Calibration and data-analysis routines for nanoindentation with spherical tips

Instrumented spherical nanoindentation with a continuous stiffness measurement has gained increased popularity in microphysical investigations of grain boundaries, twins, dislocation densities, ion-induced damage, and more. These studies rely on different methodologies for instrument and tip calibration. Here, we test, integrate, and re-adapt published strategies for tip and machine-stiffness calibration for spherical tips. We propose a routine for independently calibrating the effective tip radius and the machine stiffness using standard reference materials, which requires the parametrization of the effective radius as a function of load. We validate our proposed workflow against key benchmarks and apply the resulting calibrations to data collected in materials with varying ductility to extract indentation stress–strain curves. We also test the impact of the machine stiffness on recently proposed methods for identification of yield stress. Finally, we synthesize these analyses in a single workflow for use in future studies aiming to extract and process data from spherical nanoindentation

Micromechanical testing of ion-irradiated ferritic/martensitic steels

Ferritic/martensitic steels are the leading candidate material for structural components in the design of Gen IV reactors. They are known to exhibit high mechanical strength, ductility and toughness in the unirradiated condition and show good irradiation resistance with respect to void swelling, irradiation creep, irradiation-induced phase instabilities and high temperature helium embrittlement. Determining the mechanical properties of these structural materials after exposure to irradiation damage is essential for the safe design of the reactors. Testing neutron irradiated, bulk specimens is expensive and requires the use of a hot-cell, however, self-ion irradiation can been used as a proxy to emulate the irradiation damage caused in these materials. A disadvantage to using ion-implantation is that only a small volume of irradiated material can be achieved, hence micromechanical testing methods are required. In this work, a sample of T91 steel was irradiated using 70MeV Fe ions. Use of a high-energy accelerator provides a damage profile that extends to a depth of 6μm beneath the sample surface; a damage level of 20dpa is reached at approximately 5μm into the surface, before the Bragg peak. Although this is still a small volume of material, it provides ample material to perform micromechanical techniques including micro-cantilever bend testing and nano-indentation. Such experiments were performed both on the surface, and on cross-sections of the irradiated material. The poster reports data on results from nanoindentation experiments, both perpendicular to the irradiated surface and parallel in cross-section, as well as the yield stress measured from micro-cantilever testing. All experiments are performed in the irradiated and unirradiated regions of the sample

Scratching the surface of Lateral Size Effects (LSE): A critical comparison between indentation and scratch hardness size effects

All tribological interactions have (by definition) a shear component and it is the general experience (going back to the invention of the Mohs hardness scale) that a harder material is one that is more resistant to scratch deformation. Indentation hardness is, however, more reproducible, has a smaller footprint than a scratch test, and so has become the “go to” material parameter for predicting tribological performance. This implied relationship relies on the assumption that a material’s plastic properties depend only on the crystallographic direction of deformation and not on the test direction with respect to a free surface. Indentation size effects (ISE) and the realisation that material strength is genuinely length-scale dependent suggests anti-tribological wear applications. The question remains, however, as to whether further performance enhancement can be realised in practice and this requires study of scratch deformation and LSE itself. This talk describes the efforts we have made to determine the relationship between indentation and scratch hardness and plasticity size effects. This is not a simple as it sounds and some practical reasons for this will be presented. One such issue is the presence of viscous drag in a scratch test; absent from indentation. Clear evidence of lateral (scratch) size effects, LSE, will be presented for single and poly crystal copper and directly compared to the ISE measurements made in the same samples [1,2]. Smaller scratches are harder. By scratching with a Berkovich geometry indenter it is possible to define two different scratch geometries, Edge Forward (EF) and Face Forward (FF), with the same lateral projected area. The lateral force generated for EF and FF scratches are very different. This can be correlated to the drag coefficients of the two geometries. Please click Additional Files below to see the full abstract

Calibration and data analysis routines for nanoindentation with spherical tips

Instrumented spherical nanoindentation with a continuous stiffness measurement has gained increased popularity in material science studies in brittle and ductile materials alike. These investigations span hypotheses related to a wide range of microphysics involving grain boundaries, twins, dislocation densities, ion-induced damage and more. These studies rely on the implementation of different methodologies for instrument calibration and for circumventing tip shape imperfections. In this study, we test, integrate, and re-adapt published strategies for tip and machine-stiffness calibration for spherical tips. We propose a routine for independently calibrating the effective tip radius and the machine stiffness using three reference materials (fused silica, sapphire, glassy carbon), which requires the parametrization of the effective radius as a function of load. We validate our proposed workflow against key benchmarks, such as variation of Young's modulus with depth. We apply the resulting calibrations to data collected in materials with varying ductility (olivine, titanium, and tungsten) to extract indentation stress-strain curves. We also test the impact of the machine stiffness on recently proposed methods for identification of yield stress, and compare the influence of different conventions on assessing the indentation size effect. Finally, we synthesize these analysis routines in a single workflow for use in future studies aiming to extract and process data from spherical nanoindentation

Revealing per-grain and neighbourhood stress interactions of a deforming ferritic steel via three-dimensional X-ray diffraction

The structural performance of polycrystalline alloys is strongly controlled by the characteristics of individual grains and their interactions, motivating this study to understand the dynamic micromechanical response within the microstructure. Here, a high ductility single-phase ferritic steel during uniaxial deformation is explored using three-dimensional X-ray diffraction. Grains well aligned for dislocation slip are shown to possess a wide intergranular stress range, controlled by per-grain dependent hardening activity. Contrariwise, grains orientated poorly for slip have a narrow stress range. A grain neighbourhood effect is observed of statistical significance: the Schmid factor of serial adjoining grains influences the stress state of a grain of interest, whereas parallel neighbours are less influential. This phenomenon is strongest at low plastic strains, with the effect diminishing as grains rotate during plasticity to eliminate any orientation dependent load shedding. The ability of the ferrite to eliminate such neighbourhood interactions is considered key to the high ductility possessed by these materials

Revealing per-grain and neighbourhood stress interactions of a deforming ferritic steel via three-dimensional X-ray diffraction

The structural performance of polycrystalline alloys is strongly controlled by the characteristics of individual grains and their interactions, motivating this study to understand the dynamic micromechanical response within the microstructure. Here, a high ductility single-phase ferritic steel during uniaxial deformation is explored using three-dimensional X-ray diffraction. Grains well aligned for dislocation slip are shown to possess a wide intergranular stress range, controlled by per-grain dependent hardening activity. Contrariwise, grains orientated poorly for slip have a narrow stress range. A grain neighbourhood effect is observed of statistical significance: the Schmid factor of serial adjoining grains influences the stress state of a grain of interest, whereas parallel neighbours are less influential. This phenomenon is strongest at low plastic strains, with the effect diminishing as grains rotate during plasticity to eliminate any orientation dependent load shedding. The ability of the ferrite to eliminate such neighbourhood interactions is considered key to the high ductility possessed by these materials

Microscale stress-geometry interactions in an additively manufactured NiTi cardiovascular stent: A synchrotron dual imaging tomography and diffraction study

This study explores cardiovascular stents fabricated using laser powder bed fusion (LPBF); an emerging method to offer patient-specific customisable parts. Here, the shape memory alloy NiTi, in a near equiatomic composition, was investigated to deconvolve the material response from macroscopic component effects. Specifically, stress-geometry interactions were revealed, in-situ, for a minaturised cardiovascular stent subjected to an externally applied cylindrical stress whilst acquiring synchrotron X-ray imaging and diffraction data. The approach enabled the collection of spatially resolved micromechanical deformation data; the formation of stress-induced martensite and R-phase was evident, occurring in locations near junctions between stent ligaments where stress concentrations exist. In the as-fabricated condition, hardness maps were obtained through nanoindentation, demonstrating that the localised deformation and deformation patterning is further controlled by porosity and microstructural heterogeneity. Electron backscatter diffraction (EBSD) supported these observations, showing a finer grain structure near stent junctions with higher associated lattice curvature. These features, combined with stress concentrations when loaded will initiate localised phase transformations. If the stent was subjected to repeated loading, representing in-vivo conditions, these regions would be susceptible to cyclic damage through transformation memory loss, leading to premature component failure. This study highlights the challenges that must be addressed for the post-processing treatment of LABF-processed stents for healthcare-related applications

Scratching the surface: Elastic rotations beneath nanoscratch and nanoindentation tests

In this paper, we investigate the residual deformation field in the vicinity of nanoscratch tests using two orientations of a Berkovich tip on an (001) Cu single crystal. We compare the deformation with that from indentation, in an attempt to understand the mechanisms of deformation in tangential sliding. The lattice rotation fields are mapped experimentally using high-resolution electron backscatter diffraction (HR-EBSD) on cross-sections prepared using focused ion beam (FIB). A physically-based crystal plasticity finite element model (CPFEM) is used to simulate the lattice rotation fields, and provide insight into the 3D rotation field surrounding a nano-scratch experiment, as it transitions from an initial static indentation to a steady-state scratch. The CPFEM simulations capture the experimental rotation fields with good fidelity, and show how the rotations about the scratch direction are reversed as the indenter moves away from the initial indentation

Nano-scratch hardness and the Lateral Size Effect (LSE)

Nano-scratch testing has been used throughout this thesis in order to deepen the understanding of the processes occurring during the nano-scratch test, and to develop a method for calculating the nano-scratch hardness, from the quantifiable variables that are obtainable from the technique. Scratch testing on the macro scale is a well-established technique, however when reducing the scale of a mechanical test, whilst interest is focussed on the yield strength or hardness of the material, plasticity size effects must be explored. A literature survey concludes that size effects in nano-scratch testing have not been investigated in the past, thus this is the main subject of this thesis. In order to carry out this investigation a number of methods were trialled to calculate the nano-scratch hardness of pure, polycrystalline, oxygen free copper using both the edge forward and the face forward tip orientations of a diamond Berkovich indenter. The results obtained were compared to the indentation hardness and the most theoretically suitable method was adopted for later experiments carried out in this work; the technique for obtaining measurements was optimised, such that a genuine lateral size effect (LSE), whereby the nano-scratch hardness increases with decreasing scratch size, was observed. To further investigate the lateral size effect, nano-scratches were performed on a sample of single crystal copper in different work hardened states. It was observed that the nano-scratch hardness not only increases with decreasing scratch size, but also increases when the spacing between the dislocations in the material is reduced; when the level of work hardening in the sample increases, the density of dislocations increases, thus the spacing between these obstacles is reduced. In addition to this, the anisotropy of the nano-scratch hardness was investigated by altering the scratch direction in the (100) plane of the single crystal copper. It was found that the nano-scratch hardness is anisotropic and that the scratch hardness is largest when the scratch direction is parallel to the slip plane. It is known that the yield strength of a material increases with decreasing average grain size and therefore the effect of grain size on the nano-scratch hardness was considered. By reducing the grain size of pure, annealed, oxygen free copper, the nano-scratch hardness was observed to increase. In all experiments, the nano-scratch hardness values of scratches performed in the face forward tip orientation were larger than that of scratches performed in the edge forward tip orientation, when scratching the same sample condition. This suggests that scratch hardness is tip geometry dependent and in order to develop a method of calculating a tip orientation-independent scratch hardness, the shape of the indenter and the plastic flow of material around the indenter in that orientation, must be known and incorporated into the calculation, possibly as a drag coefficient. In addition to the geometry of the plastic flow, and therefore the plastic zone size, it was found that the nano-scratch hardness is also governed by the interaction between the geometry of the indenter and the grain boundaries in the material. Finally a number of experimental issues from the nano-scratch test are highlighted and researchers are encouraged to consider these precautions when using the nano-scratch test