Amorphous Particles Embedded in hcp Mg Grains of Melt-Quenched Mg98Cu1Gd1 Alloys

Unique precipitates with novel structures have been found in melt-quenched (MQ) Mg98Cu1Gd1 alloys by transmission electron microscopy (TEM). Amorphous spherical particles with 50-200 nm sizes are uniformly embedded in Mg grains of an MQ Mg98Cu1Gd1 alloy prepared with a relatively higher cooling rate. So-called LAL precipitates, which consist of an amorphous core sandwiched by long period stacking (LPS) phase, are formed in an MQ Mg98Cu1Gd1 alloy prepared with a lower cooling rate. TEM observations show that LAL precipitates transform into LPS phases by annealing above 450K. DSC results show that amorphous cores in LAL particles and amorphous particles crystallize at around 450 K, and partial melting of the LPS phase occurs at around 710 K prior to the melting of the Mg matrix. The composition of amorphous particles is 68 at%Mg, 26 at%Cu and 6 at%Gd, which is close to that having the highest glass forming ability (Mg65Cu25Gd10). Limited solubility of Cu and Gd in Mg and large negative mixing enthalpy between Cu and Gd are responsible for the formation of unique precipitates in rapidly solidified Mg-Cu-Gd alloys.