Cooling Rate Evaluation for Bulk Amorphous Alloys from Eutectic Microstructures in Casting Processes

Abstract

The structural features of metallic glasses depend on the cooling rate of the melt. The cooling rates for casting processes which are typically employed for preparation of bulk metallic glasses are suggested from microstructures of an eutectic Al-33 mass%Cu model alloy. The interlamellar spacing λ of eutectic Al-CuAl 2 rod-shaped specimens of 50 mm in length and 2 to 5 mm in diameter has been determined by optical microscopy and scanning electron microscopy. From the measured interlamellar spacings ranging from λ = 0.18 to 0.5 µm, the solidification front velocities and the cooling rates at different positions in the as-cast samples are derived. The decisive effect of the diameter of cast rods is confirmed. For a centrifugal casting technique, the maximum cooling rate decreases from 730 to 95 K/s when the diameter increases from 2 to 5 mm. Moreover, it is revealed that the local cooling rates decrease significantly from the bottom towards the top of the rods. From the estimated cooling rates at a fixed rod diameter the centrifugal casting technique is assessed as superior to other methods applied, namely copper-mould casting and suction casting. The estimated cooling rates are compared with literature data for glass-forming alloys.

Microstructure and mechanical properties of slowly cooled Cu47.5Zr47.5Al5

“…36 It is well known that the cooling rate decreases from the surface towards the center of the cast rods. 42 The observed microstructural features found for the 2 and 3-mmл rods suggest that these featureless darker regions are more pronounced near the specimen surface of the cylinders, which is completely the opposite of what one should expect. Rather, a mostly glassy structure is expected to form near the Cu-mold surface due to the higher cooling rate realized close to the mold, and crystalline phases should appear at the center of the specimen.…”

“…It has been reported that the cooling rate across the length of a rod decreases from the bottom to the top of the rod. 42 Even though the bulk glass formation of 5-mm-diameter rods for this alloy composition has been reported, 44 we found that the solidified microstructure can vary from glassy to fully crystalline (Figs. 2 and 5) and strongly depends on the actual cooling rate adopted along the length of the rod.…”

Ductile Metallic Glasses in Supercooled Martensitic Alloys

“…A typical microstructure of Cu 47 Zr 47 Al 6 AMI is presented in Fig. 1 22) exhibit a glassy structure, as proved by XRD ( Fig. 1(d)), DSC and TEM.…”

Impact-induced shock and the formation of natural quasicrystals in the early solar system

“…2a,d) composed of nearly stoichiometric khatyrkite (CuAl 2 , containing 0.8% Fe) and Al metal (containing 1% Cu). This texture is known to occur as a result of rapid cooling (B10 2 -10 3°C s À 1 ) of liquids in the Al-Cu system [16][17][18] . showing that the matrix was the precursor for the shocked silicates.…”