Effect of chemical composition and quenching media on recoverable strain in Cu–Zn–Al alloys

The correlation of the thermomechanical parameters and chemical composition, which enables to determine the phases providing properties related to reliable, recoverable strain, was studied for three Cu-(21.6–25.4)Zn-(3.3–5.6)Al alloys. The conventionally cast samples were hot forged and cold-rolled, followed by quenching in different media and aging treatment. The alloy A with low aluminum content (3.3%) has exhibited better tensile strength and elongation than B and C samples, in addition to good low-temperature workability and refined grain. The stress plateau recorded on the stress–strain curve during tensile testing of quenched specimens was related to the recoverable strain of up to 4.2%. Most of the examined specimens have shown shape memory recovery above 90%. The apparent activation energy for microhardness increase in the examined alloys during aging at 200 °C–500 °C for up to 30 h was (55.3 ± 3.9) kJ mol−1 compared with (55.7 ± 4.6) kJ mol−1, which was recorded for the decrease in shape memory recovery. These results suggest a common origin for both changes. The precipitate was formed along the grain boundaries and the matrix. Neither the coarsening of α-precipitate nor γ-precipitate was observed during the aging of examined alloys for up to 30 h. Martensite stabilization was avoided in A and B alloys as a result of the step quenching treatment. A relatively short life expectancy was predicted for alloy C, while good shape memory properties may be expected for alloys A and B during their exposure to temperature up to 50 °C.