Effect of Zn additions on the age-hardening of Mg–2.0Gd–1.2Y–0.2Zr alloys

We have investigated the microstructures of age hardened Mg–2.0Gd–1.2Y–xZn–0.2Zr (x = 0, 0.3, and 1.0) (at.%) alloys to understand the remarkable age-hardening and unusual plastic elongation behavior. The age-hardening of the alloys occurs through the sequential precipitations of β′ and β1 phases. The β1 phase heterogeneously nucleates at the interface of the β′ phase, and relaxes strain fields around the β′. Although the addition of Zn degrades the age-hardening response, it causes the discontinuous precipitation of a 14H-type long-period stacking (LPS) phase at grain boundaries as well as within grains in the over-aged condition, which enhances the maximum tensile elongation. The composition of the β1 phase was determined to be Mg–23.3 at.% RE–9.7 at.% Zn–2.0 at.% Zr (RE: rare-earth, Gd and Y), whereas that of the LPS is Mg–5.6 at.% RE–1.8 at.% Zn–1.0 at.% Zr.