Detailed relationship between the microstructure and properties of age-hardened Cu–4 at% Ti alloy

The detailed structure of the boundary between the Cu matrix and precipitate formed during the aging process of the Cu–4 at% Ti alloy was analyzed using atom probe tomography. The precipitate formed during peak aging was an ordered α-Cu4Ti (D1a, MoNi4-type) phase, the central part of which had an order degree of one, with a composition of 20 at% Ti. However, the outer part was a thick diffuse boundary with a gradually decreasing order degree in the radial direction and a composition range of 5–20 at% Ti. To analyze the detailed structure of the diffuse boundary, we hypothesized that a molecule-like motif (MLMh, (Cu4Ti)nTi) is the basic structure of the D1a phase, that is, the tight aggregates of MLMh form the full ordered D1a phase. MLMh can be represented by a compositional formula similar to a molecule, be dispersed in a Cu matrix, and form a diffuse boundary which is the low ordered D1a phase. MLMh is generated during the quenching process of the solution treatment, grows through subsequent aging, and aggregates via Ostwald ripening. The Ti atoms in MLMh are attracted linearly by the Ti 4sp3 orbital with a conjugated structure, which induces tetragonal distortion (a/c