Amorphous phase formation and nanostructure synthesis in aluminum-based alloys

The formation of amorphous alloys can be controlled by either nucleation or growth kinetics during solidification. Many metallic glasses including Al-based systems require rapid solidification to synthesize and often do not display a clear glass transition, Tg, signal but develop a primary crystallization reaction upon devitrification. A rapid quenching is required to suppress crystal growth and allow for the formation of amorphous phase. The assessment of the glass transition in these alloy systems has been successfully achieved by the application of modulated-temperature differential scanning calorimetry (DDSC) and has provided evidence that an amorphous phase indeed has been attained by rapid solidification processing (RSP). The amorphous structure of the ribbons made by RSP was confirmed with x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The study of the devitrification reaction in the Al₈₈Y₇Fe₅ amorphous alloy indicated that heterogeneous nucleation with a transient period governs the nucleation of the α-Al phase during primary crystallization. During the isothermal annealing of Al₈₈Y₇Fe₅ at 245°C, the estimated steady state heterogeneous nucleation rate following a transient period is about 1 × 1020 m−3 s−1. The examination of the ribbons under small angle x-ray scattering (SAXS) revealed structural heterogeneities which are believed to give raise to the high particle number density (∼10 21m−3) of α-Al nanocrystals formed during primary crystallization in Al-based amorphous alloys. The structural heterogeneities are generated during the melt-quenching process in RSP as evidenced by the comparison analysis of the amorphous phase formed by melt-spinning and by cold-rolling. The quenching rate in melt-spinning was varied to examine the formation model for amorphous alloys with particular emphasis on growth control kinetics. The reduced size of the structural heterogeneities and the decreased nucleation frequency of α-Al with increased quenching rate in RSP confirmed that Al-based amorphous alloys that exhibit a primary crystallization reaction are formed under growth control kinetics. The strategy to enhance the particle number density of α-Al nanocrystals within the amorphous matrix by the inoculation of catalytic element such as Pb has been demonstrated to enhance the particle number density by an order of magnitude (from 1021 to 1022m−3). The crystallization of the α-Al phase on the quenched-in nanocrystalline Pb particles was evidenced by the presence of Moire fringes under HRTEM examination. Moreover, the incorporation of Pb facilitated the nucleation of α-Al not only on the crystalline Pb, but also in the amorphous matrix. The deformation of melt-spun amorphous ribbons by cold-rolling has lead to the successful synthesis of amorphous and nanostructured alloys in bulk form. In addition, it has been revealed that the amorphous phase can be retained after cold-rolling the fully amorphous melt-spun ribbons free of quenched-in nuclei, but crystalline phases were detected within the amorphous matrix in the cold rolled melt-spun amorphous ribbons with quenched-in nuclei.