Mode I crack tip fields in amorphous materials with application to metallic glasses

In this work, stationary crack tip fields in amorphous materials such as metallic glasses under mode I loading are studied to understand the factors that control crack tip plasticity and in turn impart toughness to those materials. For this purpose, finite element simulations under plane strain, small scale yielding conditions are performed. A continuum elastic-viscoplastic constitutive theory, which accounts for pressure sensitivity of plastic flow as well as the localization of plastic strain into discrete shear bands, is employed to represent the material behavior. The influence of internal friction and strain softening on the plastic zone, stress and deformation fields and notch opening profile is examined. It is found that higher internal friction leads to a larger plastic zone. Also, it enhances the plastic strain ahead of the notch tip but leads to a substantial decrease in the opening stress. Thus, it appears that a higher friction parameter promotes toughening of amorphous solids. The shear band patterns within the plastic zone and brittle crack trajectories around the notch root generated from the simulations match qualitatively with those observed in experiments

Subsurface deformation during Vickers indentation of bulk metallic glasses

Bonded interface technique was employed to examine the nature of subsurface deformation during Vickers indentation in two kinds of bulk metallic glasses (BMGs), Pd42.1Ni39.77P18.13, and Zr56.69Cu26.96Al10.95Ni5.4. Quantitative information such as the shear band spacing, pile-up length, subsurface deformation zone size etc. was recorded for indentation loads ranging from 50 to 5000 g. Experimental results show that both the BMGs have an average hardness value of ~550 VHN with slightly higher hardness at low loads. Observations of deformation zones indicate that they deform appreciably through the shear band mechanism. For both bulk indentation, as well as the interface indentation, the normalized size of the deformation zone was found to be independent of the applied load. Two types of shear bands, radial, and semi-circular in nature, have been observed. These results are compared with similar studies made on ductile metals and silicate glasses

Nanoindentation studies on waveguides inscribed in chalcogenide glasses using ultrafast laser

Optical straight waveguides are inscribed in GeGaS and GeGaSSb glasses using a high repetition-rate sub-picosecond laser. The mechanical properties of the glasses in the inscribed regions, which have undergone photo induced changes, have been evaluated by using the nanoindentation technique. Results show that the hardness and elastic modulus of the photo-modified glasses are significantly lower as compared to the other locations in the waveguide, which tend to be similar to those of the unexposed areas. The observed mechanical effects are found to correlate well with the optical properties of the waveguides. Further, based on the results, the minimum threshold values of hardness and elastic modulus for the particular propagation mode of the waveguide (single or multi), has been established

Deformation morphology underneath the vickers indent in a Zr-based bulk metallic glass

Hardness and plastic deformation during Vickers indentation in as-cast, annealed, and fully crystallized Zr57Cu27Al11Ni5 bulk metallic glass was examined. Subsurface deformation morphology under the indenter tip was studied at various loads and for different annealing time. Appreciable plastic deformation, through shear banding, occurs in the as-cast and annealed alloys. Two different types of shear bands are observed. Their occurrence depends upon the amount of annealing and hence on the extent of crystallization. The fully crystallized alloy exhibits extensive cracking. Trends in the deformation zone size with load are consistent with the expanding cavity model, while the shear band morphology, particularly for the as-cast sample, attests the qualitative applicability of the slip line field theory

The indentation response of Nickel nano double gyroid lattices

The indentation response of Nickel nano double gyroid films has been measured using a Berkovich nanoindenter and the effective mechanical properties of the Ni double gyroid lattices inferred via a multi-scale finite element analysis. The 1μm thick double gyroid films were manufactured by block copolymer self-assembly followed by electrodeposition of the Ni resulting in two interpenetrating single gyroids of opposite chirality, an overall relative density of 38% and a cell size of about 45 nm. The measured hardness was ∼0.6 GPa with no discernable indentation size effect. A multi-scale finite element (FE) analysis revealed that the uniaxial compressive strength is approximately equal to the hardness for this compressible lattice. Thus, the 38% relative density Ni double gyroid has a strength equal to or greater than the strongest fully dense bulk Ni alloys. The FE calculations revealed that this was a consequence of that fact that the Ni in the 13 nm gyroid struts was essentially dislocation free and had a strength of about 5.7 GPa, i.e. approaching the theoretical strength value of Ni. The measurements and calculations reported here suggest that in spite of the nano gyroids having a bending-dominated topology they attain strengths higher than those reported for stretching-dominated micron scale lattice materials made via 3D printing. We thus argue that relatively fast and easy self-assembly processes are a competitive alternative to 3D printing manufacture methods for making high strength lattice materials

Spherical indentation response of a Ni double gyroid nanolattice

The effect of indentation strain εi upon hardness H and elastic modulus E of a Ni Double Gyroid (DG) nanolattice was investigated using a spherically-tipped nanoindenter. H remains invariant, while E decreases linearly, with increasing εi. Results reveal the progressive collapse of the DG lattice beneath the indenter. The measured values of H and extrapolated value of E at εi = 0 were used to estimate the yield strength and elastic modulus of the Ni cell walls. The latter was compared with the ideal strength of Ni, nanocrystalline films and of sub-100 nm diameter single crystals

On the variability in fracture toughness of ‘ductile’ bulk metallic glasses

The mode I fracture toughness, K_(Ic), of ductile bulk metallic glasses (BMGs) exhibits a high degree of specimen-to-specimen variability. By conducting fracture experiments in modes I and II, we demonstrate that the observed high variability in mode I, vis-à-vis mode II, is a result of highly variable propensity for the conversion of shear bands into cracks in mode I whereas in mode II, crack growth direction is fixed. Thus, the measured variability in K_(Ic) is intrinsic to the nature of BMGs

Positron annihilation investigation of thermal cycling induced martensitic transformation in NiTi shape memory alloy

Thermal cycling of a Ni excess NiTi alloy was conducted between 50 C and liquid nitrogen temperature to induce martensitic transformations and to reverse them after. The starting point was an annealed and slowly cooled state, the end point a sample thermally cycled 1500 times. Positron annihilation lifetime spectra and Coincidence Doppler Broadening profiles were obtained in various states and at various temperatures. It was found that the initial state was low in defects with positron lifetimes close to that of bulk NiTi. Cycling lead to a continuous build up of a defect structure up to 200 amp; 8722;500 cycles after which saturation was reached. Two types of defects created during cycling were identified, namely pure dislocations and vacancies attached to dislocation