Electron microscopy study of the coherent two-phase mixtures L1₀+ L1₂, in Co–Pt alloys

Abstract

Conventional and high resolution electron microscopy have been used to investigate the L1₂-L1₀, two-phase mixtures in the Co-Pt system. The domain of coexistence of the phases L1₂ (Copt₃) and L1₀ (Copt) has been determined. Three different processes of formation of the two phase mixture from the high temperature disordered state (A1) have been studied as a function of the aging temperature: (1) A1 → L1₀ → L1₀ + L1₂, (2) A1 → L1₂ → L1 ₀ + L1₂, (3) A1 → L1₀ + L1₂. In the final state, the microstructures present the same main topologies. Processes (1) and (2) involve two steps, the first one consists in ordering one of the single phases. The second step is shown to involve an interfacial wetting of the domains walls of the ordered structure formed during the first step. When the phases L1₀ and L1₂ simultaneously appear within the disordered matrix (process (3)), an archetypal microstructure is formed: it consists of a regular tiling of domains according to a precise and fascinating topology driven by long range strain interactions.