Rafael Soler

Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography

In this work, the nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties. The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 °C. Transmission electron microscopy investigations revealed structural features at various length scales: bundles (30 nm to networks of several micrometers) consisting of columnar grains (∼10 nm in diameter), grain boundary regions with a less ordered atomic arrangement, and defects including disordered clusters (∼1.5 nm in diameter) as well as stacking faults within the grains. The most prominent defect with a volume fraction of ∼0.5% is the disordered clusters, which were investigated in detail by electron energy loss spectroscopy and atom probe tomography. The results provide conclusive evidence that Ar is incorporated into the Mo2BC film as disordered Ar-rich Mo-B-C c...

In-situ TEM Study of Mechanical Size Effects in TiC Strengthened Steels

It is nowadays well understood that single crystalline metals exhibit a size dependence of the yield stress: with decreasing pillar diameters an increase in the flow stress is measured in micropillar compression tests. This size effect (“smaller is stronger”) is governed by the size of the activated dislocation sources and can be described by a power law equation relating stress and pillar diameter (σ ~ d). For single crystalline metallic pillars containing internal particles three different stress size dependencies are reported in literature: a power law [1, 2], a size-independent (i.e. constant) [3-6], or an intermediate behavior, which can be interpreted as a transition from constant to power law behavior [79]. The differences in size-dependency can be explained by two competing mechanisms, dislocation source activation and dislocation particle interaction [8, 9]. When the stress to activate dislocation sources exceeds the particle strengthening effect, pillars should show the “classical” power law dependence. In the present study we want to elaborate if we can quantitatively deduce the particle strengthening effect of e.g. shearable weak or non-shearable strong particles in metal materials from their transition regimes of size dependency. We have examined this idea in precipitation hardened steel with nanometer-sized titanium carbides (TiC) precipitates.