Interface stress relaxation in magnesium matrix composites studied by mechanical spectroscopy

Abstract

Mechanical spectroscopy has been used to study magnesium matrix composites reinforced either by long SiC fibres or randomly distributed Al₂O₃ (Saffil) short fibres. It is well known that, in metal matrix composites, thermal stresses can be built up at the interface due to the mismatch between the thermal expansion coefficients of matrix and reinforcements. In magnesium matrix composites, these thermal stresses are relaxed by dislocation motion in the matrix. This mechanism of thermal stress relaxation yields an extra transient component in the mechanical loss spectrum, which depends on the heating/cooling rate and disappears in isothermal condition in the behaviour of the shear modulus G with temperature has been observed during thermal cycling between 100 K and 500 K. The intensity of this phenomenondepends on the spatial distribution of the reinforcements in the matrix. In particular, composites reinforced with long fibres exhibit a more pronounced anomaly. This is interpreted by the modification of the interface strength when temperature is changed.