Micromechanics of fully lamellar TiAl alloys

Fully lamellar gamma titanium aluminides are very promising materials for aerospace applications, due to their increased thrust-to-weight ratios and improved efficiency under aggressive environments at temperatures up to 750 ºC. For that reason, they are projected to replace the heavier Ni- base superalloys currently used for low pressure turbine (LPT) blades manufacturing. However, their ductility is limited due to their inherent anisotropy, associated to the lamellar microstructure. The objective of this work was to study the mechanical response of single colonies of polycrystalline γ-TiAl, as a function of layer thickness and layer orientation, and to relate this mechanical response with the operative deformation mechanisms. With this aim, micropillars with lamellae oriented at 0º, 45º and 90º with respect to the loading direction were compressed at room temperature and elevated 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 45º 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 0º 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, 90º 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 mm. 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 Additionally, microtensile specimens were also milled out of single colonies and in-situ tested in the SEM, to study the role of interlamellar interfaces on the plastic deformation and fracture under tension. EBSD was used before and after the test to study the role of different type of interfaces (true twin, pseudo twin and order variant) on slip/twin transfer. This study emphasizes the complexity of the micromechanics of fully lamellar TiAl alloys, where the activation of different deformation modes is strongly affected, not only by the lamellar orientation, but also by the character of the interfaces between the different lamellae. References A.J. Palomares, M.T. Pérez-Prado, J.M. Molina-Aldareguia, Acta Mater. 123 (2017) 102-114 A.J. Palomares, I. Sabirov, M.T. Pérez-Prado, J.M. Molina-Aldareguia, Scripta Mater. 139 (2017) 17-2

New instrumentation and analysis methodology for nano-impact testing

Nanoindentation testing has become increasingly popular for mechanical characterization of materials. This is motivated by the high versatility of the technique that allows testing of small volumes that could not be tested otherwise by macroscopic techniques, with minimal test preparation. The interest on nano-/microscale characterization of materials has been also extended to the study of high strain rate mechanical behaviour. One of the available techniques is nano-impact testing. It is carried out on a pendulum-based force-actuated, displacement-sensing device with the ability of performing energy-controlled impacts. The combination of conventional nanoindentation, for which a range of strain rates from 10-3 to 10-1 s-1 can be tested, with nano-impact provides a tool for materials characterization at the nano/microscale from 10-3 to 103 s-1. Regarding the analysis of nano-impact test results, there has been no consensus in literature over what material metrics to extract from the test. Several authors base the analysis of nano-impact test on the calculation of a dynamic hardness defined as change in kinetic energy throughout the impact divided by the residual volume of indentation [1-4]. However, there are two issues with the assumptions in which this equation is based. First, it only considers the change in kinetic energy and it neglects other important contributions like the work of impulse force. Then, it assumes that hardness is constant throughout the entire impact period. While for self-similar indenters this is true in the loading part, Cheng’s dimensional analysis shows that this is not the case in the unloading [5]. Therefore, the hardness calculated from this definition is not necessarily equal to the hardness under load commonly used in the instrumented indentation literature. To this end, an alternative analysis methodology is proposed. The analysis is based on the same definition of hardness under load commonly used in the instrumented indentation literature, computed as force divided by contact area. This way, the nano-impact hardness is directly comparable with results of conventional nanoindentation that use this definition. The instrumentation of the nanoindentation device with force-sensing capability was found crucial for the implementation of the analysis methodology. In addition, and in line with the nano-impact hardness definition in literature, an energy-based hardness is presented. The technique is assessed using finite element simulations and by testing six materials covering a wide range of mechanical behaviours. The FE simulations are used to assess the two energy-based definitions of hardness, the one in literature and the one proposed in this work. It was found that the literature definition leads to values that differ significantly from the ones obtained as force divided by contact area. On the other hand, the proposed energy-based definition provides values that match the ones obtained by force-approach. The experimental results are also in line with this conclusions. The literature energy-based hardness presents significant differences compared to the force-based hardness, which are higher for the more elastic materials. Furthermore, the force-based hardness computed from nano-impact results was compared with the hardness from conventional nanoindentation. A close match is found between both set of results. References [1] J.R. Trelewicz, C.A. Schuh, The Hall–Petch breakdown at high strain rates: Optimizing nanocrystalline grain size for impact applications, Appl. Phys. Lett. 93 (2008) 171916. [2] H. Somekawa, C.A. Schuh, High-strain-rate nanoindentation behavior of fine-grained magnesium alloys, Journal of Materials Research. 27 (2012) 1295–1302. [3] J.M. Wheeler, A.G. Gunner, Analysis of failure modes under nano-impact fatigue of coatings via high-speed sampling, Surface and Coatings Technology. 232 (2013) 264–268. [4] C. Zehnder, J.-N. Peltzer, J.S.K.-L. Gibson, S. Korte-Kerzel, High strain rate testing at the nano-scale: A proposed methodology for impact nanoindentation, Materials & Design. 151 (2018) 17–28. [5] Y.-T. Cheng, C.-M. Cheng, Scaling, dimensional analysis, and indentation measurements, Materials Science and Engineering: R: Reports. 44 (2004) 91–149. doi:10.1016/j.mser.2004.05.001

Multiscale characterization of the micromechanics of pure Mg

An important limitation of wrought (rolled and extruded) Mg alloys is their inherent strong mechanical anisotropy, a consequence of their hexagonal closed- packed (hcp) lattice. Several reasons contribute to this effect. First, at room temperature, the critical resolved shear stresses (CRSSs) of basal and non-basal slip systems have very different values, spanning several orders of magnitude; second, twinning, a very common deformation mechanism in these materials, exhibits a pronounced polarity, i.e. its activation is dependent on the relative orientation between the c-axis and the applied stress; finally, both hot and cold deformation processing textures are often quite sharp and the way the activation of different slip systems is influenced by the local texture and grain boundary network is not clear. Together, these factors lead to a dependence of the dominant deformation mechanisms on the texture, grain size, testing mode (tension or compression) and the testing direction, resulting in large differences in yield stress values and strain-hardening responses. In this work, we adopt a multiscale characterization strategy to unravel the micromechanisms of pure Mg. First, we present a coupled experimental and simulation study on the nanoindentation of pure Mg at different temperatures to determine the critical resolved shear stress evolution of the different slip systems at the single crystal level [1-3]. For this, several indentations were performed at temperatures between RT and 300 °C in individual grains of a polycrystalline sheet of pure Mg with different crystallographic orientations. The deformation profile and the microstructure around the indents was analyzed by atomic force microscopy (AFM) and electron backscatter diffraction (EBSD), to determine the CRSS of the different slip systems without grain boundary effects. EBSD assisted trace analysis during in-situ SEM mechanical testing of cold-rolled polycrystalline Mg sheets was then used to account for the role of the local microstructure, such as the local texture and grain boundary network, on the activation of the different deformation modes, In particular, it was found that, with decreasing grain size, at room temperature, a clear transition from non-basal to basal-slip dominated flow takes place under tension [4] and a transition from twinning to basal slip takes place under compression [5]. On the other hand, a similar transition from twinning to basal slip takes place with increasing temperature and decreasing strain rate [6]. The emergence of basal slip as a dominant mechanism is shown to be due to increasing levels of connectivity between favorably oriented grains, which facilitate slip transfer across grain boundaries. This study emphasizes the complexity of the micromechanics of pure Mg, where the activation of different deformation modes is strongly affected, not only by their single crystal CRSS levels, but also by the local grain boundary networks and local texture emerging from processing

Understanding the links between the composition-processing-properties in new formulations of heas sintered by sps

This work presents two new compositions of high entropy alloys (HEAs) that were designed with the aim of obtaining a body-centered cubic (BCC) phase with high hardness values and a moderate density. Sintering was performed using Spark Plasma Sintering (SPS) with different heating rates to determine the influence of the processing parameters on the phase formation. The microstructural study revealed that the presence of Ni in the composition promoted phase separation, and the mechanical study confirmed a clear influence on the mechanical properties of both the composition and heating rate. The combination of microscopy with compression and nanoindentation tests at room and high temperature made it possible to advance our understanding of the relationships between the composition, processing, and properties of this emerging group of alloys

Fabrication of HfO2 patterns by laser interference nanolithography and selective dry etching for III-V CMOS application

Nanostructuring of ultrathin HfO2 films deposited on GaAs (001) substrates by high-resolution Lloyd's mirror laser interference nanolithography is described. Pattern transfer to the HfO2 film was carried out by reactive ion beam etching using CF4 and O2 plasmas. A combination of atomic force microscopy, high-resolution scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy microanalysis was used to characterise the various etching steps of the process and the resulting HfO2/GaAs pattern morphology, structure, and chemical composition. We show that the patterning process can be applied to fabricate uniform arrays of HfO2 mesa stripes with tapered sidewalls and linewidths of 100 nm. The exposed GaAs trenches were found to be residue-free and atomically smooth with a root-mean-square line roughness of 0.18 nm after plasma etching

An XFEM/CZM implementation for massively parallel simulations of composites fracture

Because of their widely generalized use in many industries, composites are the subject of many research campaigns. More particularly, the development of both accurate and flexible numerical models able to capture their intrinsically multiscale modes of failure is still a challenge. The standard finite element method typically requires intensive remeshing to adequately capture the geometry of the cracks and high accuracy is thus often sacrificed in favor of scalability, and vice versa. In an effort to preserve both properties, we present here an extended finite element method (XFEM) for large scale composite fracture simulations. In this formulation, the standard FEM formulation is partially enriched by use of shifted Heaviside functions with special attention paid to the scalability of the scheme. This enrichment technique offers several benefits since the interpolation property of the standard shape function still holds at the nodes. Those benefits include (i) no extra boundary condition for the enrichment degree of freedom, and (ii) no need for transition/blending regions; both of which contribute to maintaining the scalability of the code. Two different cohesive zone models (CZM) are then adopted to capture the physics of the crack propagation mechanisms. At the intralaminar level, an extrinsic CZM embedded in the XFEM formulation is used. At the interlaminar level, an intrinsic CZM is adopted for predicting the failure. The overall framework is implemented in ALYA, a mechanics code specifically developed for large scale, massively parallel simulations of coupled multi-physics problems. The implementation of both intrinsic and extrinsic CZM models within the code is such that it conserves the extremely efficient scalability of ALYA while providing accurate physical simulations of computationally expensive phenomena. The strong scalability provided by the proposed implementation is demonstrated. The model is ultimately validated against a full experimental campaign of loading tests and X-ray tomography analyzes.A.J., A.M., D.T., L.N. and L.W. acknowledge funding through the SIMUCOMP ERA-NET MATERA + project financed by the Fonds National de la Recherche (FNR) of Luxembourg, the Consejería de Educación y Empleo of the Comunidad de Madrid, the Walloon region (agreement no 1017232, CT-EUC 2010–10-12), and by the European Unions Seventh Framework Programme (FP7/2007–2013).Peer ReviewedPostprint (author's final draft

Microarchitected Compliant Scaffolds of Pyrolytic Carbon for 3D Muscle Cell Growth

The integration of additive manufacturing technologies with the pyrolysis of polymeric precursors enables the design-controlled fabrication of architected 3D pyrolytic carbon (PyC) structures with complex architectural details. Despite great promise, their use in cellular interaction remains unexplored. This study pioneers the utilization of microarchitected 3D PyC structures as biocompatible scaffolds for the colonization of muscle cells in a 3D environment. PyC scaffolds are fabricated using micro-stereolithography, followed by pyrolysis. Furthermore, an innovative design strategy using revolute joints is employed to obtain novel, compliant structures of architected PyC. The pyrolysis process results in a pyrolysis temperature- and design-geometry-dependent shrinkage of up to 73%, enabling the geometrical features of microarchitected compatible with skeletal muscle cells. The stiffness of architected PyC varies with the pyrolysis temperature, with the highest value of 29.57 ± 0.78 GPa for 900 °C. The PyC scaffolds exhibit excellent biocompatibility and yield 3D cell colonization while culturing skeletal muscle C2C12 cells. They further induce good actin fiber alignment along the compliant PyC construction. However, no conclusive myogenic differentiation is observed here. Nevertheless, these results are highly promising for architected PyC scaffolds as multifunctional tissue implants and encourage more investigations in employing compliant architected PyC structures for high-performance tissue engineering applications