Sci Eng Compos Mater 2016; 23(1): 1–20 Review Khushbu Dash*, Suvin Sukumaran and Bankim C. Ray The behaviour of aluminium matrix composites under thermal stresses Abstract: The present review work elaborates the behav- 1 Introduction iour of aluminium matrix composites (AMCs) under vari- ous kinds of thermal stresses. AMCs find a number of There has been a rapid growth in the development of applications such as automobile brake systems, cryostats, metal matrix composites (MMCs) since the 1960s due to microprocessor lids, space structures, rocket turbine hous- a variety of properties it offered, such as high specific ing, and fan exit guide vanes in gas turbine engines. These strength, high specific stiffness, low density, good ele- applications require operation at varying temperature vated temperature stability, and good wear resistance [1]. conditions ranging from high to cryogenic temperatures. Their commercial availability has also been increasing The main objective of this paper was to understand the steadily, and as a result, information about the thermal behaviour of AMCs during thermal cycling, under induced properties of these materials is essential for the proper thermal stresses and thermal fatigue. It also focuses on designing of MMC applications [2]. Among the MMCs, the various thermal properties of AMCs such as thermal aluminium matrix composites (AMCs) have drawn con- conductivity and coefficient of thermal expansion (CTE). siderable attention in the academic and industrial sectors CTE mismatch between the reinforcement phase and the recently. Improvement in strength and modulus has been aluminium matrix results in the generation of residual reported for AMCs over conventional metal alloys. This, thermal stress by virtue of fabrication. These thermal along with thermal stability, makes them a good choice for stresses increase with increasing volume fraction of the use as materials in aerospace and transport industries [3]. reinforcement and decrease with increasing interparticle The main property that makes MMCs unique is that they spacing. Thermal cycling enhances plasticity at the inter- offer a large variety of properties by means of combin- face, resulting in deformation at stresses much lower than ing possible combinations of matrices and the reinforce- their yield stress. Low and stable CTE can be achieved ments, which in turn enables them to adapt the material by increasing the volume fraction of the reinforcement. properties according to the application requirements [4]. The thermal fatigue resistance of AMC can be increased Using aluminium alloys as the matrix phase provides a by increasing the reinforcement volume fraction and large variety of properties such as low density, better cor- decreasing the particle size. The thermal conductivity of rosion resistance, good thermal and electrical conductiv- AMCs decreases with increase in reinforcement volume ity, high damping capacity, and ability to be strengthened fraction and porosity. by precipitation [5]. AMC finds applications in a number of structural, Keywords: aluminium matrix composite; thermal cycling; nonstructural, and functional applications in different thermal fatigue; thermal stress. fields of engineering due to their economic, environmen- tal, and performance benefits. They are now being used in DOI 10.1515/secm-2013-0185 fabricating aerospace and automotive components such Received August 7, 2013; accepted May 18, 2014; previously as ventral fins, fuel access door covers, rotating blade published online September 24, 2014 sleeves, gear parts, crankshafts, and suspension arms. In the electronic industry, they are used in the processing of integrated heat sinks, microprocessor lids, and microwave *Corresponding author: Khushbu Dash, Department of Metallurgical housing. Majority of these applications require operation and Materials Engineering, National Institute of Technology, at varying temperature conditions ranging from high to Rourkela 769008, India, e-mail: [email protected] Suvin Sukumaran and Bankim C. Ray: Department of Metallurgical cryogenic temperatures [6, 7]. Structural components and Materials Engineering, National Institute of Technology used in the aerospace industries such as aircraft wings, Rourkela, Rourkela 769008, India fuselage frames, and landing gears are frequently exposed 2 K. Dash et al.: The behaviour of aluminium matrix composites under thermal shock-a review to severe thermal loads created by aerodynamic heating compared with other reinforcement types. But the [8, 9]. Hence, it is important to study their thermal behav- mechanical properties of particulate-reinforced MMCs iour under these conditions. are less compared with that of whisker- and fibre-rein- The ceramic phase, which is generally used as rein- forced MMCs. These composites are usually isotropic forcement in MMC, has a large difference in coefficient in nature and manufactured mainly by powder metal- of thermal expansion (CTE) with the metal phase. This lurgy processing. Secondary forming operations such thermal mismatch causes large residual thermal stresses as rolling, forging, and extrusion can be performed near the interfaces of the composite when they are cooled easily on particulate reinforced MMCs. from the fabrication temperature [10, 11]. These residual 2. Short-fibre- and whisker-reinforced MMCs: Here, non- stresses have a significant influence on the mechanical continuous fibres with an aspect ratio > 5 are used. and physical properties of the composite. Thermal stresses Whisker reinforcements have better mechanical prop- are important in design because they lead to plastic erties than both continuous and particles reinforced yielding or failure of the material [12]. There are several composites do and are mainly fabricated using pow- mechanisms by which thermal stresses can be relaxed, der metallurgy or squeeze infiltration route. Short which include interface debonding, by micro plasticity fibre reinforcements find major application in use as of the metal matrix and crack initiation and propaga- piston and have intermediate properties in between tion [13]. AMCs are frequently being exposed to environ- that of continuous-fibre-reinforced MMCs and partic- ments where cyclic temperature changes prevail [10]. ulate-reinforced MMCs. Since some health hazards are Thermo-mechanical cycling (TMC) also greatly influences identified with the use of whisker as reinforcement, the thermal expansion behaviour of the composites [14]. the commercial applications of whisker-reinforced Studying the thermal expansion behaviour of composites MMCs have been limited lately. also has significant importance since most of the struc- 3. Continuous-fibre-reinforced MMCs: Continuous fibres tural components are subjected to variable temperature are used here, which can be parallel or prewoven. operation, and hence, the materials used in these compo- They have diameter normally < 20 μm. The high- nents should have lower and stable CTE and outstanding est degree of load transfer is provided by them due mechanical properties [3]. Thermal expansion behaviour to their high aspect ratio. A significant amount of of composites is affected by several material parameters, strengthening takes place along the fibre direction. such as the type of constituents, matrix microstructure, Liquid infiltration techniques are mostly employed reinforcement volume fraction and architecture, the inter- for the fabrication of continuous-fibre-reinforced nal stresses due to their CTE mismatch, thermal history, MMCs. But the main disadvantage of these compos- porosity, and interface bond strength [9]. ites is that they have low strength in the transverse Thermal fatigue study of AMCs is important since they direction compared with the longitudinal direction, are used in engine components that are subjected to thermal which results in anisotropic properties. excursions during service, such as piston crowns and valves 4. Monofilament-reinforced MMCs: Here, fibres hav- [15]. It is also important to understand properties like ing a large diameter, in the range of 100–150 μm, are thermal conductivity of MMC for various thermal manage- used as reinforcements. Chemical vapour deposition ment applications such as heat sinks in electronic devices, (CVD) and diffusion bonding techniques are normally microprocessors, power semiconductors, high-power laser used for their fabrication. Their application is limited diodes, light-emitting diodes, and micro-electro-mechani- to superplastic forming metal matrices. Like contin- cal systems [16, 17]. This paper reviews the current knowl- uous-fibre-reinforced MMCs, these composites also edge on the behaviour of AMCs under thermal stresses. have anisotropic properties. In addition to the above types, another variant called 2 Classification of MMCs hybrid AMCs having more than one type of reinforcement is also present, which is in use to some extent [6, 18, 19]. There are mainly four types of MMCs based on the type of reinforcement: 1. Particle-reinforced MMCs: These are produced by 3 Synthesis of AMCs incorporating equiaxed reinforcements in the form of particles to the metal matrix. The aspect ratio of par- Primarily, fabrication of MMCs can be classified into two ticles will be < 5, and these MMCs are less expensive processes such as solid-state processes and liquid-state K. Dash et al.: The behaviour of aluminium matrix composites under thermal shock-a review 3 processes. Solid-state processes include powder metal- the matrix and the reinforcement