Shear-Transformation Zone Activation during Loading and Unloading in Nanoindentation of Metallic Glasses (original) (raw)
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Acta Materialia, 2007
Simulated nanoindentation tests on a three-dimensional model of a binary metallic glass-forming alloy reveal how the stress field and material structure interact to control deformation beneath the indenter. Initially, the stress field follows the Hertzian solution with fluctuations due to heterogeneities. Homogeneous or localized plastic deformation arises, depending on the processing history of the material. In the case of localized deformation, the first shear band initiates sub-surface, relaxing the local shear stress. The subsequent shear band morphology is observed to be rate-dependent. Deformation induced changes in material structure are characterized in terms of short-range ordering and free volume, with the former providing significantly greater signal-to-noise.
Stucture and size of the plastic zone formed during nanoindentation of a metallic glass
Journal of Non-Crystalline Solids
Using molecular dynamics simulation, we study the plastic zone created during nanoindentation of a large CuZr glass system. The plastic zone consists of a core region, in which virtually every atom undergoes plastic rearrangement, and a tail, where the density distribution of the plastically active atoms decays to zero. Compared to crystalline substrates, the plastic zone in metallic glasses is significantly smaller than in crystals. The so-called plastic-zone size factor, which relates the radius of the plastic zone to the contact radius of the indenter with the substrate, assumes values around 1, while in crystals-depending on the crystal structure-values of 2-3 are common. The small plastic zone in metallic glasses is caused by the essentially homogeneous deformation in the amorphous matrix, while in crystals heterogeneous dislocations prevail, whose growth leads to a marked extension of the plastic zone.
Scripta Materialia, 2013
Planar flow melt spinning was used to prepare six Zr-based metallic glasses. Evidence of the effect of chemistry was found by structural and thermal characterizations. A clear influence of chemistry was also observed on the mechanical behaviour, with differences in hardness and Young's modulus measured by nanoindentation being noted. Using a statistical analysis of the data, the activation volume controlling initiation of shear bands in the localized deformation process was obtained, and a correlation between this volume and the intrinsic alloy properties is emphasized.
Indentation-induced deformation localisation in Zr–Cu-based metallic glass
Journal of Alloys and Compounds, 2014
It has been well documented that the plastic deformation of bulk metallic glasses is localised in thin shear bands. In order to comprehensively understand deformation mechanisms of BMGs, it is necessary to investigate formation of shear bands and related deformation. Nano-indentation studies with a spherical indenter were carried out to characterise shear-band localisation on a surface of Zr-Cu-based metallic glass. The load-displacement diagram of an indentation cycle reflects an elastic deformation followed by the first pop-in at around 4 mN. In addition, an alternative technique is proposed to characterise a plasticity mechanism through the evolution of localised shear bands beneath the indentation-wedge indentation. It is found that a hemicylindrical region of shear bands is formed beneath the indentation zone in this case.
Influence of stoichiometry on indentation-induced plasticity in CuZr glasses
Applied Physics A
Plasticity in metallic glasses depends on their stoichiometry. We explore this dependence by molecular dynamics simulations for the case of CuZr alloys using the compositions Cu$$_{64.5}$$ 64.5 Zr$$_{35.5}$$ 35.5 , Cu$$_{50}$$ 50 Zr$$_{50}$$ 50 , and Cu$$_{35.5}$$ 35.5 Zr$$_{64.5}$$ 64.5 . Plasticity is induced by nanoindentation and orthogonal cutting. Only the Cu$$_{64.5}$$ 64.5 Zr$$_{35.5}$$ 35.5 sample shows the formation of localized strain in the form of shear bands, while plasticity is more homogeneous for the other samples. This feature concurs with the high fraction of full icosahedral short-range order found for Cu$$_{64.5}$$ 64.5 Zr$$_{35.5}$$ 35.5 . In all samples, the atomic density is reduced in the plastic zone; this reduction is accompanied by a decrease of the average atom coordination, with the possible exception of Cu$$_{35.5}$$ 35.5 Zr$$_{64.5}$$ 64.5 , where coordination fluctuations are high. The strongest density reduction occurs in Cu$$_{64.5}$$ 64.5 Zr$$_{35...
Applied Physics Letters, 2006
Time-dependent deformation processes during nanoindentation have been investigated on a Pd 40 Cu 30 Ni 10 P 20 bulk metallic glass. Deformation under constant load has been studied as a function of prior loading rate and temperature. The constant-load displacement of the indenter into the sample shows classic relaxation kinetics and reveals the importance of anelasticity for the mechanical behavior of metallic glasses at the nanoscale.
Nanoindentation experiments were conducted on Zr 65 Cu 15 Al 10 Ni 10 bulk metallic glass samples with pre-compression deformation (PCD) under various conditions. By analyzing indentation hardness, residual morphology, pileup , and serrated flow, the effects of PCD on nanoindentation response were comparatively investigated. Experimental results indicate that 9 kN PCD induced remarkable decrease of indentation hardness, i.e. softening of the material, while for the 18 kN and 25 kN PCD samples, hardness values very close to that of the as cast sample were obtained. Furthermore, PCD also changed the mechanical heterogeneity, i.e., distribution of soft and hard regions in the bulk metallic glass. Residual morphologies and load-displacement curves of the 9 kN and 18 kN PCD samples showed opposite features in aspects of macroscopic shear bands, pileup width, pileup height, and serrated flows, suggesting different plastic flows during nanoindentation. To rationalize these phenomena, a competing mechanism between shear bands and compressive residual stress was discussed.
Mechanics of Materials, 2013
We investigate the influence of various microstructural features on the deformation behavior of binary Cu 64 Zr 36 glasses by molecular dynamics computer simulations and discuss how and why the very same modifications established for enhancing the strengths of crystalline materials, namely the insertion of solutes, precipitates and grain boundaries, can be used for tuning the mechanical properties of metallic glasses. First, by testing bulk samples with and without open surfaces under tensile load, we show that the condensation of shear transformation zones into shear bands can occur as heterogeneous but also as a homogeneous nucleation process. Then, the influence of crystalline nanoprecipitates on shear band nucleation and propagation is investigated. Finally, we study the effect of grain size and composition on the deformation behavior of nanoglasses and nanoglass composites. The results reveal that glass-glass interfaces act as structural heterogeneities ,which promote shear band formation and prevent strain localization.
Physical Review B, 2011
In this study, we characterize the mechanical properties of Cu 64 Zr 36 nanoglasses under tensile load by means of large-scale molecular dynamics simulations and compare the deformation behavior to the case of a homogeneous bulk glass. The simulations reveal that interfaces act as precursors for the formation of multiple shear bands. In contrast, a bulk metallic glass under uniaxial tension shows inhomogeneous plastic flow confined in one dominant shear band. The results suggest that controlling the microstructure of a nanoglass can pave the way for tuning the mechanical properties of glassy materials.