Multi-Scale Approaches For Understanding Deformation And Fracture Mechanisms In Amorphous Alloys

Abstract

Amorphous alloys possess attractive combinations of mechanical properties (high elastic limit, ~2%, high fracture toughness, 20-50 MPa.m1/2, etc.) and exhibit mechanical behavior that is different, in many ways, from that of the crystalline metals and alloys. However, fundamental understanding of the deformation and fracture mechanisms in amorphous alloys, which would allow for design of better metallic glasses, has not been established on a firm footing yet. The objective of this work is to understand the deformation and fracture mechanisms of amorphous materials at various length scales and make connections with the macroscopic properties of glasses. Various experimental techniques were employed to study the macroscopic behavior and atomistic simulations were conducted to understand the mechanisms at the nano level. Towards achieving these objectives, we first study the toughness of a Zr-based bulk metallic glass (BMG), Vitreloy-1, as a function of the free volume, which was varied by recourse to structural relaxation of the BMG through sub-Tg annealing treatment. Both isothermal annealing at 500 K (0.8Tg) for up to 24 h and isochronal annealing for 24 h in the temperature range of 130 K (0.65Tg) to 530 K (0.85Tg) were conducted and the impact toughness, Γ, values were measured. Results show severe embrittlement, with losses of up to 90% in Γ, with annealing. The variation in Γ with annealing time, ta, was found to be similar to that observed in the enthalpy change at the glass transition, ΔH, with ta, indicating that the reduction of free volume due to annealing is the primary mechanism responsible for the loss in Γ with annealing. Having established the connection between sub-atomic length scales (free volume) and macroscopic response (toughness), we investigated further the affects of relaxation on intermediate length scale behavior, namely deformation induced by shear bands, by employing instrumented indentation techniques. While the Vickers nano-indentation response of the as-cast and annealed glasses do not show any significant difference, spherical indentation response shows reduced shear band activity in the annealed BMG. Further, relatively high indentation strain was observed to be necessary for shear band initiation in the annealed glass, implying an increased resistance for the nucleation of shear bands when the BMG is annealed. In the absence of microstructural features that allow for establishment of correlation between properties and the structure, we resort to atomistic modeling to gain further understanding of the deformation mechanisms in amorphous alloys. In particular, we focus on the micromechanisms of strain accommodation including crystallization and void formation during inelastic deformation of glasses. Molecular dynamics simulations on a single component system with Lennard-Jones-like atoms suggest that a softer short range interaction between atoms favors crystallization. Compressive hydrostatic strain in the presence of a shear strain promotes crystallization whereas a tensile hydrostatic strain was found to induce voids. The deformation subsequent to the onset of crystallization includes partial re-amorphization and recrystallization, suggesting important mechanisms of plastic deformation in glasses. Next, a study of deformation induced crystallization is conducted on two component amorphous alloys through atomistic simulations. The resistance of a binary glass to deformation-induced-crystallization (deformation stability) is found to increase with increasing atomic size ratio. A new parameter called “atomic stiffness” (defined by the curvature of the inter-atomic potential at the equilibrium separation distance) is introduced and examined for its role on deformation stability. The deformation stability of binary glasses is found to increase with increasing atomic stiffness. For a given composition, the internal energies of binary crystals and glasses are compared and it is found that the energy of glass remains approximately constant for a wide range of atomic size ratios unlike crystals in which the energy increases with increasing atomic size ratio. This study uncovers the similarities between deformation and thermal stabilities of glasses and suggests new parameters for predicting highly stable glass compositions.