Grain refinement during high temperature tensile testing of prior cold worked and peak aged Cu-Ti alloys: Evidence of superplasticity

The high temperature tensile properties of solution treated, cold worked and peak aged Cu-1.5 wt.%Ti and Cu-4.5 wt.%Ti alloys have been investigated in the temperature range from room temperature (RT) to 550 °C. Yield strength (YS) and tensile strength (TS) of Cu-1.5Ti alloy were found to be independent of test temperature up to 350 °C and decreased thereafter, up to 550 °C. In the case of Cu-4.5Ti alloy, behaviour similar to Cu-1.5Ti alloy was observed. The decrease in strengths beyond 350 °C is attributed to coarsening of β′ precipitate particles at 450 °C and precipitation of equilibrium phase β at 550 °C. TEM analysis of the alloys confirms the presence of metastable and coherent Cu4Ti β′ precipitate when tested at 450 °C and equilibrium precipitate Cu3Ti, β at the test temperature of 550 °C. It is interesting to note that in the cold worked and peak aged Cu-Ti alloys tested at elevated temperatures, ductility (% elongation) was found to increase with the test temperature up to 550 °C which is attributed to dynamic recrystallisation. The large elongation (160%) observed in Cu-4.5Ti alloy at the test temperature of 550 °C is an indication of superplasticity. We are reporting the discovery of superplasticity in Cu-4.5Ti alloy for the first time. Ductile mode of fracture was observed in these alloys tested at elevated temperatures, i.e. at 350 and 450 °C. High temperature tensile properties of Cu-Ti alloys are found to be superior to those of Cu-Be alloys (C17200 and C17510).