Indentation study on the pressure sensitivity of a ZR-based bulk metallic glass

Spherical indentation test was conducted on as-cast and annealed Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass, and the evolution of the morphology of the deformation zone of indents upon annealing was investigated. The DSC traces of the as-cast and annealed samples show that the enthalpy change at the glass transition, ΔH, decreases with the increasing of annealing temperature, indicating the reduction of the free volume upon annealing. The morphology of the indents implies a reduced shear band activity in the annealed samples. The included angles (2θ) between two families of shear bands emanating from the edge of spherical indent in the as-cast and the annealed samples were measured to be in the range of 88-79°, which decrease with the increasing of annealing temperature, indicating pressure sensitive plasticity in the as-cast and annealed samples. By Mohr–Coulomb criterion, the pressure sensitive index, α, can be obtained on the basis of the measured 2θ. The sensitivity index increases with increasing temperature, implying an increase of 'atomistic friction' due to the reduction of the free volume upon annealing.

Plastic deformation in Zr41 Ti14 Cu12.5 Ni10 Be22.5 bulk metal glass under vickers indenter

Vickers indentation was conducted on an as-cast Zr41Ti14Cu12.5Ni10Be22.5 bulk metal glass (BMG) to study shear band formation using a bonded interface technique. The results indicate that the plastic deformation in the BMG is accommodated by the semi-circular (primary) and radial (secondary) shear bands. The inter-band spacing of the semi-circular shear bands is found to be independent of the applied load. The measured size of the deformation zone is in good agreement with the prediction of the theoretical model proposed by Zhang et al.

Plastic deformation in the annealed Zr41Ti14Cu12.5Ni10Be22.5 bulk metal glass under indenter

Vickers and nanoindentationswere carried out on an annealed Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (BMG), and the evolution of the shear bands in the annealed BMG was investigated and compared to that in the as-cast alloy. Results indicate that the plastic deformation in the BMG with the structure relaxation is accommodated by the semicircular (primary) and radial (secondary) as well as tertiary shear bands. Quantitatively, the shear band density in the annealed alloywas much lower than that in the as-cast alloy. The load-displacement curve of nanoindentation test for the annealed alloy exhibited a more flat serrated flow. The annihilation of free volume caused by the annealing was responsible for the embrittlement of the annealed sample.

Extrusion properties of a Zr-based bulk metallic glass

The extrusion behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 metallic glasses in the supercooled liquid region was investigated. Good extrusion formability was observed under low strain rates at temperatures higher than 395 °C. The metallic glasses were fully extruded without crystallization and failure within the range of T=395–415 °C under strain rates from 5×10⁻³ s⁻¹ to 5×10⁻² s⁻¹, and the deformation behavior of the metallic glasses during the extrusion was found to be in a Newtonian viscous flow mode by a strain rate sensitivity of 1.0.

Effect of relaxation on pressure sensitivity index in a Zr-based metallic glass

Shear-banding features of as-cast and annealed Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass were investigated through Rockwell indentation tests. Isothermal annealing of the as-cast samples was conducted at temperatures below its glass transition temperature, Tg. The exothermal enthalpy during continuous heating below Tg decreases with increasing annealing temperature, indicating the gradual reduction of free-volume upon annealing. The observation on the morphology of shear-banding pattern around the indents implies a reduced shear bands activity in the annealed samples. The included angles (2θ) between two families of shear bands emanating from the edge of Rockwell indent decrease from 88° for the as-cast sample to 79° for the sample annealed at 633 K for 1 h, indicating a pressure sensitive plasticity. By Mohr–Coulomb criterion, the pressure sensitive index can be obtained on the basis of the measured 2θ, which increases with increasing annealing temperature, indicating an increase of “atomistic friction” due to the reduction of the free volume upon annealing.

Theoretical study on behaviour of superplastic forming/diffusion bonding of bulk metallic glasses

Based on the behaviour of the superplastic deformation of metallic glasses and Pilling’s model, a diffusion bonding model suitable for metallic glasses is proposed in the present study. In the current model, the diffusion bonding processes consists of two stages: one is the plastic deformation stage and the other is the void shrinkage stage, in which, the atom diffusion and superplastic deformation are responsible for the void shrinkage. Applying this model to the diffusion bonding of a Zr based metallic glass, the predicted bonding time is in good agreement with the experimental result. A map for determining the bonding temperature and time to achieve high quality bonding in a Zr based metallic glass is suggested.

Effect of the degree of crystallinity on the electrochemical behavior of Mg65Cu25Y10 and Mg70Zn25Ca5 bulk metallic glasses

The effect of varying the percent crystallinity on the electrochemical behavior of Mg65Cu25Y10 and Mg70Zn25Ca5 bulk metallic glasses was studied. The alloys were heat-treated to achieve desired microstructures ranging from fully amorphous to fully crystalline, providing a systematic basis for subsequent testing. Potentiodynamic experiments in 0.01 M sodium chloride (NaCl) were used, whereby both the amorphous and partially crystallized samples were observed to have more noble corrosion potentials and lower anodic kinetics. However, this was accompanied by more rapid cathodic kinetics relative to their fully crystalline counterparts, meaning that corrosion rates were not significantly lower in the amorphous state. To describe the electrochemical response as a function of the degree of crystallinity, differential scanning calorimetry (DSC), scanning electron microscopy, x-ray diffraction (XRD), and electrical conductivity measurements were undertaken, where it was found that crystallinity alone is not necessarily the controlling factor and microchemistry that evolves upon devitrification, plays a key role in the electrochemical response of these materials.

Sensory and consumer research update : Fancy a glass of sherry?

Researchers in Germany have identified the major aroma components of Amontillado sherry wine, which is aged through both a biological and an oxidative step. The researchers used Sensomics analysis, a molecular sensory approach in which key aroma compounds are differentiated from the bulk of odourless volatiles.