Thermally Controlled Nanocrystallization in Amorphous Al Alloys

Abstract

In many amorphous Al-alloys, primary crystallization is the key reaction that controls the synthesis of high strength nanostructured bulk volumes comprised of a high density (10 2 1 - 10 2 3 m - 3 ) of nanocrystals (diameter between 7 - 20 nm) within a residual amorphous matrix. The primary crystallization kinetics, in response to pre-crystallization annealing treatments, are assessed in specific sample types that serve as model systems to evaluate primary nanocrystallization reactions. Differential scanning calorimetry (DSC) studies on powders and melt spun ribbon (MSR) samples based upon thermal cycling and annealing below the glass transition, Tg, demonstrate a strong sensitivity of the primary crystallization onset and reaction enthalpy to thermal history and the asquenched state. Although the sensitivity of conventional DSC facilities is not sufficient to detect the isothermal heat evolution at 130°C, the integrated effect of the heat evolution is clearly detected by DSC as a reduction in primary crystallization enthalpy during continuous heating. Varying heating rates in continuous heating calorimetry measurements demonstrates that a significant amount of crystallization occurs during continuous heating through the primary crystallization following annealing. The kinetics behavior highlights the important role of the as-synthesized amorphous structure, the reaction pathways and transient conditions and their interplay on the evolution of nanoscale microstructures during primary crystallization. The support of the DFG (WI 1899/1-2) and the ARO (DAAD 19-02-1-0245) is gratefully acknowledged.