The sample size effect in metallic glass deformation.

The sample size effect on deformation mode of glasses is one of the most misunderstood properties of this class of material. This effect is intriguing, since materials deemed macroscopically brittle become plastic at small size. We propose an explanation of this phenomenon for metallic glasses. A thermodynamic description of the local rearrangement zones activated under an applied stress is proposed. Using the Poisson distribution to describe the statistics of these zones and the statistical physics to associate entropy, we define a critical sample size for the change in the deformation mode. Predictions are in agreement with experimental observations and reveal hidden structural parameters describing the glassy state.

From powders to bulk metallic glass composites.

One way to adjust the properties of materials is by changing its microstructure. This concept is not easily applicable on bulk metallic glasses (BMGs), because they do not consist of grains or different phases and so their microstructure is very homogeneous. One obvious way to integrate inhomogeneities is to produce bulk metallic glass composites (BMGCs). Here we show how to generate BMGCs via high-pressure torsion (HPT) starting from powders (amorphous Zr-MG and crystalline Cu). Using this approach, the composition can be varied and by changing the applied shear strains, the refinement of the microstructure is adjustable. This process permits to produce amorphous/crystalline composites where the scale of the phases can be varied from the micro- to the nanometer regime. Even mixing of the two phases and the generation of new metallic glasses can be achieved. The refinement of microstructure increases the hardness and a hardness higher than the initial BMG can be obtained.

Influence of roughness and tilt on nanoindentation measurements: A quantitative model.

A model, which takes into account the indentation size effect, was proposed to relate hardness and its standard deviation to the quality of the sample surface, i.e., the roughness and the tilt. Stainless steel samples were mechanically polished so as to obtain various roughness and tilt. Then the quality of the surface and the hardness were measured respectively by scanning probe microscopy and by nanoindentation. The model was fitted according to those experimental data. It was shown that the standard deviation only depends on the ratio of the arithmetic roughness over the contact depth of the indent while the average hardness also depends on the contact depth and on the tilt. It was deduced that hardness measurements are performed with an uncertainty lower than 10% if: (i) the ratio of the arithmetic roughness over the contact depth of the indent is lower than 0.05, (ii) the tilt of the sample is lower than 2° and (iii) the difference of roughness between samples is lower than 50%.

Near-perfect elastoplasticity in pure nanocrystalline copper.

Ductile metals and alloys undergo plastic yielding at room temperature, during which they exhibit work-hardening and the generation of surface instabilities that lead to necking and failure. We show that pure nanocrystalline copper behaves differently, displaying near-perfect elastoplastic behavior characterized by Newtonian flow and the absence of both work-hardening and neck formation. We observed this behavior in tensile tests on fully dense large-scale bulk nanocrystalline samples. The experimental results further our understanding of the unique mechanical properties of nanocrystalline materials and also provide a basis for commercial technologies for the plastic (and superplastic) formation of such materials.

Vortex flux channeling in magnetic nanoparticle chains.

A detailed understanding of the formation of magnetic vortices in closely spaced ferromagnetic nanoparticles is important for the design of ultra-high-density magnetic devices. Here, we use electron holography and micromagnetic simulations to characterize three-dimensional magnetic vortices in chains of FeNi nanoparticles. We show that the diameters of the vortex cores depend sensitively on their orientation with respect to the chain axis and that vortex formation can be controlled by the presence of smaller particles in the chains.