Heterogeneous nucleation of β-type precipitates on nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy
Zirconium (Zr) is an important alloying element to Mg-Zn-based alloy system. In this paper, we report the formation of the β-type precipitates on the nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy during ageing at 180°C. Scanning transmission electron microscopy examinations revealed that the nanoscale Zr-rich [0001]α rods/laths are dominant in the Zr-rich core regions of the as-quenched sample after a solution treatment at 430°C. More significantly, these Zr-rich particles served as favourable sites for heterogeneous nucleation of the Zn-rich β-type phase during subsequent isothermal ageing at 180°C. This research provides a potential route to engineer precipitate microstructure for better strengthening effect in the Zr-containing Mg alloys.
Precipitation hardening of an Mg-5Zn-2Gd-0.4Zr (wt. %) alloy
Abstract Mg–5Zn–2Gd–0.4Zr alloy (wt. %) shows a significant age hardening response with a hardness increment from 62 HV to 72 HV by ageing at 200 °C for up to 80 h. Transmission electron microscopy and atom probe tomography characterizations reveal that precipitates with different morphologies and habit planes form in the alloy, including triangular-shaped ZnZr phase, rectangular-shaped Zn 2 Zr phase, [0001] Mg rods ( β ′ 1 ) MgZn 2 Laves phase, (0001) Mg plate ( β ′ 2 ) MgZn 2 Laves phase, and coherent GP zones. Furthermore, a significant partitioning of Gd into Zn-rich precipitates ( β ′ 1 and β ′ 2 ) was, for the first time, observed, which is considered to be responsible for improving the thermal stability of Zn-rich precipitates and enhancing the precipitation hardening of the alloy. A core–shell structure with the shell of Gd (∼2 at%) and the core of Zn 2 Zr phase was also observed, which is proposed to hinder the coarsening of Zn 2 Zr phase. No further change in average size of the Zn 2 Zr particles was observed after ageing over 15 h. Better understanding the partitioning of Gd into the Zn-rich precipitates and the formation of the core–shell structures are essential to control the precipitation microstructure and develop high performance Mg alloys.
First-principles calculations of the Zn–Zr system
Abstract First-principles calculations of the free energy of the intermetallic phases of the Zn–Zr system have been performed. The likely structures of the two allotropes of Zn3Zr are proposed and the structure of Zn2Zr3 is determined. The 0 K phase diagram for this system is calculated using the convex hull construction. Two compounds, ZnZr2 and Zn2Zr3, that previously were not considered to form part of the equilibrium phase diagram have been shown to become stable at high temperatures thanks to calculations of their vibrational free energy via the supercell approach within the harmonic approximation. The results from this work will later be used to develop a fully consistent thermodynamic model for this binary.