Phase selection during non-equilibrium solidification of undercooled Zr-Co melts

Kurzfassung

Zr-based alloys have attracted broad attention because of their ability to form metallic glasses even when cooling down the melts at moderate cooling rates. During cooling of the melts, glass formation competes with the solidification of crystalline phases from the undercooled liquid. From the metastable state of the undercooled liquid, apart from the stable solid phases, also metastable solid structures may be formed. Often these subsequently transform into more stable counterparts such that they cannot be found in the as-solidified material. Understanding of the solidification behavior of the competing crystalline phases is of fundamental importance to understand the glass forming ability of alloy melts. In this work we present in situ X-ray diffraction experiments on the phase selection during non-equilibrium solidification of undercooled glass-forming Zr-Co melts. In order to achieve large undercoolings, the melts were containerlessly processed under high vacuum conditions by application of the electrostatic levitation technique. This technique has been combined with the diffraction of synchrotron radiation at beamline P07 of DESY in order to determine in situ the structures of the solid phases formed during solidification. Different solidification pathways have been identified to occur during non-equilibrium solidification of Zr-Co melts. The studies on the phase selection during non-equilibrium solidification are accompanied by calculations of metastable equilibria in the Zr-Co system, which are in good agreement with the experimental results.