradually more obvious with an increase in the pulse current peak value. Moreover, the pulse current reduces the chemical potential of the reaction between SiCp and Mg matrix, thus promoting the reaction between SiCp and the alloy melt and stimulating the formation of Al4C3 along the grain boundaries. Furthermore, Al4C3 and MgO, as heterogeneous nucleation substrates, can induce heterogeneous nucleation and refine the solidification matrix structure. Finally, when increasing the peak value of the pulse current, the repulsive force between the particles increases while the agglomeration phenomenon is suppressed, which results in the dispersed distribution of SiC reinforcements.

High-Pressure Die Casting: A Review of Progress from the EPSRC Future LiME Hub

This article provides an overview of high-pressure die casting (HPDC)-related research undertaken at the EPSRC Future LiME Hub between 2015–2022. The project aimed to identify the cause of variability in the tensile ductility of die-cast structures, and to develop novel processing techniques to address this issue. Variability in tensile ductility was related to the size of large pores and non-metallic inclusions. It was proposed that these non-metallic inclusions formed during the pyrolysis of commercial plunger lubricants in the shot sleeve, and that these large pores derived from dilatational strains introduced during semi-solid deformation. Processing parameters and die design were found to significantly influence the microstructure of die-cast products, and the subsequent variability in tensile ductility. To close, recent progress on the application of intensive melt shearing to HPDC is reviewed. Intensive melt shearing was found to induce significant grain refinement in both Al and Mg alloys due to the effective dispersion of native oxide particles, and the use of these particles as heterogeneous nucleation substrates. The presence of native oxide particles also enabled the use of novel heat treatment procedures that avoided conventional issues such as surface blistering and geometrical distortion.

Microstructure evolution and enhanced mechanical properties in Al-Mn alloy reinforced by B-doped TiC particles

In this work, novel TiC particles doped by trace amounts of B, referred to as TiCB, were introduced to an Al-Mn alloy with the Al-TCB master alloy. The effects of the TiCB particles on the solidification microstructure, extrusion behavior, and mechanical properties of the Al-Mn alloy were systematically studied. The results showed that TiCB particles could significantly refine α-Al grains by providing heterogeneous nucleation substrates and hindering the growth of α-grains. For example, when the amount of added TiCB particles was 0.5 wt%, average α-Al grain size could be refined to a minimum of 40.2 μm from 410.1 μm when no particles were added. After hot extrusion, TiCB particles and fractured α-Al(Fe, Mn)Si formed streamlines along the extrusion direction. More importantly, TiCB particles could inhibit the dynamic recrystallization process during hot extrusion and retain numerous geometrically necessary dislocations. Consequently, the mechanical properties of the Al-Mn alloy at room temperature (25 °C) and elevated temperature (350 °C) were significantly improved after adding TiCB particles. When the amount of added TiCB particles was 1.5 wt%, the ultimate tensile strengths at 25 °C and 350 °C were increased by 25.6% and 32.4%. This work sheds light on the grain refinement and strengthening of non-heat-treatable wrought Al-Mn alloys.

E2EM's prediction of LaB6 as nucleation substrate for primary Mn-rich phase in Al-Si-Cu-Mn heat-resistant alloy and its refining effect

Edge-to-edge matching (E2EM) model was used to predict the potency of LaB6 as the heterogeneous nucleation substrate for primary Al13Mn4Si8 phase formed during the solidification of Al-Si-Cu-Mn heat-resistant alloy. There are five pairs of orientation relationships (ORs) between LaB6 and Al13Mn4Si8 phases which meet the criteria of E2EM model. One pair of plane ORs ((110)LaB6//(110)Al13Mn4Si8) are demonstrated by TEM observation. This strongly indicates that the LaB6 phase can act as the heterogeneous nucleation substrate for the primary Al13Mn4Si8 phase. 1.0 wt.% of Al-2La-1B master alloy was also added into Al-12Si-4Cu-2Mn alloy to evaluate the refining effect by microstructure observation and tensile test. Experimental results show that addition of Al-2La-1B master alloy can significantly refine the primary Al13Mn4Si8 phase, supporting the prediction accuracy of E2EM model. However, such refinement of primary Al13Mn4Si8 phase does not lead to an improvement in strength. This is due to the larger difference in elastic modulus between the finally formed Al13Mn4Si8 phase and aluminum matrix than that of Al15Mn3Si2 phase.

Process intensification of cooling crystallization of cholesterol from acetone solution using CO2 gas bubbles: Experiments and modeling

The present work investigates dependence of heterogeneous nucleation of solids on the gas bubble-solution interface in a process termed as "Precipitation by Gas-Bubbling" (PGB), through experiments and modeling. This process involves bubbling of CO2 gas at ambient condition through a solution of cholesterol in acetone, which is simultaneously cooled by an external coolant. The process parameters selected are coolant temperature, initial solution concentration, and CO2 flow rate. The flowing CO2 gas bubbles facilitate reduction in metastable zone width and induction time for cholesterol precipitation, rendering more efficient surface-mediated crystallization, thereby enabling process intensification of cooling crystallization. Faster precipitation of cholesterol at higher CO2 flow rate, i.e., higher CO2 bubble-solution interfacial area, demonstrates gas bubble-solution interface acting as a substrate for heterogeneous nucleation. Accordingly a mechanism for heterogeneous nucleation of cholesterol is proposed and validated by developing a model for prediction of final average particle size. The kinetic model parameters, namely, the pre-exponential factor as per the classical nucleation theory, and the crystal growth coefficient are regressed by comparing the predicted values of average particle size with the corresponding experimental data, and are found to be proportional to the surface area of gas bubbles and in turn on CO2 flow rate.

Refining effect of trace addition of B and the impurity elements in Al-Si-Cu-Mn-Ni heat-resistant alloy on primary Mn-rich phase

Refining effect of trace B addition in Al-Si-Cu-Mn-Ni alloy on primary Mn-rich phase was studied by OM, SEM, TEM and HRTEM. Addition of trace B (0.02 wt%) remarkably alters the morphology of primary Mn-rich phase from coarse dendritic to fine short-rod-like or blocky, and significantly reduces its size. TEM/HRTEM characterizations clarify that the fine particles embedded in the short-rod-like or blocky Al3MnSi2 phase are (V,Ti,Mn)B2 phase. One good matching orientation relationship between Al3MnSi2 phase and (V,Ti,Mn)B2 phase is confirmed by E2EM model calculation, indicating that (V,Ti,Mn)B2 phase has a strong potential as the heterogeneous nucleation substrate for Al3MnSi2 phase during solidification. So, trace addition of B and the impurity elements in Al-Si-Cu-Mn-Ni alloy leads to the formation of fine (V,Ti,Mn)B2 particles that greatly enhances the nucleation of primary Al3MnSi2 phase and thus significantly reduces its size.

Effects of Eu modification and heat treatment on microstructure and mechanical properties of hypereutectic Al–Mg2Si composites

To modify the coarse primary Mg2Si phase and improve mechanical properties of Mg2Si reinforced aluminum matrix composites, hypereutectic Al–18Mg2Si composites with different Eu contents were fabricated by permanent mold casting and extrusion followed by heat treatment. Results show that the as-cast Al–18Mg2Si composites consisted of primary Mg2Si and eutectic structure (Al + Mg2Si). Eu addition can obviously modify the primary Mg2Si morphology from dendrite to flat truncated octahedron shape, and the average size also decreased from ∼90 μm to ∼16 μm with increasing Eu content from 0 to 0.1%. But further increasing Eu content would cause over-modification. After T6 treatment, sharp corners of primary Mg2Si were spheroidized and the average size of primary Mg2Si were further decreased. DTA result shows the nucleation temperature of primary Mg2Si increased by Eu addition. Al2Si2Eu phase can act as the heterogeneous nucleation substrate for the primary Mg2Si phase and thus refine the size of primary Mg2Si particle. TEM results show the segregation and/or selective adsorption of Eu on the {100} facets of primary Mg2Si suppress the growth of primary Mg2Si, and thus refine and modify the composites, which proposed to the dominant reason for the refinement and modification of the primary Mg2Si phase. Tensile tests show that the mechanical properties enhanced with increasing Eu content. The elongation and ultimate tensile strength of the T6-treated Al–18Mg2Si-0.1Eu composite improved to 5.2% and 241 MPa, which enhanced 225% and 91% compared to the unmodified as-cast Al–18%Mg2Si composite. It is concluded that the morphology transition of primary Mg2Si from dendrite to tetrakaidecahedron and the refinement of primary Mg2Si size with Eu modification, the passivation of the sharp corners in primary Mg2Si particles as well as the appearance of β" precipitates all contribute to the improvement of the mechanical properties of the T6-treated Al–18Mg2Si composites.

Refining and modification effects of (Al, Zr, Si)–Al4Sr on Al–7Si–0.5 Mg alloy

The Al–9Zr-0.9Sr master alloy was used to refine and modify the Al–7Si-0.5 Mg alloy, and to improve the refining and modification effects, the Al–9Zr-0.9Sr master alloy was carried out melt spinning treatment. The second phase particles in the ribbon are smaller and more evenly distributed than those in the ingot. The Al–Zr–Sr inoculation showed excellent refining and modification effects on Al–7Si-0.5 Mg alloy, and in the range of 1 wt.%∼4 wt.%, the refining effect was gradually enhanced with the increase of the inoculant dosage. The morphology and size of particles in inoculant have significant effects on the refinement and modification effects of the inoculant. The X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and selected area electron diffraction (SAED) results showed that in Al–7Si-0.5 Mg matrix alloy, (Al, Zr, Si) instead of Al3Zr was used as the heterogeneous nucleation substrate of α-Al to refine the grains. The Sr element in the master alloy was mainly used to modify eutectic silicon, and melt spinning could effectively improve its modification effect on eutectic silicon. With the increase of inoculant dosage, the mechanical properties of the alloy were gradually improved. However, when the dosage was increased to 4 wt.%, some large rod-like particles appear due to the excessive Zr element, resulting in a decrease in elongation.

Solid solution strengthening mechanism in high pressure die casting Al–Ce–Mg alloys

A series of Al-xCe (x = 2, 4, 6, 8, 10 wt%) and Al–8Ce-yMg (y = 0, 0.10, 0.25, 0.50, 0.75 wt%) alloys were prepared by high-pressure die casting. The introduced cerium element promoted the nucleation of α-Al grains. Al11Ce3 phase served as the heterogeneous nucleation substrate of α-Al due to the small lattice mismatch of 6.72%. According to the results calculated by Nelson-Riley extrapolation function, the lattice constant of α-Al increased from 4.0511 Å to 4.0540 Å with the increasing of Mg content from 0 wt% to 0.75 wt%. Due to the solid solution strengthening effect of Mg atoms, the yield strength of Al–8Ce-yMg alloys and the hardness of α-Al matrix in the Al–8Ce-yMg alloys showed a parabolically increasing tendency, from 92 MPa to 115 MPa and 0.502 GPa–0.575 GPa, respectively. The work hardening capacity of Al–8Ce-yMg alloys was improved by the solid solution of Mg with the work hardening exponent increasing from 0.21 to 0.27. The solid solution of Mg atoms reduced the stacking fault energy of Al–8Ce-yMg alloys and suppressed the dynamic recovery process of the alloys during deformation process, which promoted the formation of dislocation tangles and dislocation networks in the α-Al matrix.

Atomic Ordering at the Liquid-Al/MgAl2O4 Interfaces from Ab Initio Molecular Dynamics Simulations

MgAl2O4 spinel particles exist inevitably in Al-Mg alloy melts and may act as potential substrates for heterogeneous nucleation of solid aluminum during solidification processing. In this paper, we investigated systematically the atomic ordering of liquid Al adjacent to liquid-Al/MgAl2O4{1 1 1} interfaces using an ab initio molecular dynamics simulation technique. Our simulations revealed that the interaction between the liquid metal and the spinel surface results in the formation of an ordered metal layer that terminates the substrate. This new terminating layer is positively charged, chemically bonded to the substrate, topologically rough, and structurally coupled with the metal sublayers beneath the outmost oxygen layer. The present results may shed new light on the role of spinel particles in Al-Mg alloys and on heterogeneous nucleation processes in general.

Grain refinement of Mg-Al alloys inoculated by MgAl2O4 powder

As a potent nucleating substrate for α-Mg grain, MgAl 2 O 4 powder was used to inoculate the Mg-Al melt in this study. The effects of MgAl 2 O 4 amount, holding time and Al content on the grain size and grain refining ratio of the inoculated Mg-Al alloys are systematically investigated. The results show that the minimum grain size of Mg-3Al alloy is achieved by adding 2 wt.% MgAl 2 O 4 powder and this alloy exhibits higher grain refining ratio than Mg-5Al and Mg-8Al alloys. The crystallographic misfit calculation indicates the well-matching and possible orientation relationships (ORs) between α-Mg and MgAl 2 O 4 . Among these predicted ORs, [10–10] α − Mg //[110] MgAl 2 O 4 in (0002) α − Mg //(1–13) MgAl 2 O 4 possesses the smallest misfit, i.e., 2.34% ( f r ). Both results of the experiment and crystallographic calculation demonstrate that the grain refinement of Mg-Al alloys is attributed to the MgAl 2 O 4 particles acting as the heterogeneous nucleation substrates for α-Mg grains.

Effects of CeO2 on the Si Precipitation Mechanism of SiCp/Al-Si Composite Prepared by Powder Metallurgy.

SiCp/Al-Si composites with different CeO2 contents were prepared by a powder metallurgy method. The effect of CeO2 content on the microstructure of the composites was studied. The mechanism of CeO2 on the precipitation of Si during sintering was analyzed by theoretical calculations. The results show that the appropriate amount of CeO2 can significantly refine the size of precipitated Si particles in the composite and increase the number of Si particles. With the increase of CeO2 content from 0 to 0.6 wt%, the number of Si particles precipitated in the composites increases gradually, and the average particle size of Si particles decreases gradually. When the CeO2 content is 0.6 wt%, the number of Si particles precipitated in the composites reaches the maximum, and the average particle size reaches the minimum. However, with the increase of CeO2 content from 0.6 wt% to 1.8 wt%, the number of Si particles precipitated in the composites began to decrease, and the average size of Si particles gradually increased. CeO2 can be used as heterogeneous nucleation substrate of precipitated Si, and the nucleation rate of precipitated Si on a CeO2 substrate is higher than that on an aluminum substrate. The proper addition of CeO2 can improve the nucleation efficiency of precipitated Si, thus increasing the amount and refining the size of precipitated Si.

Influence of Sm addition on microstructural and mechanical properties of as-extruded Mg–9Li–5Al alloy

In this study, the influence of Sm addition on microstructural evolution and mechanical properties of as-extruded Mg–9Li–5Al (LA95) alloy was characterized by OM, SEM, TEM, XRD and EDS. The morphology results showed that adding Sm resulted in the formation of Al2Sm phase, and the grains of the LA95 alloy were markedly refined with the addition of Sm. Additionally, the dislocation pile-up around the Al2Sm particles were observed by high-resolution transmission electron microscopy. The as-extruded LA95-0.4Sm alloy with granular Al2Sm phase showed optimal ultimate tensile strength of 283 MPa, which was about 25% higher than that of the matrix alloy. The main factors influencing the strength of as-extruded LA95-xSm alloys were as follows: (i) grain refinement strengthening, the lattice mismatch calculation results showed that the average lattice mismatch of (111)Al2Sm//(0001)α-Mg was 12.5%, which indicated that the Al2Sm phase served as an effective heterogeneous nucleation substrate of the α-Mg phase to refine the α-Mg phase in LA95 alloys; (ii) second phase strengthening, the fine Al2Sm particles exerted positive effects on strengthening by effectively hindering the dislocation line movement, thereby forming the dislocation pile-up adjacent to the strengthening phase Al2Sm.

Improvement of microstructure and tensile properties of Sn–Bi–Ag alloy by heterogeneous nucleation of β-Sn on Ag3Sn

Different weight fractions (0, 0.5, 1.0, 2.0, and 3.0 wt%) of sliver (Ag) were used to dope Sn–Bi eutectic alloys in this study. The microstructure and tensile and thermal properties of Sn–Bi eutectic alloys doped with Ag were investigated. The results indicate that the addition of Ag increases the solidification temperature and reduces the fusing latent heat of alloy, but it has no significant effect on the melting point of the alloy. According to the electron backscattered diffraction (EBSD) analysis and calculation of the disregistry between ε-Ag3Sn and β-Sn, ε-Ag3Sn can be used as the heterogeneous nucleation substrate of β-Sn, thus refining lamellar eutectic spacing. The orientation relationship between the ε-Ag3Sn phase and the β-Sn phase was as follows: [ 0 2‾1 ]Ag3Sn//[ 0 1 2 ]β-Sn, (0 1 2)Ag3Sn//(‾1 2‾1) β-Sn, and [‾1 2 0 ]Ag3Sn//[ 0 1 2 ]β-Sn, (2 1‾1)Ag3Sn//(‾1 2‾1)β-Sn. The tensile strength of the eutectic Sn–Bi alloy with addition of 1.0 wt% Ag reached 72 MPa, which was 20% higher than that of the original alloy. The analysis showed that Ag doping increase the tensile strength of the eutectic Sn–Bi alloy, because of fine grain strengthening and second phase strengthening mechanism

The Nature of Native MgO in Mg and Its Alloys

Native MgO particles in Mg-alloy melts have been recently exploited as potential substrates for heterogeneous nucleation during solidification, leading to significant grain refinement of various Mg-alloys. However, our current knowledge of the nature of the native MgO particles is still limited. In this work, we study both the physical and chemical nature of the native MgO in commercial purity Mg and Mg-9Al alloy by means of advanced electron microscopy. We found that as oxidation products MgO aggregates exist in the alloy melt in three different forms: dominantly young oxide film, occasionally old oxide film and ingot skin, all consisting of discrete nano-sized MgO particles. Detailed analysis shows that the native MgO particles have an octahedral or cubic morphology, a nano-scale particle size and a log-normal size distribution. The mechanisms underlying the formation of the two types of MgO were investigated, and we found that octahedral MgO is formed by oxidation of Mg melt and cubic MgO by oxidation of Mg vapor. With a large lattice misfit with α-Mg, the native MgO particles are impotent for heterogeneous nucleation, but can be made effective for grain refinement.