Phase formation maps in Zr48Cu46.5Al4Nb1.5 bulk metallic glass composites as a function of cooling rate and oxygen concentration
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A design of experiments (DoE) approach was employed to systematically characterize the effects of both oxygen concentration and cooling rate on the amorphous and crystalline phase fractions in the Zr48Cu46.5Al4Nb1.5 bulk metallic glass composite (BMGC) alloy. Specifically, four distinct oxygen concentrations and five distinct cooling rates were used. Phase quantification was achieved using Rietveld refinement of synchrotron X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) using wavelength dispersive spectroscopy (WDS) revealed that the big-cube crystalline phase may scavenge oxygen from the amorphous matrix of a 2864 +/- 47 wppm oxygen sample. Transmission electron microscopy (TEM) of the alloy containing 340 +/- 25 wppm oxygen revealed the presence of an amorphous matrix with partially-transformed colonies of B2 (Pm-3m) where the transformation product was a B19' superstructure (Cm space group) instead of the more common B19' superstructure (P2(1)/m space group); this was in agreement with the Rietveld refinement results. An orientation relationship between the B2 and B19' phases was observed as expected. Finally, it was possible to produce thicker sections (5 mm vs. 3.5 mm) with the desired composite structure ( similar to 80% amorphous fraction with similar to 20% B2/ B19') compared with previous BMGC studies on similar compositions.