Effect of cryo-rolling on age hardening of Cu–1.5Ti alloy

Cryo-rolling, i.e. deformation at liquid nitrogen temperature (LNT) has been attempted on several metals to obtain very fine grain sizes and improved strength levels. In the present study, the influence of cryo-rolling with deformation up to 90% on age hardening behaviour of Cu–1.5Ti alloy has been investigated using hardness and tensile tests and optical as well as transmission electron microscopy (TEM). Hardness increased from 75 Hv in solution treated condition to 300 Hv on 90% cryo-rolling and peak aging. The yield strength (YS) and ultimate tensile strength (UTS) reached a maximum of 824 and 854 MPa, respectively on 90% cryo-rolling and peak aging. However, elongation decreased to about 4%. In comparison, mechanical properties of the cryo-rolled and peak aged Cu–1.5Ti alloy are superior to those of the room temperature (RT) rolled and peak aged. Rolling at liquid nitrogen temperature resulted in extremely high-dislocation density, which in turn has lead to nucleation of still higher density of precipitates during aging thereby giving rise to higher strength properties. The maximum strength on peak aging was obtained due to precipitation of ordered, metastable and coherent βl, Cu4Ti phase in addition to deformation twins and dislocation cells. Overaging resulted in decrease of hardness and strength due to the formation of incoherent and equilibrium β, Cu3Ti phase. Recrystallisation leading to formation of fine grains has not been observed in the cryo-rolled Cu–1.5Ti alloy aged at 400 °C or even at 500 °C.