Effect of γ2 phase evolution on mechanical properties of continuous columnar-grained Cu–Al–Ni alloy

Effect of γ2 phase evolution on mechanical properties of Cu–14%Al–3.8%Ni (mass fraction) alloy wires fabricated by continuous unidirectional solidification technology was investigated before and after heat treated at 700–780 °C. Mechanism for the improvement of mechanical properties of the alloy was analyzed. It was found that the alloy retained continuous columnar grains after heat treatment. With the heat treatment temperatures increasing from 700 °C to 770 °C, the coarse dendrite γ2 phase evolved into the fine polygonal, ellipsoidal and spherical particles in the grains, while the long-banding, discontinuous block and ellipsoidal particles at the grain boundaries. The average size of the γ2 phase decreased from 20 μm before heat treatment down to 2 μm, and its amount reduced from 50.0% down to 0.8%. The γ2 phase was full dissolution at 780 °C. The tensile strength of the alloy treated at 700–780 °C ranged from 577 MPa to 710 MPa. The elongation of the alloy ranged from 7.5% to 23.8% and had a peak value at 760 °C. Excellent balance between strength and elongation could be obtained when the spherical γ2 phase was distributed throughout the alloy with the size smaller than 5 μm and the amount in the range of 11.4–16.6%. The nano-hardness and elastic modulus of the γ2 phase decreased gradually with the reduction of the amount, leading to the improvement of mechanical properties.