Microstructure and mechanical properties of 6061 Al alloy based composites with SiC nanoparticles

Highlights

• PM 6061 Al alloy MMCs containing 500 nm SiC particles were prepared.

• Industrial high energy ball milling and HIP as primary process was used.

• SiC nanoparticles were homogeneously distributed after extrusion.

• Improvements in Young’s modulus, strength and toughness were obtained.

• Heat treatment produces a higher increment in the MMC strength the higher the vol% nSiC.

Abstract

Materials with high specific strengths as well as damage tolerance are of great importance for automotive and aerospace applications. Ceramic reinforced metal matrix composites (MMCs) show good potential for these uses but have been hampered by insufficient ductility and production issues, both of which this work looks to resolve. Nanoparticle reinforced 6061 aluminium alloy matrix composites have been produced by a powder metallurgy route and shown to exhibit high strength and Young’s modulus alongside good ductility and low density.

A powder metallurgy route consisting of high energy ball milling, hot isostatic pressing (HIP) and extrusion has proved a highly effective process for achieving a homogeneous distribution of particles, with minimal clustering of the nanoparticles, at an industrially relevant scale. After heat treatment the composites display high strengths, owing to SiC nanoparticle reinforcement as well as the age hardening effect. The remarkable feature of nanoparticle reinforced MMCs compared to micron size reinforcements is that particle fracture does not occur and effective particle–matrix bonding can be taking place, resulting in a greater combination of strength and toughness.

The combination of properties achieved by the composites studied in this work are superior to most of the micron sized particle reinforced MMCs reported elsewhere and are well beyond what is possible with traditional aluminium alloys.

Introduction

Particle reinforced metal matrix composites (MMCs) are attractive materials for automotive and aeronautic applications due to their high strengths and low densities. They are also interesting for their high temperature behavior, good creep and wear resistance, higher stiffness than Al alloys and ageing response [1], [2], [3].

Particle containing MMCs have significant benefits in processability over continuous fiber MMCs but tend to have lower ductility. The majority of studies have concentrated on the mechanical properties of Al alloy composites reinforced with micro-sized particles [4], [5]. The effects of nano-sized reinforcements on the mechanical properties have not been thoroughly studied [6]. This class of MMCs has been shown to realise high strengths alongside respectable ductility, but are reliant on homogeneous microstructures [7] and are more complicated to process, typically using powder metallurgy (PM) techniques [8].

Aluminium alloys are regularly chosen as a matrix because of their low density, good isotropic mechanical properties, excellent corrosion resistance and reasonable cost. Amongst aluminium alloys, 6061 is an Al–Mg–Si alloy widely used for structural applications due to its good strength, weldability, corrosion resistance, immunity to stress corrosion cracking as well as heat treatability, forming precipitates that increase the strength at the cost of somewhat reduced ductility [9], [10]. SiC is commonly chosen as a reinforcement phase due to its suitable properties, such as high strength, Young’s modulus, thermal shock resistance, and because it can form a strong bond to aluminium [11]. The matrix–particle interface strength is important as it governs the efficiency of load transfer, affecting the strengthening, as well as the ease of decohesion, which has implications for the composite’s failure mechanism [12]. The formation of Al₄C₃ in Al/SiC composites degrades the particles and the fibers, and results in composites with poor mechanical properties. Therefore, during the composite fabrication, three primary techniques have been employed for controlling the deleterious interfacial reactions: (1) modification of matrix chemical composition: for example, Si was added into the aluminum alloy matrix, in order to hinder the above interfacial reaction [13]; (2) surface reinforcement modification: the surface reinforcement modification by coating or passive oxidation has been successful to some extent in preventing the detrimental interfacial reaction and enhancing the materials wettability [14], [15]; (3) processing parameter is controlled so that the extent of the interfacial reaction can be limited. Examples are controlling the processing temperature and holding time during composite fabrication. Thereby, the various composite processing methods such as compocasting, squeeze casting, semisolid forming, spray forming, and powder metallurgy can be used for the composite fabrication [8], [16], [17].

The two main parameters related to the reinforcement particles are the volume fraction and size of the reinforcing particles. With increasing particles’ volume fraction the strength is improved due to a greater number of dislocation barriers but at the cost of reduced ductility, as deformation is localised on a smaller volume of the plastic matrix which is then less able to accommodate the deformation [18]. An increase in strength can be obtained with decreasing particle size, owing to a greater number of particles for the same volume fraction, whilst at the same time ductility is preserved because, below a critical size, particles no longer fracture [4]. As well as Orowan strengthening, particles pin grain boundaries, stabilise substructure cells, can accelerate the ageing response and increase the work hardening rate [10], [19]. Particularly, an accelerated ageing process was observed in Al alloy matrix composites reinforced with SiC particles due to the thermal expansion coefficient (CTE) differences between the Al matrix and the SiC reinforcement [20].The present work aims to develop high-strength, ductile and low density composites for structural applications. It focuses on novel Al 6061 alloy composites reinforced with SiC produced by means of a powder metallurgy process using SiC nanoparticles (<500 nm) and a low temperature ageing treatment.