Spherical nanoindentation – advancements and prospects towards its application as a multifunctional testing technique

With the development of modern high-performance materials and components, cases increase where conventional testing techniques used for the mechanical characterization miss their target. Material fabrication at a bench scale, miniaturization and not least cost-effectiveness yearn for a highly reliable, fast and highly automatable testing technique. Even though uniaxial micromechanical tests on micro-pillars or -tensile samples are well suitable for the extraction of flow curves, they face the problem of elaborate specimen manufacturing. Spherical nanoindentation could be a candidate technique to overcome the mentioned drawbacks, since time needed for sample preparation is tremendously reduced. The present study will outline solutions of existing problems, which may lay the foundation for spherical nanoindentation to become a widely-used testing technique. Main objections concerning tip imperfections will be resolved by modifying the calibration procedure, and validated on a broad spectrum of materials independent of the indenter tip radius. Once the actual tip shape is available, displacement-time profiles can be designed to guarantee constant strain-rates during testing and thus permit the determination of the strain-rate sensitivity for rate-dependent materials. Finally, the comparison between nanoindentation flow curves and uniaxial tests will evidence that spherical indentation is a highly reliable technique for the extensive mechanical characterization of modern high-performance materials and show its high potential as a multifunctional standard testing technique. Please click Additional Files below to see the full abstract

Investigating thermally activated deformation mechanisms by high temperature nanoindentation – A Study on W-Re alloys

Since the advent of indentation at elevated temperatures the technique of high-temperature nanoindentation has been further developed, currently enabling testing temperatures above 1000 °C. Due to small sample sizes and a variety of different testing techniques this method provides the opportunity for alloy development at a new level regarding composition variety or efficiency. In this study the thermally activated deformation mechanisms in binary W-Re alloys will be investigated by using a high-end in-situ nanoindenter. For that purpose, three different materials were tested, namely commercially pure W, W5Re and W10Re, all of them in both, coarse grained and ultra-fine grained condition. Nanoindentation experiments were conducted from ambient temperatures up to 800 °C, thereby overcoming the critical temperature TC of tungsten at around 450 °C. With temperature increments of 100 °C a large range of the normalized temperature with respect to TC is covered, allowing general conclusions regarding the appearing deformation mechanisms in bcc metals. Additionally to constant indentation strain rate tests, strain rate jump tests were utilized to determine the mechanical properties and to evaluate the impact of temperature and microstructure on rate-dependent parameters. A strong influence of the alloying level with Re as well as the grain size on both, the thermal and athermal contribution to the flow stress, is observed. The origin and effects, such as solid solution softening for W5Re at temperatures far below TC, will be discussed in detail. Furthermore, the dominating deformation mechanisms in dependence of temperature and grain size are determined. In the coarse grained materials a change in deformation processes from kink-pair mechanism to dislocation-dislocation interaction at higher temperatures can be observed, while in ultra-fine grained materials grain boundary/dislocation interactions are responsible for the maintained time-dependent mechanical behavior

Phase transformations and local deformation mechanisms - A case study on Cu 20 m.% Sn

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Indentation unloading phase transformations in silicon: A new perspective

nanoindentation, silicon, Berkovich, contact pressure, continuous stiffness measuremen

A comprehensive study on the deformation behavior of ultra-fine grained and ultra-fine porous Au at elevated temperatures

Modern design and engineering of highly efficient devices and machines demand innovative materials to satisfy requirements such as high strength at low density. The purpose of this study was to compare mechanical properties and deformation behavior of ultra-fine grained Au and its ultra-fine porous counterpart, both fabricated from the same base material. Microstructural investigations of the foam surrendered a ligament size of approximately 100 nm consisting of ~60 nm grains in average. The ultra-fine grained Au features a mean grain size of 250 nm. Nanoindentation is a convenient technique to obtain materials properties at ambient but also at non-ambient conditions and elevated temperatures. In this work, a broad indentation test series was performed in order to determine hardness, Young’s modulus, strain-rate sensitivity, and activation volume between room and elevated temperatures up to 300 °C for both materials. Due to the small characteristic dimensions, high hardness values were noted for both materials, which rapidly drop at elevated temperatures. In addition, an enhanced strain-rate sensitivity accompanied by low activation volumes was determined, increasing with elevated temperatures for both states. This can clearly be associated with interactions between dislocations and interphases. Moreover, for ultra-fine porous Au, a considerable increase of hardness was observed after annealing, which potentially can be attributed to starvation of mobile dislocations not occurring in the ultra-fine grained state. Cross-sections of indentations in ultra-fine porous Au combined with quantitative analysis of the resulting porosity maps allow visualizing the occurring deformation of the foam properly, showing distinct differences for tests at varying conditions. While the as-fabricated material exhibits distributed plasticity underneath the indent, this changes to strongly localized failure events in the annealed condition. At increased temperature, the deformation morphology reverts to more distributed deformation favored by the additional thermal activation

Impact of temperature and hydrogen on the nanomechanical properties of a highly deformed high entropy alloy

Due to their quite attractive properties, high-entropy alloys have emerged to an intensely studied class of alloys within the past years. Besides their high strength and maintained ductility, literature reports modest sensitivity to hydrogen embrittlement for conventional microstructures. Utilizing severe plastic deformation methods, for example high-pressure torsion, it is possible to further tailor the mechanical properties by microstructure refinement to the nanometer regime, which in turn increases the hydrogen storage capability at internal defects and boundaries. Furthermore, the nanocrystalline grain size provides markedly enhanced strength values, while the high fraction of grain boundaries influences the hydrogen diffusion and storage kinetics. Within this study, the micromechanical characteristics of pure Ni and a single phase face-centered cubic CrMnFeCoNi alloy in fine and ultra-fine grained microstructural conditions, fabricated by high pressure torsion, will be investigated in detail. Moreover, electrochemical in-situ nanoindentation will be employed to determine the impact of hydrogen charging on the mechanical performance of this high-entropy alloy class and will be set into context to result found for pure Ni

Thermally activated processes in materials probed by nanoindentation - challenges, solutions, and insights

Nanoindentation experiments are widely used for assessing the local mechanical properties of materials. In recent years some new exciting developments were established for also analyzing thermally activated processes during deformation using indentation based techniques, namely nanoindentation strain rate jump and nanoindentation long term creep tests. For these different methods, control of the indenter tip movement as well as determination of the correct contact conditions are hugely important to assure reliable data. In fact, long term nanoindentation tests are prone to be strongly influenced by thermal drift, starting at room temperature but even more intensified for elevated temperatures. This talk will first focus on experimental issues and challenges, but also solutions during advanced nanoindentation testing to overcome thermal drift influences, as demonstrated for fused silica and ultra-fine grained (ufg) Au. Special focus will be on high temperature testing, different testing methodologies will be described, and it will be demonstrated how distinct indentation time and indentation depths related errors influence the basic results. In the second part different results on single crystal (sx) and ufg Cr but also on the intermetallic phase Mg17Al12 are presented. For Mg17Al12, it was observed that the deformation behavior, especially in terms of thermally activated processes, is significantly changing over temperature. While at room temperature up to 125°C deformation is dominated by jerky flow and a slight negative strain-rate sensitivity due to dislocation pinning and the Portevin - Le Chatelier effect, overcoming 150°C the material behaves remarkably different. In this regime the indentation data show significant ductile deformation behavior with large pile-up formation and a pronounced strain rate sensitivity in the superplastic regime, where the deformation is sustained by dislocation glide and climb. Sx and ufg Cr also show significant changes in deformation behavior with temperature. At ambient conditions, both microstructures show an enhanced strain-rate sensitivity due to the large thermally activated component in the flow stress. Overcoming the materials specific temperature Tc (~150°C for Cr) the behavior changes. For sx Cr the apparent strain-rate sensitivity diminishes completely, while for the ufg state the strain-rate sensitivity increases due to the increased importance of dislocation – grain boundary interactions paired with a change in the dominating deformation mechanism