Microstructure evolution during thermomechanical treatments of age-hardenable Mg sheet alloys containing Ca and Y
In the last decade, the formability of Mg sheet has been significantly improved by altering the basal-type texture to diversely distributed grain orientations. Such texture weakening can be achieved by alloying of rare earth elements or Ca with simultaneous addition of Zn. Meanwhile, the dilemma between strength and formability is one of the main reasons for the limited application of Mg alloy sheet. Mg alloys containing Ca, such as Mg-Al-Ca and Mg-Zn-Ca based alloys, show a high age hardening response such that the sheet strength can be highly improved. Likewise, Y is also well acknowledged as an effective element for improving formability of Mg alloys by texture weakening in correlation with the change in stacking fault energy, and for improved strength via precipitation hardening.
In this respect, the present work aims to develop a highly formable and high strength Mg alloy sheet by Ca and Y addition, with a focus on the microstructural evolution in relation to age-hardenability and texture development during the thermomechanical treatments. The Ca containing AX10 alloy sheet reaches the peak hardness after 1800 s ageing, while a commercial alloy without Ca addition, e.g. AZ31, does not show any age-hardening behavior. In the AXW100 sheet, thermally stable Y-containing precipitates brought about a fine grain structure even after the homogenization treatment at 500 °C and weak texture with the basal pole split towards the sheet transverse direction due to the grain boundary pinning, contrary to the coarse microstructure and basal-type texture formed in the AX10 sheet. Moreover, the AXW100 sheet showed a faster and higher aging response than that of the AX10 sheet, and the yield strength improved from 150 MPa to 214 MPa after the aging treatment for 1800 s. Based on the experimental results, the importance of strategic alloy design to result in the improved strength accompanying with a texture weakening is discussed.