High Strength Copper-Titanium Alloys: Part Two

The following has been found in Cu–Ti alloys research:

- The equilibrium phase in the age hardening Cu–Ti alloys has been reported to have the composition Cu₄Ti and may exhibit a polymorphic transformation. The high temperature phase is the Au₄Zr type orthorhombic phase with the space group Pnma and the low temperature polymorph is the tetragonal D1_a phase.

- The equilibrium phase in the Cu–Ti system is generally formed by classic Widmanstatten or cellular precipitation. The cellular or ‘‘discontinuous’’ precipitation reaction plays a central role in the overaging of these high-strength alloys. The growth kinetics of the cellular microconstituent is governed by an activation energy of less than half the activation energy for lattice diffusion of Ti in Cu.

- The formation of fine-scale coherent/semicoherent D1_a (Cu₄Ti) precipitates at high supersaturation imparts to these age hardened alloys strength levels comparable to Cu–Be alloys. The quasi-periodic arrays of D1_a particles or modulated structures coarsen according to a generalized LSW kinetic law with an activation energy of~50 kcal mol^-1 in agreement with reported values for the activation energy of Ti in Cu.

- The early stages of decomposition of supersaturated Cu–Ti alloys involves a complex interplay of ordering and clustering effects in solution. The formation of the coherent two-phase mixtures almost certainly involves non-classical nucleation or spinodal decomposition within the context of a generalized nucleation theory and clear definition of the spinodal process.

- The properties of an age-hardened copper titanium alloy are drastically improved and have only a very narrow range of variation in properties if its structure has an average crystal grain size not exceeding a certain level. This is attained by age-hardening copper titanium alloy which is subjected to a substantially full degree of solution heat treatment and thereby given a structure having an average crystal grain size not exceeding 25 microns.

This limitation is necessary to improve the formability, fatigue strength, elongation and other properties of the alloy drastically, as compared with those of the conventional age-hardened copper titanium alloy having an average crystal grain size of 40 microns or more. An average crystal grain size exceeding 25 microns does not give the alloy any substantially improved properties, and its properties have a wide range of variation.

Copper-titanium alloys consist mainly of copper and contain 2 to 6% by weight of titanium, the preferable content of titanium being 3 to 5% by weight. If they contain less than 2% by weight of titanium, no appreciable effect of age-hardening can be expected, and the addition of more than 6% by weight of titanium does not provide a correspondingly improved effect of age-hardening as the excess over 6% is increased. This mechanism is also applicable to other copper alloys containing at least one of other elements, such as Fe, Zr, Cr, B and Si, in addition to 2 to 6% by weight of titanium. Such other elements are generally contained up to 2.0% by weight in a total amount.

The hot-worked or cold-worked material is subjected to an intermediate annealing at a temperature which is lower than a solid solution-forming temperature and a recrystallization temperature. In other words, intermediate annealing is effected at a temperature lower than ordinary annealing in order to achieve the fine and uniform distribution and precipitation of a secondary phase in a master or matrix phase.

The term "master or matrix phase" as herein used means the α-phase in a binary phase diagram for a copper titanium alloy, and the "secondary phase" means the precipitate of an intermetallic compound expressed as Cu₃Ti. The "solid solution-forming temperature" is a temperature defining the boundary between the "α+Cu₃Ti" phase and the α-phase.

The use for intermediate annealing on a temperature which is lower than both the solid solution-forming and recrystallization temperatures is important for the precipitation of a fine and uniformly distributed secondary phase in the master phase. If the annealing temperature exceeds the solid solution-forming temperature, no secondary phase is precipitated in the master phase.

The fine and uniform secondary phase formed by the intermediate annealing in the master phase contributes to avoiding the coarsening of crystal grains in the master phase during the final solution heat treatment of the alloy and thereby developing a desired solution heat treated structure having an average crystal grain size not exceeding 25 microns.