Abstract
Cu-0.2, 0.4, 0.8, 1.3, 1.9 and 3.9 wt%Ti alloys were aged at 450°C and then cold-rolled by 15, 30, 60 and 90% reductions in thickness, respectively. Hardening- and softening behaviors during isothermal annealing at 450°C of the cold-rolled specimens were studied by means of hardness measurement, X-ray diffraction technique, optical and transmission electron microscopy. The following conclutions were obtained:
(1) In the age-hardened and cold-rolled Cu-1.3, 1.9 and 3.9%Ti specimens hardening occurred in the early stages of the annealing process and then two steps of softening followed. The amount of anneal-hardening increased with increasing titanium content and increasing deformation. The maximum increment of hardness obtained during annealing after cold work was about 45 Hv; ∼12% of the hardness value of the same as-rolled specimen. This type of anneal-hardening was not obtained in Cu-0.2, 0.4 and 0.8%Ti specimens.
Formation of fine particles in the early stages of annealing after cold work in the vicinity of deformation bands may be an important factor of hardening, but the mechanism of this anneal-hardening is not unknown.
(2) The anneal-softening is due to the following three processes: (i) Growth and coalescence of Cu3Ti particles (equilibrium phase), (ii) recovery and recrystallization, and (iii) formation and growth of cellular precipitates. In the case of large amount of deformation, the processes (i) and (ii) are important, and the process (iii) is the main after a small amount of deformation. In the specimens with higher titanium content, the process (i) or (iii) is promoted and softening proceeds faster.
