Preparation and properties of Mg-Cu-Y-Al bulk amorphous alloys

Bulk amorphous (Mg1-gammaAlgamma)(60)CU30Y10 alloys were prepared using a r elatively simple technique of rapid cooling of the melt in a copper wedge m ould. The temperature vs, time was recorded during the cooling and solidifi cation process of the melt and compared with a spacial and temporal numeric al simulation of that process. It is concluded that good thermal contact is maintained between the amorphous part of the solidified sample and the mou ld, while a rather poor contact develops between the crystalline part of th e sample and the mould, probably due to the appearance of a narrow gap at t he crystal-mould interface during crystallisation. The maximum amorphous la yer thickness decreases from similar to3 mm to zero when the Al content inc reases in the range from 0 to about y = 10%. The evolution of the microstru cture of the initially amorphous phase was examined by x-ray diffraction (X RD) and differential scanning calorimetry (DSC) for different alloy composi tions and annealing temperatures. On annealing into the supercooled liquid state (441 K), specimens with no Al content remain basically amorphous whil e nanoparticles are formed and remain stable also at higher temperatures in specimens containing a few percent Al. The alloy with no Al crystallises a pparently without the formation of nanoparticles. The critical cooling rate for the formation of an amorphous Mg60CU30Y10 specimen was determined expe rimentally by a combination of DSC data and temperature vs, time measuremen ts to be 60-150 K/s, in agreement with estimates from the literature. The V ickers hardness (Hv) of the amorphous material for y = 2% is higher (simila r to 360 kg/mm(2)) than for y = 0 (similar to 290 kg/mm(2)). On crystallisa tion the hardness of the latter material increases to the 400 kg/mm(2) leve l while the hardness of the former does not change.