Magnesium Grains Research Articles

Role of Ti 4+ and Sn 4+ ions in spinel formation and reactive sintering of magnesia-rich ceramics

The solid solubility of magnesia in magnesium aluminate spinel and magnesium aluminate spinel in magnesia does not change with temperature thus not creating bonds or precipitation over periclase grains in a single stage sintering process. In comparison, the precipitated spinels in magnesia-chromia refractories form complex spinel due to inversion in the position of bivalent and trivalent cations within the structure, making them more stable at high temperature than either normal or inverse spinel. Additives form low-temperature compounds that diffuse into the spinel structure and create defects that change the properties of spinel solid solution. In the present study, magnesia and alumina powders along with tetravalent oxide additives were analyzed for their role in reactive densification of spinel in a single stage firing process in order to achieve a better binding system for magnesia-based refractories. These tetravalent oxides on reaction with magnesia form spinel solid solution with MgAl 2O 4 as they have similar crystal structure. The spinel solid solution formed using oxide additives is expected to have higher solubility in magnesia than magnesium aluminate spinel, resulting in improvement of the bonding during sintering through increased in solid solubility at elevated temperatures followed by precipitation of secondary spinel phases, similar to the complex spinel in magnesia-chrome refractories. The formation of spinel during firing remains as a second phase that retards the grain growth of periclase. The changes in unit cell dimensions with temperature and amount of additive were analyzed using Reitveld method and correlated with the densification behaviour at different temperatures.

Microstructural and densification study of natural Indian magnesite in presence of zirconia additive

The sintering and microstructural evaluation of Indian magnesite was carried out in presence of zirconia. Zirconia in monoclinic form was added in the range 3–6 wt% with respect to raw magnesite and the sintering temperature selected were 1500–1600°C for 2 h. The main impurities present in the magnesite were Fe2O3, CaO, SiO2. The natural crystalline magnesite could be sintered with bulk density of 3.38 g/cc (A.P. 1.54%) at 1550°C/2 h. But the higher bulk density (3.50 g/cc) and minimum apparent porosity (A.P. 0.25%) was attained at 1550°C/2 h with the 3 wt% zirconia additive. On firing magnesite with zirconia as additive, a crystalline phase, magnesio-zirconate, was identified at the triple point regions of the direct bonded periclase grains. The morphology of the periclase grains were changed from subrounded/rounded to angular shaped in presence of zirconia as additive.

Microstructural and mechanical characterisation of WE54–SiC composites

Microstructural and mechanical characterisation of WE54 alloy reinforced with SiC particles of two sizes is studied. All the materials are produced by powder metallurgy. The composite microstructure is characterised by ultrafine magnesium grains with SiC and β particles located at the grain boundaries. The composites show better mechanical strength than the unreinforced alloy. The greater yield stress of the composites is due mainly to grain refinement and an increase in the dislocation density as a result of CTE mismatch between matrix and ceramic particles. The high-temperature mechanical strength of the composites is controlled by deformation of the matrix.

Variação do tamanho de grão da magnésia: um exemplo da complexidade de cerâmicas refratária

Concretos refratários alumina-magnésia apresentam um comportamento expansivo devido à espinelização in-situ, sendo que a fonte de magnésia e o seu tamanho de grão influenciam tal reação. Neste estudo, o aumento do tamanho de grão da magnésia utilizada resultou em expansão excessiva, gerando trincamento e deteriorando as propriedades mecânicas do material. Avaliações por microscopia eletrônica de varredura indicaram microestruturas distintas para concretos contendo diferentes tamanhos de grão (< 45 µm e < 100 µm), apσs sinterização em 1500 ºC. Para a composição contendo a magnésia mais fina, as fases comumente observadas na literatura foram detectadas: espinélio e CA6. No entanto, o concreto contendo uma magnésia mais grosseira apresentou também a formação de forsterita e monticelita ao redor de grãos de magnésia remanescentes. Sendo assim, o objetivo deste estudo é avaliar como a alteração de apenas uma variável de um concreto refratário pode afetar significativamente as suas propriedades, devido ao desenvolvimento de microestruturas distintas.

Analysis of deformation twins and the partially dehydrogenated microstructure in nanocrystalline magnesium hydride (MgH 2) powder

Cryo-stage transmission electron microscopy (TEM), supported by Density Functional Theory (DFT), is employed to explore the microstructure of magnesium hydride (MgH 2) powders. Mechanical milling results in deformation twinning of the hydride. The crystallography of the twins is established. DFT analysis shows that the twin unit cell is just as thermodynamically stable as the undeformed α-MgH 2 matrix. It is hypothesized that the twins contribute significantly to the observed milling-induced kinetic enhancement by acting as high diffusivity paths for hydrogen. Energy-filtered TEM analysis on partially desorbed MgH 2 demonstrates that nucleation and growth of metallic magnesium occurs non-uniformly. Larger powder particles are a composite of isolated magnesium grains heterogeneously nucleated on the remaining hydride. Smaller particles are either fully transformed to magnesium or remain entirely a hydride. There is little evidence for any “core–shell” structure. It is also shown that in situ hydrogen desorption in the TEM is not representative of the elevated-temperature ex situ sequence.

Grain refinement of AZ91D magnesium alloy by a new Mg–50%Al4C3 master alloy

A novel and simple method for preparing Mg–50%Al4C3 (hereafter in wt.%) master alloy has been developed by powder in-situ synthesis process under argon atmosphere. X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) results show the existence of Al4C3 particles in this master alloy. After adding 1.8% Mg–50%Al4C3 master alloy, the average grain size of α-Mg decreased from 360μm to 154μm. Based on the DTA test results and calculation of the planar disregistry between Al4C3 and α-Mg, Al4C3 particles located in the central regions of magnesium grains can act as the heterogeneous nucleus of primary α-Mg phase.

Growth and structural properties of Mg:C thin films prepared by magnetron sputtering

We investigate the growth and structure properties of Mg:C thin films. The films are prepared using a dc magnetron sputtering discharge where the electrical resistance over the films is monitored during growth in-situ with a four point probe setup. The structural properties of the films are investigated using X-ray diffraction measurements and the elemental composition and binding in the films is determined using elastic recoil detection analysis and X-ray photoelectron spectroscopy. The results show that during co-sputtering the carbon flux influences the initial stages of the film growth. The films are made of polycrystalline magnesium grains embedded in a carbon network, the size of which depends on the carbon content, but amorphous phases cannot be excluded. The XPS measurements show the presence of carbidic carbon whereas X-ray measurements find no Mg:C phases. The overall stability of the films is found to depend on the carbon content, where stable films capped with a 14 nm Pd layer cannot be obtained with carbon content above 18%.

Role of manganese on the grain refining efficiency of AZ91D magnesium alloy refined by Al4C3

Abstract A novel Mg–50% Al4C3 (hereafter in wt.%) master alloy has been developed by powder in situ synthesis process, the role of manganese on the grain refining efficiency of AZ91D magnesium alloy refined by this master alloy has been investigated. X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) results show the existence of Al4C3 particles in this master alloy. After addition of 0.6% Al4C3 or combined addition of 0.6% Al4C3 and 0.27% Mn, the average grain size of AZ91D decreased dramatically from 360 μm to 210 μm, and from 360 μm to130 μm, respectively. However, no further refinement of grain size was achieved with additional amount of Mn exceeding 0.27% for AZ91D alloy refined by 0.6% Al4C3 in the present investigation. Al–C–O–Mn–Fe-rich intermetallic particles with an Al–C–O-rich coating film, often observed in the central region of magnesium grains of the AZ91D alloy treated by the combination of Al4C3 and Mn, are proposed to be the potent nucleating substrates for primary α-Mg.

The influence of magnesium grain size and ball milling time on the phase formation ofMgB2 tapes

The influence of both Mg grain size and ball milling time on the phase formation ofMgB2 tapes has been studied. The finer grain sizes of Mg powder, under 45 µm, and the well-milled condition above 8 h improve theJc value effectively. Itwas found that the Jc values of the MgB2 tapes were consistent with the results of thermogravimetry and differential thermalanalysis and of x-ray diffraction studies, and the Mg grain sizes and the ballmilling time affect the promotion of the reaction of mixed powders and theJc value ofMgB2 tape. The highestJc value was estimatedto be about 3200 A cm−2 at 4.2 K in a magnetic field of 10 T.

Effect of Mg–TiB 2 master alloy on the grain refinement of AZ91D magnesium alloy

A novel Mg–50% TiB 2 master alloy has been developed by powder in situ synthesis process and its grain refining effect on AZ91D magnesium alloy has been investigated. X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) results show the existence of TiB 2 particles in this master alloy. After adding 1.8 wt% this master alloy, the average grain size of AZ91D decreased markedly from about 240 μm to 50 μm, and the dendritic structure of the primary phase α-Mg was obviously refined. Ti and B enriched particles, often observed in the central regions of magnesium grains, are proposed to be the potent nucleating substrates for primary Mg.

Grain nucleation and formation in Mg–Zr alloys

Zirconium is the most potent nucleant discovered to date for magnesium. This work reports the grain nucleation and formation characteristics in zirconium-inoculated magnesium melts. It was found that nucleation of magnesium grains occurred on two types of zirconium nucleants, where the first type were small and round, suggesting that they were formed by precipitation, and the other type were larger and irregularly shaped, similar to undissolved particles. Subsequent growth of the magnesium halo structures after nucleation could occur either spherically or dendritically under similar cooling conditions but under well-inoculated conditions they grew spherically in general. Hence, achieving a high level of dissolved Zr content is not only essential to the nucleation process but also important for control of the subsequent growth morphologies of the halo structures. The nucleation process can be understood by an adsorption mechanism.

Microsilica or MgO grain size: Which one mostly affects the in situ spinel refractory castable expansion?

Microsilica is commonly added to alumina–magnesia castables to counterbalance the in situ spinel expansion. This effect is attained by the generation of a low-melting temperature phase, which also affects the expansive reaction kinetics. Additionally, the MgAl 2O 4 formation depends on the grain size of the reactants. The use of coarse magnesia grains results in lower Mg 2+ dissolution and could lead, at 1500 °C, to forsterite development (Mg 2SiO 4). For finer MgO, silica was detected at the edge of the spinel grains. Considering these aspects, this work evaluated the effect of microsilica content for different magnesia grain sizes (<45 or <100 μm). Due to a faster spinel formation for the fine MgO source, microsilica counterbalanced the MgAl 2O 4 expansion. Conversely, for the coarser MgO, silica increased the Mg 2+ dissolution, speeding up the spinel formation and expansion. Therefore, microsilica presented opposite roles, pointing out that it does not always counterbalance the spinel expansion. This work also indicated the need for a systemic approach for the expanding design of alumina–magnesia refractory castables.

Influence of SiC Particles on the Grain Refinement of an Mg-Al Alloy

SiC particles are effective grain refiners in Mg-Al alloys. Several investigations, from different researchers, into their effect on a range of alloys with different Al contents has been undertaken and it has been found that the greatest reduction in grain size occurs in alloys having low Al contents. Performing grain refinement studies on a range of alloy contents also allows for further investigation into the mechanisms of grain refinement. It was found that the size of the SiC particles is also important in magnesium grain refinement. However, the presence of Mg2Si in the microstructure and the consideration of phase equilibria suggest that SiC can transform into other binary or ternary carbides. If such carbides are formed, they may also act as an effective grain refiner for Mg-Al alloys. In this study, the possibility of formation of new carbides (Al4C3, Al2MgC2, Mn7C3, Mg2C, Mg2C3, Al2CO etc.) and their ability to be good grain refiners for Mg-Al alloys is discussed.

Interaction of Al 2O 3-rich slag with MgO–C refractories during VOD refining—MgO and spinel layer formation at the slag/refractory interface

The interaction mechanisms between a pitch-bonded MgO–C refractory and an Al 2O 3 rich (∼15 wt%) stainless steelmaking slag were investigated by rotating finger tests in a vacuum induction furnace. A porous MgO layer (instead of a dense MgO layer) was observed at the hot face of the MgO–C bricks. This implies that under the present low oxygen pressure conditions, the oxygen supply from the slag is insufficient to meet the demand of reoxidising the entire amount of Mg vapor generated from the MgO–C reaction to form a fully dense MgO layer. A Mg(Al,Cr) 2O 4 spinel layer with zoning was found at the slag/brick interface in the top slag zone specimen of Test 3 (CHS3). Based on the thermodynamic analyses with and experimental data, a mechanism of Mg(Al,Cr) 2O 4 spinel formation is proposed. Initially, hot face periclase grains take up Cr 2O 3, and to a much lesser extent, Al 2O 3 from the slag. The further diffusion of Cr 2O 3 and Al 2O 3 from the slag establishes a spinel layer of three distinct compositions of the type MgAl 2(1− x) Cr 2 x O 4, with x decreasing when moving from the interior to the exterior spinel layer. Due to the low oxygen pressures, the thermodynamically less stable, dissolved Cr 2O 3 in the hot face periclase decomposes and forms chromium-rich metal droplets.

From Macro to Nanomagnesia: Designing the in situ Spinel Expansion

Magnesia grain size affects the in situ spinel formation kinetics and expansion of alumina–magnesia refractory castables. The use of coarse grains results in a large expansion and delays the reaction's starting temperature. In this work, the addition of an MgO source with different grain sizes (from macro, Dmax≤0.1 mm, to nano, D50≤20 nm) led to a broad linear expansion range (6%–2%), considering the spinel and the CA6 formation. The addition of nanomagnesia, produced by a centrifugal‐type high‐energy ball milling, to high‐alumina refractory castables affected the spinel expansion and its starting temperature. The objective to estimate the reaction starting temperature was attained, reducing it from 1200° to 900°C. Nevertheless, the decrease of spinel expansion was not totally fulfilled, as a portion of the nanoparticles partly reagglomerated before mixing and behaved as coarser magnesia grains.

Microstructural characterization of an extruded Mg–Ni–Y–RE alloy processed by equal channel angular extrusion

Equal channel angular extrusion (ECAE) was performed in a conventionally extruded Mg–Ni–Y–RE material to develop fine-grained microstructures. The microstructure of the extruded alloy consisted of agglomerated phase particles zones (APZs) aligned along the extrusion direction and phase particles free zones (PFZs) containing recrystallized magnesium grains smaller than 5 μm in size. Some PFZs contained a banded unrecrystallized structure. The global texture was very weak and described by a (1 0 0) fibre which is common in extruded magnesium alloys. PFZs grain size was reduced to less than 1 μm after ECAE deformation at temperatures of 275, 310 and 375 °C, although some PFZs remained unrecrystallized. The mechanical properties of ECAE deformed materials were correlated with grain size and texture.

Magnesia grain size effect on in situ spinel refractory castables

Alumina–magnesia castables present an expansive behavior due to in situ spinel formation and the reaction is affected by the magnesia source and its grain size. In this study, castables were prepared with different magnesia grain sizes (<45 or <100 μm) and the samples were evaluated by an assisted sintering technique, mechanical strength, creep resistance and microstructural evaluation. The expansion, the resulting phases and properties scaled with the magnesia grain size. The use of coarse MgO grain (<100 μm) led to phases which were predicted by local equilibrium phase diagrams, such as forsterite and monticellite. Due to a completely different microstructure developed for the castable containing large grains, a large number of cracks were generated, worsening the mechanical strength and creep resistance. Magnesia grain size selection is thus a key issue for alumina–magnesia castable design, as it affects the material's performance during service. As the change of a single parameter affected the final castable microstructure and properties, this study is a typical example concerning the complexity of refractory ceramic systems.

The effect of annealing on the deformation behaviour and microstructure of crystallized Mg–23.5Ni (wt.%) alloy

Rapidly solidified amorphous Mg–23.5Ni (wt.%) ribbons were crystallized at 300 and 400 °C for 90 min. After annealing at 300 °C the microstructure was heterogeneous, consisting of rounded eutectic–lamellar domains, which contained magnesium grains smaller than 500 nm. In the case of ribbons annealed at 400 °C the microstructure, however, was homogenous, and composed of well-formed magnesium grains and Mg2Ni particles. At room temperature both crystallized materials were brittle due to the high volume fraction of Mg2Ni particles, but they exhibited some ductility with increasing test temperature. Above 200 °C, the microstructure of the ribbons annealed at 300 °C was characterised by the formation of particle free zones during the tensile test. This structure was not observed in the material annealed at 400 °C. Deformation behaviour and changes in the microstructure during plastic flow of both crystallized materials were explained according to grain boundary sliding mechanisms.

Effects of Rare Earth Oxides on Microstructures and Properties of Magnesia Refractories

The effects of Y2O3, La2O3 and Nd2O3 on the sintering, microstructure and mechanical properties of magnesia refractories were investigated. Addition of rare earth oxide (ReO) to magnesia refractories increases the bulk density, decreases the porosity and improves the mechanical strength of the refractories. The improved sinterability was attributable to the vacancies generation associated with the solid-solution reactions between MgO and ReO. In the samples with ReO, rare earth silicate phases form at magnesia grain boundaries, providing additional bonding between magnesia grains and between magnesia grains and matrix. Consequently, the samples with ReO showed much higher high temperature strengths than those without ReO.

バルクメカニカルアロイング法によるマグネシウム複合材料の開発

The new bulk mechanical alloying process (Equal Channel Angular Bulk Mechanical Alloying process: ECABMA) was developed in this research. Magnesium alloy chip (AM60) mixed with 5-weight percent titanium powder was processed into a magnesium composite billet via ECABMA process. The magnesium composite billet was extruded into round bar, and several mechanical properties were investigated. It was found that the ECABMA process was effective for the mechanical fragmentation of the magnesium chip and titanium powder into small pieces and the uniform mixture of their input materials. The extruded composite had micro-scale magnesium grains finely dispersed with refined titanium powders. It had 370 MPa-0.2% proof stress, 400 MPa-tensile strength and 10%-elongation at room temperature. The ECABMA process was effective to increase 0.2% proof stress due to the grain refinement via dynamic recrystallization.