Crystalline monolayer surface of liquid Au–Cu–Si–Ag–Pd: Metallic glass former

Abstract

It is demonstrated by means of x-ray synchrotron reflectivity and diffraction that the surface of the liquid phase of the bulk metallic glass forming alloy Au 49 Cu 26.9 Si 16.3 Ag 5.5 Pd 2.3 consists of a two-dimensional crystalline monolayer phase for temperatures of up to about 50 K above the eutectic temperature. The present alloy as well as glass forming Au 82 Si 18 and Au-Si-Ge alloys containing small amounts of Ge are the only metallic liquids to exhibit surface freezing well above the melting temperature. This suggests that the phenomena of surface freezing in metallic liquids and glass forming ability are related and probably governed by similar physical properties.

Review of the highlights of X-ray studies of liquid metal surfaces

“…4 for the Au 82 Si 18 eutectic alloy 19 and which was found again for the Ag-CuSi-Ag-Pd glass forming alloy. 31 In view of the fact that this type of surface freezing is not observed for Au 72 Ge 28 , which has a similar eutectic phase and similar chemistry to Au 82 Su 18 but which does not form a metallic glass in the same way as Au 82 Si 18 , it is tempting to conclude that surface freezing is associated with the glass forming ability. This is one subject that needs further exploration.…”

“…30 Mechler et al suggested that surface freezing and glass formation might have a common origin. 31 The idea according to this line of thought is that the surface freezing effect is somehow associated with the constraint that the surface imposes on the short-range order of the bulk liquid. 31 This is clearly an issue that requires further investigation.…”

Structural features and high quasi-static strain rate sensitivity of Au49Cu26.9Ag5.5Pd2.3Si16.3 bulk metallic glass

“…The existence of icosahedral polyhedrons in the Au-based BMG was also suggested in Ref. 11. Internal structure of Au-based BMG can make a strong influence on the temperature of brittle-ductile transition.…”

“…In Ref. 11 it was discovered that in the liquid state this Au-based alloy also contains a very thin layer of a crystalline phase somewhat above the eutectic temperature. It is clear that this layer is responsible for yellow color of the rod side, while the internal amorphous phase has a typical grey metallic color as expected for the Au-alloy having a large Si content of 16.3 at.…”

“…The above-mentioned observation is in line with the finding that such a layer exists in the melt only close to the liquidus temperature. 11 As the liquid phase has lower Gibbs free energy above the liquidus temperature than a crystalline one, this process is thermodynamically unfavorable in the bulk sample and can be treated as a surface effect. Growth of this crystalline layer in the liquid state is impossible above the liquidus temperature for the thermodynamic reasons.…”