Role of segregation and precipitates on interfacial strengthening mechanisms in SiC reinforced aluminium alloy when subjected to thermomechanical processing

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Abstract

The satisfactory performance of metal matrix composites depends critically on their integrity, the heart of which is the quality of the matrix-reinforcement interface. The nature of the interface depends in turn on the processing of the metal matrix composite component. At the micro-level the development of local concentration gradients around the reinforcement, as the metal matrix attempts to deform during processing, can be very different to the nominal conditions and play a crucial role in important microstructural events such as segregation and precipitation at the matrix-reinforcement interface. These events dominate the cohesive strength and subsequent mechanical properties of the interface. The compositional variations at the matrix-reinforcement interface of a metal matrix composite are reported, emphasising the interfacial strengthening mechanisms during thermomechanical processing. A method of calculation has been applied to predict the interfacial fracture strength of aluminium and SiC interface, in the presence of magnesium segregation.

Introduction

Metal matrix composites are rapidly becoming strong candidates as structural materials for many high temperature and aerospace applications. Metal matrix composites combine metallic properties with ceramic properties, leading to high stiffness and strength with a reduction in structural weight. The main objective of using a metal matrix composite system is to increase service temperature or specific mechanical properties of structural components by replacing existing superalloys.

At present the relationship between the strength properties of metal matrix composites and the details of the thermomechanical forming processes is not well understood. The purpose of this study is to define the features which significantly affect the interfacial strength of a practical aluminium alloy/silicon carbide system and which are directly related to the forming processes currently being used by the industry. Interfacial segregation takes place by two mechanisms: equilibrium and non-equilibrium types [1]. Equilibrium segregation occurs as a result of impurity atoms relaxing in disordered sites found at interfaces such as grain boundaries. The process has been discussed and well quantified by Hondros and Seah [2] and Seah [3]. Non-equilibrium segregation arises because of imbalances in point defect concentrations set up around interfaces during non-equilibrium heat treatment processing, e.g., fast cooling. The mechanism was proposed by Aust et al. [4] and has been described and quantified by Karlsson et al. [5], Faulkner [6] and Song et al. [7].

Studies of the relationship between interfacial cohesive strength and structure have only recently become possible. This is due to remarkable advances in physical examination techniques allowing direct viewing of interface structure [8], [9], and also considerably improved theoretical treatments of grain boundary structure [10]. Recent advances relating the strength of boundaries to structure have been made by Lim and Watanabe [11] and more recently successful attempts have been made to combine models of precipitate growth at interfaces with concurrently occurring segregation in aluminium alloys of the kind studied in this project [12], [13], [14].

Section snippets

Materials

The metal matrix composite studied was an aluminium–zinc–magnesium–copper (AlZnMgCu) alloy matrix (N707) reinforced with varying amounts of silicon carbide particles of F600 grit, which has a mean diameter of approximately 10 μm.

AlZnMgCu alloy is heat treatable and is produced by spray deposition technology and in the peak aged temper T6 condition shows very high strength at room temperature without a significant loss in ductility. The AlZnMgCu alloy obtains its excellent properties from a very

Experimental

The preparation of the sample for the transmission electron microscope (TEM) was a crucial stage. The specimens, 3 mm in diameter and 1 mm thick (the tube wall thickness), were cut by spark erosion with a 3 mm diameter brass anode tube. These discs were thinned by grinding on both sides down to a thickness of 70–80 μm by using 500 and 600 grit grinding paper strips. The discs were continuously checked with a micrometer to ensure the correct thickness for subsequent polishing. The electropolishing

Results

The transmission and scanning electron microscope observations and scanning transmission electron microanalysis results are summarised in this section.

Fig. 1 presents the general structure of the spray deposited and as-extruded AlZnMgCu alloy. The large precipitates appearing in the structure are MgZn2, typical of Al–Zn–Mg alloys. Scanning transmission electron microanalysis of the fine precipitate dispersion showed that Fe2Al3 precipitates were also present in the structure, but MgZn2

Discussion

The basic idea relating interfacial fracture to the segregated state of an interface was proposed in 1979 by McMahon and Vitek [15]. They showed that the small changes in interfacial energy brought about by segregation can be translated into an effective work parameter which can be used to predict interfacial fracture strength using conventional Griffith crack type arguments. The reasoning employs the idea that certain amounts of plastic deformation are involved with crack propagation along an

Conclusions

The results demonstrate that compositional variations over 100–300 nm can be reliably measured. The presence of zinc depletion and magnesium segregation at a matrix-reinforcement interface have been measured. A method of calculation has been applied to predict the interfacial fracture strength of aluminium, in the presence of magnesium segregation. The model shows success in making prediction possible of trends in relation to segregation and intergranular fracture strength behaviour in metallic

Acknowledgements

The research was supported by the following departments and their universities: School of Engineering, Sheffield Hallam University; IPTME, Loughborough University of Technology; and Department of Engineering, University of Leicester. The material was kindly supplied by Alcan International Ltd.

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