Detailed relationship between the microstructure and properties of age-hardened Cu–4 at% Ti alloy
Kazuhiko Fukamachi
Abstract
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<1) in the face-centered cubic sublattice in D1a. The Ti–Ti axial toughness MLMh provides slip resistance toward dislocations. Therefore, after aging for 5 min, the strength of the alloy increased to 64% of the incremental strength at peak aging. In the solution-treated state, conductive electrons transfer to the orbitals of MLMh dispersed in the Cu matrix, resulting in high electrical resistivity, but the electrical resistivity decreases due to phase separation via aging. MLMh explains in detail the growth process of the aging structure, the structure of the diffuse boundary, and the cause of its properties.