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

Influence of Yttrium on the Thermal Stability of Ti-Al-N Thin Films

Ti(1-x)Al(x)N coated tools are commonly used in high-speed machining, where the cutting edge of an end-mill or insert is exposed to temperatures up to 1100 degrees C. Here, we investigate the effect of Yttrium addition on the thermal stability of Ti(1-x)Al(x)N coatings. Reactive DC magnetron sputtering of powder metallurgically prepared Ti(0.50)Al(0.50), Ti(0.49)Al(0.49)Y(0.02), and Ti(0.46)Al(0.46)Y(0.08) targets result in the formation of single-phase cubic (c) Ti(0.45)Al(0.55)N, binary cubic/wurtzite c/w-Ti(0.41)Al(0.57)Y(0.02)N and singe-phase w-Ti(0.38)Al(0.54)Y(0.08)N coatings. Using pulsed DC reactive magnetron sputtering for the Ti(0.49)Al(0.49)Y(0.02) target allows preparing single-phase c-Ti(0.46)Al(0.52)Y(0.02)N coatings. By employing thermal analyses in combination with X-ray diffraction and transmission electron microscopy investigations of as deposited and annealed (in He atmosphere) samples, we revealed that Y effectively retards the decomposition of the Ti(1-x-y)Al(x)Y(y)N solid-solution to higher temperatures and promotes the precipitation of c-TiN, c-YN, and w-AlN. Due to their different microstructure and morphology already in the as deposited state, the hardness of the coatings decreases from similar to 35 to 22 GPa with increasing Y-content and increasing wurtzite phase fraction. Highest peak hardness of similar to 38 GPa is obtained for the Y-free c-Ti(0.45)Al(0.55)N coating after annealing at T(a) = 950 degrees C, due to spinodal decomposition. After annealing above 1000 degrees C the highest hardness is obtained for the 2 mol % YN containing c-Ti(0.46)Al(0.52)Y(0.02)N coating with similar to 29 and 28 GPa for T(a) = 1150 and 1200 degrees C, respectively

Probing grain boundary relaxation in ultra-fine grained tantalum by micromechanical spectroscopy in an SEM

The study of grain boundaries (GBs) in polycrystalline materials is a field of major interest, since many physical properties, such as thermal and electrical conductivity, magnetic coercitivity, strength or fracture toughness, are influenced by the actual structure of GBs. One of the main challenges in investigating them is the fact that techniques capable to resolve their structure, for example transmission electron microscopy, require very small sample volumes. However, the necessary removal of the surrounding material might change the natural state of the GB by elimination of surrounding material constraints. To counteract this influence, one could apply indirect measurements such as internal friction to probe changes in the GB structure. However, given the ongoing trend towards miniaturization and integration, most of these macroscopic techniques are at their limit. In our current work, we developed a miniaturized technique for performing mechanical spectroscopy based on micronized bending beams in conjunction with a nanoindenter equipped with a continuous stiffness measurement module in-situ in a scanning electron microscope (SEM). We apply this miniaturized spectroscopy technique to study grain boundary relaxations of ultra-fine grained tantalum micro bending beams in-situ in the SEM, where we assess the influence of a thermal relaxation treatment on the GB structure

Effect of impurity doping on mechanical performance and microstructure in ultra-fine grained tungsten processed by HPT

due to its favorable physical properties, such as a high melting point, excellent intrinsic strength and good thermal conductivity. A comparably low fracture toughness and a high ductile-brittle transition temperature often limits the applicability and full potential of tungsten-based materials. Grain refinement by severe plastic deformation to the ultra-fine grained regime (100-500 nm) is known to improve strength as well as ductility and fracture toughness, but also for promoting intercrystalline fracture along the increased amount of grain boundaries. Enhancing the grain boundary cohesion by doping using, for example, carbon or boron might therefore lead a pathway to an additional improvement in fracture properties. In order to realize precise control of impurity content, the first challenge was to develop a fabrication route for ultra-fine grained tungsten starting from a material powder. Several issues arise from processing tungsten powder via high-pressure torsion due to the intrinsic properties of the material as well as the affinity of the powders to oxidize. These problems and their solutions are addressed in the first part of this work. To confirm the developed powder route in its eligibility, ultra-fine grained tungsten produced from a bulk precursor is then compared to the samples fabricated from powders regarding microstructural features and mechanical properties. Finally, after proving that both fabrication methods lead to comparable material properties, tungsten samples doped with various amounts of additional carbon (1-10 at.%) are fabricated and characterized extensively using nanoindentation and in-situ micromechanical testing. The effect of the carbon content on microstructure, mechanical properties and deformation behavior is thoroughly discussed in this work

Two photon lithography for synthesis of fracture mechanical specimen

The design process of any mechanically loaded device must be guided by material properties, which also includes fracture mechanical considerations. 3D lithographical techniques, such as direct laser writing through two-photon polymerization, therefore, enable completely new possibilities regarding device geometry, as well as for sample fabrication and materials testing. Please click Download on the upper right corner to see the full abstract

Femtosecond laser and FIB: A revolutionary approach in rapid micro- mechanical sample preparation

The established Focused Ion Beam (FIB) technique usually poses a bottleneck in the preparation of samples for micro-mechanical experiments. This is due to its limited material removal rate. Especially for tungsten, the sputter yield of the Ga+ ion beam is very low. Therefore the practical sample size is restricted to dimensions of a few micrometers. On the contrary a femtosecond laser offers ablation rates 4-6 orders of magnitude higher compared to a Ga+ FIB [1] and therefore allows a rapid fabrication of specimens on the meso-scale. A prototype, which combines both methods, has been developed on the basis of the Zeiss Auriga Laser platform [2]. This system consists of the main chamber, where the FIB milling is conducted, and a separated airlock chamber for the femtosecond laser ablation. This setup prevents the contamination of the main chamber with laser ablated material and allows laser processing under atmospheric, inert gas or vacuum conditions. Fracture toughness experiments on single crystal tungsten in the micro-regime [3] exhibit a different behavior compared to specimens on the macro-scale [4]. The rapid processing of specimens with the novel laser system allows to sample the transition region from a discrete flow behavior of the micro-sized cantilevers to macroscopic plasticity. An analysis of fracture experiments for sample sizes ranging from 20 x 20 x 100 µm^3 to 200 x 200 x 1000 µm^3 is conducted. In addition to that, the quality of the laser processed samples is analyzed regarding the influence of processing parameters. Please click Additional Files below to see the full abstract

Small scale mechanical testing of nanoporous tungsten tailored by reverse phase dissolution

Nanoporous metals possess a number of positive attributes such as light weight, large surface area, excellent thermal properties, and energy absorption capability, making them a good candidate as future radiation shielding materials [1]. Tungsten seems to be ideally suited as the base material for such a foam, as it is commonly used in nuclear facilities, medical diagnosis systems and a number of other circumstances in order to protect personnel and sensitive equipment from radiation [2]. Therefore, it is of great value and interest to tailor such novel nanoporous tungsten, in order to combine the beneficial properties of tungsten with the positive attributes of nanoporous foams. In this work, nanoporous tungsten foams with relative densities ranging from 20 to 50 % were created on a bulk scale through a unique route involving severe plastic deformation of a coarse-grained tungsten-copper composite, followed by the selective dissolution of the nobler copper phase. Scanning electron microscopy and high-resolution transmission electron microscopy were utilized for characterizing the microstructural evolution and analyzing the way the etching solutions affect the resulting nanoporous structures. The mechanical properties, which are an important consideration in fusion reactor applications, were investigated by employing nanoindentation and other small-scale testing techniques in situ in the SEM. Based on this, the elemental plasticity mechanisms governing the mechanical behavior were elucidated. This work for the first time provides an innovative and adaptive approach to create bulk nanoporous tungsten. The developed reverse phase dissolution method is generally applicable and can be transferred to other refractory metal materials in the future. The promising mechanical results of nanoporous tungsten will serve as foundation for forthcoming related scientific studies and engineering applications. [1] S. Xu. M. Bourham, A. Rabiei. A novel ultra-light structure for radiation shielding. Materials & Design. 31 (2010), 2140-2146. [2] S. Kobayashi, N. Hosoda, R. Takashima. W alloys as radiation protection materials. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 390 (1997), 426-430