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

Abstract 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 oC to 500 oC for up to 30 hours 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 hours. 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 oC.