Precipitation-hardening in Cu-3.6 wt PCT Ti

Abstract

The room temperature tensile properties of age-hardened Cu-3.6 wt pct Ti were investigated. The resulting structures of both single- and double-aged samples were examined by transmission electron microscopy. Emphasis was placed on the correlation of structure with mechanical properties. Isothermal aging at 400°C resulted in an increase in yield strength from that of 40 ksi in the solution-treated alloy to a maximum of 98 ksi after aging for 10,000 min (167 hr). The structure associated with maximum strength was that of an aligned disordered cubic coherent precipitate. Examination of deformed specimens showed that slip was heterogeneous both within a grain and along a given shear band. Aging at 550°C resulted in the same aligned cubic coherent precipitate as at 10 min, but continued-isothermal holding caused the growth of a weaker recrystallized structure and thus a decrease in strength. It was concluded that overaging in this system was due entirely to the consumption of the continuous precipitate by the discontinuous. This recrystallized constituent was found to be composed of a copper-rich solid solution and Cu3Ti arranged in alternate parallel lamellae. The hcp Cu3Ti precipitate had its close-packed plane parallel to the {Ill} plane of the matrix, and edge dislocations were found to take up the misfit between these two structures. A series of single- and multiple-aging treatments gave three completely recrystallized structures with approximately the same yield strength. However, the total elongation of these structures decreased as the final aging temperature was reduced. This was attributed to an ordering reaction in the Cu3Ti plates at lower temperatures. Direct evidence of a change in deformation characteristics was obtained by examining specimens deformed to the same extent, but having different degrees of order induced by varying the final aging temperature. The disordered plates were shown to plastically deform along with the matrix, but the ordered plates fractured.