Fatigue of Metal Matrix Composites
This thesis treats the mechanical properties of discontinuously reinforced metal matrix composites, DMMCs, in general and their fatigue properties in particular. Methods of manufacture, microstructures and resulting mechanical properties of DMMCs are reviewed in the introductory part. The incorporation of short ceramic fibres or particles in light alloys renders composites that are stiffer, stronger and in some respects more fatigue resistant than their parent matrix alloys. However, these beneficial effects have to be paid for by a reduction in toughness. Theories proposed for the explanation of these experimental observations are discussed.
The appended papers deal with the fatigue crack growth characteristics and the low cycle fatigue properties of composites based on the AA6061-alloy containing 15 vol.% SiC-particles or 20 vol.% Saffil- (Al2O3) fibres. The processing of the particulate composite involved casting and extrusion whilst the Saffil composite was made by squeeze-casting. AA6061-matrix alloys in wrought and squeeze-cast conditions are included as well.
The fatigue crack growth experiments showed that the composites are markedly superior to the matrix alloys in the threshold region: a more than 50 % increase of DKth over the matrix alloys was measured. This increase is partly due to increased crack closure of the composites, resulting i.a. from increased elastic modulus. Quantitative fractographic measurements allowed for a coupling of the altered fatigue crack growth resistance to changes in the appearance of fatigue fracture surfaces, induced by particles and fibres. The importance of crack topography, elastic modulus and crack branching for the net driving force for fatigue crack growth was quantified. Reduced crack tip opening displacement in the SiCp-composite, compared with the wrought matrix alloy, was indirectly measured. This promotes further reduction in crack growth rate, in addition to what is obtained from enhanced surface roughness and elastic modulus.
Low cycle fatigue experiments, LCF, were performed on the two composites and the cast matrix alloy. The SiCp-composite showed the largest tolerance against cycling at smaller plastic strains, while the matrix alloy was superior at higher plastic strain ranges. A systematic study of surface crack initiation and growth during LCF demonstrated that crack initiation from fractured large fibres or particles is instantaneous (Saffil composite) or almost instantaneous (SiCp-composite). A 3D FEM-simulation demonstrated that the locally inhomogeneous state of stresses and strains in and around a stiff particle is enhanced, when the particle intersects a free surface compared to when the particle is entirely contained in the matrix. It was observed that the growth properties of individual microcracks determine the LCF-life in the composites. The growth rates of such cracks were an order of magnitude faster than those of long through cracks, at corresponding values of the growth parameter DJ.