The mechanism of anneal-hardening phenomenon in extruded Zn–Al alloys

An anomalous anneal-hardening phenomenon was studied in extruded Zn–Al alloys. The extruded Zn–Al alloys were isothermally heated above the eutectoid point for 10 h, and then cooled with furnace. The anneal-hardening phenomenon was observed in specimens with aluminum content ranging from 5 wt% to 25 wt%. Microstructure evolution during the annealing process was investigated. The annealing–hardening phenomenon was accompanied by phase transformation from a suspensive Zn-rich α2 phase to an equilibrium Al-rich α1 phase and a Zn-rich η phase. The hardness of the alloys increased with the decreasing volume fraction of the α2 phase. The hardness of the α1 phase and the η phase was measured and the hardness of the α2 phase was calculated. The mechanism of the anneal-hardening phenomenon in extruded Zn–Al alloys can be attributed to phase transformation from the soft α2 phase to the harder α1 + η phases.