Critical concentrations of Zn and Mg for enhanced diamagnetism in Al-Zn-Mg alloys

Temperature and time dependences of the magnetization of Al-Zn-Mg alloys with varying Zn to Mg ratios (Zn/Mg = 0.25, 0.5, 1, 2, 5.5, and 9, keeping the total concentration of Zn plus Mg to be 5 at. %) were studied in the range from 10 to 310 K after various periods of natural aging. In particular, for Al1-y(Mg2Zn11)(y) alloys, the total concentrations of Zn and Mg were also varied from 2 to 20 at. % (y = 0.02, 0.03, 0.04, 0.05, 0.1, and 0.2). The largest time variant enhanced diamagnetism was observed for Al-0.95(Mg2Zn11)(0.05) as a result of solution heat treatment/quenching and natural aging. Isothermal measurements of magnetization vs time for natural aging temperatures from 260 to 300 K for Al-0.95(Mg2Zn11)(0.05) provided activation energies for solute clustering: 0.69 +/- 0.05 eV (for stages I and II) and 0.78 +/- 0.03 eV (for stages II and III). The mechanical hardness vs time at 273 K for Al-0.95(Mg2Zn11)(0.05) confirmed that the time variation of magnetization was related to the precipitation process of Zn/Mg/vacancy zones. Additionally, temperature dependences of the magnetization of Mg21Zn25, Mg4Zn7, MgZn2, and Mg2Zn11 were examined. The observed magnetization for the Mg-Zn compounds was found to be too small to account for the enhanced diamagnetic contributions to magnetization of Al-Zn-Mg alloys. A possible Zn-Mg-vacancy atomic arrangement responsible for the enhanced diamagnetism is discussed.