How the toughness in metallic glasses depends on topological and chemical heterogeneity

Abstract

To gain insight into the large toughness variability observed between metallic glasses (MGs), we examine the origin of fracture toughness through bending experiments and molecular dynamics (MD) ngineering ceramics are strong, with high yield strength, but suffer from brittleness. In contrast, crystalline metals tend to have high fracture toughness because dislocation motion promotes plastic deformations that suppress cracks propagations, but concomitantly this dislocation motion reduces yield strength. Metallic glasses (MGs) tend to have high strength, and for some compositions, the high strength is accompanied by a high fracture toughness, making MGs promising engineering materials (1). The fracture toughness in MGs, which is accommodated by shear banding and limited by cavitation, is thought to arise from initiation of a crack opening at the core of an extending shear band (2-4). Then, new high-strength and high-toughness MGs may be designed by identifying compositions capable of suppressing cavitation during shear band extension. However, the complex physics of cavitation in MGs has obscured the development of models to illustrate cavitation's origin.For MGs, cavitation leads to the crack opening process that controls directly the fracture toughness, a fundamental property for material design and applications. Since the first amorphous alloy (Au 75 Si 25 ) reported at the California Institute of Technology in 1960 (5), tremendous effort has been dedicated to understand why the amorphous structure leads to such excellent mechanical properties as high elastic limit, yield strength, and hardness (2, 6). However, toughness, which varies dramatically between MG compositions, ranging from values typical of brittle ceramics to those typical of engineering metals (2, 6, 7), is still poorly understood. More recently, improved alloys have been developed that demonstrate very high toughness, including a bulk Pd-rich, Si-bearing glass, Pd 79 Ag 3.5 P 6 Si 9.5 Ge 2 (7), and a bulk Zr-rich, Cu/Al-bearing glass, Zr 61 Ti 2 Cu 25 Al 12 (8), in which shear band plasticity suppresses crack opening.The fracture resistance of MGs is understood to arise from a competition between two processes: shear band plasticity and void nucleation. Currently, the process of shear band plasticity is widely recognized to be accommodated by the cooperative shearing of local atomic clusters [shear transformation zones (STZs)] (9, 10). However, to describe the fracture process, a condition for cavitation is needed coupled with the description of shear band plasticity to account for a crack opening along an operating shear band. Recently, Rycroft and Bouchbinder (11) coupled a continuum STZ model with a condition for cavitation to describe the fracture of MGs. The authors found that cavitation plays an essential role in the initiation of fracture, where they found a crack to evolve by successive void nucleation events along an operating shear band. In the context of molecular dynamics (MD) simulations, we and others proposed that cavitation prec...

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