Microstructure and mechanical properties of in-situ nano γ-Al₂O_3p/A356 aluminum matrix composite

Highlights

• Co₃O₄ power can react with Al to produce nanometer γ-Al₂O₃ particles in A356 melt.

• The produced Al₂O₃ and cobalt can modify Si phase and refine α-Al dendrite.

• The best addition amount of γ-Al₂O₃ is 0.6 vol% and cobalt is 0.57%.

• The UTS increases by 43.6% and the El increases by 85.7% compared with A356 matrix.

Abstract

In this study, the nanometer in-situ γ-Al₂O₃ particles reinforced aluminum matrix composites was successfully fabricated by the A356 aluminum alloy and Co₃O₄ powder at 850 °C by means of in-situ reaction in the high temperature melt. The effects of γ-Al₂O₃ particle and cobalt element on the microstructure and properties of the composites were investigated in detail. The endogenous γ-Al₂O₃ and cobalt could modify the coarse plate like eutectic Si into fine fibrous like and refine the second dendrite arm spacing (SDAS) of the grains simultaneously. When the content of cobalt was lower than 0.57% and content of γ-Al₂O₃particles was lower than 0.6 vol%, the particles distributed uniformly in the grain, however, when the particulate amount was up to 0.8 vol%, the particles would distribute at the grains boundaries or form as a rodlike phase. The grain size was sharply refined to 200 μm when the particulate content was 0.8 vol%. It was found that the ultimate tensile strength (UTS), elongation of the 0.6 vol% γ-Al₂O₃/A356 composites had arrived at the maximum that had enhanced by 43.6% and 85.7% in comparison to the matrix respectively, which were attributed to the homogeneous distribution of the nanoparticles, the refinement of the SDAS of the α-Al grains and the modification of the eutectic Si phases.

Introduction

Since the beginning of the 21st century, people have started to pay attention to the automobile lightweight, due to the critical problems as climate change and energy crisis. Usage of MMCs(Metal matrix composites), such as aluminum alloy composites and magnesium alloy matrix composites, has been enormously increasing because of its high strength to weight ratio, light density and high specific stiffness [[1], [2], [3], [4]]. In comparison to the pure metal and alloy, MMCs possess a higher strength to weight ratio, wear resistance, lower shrinkage, lower coefficient of thermal expansion, enhanced mechanical and thermal properties [[5], [6], [7], [8], [9]]. MMCs have become really attractive for automotive and aerospace applications. The particle form reinforcement presents in the composites exhibits an isotropic property when compared with other geometries of reinforcement such as fiber and whisker. Fiber and whisker composites are generally anisotropic. Besides this, the advantages of particulate composites further include mechanical strength improvement, operating temperature increase, oxidation resistance enhancement, easy production and so on [[10], [11], [12]].

In conventional fabrication processes of AMCs (Aluminum Matrix Composites), the reinforcements have been added in the two modes of external or in-situ and formed as ex-situ or in-situ composites. A variety of hard ceramic particles have been introduced to be the reinforcements, such as SiC [13], TiC [14], ZrB₂ [4,[7], [8], [9], [10],15], TiB₂ [16], AlN [17] and Al₂O₃ [3,[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]], some of which also can act as the nucleating sites for grain refinement, and as the propeller in strength, hardness, and wear resistance [12]. These reinforcements have exhibited several classic characteristics as high melting points, high hardness, high modulus of elasticity, and low coefficient of thermal expansion.

As for the efficiency and productivity of the ex/in situ methods, there are some defects and difficulties to fabricate composites by ex-situ methods. For example, the particulate morphology, is difficult to be controlled at nanoscale, the fine particles are difficult to add, the interfacial reactions are likely to occur between the reinforcement and the matrix during the fabrication process [23]. The weak matrix/reinforcement interface, inhomogeneous distribution of reinforcement particles, low wettability between particle and matrix, in most cases, make a deadly effect on material properties. In the in-situ process, the reinforcement phases are formed in metallic matrix through the salt reaction or oxidation-reduction reaction between the additive particles and matrix during composite fabrication process. There are many advantages for AMCs synthesized by in-situ process. For example, the in-situ formed reinforcements are thermodynamically stable, free of surface contamination and homogeneous distribution in matrix and strong particle–matrix bonding.

For aluminum matrix composites, the problem of reinforced particles agglomeration has always existed since the invention of composite and is difficult to be solved. The agglomeration phenomenon is affected by many factors. The most important factors are the particulate sizes and the wettability of the interface between the particles and the matrix. When the particulate size is less than eighty percent of critical radius, the particle can be engulfed by the grain during the solidification process [24]. The approaches to improving the wettability included: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy [24]. Till to now, Mg element can decrease the Al alloy surface tension and the effect of Si element is not as effective as Mg [25]. Therefore, A356 was selected as the matrix alloy in this study because of its components.

In the previous study, it was found that many metallic oxides CuO, SnO₂, ZnO, Cr₂O₃ had been used for producing Al₂O₃ [26]. They found that in order to produce adequate Al₂O₃ the metal produced frequently exceeded the dissolution limit of the metal in aluminum [20]. Thus superfluous amount of intermetallic compounds such as CuAl₂ will form in matrix. Intermetallic compounds are brittle phrase, which often result in the decline of mechanical properties. Most of the metal oxides can react with aluminum by means of oxidation-reduction reaction, but Co₃O₄ is hopeful to affect the morphology of silicon after being reduced. Therefore, in this study A356 and Co₃O₄ were selected as raw materials to investigate the synergistic effect of in-situ γ-Al₂O₃ and cobalt element on the microstructure and mechanical properties of A356 alloy.

A356 is a hypoeutectic aluminum alloy, consisting of plate-like eutectic silicon and α-aluminum dendrite. With trace additions of certain chemical modifiers, like Na and Sr, the eutectic Si phase can be refined from coarse plate-like to fine fibrous [27]. The morphological change of Si, which is induced by impurity atom modification, can make an improvement of the mechanical properties of Al–Si alloys. S. Lu implied many elements had similar functions as sodium and strontium [28]. Eu [29] and Sc [30] have been proved to be effective modifier, however, others have not been researched, for example cobalt. Layer growth [28] and twin plane re-entrant edge (TPRE) growth [31] are the two postulations, which have been widely accepted by publication, of the growth model of eutectic Si. For the layer growth, the driving force is entropy of fusion per atom [28]. For the twin plane re-entrant edge (TPRE) growth, the re-entrant corners are consisted of the {111} twin planes intersect surfaces of silicon phase [32]. Atoms attach preferentially to the kink sites of steps in layer growth and re-entrant corners in TPRE growth [33]. Therefore, both of the two models can explain the morphology of Si phase. Si crystals grow preferentially in<110> directions, and present anisotropy.

There are two main mechanisms of chemical modification eutectic Si phase in Al–Si alloys which are widely accepted. In the Al–Si alloys without modifier added condition, there are three possible sites for Si phase to nucleate [34]. Almost all of commercial cast aluminum alloys contain the Fe element more or less, the Fe can form intermetallic β-Al₅FeSi with aluminum and silicon during solidification [35]. This is the first site that can be the nucleation site of Si phase. In the former researches, some foundry added phosphorus reacted with aluminum form AlP, which was the second site for Si phase nucleation [36]. During the smelting process, aluminum melt sucked the oxygen in the air to react to form an oxide film. The oxide films presented in front of the solidifying Al dendrites was regarded as the third site that Si may nucleate [37]. For β-Al₅FeSi phase, regarding as the Si nucleation site, the addition of Sr can decrease the forming temperature of β-Al₅FeSi phase and Al-Si phase, result in a significant undercooling of the inter-dendritic liquid compare with the eutectic temperature [36,38]. The eutectic phase presents a refined morphology. For the AlP phase, after the addition of Sr, they will react with Al and Si to form Al₂Si₂Sr which precipitates on the pre-existing AlP compound, result in a condition of that the nucleate site of Si phase decrease, forcing the eutectic Si to nucleate at larger undercooling [36]. Sr and Na can react with P by forming Sr₃P/Na₃P [38,39]. The effect of Sr/Na addition on the two nucleation sites belongs to the constitutional undercooling mechanism.

For the layer growth model, the large impurity atoms displace {111} monolayer atoms to the change the Si atoms stacking sequence to form Si twins. This twin generation mechanism by introducing impurity atoms is named impurity-induced twinning (IIT) mechanism [40]. For the TPRE mechanism, with sodium or strontium addition, Sr/Na atoms concentrate at the re-entrant edges, which remove the growth advantage of the TPRE mechanism, the anisotropic growth habit of silicon on specific crystallographic planes is reduced as well [41]. A recent research by electron back-scattered diffraction(EBSD) revealed that the TPRE growth in unmodified Al–Si alloys was on the <110> direction, and the exposure of Si-{111} planes to the melt to reduce interfacial energy. The microscopic <01$1(\_)$>growth direction is realized by a paired <11$2(\_)$> zigzag growth at atomic scale [42].

The present paper is to fabricate in-situ Al₂O₃ particle-reinforced aluminum matrix composites and investigates the effect of in-situ γ-Al₂O₃ particles and reduced cobalt on the microstructure and mechanical properties of A356 alloy.