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Many metallic glasses undergo glass transitions at Tg some times up to more than
100 K below their crystallisation temperatures Tx. They thus possess
a so-called super-cooled liquid region ΔT = Tx - Tg to which they
can be heated in principle without crystallization. This temperature region has been
used for shaping and joining metallic glasses or for writing motifs on them.
The flow stress σflow required to achieve such shape changes in the ΔT temperature
range depends on the viscosity which η is strongly temperature dependent near Tg.
Here we report microscopic shape changes occurring under no applied stress simply due to surface tension g. Surfaces of melt-spun metallic glasses carry traces of shape instabilities. These surface defects are usually deep pockets of gas trapped on the cooling substrate (wheel) side and very thin (few microns) elongated "wave-like" defects on the free surface side. Pd-based melt-spun ribbons were heated in protective atmosphere to 583K which is just above Tg and the free surfaces photographed after different times. Surface defects were found to disappear or "self-heal" in the ΔT temperature range. The results are discussed in terms of the experimental self-deformation rates and viscosity. |
full paper (pdf, 186 Kb)