Magnesium Grains Research Articles

Microstructural Evolution of Cast Mg-Al-Ca Alloy during Friction Stir Processing

A thixomoulded magnesium alloy containing thermally stable Al2Ca phase was friction stir processed, and its microstructure and mechanical property were investigated. In friction stir zone, the cast structure of the base metal is replaced by fine magnesium grains containing fine dispersoides of Al2Ca. Due to the refined grain size of 0.8-1 μm, hardness of friction stir zone is higher than that of base metal. Texture was developed at top and bottom surfaces, (0002) being parallel to the plate surface, whereas (0001) plane is randomly oriented in the center of friction stir zone.

The effect of solidification rate on microstructural evolution of a melt-spun Mg–20Ni–8Mm hydrogen storage alloy

Mg-based Mg–20Ni–8Mm (wt%) (Mm = Ce, La-rich Mischmetal) hydrogen storage alloy with microcrystalline, nanocrystalline, and amorphous microstructures was obtained by controlling the solidification rate of the melt-spun ribbon. The microstructure was remarkably refined by rapid solidification compared with that of a conventionally prepared polycrystalline alloy. The optimal microstructure with a considerable amount of nanocrystalline Mg and Mg 2Ni in an amorphous matrix was expected to have the highest hydrogen storage capacity. Mischmetal-containing Mg 12Mm intermetallic precipitated preferentially at the boundary of magnesium grains in the microcrystalline microstructure, thus providing possible pathways for the enhanced hydrogen diffusion into the Mg.

Effect of extrusion temperature on superplasticity of PM-WE54

The effect of the extrusion temperature on the microstructure and mechanical behaviour of the powder metallurgy WE54 alloy has been investigated between 300 and 400 °C. After extrusion, the microstructure consists of magnesium grains and Mg 14YNd 2 particles located at grain boundaries. The alloy exhibits superplastic behaviour at high strain rates, attaining elongations to failure around 500%.

Microstructure and tensile creep behavior of Mg–4Al based magnesium alloys with alkaline-earth elements Sr and Ca additions

The microstructure, tensile property and creep behavior of three Mg–Al–Sr (AJ) and two Mg–Al–Sr–Ca (AJC) based alloys have been investigated. The as-cast microstructure of Mg–4Al–(1–3)Sr based alloys consists of the dendritic α-matrix and grain boundary second-phases. In the alloy with 1% of strontium added a lamellar eutectic Al 4Sr compound is observed at grain boundaries. A ternary Mg–Al–Sr interphase exists at grain boundaries besides eutectic Al 4Sr in the alloys containing 2–3% of strontium. Calcium addition to Mg–4Al–(1–2)Sr based alloys results in the formation of Mg 2Ca and (Mg, Al) 2Ca, which shows two morphologies, lamellar eutectic at grain boundaries and granular in the matrix grains. Creep properties of the Mg–Al–Sr based alloys at the temperatures between 150 and 200 °C and applied stresses between 50 and 80 MPa are significantly improved with the increase of strontium addition and higher creep resistance is obtained from the Mg–4Al–(1–2)Sr based alloys with 1% of calcium addition. The improvements on creep resistance for these alloys studied are mainly attributed to the formation of thermally stable interphases, Al 4Sr and Mg–Al–Sr ternary phase as well as Mg 2Ca and (Mg, Al) 2Ca, which provide the pinning effect at the magnesium grain boundaries. Primarily assessments of creep mechanism based on stress dependence of secondary creep rate at 175 °C and temperature dependence of secondary creep rate at 70 MPa seem that both grain boundary sliding and dislocation climb contribute to the creep deformation.

Effect of extrusion temperature on the microstructure and plastic deformation of PM-AZ92

The effect of extrusion temperature on the microstructure and mechanical behaviour of powder metallurgy AZ92 alloy has been investigated. After extrusion the microstructure consists of magnesium grains and γ(Mg 17Al 12) particles located at grain boundaries. The appearance of Al 8Mn 5 particles in the original powders is also observed. As the extrusion temperature decreases, the yield stress increases due to grain size refinement and a higher volume fraction of γ(Mg 17Al 12) particles.

The Effect of Manganese on the Grain Size of Commercial AZ31 Alloy

The effect of manganese on grain refinement of a commercial AZ31 alloy has been investigated using an Al-60%Mn master alloy splatter as an alloying additive at 730 °C in aluminium titanite crucibles. It is shown that grain refinement by manganese is readily achievable in AZ31. Electron microprobe analyses reveal that prior to the addition of extra manganese the majority of the intermetallic particles found in AZ31 are of the Al8Mn5 type. However, after the addition of extra manganese in the range from 0.1% to 0.8%, the predominant group of intermetallic particles changes to the metastable AlMn type. This leads to a hypothesis that the metastable AlMn intermetallic particles are more effective than Al8Mn5 as nucleation sites for magnesium grains. The hypothesis is supported by the observation that a long period of holding at 730 °C leads to an increase in grain size, due probably to the transformation of the metastable AlMn to the stable Al8Mn5. The hypothesis has also been used to understand the mechanism of grain refinement by superheating.

Grain refinement efficiency and mechanism of aluminium carbide in Mg–Al alloys

Detailed microscopic examination using optical and electron microscopes suggests that Al4C3, often observed in the central regions of magnesium grains on polished sections, is a potent substrate for primary Mg. Calculations of the crystallographic relationships between magnesium and Al4C3 further support the experimental observations. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Crystallography of grain refinement in Mg–Al based alloys

An edge-to-edge matching model has been used to analyze the crystallographic features between magnesium matrix and three commonly accepted heterogeneous nucleants for magnesium grains, Al 4C 3, Al 2CO and Al 8(Mn,Fe) 5. Although certain orientation relationships between magnesium and each of these compounds have been predicted, the potency of each of these three compounds as nucleating substrates for magnesium grains is different. Based on calculated interatomic spacing misfits along matching directions and d-value mismatches between matching planes, the model has predicted that Al 2CO is the most effective nucleant of the three compounds assessed for Mg grains and is probably responsible for the grain refinement in both the superheating and carbon inoculation processes. Al 4C 3 can be a nucleant for Mg grains, but it is not as effective as Al 2CO. This prediction is consistent with most experimental results reported recently. The Al 8(Mn,Fe) 5 intermetallic compound is predicted to have the lowest efficiency as a nucleant for magnesium grains. The model may be used to provide theoretical guidance for selection of new and more powerful grain refiners for Mg–Al based alloys.

Composition of Periclase and Calcium‐Silicate Phases in Magnesia Refractories Derived from Natural Microcrystalline Magnesite

Mineralogical and microstructural properties of sintered and fused magnesia grains and refractory bricks derived from natural magnesite were examined. Although their CaO/SiO2 ratios can be used to approximate their mineralogical compositions, detailed X‐ray diffraction (XRD) and scanning electron microscopy (SEM) studies revealed significant deviations from the expected phase assemblages. Their microstructures vary widely in terms of proportions of direct periclase bonding, amounts and phase constitution of the siliceous bonding, and the size and composition of the periclase crystals. An exceptionally high solid solubility of CaO in periclase crystals was detected. Most of the phases of the siliceous bond exhibit nearly stoichiometric compositions and show intriguing substitutions.

Microstructural evolution of a heat-resistant magnesium alloy due to friction stir welding

Microstructures of a heat-resistant magnesium alloy prepared by friction stir welding were investigated. In the friction stir zone, the microstructure of the thixomoulded parent material is replaced by fine magnesium grains and small particles of intermetallic compounds (Al2Ca). Due to microstructural refinement, the microhardness of the friction stir zone is higher than that of the base material.

The Wear of Tundish Stopper Refractories by Inclusion Slags

Liquid inclusions in the steel may play an important role in the wear of the stopper tip refractories in tundish operations. In the present study, the attack and wear of the tundish stopper refractories by inclusion slagsin OneSteel Whyalla billet product have been investigated. Three stopper tip refractories, Al 2 O 3 -C, ZrO 2 -C and MgO-C, have been tested with inclusion slags (SiO 2 -Al 2 O 3 -CaO-MnO-FeO) under argon at 1 570-1 610°C for 1-4 h using an in-situ gravimetric technique. The in-situ gravimetric data, combined with autopsy of the samples after the tests, provide valuable insights into the dynamic processes of the slag-refractory interactions and the mechanisms for refractory wear. It was found that the performance of the three refractories differed considerably and was determined predominantly by the resistance to the chemical attack by the liquid inclusions and the extent of internal carbon-oxide reactions. The FeO in the inclusion had a detrimental effect on the oxidation of carbon in the refractory, causing vigorous reaction and severe wear. The MnO in the slag also reacted with carbon, but to a much lesser extent, while severe wear only occurred when slag attacked the refractory grains. The weight loss, due to internal carbon-oxide reactions, appeared to be an important issue. Of the refractories tested, the MgO-C performed the worst, suffering from severe inclusion attack on both the carbon and the periclase grains, and considerable weight loss due to internal carbon reaction. The ZrO 2 -C showed a reasonable resistance to the chemical attack from the inclusion, with MnO up to 33wt%; but showed extensive weight loss due to internal carbon reaction. The Al 2 O 3 -C was the best performer in terms of resistance to both the inclusion attack and internal carbon reaction. The results were found to be in broad agreement with plant experience and observations.

Effect of iron on grain refinement of high-purity Mg–Al alloys

The effect of iron on the grain refinement of high-purity Mg–3%Al and Mg–9%Al alloys has been investigated using anhydrous FeCl 3 as an iron additive at 750 °C in carbon-free aluminium titanite crucibles. It was shown that grain refinement was readily achievable for both alloys. Fe- and Al-rich intermetallic particles were observed in many magnesium grains.

MlNERALOGICAL STUDY AND PROPERTIES OF MAGNESIA REFRACTORIES DERIVED FROM EVIAN MAGNESITE

In order for the Greek magnesia industry to retain a high position in the world market, the basic refractories derived from Greek magnesite must remain at the forefront of the international developments. A mineralogical study of magnesia materials produced from Evian magnesite has been carried out with the aim a) to provide detailed characterization of products and microstructures derived from the firing processes of magnesia raw materials and b) to contribute to the development of new magnesia-spinel refractory materials from natural Greek magnesite. The magnesite of N. Evia, Greece, is micro- crystalline and has been used for the production of basic refractories because of the very low amounts of impurities such as CaO, S1O2, FeO(tot), B2O3 it contains . Magnesia materials studied here are: 1) Raw materials. The dead- burned magnesia grains examined are divided into two groups according to the mode of beneficiation of the raw magnesite: a) Dead burned magnesias produced from natural microcrystalline magnesite (Group A). b) Dead burned magnesias of high purity produced from natural microcrystalline magnesite (Group B). The chemical composition of these materials lies essentially in the MgO-CaO-SiC>2 system, since they contain only trace amounts of Fe2Û3 and AI2O3. Their microstructures vary widely in terms of proportions of direct MgO-MgO bonding, amounts and types of phases of the siliceous bonding and size of the periclase crystals. Dead burned magnesias of high purity are better sintered and contain lower amounts of secondary phases compared to magnesias of lower purity. 2) Commercial magnesia bricks derived from dead burned magnesia of high purity and magnesia chromite bricks derived from raw materials of dead burned magnesia of high purity and chromite from Africa. 3) New spinel-based composition and new magnesia-spinel refractory materials which have been synthesized for the needs of this study. The newly synthesized spinel-based composition (70wt% Al2O3-30wt% MgO) shows an increase in the bulk density as well as in the amount of spinel formed compared to available commercial qualities (50%wt AI2O3). 4) Using this new spinel-based composition and dead burned magnesia of high purity, two new magnesia-spinel refractory materials containing 10 and 20wt% AI2O3 were produced, with the aim to obtain more friendly to the environment magnesia refractories to substitute for the magnesia chromite bricks. The compositions of the magnesia spinel refractories thus produced were expected to show endurance in thermal shocks as well as in the corrosion from slags and friction, in order to have a wide application in steel, cement industry etc .

Effect of compositional variation on the synthesis of magnesite–chrome composite refractory

Magnesite–chrome composites have been prepared by utilizing sintered magnesite and friable chrome ore in presence of titania as additive. Three types of batch compositions containing 5% Cr 2O 3, 18% Cr 2O 3 and 30% Cr 2O 3 have been selected for developing mag–chrome composites. The aggregates were developed by first vibro milling the raw materials and then additives were incorporated in wt.% along with 5% PVA solution as binder to the different batches. It was then properly mixed by fluidized bed mixer and after sieving in 20-mesh BS sieve briquettes were formed uniaxially at a pressure of 100 MPa. The briquettes were first air dried and subsequently oven dried at 110±5 °C for 24 h. Finally, the green briquettes were fired at 1700 and 1750 °C in electrically program controlled muffle furnace. The physical properties as well as thermo-mechanical properties and microstructural studies of the sintered aggregates have been evaluated. The common feature observed in the microstructures is the exsolution of spinel phase in magnesia grains. The hot properties of the composites are superior when the Cr 2O 3 content is increased to 30%. Additives played a significant role in improving the bulk density and strength.

Quasi-crystalline grain-boundary phase in the magnesium die-cast alloy ZA85

The microstructures of a fine and a coarse-grained magnesium alloy with 8 wt.% Zn and 5 wt.% Al were investigated. Both microstructures consist of dendritic magnesium grains with a coarse intermetallic phase at the grain boundaries. Our investigations showed that during solidification a quasi-crystalline grain-boundary phase forms which exhibits a pronounced thermal and mechanical stability.

Characterization and sinterability of chemically precipitated phosphate-bearing magnesia grains

Two brucite powders having different impurity levels and doped with 0.5 mol% P 2O 5 were coprecipitated from MgCl 2-rich solutions derived from natural magnesite rock. The precipitated powders were characterized for their chemical and mineralogical compositions, thermal analyses and particle size distribution as well as particle morphology. The effect of P 2O 5 as well as other impurities on densification, phase composition and microstructure of the dense-magnesia grains processed by two-stage firing process up to 1300–1550°C was discussed. Phase composition was qualitatively determined by XRD, whereas a SEM attached with an EDS unit was applied for investigating microstructure. The results reveal that dense periclase ceramics containing 93–96% MgO with variable degree of direct bonding could be obtained by firing the precipitated brucite pellets up to 1300–1550°C. Brucite doped with 0.5 mol% P 2O 5 has contributed in densification of magnesia grains in the presence of magnesia-rich calcium silicate, ferrite and phosphate liquid phase. As the impurity content increases, densification is accelerated and occurs as low as 1300°C with relative decrease in bulk density and degree of direct bonding.

Contribution of the silicon carbide particles to the process of forming the structure of magnesium cast composites

The work presents results of investigations and microstructural analyses for Mg- SiCp cast composites. The metallographic examination shows that the distribution of SiC particles is uniform. Quantitative image analysis of the microstructure shows the magnesium grain size to be smaller and more uniform than in the base metal. The studies at the level of the interfaces suggest that the bonding between the matrix and the reinforcement is of the adhesive type. The presence of the Mg2 Si compound in the composites has also been revealed. Additionally, some compounds seem to be present at the SiC/matrix interface. The performed analyses have allowed to propose a model description of the process of the formation of the Mg-SiC composite structure. Mechanical testing has shown that composites have increased YoungOs modulus and proof stress (R) and decreased impact strength.

Penetration and corrosion of magnesia grain by silicate slags

AbstractPenetration and corrosion resistance of high purity sintered and fused magnesia grain by model EAF (CaO/SiO2 = 1·38) and BOF ‘late’ slags (CaO/SiO2 = 3·29) at 1600 and 1700°C were investigated by SEM, EDS, and XRD analysis. Thermodynamic calculations were performed to assist interpretation of the reaction processes involved. At the test temperatures, Fe andMn ions from both model slags diffused into themagnesia grain to form a magnesiowustite, (Mg,Fe,Mn)O. The magnesiowustite directly adjacent to the slag had a much larger crystal size than that of the bulk MgO far from the MgO/slag interface. The large magnesiowustite grains limit the potential for grain boundary penetration into the sintered magnesia. The magnesiowustite layer formed with the EAF slag took up more FexO from the slag than that formed with the BOF slag, which was partially responsible for a lower slag penetration into sintered magnesia grain since the remaining silica rich local liquid was rendered more viscous. The EAF slag was n...

Microstructure and heterogeneous nucleation phenomena in cast SiC particles reinforced magnesium composite

The microstructure and heterogeneous nucleation phenomena in cast SiC particles reinforced magnesium composite have been studied using optical and transmission electron microscopy (TEM). The microstructure of magnesium composites showed that most SiC particles were pushed and segregated at the grain boundaries while few SiC particles (3%) were entrapped in the magnesium grain. The SiC particles in the magnesium composite were identified as 6Hα-SiC structure. Stacking faults and pits defects were also observed within the SiC particles. The primary magnesium phase which heterogeneously nucleated on the SiC particle surface has been identified with a small lattice disregistry (2.3%) while their crystallographic orientation relationship was (10 1 ̄ 0) Mg//(0001) SiC. Examination of the composite interface indicated that the eutectic and Cu 5Zn 8 phases were able to wet the SiC particles and heterogeneously nucleate on the SiC particles while the crystallographic orientation relationships between them were (01 1 ̄ 1) SiC//(110) Mg17Al12 and (0001) SiC//(001) Cu5Zn8, respectively. Finally, the factors that influence the heterogeneous nucleation of primary magnesium phase, eutectic and Cu 5Zn 8 phases on the SiC particles are discussed.

Segregation in the MgO–MgAl2O4 system processed from nitrate precursors

The occurrence of segregation and its influence on microstructural and phase evolution have been studied in MgO–MgAl2O4 powders synthesized by thermal decomposition of aqueous nitrate precursors. When the nitrate solutions of Mg and Al were spray-pyrolyzed on a substrate held at 673 or 573 K, homogeneous mixed oxides were produced. Spraying and drying the nitrate solutions at 473 K resulted in the formation of compositionally inhomogeneous, segregated oxide mixtures. It is suggested that segregation in the dried powders was caused by the difference in solubility of the individual nitrate salts in water which caused Mg-rich and Al-rich salts to precipitate during dehydration of the solutions. The occurrence of segregation in the powders sprayed at 473 K and not 573 or 673 K is ascribed to the sluggish rate at which the early stages of decomposition occurred during which the cations segregated. The phase evolution in segregated and segregation-free MgO–MgAl2O4 powders has been compared. The distinguishing feature of the segregated powders was the appearance of stoichiometric periclase grain dimensions in excess of 0.3 μm at temperatures as low as 973 K. By comparison, the segregation-free powders displayed broad diffraction peaks corresponding to fine-grained and nonstoichiometric periclase. The grain size was in the range 5–30 nm at temperatures up to 1173 K. The key to obtaining fine-grained periclase was the ability to synthesize (Mg Al)O solid solutions with the rock salt structure. In the temperature range 973–1173 K, spinel grain size varied from 5 to 40 nm irrespective of its composition and did not appear to be influenced by segregation.