Fracture Modes for Self-Fragmentation of Al-Based Quasicrystals Contaminated by Carbon

Abstract

Cast ingots of quasicrystal forming composition Al 62 Cu 26 Fe 12 and Al 72 Ni 12 Co 16 were prepared in an inductive melting apparatus using graphite crucible in the aim of investigating the effect of carbon on the stability of the single icosahedral (i) and decagonal (d) quasicrystalline phases, respectively. Owing to this process the contamination of carbon was expected to occur during melting by diffusion from the crucible into the quasicrystalline ingots. The detected amounts of carbon in the i- and d-phase were 0.15 mass% and 0.88 mass%, respectively. Carbon introduced by this method was first found to disintegrate spontaneously these monolithic i-AlCuFe and d-AlNiCo ingots, i.e., a detrimental effect of self-fragmentation of quasicrystals induced by carbon. Our experimental results indicated that the self-fragmentation occurring in the carbon contaminated i- and d-phases did not appear immediately after solidification, but after a certain incubation time. During the self-fragmentation process, the quasicry stalline samples with an initial size of 20 mm diameter bar ingot were disintegrated to form small fragments of about 200 μm after 50 days. Fractography was carried out to analyze fracture modes in the i- and d-phases. The fracture surfaces of the i phase exhibited a transgranular cleavage pattern without trace of cleavage steps and river patterns, while the d-phase showed both transgranular cleavage and intergranular fracture. Such unique fracture modes in the i- and d-phases are thought to arise from an internal mechanism of carbon-assisted fracture.