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

Abstract

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 Cu 4Ti β′ precipitate when tested at 450 °C and equilibrium precipitate Cu 3Ti, 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).