Nanocrystallization Reactions in Amorphous Aluminum Alloys

Abstract

Primary crystallization is the key reaction that controls the synthesis of nanostructured bulk volumes comprised of a high density (10²¹–10²³ m⁻³) of nanocrystals (7–20 nm) within an amorphous matrix. The primary crystallization kinetics in response to the annealing and the deformation of amorphous Al alloys are assessed in specific sample types and selected thermal treatments to evaluate primary nanocrystallization reactions. All amorphous Al alloy compositions are hypereutectic so that the initial phase selection of primary Al proceeds at a reduced driving free energy compared to thermodynamically favored intermetallic phases. Differential scanning calorimetry (DSC) studies on powders and melt spun ribbon (MSR) samples based upon thermal cycling and annealing below the glass transition, T_g, demonstrate a strong sensitivity of the primary crystallization onset and reaction enthalpy to thermal history and the as-quenched state. Microcalorimetry investigations and careful analysis of nanocrystal size distributions for Al₉₂Sm₈ MSRs following sub-T_g anneals reveal a partial nanocrystallization reaction resulting from a transient, decaying nucleation rate and a limited supply of heterogeneous nucleation sites. While crystallization is generally thought of as a thermally activated process, it can also be induced in response to external forcing such as irradiation or mechanical alloying. Intense deformation of amorphous Al₈₈Y₇Fe₅ MSR, for example, yields a distribution of Al-nanocrystallites in the amorphous matrix without thermal annealing. Moreover, the results of cold-rolling experiments with melt-spun amorphous Al₈₅Ni₁₀Ce₅ ribbons show that the deformation process can alter the phase selection upon annealing. These results suggest that the shear process during rolling effects a local rearrangement of atoms in the amorphous matrix. The kinetics behavior highlights the important role of the as-synthesized amorphous structure, reaction pathways and transient conditions on the evolution of nanoscale microstructures during primary crystallization.