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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

2.6. Aims and Objectives

2.6. Aims and Objectives Particulate preforms as a basis for preform MMCs are promising due to their advantages regarding cost, availability and knowledge of processing routes. Nevertheless, few papers have been published on infiltration of Al alloy melts into Al2O3 and TiO2 and none into MgO preforms. The ceramic volume fraction range between the upper limit of chopped Al2O3 fibres (0.25) and the lower limit of metal toughened ceramics (0.60) has not been investigated, yet it is suggested MMCs within this range might offer improved tribological and mechanical properties. This study is aimed at investigating the properties of MMCs with a ceramic volume fraction between 0.30 and 0.40 in order to get sufficient strengthening and keep the ductility as high as possible. As it was found that there is a lack of knowledge of the influencing factors on preform infiltration, the study of the preform infiltration behaviour was a further objective of the present study. As reported, the infiltration process is subdivided into two steps: initiation of the flow into the capillaries, characterised by the wetting behaviour, and further flow to fill the pores. Below process temperatures of 1000°C, fully non-wetting conditions (θ = 180°) were proposed for most Al-ceramic systems (53,114) . A question of current interest is whether reactions influence the infiltration performance. The data found in the literature for different reactive melt-ceramic systems, either in static or dynamic wetting experiments, indicated that reactions do not improve the infiltration process. Even though for further pore filling commonly the slug-flow assumption was proposed, in recent publications some inconsistencies of this model could be better explained using the Richards model, originating from soil sciences, which assumes unsaturated flow. Both reactive wetting and further filling of the preform are of interest and to be investigated in the constant pressure and the constant flux modes. 61

3. EXPERIMENTAL PROCEDURE The influence of reactivity between the metal melt and the ceramic reinforcement on infiltration behaviour was investigated. Thermodynamic calculations were undertaken to determine the equilibrium standard free energies of different aluminium alloy-oxide ceramic systems. From this analysis, promising systems were chosen for experimental investigation of static and dynamic wetting. The contact angles were determined using sessile drop experiments, whereas the dynamic contact angles and the fluid flow behaviour were evaluated during infiltration of the porous preforms with the selected aluminium alloys. The procedure of preform preparation and characterisation and their infiltration using gas pressure (GP), direct squeeze casting (DSQC) and high pressure die casting processes (HPDC) are presented. The resulting MMC materials were characterised using non-destructive X-ray methods as well as destructive metallographic methods. The Young’s modulus, strength and fracture toughness of the MMCs and their tribological behaviour were measured. Details of these experimental methods are given in this chapter. 3.1. Thermodynamic calculations The standard free energies of formation for different liquid aluminium alloy-oxide ceramic systems were calculated using the commercial thermodynamic modelling software FACTSAGE of GTT Technologies in Herzogenrath, Germany. The calculations were performed using the module 'Equilib' which is based on minimizing the Gibbs energy G by the method of complex system-equilibria as introduced by Eriksson and Hack (147) and given by the relation: = ∑ νi µ = min Equation 38 G i 62

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