STRUCTURE OF CU-AU ALLOY NANOSCALE PARTICLES AND THE PHASE-TRANSFORMATION

Atomic structure of nanoscale particles of a Cu-Au alloy with a nomina l composition Cu3Au and the phase transformation therein are studied b y means of high-resolution electron microscopy and electron diffractio n. The particles 8.8 nm in diameter on average prepared by simultaneou s vacuum deposition of the constituent elements exhibit as a whole an fee structure of an alloy with a composition of 26.4 at.% Au. The allo y particles are ordered into the L1(2)-type superlattice structure whe n heat-treated at 563 K for 1 h. The superlattice reflections disappea r upon heating up to 773 K. In a particle about 10 nm in size an antip hase domain boundary is observed. It thus appears that the nanoscale p articles of the alloy undergo the order-disorder transition, as in the bulk. However, the critical temperature T-c for the order-disorder tr ansition of the nanoscale particles is found to be by about 90 K lower than that of the bulk. The minimum size of the alloy particles in whi ch lattice fringes whose spacing corresponds to the interplanar spacin g of the {100}-type superlattice planes are observed is about 4 nm. Th ese facts suggest that a certain critical size and size effects are pr esent for atomic ordering in the Cu-Au alloy particles.