Mg2Si Compounds Research Articles

Microstructural Characterization of Cast Magnesium Matrix Composites by Raman Microscopy

Abstract Cast magnesium matrix composites reinforced with silicon carbide particles were investigated by using Raman microscopy. 3C, 4H and 6H polytypes of SiC particles were identified in the investigated composites. Additionally, Mg2Si compound was detected by Raman microscopy in the composites microstructure.

Microstructures and mechanical properties of the Mg–8Gd–4Y–Nd–Zn–3Si (wt%) alloy

Microstructures and mechanical properties of Mg–8Gd–4Y–Nd–Zn–3Si alloy were systematically investigated using OM, SEM, XRD, TEM, hardness measurements, tensile and resonance test. The as-cast microstructure consists of α-Mg, Mg3(GdYZn), Mg12(GdY)Zn, Mg2Si, Gd5Si3 and YSi2 phases. The long period stacking ordered (LPSO) structure is found in the as-cast sample, which disappears after solution treatment. The mechanical properties of the extruded-T5 alloy are as follows: Young's modulus E=58.5GPa, yield tensile strength σ0.2=328MPa, ultimate tensile strength σb=386MPa and elongation δ=5.9%. The high Young's modulus of the alloy is attributed to the existence of Mg2Si, Gd5Si3 and YSi2 compounds with high Young's modulus. The weakening of age hardening response is ascribed to the formation of vast Gd5Si3 and YSi2 compounds, which consumed plenty of rare earth atoms in the matrix and decreased the amount of β″ and β′ precipitates.

液相－固相反応法で合成したMg<sub>2</sub>Si 系化合物の焼結性と熱電特性に及ぼすMg仕込量とB添加量の影響

Dimagnesium silicide (Mg2Si) is a promising n – type thermoelectric compound suitable for thermoelectric power generation using waste heat of intermediate temperature range (600 − 900 K). However, there are two problems in making the Mg2Si compound. One is difficulty in synthesizing the compound due to ease of Mg evaporation in fusion, resulting in existence of non-reacting Si. Another is difficulty in sintering the synthesized powder. In order to overcome the two problems, we synthesized the Mg2Si compound on Mg – rich state by the liquid-solid phase reaction and sintered the synthesized powder by the pulse discharge sintering (PDS). The influences of Mg/Si ratio and B addition on the sinterability and the thermoelectric properties of the synthesized Mg2Si compounds were investigated. The sinterability of the synthesized Mg2Si compounds were improved by the increase of Mg amount. And, electrical resistivities of sintered samples were decreased by B addition. Therefore, thermoelectric properties were strongly affected by both Mg/Si ratio and B addition. The sample synthesized with feed amount of Mg of 67.00 at% and additive amount of B of 1.0 at% had the maximum dimensionless figure of merit of about 0.39 at 773 K.

Thermoelectric Properties of Solid-State Synthesized Magnesium Silicides Doped with Group B<sup>I-III</sup> Elements

Group BI(Cu, Ag)-, BII(Zn)- and BIII(Al, In)-doped Mg2Si compounds were synthesized by solid state reaction and mechanical alloying. Electronic transport properties (Hall coefficient, carrier concentration and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, power factor, thermal conductivity and figure of merit) were examined. Mg2Si powder was synthesized successfully by solid state reaction at 773 K for 6 h and doped by mechanical alloying for 24 h. It was fully consolidated by hot pressing at 1073 K for 1 h. The electrical conductivity increased by doping due to an increase in the carrier concentration. However, the thermal conductivity did not changed significantly by doping, which was due to much larger contribution of the lattice thermal conductivity over the electronic thermal conductivity. Group BIII(Al, In) elements were more effective to enhance the thermoelectric properties of Mg2Si.

Reaction Sintering of Mg<sub>2</sub>Si Intermetallic Compound by Microwave Irradiation

In order to reduce the oxidizing and volatilizing caused by Mg element in the traditional methods for synthesizing Mg2Si compounds, solid state phase reaction at low temperature was introduced by microwave field. XRD was used to characterize the powders. At the same time, the influences of parameters during the synthesis processing were discussed. The results suggest that the heating profile is also dependent on the initial green density and higher green density provides lower heating rate while power setting are fixed and the oxidation of Mg can be rest rained by changing microwave heating programs. It was found that high purity Mg2Si intermetallic compound can be obtained with excessive content of 8at% Mg from the stoichiometric Mg2Si, 853K and 30min

Microstructure and compressive property of in situ Mg2Si reinforced Mg-microballoon composites

The in situ Mg2Si particles reinforced AZ91-microballoon composites were fabricated by preforming pressure-infiltrated process using hollow flyash cenospheres (FAC, 180–250μm) as the preform. The microstructure and chemical components of the composites were observed and analyzed by optical microscopy (OM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The compressive properties were investigated by compression test. The fracture mechanisms of the composites were concluded by fractography observation. The experiment results show that the FAC distributed homogeneously in the composites. Many FAC were hollow, and some FAC were filled with Mg alloy matrix. The shapes of most of the Mg2Si compounds in situ formed were polygons, and their sizes were 10–25μm. The AZ91-microballoon composites display a typical behavior of brittle foam material in the compression test. There are three main failure modes according to the fractography study: microballoon cracking, debonding of the interface between FAC microballoon and matrix and debonding of the interface between inner matrix and FAC shell. Among these modes, the microballoon cracking is the primary fracture mechanism.

The effect of crystallite size on thermoelectric properties of bulk nanostructured magnesium silicide (Mg2Si) compounds

In nanostructured bulk materials, the additional interfaces in the material enhance phonon scattering and reduce the thermal conductivity. However, interfaces also scatter electrons and deteriorate charge carrier transport. In order to benefit from the interfacial effects, the crystallite size in the material must be small compared with phonon mean free path (PMFP) and large compared with the charge carrier mean free path (CMFP). In this paper, we solve the Boltzmann transport equation for charge carriers and phonons. We show that bulk nanostructuring of Mg2Si is not an efficient method to enhance the figure-of-merit as the PMFP and CMFP are in the same range.

Microstructure and Damping Capacity of Mg<sub>2</sub>Si/Mg–Al–Si–(Bi) Composites

Influences of Bi addition on microstructures and damping capacities of Mg2Si/Mg­Al­Si­(Bi) composites have been investigated. The Bi addition is beneficial to modify Mg2Si phase from coarse Chinese script shape to fine polygonal shape. Even though the Bi-containing alloy has modified polygonal Mg2Si compounds, its damping capacity shows similar level with that of the Bi-free alloy. This would be attributed to the increased solute concentration related to high solubility of Bi in Mg, which eventually offsets the damping increment in response to Mg2Si modification. [doi:10.2320/matertrans.M2012255]

Thermoelectric Properties of Sb-doped Mg2Si Prepared by Solid-State Synthesis

Sb-doped Mg2Si was prepared by solid-state synthesis and thermoelectric properties were examined. All the Sb-doped Mg2Si compounds showed n-type conduction, indicating that the electrical conduction is mainly due to electrons. The electrical conductivity significantly increased and the absolute value of Seebeck coefficient decreased with increasing Sb content because electron concentration considerably increased from ~1016 to ~1020 cm−3 by Sb doping. The thermal conductivity did not changed significantly by Sb doping. The maximum value of thermoelectric figure-of-merit of 0.62 was obtained for Mg2Si:Sb0.02 at 823 K.

Synthesis and Thermoelectric Properties of n-Type Mg2Si

Group BV(Bi, Sb)- and BVI(Te, Se)-doped Mg2Si compounds were synthesized by solid state reaction and mechanical alloying. Electronic transport properties (Hall coefficient, carrier concentration and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity and figure-of-merit) were examined. Mg2Si powder was synthesized successfully by solid state reaction at 773 K for 6 h and doped by mechanical alloying for 24 h. Powder was fully consolidated by hot pressing at 1073 K for 1 h. All doped Mg2Si compounds showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased greatly by doping due to an increase in the carrier concentration. However, the thermal conductivity did not change significantly by doping, which was due to the much larger contribution of the lattice thermal conductivity over the electronic thermal conductivity. Group BV (Bi, Sb) elements were much more effective at enhancing the thermoelectric properties of Mg2Si than group BVI(Te, Se) elements.

Nano-Engineering Approach to Destabilization of Magnesium Hydride (MgH<SUB>2</SUB>) by Solid-State Reaction with Si

We studied a possibility of destabilization of MgH2 by chemical reaction with Si by using a nano-engineering method for reducing diffusion distances and increasing surface area. The structure, morphology, chemical composition and dehydriding properties were investigated by XRD, SEM, EDS, DTA-TG and the volumetric Sievert method. The commercial MgH2 and Si powder mixture corresponded to the stoichiometry of the ideal Mg2Si intermetallic compound was ball-milled under argon atmosphere to reach a nanocrystalline composite structure (< 50 nm). The ball-milling of the MgH2-Si mixture leads to the formation only a small amount of the Mg2Si compound. Microstructural studies showed that Si after ball-milling is heterogeneously distributed on the surface of MgH2 particles and incorporated in the nanocrystalline MgH2 matrix, forming a nanocomposite structure. The sluggish destabilization of MgH2 by solid-state reaction with Si forming the Mg2Si intermetallic compound was observed at 250 degrees C. The XRD and EDS analysis confirmed that the Mg2Si compound is formed after the dehydrogenation of the synthesized MgH2-Si composite. The activation energy of the destabilization reaction for the investigated composite significantly decreased (162 kJ/mol) as compared with unmilled MgH2-Si powder mixture (213 kJ/mol).

Fabrication of Mg2Si Thermoelectric Materials by Mechanical Alloying and Spark-Plasma Sintering Process

A mixture of pure Mg and Si powders with an atomic ratio 2:1 has been subjected to mechanical alloying (MA) at room temperature to prepare the Mg2Si thermoelectric material. Mg2Si intermetallic compound with a grain size of 50 nm can be obtained by MA of Mg66.7Si33.3 powders for 60 hours and subsequently annealed at 620 degrees C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies up to 800-900 degrees C under 50 MPa. The shrinkage of consolidated samples during SPS was significant at about 250 degrees and 620 degrees C. X-ray diffraction data shows that the SPS compact from 60 h MA powders consolidated up to 800 degrees C consists of only nanocrystalline Mg2Si compound with a grain size of 100 nm.

Fabrication of Mg2Si Thermoelectric Materials by Mechanical Alloying and Spark-Plasma Sintering Process

A mixture of pure Mg and Si powders with an atomic ratio 2:1 has been subjected to mechanical alloying (MA) at room temperature to prepare the Mg2Si thermoelectric material. Mg2Si intermetallic compound with a grain size of 50 nm can be obtained by MA of Mg66.7Si33.3 powders for 60 hours and subsequently annealed at 620 degrees C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies up to 800-900 degrees C under 50 MPa. The shrinkage of consolidated samples during SPS was significant at about 250 degrees and 620 degrees C. X-ray diffraction data shows that the SPS compact from 60 h MA powders consolidated up to 800 degrees C consists of only nanocrystalline Mg2Si compound with a grain size of 100 nm.

Nanogranular MgB2 thin films on SiC buffered Si substrates prepared by an in situ method

MgB2 thin films were deposited on SiC buffered Si substrates by sequential electronbeam evaporation of a B–Mg bilayer followed by in situ annealing. Theapplication of a SiC buffer layer enables the maximum annealing temperature of830 °C to be reached. Transmission electron microscopy analysis confirms the growth of a nanogranularMgB2 film and thepresence of a Mg2Si compound at the surface of the film. The 150–200 nm thickfilms show a maximum zero resistance critical temperatureTC0 above 37 K and acritical current density JC∼106 A cm−2 at 11 K.

Ｍｇ２Ｓｉ粒子添加によるマグネシウム基複合材料の作製およびその機械的特性

Mg-Si alloys have high potential as heat resistant light metals because Mg2Si dispersoids exhibit a high melting temperature, low density, low thermal expansion coefficient, and a reasonably high elastic modulus.In this study, Mg2Si compounds are formed in the solid-state by spark plasma sintering process, in employing the elemental powder mixture of Mg-33.33 mol% Si. The magnesium composite alloys including a large amount of Mg2Si particles were fabricated. The relative density of the composite alloys decreases with increase in the amount of Mg2Si particles because the dispersoids cause not only pores at primary particle boundaries, but poor sinterability between magnesium alloy powder due to their thermal stability. The elastic coefficient, however, increases in increasing the Mg2Si content of the composite alloys.

Innovative Reuse of Agricultural Wastes as Industrial Raw Materials to Form Magnesium Composites

Innovative reuse process of rice husks (RH), being one of the representative agricultural wastes, has been developed to fabricate magnesium composites by solid-state reaction to form magnesium silicide (Mg2Si) reinforcements, having high hardness and Young’s modulus. From a view point of the biomass energy, rice husks, including about 70 mass% organics, could be effectively used as environmentally benign fuels to supply electric power or biomass ethanol. Considering that the most of the rice husk wastes after absorbing organics is SiO2, the solid-state synthesis of Mg2Si compounds by reaction of SiO2 with magnesium was suggested in this study. First of all, the effect of the acid washing treatment to remove organics of rice husks on the crystallization of amorphous SiO2 was evaluated by TG-DTA and XRD analysis. The carbon content of 0.05% in the wastes was drastically reduced by using acid-washed rice husks, compared to that of non acid-washed ones (0.19%). When employing the wastes with and without the acid washing treatment, the crystallization temperature is about 1273 and 1073 K, respectively. The latter showed the lower crystallization temperature because the reaction of the alkaline contents, such as K, P, and Ca, with SiO2 during heating caused to decrease the SiO2 melting point. Rice husk wastes burned at 1273 K after the acid treatment was mixed with AZ31 magnesium alloy powder to fabricate the magnesium composites including Mg2Si dispersoids. Microscope observation and XRD analysis indicated that the amorphous SiO2 included the wastes was more effective to promote Mg2Si formation.

Improvement of Wear Resistance of Magnesium by Laser-Alloying with Silicon

The surface modification of magnesium by laser alloying using a powder injection method was carried out for the purpose of improving its wear resistance. Silicon powder was used as the feeding powder. The applied CO2 laser power conditions were 2 kW-1pass and 2 kW-2pass, while the moving speed of the substrate was 8.3 mms � 1 in all the cases. The silicon powder reacts easily with molten magnesium to form fine Mg2Si compound in the modified layer. Under both laser power conditions of 2 kW-1pass and 2 kW-2pass, the modified layer becomes thick as the powder feeding rate increases, and the modified layer obtained using 2 kW-2pass is thicker than that of 2 kW-1pass at all powder feeding rates. Furthermore, fine and homogeneously distributed Mg2Si compound crystallizes on the whole modified layer in the 2 kW-2pass. At the laser power condition of 2 kW-1pass, the area fraction of Mg2Si compound increases with an increase in the powder feeding rate, while at 2 kW- 2pass, the area fraction of Mg2Si compound is almost the same regardless of the powder feeding rate. Wear resistance of the modified layer improves with increasing area fraction of Mg2Si compound held in the soft matrix of magnesium solid solution, while there is almost no wear of the pin. Coefficient of friction rapidly decreases within a few minutes and then remains virtually constant during sliding wear test. Microstructure of the wear-tested surface reveals that the soft substrate of magnesium is worn by pin at the initial stage and pits which subsequently serve as oil pockets are developed, resulting in the remarkable improvement of wear resistance.

&lt;I&gt;In-situ&lt;/I&gt; Synthesis of Mg&lt;SUB&gt;2&lt;/SUB&gt;Si Intermetallics via Powder Metallurgy Process

The solid-state synthesis of Mg2Si intermetallic compounds has been evaluated in the present paper. The elemental magnesium and silicon powder mixture was employed as starting raw materials. In particular, the influence of the silicon particle by the repeated plastic working (RPW) on the in-situ formation of Mg2Si is discussed, based on the thermal and structural analysis results by DSC thermal analysis and XRD, respectively. Refined Si particles embedded uniformly in the magnesium matrix via RPW, can drive progressing Mg2Si synthesis at extremely low temperature; for example, the ignition temperature, Ts=773 K, to commence the solid-state synthesis of raw powder mixture, was reduced to about 400–430 K. Further plastic working on the mixture causes in-situ formation of Mg2Si intermetallic compounds with a crystallite size of 30–80 nm. However, the ignition temperature to synthesize Mg2Si shifts to a higher temperature again. Regarding to the thermal stability of Mg2Si compounds, the particle size of in-situ synthesized Mg2Si compounds after annealing at 573 K for 900 s is about 30–100 nm, that is, the remarkable coarsening does not occur during annealing.

レーザクラッディングによるＡＺ９１Ｄマグネシウム合金の表面改質

Laser cladding using Al–30%Si alloy powder was applied to AZ91D alloy in order to improve its wear resistance. Area fractions of Mg2Si and Mg17Al12 compounds with relatively high hardness in the cladding layer with a width of 4 mm and a depth of 1.5 mm increase with an increase in powder feeding rate, resulting in high hardness of the cladding layer. Further increase in the powder feeding rate leads to crystallization of aluminum solid solution with low hardness as the matrix phase of the cladding layer together with further increase in the area fraction of Mg2Si compound. Therefore, the hardness of the obtained cladding layer becomes lower than that of the layer consisted of Mg17Al12 + Mg2Si compounds. With the increase in an area fraction of Mg2Si compound, the wear depth of the cladding layer decreases, but that of Cr-plated pin used as the counter material increases. The wear depth of the layer consisting of large area fraction of Mg2Si compound and aluminum solid solution is almost same as that of the Cr-plated pin, and this indicates good wear resistance. However, the presence of a large amount of brittle Mg17Al12 compound causes flaking of Mg2Si compound during wear test and, consequently, leads to ternary abrasive wear, resulting in large wear depth.

Fabrication of aluminum borate whisker reinforced AZ91D magnesium alloy composites by semi-solid process and their mechanical properties

In order to develop a light weight piston for an automobile engine, aluminum borate whisker reinforced AZ91D magnesium alloy composites were fabricated by a semi-solid material process using squeeze casting. By controlling the solid fraction, f s, and the rate of the infiltration into the preform, we succeeded in reinforcing a part of the cast alloy without deformation of the preform. The tensile strength of the alloy/composites interface fabricated by semi-solid (50% f s, solid fraction in the semi-solid slurry) process was 120 MPa, and it was equal to the strength by conventional 0% f s process. However, the flexural strength of the composite by the 50% f s process was about 10% lower than the strength of that from the 0% f s process, 498 MPa. After solid solution and aging treatment, the bending strength of the composites in both 0% and 50% f s process was decreased. This might be caused by the notch effect of Mg2Si compound produced between SiO2 binder of the preform and magnesium during heat treatment.