Creep behaviour of Al-based Metal Matrix Composites

High Tech engineering applications require materials with property profiles, where light weight, stiffness, conductivity, corrosion resistance, strength up to elevated temperatures, fatigue and creep resistance are combined. The development of Metal Matrix Composites (MMCs) during the last decades has been trying to cope with these requirements providing a variety of combinations of matrix alloys and reinforcements of different types, volume fractions, shapes and architectures. MMCs exhibit significant improvements with respect to conventional alloys, but as well some disadvantages like increased cost. Al-based MMCs with discontinuous ceramic reinforcement have reached applications due to their high specific mechanical properties such as Young`s modulus and strength and relatively moderate price. However, the behaviour of Al-based discontinuously reinforced MMCs at elevated temperatures needs further investigations to understand the heterogeneous thermal activation of the matrix offering a knowledge based potential for improvement. The creep resistance of this kind of materials depends essentially on the size, volume fraction, shape and architecture of the ceramic reinforcement. The available literature as well as own studies show that ceramic short fibres enhance the creep resistance with respect to the unreinforced matrix alloys, while particles reduce the creep resistance in some cases. Focusing on these contradictory results, the proposed project aims to achieve a basic and generally applicable understanding of the processes and parameters influencing the long-term creep behaviour of discontinuously reinforced Al-MMCs. The investigations will be carried out for model MMCs, prepared in co-operation with international partners, and MMCs resembling commercial grades: 1) an unreinforced and particle reinforced Al-Cu alloy using two different particle types with different volume fractions and reinforcement sizes. 2) unreinforced and short fibre reinforced Al-Si alloys to study the effect of short fibres as well as that of the three dimensional hybrid Si-short fibre network formed in these composites. Prior and in the course of the creep tests, the materials will be characterised by means of Young`s modulus measurements, high temperature tensile tests, differential scanning calorimetry (DSC), dilatometry and metallographic methods in order to determine mechanical properties and microstructural features. Internal stresses and pores induced during production and their evolution during long-term creep will be studied by neutron and synchrotron diffraction as well as by X-ray and synchrotron computed tomography. These "non-destructive" techniques provide a three dimensional insight into the internal adaptation of these heterogeneous materials to long- term creep exposure. The experimental results will provide input data for the modelling of the creep behaviour of the studied matrices and composites, while the modelling results will be verified by experiments and shall provide the means for the prediction of material improvements.

The advantages for application of discontinuously reinforced aluminium matrix composites are the increased specific stiffness and wear resistance with respect to conventional alloys. Such a property profile can be exploited for combustion engine components and break systems. Because of the elevated Service temperature, the Jong term stability of the material has to be assured, that was experimentally verified by long term creep tests at 300C. The price of such MMC still restricts the market volume, but the benefits in weight savings and environmental Impact are steadily increasing. The project was divided into 2 parts: 1) Particle reinforced wrought aluminium alloys (PRM): The aim was to identify the effect of the size distribution of ceramic particles used to reinforce aluminium wrought alloys. The influence an the creep resistance is discussed controversially in literature. The hypothesis Gould be verified, that particles smaller than 1 m increase the creep resistance acting as obstacles for conservative and thermally activated dislocation movement similar as in dispersion strengthened alloys. Particles bigger than 5 m enhance the creep rate by acting as dislocation sources, as demonstrated by PRM produced by powder metallurgical wer blending of aluminium powder with ceramic particles. Samples produced by ball milling of aluminium powder with different size classes of ceramic particles bigger than 5 m contain dispersoids smaller than 1 m due to the fragmentation of the added particles. A fraction of such dispersoids in the range -2-4 vol% was found to be large enough to improve the creep resistance by an order of magnitude in creep rate and a factor of two with respect to the load stress. Sub-pm synchrotron holotomography has been successfully carried out for the first time to describe the particle distribution in the PRM. The dispersoids were identified by transmission electron microscopy. 2) Short fibre reinforced Gast aluminium alloys (SFRM): Porous preforms of ceramic short fibres were infiltrated with the melt of an AI piston alloy by squeeze casting. The specific stiffness and elevated temperature strength as well as the wear resistance of the alloy are significantly increased by the short fibre reinforcement. Long term creep tests up to more than one year indicated an improvement of the creep resistance during exposure. lt Gould be shown by synchrotron tomography, that the rigid phases of the alloy connect the short fibres by diffusion creating a 3D network. The load carrying capacity of such a percolating structure Gould be measured by in situ stress measurements by neutron diffraction of the matrix and the reinforcing phases. On the basis of the 3D micro- structural characterisation, a simplified architecture was modelled for Simulation of the creep resistance. The measured improvement of creep resistance Gould be exploted by the in situ development of an interpenetrating composite. Results have already been reported in scientific journals and in one book chapter and will be communicated to relevant material producers and potential users, who can expect longer life time of lightweight products, the value of which is increasing.