Evidence for the Formation of a Metastable Crystalline Phase during Crystallization of Zr-Ni Metallic Glasses of Approxi
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EVIDENCE FOR THE FORMATION OF A METASTABLE CRYSTALLINE PHASE DURING CRYSTALLIZATION OF Zr-Ni METALLIC GLASSES OF APPROXIMATELY 57-63.2 at. % Zr* C. G. MCKAMEY, D. S. EASTON, AND D. M. KROEGER Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT The crystallization of rapidly solidified Zr-Ni metallic glasses has been studied by differential scanning calorimetry (DSC), x-ray diffraction Evidence has been found (XRD), and transmission electron microscopy (TEM). for a metastable phase which is the first crystalline phase to form upon heating in compositions of 57 to 63.2 at. % Zr. This phase seems to be more easily formed and is most stable in the region of 58 to 59 at. % Zr. Upon subsequent heating to higher temperatures, the metastable phase transforms to the equilibrium phases ZrNi and Zr 2 Ni. The relationship of this new phase to phase separation at the eutectic composition of 63.5 at. % Zr is discussed in support of the conclusions drawn from low temperature specific heat studies reported previously [1,2]. INTRODUCTION In the Zr-Ni metallic glass system, the absence of XRD evidence for crystallization in compositions around the eutectic at 63.5 at. % Zr after heating through the first exothermic DSC peak has caused an increased interest in the nature of the transformations at these compositions [3,4]. Researchers have suggested that either a transition is occurring within the amorphous state (phase separation) [3-5] or very fine grained microcrystals
are precipitating from the amorphous matrix [6]. Our previous studies on Zr-Ni glasses of compositions between 55 and 74 at. % Zr support the theory of phase separation [1,2]. From lowtemperature specific heat data, evidence for two superconducting phases was observed in a splat quenched sample of composition Zr6 2 .gNi 3 7 .j. From measurements of the density of states at the Fermi level, superconducting transition temperatures, and AH on crystallization, we concluded that the transitions were representative of two phases having compositions near 4 o(Zr 3 Ni 2 ) and Zr 6 6 . 7Ni 3 3 3 (Zr 2 Ni). Recently a study of the crystallization processes of these alloys using differential scanning calorimetry (DSC), x-ray diffraction (XRD),
Zr 6 oNi
and
transmission electron microscopy (TEM) was conducted [7,8]. During these studies, evidence was found for the formation of a metastable phase which is the first crystalline phase to form upon heating in compositions of 57 to 63.2 at. % Zr [9]. Upon further heating to higher temperatures, the metastable phase transforms to the equilibrium phases ZrNi and Zr 2 Ni.
This report summarizes the evidence accumulated to date for the presence of this metastable phase. *Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract DE-ACO5-840R21400 with Martin Marietta Energy Systems,
Inc.
Mat. Res. Soc. Syrup. Proc. Vol. 58. ý 1986 Materials Research Society
94
EXPERIMENTAL PROCEDURES Alloy buttons of Zr-Ni compositions between 55 and 70 at. % Zr were p
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