ECAP consolidation of Al matrix composites reinforced with in-situ γ-Al₂O₃ nanoparticles

Abstract

This work is aimed at proposing a method to prepare aluminum matrix composites reinforced with γ-Al₂O₃ nanoparticles and at describing the effects of an in-situ reaction on the resulting nano-reinforcement dispersed throughout the metal matrix. Al nano- and micro-particles were used as starting materials. They were consolidated by equal channel angular pressing (ECAP) in as-received conditions and after undergoing high-energy ball milling. Further, γ-Al₂O₃ reinforcing nanoparticles were produced in-situ from the hydroxide layer that covered the Al powder particles. The powder particle morphology and the composites microstructures were investigated by electron microscopy. The transformation process was monitored by X-ray diffraction, differential scanning calorimetry and thermo-gravimetric analysis.

Introduction

Metal matrix nanocomposites (MMnCs) are considered as potential materials for uses in a large number of industrial applications. They provide opportunities to design lightweight structures with tailored balance of mechanical and physical properties, improvement of tribological performance and high temperature strength [1], [2], [3]. The reinforcement nanoparticles (NPs) are generally stable at elevated temperatures, making these materials suitable for high temperature applications [4], [5]. Thanks to the very small size, the nano-fillers are able to obstruct the movement of dislocations, enabling the Orowan strengthening mechanism [6], [7], [8].

Notwithstanding the prospective properties of MMnCs, some aspects of their processing need to be improved. Their fabrication is much more complicated than that of conventional composites reinforced with fibers or micro-sized particles. When the particles scale down from the micro- to the nano-level sizes, new challenges have to be faced. In particular, the high surface energy of NPs readily leads to the formation of clusters which are not effective in hindering the movement of dislocations and result in poor bond with the matrix, thus reducing significantly the strengthening capability of nanoparticles. A number of novel processing routes have been proposed for the synthesis of MMnCs [1], [2], [3]. The methods are based either on powder sintering or on liquid processing. Consolidation of particles, generally preceded by high-energy ball milling, can be carried out either by conventional techniques such as hot isostatic pressing and cold isostatic pressing followed by heat treatment or by plastic deformation including equal channel angular pressing (ECAP) and hot extrusion [9], [10], [11], [12], [13], [14], [15], [16], [17]. Among the liquid processes, promising results have been achieved by ultrasonic assisted casting [18], [19], [20]. Both liquid and solid synthesis methods can be categorized into two classes: the ex-situ and the in-situ synthesis routes. The former refers to those processes in which the nano-reinforcement is added to liquid or powder metal whereas the latter refers to those methods that lead to the formation of nano-sized reinforcement during the process itself, e.g. through reacting gases [1], [2], [3].

ECAP is one of the most effective severe plastic deformation techniques, being able to produce metallic materials with ultra-fine grained (UFG) structure. It relies on the introduction of extremely high shear strain during the deformation. Moreover, it has also been used for powder consolidation [21]. Traditional sintering methods are based on diffusion of atoms at the solid state. In contrast, ECAP consolidation mainly relies on very high plastic deformation (especially shear strains) [22]. The high deformation breaks the brittle surface oxide film that covers the particles and leads to direct contact between fresh metal surfaces so that bonding between the particles can occur instantaneously [22]. Fully dense bulk materials are produced thanks to the material flow accompanying the deformation, which is believed to be able to close the pores between the particles [23]. The application of a back-pressure (BP) is very useful during ECAP consolidation of powdered metals at low temperatures. With the application of a BP, the particles are pushed against each other, generating more friction when passing the shear zone and causing the particles to deform more efficiently instead of just sliding over each other [22].

In this work, Al nano- and micro-particles were employed to prepare, by back-pressure ECAP, Al matrix nanocomposites reinforced with in-situ γ-Al₂O₃ NPs. The pure Al powder was consolidated in the as-received condition and after ball-milling. A milled mixture of Al nano- and micro-powder was also used. The in-situ γ-Al₂O₃ NPs were produced by exploiting the oxide/hydroxide layer that covers the Al particles. The Al nano-sized particles possess higher surface than the micro-sized counterpart. This means that they can potentially lead to the production of nanocomposites reinforced with a much higher content of in-situ reinforcement. The morphology of the Al particles, before and after mechanical milling, and the microstructures of the composites were investigated by electron microscopy and X-ray diffraction. The mechanical properties of the final composites were evaluated by hardness and compression tests. Particular attention is given to the study of the in-situ phase transformation of the oxide layer into γ-Al₂O₃ NPs, using XRD, differential scanning calorimetry (DSC), and thermo-gravimetric (TG) analysis.