Thermal stability and hardness of Mg–Cu–Au–Y amorphous alloys

In this work the effect of Au addition to the GFA of the Mg65Cu25Y10 alloy will be discussed. Mg65Cu25Y10 and Mg65Cu15Au10Y10 amorphous alloys were obtained by rapid solidification. Copper mould casting gave a fully amorphous phase for Mg65Cu25Y10, whereas the equilibrium crystalline phases were observed for Mg65Cu15Au10Y10. A single eutectic melting reaction was observed for Mg65Cu25Y10 but for Mg65Cu15Au10Y10 melting is clearly off-eutectic. The presence of an off-eutectic melting of the alloy allows glass formation only by rapid solidification and reduces the glass forming ability. The addition of Au changes the crystallization mechanism of Mg65Cu25Y10 amorphous alloy from polymorphic to primary. Two well-separated crystallization peaks were observed for Mg65Cu25Y10, whereas for Mg65Cu15Au10Y10 the crystallization is more complicated and two overlapped crystallization signals were obtained, followed by a broad exothermal peak at higher temperature. Y-containing Mg2Cu nanocrystals are formed as first crystallization product for both alloys. The Au-containing sample produces also a nanocrystalline AuCu3 phase. From the indentation tests, a significant increase of hardness was observed in the Au-containing amorphous alloy during crystallization, because of the formation of nanocrystalline phases.