Wendelin J. Wright

Localized heating during serrated plastic flow in bulk metallic glasses

We have studied the serrated plastic flow observed in Zr40Ti14Ni10Cu12Be24 and Pd40Ni40P20 bulk metallic glass alloys tested in uniaxial compression. Quantitative measurements with sufficient temporal resolution to record the fine-scale structure of the data are reported. These data are used to predict temperature increases in single shear bands due to local adiabatic heating caused by the work done on the sample during plastic deformation. Since the predicted temperature increases are on the order of only a few degrees Kelvin, it seems unlikely that localized heating is the primary cause of flow localization. Instead, changes in viscosity associated with increased free volume in the shear band seem more consistent with experiment. Substantial shear band heating is, however, predicted for final failure, as corroborated by evidence of melting on the fracture surface.

Free volume coalescence and void formation in shear bands in metallic glass

We have investigated the possibility of void nucleation from the coalescence of excess free volume generated in shear bands during deformation of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass. Excess free volume in a shear band results in excess free energy relative to a relaxed glass with less free volume. To calculate the free energy of the material in a shear band with excess free volume, we model it as having the same structure as a glass solidified at an elevated glass transition temperature, which we call the fictive temperature. This excess free energy can be correlated with a free volume chemical potential that provides a driving force for void nucleation. The results of this modeling indicate that any free volume generated in the shear band during deformation is unstable, with the consequence that voids are predicted to form spontaneously from the coalescence of free volume. These voids are then expected to coarsen. Under tension, void growth and linkage would be facilitated by a tensile...

Enhancement of strength and stiffness of Nylon 6 filaments through carbon nanotubes reinforcement

We report a method to fabricate carbon nanotube reinforced Nylon filaments through an extrusion process. In this process, Nylon 6 and multiwalled carbon nanotubes (MWCNT) are first dry mixed and then extruded in the form of continuous filaments by a single screw extrusion method. Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies have indicated that there is a moderate increase in Tg without a discernible shift in the melting endotherm. Tensile tests on single filaments have demonstrated that Young’s modulus and strength of the nanophased filaments have increased by 220% and 164%, respectively with the addition of only 1wt.% MWCNTs. SEM studies and micromechanics based calculations have shown that the alignment of MWCNTs in the filaments, and high interfacial shear strength between the matrix and the nanotube reinforcement was responsible for such a dramatic improvement in properties.

An improved analysis for viscoelastic damping in dynamic nanoindentation

The conventional analysis for stiffness and damping of the contact during nanoindentation models the indented material as a Voigt solid. With the assumption of the Voigt solid, the model can not capture instantaneous elastic recovery and the contact damping is underestimated. In this work, the material is modelled as a standard linear solid and the characteristic equations for motion of the system during dynamic nanoindentation, including the sample and instrument response, are solved to determine the stiffness and damping of the contact. These parameters are functions of the contact area and are not functions of the frequency of the imposed oscillation. The storage and loss moduli are computed and compared to results from the conventional indentation analysis.

The Prospects for Mechanical Ratcheting of Bulk Metallic Glasses

The major mechanical shortcoming of metallic glasses is their limited ductility at room temperature. Monolithic metallic glasses sustain only a few percent plastic strain when subjected to uniaxial compression and essentially no plastic strain under tension. Here we describe a room temperature deformation process that may have the potential to overcome the limited ductility of monolithic metallic glasses and achieve large plastic strains. By subjecting a metallic glass sample to cyclic torsion, the glass is brought to the yield surface; the superposition of a small uniaxial stress (much smaller than the yield stress) should then produce increments in plastic strain along the tensile axis. This accumulation of strain during cyclic loading, commonly known as ratcheting, has been extensively investigated in stainless and carbon steel alloys, but has not been previously studied in metallic glasses. We have successfully demonstrated the application of this ratcheting technique of cyclic torsion with superimposed tension for polycrystalline Ti-6Al-4V. Our stability analyses indicate that the plastic deformation of materials exhibiting elastic--perfectly plastic constitutive behavior such as metallic glasses should be stable under cyclic torsion, however, results obtained thus far are inconclusive.