Static and in situ TEM investigation of phase relationships, phase dissolution, and interface motion in Ag-Au-Cu alloy nanoparticles

Ternary nanoparticles of Ag-Au-Cu alloy were fabricated by physical vapor deposition of a ternary alloy source onto a heated amorphous carbon film in ultra-high vacuum. The as-cooled nanoparticles were studied using bright-field imaging, selected-area diffraction, energy-dispersive X-ray spectroscopy and high-resolution imaging in the transmission electron microscope (TEM). These analyses revealed the presence of two phases in the 20-50 nm diameter nanoparticles: a Ag-rich fcc α₁ phase and an ordered L1 ₂ phase (AuCu₃ prototype). The phases were found to have a cube-on-cube (or parallel) orientation relationship with α₁- L1₂ interfaces that were parallel to {1 0 0}. In situ heating of the particles in the TEM showed that the α₁-L1₂ {1 0 0} interface dissolved by steps one moiré-fringe spacing high, consistent with recent theories of interface movement. In contrast to previous studies on the stability of ordered phases in Au-Cu alloys, the ordered L1₂ phase in the Ag-Au-Cu alloy remained at temperatures several hundred degrees higher than predicted from the equilibrium bulk phase diagram. This may be caused by elastic stabilization of the phase due to the small size of the particles and/or composition modulations induced by the surface.