Influence of transition metals on the solid solution strengthening and creep behavior of Nickel studied by ultra-high temperature nanoindentation testing

In recent years, nanoindentation systems have been developed which can operate at ever higher temperatures (up to 1000°C) [1]. This now allows to characterize individual phases of high temperature materials such as nickel-based superalloys at their operating temperature. The influence of rhenium on the mechanical properties of Ni alloys at room temperature and elevated temperatures has already been investigated by using nanoindentation testing [2, 3]. However, for the mechanical characterization at temperatures above 1000 °C, macroscopic test methods had to be applied until now. Testing at these high temperatures present special challenges for the tip material used as well as the temperature stability of the system. Therefore, a new high temperature nanoindentation system with a maximum test temperature of 1100 °C was developed to overcome this limitation. The system is capable to perform tests at relatively high indentation depths due to the combination of a 1 N actuator and a comparatively high frame stiffness even at temperatures above 1000 °C. In our study, the influence of rhenium, tantalum and tungsten on the solid solution strengthening of single crystalline nickel at temperatures up to 1100 °C were investigated by nanoindentation testing. In addition to experiments with constant strain rate, strain rate jump tests, creep experiments were also performed and compared with macroscopically determined data [3]. Furthermore, the principle of the recently developed Constant Contact Pressure (CCP) creep method is presented [4]. In contrast to conventional nanoindentation creep methods, the contact pressure instead of the load is kept constant. This avoids a simultaneous relaxation of hardness and strain rate and offers the possibility of performing long-term creep experiments. References: 1. Gibson, J.S.K.L., et al., On extracting mechanical properties from nanoindentation at temperatures up to 1000 °C. Extreme Mechanics Letters, 2017. 17: p. 43-49. 2. Durst, K. and M. Göken, Micromechanical characterisation of the influence of rhenium on the mechanical properties in nickel-base superalloys. Materials Science and Engineering: A, 2004. 387-389: p. 312-316. 3. ur Rehman, H., et al., On the temperature dependent strengthening of nickel by transition metal solutes. Acta Materialia, 2017. 137: p. 54-63. 4. Prach O., et al., A new nanoindentation creep technique using constant contact pressure. Journal of Materials Research, in press

Assessment of thermally activated dislocation mechanisms via novel indentation approaches

The efficiency of gas turbines and jet engines used for energy generation and transportation can be increased by raising their combustion temperature. However, this is often limited by the materials used. For the development of new high-temperature materials, knowledge of the local mechanical properties of, for instance, individual phases in Ni-based superalloys is therefore of great importance. These properties are largely unknown, as they are not accessible with conventional macroscopic test methods. In the present work, the depth-sensing indentation testing technique was applied to assess the thermally activated deformation mechanisms on a local scale. For this purpose, a new in-situ indentation device was developed, which for the first time allows dynamic indentation experiments to be carried out on a small scale at temperatures of up to 1100 °C. Furthermore, a new indentation creep loading protocol was developed using a constant contact pressure approach similar to conventional uniaxial creep experiments. For indentation testing at high temperatures, a new step load method has been presented that allows a significant reduction of the contact time, thus minimizing the wear of the indenter tips. The method is suitable for the investigation of transients in material behavior at high to medium strain rates. In addition, a new approach for determining the brittle-ductile-transition temperature of body centered cubic metals was presented. In this approach, the change in the temperature-dependent activation volume was used to determine an intersection temperature that agrees well with the brittle-to-ductile-transition temperature from conventional Charpy pendulum impact tests

Solid solution hardening effects on structural evolution and mechanical properties of nanostructured high entropy alloys

The equiatomic Cantor alloy and Ni-enriched derivates of it ((CrMnFeCo)xNi1-x with x = 0.8, 0.4, 0.08 and 0) were deformed by high pressure torsion to the saturation regime and subsequently annealed up to 900°C. The HEA alloy compositions exhibits the highest solid solution strengthening, leading to the smallest saturation grain size as well as highest thermal stability, but a phase decomposition at intermediate temperatures. The differences in microstructural stability are also reflected by the mechanical properties as studied via Nanoindentation strain rate jump and constant contact pressure experiments. All alloys show pronounced pileup around the indentation, which is even increasing after annealing, leading seemingly to a peak in hardness and modulus of the HEA alloys. After correction, the hardness of the HEA type alloys remains constant up to 450 °C (Ni60) or even increases up to 500 °C (Ni20) followed by a softening at higher annealing temperatures. Transients are observed during strain rate change, with a slightly enhanced rate sensitivity of the dilute alloys. Furthermore, stress reduction experiments indicate higher rate sensitivities at low applied contact stresses and small deformation rates. However, the Ni-based alloys remain fairly stable at RT deformation

Ductile‐brittle‐transition of flash annealed Fe‐based metallic glass ribbons

Fe-based metallic glasses show a ductile-brittle-transition (DBT) after annealing above a critical temperature, which makes the further processing of annealed ribbons complicated. However, the annealing step is necessary to improve the soft magnetic properties of these materials for industrial applications. For the future development of ductile (partial-) nanocrystalline Fe-based ribbons with excellent soft magnetic properties, it is important to understand the mechanisms behind the DBT. Therefore, tensile and bending tests were performed to determine the DBT of 15-20 µm thin, flash annealed Fe85.2B9.5P4Cu0.8Si0.5 ribbons in terms of critical stress intensity factor and bending ductility. Microstructure analysis has been done via X-ray diffraction (XRD), differential scanning calorimetry (DSC) and atom probe tomography (ATP). Please click Additional Files below to see the full abstract

Nanoindentation creep testing: Advantages and limitations of the constant contact pressure method

Different loading protocols have been developed in the past to investigate the creep properties of materials using instrumented indentation testing technique. Recently, a new indentation creep method was presented, in which the contact pressure is kept constant during the creep test segment, similar to the constant stress applied in a uniaxial creep experiment. In this study, the results of constant contact pressure creep tests are compared to uniaxial and constant load hold indentation creep experiments on ultrafine grained Cu and CuAl5. The constant contact pressure method yields similar stress exponents as the uniaxial tests, down to indentation strain rates of 10⁻⁶ s⁻¹, whereas the constant load hold method results mainly in a relaxation of the material at decreasing applied pressures. Furthermore, a pronounced change in the power law exponent at large stress reductions is found for both uniaxial and constant contact pressure tests, indicating a change in deformation mechanism of ultrafine grained metals

Direct Observation of Quadrupolar Strain Fields forming a Shear Band in Metallic Glasses

For decades, scanning/transmission electron microscopy (S/TEM) techniques have been employed to analyze shear bands in metallic glasses and understand their formation in order to improve the mechanical properties of metallic glasses. However, due to a lack of direct information in reciprocal space, conventional S/TEM cannot characterize the local strain and atomic structure of amorphous materials, which are key to describe the deformation of glasses. For this work, 4-dimensional-STEM (4D-STEM) is applied to map and directly correlate the local strain and the atomic structure at the nanometer scale in deformed metallic glasses. Residual strain fields are observed with quadrupolar symmetry concentrated at dilated Eshelby inclusions. The strain fields percolate in a vortex-like manner building up the shear band. This provides a new understanding of the formation of shear bands in metallic glass

Simultaneous mapping of magnetic and atomic structure for direct visualization of nanoscale magnetoelastic coupling

Achieving a correlative measurement of both magnetic and atomic structures at the nanoscale is imperative to understand the fundamental magnetism of matters and for fostering the development of new magnetic nanomaterials. Conventional microscopy methods fall short in providing the two information simultaneously. Here, we develop a new approach to simultaneously map the magnetic field and atomic structure at the nanoscale using Lorentz 4-dimensional scanning transmission electron microscopy (Ltz-4D-STEM). This method enables precise measurement of the characteristic atomic and magnetic structures across an extensive field of view, a critical aspect for investigating real-world ferromagnetic materials. It offers a comprehensive visualization and statistical evaluation of the different structural information at a pixel-by-pixel correlation. The new method allows to directly visualize the magnetoelastic coupling and the resulting complex magnetization arrangement as well as the competition between magnetoelastic and magnetostatic energy. This approach opens new avenues for in-depth studying the structure-property correlation of nanoscale magnetic materials.Comment: 28 pages, 14 figure

Influence of densification on the indentation cracking behaviour

Nanoindentation is a versatile method to study the plastic deformation and cracking behavior of glasses on various length scale. For fused silica, plastic deformation occurs by volume conservative shear flow and inelastic densification. The Drucker-Prager-Cap (DPC) plasticity finite element analysis approach was used to describe the yield surface of fused silica by an ellipsis. This approach was extended by the implementation of a sigmoidal hardening behavior to take densification saturation into account. Cohesive Zone (CZ) FEM was used to model indentation cracking along median/radial axis. By using Raman spectroscopic mapping of indents and literature data on high pressure densification, the behavior of the finite element analysis approach to describe the densification profiles of indents is determined. Further the sensitivity of estimating densification from shifts in the Raman signal was investigated for different indent sizes. The results show that the precision of the densification estimate increases with indentation size and a rule of thumb for an appropriate experimental set-up is proposed. The extended Drucker-Prager-Cap approach in FEA delivers an accurate description of the densification field of a pyramidal indentation (i.e. Berkovich or Vickers) of silica glass and reproduces experimental data remarkably better than the conventional model. In CZ-FEM densification inhibits the crack extension by a factor of 15 % compared to the case of pure shear flow. This factor however is significant smaller than improvements in fracture behavior, which are often attributed to densification found in literature. For pillar splitting densification plays a negligible role

Dislocation toughening in single‐crystal KNbO₃

The growing research interest in dislocation‐tuned functionality in ceramics is evident, with the most recent proofs‐of‐concept for enhanced ferroelectric properties, electrical conductivity, and superconductivity via dislocations. In this work, we focus on dislocation‐tuned mechanical properties and demonstrate that, by engineering high dislocation densities (up to 10¹⁴ m⁻²) into KNbO₃ at room temperature, the fracture toughness can be improved by a factor of 2.8. The microstructures, including dislocations and domain walls, are examined by optical microscopy, electron channeling contrast imaging, piezo‐response force microscopy, and transmission electron microscopy methods to shed light on the toughening mechanisms. In addition, high‐temperature (above the Curie temperature of KNbO₃) indentation tests were performed to exclude the influence of ferroelastic toughening, such that the origin of the toughening effect is pinpointed to be dislocations

Room‐temperature dislocation plasticity in SrTiO₃ tuned by defect chemistry

Dislocations have been identified to modify both the functional and mechanical properties of some ceramic materials. Succinct control of dislocation-based plasticity in ceramics will also demand knowledge about dislocation interaction with point defects. Here, we propose an experimental approach to modulate the dislocation-based plasticity in single-crystal SrTiO₃ based on the concept of defect chemistry engineering, for example, by increasing the oxygen vacancy concentration via reduction treatment. With nanoindentation and bulk compression tests, we find that the dislocation-governed plasticity is significantly modified at the nano-/microscale, compared to the bulk scale. The increase in oxygen vacancy concentration after reduction treatment was assessed by impedance spectroscopy and is found to favor dislocation nucleation but impede dislocation motion as rationalized by the nanoindentation pop-in and nanoindentation creep tests