Fracture modes for self-fragmentation of Al-based quasicrystals contaminated by carbon

Cast ingots of quasicrystal forming composition Al62Cu26Fe12 and Al72Ni12Co 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 si ngle icosahedral (i) and decagonal (d) quasicrystalline phases, respectivel y. Owing to this process, the contamination of carbon was expected to occur during melting by diffusion from the crucible into the quasicrystalline in gots. 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 fo und to disintegrate spontaneously these monolithic i-AlCuFe and d-AlNiCo in gots, i.e., a detrimental effect of self-fragmentation of quasicrystals ind uced by carbon. Our experimental results indicated that the self-Fragmentat ion occurring in the carbon contaminated i- and d-phases did not appear imm ediately after solidification, but after a certain incubation time. During the self-fragmentation process, the quasicrystalline samples with an initia l size of 20 mm diameter bar ingot were disintegrated to form small fragmen ts of about 200 mum 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 transganular cleavage pattern without trace of cleavage steps and river patterns, while the d-phase showed both transgranular cleavage an d intergranular fracture. Such unique fracture modes in the i- and d-phases are thought to arise from an internal mechanism of carbon-assisted fractur e.