Fatigue in magnesium alloy AZ91-[formula omitted]Alumina fiber composite studied by internal friction measurements

Abstract

Compared to their matrix metals Magnesium Matrix Composites (MMCs) show higher stiffness, strength, improved tribological properties, lower thermal expansion coefficient, improved wear resistance, enhanced strength and creep resistance. Moreover they own low density and good machinability. Investigation of their physical and mechanical properties is important not only for applications but also for better understanding of the processes responsible for their behavior. Squeeze casting as an advanced technology of MMCs production became possible to combine metallic materials with ceramic reinforcing fibers and so to modify not only mechanical and thermal characteristics but also damping capacity. On the other hand damping capacity measurements are suitable tools to detect changes in the microstructure of thermally or mechanically loaded composites. Therefore Magnesium alloy AZ91 with 15 vol. % γ- Al2O3 fibers as reinforcing phase produced by squeeze casting was thermally cycled between room temperature and increasing upper temperatures. After thermal cycling the amplitude dependence of damping in terms of the logarithmic decrement was measured. Dislocations present in the material turned out to be the main reason for damping. Thermal stresses induced in the investigated composites due to the considerable difference between the thermal expansion coefficients of the matrix and the ceramic fibers create new dislocations on cooling from elevated to ambient temperature. The thermal stresses can achieve the yield stress of the matrix and micro—glide of newly created dislocations as well as their annihilation can occur. Thermodynamic processes in the matrix may influence these effects. The density and arrangement of dislocations may be changed also due to mechanical cycling. Therefore the influence of cyclic bending on the damping behavior of magnesium alloy based MMCs was determined at room temperature. The logarithmic decrement of free decaying vibrations of bending beams as a function of the number of cycles was studied. During cycling damping first increased up to 4×103 cyles stayed more or less constant and decreased again for cycle numbers >106. Like in thermal cycling this behavior can be explained by similar dislocation effects. For numerousness cycles >8×107 the damping again steeply increased with further cycling. With the help of a rheological model developed for crack nucleation and crack growth in unreinforced Magnesium alloy AZ91 this increase could be explained by nucleation and progressing growth of cracks. Relative maxima occurred in this cycling region on the damping versus strain amplitude curves like in unreinforced Magnesium alloy AZ91. Again these maxima can be qualitatively explained and analyzed with the rheological model assuming crack nucleation and crack growth as the source of damping. With increasing numbers of cycles the maximum increased up to fracture of the sample. Cracks in the vicinity of the fractured surface nucleated at fibers or coarse precipitates.