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A high-throughput methodology is proposed, based on the combination of diffusion couples and advanced nanomechanical testing methods, to directly measure alloying effects on the critical resolved shear stress (CRSS) of individual deformation modes in Mg alloys. The methodology is tested in Mg-Zn alloys by assessing the alloying effects, up to Zn contents of 2 at.%, on basal slip, extension twining and pyramidal slip in two metallurgical conditions: as-quenched, for which the Zn solute atoms remain homogenously dispersed in solid solution; and peak-aged, for which the Zn atoms form rod-shape β1′ (MgZn2) precipitates. A combined approach including micromechanical testing, transmission Kikuchi diffraction, and high-resolution transmission electron microscopy was performed to reveal the corresponding deformation mechanisms. It was found that the CRSS enhancement for basal slip and extension twinning by MgZn2 precipitates is considerably larger than the effect of Zn in solid solution, while the strengthening of pyramidal slip is similar in both cases. As a result, the anisotropy ratios remain high and similar to pure Mg in the solid solution strengthened Mg-Zn alloys. However, they are substantially reduced in precipitation strengthened Mg-Zn alloys.

Al/SiC nanolaminates with equal nominal thicknesses of the Al and SiC layers (10, 25, 50 and 100 nm) were manufactured by magnetron sputtering. The mechanical properties were measured at 25 °C and 100 °C by means of nanoindentation and micropillar compression tests and the deformation mechanisms were analyzed by in situ micropillar compression tests in the transmission electron microscope. In addition, finite element simulations of both tests were carried out to ascertain the role played by the strength of the Al layers and by the elastic constraint of the ceramic layers on the plastic flow of Al in the mechanical response. It was found that the mechanical response was mainly controlled by the constraint during nanoindentation or micropillar compression tests of very thin layered (≈10 nm) laminates, while the influence of the strength of Al layers was not as critical. This behavior was reversed, however, for thick layered laminates (100 nm). These mechanisms point to the different effects of layer thickness during nanoindentation and micropillar compression, at both temperatures, and showed the critical role played by constraint on the mechanical response of nanolaminates made of materials with a very large difference in the elasto-plastic properties.

The aim of this study is to determine the influence of lamellar orientation on the strength and operative deformation mechanisms of a fully lamellar Ti-45Al-2Nb-2Mn (at.%) + 0.8(vol.%) TiB2 (Ti4522XD) alloy. With this aim, micropillars with lamellae oriented at 0°, 45° and 90° with respect to the loading direction were compressed at room temperature. The results revealed a large plastic anisotropy, that was rationalized, based on slip/twin trace analysis, according to the relative orientation of the main operative deformation modes with respect to the lamellar interfaces. Loading at 45° resulted in the activation of soft longitudinal deformation modes, where both the slip plane and the slip direction were parallel to the interfaces, and therefore, little interaction of dislocations with lamellar interfaces is expected. At 0° loading, deformation was mainly accommodated by harder mixed deformation modes (with an oblique slip plane but a slip direction parallel to the lamellar interfaces), although the lamellar interfaces seemed to be relatively transparent to slip transfer. On the contrary, 90° loading represented the hardest direction and deformation was accommodated by the activation of transverse deformation modes, confined to individual lamellae, together with longitudinal modes that were activated due to their softer nature, despite their very small Schmid factors. Finally, a thorough study of pillar size effects revealed that the results were insensitive to pillar size for dimensions above 5 μm. The results can therefore be successfully applied for developing mesoscale plasticity models that capture the micromechanics of fully lamellar TiAl microstructures at larger length scales.

Metal-ceramic nanolaminate composites show promise as high strength and toughness materials. Micropillar compression was used to characterize the mechanical behavior of Al SiC multilayers in different orientations including loading at 0°, 45° and 90° with respect to the direction of the layers. The 0° orientation showed the highest strength while the 45° orientation showed the lowest strength. Each orientation showed unique deformation behavior. Effects of pillar size and aspect ratio were also studied. Higher compressive strengths were observed in smaller pillars for all orientations. This effect was shown to be due to a lower probability of flaws using Weibull statistics. Additionally, changes in the aspect ratio was shown to have no significant effect on the behavior except an increase in the strain to failure in the 0° orientation. Finite element analysis (FEA) was used to simulate and understand the effect of these parameters on the deformation behavior.

Amorphous carbon (aC), in particular diamond-like carbon (DLC), is one of the most studied and promising coating materials, but many of the atomic-scale mechanisms involved in their plastic deformation process are not fully understood. The mechanical response of non-hydrogenated DLC films with different sp 3 concentrations is investigated using nanoindentation experiments, ab-initio simulations, and classical molecular dynamics simulations. Experimental characterization is carried out with Raman spectroscopy and Electron Energy Loss Spectroscopy (EELS). Ab-initio and classical simulations show good agreement for sp 1 , sp 2 and sp 3 content of in-silico samples. Elastic modulus and hardness of DLC films increase with sp 3 content, for sp 3 between 10% and 55%, and excellent agreement is obtained between experiments and simulations. Simulated strain distributions are shown to be highly anisotropic, unlike continuum-scale predictions for the use of a perfectly spherical indenter in an amorphous solid. MD simulations also reveal two different plasticity modes depending on the sp 3 level of the indented sample. For films with sp 3 concentrations less than 40%, plasticity is mainly mediated by the sp 2 to sp 3 transition. For larger sp 3 concentrations, DLC plastic deformation is attributed to densification due to bond rearrangement. All in all, our work offers a comprehensive study of DLC, revealing unexpected plastic deformation mechanisms that had not been considered before. Our study might help to the fundamental understanding of amorphous carbon coatings for both scientific and technological purposes.

A multiscale modeling strategy is presented to determine the effective mechanical properties of polycrystalline Ni-based superalloys. They are obtained by computational homogenization of a representative volume element of the microstructure which was built from the grain size, shape and orientation distributions of the material. The mechanical behavior of each grain was simulated by means of a crystal plasticity model, and the model parameters that dictate the evolution of the critical resolved shear stress in each slip system (including viscoplastic effects as well as self and latent hardening) were obtained from compression tests in micropillars milled from grains of the polycrystal in different orientations suited for single, double (coplanar and non coplanar) and multiple slip. The multiscale model predictions of the compressive strength of wrought IN718 were in good agreement with the experimental results.

Tensile twinning is a key deformation mode in magnesium and its alloys, as well as in other hcp metals. However, the fundamentals of this mechanism are still not fully understood. In this research, instrumented nanoindentation and crystal plasticity finite element simulations are utilized to investigate twin formation and propagation in pure Mg. With that purpose, several nanoindentations at different indentation depths were performed in pure Mg single crystals with a wide range of crystallographic orientations. A careful analysis of the deformation profile, by atomic force microscopy, and of the microtexture, by electron backscatter diffraction, in areas around and underneath the indents, reveals that twinning is subjected to strong size effects, i.e., that the relative activity of twinning increases dramatically with the indentation depth. Furthermore, the twin volume fraction is found to be related to the pile-up or sink-in areas close to the indentations. A decrease in hardness in orientations where the twinning activity is high was confirmed both experimentally and by crystal plasticity finite element simulations. Finally, our results support the thesis that twin activation is an energetic process that demands a concentration of high stresses in a certain activation volume.

The aim of this paper is to elucidate the origin of the transition from twinning to slip dominated flow in pure Mg. With that purpose, two polycrystals with average grain sizes of 19 and 5 μm were prepared by rolling and annealing and they were tested in compression along the rolling direction at strain rates ranging from 10 −3 to 10 −5 s −1 and at temperatures comprised between 50 and 250 °C. Twinning was evaluated by conventional electron backscatter diffraction (EBSD) and the activity of different slip systems was measured by an exhaustive EBSD-assisted slip trace analysis. A transition from twinning to basal slip, localized along deformation bands, was found to take place with decreasing grain size, with decreasing strain rate and with increasing temperature. The emergence of basal slip as the dominant deformation mechanism is promoted in all three cases by increasing levels of connectivity between favorably oriented grains, which facilitate slip transfer across grain boundaries. Such connectivity is related to the fraction of grain boundaries (GBs) with misorientations smaller than a threshold value ( f θ θ th ) as well as to their local arrangement. Since processing for grain refinement results in larger f θ θ th values and both decreasing strain rate and increasing temperature increase θ th , in all cases twinning is eventually replaced by basal slip.

The aim of this work is to investigate the effect of grain size on slip activity, a critical issue for materials design that remains unsolved. Three pure magnesium polycrystals with average grain sizes ( d ) of 36, 19 and 5 μm and with very similar textures and grain boundary (GB) misorientation distributions were designed by selecting appropriate rolling and annealing conditions. The three microstructures were tested in tension along the rolling direction (RD) at 50 °C and at an strain rate of 10 −3 s −1 and the incidence of the different deformation mechanisms was evaluated by electron backscattered diffraction (EBSD)-assisted slip trace analysis. A clear transition from non-basal to basal slip-dominated flow takes place when d decreases below 36 μm, i.e. at grain size ranges that are very relevant for most structural applications. This change in slip activity is shown to be consistent with an increase in the CRSS non-basal /CRSS basal ratio with decreasing d , i.e. with a more potent GB strengthening of non-basal than basal systems. Accordingly, a transition in the mechanisms responsible for the accommodation of local stress concentrations at GBs takes place with decreasing grain size. Intergranular compatibility becomes increasingly difficult with grain refinement due to the progressively lower availability of active deformation mechanisms. Macroscopically, these phenomena translate to a reduction of work hardening and ductility with decreasing grain size.

Compression tests of 〈1 1 1〉-oriented LiF single-crystal micropillars 1–5 μm in diameter were carried out from 25 °C to 250 °C. While the flow stress at ambient temperature was independent of the micropillar diameter, a strong size effect developed with elevated temperature. This behavior was explained by rigorously accounting for the different contributions to the flow stress of the micropillars as a function of temperature and pillar diameter: the lattice resistance, the forest hardening; and the size-dependent contribution as a result of the operation of single-arm dislocation sources. This was possible because the micropillars were obtained by chemically etching away the surrounding matrix in directionally solidified LiF–NaCl and LiF–KCl eutectics, avoiding any use of focused ion beam methods, yielding micropillars with a controlled dislocation density, independent of the sample preparation technique. In particular, the role of the lattice resistance on the size effect of micrometer-size single crystals was demonstrated unambiguously for the first time. This result rationalizes the different values of power-law exponent for the size effect found in the literature for face-centered cubic and body-centered cubic metals as well as for covalent and ionic solids.

A novel methodology based on instrumented indentation is developed to determine the mechanical properties of amorphous materials which present cohesive-frictional behaviour. The approach is based on the concept of a universal hardness equation, which results from the assumption of a characteristic indentation pressure proportional to the hardness. The actual universal hardness equation is obtained from a detailed finite element analysis of the process of sharp indentation for a very wide range of material properties, and the inverse problem (i.e. how to extract the elastic modulus, the compressive yield strength and the friction angle) from instrumented indentation is solved. The applicability and limitations of the novel approach are highlighted. Finally, the model is validated against experimental data in metallic and ceramic glasses as well as polymers, covering a wide range of amorphous materials in terms of elastic modulus, yield strength and friction angle.

A testing technique based on cross-sectional nanoindentation has been used to assess the mechanical reliability of interconnect structures. A Berkovich indenter was used to initiate fracture in a silicon substrate and cracks propagated through the structure. To better control crack growth and to convert the problem into two dimensions, a trench parallel to the indentation surface was previously machined using a focused ion beam. The crack lengths obtained for different material systems in the interconnect structure correlate well with the fracture energies measured for the same materials in blanket films. Finite element model simulations incorporating cohesive elements have been used to model the fracture processes and to explain the different cracking behaviour observed.