Effect of testing temperature on the strength and fracture behavior of Al-B 4 C composites

Abstract

The present study was performed on two Al-10 vol. % B₄C composite containing in common 0.48%Ti and 0.25%Zr. The matrix in the first composite was commercially pure aluminum and in the other was experimental 6063 alloy. The molten composite was cast in the form of slabs (25 × 20 × 400 mm). Prior to hot rolling, the slabs were annealed at 540℃ for 16 h and the last two rolling passes were done at room temperature. Final sheet thickness was about 2 mm. Samples for tensile testing were prepared from the rolled sheet using wire cutting technique. The tensile samples were solution heat treated at 540℃ for 8 h followed by furnace cooling. Samples for microstructure as well as fractography were also examined. Tensile samples were tested in the temperature range of 25–500℃ at a strain rate of 5 × 10⁻⁴ s⁻¹. The results show that when composite samples were tested following solution heat treatment, the role of the matrix composition in controlling the preservation of the composite strength at high temperature is nil. Increasing the testing temperature caused rapid decrease in the composite strength in a non-linear pattern. The fracture surface of the deformed composites consisted mainly of well-defined dimple structure. The size of these dimples is a direct function of the testing temperature. The presence of Ti-Zr rich protecting layers surrounding the B₄C particles resulted in good particle/matrix wettability throughout the entire testing temperatures (25–500℃). Cracks were observed to be initiated at the particle/matrix interface and propagate through the B₄C particles. The length of crack propagation was mainly dependent on the particle size. Increasing the testing temperature appeared in the gradual fracturing of the reinforcement particles.