Formation of Magnesium Silicide by Mechanical Alloying

Elemental Mg and Si powders were mechanically alloyed in a planetary ball mill. The formation of magnesium silicide as well as the formation of magnesium oxide and hydride in the milled powders was studied in detail by X-ray diffraction and scanning differential calorimetry. It was found that direct formation of the magnesium silicide, Mg₂Si, occurred after 10 hours of milling and the content of Mg₂Si increased with increasing the milling duration. The activation energy for the formation of Mg₂Si was calculated by the Kissinger approach to be 215 kJ/mol. Besides oxidizing and hydrizing of Mg by decomposed organic additives during mechanical alloying, an increased contamination of powders from steel and alumina milling tools with increasing milling duration was detected. A short milling duration followed by a thermal treatment was thus suggested to synthesize magnesium silicide.     

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.     

Effects of annealing on the formation of Mg2Si film prepared by resistive thermal evaporation method

The effects of annealing time and temperature on the formation and structure of magnesium silicide (Mg₂Si) films were investigated. Magnesium films of 380 nm thickness were deposited on Si (111) substrates using resistive thermal evaporation method. The films were then annealed in an annealing furnace under a low vacuum atmosphere of 10^?1–10^?2 Pa. The results showed that the crystallization quality of Mg₂Si films was strongly affected by the annealing time and temperature. Annealing at 400 °C for 4 h was the optimal preparation conditions for Mg₂Si films.     

High-resolution elemental maps for three directions of Mg2Si phase in Al-Mg-Si alloy

Elemental maps of the Mg and Si sub-lattices of the Mg₂Si phase in an Al-1.0mass% Mg₂Si alloy were produced using an energy-filtering transmission electron microscope (EFTEM). Low magnification elemental maps were obtained using both low and high energy loss edges, and the intensities of the high energy loss edges were sufficiently high to allow the Mg₂Si phase to be observed at high magnification. High-resolution core-loss images of Mg and Si-K edges were taken parallel to [001], [111] and [110] of the Mg₂Si phase. In the [110] direction, Mg and Si atoms were successfully identified as sub-lattices. The Mg atoms formed a 0.39 nm diamond network, whereas the Si atoms formed a 0.32 nm by 0.22 nm rectangular network. This result is in good agreement with the projected potential of the Mg₂Si phase in the [110] direction. This is the first report of magnesium and silicon atoms in the Mg₂Si phase being successfully identified at the atomic level by EFTEM.     

Thermal stability and thermoelectric properties of Mg2Si0.4Sn0.6 and Mg2Si0.6Sn0.4

Compounds of Mg₂Si_1?x Sn _x are environmentally friendly, inexpensive and high-efficiency thermoelectric materials for energy conversion in the temperature range 300–550 °C. In this study, the thermal stability is investigated of fine powders and sintered pellets of the compounds Mg₂Si_0.4Sn_0.6 and Mg₂Si_0.6Sn_0.4 by heating the samples from room temperature to ~400 °C in air, while measuring powder X-ray diffraction patterns. The diffractograms of the pellets show no significant changes upon heating for several hours, while the powder samples show increasing emergence of a Mg₂Sn-rich, Mg₂Si_1?x Sn _x phase, and other impurities upon heating for only several minutes. This is attributed to the larger amount of surface area in the powder samples. The appearance of the Mg₂Sn-rich phase is most pronounced for the Sn-rich composition. In addition, the thermal expansion coefficients were extracted from the powder diffraction patterns. All materials have been synthesized by induction-melting followed by ball milling and spark plasma sintering. The thermal conductivity, Seebeck coefficient, electrical resistivity and Hall carrier concentrations have been measured from room temperature to 400 °C on the pellets.     

Equilibrium pseudobinary Al–Mg2Si phase diagram

Abstract

Preliminary experiments and phase diagram calculations were conducted to determine the equilibrium phase diagram of the Al–Mg₂Si pseudobinary section. It was found that there is a narrow ternary phase field of Al+Mg₂Si+liquid in the diagram. At the pseudoeutectic composition of Al–13.9 wt-%Mg₂Si, a pseudoeutectic reaction takes place between the temperatures of 583.5 and 594°C. The solubility of Mg₂Si in Al at 583.5°C is calculated as 1.91 wt-%.     

Sintering behavior of mechanically alloyed Ti-48Al-2Nb aluminides

Ti-Al intermetallics have been produced using mechanical alloying technique. A composition of Ti-48Al-2Nb at % powders was mechanically alloyed for various durations of 20, 40, 60, 80 and 100 h. At the early stages of milling, a Ti (Al) solid solution is formed, on further milling the formation of amorphous phase occurs. Traces of TiAl and Ti₃Al were formed with major Ti and Al phases after milling at 40 h and beyond. When further milled, phases of intermetallic compounds like TiAl and Ti₃Al were formed after 80 hours of milling and they also found in 100 h milled powders. The powders milled for different durations were sintered at 785°C in vacuum. The mechanically alloyed powders as well as the sintered compacts were characterized by XRD, FESEM and DTA to determine the phases, crystallite size, microstructures and the influence of sintering over mechanical alloying.     

Mill setting and microstructural evolution during mechanical alloying of Mg2Si

The mechanical alloying behaviour of magnesium and silicon to form the intermetallic compound Mg₂Si and the optimum setting of a planetary ball mill for this task, were examined. For the ductile–brittle magnesium–silicon system it was found that the efficiency of the mill is mostly influenced by the ratio of the angular velocity of the planetary wheel to that of the system wheel and the amount of load. The examination of the kinetics inside the planetary ball mill for different mill settings showed that a ratio of angular velocities of at least 3 is necessary to compensate the reduction of efficiency due to slip. The optimum powder load for the 500 ml vial was found to be 10–20 g. The milling process starts with elemental magnesium and silicon bulk particles. During the milling, the silicon pieces are rapidly diminished and together with the constantly forming Mg₂Si they act as an emery powder for the magnesium bulk pieces. Simultaneous to the diminution of the magnesium, alloying occurs.     

Solid state reaction and formation of nano-phase composite hydrogen storage alloy by mechanical alloying of MmNi3.5(CoMnAl)1.5 and Mg

In this work MmNi_3.5(CoMnAl)_1.5 and Mg were mechanically alloyed to prepare composite hydrogen storage alloys. The microstructural variation of the alloy resulted from the mechanical alloying was characterized by X-ray diffraction, SEM and TEM analysis. It was found that solid state reaction occurred between MmNi_3.5(CoMnAl)_1.5 and Mg components, resulting in Mm₂Mg₁₇ phase formation. The alloy obtained by ball milling contains homogeneously distributed Mg, MmNi_3.5(CoMnAl)_1.5 and Mm₂Mg₁₇ phases of nanometer size. It was also found that Mg₂Ni was formed after the mechanically alloyed samples were annealed. The mechanism of the reaction has been proposed based on the estimation of the heat of formation using Miedema's theory and is in accordance with the experiment result of lattice constant measurement of ball milled sample and its structure variation during annealing.     

Spark plasma sintering and thermoelectric evaluation of nanocrystalline magnesium silicide (Mg2Si)

Recently magnesium silicide (Mg₂Si) has received great interest from thermoelectric (TE) society because of its non-toxicity, environmental friendliness, comparatively high abundance, and low production material cost as compared to other TE systems. It also exhibited promising transport properties, including high electrical conductivity and low thermal conductivity, which improved the overall TE performance (ZT). In this work, Mg₂Si powder was obtained through high energy ball milling under inert atmosphere, starting from commercial magnesium silicide pieces (99.99 %, Alfa Aesar). To maintain fine microstructure of the powder, spark plasma sintering (SPS) process has been used for consolidation. The Mg₂Si powder was filled in a graphite die to perform SPS and the influence of process parameters as temperature, heating rate, holding time and applied pressure on the microstructure, and densification of compacts were studied in detail. The aim of this study is to optimize SPS consolidation parameters for Mg₂Si powder to achieve high density of compacts while maintaining the nanostructure. X-Ray diffraction (XRD) was utilized to investigate the crystalline phase of compacted samples and scanning and transmission electron microscopy (SEM & TEM) coupled with Energy-Dispersive X-ray Analysis (EDX) was used to evaluate the detailed microstructural and chemical composition, respectively. All sintered samples showed compaction density up to 98 %. Temperature dependent TE characteristics of SPS compacted Mg₂Si as thermal conductivity, electrical resistivity, and Seebeck coefficient were measured over the temperature range of RT 600 °C for samples processed at 750 °C, reaching a final ZT of 0.14 at 600 °C.     

Effect of Sr on microstructure,tensile properties and wear behavior of as-cast Mg–6Zn–4Si alloy

The microstructure, tensile properties and wear behavior of as-cast Mg–6Zn–4Si alloy with strontium additions at ambient and elevated temperature were investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), standard high temperature tensile testing and a pin-on-disc type apparatus. The results indicated that the grain size of the primary Mg₂Si decreased initially and then gradually increased with increasing Sr amount. Meanwhile, the morphology of the primary Mg₂Si in the alloys changed from large dendritic to polygonal or fine block, and that of the eutectic Mg₂Si phase turned to fine fibre with increasing Sr content. Tensile testing results showed that Sr addition improved the ultimate tensile and elongation of the Mg–6Zn–4Si alloys at both ambient temperature and 150 °C. Dry sliding wear tests indicated that the change trend of wear rate was basically coincident with that about the average size of the primary Mg₂Si phases. Optimal mechanical properties and wear behavior could be achieved by a Sr addition of 0.5%. An excessive Sr addition resulted in the formation of the needle-like SrMgSi compound, which was detrimental to the tensile properties and wear behavior of the alloys.     

Synthesis of a lightweight Ti–10Al–5Mg (wt.%) alloy by mechanical alloying followed by compaction and sintering

A lightweight Ti–10Al–5Mg (wt.%) alloy was synthesized by mechanical alloying followed by cold compaction and sintering. A metastable hcp α-Ti(Al,Mg) solid solution was obtained after 5 h of milling. The c/a ratio of the hcp structure was found to increase with milling time. The particle size distribution of the mixture becomes broader and the dp50 value decreases as the milling time increases. The structure of the 900 and 1,100 °C sintered samples consist of an α-Ti(Al) solid solution matrix with α₂-Ti₃Al and MgO precipitates. Values of 11 and 188 GPa were obtained for the Hardness and Young’s modulus of 1,100 °C sintered sample, respectively, confirming the strength improvement of the Ti-based alloy.     

Mechanical properties of thermoelectric Mg2Si using molecular dynamics simulations

In this paper, the mechanical properties of thermoelectric Mg₂Si are investigated using molecular dynamics (MD) method, with Mg₂Si in the forms of bulk, nanofilm, and nanowire, respectively. The effects of the Mg vacancy on the mechanical properties of Mg₂Si in these three forms are studied in details. First of all, the equilibrium state of Mg₂Si is simulated after choosing the proper potential function, boundary conditions, and the speed algorithm. Nondimensionalization is also implemented during the simulations. This part of simulation aims to verify the correctness of the crystal model established via the use of the molecular dynamics analysis. Next, the models of Mg₂Si in the forms of bulk, nanofilm, and nanowire are established with different Mg vacancy proportions, and then the mechanical properties of each model are studied via the uniaxial tensile test. Finally, the stress–strain curve and subsequently the ultimate tensile strength are obtained for each model. Simulation results indicate that the ultimate tensile strength of Mg₂Si in each model is decreased with the increase of the Mg vacancy proportion. Moreover, through the comparison of the ultimate tensile strengths of Mg₂Si bulk, nanofilm, and nanowire, it is found that low-dimensionalization significantly reduces the ultimate tensile strength of thermoelectric Mg₂Si. Results obtained in this paper can provide valuable guidance to the future applications of thermoelectric devices.     

Microwave synthesis and sintering of Mg4Nb2O9 nanoceramics

Mg₄Nb₂O₉ nanopowders were prepared from MgO and Nb₂O₅ mixtures by using a high energy ball milling method, combined with subsequent annealing at low temperatures by microwave heating. After milling for 20 h, pure phase Mg₄Nb₂O₉ nanopowders with an average grain size of 127 nm were obtained at 850, 130 °C lower than that required by a conventional solid state reaction process. Mg₄Nb₂O₉ ceramics sintered at a low temperature of 1,300 °C using microwave heating showed almost full density and excellent microwave dielectric properties (ε_r = 12.9, Q × f = 174,200 GHz and τ_f = ?68).     

Nanocrystalline Al/Al12(Fe,V)3Si alloy prepared by mechanical alloying: Synthesis and thermodynamic analysis

Al–Al₁₂(Fe,V)₃Si nanocrystalline alloy was fabricated by mechanical alloying (MA) of Al–11.6Fe–1.3V–2.3Si (wt.%) powder mixture followed by a suitable subsequent annealing process. Structural changes of powder particles during the MA were investigated by X-ray diffraction (XRD). Microstructure of powder particles were characterized using scanning electron microscopy (SEM). Differential scanning calorimeter (DSC) was used to study thermal behavior of the as-milled product. A thermodynamic analysis of the process was performed using the extended Miedema model. This analysis showed that in the Al–11.6Fe–1.3V–2.3Si powder mixture, the thermodynamic driving force for solid solution formation is greater than that for amorphous phase formation. XRD results showed that no intermetallic phase is formed by MA alone. Microstructure of the powder after 60 h of MA consisted of a nanostructured Al-based solid solution, with a crystallite size of 19 nm. After annealing of the as-milled powder at 550 °C for 30 min, the Al₁₂(Fe,V)₃Si intermetallic phase precipitated in the Al matrix. The final alloy obtained by MA and subsequent annealing had a crystallite size of 49 nm and showed a high microhardness value of 249 HV which is higher than that reported for similar alloy obtained by melt spinning and subsequent milling.     

The microstructure and mechanical properties of low-temperature and low-speed extruded Mg–3Sn–2Al–1Zn alloy

ABSTRACT

The microstructure and mechanical properties of the Mg–3Sn–2Al–1Zn (TAZ321) alloy extruded at 180–250°C and at a ram speed of 0.08?mm/s were investigated by optical microscope, X-ray diffraction, scanning electron microscope, transmission electron microscope, tensile testing and microhardness tester. The results indicate that the as-extruded alloy shows ultrafine dynamic recrystallisation grains, nanoscale Mg₂Sn precipitates of 50–80?nm, strong texture and excellent mechanical properties, the degree of which mainly depends on the extrusion temperature. The tensile yield strength and elongation vary from 413?MPa and 6.7% of TAZ321 extruded at 180°C to 312?MPa and 14.1% of TAZ321 extruded at 250°C, which can be regulated flexibly.     

Experimental investigations on energy absorption capacity of selected Al–Si–Mg casting alloys

Three hypoeutectic Fe containing Al–Si–Mg alloys for casting crash relevant automotive components are experimentally investigated. The comparatively short heat treatments include solutionising at 540°C for 5 min or at 465°C for 60 min respectively, compressed air quenching and artificial aging at 223°C for 120 min. Characteristic mechanical parameters are determined by tensile, plate bending and Charpy pendulum impact tests. Intermetallic phases are identified by scanning electron and by light microscopy. The results show that increasing the Mg content promotes the precipitation of Mg₂Si particles, which enhance the strength. Increasing the Fe content promotes the formation of intermetallic Fe bearing particles which reduce the energy absorption capacity and the ductility. Increasing the Si content has the similar effect, since the volume fraction of the eutectic phase and the size of the intermetallic particles increase.     

Transient liquid phase diffusion bonding of Al/Mg2Si metal matrix composite

As-cast Al/Mg₂Si metal matrix composite was joined by transient liquid phase diffusion bonding using Cu interlayer at various bonding temperatures and durations. This metal matrix composite contained 15% Mg₂Si and was produced through in situ technique by gravity casting. Specific diffusion bonding process was applied as a low vacuum technique. The microstructure of joints consisted of Al-α, CuAl₂ and Mg₂Si or Al-α and Mg₂Si depending on bonding temperature and duration. The maximum shear strength was achieved when samples were bonded at 580 °C for 120 min. Micro-hardness and compositional homogeneity of joints across the bonded interface were improved with increasing the bonding duration at 560 °C and had no significant changes at 580 °C.     

A study on the influence of Mo, Al and Si additions on the microstructure of annealed dual phase Cr–Ta alloys

The effects of additions of 5 at.% Mo, Al and Si on the long-term annealed microstructures of a two phase Cr–Cr₂Ta alloy have been studied. Following 200 h at 1300 °C, the lamellar eutectic constituent of all the alloys disintegrated into discrete particles of the Laves phase embedded within a Cr-rich solid solution phase, along with the formation of fine Laves phase precipitates. One of the predominant differences between the three alloying additions was the extent of the C14 to C15 polytypic transformation of the Cr₂Ta-based Laves phase. With Mo and Al additions, the Cr₂Ta Laves phase transformed from C14 to either C15 or intermediate hexagonal polytypes following 200 h annealing at 1300 °C. In contrast, Si additions stabilised the C14 polytype, with no transformation to other polytypes observed after prolonged annealing at 1000, 1100 and 1300 °C.     

Sintering behaviour of Nb–16Si–25Ti–8Hf–2Cr–2Al alloy powder fabricated by a hydrogenation–dehydrogenation method

The present study investigated the sintering behaviour of Nb–16Si–25Ti–8Hf–2Cr–2Al alloy powders. The alloy powders were produced using ahydrogenation–dehydrogenation method and featured an irregular morphology. Powders sintered at 1500°C and 1600°C exhibited pores in their microstructures, while powders sintered at 1700°C for 4?h were fully densified and poreless. The cast ingot and powders were composed of three phases: Nb_ss, Nb₅Si₃, and Nb₃Si. However, the Nb₃Si phase was not observed, while HfO₂ oxides formed in the sintered compact. The hafnium and oxygen reacted to form an HfO₂ oxide during the high-temperature sintering process. From the result of the thermodynamic calculation, Hf oxide formed after sintering because Hf has the highest driving force for oxidation among the elements constituting the alloy.