Effect of annealing on damping capacities of as-cast ZA27 alloy

ZA27 alloy was prepared by casting with permanent mold and then annealed at 250°C for 1-4 h. The damping capacity of the alloy was measured using a testing apparatus based on the cantilever beam technique. It was found that the as-cast ZA27 alloy possesses high damping capacity with the value of 1.3 × 10^-3 at 320 Hz. After annealed at 250°C for 1h, the damping capacity decreases to 1.1 × 10^-3 and then remains constant even when the annealing time is increased to 4h. The microstructure of the as-cast ZA27 alloy consists of large dendrites of Al-rich primary α-phases, eutectoid (α + η) and nonequilibrium eutectic phases (α + η + ε). After annealing at 250°C for 1h, the ε phase disappears due to dissolution into the matrix, and the spacing between the flakes of eutectoid increases. The further increase in the annealing time has little effect on the spacing. The damping mechanism of the alloy was discussed considering the thermoelastic damping and defect damping. The value of thermoelastic damping accounts only for 7%-8% in the overall damping in cantilever beam damping measurements and the damping capacity of the ZA27 alloy came mainly from defect damping.