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Silicon is of great economic importance for the semiconductor industry as well as of academic interest because of its high-pressure phase transformations. These transformations also occur during the indentation of silicon. To further investigate these transformations, a modified method using the continuous stiffness measurement (CSM) during unloading is presented in this work. The use of the CSM signal allows directly calculating the mean contact pressure while unloading. The measurements will be compared to conventional indentation tests and data from high-pressure cell experiments reported in the literature. Furthermore, the influence of constant load holding segments on the phase transformation during unloading is investigated.

Despite a lot of research activities, the influence of hydrogen on the plastic deformation process is controversially discussed and often underestimated. Therefore, in this work strain rate jump tests were performed, using an electrochemical nanoindentation setup to investigate the deformation processes in a nickel-based alloy 725 under the influence of hydrogen, with the aim of determining thermally activated parameters such as strain rate sensitivity and activation volume. A hydrogen-induced hardness increase of about 8% was detected for all applied strain rates. The measured increase in strain rate sensitivity and the decrease in activation volume could be related to short-range order effects, which can lead to a more localized deformation. Furthermore, the optical evaluation of the remaining imprints showed a clear change in the formation of the plastically deformed zone during hydrogen charging. These insights into the deformation behaviour give further understanding regarding hydrogen-induced localized plasticity.

Purpose Correlation of characteristic surface appearance and surface roughness with measured air kerma (kinetic energy released in air) reduction of tungsten-rhenium (WRe) stationary anode surfaces. Methods A stationary anode test system was developed and used to alter nine initially ground sample surfaces through thermal cycling at high temperatures. A geometrical model based on high resolution surface data was implemented to correlate the measured reduction of the air kerma rate with the changing surface appearance of the samples. In addition to the nine thermally cycled samples, three samples received synthetic surface structuring to prove the applicability of the model to nonconventional surface alterations. Representative surface data and surface roughness values were acquired by laser scanning confocal microscopy. Results After thermal cycling in the stationary anode test system, the samples showed surface features comparable to rotating anodes after long-time operation. The established model enables the appearance of characteristic surface features like crack networks, pitting, and local melting to be linked to the local x-ray output at 100 kV tube voltage ,10° anode take off angle and 2 mm of added Al filtration. The results from the conducted air kerma measurements were compared to the predicted total x-ray output reduction from the geometrical model and show, on average, less than 10 % error within the 12 tested samples. In certain boundaries, the calculated surface roughness Ra showed a linear correlation with the measured air kerma reduction when samples were having comparable damaging characteristics and similar operation parameters. The orientation of the surface features had a strong impact on the measured air kerma rate which was shown by testing synthetically structured surfaces. Conclusions The geometrical model used herein considers and describes the effect of individual surface features on the x-ray output. In close boundaries arithmetic surface roughness Ra was found to be a useful characteristic value on estimating the effect of surface damage on total x-ray output.

Abstract Due to their outstanding properties, ultra-fine-grained tungsten and its alloys are promising candidates to be used in harsh environments, hence it is crucial to understand their high temperature behavior and underlying deformation mechanisms. Therefore, advanced nanoindentation techniques were applied to ultra-fine-grained tungsten–rhenium alloys up to 1073 K. A continuous hardness decrease up to 0.2 $$T_{\text{m}}$$ T m is rationalized by a still dominating effect of the Peierls stress. However, the absence of well-established effects of Rhenium alloying, resulting in a reduced temperature dependence of strength for coarse-grained microstructures, was interpreted as an indication for a diminishing role of kink-pair formation in ultra-fine-grained metals with sufficiently fine grain size. Despite slight grain growth in W, dislocation–grain boundary interaction was identified as the dominating deformation mechanism above 0.2 $$T_{\text{m}}$$ T m . Interaction and accommodation of lattice dislocations with grain boundaries was affected by a reduced boundary diffusivity through alloying with Re. Graphic abstract

An electrochemical charging setup was implemented in a nanoindentation system to evaluate the sensitivity of technically relevant materials to hydrogen embrittlement. Corresponding changes in the nanomechanical properties such as the hardness, Young’s modulus and pop-in load can be measured and interpreted. A nickel-based alloy was examined in solution-annealed and aged condition. A hydrogen-induced hardness increase of 15% was measured for the solution-annealed sample. Aging the alloy leads to a reduced influence of hydrogen, lowering the hardness increase to 5%. For the solution-annealed sample, hydrogen charging-induced surface steps were observed and characterized with laser confocal microscopy and electron backscatter diffraction to reveal plastic deformation.

Supercrystalline nanocomposites (SCNCs) are inorganic-organic hybrid materials with a unique periodic nanostructure, and as such they have been gaining growing attention for their intriguing functional properties and parallelisms with hierarchical biomaterials. Their mechanical behavior remains, however, poorly understood, even though its understanding and control are of paramount importance to allow SCNCs implementation into devices. An important aspect that has not been tackled yet is their time-dependent deformation behavior, which is nevertheless expected to play an important role in materials containing such a distribution of organic phase. Hereby, we report on the creep of ceramic-organic SCNCs with varying degrees of organic crosslinking, as assessed via nanoindentation. Creep strains and their partial recoverability are observed, hinting at the co-presence of viscoelasticity and viscoplasticity, and a clear effect of crosslinking in decreasing the overall material deformability. We rationalize our experimental observations with the analysis of stress exponent and activation volume, resulting in a power-law breakdown behavior and governing deformation mechanisms occurring at the organic sub-nm interfaces scale, in terms of organic ligands rearrangement. The set of results is reinforced by the evaluation of the strain rate sensitivity via strain rate jump tests, and the assessment of the effect of oscillations during continuous stiffness measurement mode.

Molybdenum disilicide (MoSi$_2$) is a very promising material for high temperature structural applications due to its high melting point (2030 {\deg}C), low density, high thermal conductivity and good oxidation resistance. However, MoSi$_2$ has limited ductility below 900 {\deg}C due to its anisotropic plastic deformation behaviour and high critical resolved shear stresses on particular slip systems. Nanoindentation of MoSi$_2$ microalloyed with aluminium, niobium or tantalum showed that all alloying elements cause a decrease in hardness. Analysis of surface slip lines indicated the activation of the additional {1 1 0} slip system in microalloyed MoSi$_2$, which is not active below 300 {\deg}C in pure MoSi$_2$. This was confirmed by TEM dislocation analysis of the indentation plastic zone. Further micropillar compression experiments comparing pure MoSi$_2$ and the Ta-alloyed sample enabled the determination of the critical resolved shear stresses of individual slip systems even in the most brittle [0 0 1] crystal direction.

Knowledge about the adhesion of protective hard coatings on tool materials is of great importance to understand their failure mechanisms in metalworking. Until now, common techniques such as scratch and indentation tests are used to establish a qualitative ranking of a coating’s adhesion on various substrate materials. Nevertheless, there is a lack of quantitative measures to describe the strength of the interfaces between individual microstructural constituents of substrate-coating composites. The current work investigates the interfacial strength and thus the adhesion of TiN deposited as a hard coating on an MC-type carbide, an M6C-type carbide and on martensite being constituents of high speed steels. Tensile stresses were introduced at the interface between TiN and the individual microstructural constituents of a high speed steel via micromechanical testing of a novel MSC specimen within a scanning electron microscope. The tested MSC specimens were subsequently investigated in detail by scanning electron microscopy. Evaluation of the interface stress at fracture via finite element analysis yielded a ranking in interface strength and therefore coating adhesion in a sequence from high to low strength values from MC/TiN over M6C/TiN to martensite/TiN.

Dislocation-grain boundary interaction is widely accepted as the rate-controlling process for ultra-fine grained bcc metals in their high temperature deformation regime above the critical temperature. However, the influence of different types of grain boundaries remains widely unexplored so far. To this end we present here an advanced high temperature nanoindentation study on Ta2.5W specimens consisting of two distinctively different grain boundary types, but with similar submicron average spacing. While one set of samples consisted of a predominant fraction of high-angle boundaries, the second set contained mainly low-angle boundaries. Fully recrystallized samples served as a coarse grained reference batch. Using advanced nanoindentation at elevated temperatures up to 823 K, we find a temperature invariant hardness in the case of the low- and a strongly pronounced temperature dependence for the high-angle grain boundary samples. This underlines the importance of grain boundary diffusivity for the predominant process of interfacial stress relaxation. Pronounced interaction of dislocations with oxygen impurity atoms was observed from 473 to 773 K for the coarse grained microstructure, yielding serrated flow as an indicator for a Portevin-Le Chatelier effect up to 573 K. Both grain boundary types showed a significant influence to the dislocation-impurity interaction, whereby the high-angle grain boundaries suppress discrete flow characteristics.

Hydrogen embrittlement (HE) is one of the main limitations in the use of advanced high-strength steels in the automotive industry. To have a better understanding of the interaction between hydrogen (H) and a complex phase steel, an in-situ method with plasma charging was applied in order to provide continuous H supply during mechanical testing in order to avoid H outgassing. For such fast-H diffusion materials, only direct observation during in-situ charging allows for addressing H effects on materials. Different plasma charging conditions were analysed, yet there was not a pronounced effect on the mechanical properties. The H concentration was calculated while using a simple analytical model as well as a simulation approach, resulting in consistent low H values, below the critical concentration to produce embrittlement. However, the dimple size decreased in the presence of H and, with increasing charging time, the crack propagation rate increased. The rate dependence of flow properties of the material was also investigated, proving that the material has no strain rate sensitivity, which confirmed that the crack propagation rate increased due to H effects. Even though the H concentration was low in the experiments that are presented here, different technological alternatives can be implemented in order to increase the maximum solute concentration.

The interaction between strain-induced precipitation, recovery and recrystallization governs the properties of the particle strengthened Mo-Hf-C alloy MHC. Complex, multi-staged thermomechanical processes have become the key approach for further improvement of the high temperature properties of MHC products within recent years. An investigation of those processes, including a wide field of parameters, usually turns out to be both difficult and time consuming. With multi-pass compression tests it is possible to analyze complex thermomechanical processes on small scale samples in a straightforward manner. The evaluation of the gathered information is demonstrated and the results are compared to microstructural investigations, conventional mechanical tests as well as relevant literature. Furthermore, the influence of different particle size distributions in MHC is interpreted. Precipitation between individual deformation steps resulted in minor additional hardening of the final material. Intermediate recrystallization during the thermomechanical processing showed high softening and ductilization of the final condition.

Understanding and linking mechanical properties obtained by spherical indentation experiments to uniaxial data is extremely challenging. Since the first attempts in the early 20th century numerous advances gradually allowed to expand the output of indentation tests. Still, the extraction of flow curves from spherical nanoindentation has not yet been fully established, as tip shape problems and size effects impede a straight-forward implementation. Within this study, we show new calibration procedures originating from fundamental geometrical considerations to account for tip shape imperfections. This sets the base for strain-rate controlled tests, which in turn enables us to measure rate-dependent material properties either with constant strain-rate or by strain-rate jump tests. Finally, experimental evaluation of the constraint factor in consideration of the mechanical properties and induced strain enables the extraction of flow curves. Testing materials with refined microstructures ensures the absence of possible size-effects. This study contributes to a significant improvement of current experimental protocols and allows to move flow curve measurements from single spherical imprints one step closer to its implementation as a standard characterization technique for modern materials. Keywords: Spherical nanoindentation, Tip calibration, Constant strain-rate tests, Strain-rate jump tests, Nanoindentation flow curves

Steel sheets of Fe-0.2C-2Mn-0.2Si-0.03Ti-0.003B (m%) for the automotive industry were isothermally heat-treated, comprising austenitizing and subsequent isothermal annealing at temperatures between 300°C and 500°C. As a consequence, microstructures ranging from granular bainite over lower bainite to auto-tempered and untempered martensite were obtained. In tensile, hole expansion and bending tests, the performances in different forming conditions were compared and the changes of microstructure and texture were studied by complementary electron backscatter diffraction (EBSD) analyses. Samples with granular bainitic microstructures exhibited high total elongations but lower hole expansion ratios; in subsequent EBSD and texture analyses, evidence for inhomogeneous deformation was found. In contrast, the lath-like bainitic/martensitic microstructure showed higher strength and lower elongation to fracture. This results in a reduced bendability, but also in a high tolerance against damage induced by the shearing of edges, and, thus, allows homogeneous deformation to higher strains in the hole expansion test.

Grain boundary engineering (GBE) plays an important role in the design of new polycrystalline materials with enhanced mechanical properties. This approach has been shown to be very effective in design of Ni-base alloys, where grain boundary segregation is expected to play a central role in defining their mechanical behavior. In the present work, we apply a powerful combination of advanced experimental and theoretical methods to reveal the grain boundary chemistry of the 725 Ni-base alloy at the atomic level. The methods of investigation comprise atom probe tomography (APT) measurements and density functional theory (DFT) calculations. We also propose a way to cross-validate DFT and APT results in a DFT-based model approach for evaluation of the interfacial excess as a function of the heat treatment history of the material and its chemistry. Both theoretical and experimental methods are applied to a detailed analysis of the GB chemistry of three modifications of the 725 alloy and the results of this investigation are presented and discussed in detail.

Grain boundary engineering plays a major role for controlling the properties of modern high-performance materials. Especially Mo and its alloys have advantageous high-temperature structural properties as well as a number of attractive functional properties. However, depending on the processing state, technically pure Mo is prone to intercrystalline failure at low temperatures. The addition of B and/or C is known to improve interface cohesion, allowing for a targeted improvement of mechanical properties through segregation engineering. In this work, the early stages of crack initiation of technically pure and B micro-doped Mo are investigated by scanning electron microscopy on the tension-loaded surface after three-point-bending of mm-sized specimens. Increased grain boundary cohesion is evident from a drastically reduced relative length of separated interfaces in the B-doped material. The presence of B at the grain boundaries is confirmed via atom probe tomography experiments.

Nanoindentation became a versatile tool for testing local mechanical properties beyond hardness and modulus. By adapting standard nanoindentation test methods, simple protocols capable of probing thermally activated deformation processes can be accomplished. Abrupt strain-rate changes within one indentation allow determining the strain-rate dependency of hardness at various indentation depths. For probing lower strain-rates and excluding thermal drift influences, long-term creep experiments can be performed by using the dynamic contact stiffness for determining the true contact area. From both procedures hardness and strain-rate, and consequently strain-rate sensitivity and activation volume can be reliably deducted within one indentation, permitting information on the locally acting thermally activated deformation mechanism. This review will first discuss various testing protocols including possible challenges and improvements. Second, it will focus on different examples showing the direct influence of crystal structure and/or microstructure on the underlying deformation behavior in pure and highly alloyed material systems.

A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a 〈100〉-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young’s modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100–150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution.

The equiatomic high-entropy alloy (HEA), CrMnFeCoNi, has recently been shown to be microstructurally unstable, resulting in a multi-phase microstructure after intermediate-temperature annealing treatments. The decomposition occurs rapidly in the nanocrystalline (NC) state and after longer annealing times in coarse-grained states. To characterize the mechanical properties of differently annealed NC states containing multiple phases, nanoindentation was used. The results revealed besides drastic changes in hardness, also for the first time significant changes in the Young's modulus and strain rate sensitivity. Nanoindentation of NC HEAs is, therefore, a useful complementary screening tool with high potential as a high throughput approach to detect phase decomposition, which can also be used to qualitatively predict the long-term stability of single-phase HEAs. Keywords: Nanoindentation, High-entropy alloys, Severe plastic deformation, High-pressure torsion, Phase stability, Nanocrystalline

A new field of application for high temperature nanoindentation as a complimentary method to understand the mechanics of plasticity upon bulk phase transformations in thermodynamic equilibrium is introduced. The feasibility is outlined on polycrystalline Cobalt involving a low-temperature hexagonal closed packed phase, and above 700K, a high-temperature face centered cubic phase, which was conventionally characterized by means of differential scanning calorimetry and high temperature X-ray diffraction. Strain rate sensitivity, activation volume and activation energy of plastic deformation were determined up to 873 K to identify the rate-controlling deformation mechanism. From RT to 473 K plasticity was found to be controlled by lattice friction on the basal plane, where dislocations have to overcome the Peierls barrier and deformation twinning was apparent in the hexagonal phase. The thermal activation of cross slip lead to reduced twinning actions when the phase transition temperature is approached. In the high temperature face centered cubic phase deformation was found to be controlled by cutting of forest dislocations and no deformation twinning was observed.

The presence of interfaces with nanoscale spacing significantly enhances the strength of materials, but also the rate controlling processes of plastic flow are subject to change. Due to the confined grain volumes, intragranular dislocation-dislocation interactions, the predominant processes at the micrometer scale, are replaced by emission of dislocations from and their subsequent accommodation at the interfaces. Both processes not only depend on the interfacial spacing, but also on the atomistic structure of the interface. Hence, a thorough understanding how these processes are affected by the interface structure is required to predict and improve the behavior of nanomaterials. The present study attempts to rationalize this effect by investigating the thermomechanical behavior of samples consisting of three different interfaces. Pure nickel samples with predominant fractions of low- and high-angle as well as twin boundaries with a similar average spacing around 150 nm are investigated using high temperature nanoindentation strain rate jump tests. Depending on the interface structure, hardness, strain rate sensitivity and apparent activation volumes evolve distinctively different with testing temperature. While in case of high-angle boundaries for all quantities a pronounced thermal dependence is found, the other two interface types behave almost athermal in the same temperature range. These differences can be rationalized based on the different interfacial diffusivity, affecting the predominant process of interfacial stress relaxation.

This article was updated to correct an error in the Editor’s note introduced during production.

Advanced die application materials, utilizing pressure-less sintered copper, show great prospects regarding cost effectiveness, power density, withstanding high switching speeds and temperature loading for novel eco-friendly and high efficiency semiconductors. In general, to preserve high reliability in combination with electrical functionality the design of elastic as well as electrical material parameters is of great importance. Here, we present a multi-method characterization approach to understand the impact of the morphology on the elastic as well as electrical behavior, which facilitates a strategy to design the relevant material parameters by tuning the morphology. Nano-SEM/FIB tomography and SEM/EBSD are applied to probe the morphology of three representative copper films. Nanoindentation and 4-point probe are used to extract the elastic modulus and specific electrical resistivity, respectively. The evaluated material parameters are compared with modeling results using the analyzed image data as an input. For the crucial image analysis, we develop a validated objective image analysis workflow. We obtain a quantified insight about the effect of the heterogeneous morphologies on the elastic modulus and specific electrical resistivity, thereby delivering important information about the necessary homogeneous copper morphology- and nano-scale pore-design. The strategy shall provide design guidelines to ensure reliable and high-performance die attachments.

This paper features four microstructurally different tungsten 10 wt% rhenium (W10Re) alloys tested by thermal cycling at high temperatures in a conventional electron beam welding machine. The sample surfaces undergo minimum temperatures of 1700–1750 °C with 3.000–180.000 additional temperature jumps of 170–200 °C. The used materials show microstructural changes as well as surface damage related to the exposure time and the number of applied temperature jumps. The loaded surfaces show formation of slip bands, grain boundary bulging, pitting, thermal grooving as well as crack formation after the cyclic thermal loading. An initial columnar grain structure reduced pitting of grains at the surface by influencing the preferential crack direction, while on the other hand increasing surface swelling. Introducing HfC into the W10Re matrix led to a smaller final grain size after recrystallization as well as decreasing surface swelling and pitting. A larger initial grain size has shown increased surface degradation and large amounts of swelling. The changes in microstructure were characterized by classical metallographic means including light optical microscopy and hardness testing. The surface damage was investigated in detail by using laser scanning microscopy. Differences in surface damage mechanisms were characterized by electron back scatter diffraction and scanning electron images. The combination of temperature measurements with finite element modeling enabled to calculate the temperatures and loading conditions of the samples.

Thermally activated deformation mechanisms in three different W-Re alloys were investigated by performing high temperature nanoindentation experiments up to 800 °C. With increasing Re content the athermal hardness increases, while the temperature-dependent thermal contribution is strongly decreased. This results in a reduced strain rate sensitivity for W-Re alloys compared to pure W. The origin of this effect is a reduction of the Peierls potential due to Re, manifesting in an increased activation volume at lower temperatures. This gives rise to a solid solution softening effect, while at high-temperature application the mechanical behavior is governed by dislocation-dislocation interaction and solution strengthening. Keywords: Refractory metals, High temperature deformation, Plastic deformation, Strain rate sensitivity

Additive manufacturing 32, 101027 -101039 (2020). doi:10.1016/j.addma.2019.101027, Synthesis of multi-metal hybrid structures with narrow heat affected zones, limited residual stresses and secondary phase occurrence represents a serious scientific and technological challenge. In this work, liquid dispersed metal powderbed fusion was used to additively manufacture a multilayered structure basedon alternating Inconel 625 alloy (IN625) and 316L stainless steel (316L) layers on a 316L base plate. Analytical scanning and transmission electron microscopies, high-energy synchrotron X-ray diffraction and nanoindentation analysis reveal sharp compositional, structural and microstructural boundaries between alternating 60μm thick alloys’ subregions and unique microstructures at macro-, micro- and nano-scales. The periodic occurrence of IN625 and 316L sub-regions is correlated with a cross-sectional hardness increase and decrease and compressive stress decrease and increase, respectively. The laser scanning strategy induced a growth of elongated grains separated by zig-zag low-angle grain boundaries, which correlate with the occurrence of zig-zag cracks propagating in the growth direction. A sharpfiber texture within the 316L regions turns gradually into afiber texture intheIN625regions.TheoccurrenceoftheC-likestressgradient withapronouncedsurfacetensile stress of about 500MPa is interpreted by the temperature gradient mechanism model. Chemical analysis indicates a formation of reinforcing spherical chromium-metal-oxide nano-dispersoids and demonstrates a possibility for reactive additive manufacturing and microstructural design at the nanoscale, as a remarkable attribute of the deposition process. Finally, the study shows that the novel approach represents an effective tool to combine dissimilar metallic alloys into unique bionic hierarchical microstructures with possible synergetic properties., Published by Elsevier, Amsterdam [u.a.]

International audience; • Nanoindentation strain rate jump tests of 3 BMGs and 3 BMGCs • Development of new method to determine activation energy and volume of shear transformation zones • Testing temperature strongly influences activation energy and volume • Rejuvenation (by HPT and thermal cycling) does not influence activation energy and volume G R A P H I C A L A B S T R A C T a b s t r a c t Nanoindentation strain-rate jump testing was used to determine activation volumes and energies of various metallic glasses and composites. Three different single phase metallic glasses and three composites (two with amor-phous/crystalline Cu and one with an amorphous/amorphous structure) were investigated. The state of the materials was additionally changed by varying the testing temperature between room temperature and 430 °C, by performing high pressure torsion and by thermal cycling. The results show that testing temperature is the main parameter controlling activation volume and energy, whereas material modifications by high pressure torsion and thermal cycling do not significantly affect them.

International audience; Keywords: A metallic glass B mechanical properties B phase stability B bonding C severe plastic deformation D microstructure A B S T R A C T Two different metallic glass powders were consolidated and deformed via high-pressure torsion to synthesize amorphous dual phase composites. The influence of volume fraction of the two amorphous phases and the applied shear strain was investigated. By varying the applied strain, the dimensions of the phases could be systematically varied from the micro-to the nanometer regime and at the highest applied strain even a transition to a single phase state could be observed. The study illustrates the potential of producing novel bulk metallic glasses by deformation-induced mixing which are not accessible by the classical casting route.

Journal of alloys and compounds 695, 2285 - 2294 (2017). doi:10.1016/j.jallcom.2016.11.085, A Cu-37 at%Ag composite was produced by high-pressure torsion processing of elemental Cu and Agpowders at room temperature. The initial micrometer-sized powder particles were compressed directlyin the high-pressure torsion tool and subsequently deformed to different strain levels. The microstructuralevolution was studied in detail by scanning and transmission electron microscopy and synchrotronX-Ray measurements, and related to the mechanical properties by microhardness and nanoindentationmeasurements. The HPT process led to an alignment of Cu and Ag into a lamellar composite microstructure.With increasing applied strain the Cu and Ag lamellae were continuously thinned andsimultaneously an ultrafine-grained microstructure was formed in the separate Cu and Ag lamellae.When the lamella spacing reached values lower than the respective grain sizes inside the lamellae, afurther lamella thinning occurred causing a significant hardness increase of the composite. At lamellaspacings below 50 nm deformation started to localize in 150e300 nm broad shear bands, which surprisinglyexhibited no softening. Instead, the steady formation of new shear bands aided to transform thelamellar structure into a nanocrystalline equi-axed microstructure and additionally rotated the lamellarmatrix towards the shear plane. This process led to an additional refinement of the alloy and a hardnessincrease until a constant hardness level was obtained. Combined analyses by synchrotron X-ray andtransmission electron microscopy measurements revealed that, after reaching the saturation microhardnesslevel, mechanical mixing of Cu and Ag occurred in the shear bands, which can be attributed tothe enormous strains accommodated in the shear bands. Due to the localized deformation by shearbands, structural and chemical homogenization of the alloy was not achieved even at very high appliedstrains. The final microstructure was composed of nanocrystalline single-phase supersaturated regionsembedded in a residual nano-lamellar matrix., Published by Elsevier, Lausanne

Abstract Micromechanical testing techniques can reveal a variety of characteristics in materials that are otherwise impossible to address. However, unlike to macroscopic testing, these miniaturized experiments are more challenging to realize and analyze, as loading and boundary conditions can often not be controlled to the same extent as in standardized macroscopic tests. Hence, exploiting all possible information from such an experiment seems utmost desirable. In the present work, we utilize dynamic in situ microtensile testing of a nanocrystalline equiatomic CoCrFeMnNi high entropy alloy in conjunction with initial feature tracking to obtain a continuous two-dimensional strain field. This enables an evaluation of true stress–strain data as well as of the Poisson’s ratio and allows to study localization of plastic deformation for the specimen. We demonstrate that the presented image correlation method allows for an additional gain of information in these sophisticated experiments over commercial tools and can serve as a starting point to study deformation states exhibiting more complex strain fields. Graphic abstract

Lightweighting of structural materials has proven indispensable in the energy economy, predicated on alloy design with high strength-to-weight ratios. Modern aluminum alloys have made great strides in ambient temperature performance and are amenable to advanced manufacturing routes such as additive manufacturing, but lack elevated temperature robustness where gains in efficiency can be obtained. Here, we demonstrate the intentional design of disorder at interfaces, a notion generally associated with thermal runaway in traditional materials, in a segregation-engineered ternary nanocrystalline Al–Ni–Ce alloy that exhibits exceptional thermal stability and elevated temperature strength. In-situ transmission electron microscopy in concert with ultrafast calorimetry and X-ray total scattering point to synergistic co-segregation of Ce and Ni driving the evolution of amorphous intergranular films separating sub-10 nm Al-rich grains, which gives rise to emergent thermal stability. We ascribe this intriguing behavior to near-equilibrium interface conditions followed by kinetically sluggish intermetallic precipitation in the confined disordered region. The resulting outstanding mechanical performance at high homologous temperatures lends credence to the efficacy of promoting disorder in alloy design and discovery.

The applicability of nano-crystalline W/Cu composites is governed by their mechanical properties and microstructural stability at high temperatures. Therefore, mechanical and structural investigations of a high-pressure torsion deformed W/Cu nanocomposite were performed up to a temperature of 600 °C. Furthermore, the material was annealed at several temperatures for 1 h within a high-vacuum furnace to determine microstructural changes and surface effects. No significant increase of grain size, but distinct evaporation of the Cu phase accompanied by Cu pool and faceted Cu particle formation could be identified on the specimen′s surface. Additionally, high-temperature nanoindentation and strain rate jump tests were performed to investigate the materials mechanical response at elevated temperatures. Hardness and Young′s modulus decrease were noteworthy due to temperature-induced effects and slight grain growth. The strain rate sensitivity in dependent of the temperature remained constant for the investigated W/Cu composite material. Also, the activation volume of the nano-crystalline composite increased with temperature and behaved similar to coarse-grained W. The current study extends the understanding of the high-temperature behavior of nano-crystalline W/Cu composites within vacuum environments such as future fusion reactors.