Abstract
The crystallization process, both at the initial and subsequent stages, of amorphous Al88-RE4-Ni8 alloys (RE = Y, Sm and Ce) has been studied. Additionally, the consequences of adding 1 at.% Cu replacing Ni or Al were studied. The stability of the amorphous structure in melt spun ribbons was thermally studied by differential scanning calorimetry, with Ce alloys being the most stable. The effect of Cu to reduce the nanocrystal size during primary crystallization was analyzed by transmission electron microscopy. This latter technique and x-ray diffraction showed the formation of intermetallic phases at higher temperatures. A clear difference was observed for the Ce alloy, with a simpler sequence involving the presence of Al3Ni and Al11Ce3. However, for the Y and Sm alloys, a more complex evolution involving metastable ternary phases before Al19RE5Ni3 appears, takes place. The shape of the intermetallics changes from equiaxial in the Ce alloys to elongate for Y and Sm, with longer particles for Sm and, in general, when Cu is added to the alloy.
Highlights
The disordered or amorphous structure typical of the liquid state can be maintained in solid metals by heating above their melting temperature and very quick cooling to room temperature [1,2].Only particular compositions are appropriate to form metallic glasses, and depending on the composition, the necessary cooling rate varies
Y alloy, a small shoulder appeared for 2θ ~44◦, because of the presence of some local order [41,45,46], corresponding to the initial stages of disappearance of the amorphous structure, while still being maintained
Amorphous structure formation in Al88 -RE4 -Ni8 alloys was quite similar for the Y, Sm, and Ce-containing alloys, the Sm and Ce alloys seemed to show a higher glass forming ability (GFA)
Summary
The disordered or amorphous structure typical of the liquid state can be maintained in solid metals by heating above their melting temperature and very quick cooling to room temperature [1,2].Only particular compositions are appropriate to form metallic glasses, and depending on the composition, the necessary cooling rate varies. Aluminum mixtures with at least other two chemical elements ( it is possible with only one, i.e., [5,6]) can reach the amorphous state by quick cooling, attaining high strengths with respect to conventional aluminum alloys [7]. Properties such as elastic strain [8], or the possibility of superplastic forming in a supercooled liquid region [9] are remarkable. Al amorphous alloys with a composition Al-Fe-Si [10,11] showed brittleness problems that were subsequently solved by changing for instance to Al-Ni-Zr compositions [12]
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