High-strength age hardening copper–titanium alloys: redivivus

In this review the decomposition of supersaturated Cu–Ti solid solutions and subsequent microstructural evolution are discussed in terms of modern views of precipitation from solid solution. This update is motivated by an anticipated emergence of these alloys as technologically significant high-strength, high-conductivity, precipitation hardened alloys over the next decade replacing conventional Cu–Be alloys in numerous applications. The decomposition of Cu–Ti alloys is shown to involve a complex interplay between clustering and ordering effects including a synergy between LRO and SRO similar to those observed in Ni 4Mo-type concentrated solutions. New perspectives regarding metastable and stable phase equilibria in the system are discussed including the polymorphic nature of the Cu 4Ti precipitate phase. The role of non-classical nucleation and spinodal decomposition in the initial breakdown of the supersaturated state is addressed within the context of a generalized nucleation theory. The fine-scale coherent/semicoherent two-phase mixtures which emerge coarsen according to a LSW coarsening law with an activation energy for the diffusion of Ti in Cu of approximately 50 kcal mol −1 in excellent agreement with reported values from diffusion studies in the Cu–Ti system. Overaging in Cu–Ti age hardening alloys is associated with the emergence of a coarse lamellar microconstituent which nucleates at the grain boundaries of the parent matrix phase and the growth of these cells consumes the metastable, fine-scale coherent/semicoherent phase mixtures leading to a rapid degradation of mechanical properties. The activation energy for the growth of the cellular microconstituent is less than half that for bulk diffusion indicating interfacial/boundary diffusion control. It is suggested that to enhance and optimize the mechanical properties of the precipitation hardened Cu–Ti alloys, alloying and thermomechanical processing strategies should focus on controlling the nucleation and growth of the cellular or “discontinuous” precipitation reaction.