Natural Aging in Mg-Zn(-Cu) Alloys
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INTRODUCTION
MAGNESIUM alloys containing Zn as the major alloying element are known to have a marked response to precipitation hardening. Due to problems with hot shortness, brittleness, and coarse and uneven grain size in binary Mg-Zn alloys, commercially interesting alloys are always grain refined by the addition of Zr (ZK series alloys). Alloying with rare earth elements such as Ce, Nd, La, Pr, and Gd, often combined in the form of misch metal, improves both casting characteristics and mechanical properties at elevated temperatures (ZE and EZ series);[1] however, this also increases the alloy cost. Alloying with Cu is cost effective, and as Unsworth and King[2,3] reported, Mg-Zn-Cu alloys (ZC series), apart from having good casting characteristics, also have a significantly increased age-hardening response compared to binary Mg-Zn alloy. Addition of Cu also increases the eutectic temperature and permits the use of higher solution treatment temperatures.[2,3] The precipitation sequence in the Cu-containing Mg-Zn alloys above 150 C[4] has been reported to be the same as the most probable aging sequence for a binary Mg-Zn alloy:[5–12] SSSS ! solute clusters ! GP zones/pre-b0 ! b01 rods, blocks ? f0001gMg ; (possibly Mg4 Zn7 ) ! b02 discsjjf0001gMg ; laths ? f0001gMg ; (MgZn2 ) ! b equilibrium phase (MgZn or Mg2 Zn3 )
J. BUHA, formerly JSPS Fellow, National Institute for Materials Science, is Research Fellow, School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia. Contact e-mail: [email protected] T. OHKUBO, Group Leader, is with the National Institute for Materials Science, Tsukuba 305-0047, Japan. Manuscript submitted February 13, 2007. Article published online May 22, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
The structure, composition, and thermal stability of some of these phases have not yet been fully clarified and agreed upon; however, a number of reports agree that the maximum hardening due to the precipitation in Mg-Zn alloys is associated with the formation of the rod-shaped transition b¢1 phase. Earlier reports stated that this phase formed possibly via another transition phase denoted pre-b¢ (still uncharacterized) and had a composition and a crystal structure indistinguishable from that of the Laves MgZn2 phase.[5,9] A more recent study, however, shows that the crystal structure of the b¢1 is similar to that of Mg4Zn7 phase.[11] A smaller fraction of this phase may also exhibit a blocky morphology.[11] In addition to the b¢1 rods, a lathshaped form of the b¢2 phase (MgZn2) may also form in the artificially aged condition.[11] On overaging, the rods and laths are gradually replaced by a coarse plateshaped form of the b¢2 phase.[5,9] The equilibrium b phase, MgZn[5] or Mg2Zn3,[9] may form upon high overaging; however, its crystal structure is still unknown. In the Cu-containing alloys, the precipitates were found to be finer, and more uniformly and far more densely distributed, than in the binary alloy.[4] This was one of the main reasons for the superio
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