Materials Science and Engineering A 527 (2010) 2207–2213 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Model experiments to mimic fracture surface features in metallic glasses Lisa A. Deibler, John J. Lewandowski ∗ Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, United States article info Article history: Received 24 September 2009 Accepted 28 October 2009 Keywords: Metallic glasses Viscosity effects Tension test Fracture Image analysis abstract The vein-like fracture surface features which occur in metallic glasses have been modeled via experiments in which a thin layer of grease is tested in tension. The effects of layer thickness, substrate shape, viscosity of the grease layer, and stress state on the fracture surface features are investigated. The trend discovered in the relationship between the viscosity of the viscous layer and the fracture surface feature size in the model experiments is consistent with what is found in metallic glasses. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The amorphous structure of metallic glasses precludes the occurrence of the well-understood deformation and fracture mech- anisms present in crystalline metals. In order for metallic glasses to become more widely used, their failure mechanisms need to be better understood so that their mechanical properties can be improved. Since the flow and fracture of metallic glasses were first studied, it has been shown that homogeneous flow occurs at high temperatures relative to the glass transition temperature and at low strain rates, and that inhomogeneous flow (shear banding) occurs at lower temperatures over a wider range of strain rates [1]. In the inhomogeneous flow regime, shear induced disordering creates a thin layer of material with significantly reduced viscosity between two solid layers [1–4]. It was first reported by Pampillo [5] in 1974 that this situation is analogous to the features formed by separa- tion of grease between plates of materials. Pampillo conducted a set of model experiments with grease between glass slides and ref- erenced the Taylor instability [6] as the cause of the “fingers” or “veins” which develop. The Taylor instability indicates that when a viscous fluid is driven through a void by a lower viscosity medium, the interface between the two fluids becomes unstable and develops “finger- like” features. In the case of metallic glasses, the reduced viscosity metallic glass in the shear band is pushed between the layers of high viscosity metallic glass by air (i.e. the lower viscosity medium). A schematic of this process was produced by Spaepen [7] and is ∗ Corresponding author. Fax: +1 2163683209. E-mail address: jjl3@case.edu (J.J. Lewandowski). redrawn in Fig. 1. This provides an interesting opportunity to con- duct model experiments with grease between plates of different geometry in order to better understand the Taylor instability in such systems. Spaepen [7] utilized the Taylor instability idea and developed an analysis of the wavelength between the tributaries to estab- lish a relationship between the fracture stress of the metallic glass and the spacing between the veins. Argon [8] modified the model to include the non-linearity stemming from the non-Newtonian behavior in the fluid. Much more recently, Pan [9] used Argon’s analysis to analyze vein patterns and fracture strengths exhibited by a Mg 65 Cu 15 Ag 5 Gd 10 metallic glass, concluding that in the par- ticular Mg alloy studied, multiple voids were coalescing ahead of the crack tip to create very small veins. In this paper, model experiments are conducted on viscous materials deformed between substrates of different dimensions. The effects of changes in sample geometry, stress state, and vis- cosity are particularly examined for their effects on the resulting fracture surface appearance. The results of the model experiments are then compared to preliminary data reported on various metallic glasses and compared to Spaepen’s model [10]. 2. Experimental procedures 2.1. Viscosity measurements The three greases used in this set of experiments were Dow Corning Silicon Vacuum grease, Jet-Lube AP-1 multipurpose grease, and Ultra-High Temperature synthetic grease distributed by McMaster Carr supply. The viscosity of the greases was measured by an Anton Paahr 501 parallel plate viscometer in the Macromolec- 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.10.072