The Analysis of Thermal Stability and Crystallization of Structural Amorphous Alloys
Abstract
Controlled synthesis of bulk nanostructured volumes comprised of a high density (10(exp 21)-10(exp 23) m(exp -3)) of nanocrystals (7-20nm) dispersed throughout an amorphous matrix requires a thorough understanding of the primary crystallization reactions responsible for their transformation. The primary crystallization of Al from amorphous Al alloys generally is observed in the competitive kinetics of devitrification in spite of a reduced driving free energy, which results from a hypereutectic composition within the amorphous matrix. Differential scanning calorimetry (DSC) studies of Al(sub 92)Sm(sub 8) and Al(sub 87)Ni(sub 10)Ce(sub 3) alloys, based upon sub-T(sub g) annealing treatments, demonstrate a strong sensitivity of the primary crystallization onset and reaction enthalpy to thermal history and the as-quenched state. Calorimetry investigations, careful analysis of nanocrystal size distributions, small angle x-ray scattering studies, and fluctuation electron microscopy investigations of Al(sub 92)Sm(sub 8) MSR (melt spun ribbon) following sub-Tg anneals reveal a transient, decaying nucleation rate and a limited supply of heterogeneous nucleation sites that may originate from the medium range order (MRO) detected in the as-quenched state. In Al(sub 88)Ni(sub 8)Sm(sub 4) melt-spun ribbon (MSR), incremental substitutions of Cu for Ni (0-1at%) affect the thermal stability of the material (crystallization onset shifts to lower temperature) and refine the size of the primary phase nanocrystals. However, continuous heating calorimetry measurements indicate that the primary crystallization enthalpy remains approximately constant with increased Cu substitution. From a structural analysis standpoint, quantitative microstructure examinations applied in parallel with calorimetry measurements have been employed to characterize the as-quenched volume of MSR samples.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 06, 2006
- Accession Number
- ADA442462
Entities
People
- John H. Perepezko
Organizations
- University of Wisconsin–Madison