Abstract
The effect of long period stacking ordered (LPSO) phase and γ′ precipitates on the ageing behavior and mechanical properties of the extruded Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt.%) alloy was investigated. The results show that more β′ phases precipitate during ageing treatment in the LPSO phase containing alloy so that the LPSO phase containing alloy exhibits a higher age-hardening response than the γ′ precipitates containing alloy. The precipitation strengthening induced by β′ precipitates is the greatest contributor to the strength of the peak-aged LPSO-containing alloys. Higher strength is achieved in γ′ precipitates containing alloy due to the more effective strengthening induced by dense nanoscale γ′ precipitates than LPSO phases as well as the higher volume fraction of coarse unrecrystallized grains with strong basal texture. The extruded alloy containing γ′ precipitates after T5 peak-ageing treatment shows ultra-high tensile yield strength of 462 MPa, high ultimate tensile strength of 520 MPa, and superior elongation to failure of 10.6%.
Highlights
1~2 wt.% Zn addition to Mg–6Gd–0.6Zr alloy remarkably enhance the age hardening response by the formation of dense basal precipitate plates[8]
For the Mg-RE-Zn alloy system, when the rare-earth element is less than 10 wt.%, only long period stacking ordered (LPSO) phase/γ′precipitates are formed through specific heat treatment[17]
The dominant strengthening mechanism would be dependent on the precipitate phase formed in the alloys and it is expected that high strength can be achieved by simultaneous precipitation of plate-shaped LPSO phases and/or γ′precipitates on basal and β′phase on prismatic planes of α-Mg alloys
Summary
∆τbLaPsaSlO and ∆τbγa′sal in the unDRXed regions for the FEA and QEA are calculated to be 1.6 MPa and 11 MPa, respectively, suggesting that the finer γ′precipitates exhibits much higher strengthening effect than the coarse LPSO phases. The strengthening caused by the β′precipitates is slightly higher in the FEA than QEA and prismatic precipitates are much effective than the LPSO phases and γ′precipitates in blocking the basal dislocation slip. While the QE and QEA with low DRX ratio of 31% in the present study show reversed yield anisotropy, the values are almost identical to those values for the FE and FEA with plate-shaped LPSO phases This indicates that the dense γ′precipitates may show higher strength in compression in tension as LPSO phases and further research is needed to clarify the mechanism. The peak-aged γ′precipitates containing alloy exhibits tensile yield strength of 462 MPa, ultimate tensile strength of 520 MPa and elongation to failure of 10.6%
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