Mechanical and fracture behaviour of aluminium-based discontinuously reinforced composites at hot working temperatures

In this paper, the tensile properties and the low-cycle fatigue behavior of the 7005 aluminum alloy reinforced with 10 vol% of Al2O3 particles (W7A10A composite) and 6061 aluminum alloy reinforced with 20 vol% of Al2O3 particles (W6A20A composite) were studied. The microstructural analyses showed clustering of Al2O3 particles, irregularly shaped and with a non-uniform size. A significant increase of the elastic modulus and tensile strength in the MMCs, respect to the unreinforced alloys, was evidenced by the tensile tests, while the elongation to fracture decreased. The temperature effect on the tensile properties was not relevant up to 150 °C, while strength significantly decreased at 250 °C, mainly in the composite with the lower content of the ceramic reinforcement. The low-cycle fatigue tests showed no evidence of isotropic hardening or softening for the W7A10A, and a slight cyclic softening for the W6A20A. SEM analyses of the fracture surfaces showed that both the tensile and fatigue fracture was controlled by interfacial decohesion (expecially for the W7A10A composite), fracture of reinforcing particles (mainly for the W6A20A composite), void nucleation and growth. Also the presence of the MgAl2O4 spinel, probably, played a significant role in the mechanisms of failure in the W6A20A composite, by promoting void nucleation at the particles–matrix interfaces, interfacial decohesion, and also failure of the particles. These effects can be responsible of the slight softening observed in the W6A20A, under the low-cycle fatigue conditions.