La-based bulk metallic glasses with critical diameter up to 30 mm

We report composition optimization, thermal and physical properties of new La-based bulk metallic glasses with high glass forming ability (GFA) based on a ternary La62Al14Cu24 alloy. By refining the (Cu, Ag)/(Ni, Co) and La/(Cu, Ag) ratios in the La-Al-(Cu,Ag)-(Ni, Co) pseudo-quaternary alloy, the formation of 30 mm diameter of La65Al14(Cu5/6Ag1/6)(11)(Ni1/2Co1/2)(10) bulk metallic glass (BMG) alloy is achieved using water quenching. The origin of the high GFA was investigated from the kinetic, structural and thermodynamic points of view, and was found to be due to the smaller difference in Gibbs free-energy between the amorphous and crystalline phases in the pseudo-quaternary alloy. These alloys exhibit low glass transition temperatures, below 430 K, and relatively wide supercooled liquid regions of 40-60 K. Mechanical tests on these alloys show a fracture strength of 650 GPa, Vicker's hardness 200 kg mm(-2), Young's modulus 35 GPa, shear modulus 13 GPa and Poisson ratio 0.356. The La-based BMGs are useful for both scientific and engineering applications. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.