| The recent discovery of Al-rich glasses containing {dollar}approx{dollar}85 at% Al and a combination of transition and rare earth element additions has yielded microstructures of Al nanocrystals in an amorphous matrix with nanocrystal volume fractions approaching 40% and excellent mechanical properties. This characteristic microstructure is synthesized by a primary crystallization reaction that yields a density of {dollar}{lcub}>{rcub}10sp{lcub}20{rcub}{dollar} m{dollar}sp{lcub}-3{rcub}{dollar} nanocrystals and limited growth. Although it was reported that the mechanical properties was strongly related to the size and density of the nanocrystals, information about the formation and decomposition of the Al-rich glasses was limited. In the present study, the aluminum-rare earth-transition metal (Al-RE-TM) systems were found to require high {dollar}rm(Delta T>500spcirc C){dollar} levels of undercooling to form an amorphous phase. The high level of undercooling required to reach the glass transition, along with the number and potency of sites active in this study have prevented the formation of an amorphous phase in droplet samples. In contrast, rapid solidification processing with cooling rates on the order of {dollar}10sp5{lcub}-{rcub}10sp6spcirc{dollar}C/sec of Al-Y-Fe, Al-Y-Ni and Al-Sm alloys has produced microstructures that are predominately amorphous. Thermal analysis has shown that even though the x-ray diffraction (XRD) and transmission electron microscopy (TEM) do not show any evidence of crystalline structure, quench-in nuclei that develop into Al-nanocrystals with an approximate density of {dollar}10sp{lcub}21{rcub}{lcub}-{rcub}10sp{lcub}22{rcub}{dollar} m{dollar}sp{lcub}-3{rcub}{dollar} are formed in the Al-Y-Fe and Al-Sm systems. The growth of the Al nanocrystals occurs below a characteristic thermal peak that is associated with the glass transition (T{dollar}rmsb{lcub}g{rcub}){dollar} and additional Al nanocrystal formation. Above T{dollar}rmsb{lcub}g{rcub}{dollar} the governing diffusion conditions allow for additional nucleation of Al nanocrystals and rapid diffusion field impingement. Further annealing at temperatures above the characteristic peak will result in the formation of intermetallic which will eventually consume the amorphous material. The identification of the factors that govern amorphous phase formation in Al-RE-TM along with the decomposition events that are associated with nanocrystalline Al formation has allowed for a formulation of strategies for Al-RE-TM alloy design. |
