Florian H. Dalla Torre

Negative strain rate sensitivity in bulk metallic glass and its similarities with the dynamic strain aging effect during deformation

Detailed investigations were carried out on the deformation behavior of Zr-based monolithic bulk metallic glass and bulk metallic glass matrix composites. The latter, due to splitting and multiplication of shear bands, exhibits larger compressive strains than the former, without significant loss of strength. Serrated flow in conjunction with a negative strain rate sensitivity was observed in both materials. This observation, together with an increase in stress drops with increasing strain and their decrease with increasing strain rate, indicates phenomenologically close similarities with the dynamic strain aging deformation mechanism known for crystalline solids. The micromechanical mechanism of a shear event is discussed in light of these results.

Temperature-dependent shear band dynamics in a Zr-based bulk metallic glass

Flow serrations recorded during inhomogeneous deformation of Zr52.5Ti5Cu17.9Ni14.6Al10 (Vit105) were studied during compression testing at temperatures between −40 and 60 °C. The shear band velocities determined exhibit a pronounced temperature dependence covering nearly two orders of magnitude. The velocities follow an Arrhenius-type behavior with an associated activation energy of 0.3±0.05 eV. The results demonstrate a thermally activated mechanism of shear band propagation, which is similar to the behavior of other, nonmetallic amorphous materials.

Large-scale synthesis of defined cobalt nanoparticles and magnetic metal-polymer composites

We present a method where epsilon-cobalt nanoparticles with an average diameter of 4.5 nm can be synthesized in a controlled process and in significantly larger quantities than previously reported in the literature, based on the thermal decomposition of dicobaltoctacarbonyl in the presence of oleic acid and trioctylphosphine oxide (TOPO). Moreover, since the resulting particles are coated with an oleate layer, as shown by infrared (IR) spectroscopy, the colloids can be re-dispersed in organic solvents. These dispersions are suitable for the preparation of nanocomposites by a simple procedure, i.e. mixing of the cobalt dispersion with a polymer solution followed by casting and solvent evaporation. Magnetization measurements confirm the expected superparamagnetic behavior for both the cobalt nanoparticles and the metal-polymer nanocomposites.

Metallic foams from nanoparticle-stabilized wet foams and emulsions

We present a method to prepare metallic foams with a unique architecture of small pores and thin struts using wet foams and emulsions stabilized by metallic nanoparticles as templates.

On the Influence of the Misorientation of Grains, Grain Size and Boundary Volume on the Strength and Ductility of Ultrafine Grained Cu

The influence of the microstructure and the misorientation relationship of grains on mechanical properties is investigated in specimens of ultrafine-grained copper processed by equal channel angular extrusion (ECAE) route B c for 1, 4 and 12 passes. XRD texture analyses have shown that the major texture component is developed during the first pass of ECAE, and remains approximately constant with greater number of passes. EBSD measurements indicate that the majority of grain boundaries are still of low angle (>15°), while after four and twelve passes more than 50 % of all boundaries are high angle ones. TEM analyses have shown that the microstructure evolves from microbands and elongated cells towards a more equiaxed homogenous microstructure. On the microscale observed by TEM the degree of misorientation among subgrains/cells increases and the width of boundaries decreases while the cell/subgrain size remains approximately constant as the number of passes increases. The mechanical properties show a saturation level with a maximum in the yield stress and UTS after 4 passes. The strength of the material decreases between the fourth and the twelfth passes and the uniform elongation increases. It is suggested that the increase in ductility (and decrease in strength) is associated with the decrease in width of boundaries leading to an increase in the mean free path of dislocations.