High-temperature fracture test using chevron-notched tungsten microcantilevers

The combination of focused-ion beam (FIB) based sample preparation and nanoindentation allows fracture tests to be conducted at the micro-scale. Micro-fracture tests are of great interest to the nuclear materials community, as it allows direct measurements of fracture toughness within thin ion-irradiated layers and significantly reduces the volume of radioactive samples required as compared to working with neutron irradiated samples. The main drawback of existing micro-fracture tests is its limitation to brittle materials, as only linear-elastic fracture mechanics (LEFM) solutions have been developed so far. Tungsten-tantalum (W-Ta) alloy, is the primary candidate material for the plasma facing components of a future fusion reactor divertor, however is a semi-brittle material. LEFM solutions neglect any local plastic deformation that contribute to the blunting of the crack tip, therefore underestimate the true fracture toughness. Elastic-plastic fracture mechanics (EPFM) is necessarily to quantitatively analyse the complete fracture process, this greatly complicates both sample manufacture and experimental analysis. This research introduces a novel chevron-notch design to the W-Ta micro-cantilevers to promote stable crack growth which is a requisite for the EPFM approach. Cantilevers, manufactured using FIB machining, were loaded via a cyclic method, using a G200 Nanoindenter to monitor the stiffness in each cycle. By monitoring the decrease in stiffness of the cantilever through the cycles, crack length can be measured. Given detailed information of the crack length and the cantilever geometry, the complete fracture process of the semi-brittle W-Ta alloy can be quantitatively analysed. Initial results showed the fracture toughness of W-1%Ta alloy at room temperature is 2.7 MPa·m0.5, showing no significant R-curve behaviour before the onset of unstable fracture. This revealed no crack tip blunting occurred when tested at room temperature. This result is consistent with previous macro-scale fracture tests of W-1%Ta alloy at room temperature. The future goal is to extend this technique at elevated temperatures using our hot nanoindenter (up to 750 °C). This will provide quantitative analysis of the fracture process of W-Ta alloys at real reactor operating environment. By comparing micro- with macro-fracture toughness, this will also shed light on the feasibility of using micro-fracture tests to probe bulk fracture toughness

An iterative method for reference pattern selection in high-resolution electron backscatter diffraction (HR-EBSD)

For high (angular) resolution electron backscatter diffraction (HR-EBSD), the selection of a reference diffraction pattern (EBSP0) significantly affects the precision of the calculated strain and rotation maps. This effect was demonstrated in plastically deformed body-centred cubic and face-centred cubic ductile metals (ferrite and austenite grains in duplex stainless steel) and brittle single-crystal silicon, which showed that the effect is not only limited to measurement magnitude but also spatial distribution. An empirical relationship was then identified between the cross-correlation parameter and angular error, which was used in an iterative algorithm to identify the optimal reference pattern that maximises the precision of HR-EBSD

Characterisation of slip and twin activity using digital image correlation and crystal plasticity finite element simulation:Application to orthorhombic αα-uranium

Calibrating and verifying crystal plasticity material models is a significant challenge, particularly for materials with a number of potential slip and twin systems. Here we use digital image correlation on coarse-grained $\alpha$-uranium during tensile testing in conjunction with crystal plasticity finite element simulations. This approach allows us to determine the critical resolved shear stress, and hardening rate of the different slip and twin systems. The constitutive model is based on dislocation densities as state variables and the simulated geometry is constructed from electron backscatter diffraction images that provide shape, size and orientation of the grains, allowing a direct comparison between virtual and real experiments. An optimisation algorithm is used to find the model parameters that reproduce the evolution of the average strain in each grain as the load is increased. A tensile bar, containing four grains aligned with the load direction, is used to calibrate the model with eight unknown parameters. The approach is then independently validated by simulating the strain distribution in a second tensile bar. Different mechanisms for the hardening of the twin systems are evaluated. The latent hardening of the most active twin system turns out to be determined by coplanar twins and slip. The hardening rate of the most active slip system is lower than in fine-grained $\alpha$-uranium. The method developed in the present research can be applied to identify the critical resolved shear stress and hardening parameters of other coarse-grained materials

3D Studies of Damage by Combined X-ray Tomography and Digital Volume Correlation

AbstractThe combined use of high resolution X-ray computed tomography with digital image correlation allows quantitative observations of the three-dimensional deformations that occur within a material when it is strained. In suitable microstructures, the displacement resolution is sub-voxel (a voxel is the three-dimensional equivalent of a pixel), and both elastic and plastic deformations can be studied. This paper reviews recent work in which three-dimensional in situ observations of deformation have provided unique insights that support both continuum and heterogeneous microstructure-dependent models of damage development in a range of materials. The examples presented include; crack propagation in a quasi-brittle porous material (polygranular graphite), sub-indentation radial and lateral cracking in a brittle polycrystalline ceramic (alumina); plastic deformation and damage development underneath indentations in a ductile metal (Al-SiC composite) and a ceramic matrix composite (SiC-SiCfibre). These examples show how material properties can be obtained by analysis of the displacement fields, how such measurements can be used to better define the applied loading on small test specimens and how crack opening magnitude and mode may be extracted also. Some new directions for research are outlined, including the combined use of diffraction and imaging techniques on synchrotron X-ray facilities to map both elastic and inelastic strains

HR-EBSD analysis of in situ stable crack growth at the micron scale

Understanding the local fracture resistance of microstructural features. such as brittle inclusions, coatings, and interfaces at the microscale under complex loading conditions is critical for microstructure-informed design of materials. In this study, a novel approach has been formulated to decompose the J-integral evaluation of the elastic energy release rate to the three-dimensional stress intensity factors directly from experimental measurements of the elastic deformation gradient tensors of the crack field by in situ high (angular) resolution electron backscatter diffraction (HR-EBSD). An exemplar study is presented of a quasi-static crack, inclined to the observed surface, propagating on low index {hkl} planes in a (001) single crystal silicon wafer

Data related to the mesoscopic structure of iso-graphite for nuclear applications

The data in this article are related to the research article “Mesoscopic structure features in synthetic graphite” (Maerz et al., 2018) [1]. Details of the manufacture of isostatically moulded graphite (iso-graphite), thin foil preparation by focused ion beams (FIB) for analysis, and characterisation methods are provided. The detailed structures of coke filler and binding carbon are presented through scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and Raman spectroscopy characterisation. Atomistic modelling results of mesoscopic structural features are included

Mesoscopic structure features in synthetic graphite

The mesocopic structure features in the coke fillers and binding carbon regions of a synthetic graphite grade have been examined by high resolution transmission electron microscopy (TEM) and Raman spectroscopy. Within the fillers, the three-dimensional structure is composed of crystal laminae with the basal plane dimensions (La) of hundreds nanometres, and thicknesses (Lc) of tens of nanometres. These laminae have a nearly perfect graphite structure with almost parallel c-axes, but their a-b planes are orientated randomly to form a “crazy paving” structure. A similar structure exists in the binding carbon regions, with a smaller La. Significantly bent laminae are widely seen in quinoline insoluble inclusions and the graphite regions developed around them. The La values measured by TEM are consistent with estimates from the intensity ratios of the D to G Raman peak in these regions. Atomistic modelling finds that the lowest energy interfaces in the crazy paving structure comprise 5, 6 and 7 member carbon rings. The bent laminae tend to maintain the 6 member rings, but are strained elastically. We suggest that a 7 member carbon ring leaves a cavity representing an arm-chair graphite edge contributing to the Raman spectra D peak

Plasma-sprayed thermal barrier coatings: numerical study on damage localization and evolution

Thermal barrier coatings (TBCs) are advanced material systems used to enhance performance and in-service life of components operated at high temperatures in gas turbines and other power generation devices. Because of complexity, numerical methods became important tools both for design of these coatings and for in-service life estimations and optimization. In this contribution, two main features that affect the TBCs’ performance, namely the roughness of the bond coat and the microstructure of the ceramic top coat, are discussed based on Finite Element Method (FEM) and Finite Element Microstructure MEshfree (FEMME) simulations that were used to calculate stresses and assess damage within the coating. Roughness data obtained from plasma-sprayed CoNiCrAlY + YSZ coated samples are supplemented to discuss assumptions and results of employed numerical models