Degree of short-to-medium-range order in metallic glasses

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Nontechnical abstractMost metals and alloys in use are crystals, with an internal structure characterized by long-range order. Their structures have been well studied over the past century, providing concrete basis of their properties. Such a structure-property paradigm is well established in the field of materials science. In contrast, metallic glasses (MGs) obtained by fast cooling of their parent liquids have a liquid-like amorphous structure with no discernable microstructure inside. Though lacking long-range periodicity, MGs do have structure, at the level of nearest-neighbor atomic coordination and positional correlations on the length scale of a couple of nanometers. This structural organization is difficult to characterize and remains poorly understood. As a result, the understanding of the links between the atomic structure and properties in MGs lags far behind that achieved for conventional metals. This project aims to resolve the short-to-medium range order in MGs, using computer simulation approaches. The detailed structural information is expected to shed light on the macroscopic behavior of these glasses, enabling better judgement in designing their applications. In terms of education, this research has significance in enriching key materials science subjects, in particular the core subject of Structure of Materials. Many of the computer molecular dynamics simulations are amenable to the participation of undergraduate students in Senior Design/Research Experience courses, providing interesting case studies to compare with the familiar crystal symmetry learnt previously in the classroom. Technical abstractThe goal of this project is to 1) quantify the degree and extent of short-range order (SRO) realizable in metallic glasses (MGs), and 2) reliably identify the characteristic patterns of medium-range order (MRO) in different types of MGs. This basic understanding of the ordering in amorphous solids is a fundamental materials science issue, but has been difficult to achieve so far because the MG configurations constructed to date, either by fitting to scattering data or by liquid quenching in molecular dynamics (MD) simulations, are inherently far less ordered than experimental glasses. The project aims to develop efficient MD simulation routes that are capable of producing MG structures having potential energy, atomic density, elastic constants, and therefore degree of order all similar to real-world MGs. A focus is algorithm development to accelerate MD in the search of deeper energy basins. The plan is to use 1) prolonged aging, 2) cyclic-stress-assisted aging, and 3) a new deletion-addition protocol, building upon amorphous structures obtained from simulated collapse of crystals or vapor deposition. Preliminary results suggest that these approaches (and their combination) can access low-energy glassy states along the way towards eventual crystallization. The configurations captured provide an unprecedented opportunity to uncover the SRO/MRO at a level of make-up and saturation that resembles those in laboratory glasses, to allow connections with atomistic mechanisms underlying shear transformations, relaxation and other glass properties. The results are expected to be a major step-forward towards establishing concrete structure-property relations in MGs.