Solidification Of Mg Research Articles

Interactions of CaO with pure Mg and Mg-Ca alloys—an in situ synchrotron radiation diffraction study

CaO additions are used as an inexpensive replacement for Ca in Mg alloys. CaO dissociation in Mg has been reported in literature without a clear mechanism as to why this occurs. In situ synchrotron radiation diffraction investigation of the melting and solidification of Mg with CaO shows, that the stability of CaO was overestimated in Mg melts compared with MgO. The experiments that were performed on the Mg-20CaO and Mg-xCa-6CaO (x = 6 and 16 wt.%) alloys, show the dissociation and formation of various phases during melting and solidification. The results indicate that Mg can reduce CaO even in the solid state, which is the opposite of that proposed by the Ellingham diagrams for stoichiometric reaction. Phase formations during the in situ experiment are compared with published thermodynamic calculations for the interaction between Mg-Ca alloys and oxides.

Al–Mn–Fe intermetallic formation in AZ91 magnesium alloys: Effects of impurity iron

The influence of iron on the formation of Al–Mn–Fe intermetallic compounds (IMCs) has been investigated in the solidification of Mg–9Al-0.7Zn-0.2Mn (wt.%, AZ91) with iron contents ranging from ∼0.001 to > 0.01 wt.% Fe. Four Al–Mn–Fe IMCs formed depending on the Fe-content and location in the crucible: B2–Al(Fe,Mn), Al8Mn5, Al11Mn4 and, at the bottom of crucibles, Al5Fe2. The four IMCs nucleated and grew on one another, producing multiphase particles. These usually contained numerous orientations that were all interrelated through simple orientation relationships that are discussed in terms of the similarities between the IMC crystal structures. The iron content affected the IMC phase fractions and the multiphase particle morphology. At low iron content, the Fe-rich B2 phase was encapsulated by a low-Fe Al8Mn5 shell. With increasing iron content, the Fe-rich phases (B2 and Al5Fe2) gradually became in direct contact with the α-Mg. The threshold Fe:Mn content for adequate corrosion performance is found to correlate approximately to where B2–Al(Fe,Mn) first becomes exposed to the α-Mg matrix.

A comparative study of the role of solute, potent particles and ultrasonic treatment during solidification of pure Mg, Mg–Zn and Mg–Zr alloys

Ultrasonic treatment (UST) applied during the solidification of pure Mg, eutectic (Mg – Zn) and peritectic (Mg – Zr) alloys was investigated in order to explore the grain refinement mechanisms. Temperature dependent grain refinement is observed in pure Mg where decreasing the superheat temperature (at which UST is applied from above the melting temperature, TM) from 100 °C to 40 °C produces significant refinement with a uniform grain structure. The presence of solute reduces the temperature dependence of the UST refinement and excellent grain refinement is obtained regardless of the superheat temperature (100 °C or 40 °C) and even with the use of preheated sonotrode in the Mg – 6 wt.% Zn alloy. A further improvement in grain refinement is achieved when the alloy contains potent particles that introduce additional nucleation of grains in Mg – 0.5 and 1.0 wt.% Zr alloys (producing an average grain size of ≤100 µm). At 40 °C superheat, UST of Mg – Zn alloys produces excellent refinement (average grain size <200 µm) with non-dendritic grains, which is normally achieved only with the addition of grain refining master alloy in the as-cast condition. The enhanced refinement observed in the eutectic alloy is explained through the undercooling imposed by a relatively cold sonotrode combined with high frequency vibrations and acoustic streaming. The advantages of using a cold sonotrode, a low superheat and solute are demonstrated for achieving significant refinement during solidification of Mg alloys under UST without or with a lower addition of grain refining master alloys.

Solidification of Mg–Zn–Zr Alloys: Grain Growth Restriction, Dendrite Coherency and Grain Size

The solidification characterization of Mg–xZn–0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys has been extensively investigated through thermal analysis, microstructure characterization and thermodynamic calculations. The impact of Zn content on the grain growth restriction, dendrite coherency and thus the final grain size has been investigated and discussed. Increasing Zn content, the grain size of Mg–xZn–0.5Zr alloy was firstly refined and then coarsened with the finest grain size of ~ 50 μm for the Mg–3Zn–0.5Zr (ZK31) alloy. Significant effects of the grain size on the mechanical properties were observed in the investigated alloys. The combination of growth restriction factor theory and dendrite coherency point provides a reasonable explanation of the grain size results. It helps to further understand the mechanisms of grain refinement and grain coarsening related to solute content, providing reference for alloy design and grain size prediction.

Coupling in situ synchrotron X-ray radiography and phase-field simulation to study the effect of low cooling rates on dendrite morphology during directional solidification in Mg–Gd alloys

The effect of low cooling rate with a fixed temperature gradient during directional solidification of Mg–Gd alloys is investigated by coupling in situ X-ray synchrotron radiography and phase-field method. Results show that the primary dendrite spacing decreases with the increase of cooling rates, and the relationship between the logarithm of primary dendrite arm spacing and the logarithm of cooling rate is linear. After fitting, the slope of primary dendrite arm spacing is −0.71, while that of phase field simulation is −0.74, which indicates the experimental statistics are basically consistent with that of phase field simulation. The simulation thoroughly studies the kinetic transition process of seaweed-like morphology and the evolution of solute distribution for dendrites, both the solid volume fraction and the velocity of liquid-solid interface increase with the increase of cooling rate. The simulation is in good agreement with experimental results.

The Mg–Ca–O system: Thermodynamic analysis of oxide data and melting/solidification of Mg alloys with added CaO

AbstractSome published experimental results on the thermodynamic data of pure solid CaO and MgO significantly contradict the widely used data in review books or tables, such as JANAF data. We scrutinized all the original experimental data forming the basis in review books and tables in order to establish a more realistic uncertainty range. The two Gibbs energy functions of CaO and MgO are combined with those of all other relevant phases to develop a thermodynamic database for the Mg–Ca–O system. The first systematic study of the calculated Mg–Ca–O phase diagram is presented. This phase diagram is validated by recently obtained dedicated experimental data using in-situ synchrotron radiation diffraction to study phase formation and reactions during melting and solidification of Mg and Mg–Ca alloys with CaO-additions below 1050 K. Intricacies in the solidification path of Mg-rich ternary alloys are revealed by the present thermodynamic calculations. Mg-rich alloys are categorized in four groups according to their participation in different invariant four-phase reactions. The development of non-equilibrium constitution of four phases, (Mg) + Mg2Ca + MgO + CaO, in the fully solidified sample is explained.

Effect of mechanical vibrations on grain refinement of AZ91 Mg alloy

AZ91 Mg alloys are extensively being consumed in the automotive, aerospace, electrical and communication industries. The large freezing range of this alloy allows the formation of coarse-grained structures. To overcome this problem magnesium alloys are, therefore, treated for grain refinement. In this study, mechanical vibrations in the frequency range from 0 to 25 Hz at a constant amplitude of 2 mm were imposed during solidification of Mg–9Al–1Zn alloy. Optimum results were obtained at 15 Hz. It was observed that mechanical vibration during solidification had a good refining effect on the grain morphology of the alloy. The alloy was characterized by optical metallography, scanning electron microscopy and mechanical properties such as hardness and tensile strength. It was observed that, by applying vibrations to the melt, a uniform distribution of the fine grains of the primary α-Mg phase was achieved and the properties were enhanced.

Fast synchrotron X-ray tomographic quantification of dendrite evolution during the solidification of Mg[sbnd]Sn alloys

The evolution of dendritic microstructures during the solidification of a Mg-15 wt%Sn alloy was investigated in situ via fast synchrotron X-ray microtomography. To enable these in situ observations a novel encapsulation method was developed and integrated into a fast, pink beam, imaging beamline at Diamond Light Source. The dendritic growth was quantified with time using: solid volume fraction, tip velocity, interface specific surface area, and surface curvature. The influence of cooling rate upon these quantities and primary phase nucleation was investigated. The primary dendrites grew with an 18-branch, 6-fold symmetry structure, accompanied by coarsening. The coarsening process was assessed by the specific surface area and was compared with the existing models. These results provide the first quantification of dendritic growth during the solidification of Mg alloys, confirming existing analytic models and providing experimental data to inform and validate more complex numeric models.

Microstructure and room temperature mechanical properties of directionally solidified Mg–2.35Gd magnesium alloy

Directional solidification of Mg–2.35Gd (mass fraction, %) magnesium alloy was carried out to investigate the effects of the solidification parameters (growth rate v and temperature gradient G) on microstructure and room temperature mechanical properties under the controlled solidification conditions. The specimens were solidified under steady state conditions with different temperature gradients (G=20, 25 and 30 K/mm) in a wide range of growth rates (v=10–200 μm/s) by using a Bridgman-type directional solidification furnace with liquid metal cooling (LMC) technology. The cellular microstructures are observed. The cellular spacing λ decreases with increasing v for constant G or with increasing G for constant v. By using a linear regression analysis the relationships can be expressed as λ=136.216v−0.2440 (G=30 K/mm) and λ=626.5630G−0.5625 (v=10 μm/s), which are in a good agreement with Trivedi model. An improved tensile strength and a corresponding decreased elongation are achieved in the directionally solidified experimental alloy with increasing growth rate and tempertaure gradient. Furthermore, the directionally solidified experimental alloy exhibits higher room temperature tensile strength than the non-directionally solidified alloy.

The anisotropy of hexagonal close-packed and liquid interface free energy using molecular dynamics simulations based on modified embedded-atom method

This work aims to comprehensively study the anisotropy of the hexagonal close-packed (HCP)-liquid interface free energy using molecular dynamics (MD) simulations based on the modified-embedded atom method (MEAM). As a case study, all the simulations are performed for Magnesium (Mg). The solid–liquid coexisting approach is used to accurately calculate the melting point and melting properties. Then, the capillary fluctuation method (CFM) is used to determine the HCP-liquid interface free energy (γ) and anisotropy parameters. In CFM, a continuous order parameter is employed to accurately locate the HCP-liquid interface location, and the HCP symmetry-adapted spherical harmonics are used to expand γ in terms of its anisotropy parameters (ε20, ε40, ε60 and ε66). Eight slip and twinning planes (basal, two prismatic, two pyramidal, and three twinning planes) are considered as the HCP-liquid interface planes. An average HCP-liquid interface free energy of 122.2 (mJ/m2), non-zero ε20, ε40, and ε66 parameters, and approximately zero ε60 parameter for Mg are predicted. Using these findings, the first preferred dendrite growth direction in solidification of Mg is predicted as [112¯0], which is in agreement with experiments. Also, a second preferred dendrite growth direction for Mg is predicted as [336¯2].

On the stable eutectic solidification of iron–carbon–silicon alloys

Extensive effort was expanded to elucidate the growth and morphology of the stable eutectic grains during early solidification of continuous cooled Fe–C–Si alloys. To this purpose, quenching experiments at successive stages during solidification have been carried out on five cast irons with various magnesium and titanium levels designed to produce graphite morphologies ranging from lamellar to mixed compacted–spheroidal.The graphite shape factors were measured on the metallographic samples, and their evolution as a function of the chemical composition and the solid fraction was analyzed. Extensive scanning electron microscopy was carried on to evaluate the change in graphite shape during early solidification, to establish the fraction of solid at which the transition from spheroidal-to-compacted-to-lamellar graphite occurs, and to outline the early morphology of the eutectic grains. It was confirmed that solidification of Mg containing irons started with the development of spheroidal graphite even at Mg levels as low as 0.013 mass%. Then, as solidification proceeds, when some spheroids developed one or more tails (tadpole graphite), the spheroidal-to-compacted graphite transition occurs.The new findings were then integrated in previous knowledge to produce an understanding of the eutectic solidification of these materials. It was concluded that in hypoeutectic lamellar graphite iron austenite/graphite eutectic grains can nucleate at the austenite/liquid interface or in the bulk of the liquid, depending on the sulfur content and on the cooling rate. When graphite nucleation occurs on the primary austenite, several eutectic grains can nucleate and grow on the same dendrite. The primary austenite continues growing as eutectic austenite and therefore the two have the same crystallographic orientation. Thus, a final austenite grain may include several eutectic grains. In eutectic irons the eutectic grains nucleate and grow mostly in the liquid. The eutectic austenite has different crystallographic orientation than the primary one.The solidification of the austenite/spheroidal graphite eutectic is divorced, with graphite spheroids growing on primary austenite dendrites. The eutectic austenite grows on the primary austenite and has the same crystallographic orientation. The result is large austenite (primary and eutectic) dendrites that incorporate numerous graphite spheroids. A eutectic grain cannot be defined.

Study on Dendritic Growth in Pressurized Solidification of Mg–Al Alloy Using Phase Field Simulation

A multi-grain phase field model coupled with thermodynamic calculation was adopted to describe the dendritic growth in pressurized solidification of Mg–Al alloy during squeeze casting, in which the effects of the pressure on the Gibbs free energy and chemical potential of solid and liquid phases, the solute diffusion coefficient, and the solute partition coefficient were considered. The individual effect of solute diffusion coefficient, and the Gibbs free energy on the dendritic growth was studied. With the comparison of the dendritic growth under atmospheric and elevated pressures, the effect of pressure on the microstructure evolution was discussed. The results showed that the grains are refined, the dendritic growth rate tends to increase and the secondary dendrite arms are more developed as the pressure is increased from 0.1 to 100 MPa, which showed a good agreement with the experimental results of direct squeeze casting of Mg–Al alloy. As the pressure increases, the largest dendritic growth rate can be obtained under the pressure between 200 and 250 MPa, while the growth rate decreases with a further increase of pressure.

Formation and Magic Number Characteristic of Clusters during Rapid Solidification of Mg&lt;sub&gt;2&lt;/sub&gt;Ca

A molecular dynamics simulation study has been performed for a system consisting of 15,000 atoms to investigate the formation and magic number characteristics of various clusters formed during the rapid solidification. Results indicate that the icosahedral cluster (12 0 12 0) plays key role in the glass transition. The size distribution of clusters in the system showing magic number characteristics, and the magic number sequence in the Mg2Ca system is 13, 19, 25, 36, 37, ....This magic number sequence is quite similar with that of the system of metal Al.

Characterization of late-stage equiaxed solidification of alloys

We use a two-dimensional phase-field model coupled to a nucleation mechanism to study the evolution of interdendritic liquid pools during late-stage solidification of Mg–Al alloys under spatially uniform temperature and constant cooling rates. We obtain the channel size distribution (CSD) of liquid pools at solid fractions close to those where eutectic phase is expected to form and investigate the influence of cooling rate on the morphology of the CSD at different solidification stages. Our results show that the CSD is unimodal, exhibiting a peak at small channel widths followed by a shoulder and longer decay tail at large channel widths. This feature is correlated to the presence of two distinct liquid regions, small channels between secondary branches of the primary phase and larger channels between adjacent grains. We construct a cooling-rate/solid-fraction morphology diagram that shows the relative importance of the shoulder in the CSD. We characterize the mean and standard deviation of the CSD and show that, within the range of data examined, the mean channel size vs. cooling rate curves scale with solid fraction. The numerical tools developed for this work can also be used to analyze experimental results. We include the analysis of two experimental micrographs previously published by Paliwal et al.

Grain refinement of as cast Mg–Mn alloy by simultaneous addition of trace Er and Al

A novel method for the grain refinement of as cast Mg–1·8Mn alloy by simultaneous addition of trace Er and Al was developed. A series of Mg–Mn alloys with systematic additions of Er alone (Er) or simultaneously with Al (Er/Al) were designed in order to illustrate the effects of Er/Al on the microstructure of the Mg–Mn alloy. The results showed that significant grain refinement was obtained by simultaneous addition of Er/Al. The grain size decreased remarkably from over 800 μm in the unrefined alloy to 150 μm with the addition of 0·4 wt-%Er/Al. Adding Er alone to the Mg–Mn alloy did not evidently influence the grain size. The Al3Er phase was identified to be the dominant phase in the Er and Al containing alloys, besides elemental Mn particles. Based on the calculation of the planar disregistry between Al3Er and α-Mg, Al3Er particles could act as efficient heterogeneous nuclei of primary α-Mg during the solidification of Mg–Mn alloy.

Nanosize icosahedral quasicrystal in Mg90Ca10 glass: An ab initio molecular dynamics study

Rapid solidification of Mg(90)Ca(10) from its liquid state is studied by means of an ab initio molecular dynamics technique, and its local structure is investigated by various analyzing methods. The liquid and amorphous states are found to have slightly different short range order even though the perfect and defective icosahedral bonding environments are major bonding elements of both liquid and amorphous states. Perfect icosahedrons with a small frequency exist in the liquid state, more develop during the cooling process and they become the leading building units in the glass state, indicating an icosahedral short range order in Mg(90)Ca(10) glass. Also the linked icosahedrons lead to an icosahedral medium range order. Furthermore, an ordered arrangement of some icosahedrons in the hexagonal symmetry is observed in the glass model, representing a nanoscale icosahedral quasicrystalline phase in Mg(90)Ca(10) glass.

Correlation between hydrogen migration and microstructure in cast Mg alloys

A simple casting process has been applied to produce binary Mg–X (X = Ni, Cu or Al) in situ composites for hydrogen storage. Besides a major single phase of primary Mg, nanostructured eutectic networks of Mg–Mg 2Ni, Mg–Mg 2Cu and Mg–Mg 17Al 15 were formed during solidification of Mg–Ni, Mg–Cu and Mg–Al alloys, respectively. Each intermetallic phase has different effects on the hydrogen sorption kinetics of the Mg alloys due to their different reactivities with hydrogen. In this study, the relationship between hydrogen migration and microstructure of the cast Mg alloys is investigated. The influence of hydrogenation/dehydrogenation cycling on the formation of porous structures is further discussed.

The effect of Al 8Mn 5 intermetallic particles on grain size of as-cast Mg–Al–Zn AZ91D alloy

Al 8Mn 5 intermetallic particles are usually present in most of commercial Mg–Al based alloys. In recent years, the ability for the Al 8Mn 5 particles to act as potent nucleation sites for the α-Mg grains during the solidification of the Mg–Al alloys has been the subject of debate. In this work, the effect of Al 8Mn 5 particles on grain size of an Mg–9Al–1Zn (AZ91D) alloy was assessed. The experimental result from two AZ91D alloys with different Mn concentrations, i.e., 0.02 wt.% and 0.22 wt.% respectively, indicated that the grain size did not show any notable change. Further, high resolution transmission electron microscopy (HRTEM) was conducted on samples with concentrated Al 8Mn 5 particles collected by pressurised filtration of the AZ91D alloy melt. The extensive TEM examination on the Al 8Mn 5/α-Mg interfaces revealed no crystallographic orientation relationship (OR) between the Al 8Mn 5 and α-Mg crystals. It is therefore concluded that Al 8Mn 5 particles are unlikely to act as effective nucleation sites for the α-Mg during solidification of Mg–Al based alloys.

Shear enhanced heterogeneous nucleation in some Mg- and Al-alloys

Intensive shearing was applied to alloy melts at temperatures above their liquidus by using a twin-screw mechanism. The sheared melt was then cast into a TP1 mould for microstructural examination. Alloy melts with or without shearing were also filtered using the Prefil technique developed by N-Tech Ltd in order to analyse oxides and other second phase particles. The experimental results showed a significant grain refinement through enhancement of heterogeneous nucleation. The intensive melt shearing converted oxide films and agglomerates into well dispersed fine particles with a narrow size distribution. It was confirmed that the fine oxide particles can act as potent sites for nucleation during the solidification of the sheared melt. This paper presents the experimental results and theoretical analysis of shear enhanced heterogeneous nucleation during solidification of Mg- and Al-alloys. A multi-step heterogeneous nucleation mechanism has been proposed and discussed.

Solidification paths and eutectic intermetallic phases in Mg–Al–Ca ternary alloys

The solidification of Mg–Al–Ca ternary alloys in the α-Mg solidification region was investigated by microstructural and thermal analysis, and the liquidus projection was determined. The liquidus temperature decreases with additions of Al and Ca. Solidification is terminated with the formation of C14 (Mg 2Ca), C36 ((Mg,Al) 2Ca) or A12 (β-Mg 17Al 12) eutectic compounds, in the order of increasing Al content. The L → α + C36 eutectic has a saddle point at 807 K, and with decreasing temperature, two ternary invariant transformations, L → α + C14 + C36 (787 K) and L + C36 → α + A12 (725 K) are observed. The stability of the intermetallic C36 phase during annealing at 573 K was examined in die-cast Mg–5Al–3Ca based alloys. The C36 phase transformed to the C15 (Al 2Ca) phase by a shear-assisted mechanism.