Microstructure Selection in Severely Deformed Al-Base Systems

Abstract

Many Aluminum base glass forming alloys require extremely high cooling rates for vitrification and often do not display a clear glass transition signal upon reheating. This marginal glass formation behavior is related mainly to high nucleation rates coupled with 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 (10 2 2 m - 3 ) of nanocrystals (diameter about 10-20 nm) during a primary crystallization reaction - the so-called nanocrystallization - upon reheating. With alternate synthesis routes based upon solid state alloying resulting from severe plastic deformation, e.g. by repeated cold rolling and folding, the kinetic pathways to glass formation can be altered to avoid nanocrystallization reactions in the marginal glass forming alloys. In other systems, a deformation-induced nanocrystal synthesis can be observed during repeated cold rolling of amorphous ribbons. The microstructure selection in dependence of the processing pathway was monitored by X-ray diffraction and electron microscopy techniques. Additionally, modulated-temperature calorimetry has been used to detect the calorimetric signature of the glass transition on melt-quenched samples directly. The results are discussed with respect to the origin of the nanocrystallization, especially the formation of nanocrystal precursors during rapid melt quenching. Moreover, the development of the microstructure during cold-rolling and the comparison of the compositional ranges that allow the synthesis of vitreous product structures by severe plastic deformation or rapid quenching are analyzed in terms of the major thermodynamic - and mechanical properties that govern intermixing during the deformation process.