Aluminum matrix composites reinforced by in situ Al₂O₃ and Al₃Zr particles fabricated via magnetochemistry reaction

ZHAO Yu-tao(赵玉涛), ZHANG Song-li(张松利), CHEN Gang(陈 刚)

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China

Received 23 October 2009; accepted 16 September 2010

Abstract: Aluminum matrix composites reinforced by in situ Al₂O₃ and Al₃Zr particles are fabricated from A356-Zr(CO₃)₂ system via magnetochemistry reaction, and the morphologies, sizes and distributions of the in situ particles as well as the microstructures, mechanical mechanisms of the composites are investigated by XRD, SEM, TEM and in situ tensile tests. The results indicate that with the pulsed magnetic field assistance, the morphologies of the in situ particles are mainly with ball-shape, the sizes are in nanometer scale and the distributions in the matrix are uniform. The interfaces between the in situ particles and the aluminum matrix are net and no interfacial outgrowth is observed. These are due to the strong vibration induced by the applied magnetic field in the aluminum melt, which in turn, accelerates the melt reactions. The effects of the magnetic field on the above contributions are discussed in detail.

Key words: aluminum matrix composites; magnetochemistry reaction; Al₂O₃; Al₃Zr; in situ tensile

1 Introduction

Particle-reinforcement aluminum metal matrix composites (PRAMMCs) are attractive for the materials scientists to meet the increasing demands of applying advanced structures in aerospace, military, automobile and electronic areas because of their low density, high specific stiffness, strong wear resistance, reduced thermal expansion coefficient and high thermal conductivity, such as aerofoil, motor cylinder body, braking pan[1-5]. PRAMMCs can be fabricated by ex-situ synthesis (e.g. liquid ingot casting and powder metallurgy) where particles are added to the melt as a powder, and also by in-situ synthesis (e.g. exothermic dispersion, reactive hot pressing, reactive infiltration and direct melt reaction) where particles are synthesized within the melt[6-12]. In ex-situ methods, the particles are added into the metal melt from the outside, which induces interface pollution, poor wetting ability and weak properties of the composites. In in-situ process, the reinforcement phases are formed in the matrix metal from chemical reactions or from exothermic reactions between elemental powders of the composite under hot pressing conditions, which are thermodynamically stable, free of surface contamination and disperse more uniformly, leading to stronger particle-matrix bonding. Such unique properties make the in-situ PRAMMCs possess excellent mechanical properties and economical viability than their ex-situ counterparts. In particular, the direct melt reaction (DMR) process is the most promising in-situ synthesis techniques for commercial applications due to its simplicity, low cost and near net-shape forming capability. In the conventional DMR processing, long agitating time at high temperature is necessary for obtaining full incorporation and thorough reaction of the added reactants with the molten metal. These lead to the in situ particles growing up or agglomerating together and more melt being lost, and induce the properties decreasing. In order to shorten the long mixing time at high temperature, some researches have been done[13-15].

In the present work, the aluminum matrix composites reinforced by Al₂O₃ and Al₃Zr particles are fabricated from an A356-Zr(CO₃)₂ system through magnetochemistry reaction, and the morphologies, sizes and distributions of the in situ particles as well as the microstructures, mechanical mechanisms of the composites are investigated by XRD, SEM, TEM and in-situ tensile observations. The effects of the magnetic field on the above contributions are discussed in detail.

2 Experimental

The starting materials were commercial A356 alloy ingots and inorganic salt Zr(CO₃)₂ powder.

Firstly, Zr(CO₃)₂ powders were dehydrated at 250 °C for 3 h in an electric furnace. Then aluminum ingots were heated to 900 °C and melted, the dried Zr(CO₃)₂ powders with the mass fraction of 20% to the total aluminum melt were added and pressed into the aluminum melt with a campanulate graphite mantle. During the in situ reaction, the impulse magnetic field system (WY1600) was turned on and the frequency was fixed at 25 Hz and the magnetic current intensity was 150 A. The parameters of pulsed magnetic field were voltage 550 V, current 10 A, frequency 100 kHz and pulse width 10 ms. After 20 min, the melt was degassed by hexachloroethane degasifying agent, deslagged, and then poured into a mould with 14 mm inner diameter. After being cooled to room temperature in air, the specimens for XRD, SEM and TEM analyses, mechanical properties testing and in situ tension test were processed. The tensile properties are the average values of three tests.

SEM(JOEL-JXA-840) with a microtensile holder was used to investigate the microstructure of the as- prepared specimens and the in-situ tensile test, and TEM(JSM2010) for the morphologies and the interfaces. Tensile properties tests of the specimens with a gauge diameter of 6.35 mm and length of 25.4 mm were carried out at room temperature by a computer-controlled electronic tensile testing machine (DWD-200) at a strain rate of 1.67×10^-4 s^-1 according to the ASTM E8 standard. The Brinell Hardness was determined by THI100 electronic-controller type hardness testing device according to the ASTM E10 standard. The values of the tensile properties and Brinell hardness are the average of three tests at each condition, respectively.

3 Results and discussion

3.1 XRD pattern

Fig.1 shows the XRD pattern of the composites fabricated with pulsed magnetic field assistance. It indicates that Al, Al₂O₃ and Al₃Zr phases are obtained. These mean that some chemical reactions occur in the aluminum melt: the Zr(CO₃)₂ are decomposed to ZrO₂ particles and CO₂ gas. The decomposed products ZrO₂ react with aluminum, and CO₂ gas is partly escaped out of the aluminum melt and the residual is degassed by degasifying agent. Zirconium atom in ZrO₂ is substituted by aluminum atom and Al₂O₃ particles are formed. The substituted zirconium atom in aluminum melt also reacts with aluminum atom, and Al₃Zr particles are formed. The total melt chemical reactions can be expressed as:

3Zr(CO₃)₂+13Al(l)→6CO₂↑+3Al₃Zr+2Al₂O₃      (1)

Fig.1 XRD pattern of composites fabricated with pulsed magnetic field assistance

3.2 SEM images

Fig.2 shows the SEM images of the composites prepared without and with pulsed magnetic field assistance. It can be clearly seen that with the assistance of pulsed magnetic field, the distributions of the in-situ Al₂O₃ and Al₃Zr particles become more uniform, the size gets more smaller, and the amount of the in-situ particles is increased. The size of particles is in the range of 0.08-0.13 μm, which is much finer than that formed without pulsed magnetic field assistance. These are due to the effects of pulsed magnetic field on the in-situ melt chemical reactions.

Fig.2 SEM images of in-situ particles reinforced composites fabricated without pulsed magnetic field assistance (a) and with pulsed magnetic field assistance (b)

3.3 TEM micrographs

Fig.3 shows the TEM micrograph of the Al₃Zr/Al and α-Al₂O₃/Al interfaces and the electron diffraction patterns, respectively. The diffraction patterns of the A, B, C and D regions are determined as Al₃Zr phase, Al phase, α-Al₂O₃ phase and Al phase according to the standard diffraction patterns. Thus, the interfaces between A and B, C and D regions are the Al₃Zr/Al and α-Al₂O₃/Al interfaces, separately. As shown in Figs.3(a) and (b), the interfaces are smooth, round and net. These contribute to the interface bond strength.

3.4 Effects of magnetic field on microstructure of composite

In aluminum melt, when the pulsed magnetic field is applied, the alternating current generates the pulsed magnetic field B in the melt, which in turn, induces the pulsed eddy current J_e in melt according to laws of Faraday electromagnetism induction. Therefore, the aluminum melt is subjected to electromagnetic body forces f caused by the interaction of the induced current J_e and the magnetic filed B. Based on Maxwell’s equations,the electromagnetic forces f, which is a dependent variable to the volume of the aluminum melt, can be described by the following equation:

                    (2)

where μ₀ is the permeability of the aluminum melt, and B the magnetic filed intensity. According to Maxwell equation, Eq.(2) integrated to the volume can be written as:

                (3)

where b is the unit vector in the direction of the impulse magnetic field, n the outer-normal unit vector of the integral unit, θ the separation angle between b and n. The magnetic intensity in the inner of the aluminum melt is generated by the two parts indicated in the right of Eq.(3). The pulsed magnetic field B changes at a high rate. These generate strong vibration in the aluminum melt, which in turn, induces violence convection current. These contribute to the uniform distribution of the reinforcements in the aluminum matrix.

Fig.3 TEM micrographs of Al₃Zr/Al and α-Al₂O₃/Al interfaces and electron diffraction patterns

In the Al-Zr(CO₃)₂ system, the following chemical reaction is important to the formation of Al₂O₃ and Al₃Zr phases:

4Al(l)+3ZrO₂→2Al₂O₃+3[Zr]                  (4)

Reaction (4) can be expressed as:

4Al+3ZrO₂→[3Zr·6O·4Al]→2Al₂O₃+3Zr        (5)

The reaction (5) indicates that ZrO₂ particles are wetted with aluminum molten, and then transformed into intermediate products. After complex chemical transition, the simple substance Zr and Al₂O₃ particles are produced, and promptly diffused out of the reaction transition regions. The Zr reacts with aluminum to form Al₃Zr particles.

When pulsed magnetic field is applied, the practical reaction rate coefficient K can be written as K=K_t+K_m, where K_m comes from the magnetic field contributions. The reaction rate, which is represented by the volume mole concentration of Al₂O₃ particles can be expressed as:

       (6)

Thus, the reaction coefficient is magnified due to the K_m and the chemical reaction is accelerated.

3.5 Mechanical properties

The mechanical properties of the as-prepared composites fabricated with and without the assistance of pulsed magnetic field are listed in Table 1. It indicates that the values of the tensile strength, yield strength and the hardness are increased with the pulsed magnetic field assistance, reaching 393.87 MPa, 339.74 MPa and 139.8 N/mm², respectively, and the values of the elongation show no obvious variations.

Table 1 Mechanical properties of composites fabricated with and without pulsed magnetic field assistance

Fig.4 shows the SEM micrographs of the crack initiating course and the crack growing course for the as-prepared composites. In the initiating stage shown in Figs.4(a)-(c), the white-fine interleaving slipping bands with obvious convex-concave marks are clearly seen. The observed short-tenuous breaches are grown and joined together, which induces to the micro-cracks birth and growth. When the master cracks expand and meet with the rich-particle areas, the extending direction turns to the poor-particle areas, the bottleneck of the cracks is generated, and the crack expanding is continued along the boundary between the particle-rich and the particle-poor areas. With the load increasing, the rich-particle areas are desquamated from the aluminum matrix and the composites are breakdown. Thus, the tensile strength is inactivation.

Fig.5 shows the SEM images of the tensile fracture surface of the as-prepared composites synthesized without and with pulsed magnetic field assistance, respectively. The obvious dimple-shape appearances of the fracture surfaces are seen in the fracture surface. With the pulsed magnetic field assistance, the amounts of the observed dimples increases, and the sizes reduce, comparing to that without pulsed magnetic field assistance. These contribute to the mechanical properties of the composites.

Fig.4 SEM micrographs of crack initiating and growing course for (Al₃Zr+Al₂O₃)/A356 composites

Fig.5 SEM images of tensile fracture surface of as-prepared composites synthesized without (a) and with (b) pulsed magnetic field assistance

4 Conclusions

The aluminum matrix composites reinforced by in situ Al₂O₃ and Al₃Zr particles are fabricated from A356-Zr(CO₃)₂ system via magnetochemistry reaction, and the morphologies, sizes and distributions of the in situ particles as well as the microstructures, mechanical mechanisms of the composites are investigated. The results indicate that with the pulsed magnetic field assistance, the morphologies of the in situ particles are ball-shape, the sizes are in nanometer scale and the distributions are uniform in the matrix. The interfaces between the in situ particles and the aluminum matrix are net and no interfacial outgrowth is observed. The values of the tensile strength, yield strength and the Brinell hardness reach 393.8 MPa, 339.74 MPa and 139.8 N/mm², respectively. The in situ tensile observations show that the cracks are initiated by the dislocation accumulation effects, the cavity-nucleation effects and the matrix fracture effects; the crack growth behaviors are very much dependent on the microstructure of the composites. These are due to the strong vibration induced by the applied magnetic field in the aluminum melt, which in turn, accelerates the melt reactions.

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(Edited by LAI Hai-hui)

Foundation item: Project(2007AA03Z548) supported by High-Tech Research and Development Program of China; Project(50971066) supported by the National Natural Science Foundation of China; Project(1283000349) supported by the Jiangsu University Research Fund for Advanced Scholars, China

Corresponding author: ZHAO Yu-tao; Tel: +86-511-88797658; E-mail: zsl@ujs.edu.cn; zhaoyt@ujs.edu.cn

DOI: 10.1016/S1003-6326(09)60429-5