Microstructure of Gold Nano-Crystals from Nanometer to Micrometer Lengthscale in Gold Bulk Metallic Glass

Gold based bulk metallic glass was synthesized at with different quenching rates. Au54Ag5Pd2Cu24.3Si14.7 alloy ingots were cast into wedge shape and cone shape samples. The critical casting thickness of this alloy composition was found to be 4 mm when cast in copper mold. Completely amorphous structure as found in sections thinner than 4 mm when considering a infinite copper plate geometry. In locations with slower cooling rates, there are small fractions of transformed nanocrystals. The transmission electron microscope (TEM) study of Au54Ag5Pd2Cu24.3Si14.7 solidified microstructure could offer snap shots of nucleation and growth of crystals from nanometer to micrometer length-scale as evolved with different cooling rates. The phase evolution was recorded by transmission electron microscope study, electron diffraction and metallography using scanning electron microscope. The specimens comprised of both fully amorphous solid and nanocomposites which were the combination of nano-crystals of controlled sizes in amorphous matrix. Vickers hardness values increased to 350-450 Hv, when compared to sub 100-150 Hv level of conventional 18-k gold alloys as cast. The study offers a revolutionary way to create ultrahigh strength bulk metals that comprise of large percentage of nano-crystals in amorphous matrix.