Shear Transformation Zone Activation During Deformation in Bulk Metallic Glasses Characterized Using a New Indentation Creep Technique

Abstract

We have developed a novel type of nanoindentation creep experiment, called broadband nanoindentation creep (BNC), and used it to characterize the thermal activation of shear transformation zones (STZs) in three BMGs in the Zr-Cu-Al system. Using BNC, material hardness can be determined across a wide range of strain rates (10 –4 to 10 s –1 ) in a single experiment at room temperature. This data can be used to characterize the kinetics of deformation, including the free energy function ( �G) for thermally-activated deformation. We have found that the activation energy agrees with the theory of Johnson and Samwer, in which �G ∝ [( τc – τ)/ τc] 3/2 , where τ is the flow stress and τc is the flow stress at 0K. In the context of their model, we estimate that the volume of an STZ is ~ 100-300 atomic volumes and the activation energies for low-stress deformation are 5-10 eV. From these measurements, it is possible to reproduce the temperature dependence of the flow stress. BACKGROUND Plastic deformation in metallic glasses (MGs) is thought to be the result of cooperative rearrangements of atoms in susceptible regions of the glass under the influence of an applied shear stress [1-4]. This reorganization could be restricted to a single atom making a diffusive jump [1], or it could encompass a few dozen atoms changing to a sheared configuration [2-4]. Rearrangements of the second type are the basis for the shear transformation zone (STZ) [5] description of MG defects. The unique deformation properties that MG alloys exhibit [6] are the consequence of the dynamics of STZ activation and, in turn, the structural details of the glass. With the discovery of bulk metallic glasses (BMGs) [7], there is a need for mechanical characterization techniques that provide insight into deformation mechanisms present in MGs. Nanoindentation is a common technique [8] which is typically used to determine the “rateindependent” hardness (H0) and the elastic modulus ( E) of samples of material whose geometry or properties preclude conventional testing, such as embedded phases or brittle materials. Less well-known are the uses of nanoindentation to study the time-dependent (creep) properties of materials, which can provide insight into the nature and dynamics of plastic deformation. These techniques can be broadly classified under the heading “nanoindentation creep” [9, 10], and have recently been applied to MGs [11-13]. The dynamics of defect-based MG deformation imply a constitutive plastic deformation law in which the overall strain rate ( e& ) is proportional to the rate of defect activation, such as