Devitrification behavior around supercooled liquid region of an Al 85 Ni 5 Y 8 Co 2 metallic glass
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M. Ohnuma National Institute for Materials Science, Tsukuba 305-60047, Japan (Received 6 December 2005; accepted 1 March 2006)
Melt-spun and annealed Al85Ni5Y8Co2 metallic glass, with a large supercooled liquid region, were investigated by means of differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and small-angle x-ray scattering (SAXS). TEM studies revealed that the as-quenched ribbons were fully amorphous. Further, the SAXS measurements showed that no evidence for compositional inhomogeneities associated with amorphous phase separation was found in the as-quenched state and the early stage annealing prior to devitrification. The primary crystallization of this glass was characterized, intriguingly, which appeared to be proceeded with an initial thermal relaxation, then ␣-Al nanocrystal nucleation with a limited number, finally a high density of nanocrystals nucleation and growth.
I. INTRODUCTION
The discovery of lightweight amorphous aluminum alloys has stimulated great interest within the past decade due to their attractive potential in structural applications. Ultimate tensile strength exceeding 1000 MPa was reported for a series of Al-RE-TM (RE, rare earth element; TM, transition metal) amorphous alloys.1 Their strengths can be essentially improved by controlled quenching, which promotes the microstructural formation of nanometer-sized Al crystallites dispersion-embedded in the residual amorphous matrix. It is recognized that the nanostructures frequently observed, with grain densities above 1020/m3 and grain sizes of 5–20 nm, exhibit high nucleation rates and low growth velocities. With regard to the essence of such nanocomposites, firstly, it is understood with the classical theory of nucleation2,3 (e.g., nucleation being heterogeneous, since the nucleation rate tends to saturate with increasing annealing time). For heterogeneous nucleation, the active sites must be present at high concentrations to account for the observed particle density. But so far there are not sufficient data regarding the as-quenched structure. Lately, a very different mechanism of amorphous phase separation on nanoscale prior to crystallization was proposed.4 Transmission electron microscope (TEM) observations showed
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0295 J. Mater. Res., Vol. 21, No. 9, Sep 2006
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preferential nucleation of Al nanocrystals near the boundaries of phase separated regions. Phase separation in a glass is possible if the free-energy versus concentration curve of the glass is convex upwards with two minima. This seems to be difficult to occur in many of Al-RE-TM glassy systems since some of repulsive atomic pairs are hardly observed in these systems. Therefore, to reveal the nature of nanostructure formation, a reaffirmation of the occurrence of phase separation in the precursor stage of crystallization for these glasses is worthwhile. For amorphous alloys with good glass-for
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