Effect of thermal cycling on the mechanical properties of Cf/Al composites

An unidirectional ultra-high modulus M40J carbon fiber reinforced 5A06 aluminum alloy with volume fraction of 55% was fabricated by the pressure infiltration method. The effect of thermal cycling (-70 to 120°C and -196 to 120°C) on the mechanical properties of Cf/Al composites was investigated. Bending modulus increased abruptly after five thermal cycles and then reached a platform. However, after 50 cycles, the bending modulus decreased with the increase of cycles due to generation and propagation of microcracks at C fiber/Al interface. The bending strength of composites showed fluctuations between 800 and 1000MPa, and the amount and length of pull-out fiber increased with the increase of the thermal cycling times regardless of temperature difference. Microstructure observation revealed the formation of microcracks and interface debonding at the C fiber/Al interface after thermal cycling treatment, and the amount of the microcracks and interface debonding increased with the temperature difference. The main interfacial products in annealed Cf/Al composites were rod-like Al₃Mg₂ and bulky Al₅₈Mg₄₂, and no significant Al₄C₃ was observed at C fiber/Al interface. The thermal cycling treatment used in the present work had little effect on the interface microstructure of Cf/Al composites. Very few dislocations were observed in the Al matrix of annealed Cf/Al composites. However, the amount of dislocations in the Al matrix increased gradually with the increase of thermal cycles. Moreover, the amount of dislocations would decrease after a certain thermal cycles due to movement and annihilation of unlike-sign dislocations.