Investigation of precipitation behavior in age-hardenable Cu-Ti alloys by an extraction-based approach

We investigated the precipitation processes in Cu-4 mol pct Ti alloy specimens aged at 723 K (450 °C), by means of X-ray diffraction and chemical analyses of the precipitates extracted from the parent alloy specimens. Aging-induced precipitate particles of a spinodally decomposed disorder, α'; those of a metastable order, β'-Cu₄Ti; and those of a stable order, β-Cu₄Ti, were continuously formed in the aged specimens. The extraction of the precipitate particles from the aged specimens by submergence in a nitric solution allowed for not only the structural analyses of the constituent precipitate phases but also the quantitative evaluation of their chemical compositions and volume fractions. Early during the aging process, the supersaturated Cu solid solution decomposes spinodally in a continuous manner, and an unstable disorder, α', appears. Then, fine needle-shaped β'-Cu₄Ti particles, which have a Ti content of approximately 37.5 mol pct, form in the Cu matrix. During prolonged aging, coarse cellular components composed of the terminal Cu solid solution and stable β-Cu₄Ti particles which have a Ti content of 20.5 mol pct nucleate and grow, primarily in the grain boundaries, at the expense of the metastable β'-Cu₄Ti particles. The volume fraction of the β'-Cu₄Ti particles in the alloy reaches a maximum of approximately 1.7 pct after aging for 24 hours, while that of the β-Cu ₄Ti particles increases steadily to more than 18 pct after 480 hours. The volume fraction of the fine β'-Cu₄Ti particles in the alloy specimens remained constant throughout the age-hardening, indicating that the hardening is primarily owing to the fine dispersion of the β'-Cu ₄Ti particles and not because of the large volume fraction of coarse β-Cu₄Ti particles.