Primary and secondary phase separation in Cu–Zr–Al bulk metallic glass by control of quenching conditions

Abstract

Here, we report primary and secondary nanoscale phase separation in Cu46Zr46Al8 (at.%) bulk metallic glass (BMG) achieved by altering the casting temperature. The BMG rods were cast by using the melting currents of 160 A and 220 A which provide, respectively, higher and lower cooling rates. The transmission electron microscopy and energy-dispersive X-ray analysis revealed that the higher cooling rate conditions brings a primary phase separation of nanoscale Cu-rich amorphous regions which are evenly dispersed in the Cu-depleted amorphous matrix phase. However, lower cooling rates results in a bimodal microstructure comprising coarsened primary-separated nanograins and very fine secondary-separated nanograins throughout the amorphous matrix containing nanocrystals of smaller than 5 nm in size. This unique microstructure developed under lower cooling rates gives lower relaxation enthalpy, higher short-range ordering, and enhanced strain-hardening. Our study emphasizes the significant role of casting temperature on controlling the morphology of nanoscale phase separation in BMGs.