NANOCRYSTAL DEVELOPMENT IN ALUMINUM-BASED METALLIC GLASSES

摘要

Many aluminum-base glass forming alloys such as Al_(92)Sm_8 and Al_(88)Y_7Fe_5 require rapid quenching for vitrification and often do not display a clear glass transition signal upon reheating. This marginal glass formation behavior is related mainly to growth limitations that prevent a complete crystallization during quenching. The same kinetic control also provides the basis for the development of a high number density of nanocrystals during a primary crystallization reaction upon reheating. In order to investigate the kinetics of the nanocrystallization process quantitatively, the crystallite size distributions of annealed Al_(92)Sm_8 samples have been obtained by transmission electron microscopy techniques. Moreover, the nanocrystallization process has been monitored isother-mally at low crystal growth rates on long time scales to include the early stages of the nanocrystallization process into the kinetics analysis. Additionally, modulated-temperature calorimetry has been used to separate the reversible contributions to the calorimetric signal due to the glass transition from irreversible contributions due to the formation of nanocrystals. These experimental results serve as a basis for a detailed description of the kinetics of the nanocrystal development and a comparison with respect to different theoretical models for the nanocrystallization process in amorphous aluminum-rich alloys.