Nanoindentation based techniques were significantly enhanced by continuous stiffness monitoring capabilities. In essence, this allowed to expand from point-wise discrete measurement of hardness and elastic modulus towards advanced plastic characterization routines, spanning the whole rate-dependent spectrum from steady state creep properties via quasi static flow curves to impact or brittle fracture. While representing a significant step forwards already, these techniques can tremendously benefit from additional or complementary input provided by in situ or operando experiments. In fact, by combining and merging these approaches, impressive advances were made towards well controlled nanomechanical investigations at various non-ambient conditions. Here we will discuss some novel experimental avenues facilitated by deliberate extreme environments, and also indicate how future improvements and enhancements will potentially provide previously unseen insights into fundamental material behavior at extreme conditions. 

Recent advances in nanomechanical and in situ testing techniques: Towards extreme conditions

Tuning mechanical properties of ultrafine-grained tungsten by manipulating grain boundary chemistry

Helium-induced swelling and mechanical property degradation in ultrafine-grained W and W-Cu nanocomposites for fusion applications

Small-scale fracture mechanical investigations on grain boundary doped ultrafine-grained tungsten

W-Cu composites are commonly used as heat-sinks or high-performance switches in power electronics. To enhance their mechanical properties and mutually their usability, grain refinement of the initially coarse-grained microstructure was realized using high–pressure torsion. This leads to different microstructural conditions, exhibiting fine-, ultrafine-grained or nanocrystalline microstructures. Scanning as well as transmission electron microscopy was performed to analyze the respective grain size and microstructures. The hardness and Young’s modulus of the deformed specimens were quantified by nanoindentation testing. Furthermore, X–ray diffraction indicated a decreased grain size and changed lattice spacings upon increasing the deformation ratio. The deformed specimens were tested for their fracture behaviour by continuous stiffness measurements during in-situ microcantilever bending experiments. Here, mean J–integral values of 288 ± 38 J/m2 and 402 ± 89 J/m2 were determined for the 5 and 50 times turned specimens, respectively. The combination of different characterization methods applied on a W–Cu composite allows to identify both, beneficial and unfavourable microstructural components regarding the fracture properties. 

In-situ micromechanical analysis of a nano-crystalline W-Cu composite

Enhancing Mechanical Properties of Ultrafine-Grained Tungsten for Fusion Applications

/pdf/ultrafine-grained-tungsten-by-high-pressure-torsion-bulk-4tlig4jvo0.pdf

Ultrafine-grained Tungsten by High-Pressure Torsion - Bulk precursor versus powder processing route

Nanocomposite materials containing a soft and hard metal phase are a promising strategy to combine ultra-high strength, ductility and fracture toughness. However, given the rather brittle intercrystalline fracture mode, the true potential of these materials is only accessible after strengthening the vast number of interfaces within the composite. In this work, this is realized by doping a W–75Cu nanocomposite with either C, B, Hf or Re, elements that show promising effects on grain boundary cohesion in ab-initio calculations. The samples are fabricated from powders using severe plastic deformation and characterized using electron microscopy. Subsequently, various small-scale mechanical experiments are utilized to investigate the effect of the doping on strength, ductility and fracture toughness. While doping with C and B only leads to slight changes in mechanical properties, it was found that Hf increases the strength of the composite tremendously, most likely via the formation of nanosized oxides. Doping with Re showed an increase in strength and a major improvement in bending ductility, exhibiting “super-ductile” behavior in some cases. In microtensile tests this behavior was reduced, yet an increase in strength and ductility compared to the undoped composite was also apparent in these experiments. Interestingly enough, the fracture toughness of all doped variants did not change compared to the undoped W–Cu composite. This indicates that doping with Re improves resistance against crack initiation but not against crack propagation, making the materials properties highly sensitive to pre-existing defects and probed sample volume. 

Mechanical performance of doped W–Cu nanocomposites

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Tuning mechanical properties of ultrafine-grained tungsten by manipulating grain boundary chemistry

Enhancing Mechanical Properties of Ultrafine-Grained Tungsten for Fusion Applications

Ultrafine-grained Tungsten by High-Pressure Torsion - Bulk precursor versus powder processing route

Mechanical performance of doped W–Cu nanocomposites