Microstructure and tensile creep resistance of Mg-5.5%Zn-(0.7%, 1.5%, 3.5%, 7.5%)Y alloys

Abstract

The tensile creep resistance of Mg-5.5%Zn-(0.7%, 1.5%, 3.5%, 7.5%)Y (mass fraction, %) gravity-casting alloys was investigated systematically. The corresponding physical models were established for analyzing the microstructure evolution and creep mechanism. The results show that four second phases are found in Mg-5.5%Zn-(0.7%, 1.5%, 3.5%, 7.5%)Y alloys, including Mg₃ZnY, Mg₃Zn₆Y, Mg₃Zn₃Y₂ and Mg₇Zn₃, where the rare earth rich phase (Mg₃ZnY, Mg₃Zn₆Y, Mg₃Zn₃Y₂) with high melting point can more effectively improve the creep resistance properties of alloys than Mg₇Zn₃. With the increasing of Y content, the creep resistance of alloys is improved correspondingly. The alloys with (1.5%, 3.5%)Y addition exhibit high creep resistance at temperatures from 175 °C to 200 °C and load from 55 MPa to 70 MPa. The 7.5%Y added alloy presents excellent creep resistance even at 275 °C and 55 MPa. The second phase which shows discontinuous distribution at the grain boundary of (0.7%, 1.5%, 3.5%)Y added alloys has preferred orientation and clogs in triple junctions of grain boundary. Simultaneously, the arrangement of second phase particles along tensile direction and the formation of denuded zones are observed during the creep process. Moreover, the crack initiates in these areas and propagates along grain boundary. Compared with discontinuous second phase, the continuous skeleton-like second phase of 7.5%Y added alloy at grain boundary has a better effect on improving the creep resistance properties of alloys.

Keywords

creep resistance / Mg-Zn-Y alloy / clustering phases / creep model / denuded zones

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