Glass Formation and Nanostructure Development in Al-Based Alloys
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Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
EXPERIMENTAL DETAILS Bulk alloy ingots with nominal compositions of Al-7at%Y-5at%Fe and AI-8at%Sm were prepared by repeated arc-melting high purity elements (Al: 99.999%; Y: 99.9%; Fe: 99.995%; Sm: 99.9%) at proper mixtures in an argon-protected environment. Amorphous ribbons were produced by rapid solidification processing using a single roller melt-spinning facility with a tangential wheel speed of 33m/s. The production of Al-Y-Fe-Pb melt-spun ribbons follows the procedures described above except that Pb is not included in the ingot during arc-melting due to its much lower melting point compared with the other three constituent elements. High purity Pb (99.99%) of desired weight was incorporated into the liquid alloy during melt-spinning. The amorphous melt-spun ribbon (MSR) samples were devitrified by continuous heating or by isothermal annealing in a differential scanning calorimetry (DSC, Perkin-Elmer DSC7). Mechanical intermixing was applied to fabricate amorphous A192Sm 8 sample at ambient temperatures by manually cold rolling stacked arrays of elemental foils of high purity Al (99.999%) and Sm (99.9%) at suitable composition mixture [6]. Phase identification was carried out with standard X-ray diffraction method (XRD, Philips XRD) in the reflection mode using Cu-Kcc radiation. Microstructural characterization of the samples was performed by transmission electron microscopy with selected area electron diffraction (TEM/SAED, Philips CM200). The TEM samples were prepared by standard thinning procedures using a Gatan ion mill with liquid nitrogen cooling. The size and volume fraction of the nanocrystalline Al dispersions in different samples were determined from the TEM bright-field micrographs with image analysis software (ImagePro). Calorimetric signals from phase transformations during heating and isothermal annealing were monitored by DSC. Moreover, modulated-temperature DSC (DDSC) was applied to assess the glass transition temperature of the amorphous alloys. RESULTS AND DISCUSSION The TEM bright-field image of the as-spun ribbon sample of A188Y7Fe 5 in Figure 1-a shows a homogeneous and fully amorphous structure with no evidence of a crystalline fraction. The diffuse ring in the corresponding selected area electron diffraction (SAED) pattern and the diffuse scattering maximum in the XRD trace in separate experiments indicate that the ribbon samples are amorphous. However, the DSC trace in Figure 1-b of an AlssY 7Fe5 as-spun sample during continuous heating at 40°C/min displays two exothermic peaks but no indication of an endothermic reaction that corresponds to the glass transition. XRD study indicated the first exothermic peak is the primary crystallization reaction of a-Al [12]. Moreover, TEM investigation on the samples that were isothermally annealed at 2450 C revealed a high density 0/
ST
-2 Primary Crystallization •273 0 C
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