Kinetics and thermodynamics associated with age hardening of Cu-4at% Ti alloy

In this study, age-hardened Cu-4at%Ti alloys were studied using high-resolution scanning transmission electron microscopy and high-power X-ray diffraction to identify the hardening structures and their formation mechanisms. The formation of ordered alpha-Cu4Ti phase (D1(a), MoNi4-type) D1(a) domains during quenching following solution treatment was attributed to the isotropic random diffusion of Ti in the solution via a so-called reaction-diffusion mechanism. This first nucleation stage of the domains forms a Ti body-centered cubic sublattice on a Cu face-centered cubic lattice, thereby generating a fine D1(a) domains with a short-range-ordered superlattice. Subsequent anisotropic diffusion occurs because of variations in the radii of the fine domains, leading to Ostwald ripening during the second coarsening stage of the D1(a) domain formation process. During the early stage of aging, the number of domains sharply decreases until the peak aging condition. Ostwald ripening is considered to progress because D1(a) is a metastable and substitutional-solid-solution phase. As a result, diffusional transformation comprising reaction-diffusion and subsequent Ostwald ripening proceeded with aging to form a hardened structure. The domain diameters at peak aging ranged from 10 to 20 nm. It is proposed that this hardening mechanism, for which no spinodal decomposition is observed, be called ordered-domain dispersion hardening.