Influence of microalloying on glass formation and crystallization

The influence of extremely small amounts of early transition metals on glass formation has recently been demonstrated in Al-Y-Fe alloys. Although rapidly quenched samples of Al 88 Y 7 Fe 5 have X-ray diffraction patterns that are typical of amorphous metallic alloys, the isothermal differential scanning calorimetry (DSC) data do not show the expected nucleation and growth peak associated with the formation of nano-sized grains of α-Al. Samples prepared with Ti (Al 88-x Y 7 Fe 5 Ti x with x=0.5, 1, 2) transform at higher temperatures and show the DSC isothermal peak expected for glass crystallization. It is often claimed that the absence of an isothermal DSC peak indicates that the samples are not glasses, but are actually fully transformed nanocrystal composites, with the monotonic DSC trace indicating a coarsening of the nanocrystals. However, calculations presented here show that it could also be explained by diffusion-controlled nucleation and growth, with a high nucleation rate due to a change in the local structure of the glass.