Desgranges Caroline, Delhommelle Jerome
Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202, USA.
Phys Rev Lett. 2018 Mar 16;120(11):115701. doi: 10.1103/PhysRevLett.120.115701.
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_{6}Cu_{4}, which has a positive heat of mixing, and CuZr, characterized by a large negative heat of mixing. Our results allow us to identify this unusual behavior as directly correlated with a nonmonotonic 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.
通过分子模拟，我们揭示了金属合金中晶体成核的潜在机制，并阐明了晶体成核与玻璃转变之间的相互作用，因为结晶条件接近这种转变。虽然降低结晶温度通常会导致更低的自由能垒，但我们发现，随着结晶温度接近玻璃转变温度，形成玻璃的合金会出现意想不到的行为反转。为此，我们模拟了两种形成玻璃的铜合金的结晶过程，一种是具有正混合热的Ag₆Cu₄，另一种是以大的负混合热为特征的CuZr。我们的结果使我们能够确定这种异常行为与连接二十面体结构的形成能的非单调温度依赖性直接相关，这些结构与晶体有序性不相容，并阻碍晶核的发展，导致低温下出现意想不到的更大自由能垒。反过来，这促进了主要由紧密堆积的临界核的形成，缺陷更少，从而提出了一种控制晶核结构的新方法，这在催化中至关重要。
