Formation of micrometer sized quasicrystals in slowly cooled Zr-Ti-Nb-Cu-Ni-Al alloys

The solidification behavior of copper mold cast Zr60Ti6Nb2Cu14Ni9Al9 and Zr60Ti2Nb6Cu14Ni9Al9 alloys has been investigated. The phase formation of the alloy with a ratio of Ti: Nb = 3: 1 depends very sensitively on changes of the cooling conditions, exhibiting three competing phases: a glassy, an icosahedral quasicrystalline (i phase) and a crystalline phase, which are arranged very closely in the part of the sample with the highest cooling rate. Decreasing the cooling rate leads to a fully crystalline microstructure. In contrast, the alloy with a ratio of Ti: Nb = 1: 3 forms predominantly an i phase and presents only a low volume fraction of a residual glassy phase at the interfaces of the quasicrystals. Besides a specific cooling rate a content of Cu + Ni + Al of about 32 at% and a content of Ti + Nb of about 8 at% is required for the formation of an as-cast i phase in these Zr-based alloys, because of a decrease of their glass-forming ability linked with more pronounced icosahedral short-range ordering in the melt. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.