Preparation and Properties of Mg–Cu–Y–Al Bulk Amorphous Alloys

Abstract

Bulk amorphous (Mg 1-y Al y ) 60 Cu 30 Y 10 alloys were prepared using a relatively simple technique of rapid cooling of the melt in a copper wedge mould. The temperature vs. time was recorded during the cooling and solidilication process of the melt and compared with a spacial and temporal numerical simulation of that process. It is concluded that good thermal contact is maintained between the amorphous part of the solidified sample and the mould, while a rather poor contact develops between the crystalline part of the sample and the mould, probably due to the appearance of a narrow gap at the crystal-mould interface during crystallisation. The maximum amorphous layer thickness decreases from ∼3 mm to zero when the Al content increases in the range from 0 to about y = 10%. The evolution of the microstructure of the initially amorphous phase was examined by x-ray diffraction (XRD) and differential scanning calorimetry (DSC) for different alloy compositions and annealing temperatures. On annealing into the supercooled liquid state (441 K), specimens with no Al content remain basically amorphous while nanoparticles are formed and remain stable also at higher temperatures in specimens containing a few percent Al. The alloy with no Al crystallises apparently without the formation of nanoparticles. The critical cooling rate for the formation of an amorphous Mg 60 Cu 30 Y 10 specimen was determined experimentally by a combination of DSC data and temperature vs. time measurements to be 60-150 K/s, in agreement with estimates from the literature. The Vickers hardness (H V ) of the amorphous material for y = 2% is higher (∼360 kg/mm 2 ) than for y = 0 (∼290 kg/mm 2 ). On crystallisation the hardness of the latter material increases to the 400 kg/mm 2 level while the hardness of the former does not change.