Cooling rate controlled basal precipitates and age hardening response of solid-soluted Mg–Gd–Er–Zn–Zr alloy

Abstract

The precipitation and age hardening response of the solid-soluted Mg–10Gd–1Er–1Zn–0.6Zr (wt.%) alloy performed by water-quenching (QC), air-cooling (AC) and furnace-cooling (FC) in terms of the volume fraction of precipitates and tensile properties were investigated in present paper. Results indicated the solid-soluted alloy contained stacking faults (SFs) and long period stacking ordered (LPSO) phase on the basal planes regardless of the cooling rate, but a larger volume fraction of the LPSO phase was formed with decreasing in the cooling rate. After aging, β′ and β1 phases precipitated on the prismatic planes, and their number density decreased but mean particle size increased with decreasing in the cooling rate. The solid-soluted alloys (QC, AC and FC samples) showed no apparent difference in yield strength (YS), but their correspondent peak-aged alloys exhibited sharp difference in hardening response. The strongest hardening response took place in the QC sample and showed 82 MPa enhancement in YS, which was much larger than that of AC (+26 MPa) and FC samples (+5 MPa). The reason lies in that the higher cooling rate promotes the precipitation and reduces the average size of β′ precipitate. A novel cooling-rate controlled precipitation model with respect to the correlation of precipitates on basal and prismatic planes was established. From this model, the basal precipitates showed a restrictive effect on the growth and/or coarsening of β′ precipitate, and composite precipitates containing the β′ phase with fine size as well as high area-number density and lower volume fraction of the LPSO phase are preferred to strengthen the Mg–10Gd–1Er–1Zn–0.6Zr alloy.

Keywords

• Magnesium alloy
• Long period stacking ordered (LPSO)
• Stacking faults (SFs)
• Cooling rate
• Tensile properties