Understanding the atomically precise evolution of the miscibility of newly prepared face-centered cubic W-Cu nanoalloys and its asymmetry

TL;DR

This study demonstrates the atomically precise synthesis of face-centered cubic W-Cu nanoalloys with a large miscibility gap, revealing size-dependent phase behavior and a strain-induced mechanism that challenges classical theories.

Contribution

It provides the first comprehensive nanoalloy phase diagram for W-Cu, showing size effects on miscibility and introducing a strain-based mechanism for alloy stability.

Findings

W-Cu nanoalloys with ~2280 atoms exhibit FCC structure.

A large miscibility gap with a critical size of ~6000 atoms was identified.

DFT calculations suggest strain-induced surface energy reduction as key mechanism.

Abstract

According to classical Miedema theory, reducing crystals to the order of nanometer sizes might greatly modulate the mixing enthalpy of elements, thus enabling the invention of a lot of new bulk-immiscible alloys. Although numerous alloys with higher mixing enthalpies remain unexplored, this strategy is approaching its limit, as reflected by the critical diameter of recent alloys of 1.8 nm, which corresponds to ~150 atoms and hardly provides a crystalline order. Future development requires not only even smaller atomic-scale control but also a new surface energy-saving mechanism. Here, we report the formation of W-Cu nanoalloys with a very large miscibility gap in the bulk via the use of an atomically size-selected cluster beam source as an example. The face-centered cubic (FCC) structure was demonstrated through electron diffraction, which indicated a lattice constant of 3.88{\AA} for W0.85Cu0.15 nanoalloys (~2280 atoms). In this comprehensive study that covers a large parameter space of W/Cu compositions and numbers of atoms, an asymmetric miscibility nanophase diagram in which W-rich compositions favor mixing and the critical size is approximately 6000 atoms, which far exceeds the approximately tens of atoms predicted via classical theory, was obtained for the first time. Density functional theory (DFT) calculations revealed a mutual strain-induced mechanism that simultaneously lowers the surface energies while reducing the size to the atomic scale. This approach paves the way for the development of new high-performance nonequilibrium phase alloys.