Fatigue behaviour of short alumina fibre reinforced AZ91 magnesium alloy metal matrix composite

Magnesium alloy metal matrix composites (MMCs) have been produced by squeeze casting using AZ91 magnesium alloy with ceramic reinforcements based on alumina (Saffil) fibre two-dimensional preforms. MMCs have been cast with 0.05, 0.16 and 0.25 volume fraction alumina reinforcements. Some tensile test results were obtained for the 0.25 volume fraction Saffil composite. The heat treatment widely used for AZ91 is unsuitable for the composite material where heating to 420°C has changed the nature of the tensile failure from a planar fracture to one exhibiting extensive fibre pull-out. The room temperature rotating bending fatigue strength for 0.05, 0.16 and 0.25 volume fraction Saffil MMCs has increased significantly with increasing volume fraction of reinforcement. This improvement appears to result from a higher fatigue initiation resistance caused by the alumina fibres impeding long-range slip. There is, in effect, a change from stage I fatigue to a 'stage II' fatigue. The fatigue crack growth rate of the composite material, on the contrary, is inferior to that of the matrix material in the high ΔK region (above 5 MPa m 1/2 ).