Effect of cooling rate on the precipitation of quasicrystals from the Zr-Cu-Al-Ni-Ti amorphous alloy

The Zr57Cu20Al10Ni8Ti5 alloy solidifies into an amorphous phase upon rapid quenching or casting at low cooling rates. However, the amorphous alloys formed at different cooling rates exhibit different crystallization behavior. The slowly cooled bulk amorphous alloys prepared by copper mold casting reveal a first crystallization peak at 715 K upon heating at 0.33 K/s and crystallize via precipitation of an icosahedral quasicrystalline phase in the first crystallization step. The rapidly quenched ribbons exhibit a first crystallization peak at 720 K and crystallize by simultaneous precipitation of the quasicrystalline phase together with Zr2Cu and Zr2Ni intermetallic phases in the first stage of crystallization. It is supposed that the undercooled Zr57Cu20Al10Ni8Ti5 melt has a tendency to develop an icosahedral short-range order, which is favored by low cooling rate. As a result, the bulk amorphous alloy has a short-range order close to a quasicrystalline phase. In contrast, the structure of the ribbon is more similar to that of the melt at high temperature and the atoms are oriented more randomly. (C) 1998 American Institute of Physics. [S0003-6951(98)01541-1].