简介概要

Microstructure and hardness of Mg-based composites reinforced with Mg₂Si particles

来源期刊：Rare Metals2009年第4期

论文作者：Mustafa Aydin Cem zgür Osman San

文章页码：396 - 400

摘要：Magnesium powders were mechanically alloyed with SiO2 powder particles having different particle sizes using high-energy ball milling techniques under Ar atmosphere for 1 h. The powders were consolidated with cold pressing under 560 MPa. They were then sintered at 550°C for 45 min under Ar atmosphere. The composites obtained on the Mg-SiO2 system were investigated using the Archimedes principle, a differential scanning calorimeter, X-ray diffraction, optic microscopy, and scanning electron microscopy. For the mechanically alloyed powders, the solid-state reaction of the synthesis of Mg2Si and MgO progressed further during sintering of the materials. The results showed that the strengthening mechanisms were dependent on dispersion hardening of fine Mg2Si and MgO particulates dispersed homogeneously in the matrix.

Rare Metals 2009,28(04),396-400

Microstructure and hardness of Mg-based composites reinforced with Mg₂Si particles

Mustafa Aydin Cem zgür Osman San

Mechanical Engineering Department, Dumlupinar University

Ceramic Engineering Department, Dumlupinar University

作者简介：Mustafa Aydin E-mail: m_aydin@dumlupinar.edu.tr;

收稿日期：21 January 2009

Microstructure and hardness of Mg-based composites reinforced with Mg₂Si particles

Abstract：

Magnesium powders were mechanically alloyed with SiO2 powder particles having different particle sizes using high-energy ball milling techniques under Ar atmosphere for 1 h. The powders were consolidated with cold pressing under 560 MPa. They were then sintered at 550°C for 45 min under Ar atmosphere. The composites obtained on the Mg-SiO2 system were investigated using the Archimedes principle, a differential scanning calorimeter, X-ray diffraction, optic microscopy, and scanning electron microscopy. For the mechanically alloyed powders, the solid-state reaction of the synthesis of Mg2Si and MgO progressed further during sintering of the materials. The results showed that the strengthening mechanisms were dependent on dispersion hardening of fine Mg2Si and MgO particulates dispersed homogeneously in the matrix.

Keyword：

Mg-based composites; SiO2; mechanical alloying; strengthening mechanism;

Received： 21 January 2009

1. Introduction

In recent years,there has been increasing use of light-weight materials as structural materials in engineering applications such as in the automobile,aerospace,and electronic industries [ 1, 2, 3] .Magnesium and its alloys are most effective for reducing the weight of components [ 4, 5] .The density of magnesium is 35%less than that of aluminum and 77%less than that of steel.However,these alloys have limited use in the aircraft and automotive industries due to their deficient mechanical properties [ 3, 4] .

A comparative survey reveals that the number of investigations on these materials is low compared to that on Al-MMC [ 6] .The production of Mg-Si O₂ composites by conventional methods such as ingot metallurgy and casting usually results in an inhomogeneous microstructure with coarse grains due to a large difference in the melting points between magnesium(650°C)and silicon(1713°C) [ 5] .

Recently,it has been shown that Mg and its alloys containing Mg₂Si particles have high potential because Mg₂Si exhibits low density and high hardness [ 7] .The major disadvantage of Mg₂Si is its brittleness [ 1] .Unfortunately,however,the composites even have a lower ductility than those obtained from the casting method [ 8] .The current mechanical properties of this composite material can be improved by forming Mg₂Si and MgO phases within the magnesium matrix as intermetallic phases [ 5, 9] .When Mg₂S and MgO are formed as the intermetallic phase in a magnesium matrix,the mechanical properties of the composites can be enhanced.Akihiro et al. [ 10] reported that pure magnesium powder and Si O₂ powder were mechanically alloyed using a planetary ball mill under Ar atmosphere This work indicated that such a solid-state reaction progressed further during pressing and heating of the P/M materials,and the hardness of the composites increased.Thus Mg-Si O₂ P/M materials can be strengthened by in-situ formation of Mg O and Mg₂Si.Thus,the optimization of al parameters such as volume fraction,interparticle distance and particle distribution is effective in improving the properties of these materials,although it remains a major problem [ 6] .The optimization of these parameters would increase the industrial application of magnesium and its alloys.It is known that the mechanical properties of the Mg-SiO₂ composite can be improved by refining the grains and homogeneous distribution of particles in the matrix [ 1, 5] .In the literature,there are few studies on the effect of particle size on Mg-Si O₂ composites.

The aim of this work is to investigate the influence of the reinforced particle size on the microstructural changes and mechanical properties of the Mg-based MMC.For this purpose,pure Mg matrix is reinforced with Si O₂ particles by mechanical alloying for the formation of the Mg₂Si and Mg O intermetallic phases.

2. Experimental

Pure Mg powder(>99%)and SiO₂powders with different particle sizes were used as the starting materials.The median particle size of Mg was about 40?m,and the sizes of the Si O₂ powders were 3,45,100,190,and 375?m.The production route of magnesium composites is given as a schematic in Fig.1.Five different composite powder mixtures were prepared with the addition of 1 wt.%stearic acid.Then the mixed powders were pressed and sintered.The ball milling was carried out for 1 h in a vial cup with alumina balls at a speed of 1000 r/min.The Mg-5wt.%SiO₂ powders were balanced in the glove-box and mixed using a Turbola Spex 8000 high energy ball milling machine,with ball to powder weight ratio of 5:1.

Fig.1.Schematic illustration for production processing of Mg-SiO₂ composites.

After ball milling,the powders were cold compacted using the hydraulic press to a size of 30-mm diameter and5-mm height under 560 MPa pressure.Diffraction scanning calorimeter analyses were carried out for determination of the sintering temperature with a Perkin Elmer diamond diffraction scanning calorimeter machine.The phases,microstructures and morphology of the powders were characterized by X-ray diffraction on a diffractometer(Rigaku mini flex)with Cu K_αradiation,scanning electron microscopy before and after sintering by the FESEM(Zeiss Supra 50 VP)and optic microscopy using Nikon ECLIPSE L 150,respectively.Densities and porosities of Mg-Si O₂ composite specimens were determined by the Archimedes principle.The X-ray diffraction analysis of compacted samples was carried out for determination of the effect of mechanical ball milling before and after the mixing process.

3. Results and discussion

3.1. Structural evolution

A differential scanning calorimeter was employed to determine the sintering temperature.The sintering temperature was affected by the formation of Mg₂Si and Mg O phases.Fig.2(a)shows the differential scanning calorimeter thermograms of Mg-SiO₂ before milling and Fig.2(b)shows the same after milling for 1 h.Before milling,there is no exothermic peak on the differential scanning calorimeter graph in Fig.2(a).However,after milling for 1 h,there is a sharp and distinct exothermic peak at around 450-550°C.In Fig.2(b),this exothermic peak was clearly ascribed to the solid-state reaction of Mg to SiO₂ to form Mg₂Si.In differential scanning calorimeter analyses for 3,45,100,190,and375?m,the same exothermic peaks on the differential scanning calorimeter curves are also seen for all of the milled powders,because during the milling process the sizes of powders in composite were also decreased and became similar for all the samples.

Fig.2.Differential scanning calorimeter curves for the un-milled powder(a)and for the Mg-SiO₂ powders milled for 1 h(Mg-5wt.%Si O₂ for 3?m)(b).

X-ray diffraction analyses for the high-energy ball milled samples were carried out to determine the solid-state reaction on the effect of the formation of Mg₂Si before and after sintering.There are no Mg₂Si or Mg₂Si/Mg O phases(Fig.3(a))before sintering.Hence,it can be concluded that the Mg₂Si-Mg O phases are not formed by mechanical activation However,mechanical activation accelerates the formation of the Mg₂Si phase.Typical X-ray diffraction spectra of the mechanically alloyed and sintered composite samples are shown in Fig.3.The pattern(a)in Fig.3 shows that Mg₂Si and other phases were not formed before sintering.Those phases were also not detected in other samples that have different SiO₂particle sizes.But after sintering at 550°C for all the specimens,the SiO₂ phase could not be detected.That meant that all the Si O₂particles reacted with the Mg particles and occurred in new phases as Mg₂Si and MgO by sintering,and they were dispersed homogenously in the Mg matrix.For all particle sizes the results are almost the same as shown by the patterns(b)-(d)in Fig.3.Formation of Mg₂Si phases was detected in the specimen after milling for1 h and sintering at 550°C for 45 min.It is obvious tha Mg₂Si and MgO do not form during the mechanical milling process,implying that the formation of Mg₂Si and MgO was accomplished after the sintering process.

Fig.3.X-ray diffraction patterns of the mechanically milled Mg-SiO₂ powders for 45 min in different particle sizes,before sinter-ing(a),and after sintering at 550°C for 1 h with SiO₂ particle sizes of 100?m(b),3?m(c),and 375?m(d).

The porosities of composite materials were measured based on the Archimedes principle and the results are given in Fig.4.According to Fig.4,the 3-?m particle-reinforced composite has a higher density than that reinforced with375-?m particles.The highest density was obtained when the particle size of the reinforcement material was 46?m,which is close to the particle size of the matrix material.The measurements revealed that when the particle size of the reinforcement material is not close to the particle size of the matrix,the porosities of materials increase.

Fig.4.Porosities of Mg-Si O₂ composite samples measured according to the Archimedes principle.

At the end of the process in Fig.1,it is seen that the Mg₂Si phase was formed after the sintering.Si O₂ particles are supplied as starting material to synthesize Mg₂Si and Mg O via the deoxidization by magnesium.These occur based on the following equations:

3.2. Microstructure

Fig.5 shows the microstructures of the specimens employing Mg-5wt.%SiO₂(the particle sizes of SiO₂ are 3 and190?m,respectively)after mechanically milling for 1 h,pressing at 560 MPa and sintering for 45 min at 550°C under argon atmosphere inside a tube furnace.Fig.5 also reveals that particles in the optic microstructure are homogenously distributed within the matrix and the mean particle size is almost the same.Fig.5 clearly shows Mg₂Si phases occurring within the Mg matrix.The presence of these phases supports the X-ray diffraction analysis.Thus,these phases will help improve the mechanical properties of the produced composite material.

Fig.5.Microstructures of the Mg-Si O₂composite specimens of Mg-5wt.%Si O₂:the particle sizes of Si O₂ are 3?m(a)and190?m(b),respectively.

Scanning electron microscopy observations on the obtained composites were carried out to confirm X-ray diffraction results.As seen from the scanning electron microscopy microstructures in Fig.6,agglomeration occurs in the 3-?m particle-reinforced composite material,and there is both reduction in particle size and increase in porosity in the375-?m particle-reinforced composite material.Lim et al. [ 11] reported that the fine particulate size is able to suppress the occurrence of agglomeration.Fig.6 shows that the increase in the reinforcement particle size also increased the particle size of the emergent Mg₂Si phase.However,this increase in Mg₂Si phase size led to an increase in the distance among the particles of this phase formed within the matrix.Thus,the mechanical properties of the composite material would decrease.As a result,the porosity would negatively affect the mechanical properties of the composites.As shown in Fig.3,despite the increase in particle size,it is clear from X-ray diffraction peaks that ratios of Mg₂Si and Mg O phases occurring within the matrix do not change.However,as shown in the scanning electron microscopy microstructure(Fig.6),porosities and gaps are increased.In Fig.6,agglomeration in the sample with 3?m particle sizes is greater than that in the sample with 45?m particle sizes.

Mg₂Si and Mg O phases occurring during the sintering process affect the mechanical properties of the composite material.These phases homogenously distributed within the matrix improve the mechanical properties of the composite with the help of a dispersion hardening mechanism.The particle size control of the SiO₂ powders in the green compacts is very important for producing magnesium composites with fine Mg₂Si or Mg₂Si/Mg O dispersions.Fig.7 illustrates microstructures of conventional Mg alloys(Fig.7(a))and Mg-SiO₂ composite material(Fig.7(b)).Kondoh et al [ 12] and Wang et al. [ 9] indicated that the uniform distribution of in-situ formed Mg₂Si contributes not only to the improvement of mechanical properties but also to the corrosion and wear resistance of the magnesium composite.As indicated by other studies,these phases can improve wear properties [ 7] and corrosion resistance of the composite [ 4] ;the solid-state reaction progressed further during pressing and heating of the P/M materials and hardness of composite parts was increased.As shown in Fig.7(b),the Mg-Si O₂P/M materials can be strengthened by in-situ formation of Mg O and Mg₂Si into the Mg matrix.

Fig.6.Scanning electron microscopy microstructures of Mg-Si O₂ composite samples with different Si O₂ particle sizes:(a)3?m;(b)375?m.

Fig.7.Illustration of the microstructure of conventional Mg alloys(a)and Mg-SiO₂ composite with fine Mg₂Si/Mg O dispersoids(b).

3.3. Microhardness test

The microhardness of pure Mg and Mg-SiO₂composites after sintering are shown in Fig.8.As shown in the Figure all of the SiO₂ reinforced composite materials have a higher microhardness value than the pure Mg material.Compared with pure Mg,the hardness of the composite with the distribution of the Mg₂Si and Mg O phases into the Mg matrix was increased.As shown in Fig.3(XRD patterns),the Mg₂Si and MgO phase intensity was not increased with the increment of particle size.However,the hardness of the composite decreased due to the increase in porosity of the composite material.

Fig.8.Microhardness(HV)of pure Mg and Mg-SiO₂ MMC parts after sintering(experimental conditions:0.49 N;10 s).

4. Conclusion

In this study,pure Mg and Si O₂ powders with differen particle sizes were mixed by mechanical alloying to form Mg₂Si and MgO intermetallic phases.The desired intermetallic phases were formed due to the combination of mechanical alloying and sintering processes.The phases did not form during only mechanical alloying or sintering.The particle size of the SiO₂ powders has no significant effect on the formation of the Mg₂Si and MgO phases.However,it is important for producing the Mg composites with fine and homogeneously dispersed reinforcement phases.Further,the increase in particle sizes of SiO₂ led to an increase in porosity of the composite material.Therefore,the hardness of the composite decreased.It can be concluded that approximation of powder size ratio of matrix material to that of reinforcement element further improves the properties of the produced composite material