Dresden 2003 – wissenschaftliches Programm

Bereiche | Tage | Auswahl | Suche | Downloads | Hilfe

M: Metallphysik

M 7: Quasikristalle I

M 7.3: Vortrag

Montag, 24. März 2003, 10:45–11:00, IFW B

Formation of quasicrystals in Zr-Ti-Nb-Cu-Ni-Al alloys by casting or annealing — •Uta Kuehn, Juergen Eckert, Norbert Mattern, and Ludwig Schultz — IFW Dresden, P.O. Box 270016, D-01171 Dresden

Zr-Ti-Nb-Cu-Ni-Al alloys were prepared by melt spinning and copper mould casting to achieve different cooling rates and, therefore, different microstructures of the as-cast states. Depending on the actual composition, a glassy and/or an icosahedral quasicrystalline phase is obtained. Additionally, the crystallization behaviour of the as-cast ribbons and cylindrical rods was studied by thermal analysis. The devitrification of the samples is characterized by the formation of a metastable quasicrystalline phase during the first stage of the crystallization process. The size and morphology of the quasicrystalline phase is clearly different for the two preparation routes, which is correlated with the asymmetry of nucleation and growth upon cooling or heating. The quasicrystals formed upon slow cooling from the melt are on a micrometer scale. The size of the quasicrystals formed by annealing is in the range of about 5 to 200 nm, strongly depending on the alloy composition and the actual annealing treatment. It has been found that the higher the ability of an alloy to form a quasicrystalline phase during solidification from the melt the smaller is the size of the quasicrystalline precipitates formed by annealing the glassy phase connected with a decrease in thermal stability. We assume that the existence or the absence of an icosahedral short-range order in the supercooled liquid may be responsible for the different (quasi)crystallization behaviour.