Unusual crystallization behavior close to the glass transition

TL;DR

This study uses molecular simulations to explore how crystal nucleation in metal alloys behaves near the glass transition, revealing unexpected reversal in crystallization trends linked to structural energetics.

Contribution

It uncovers the non-monotonic temperature dependence of formation energy of icosahedral structures affecting nucleation near the glass transition in copper alloys.

Findings

Unusual increase in free energy barrier at low temperatures near glass transition.

Correlation between icosahedral structures and impeded crystal nucleation.

Potential to control crystal structure for catalytic applications.

Abstract

Using molecular simulations, we shed light on the mechanism underlying crystal nucleation in metal alloys and unravel the interplay between crystal nucleation and glass transition, as the conditions of crystallization lie close to this transition. While decreasing the temperature of crystallization usually results in a lower free energy barrier, we find an unexpected reversal of behavior for glass-forming alloys as the temperature of crystallization approaches the glass transition. For this purpose, we simulate the crystallization process in two glass-forming Copper alloys, $Ag_6Cu_4$, which has a positive heat of mixing, and in $CuZr$, characterized by a large negative heat of mixing. Our results allow us to identify that this unusual behavior is directly correlated with a non-monotonic temperature dependence for the formation energy of connected icosahedral structures, which are incompatible with crystalline order and impede the development of the crystal nucleus, leading to an unexpectedly larger free energy barrier at low temperature. This, in turn, promotes the formation of a predominantly closed-packed critical nucleus, with fewer defects, thereby suggesting a new way to control the structure of the crystal nucleus, which is of key importance in catalysis.