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Abstract The anisotropic deformation behaviour of ZrB 2 grains of basal and prismatic orientations in a polycrystalline ZrB 2 sample was studied during in-situ micropillar compression at room temperature (RT), 350 °C and 500 °C. Micropillars were milled out of large grains by focused ion beam machining; the basal and prismatic orientations were previously identified using electron backscatter diffraction maps. The basal pillars showed brittle behaviour and cracking on the 10 1 ¯ 0 and 2 1 ¯ 1 ¯ 0 type prismatic planes that lead to their collapse. For this orientation, the yield/rupture stress ranged between 8 and 14 GPa and it exceeded 12 GPa even at 500 °C. In the prismatic orientation, considerable plasticity was observed, showing a temperature dependent yield stress from 6 to 8 GPa at room temperature to ~2GPa at 500 °C. The activated slip system family was identified as 10 1 ¯ 0 11 2 ¯ 3 type for all of the test temperatures (RT, 350 °C and 500 °C). Analysis of the Schmid-factors revealed that the plasticity of the prismatic micropillars is controlled by the temperature dependence of the critical resolved shear stress of the 10 1 ¯ 0 11 2 ¯ 3 slip systems. The brittleness of the basal micropillars was influenced by their defect structure (e.g. microcracks) and it was temperature independent due to the absence of slip activation. Highlights • Anisotropic deformation was observed in ZrB 2 micropillars from room temperature to 500°C. • Basal pillars showed cracking on the 10 1 ¯ 0 and 2 1 ¯ 1 ¯ 0 type prismatic planes with rupture stress ranged between 8 and 14 GPa. • Prismatic pillars exhibited considerable plasticity and temperature dependent yield stress from 6-8 GPa to ~2 GPa. • The activation of the 10 1 ¯ 0 11 2 ¯ 3 type slip system was identified for all of the test temperatures (RT, 350°C and 500°C). [ABSTRACT FROM AUTHOR]

Abstract Micro-pillar splitting at temperatures up to 500 °C was used to evaluate the toughness for a series of thin physical vapour deposited ceramic-nitride coatings. When compared to ion beam notched geometries, this testing approach reduces the likelihood of ion impregnation within a material volume where fracture may initiate. A toughness increase with elevated temperature was observed for the nanostructured ceramic coatings; the magnitude of which varied between the coatings. A transmission electron microscopy investigation showed deposition-specific periodic nano-layering within the films as a result of the deposition process, which may explain the observed differences in the toughness trends. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Micromechanical testing is normally limited to the quasi-static strain rate regime: 10 −5 to 10 −2 s −1 . In this work, a nanomechanical testing device has been developed that allows indentation and microcompression at strain rates from the quasi-static regime continuously up to the high strain rate regime. To reach the highest strain rates, the conventional, strain gage-based load cell was replaced with a new piezo-based sensor. The sensor's concept, calibration methods, and measurement strategies are detailed. It is shown, using nanocrystalline Nickel as a case study material, that this new high strain rate device can measure precisely mechanical properties at strain rates up to 1000 s −1 by nanoindentation, and strain rates up to 100 s −1 by microcompression, enabling the device to measure strain rates over 9 orders of magnitude. [ABSTRACT FROM AUTHOR]

In this study, we probe the activation parameters and time-dependent plastic behavior of a nanocrystalline Palladium-Gold alloy by means of elevated temperature micropillar compression experiments. All tests were conducted on a stable, displacement-controlled in situ SEM indenter. A series of strain rate jump tests, where the strain rates were varied from 10 −4 s −1 up to 10 −2 s −1 , allowed us to extract strain rate sensitivity and average activation volumes in the order of 8 b 3 . Repeated load relaxation tests yielded lower activation volumes in the range of 4 b 3 . From the variation of test temperatures up to 125 °C, we extracted an apparent activation energy of ∼0.77 eV. The possible deformation mechanisms at such extremely small crystallite sizes are discussed within the framework of pertinent literature. [ABSTRACT FROM AUTHOR]

The high temperature mechanical behaviour of plasma-nitrided tool steel is investigated at variable temperatures using in situ micro-pillar compression. It is observed that the strengthening brought about by nitriding is steadily, but reversibly, reduced when increasing the testing temperature, until it is completely negated at 500 °C. [ABSTRACT FROM AUTHOR]

Silicon is brittle at ambient temperature and pressure, but using micro-scale samples fabricated by focused ion beam (FIB) plasticity has been observed. However, typical drawbacks of this methodology are FIB-damage and surface amorphization. In this study, lithographic etching was employed to fabricate a large number of 〈100〉-oriented Si pillars with various diameters in the micro-scale. This allowed quantitative study of plasticity and the size effect of FIB-free Si in the brittle temperature range (25–500 °C) by conducting monotonic and transient microcompression in situ in the scanning electron microscope (SEM). Lithographic pillars achieved the ideal strength in temperature range of 25–100 °C and displayed significantly higher strengths (30–60%) than FIB-machined pillars because of the undamaged surface and the oxide layer confinement. The activation energy of deformation revealed a transition in dislocation mechanisms as a function of temperature. Strain rate sensitivity and activation volume measured from strain rate jump and stress relaxation tests indicated the surface nucleation of kink-pairs associated with the constricted dislocation motion in Si during deformation at temperatures below the brittle-ductile transition. A modified analytical model is proposed to accurately evaluate the size-dependent strength of covalent crystalline Si. The weak size effect observed in Si is attributed to the surface nucleation of dislocations and high lattice friction during their motion. Unlabelled Image • Lithographic Si pillars achieve near ideal strength, 60% stronger than FIB pillars. • Activation parameters indicate kink-pair nucleation in Si at low temperatures. • Si transitions from full to partial dislocations gradually from 150 to 300 °C. • A modified analytical model is proposed for the size-dependent strength of Si. [ABSTRACT FROM AUTHOR]

The micromechanical fracture behavior of Si [100] was investigated as a function of temperature in the scanning electron microscope with a nanoindenter. A gradual increase in KC was observed with temperature, in contrast to sharp transitions reported earlier for macro-Si. A transition in cracking mechanism via crack branching occurs at -300 °C accompanied by multiple load drops. This reveals that onset of small-scale plasticity plays an important role in the brittle-to-ductile transition of miniaturized Si. [ABSTRACT FROM AUTHOR]

Abstract The distribution of strain in hard mode oriented lamellar stacks of the two-phase γ-TiAl/α 2 -Ti 3 Al alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol% TiB 2 was measured at several temperatures up to 633 °C by in situ micropillar compression, complemented by electron backscatter diffraction orientation mapping and digital image correlation strain mapping of a thermally stable surface Pt speckle pattern. Post-mortem transmission electron microscopy further identified the finest scale deformation structures. It was found that slip and twinning transverse to the lamellae operates within discreet bands that zigzag across the lamellar structure. The shear strain within each band is approximately constant across the pillar width. This is inconsistent with current energetic models for transverse twin formation in γ-TiAl, which assume independent, non-interacting twins. This is explained using a mathematical formulation for the stress required to operate this transverse mechanical twinning as a function of strain. This study has elucidated how the multi-scale combination of several transverse twinning systems on different {111} planes in γ-TiAl lamellae can relieve the elastic stresses generated at a lamellar interface by the primary (highest Schmid factor) twinning system. It is thought that the facilitation of this mechanism will promote the ductilisation of lamellar γ-TiAl alloys. This is crucial for an increased damage tolerance and ease of component manufacture, leading to a more widespread use of γ-TiAl alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The occurrence of plasticity below the macroscopic yield stress during tensile monotonic loading of nearly lamellar Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB 2 at both 25 °C and 700 °C, and in two conditions of lamellar thickness, was measured by digital image correlation strain mapping of a remodelled Au surface speckle pattern. Such initial plasticity, not necessarily related to the presence of common stress concentrators such as hard particles or cracks, could occur at applied stresses as low as 64% of the general yield stress. For a same applied strain it was more prominent at room temperature, and located as slip and twinning parallel to, and near to or at (respect.) lamellar interfaces of all types in soft mode-oriented colonies. These stretched the full colony width and the shear strain was most intense in the centre of the colonies. Further, the most highly operative microbands of plasticity at specimen fracture were not those most active prior to yielding. The strain mapping results from polycrystalline tensile loading were further compared to those from microcompression testing of soft-mode stacks of lamellae milled from single colonies performed at the same temperatures. Combined with post-mortem transmission electron microscopy of the pillars, the initial plasticity by longitudinal dislocation glide was found to locate within 30–50 nm of the lamellar interfaces, and not at the interfaces themselves. The highly localised plasticity that precedes high cycle fatigue failure is therefore inherently related to the lamellar structure, which predetermines the locations of plastic strain accumulation, even in a single loading cycle. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Optimally, hard protective coatings should effectively absorb impact energy to reduce the likelihood of failure events. In this work, an arc-PVD approach was utilised for the deposition of thick ceramic multilayer AlCrTiN/CrN-based coatings containing a distribution of metallic nickel inclusions throughout sequential CrN-based interlayers. The aim of such coatings is to provide resistance to impact loading in intensive application environments, such as drop forging of steel and turbine blades exposed to abrasive particles. The structure and micro-structural development of the ceramic was first investigated using transmission electron microscopy, where discrete Ni inclusions were observed as both larger discs (d ~ 600–800 nm, h ~ 200 nm) and smaller (d ~ 50 nm) nanoclusters. Confirmation of the distribution and nanocluster chemistry was achieved using atom probe tomography. For mono-block coatings, XRD data showed drastically reduced internal stresses as a result of the Ni inclusions, enabling the creation of thicker protective coatings which minimise substrate stress concentration upon loading. Nickel inclusion additionally provides a softening of the containing hard ceramic layer, allowing for tuning of mechanical properties. Supporting this idea, in situ micropillar compression measurements of the multi-layered coating systems showed that Ni clusters hindered crack propagation through the coating during failure, while the fracture strength could be increased by incorporating both Ti and Ni in the softer CrN-based layer. High-load impact testing highlighted the influence of Ni 'shock absorbers' in reducing circumferential cracking, which was further confirmed by an industrial die forging test that demonstrated a 15–22% life-time increase compared to a much thicker hard-chrome plated reference. The results of this study demonstrate an exciting way to produce Ni nanocluster integrated hard ceramic multi-layers using arc-PVD in a single processing step. Such tuneable thin-film composite systems show great promise in minimising damage from impact loading, even under severe working conditions such as in hot forging. Highlights • One-step process for 'thick' multilayer ceramic coatings containing Ni-nanoclusters • Ni addition reduces the residual stress in ceramic thin-films. • Inclusion of Ni alters the microstructure and mechanical performance. • Improved life-time during industrial forging tests compared to industry standard [ABSTRACT FROM AUTHOR]

The stress required to activate twinning of the longitudinal < 11 2 ¯ ] { 111 } system in the lamellar γ-TiAl phase of the alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol.% TiB 2 was measured at several temperatures up to 700 °C by in situ micropillar compression of soft mode oriented γ-TiAl/α 2 -Ti 3 Al lamellar stacks. The lamellae undergoing deformation twinning were identified by electron backscatter diffraction orientation mapping. In some cases, such lamellae were not constrained by domain or colony boundaries and longitudinal twinning was the only deformation mechanism observed based on digital image correlation strain maps. The resolved shear stress for such unconstrained twinning was found to increase monotonically with temperature from 25 °C to 700 °C. This is consistent with the stacking fault energy increasing with temperature as found in many metallic alloys, suggesting that the increased ease of deformation twinning at high temperature in bulk TiAl alloys is due to the increased ease with which the twinning shear can be accommodated by the neighbouring domains and lamellae with increasing temperature, rather than a thermal softening of the intrinsic twinning mechanism. [ABSTRACT FROM AUTHOR]

The third body formed in a contact between HS25 cobalt-based superalloy versus ceramic under fretting wear (small reciprocating displacements) was investigated. This tribomaterial, commonly called “glaze layer”, was created from nanosized, compacted and sintered wear debris and adheres on both rubbed parts. The glaze layer was investigated both from tribological and rheological points of view. In terms of tribology, the glaze layer was found stable above 450 °C, providing low friction and very low wear rate in the interface. To study the mechanical properties of the third body, micropillars have been FIB-machined within the glaze layer and compressed between 25 °C and 500 °C. Low temperature testing showed a brittle and hard behavior for the glaze layer which was confirmed by nanoindentation. By contrast, glaze layer at 500 °C evidenced a perfect ductile response with high strain rate during fretting and no brittleness. Made of 10–20 nm nanocrystals embedded in a ceramic-metallic amorphous matrix, the glaze layer is put forward in the brittle to ductile transition: the amorphous matrix may act as a metallic glass. This allows the correlation between glaze layer ability to plastically accommodate the strain without fracturing processes, thus without being damaged through debris generation. The ejected debris flow is stopped and the wear rate becomes negligible. [ABSTRACT FROM AUTHOR]

Based on a combination of micropillar compression experiments and modelling of the secondary cell wall (cw) using continuum micromechanics, the shear yield stress of the polymer matrix is identified for both normal and compression wood of Norway spruce. It is shown that the model is able to capture the differences in mechanical properties between the two tissues based on the knowledge of composition of the samples, microfibril angle, as well as phase properties on the nanometer scale. By testing an isolated piece of the cell wall with a homogeneous and uniaxial stress field on the micrometer scale and using the micromechanical model to determine average stress fields on the nanometer scale, it is possible to identify the shear yield stress of the polymer matrix in wood, which was found to be in the range of 14.9–17.5 MPa for normal and compression wood. It was shown that this corresponds to a stress in the lignin phase of approx. 17 MPa. This combined study thus demonstrates a new approach for validating multiscale models predicting yield properties with uniaxial experiments at the microscale and measuring phase properties of inhomogeneous materials by a combination of modelling and experimental approaches. [ABSTRACT FROM AUTHOR]

While small-scale fracture toughness measurements provide an extremely useful material parameter, notch preparation and ion-beam damage continue to be contentious issues. Herein we utilize a conventional gallium source for notching ceramic cantilevers, and compare against notches fabricated using xenon and helium ions from commercial focused ion microscopes. This is the first time ions other than gallium have been used for notches in such measurements. We observe some statistical dependence of the measured toughness on the utilized notching ion, bringing into question the influence of ion-material interactions in creating residual stresses or embrittlement in the small volumes around the notch root. [ABSTRACT FROM AUTHOR]

Nanocrystalline and ultrafine-grained materials show enhanced strain rate sensitivity (SRS) in comparison to their coarse grained counterparts. Majority of SRS measurements on nanocrystalline thin films reported in literature have focused on nanoindentation-based approaches. In this paper, micropillar strain rate jump tests were demonstrated on an electrodeposited nanocrystalline nickel film from 25 to 100 °C. SRS exponent,m,and activation volume,V, values were determined as a function of temperature. The measured values were found to be in good agreement with previously reported literature on bulk and nanoindentation measurements. Apparent activation energy for deformation was found to be about 100 kJ/mol, which is close to that for grain boundary diffusion in nickel. Grain boundary sliding was observed in the deformed pillars from scanning electron microscopy images. [ABSTRACT FROM PUBLISHER]