A nanoindentation investigation of local strain rate sensitivity in dual-phase Ti alloys

Using nanoindentation we have investigated the local strain rate sensitivity in dual-phase Ti alloys, Ti-6Al-2Sn-4Zr-xMo (x=2 and 6), as strain rate sensitivity could be a potential factor causing cold dwell fatigue. Electron backscatter diffraction (EBSD) was used to select hard and soft grain orientations within each of the alloys. Nanoindentation based tests using the continuous stiffness measurement (CSM) method were performed with variable strain rates, on the order of 10−1 to 10−3s−1. Local strain rate sensitivity is determined using a power law linking equivalent flow stress and equivalent plastic strain rate. Analysis of residual impressions using both a scanning electron microscope (SEM) and a focused ion beam (FIB) reveals local deformation around the indents and shows that nanoindentation tested structures containing both α and β phases within individual colonies. This indicates that the indentation results are derived from averaged α/β properties. The results show that a trend of local rate sensitivity in Ti6242 and Ti6246 is strikingly different; as similar rate sensitivities are found in Ti6246 regardless of grain orientation, whilst a grain orientation dependence is observed in Ti6242. These findings are important for understanding dwell fatigue deformation modes, and the methodology demonstrated can be used for screening new alloy designs and microstructures

Optimizing broad ion beam polishing of zircaloy-4 for electron backscatter diffraction analysis

Microstructural analysis with electron backscatter diffraction (EBSD) involves sectioning and polishing to create a flat and preparation-artifact free surface. The quality of EBSD analysis is often dependant on this step, and this motivates us to explore how broad ion beam (BIB) milling can be optimised for the preparation of zircaloy-4 with different grain sizes. We systematically explore the role of ion beam angle, ion beam voltage, polishing duration and polishing temperature and how this changes the surface roughness and indexing quality. Our results provide a method to routinely prepare high-quality Zircaloy-4 surfaces, and methods to optimise BIB polishing of other materials for high-quality EBSD studies

High-Angular Resolution Electron Backscatter Diffraction as a New Tool for Mapping Lattice Distortion in Geological Minerals

Analysis of distortions of the crystal lattice within individual mineral grains is central to the investigation of microscale processes that control and record tectonic events. These distortions are generally combinations of lattice rotations and elastic strains, but a lack of suitable observational techniques has prevented these components being mapped simultaneously and routinely in earth science laboratories. However, the technique of high-angular resolution electron backscatter diffraction (HR-EBSD) provides the opportunity to simultaneously map lattice rotation and elastic strain gradients with exceptional precision, on the order of 0.01° for rotations and 10−4 in strain, using a scanning electron microscope. Importantly, these rotations and lattice strains relate to densities of geometrically necessary dislocations and residual stresses. Recent works have begun to apply and adapt HR-EBSD to geological minerals, highlighting the potential of the technique to provide new insights into the microphysics of rock deformation. Therefore, the purpose of this review is to provide a summary of the technique, to identify caveats and targets for further development, and to suggest areas where it offers potential for major advances. In particular, HR-EBSD is well suited to characterizing the roles of different dislocation types during crystal plastic deformation and to mapping heterogeneous internal stress fields associated with specific deformation mechanisms/microstructures or changes in temperature, confining pressure, or macroscopic deviatoric stress. These capabilities make HR-EBSD a particularly powerful new technique for analyzing the microstructures of deformed geological materials.D. Wallis, L. N. Hansen, and A. J. Wilkinson acknowledge support from the Natural Environment Research Council grant NE/M0009661. T. B. Britton acknowledges support for his research fellowship from the Royal Academy of Engineering

Correlative statistical microstructural assessment of precipitates and their distribution, with simultaneous electron backscatter diffraction and energy dispersive X-ray spectroscopy

Modern engineering alloys have bespoke microstructures, where features such as precipitates are used to control properties. In many Ni-based alloys, carbo-nitride precipitates are introduced to strengthen and improve performance. These precipitates can be distributed throughout the microstructure and niobium rich carbides are often found at grain boundaries. In this work, we used combined energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) to characterise the population of these precipitates. Processing of the EDS signal is used to label the Mo/Nb-rich precipitates, and their size and location are measured from maps using a circular Hough transform. This label map is combined with the grain boundary network (from EBSD analysis). Statistical analysis, using ANOVA testing, reveals differences in chemistry between carbides found in Ni-rich matrix grain interiors, on random high angle boundaries and on special boundaries (Σ3 and Σ9). These results are compared between wrought and power metallurgy product forms. These distributions are discussed in the context of their performance within demanding environments, such as reactor core internals

Mechanical and microstructural investigations of tungsten and doped tungsten materials produced via powder injection molding

The physical properties of tungsten such as the high melting point of 3420°C, the high strength and thermal conductivity, the low thermal expansion and low erosion rate make this material attractive as a plasma facing material. However, the manufacturing of such tungsten parts by mechanical machining such as milling and turning is extremely costly and time intensive because this material is very hard and brittle. Powder Injection Molding (PIM) as special process allows the mass production of components, the joining of different materials without brazing and the creation of composite and prototype materials, and is an ideal tool for scientific investigations. This contribution describes the characterization and analyses of prototype materials produced via PIM. The investigation of the pure tungsten and oxide or carbide doped tungsten materials comprises the microstructure examination, element allocation, texture analyses, and mechanical testing via four-point bend (4-PB). Furthermore, the different materials were characterized by high heat flux (HHF) tests applying transient thermal loads at different base temperatures to address thermal shock and thermal fatigue performance. Additionally, HHF investigations provide information about the thermo-mechanical behavior to extreme steady state thermal loading and measurements of the thermal conductivity as well as oxidation tests were done. Post mortem analyses are performed quantifying and qualifying the occurring damage with respect to reference tungsten grades by metallographic and microscopical means

Advancing characterisation with statistics from correlative electron diffraction and X-ray spectroscopy, in the scanning electron microscope.

The routine and unique determination of minor phases in microstructures is critical to materials science. In metallurgy alone, applications include alloy and process development and the understanding of degradation in service. We develop a correlative method, exploring superalloy microstructures, which are examined in the scanning electron microscope (SEM) using simultaneous energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). This is performed at an appropriate length scale for characterisation of carbide phases' shape, size, location, and distribution. EDS and EBSD data are generated using two different physical processes, but each provide a signature of the material interacting with the incoming electron beam. Recent advances in post-processing, driven by 'big data' approaches, include use of principal component analysis (PCA). Components are subsequently characterised to assign labels to a mapped region. To provide physically meaningful signals, the principal components may be rotated to control the distribution of variance. In this work, we develop this method further through a weighted PCA approach. We use the EDS and EBSD signals concurrently, thereby labelling each region using both EDS (chemistry) and EBSD (crystal structure) information. This provides a new method of amplifying signal-to-noise for very small phases in mapped regions, especially where the EDS or EBSD signal is not unique enough alone for classification

In situ micropillar deformation of hydrides in Zircaloy-4

Deformation of hydrided Zircaloy-4 has been examined using in situ loading of hydrided micropillars in the scanning electron microscope and using synchrotron X-ray Laue microbeam diffraction. Results suggest that both the matrix and hydride can co-deform, with storage of deformation defects observed within the hydrides, which were twinned. Hydrides placed at the plane of maximum shear stress showed deformation within the hydride packet, whilst packets in other pillars arrested the propagation of shear bands. X-ray Laue peak broadening, prior to deformation, was associated with the precipitation of hydrides, and during deformation plastic rotation and broadening of both the matrix and hydride peaks were observed. Post-mortem TEM of the deformed pillars has indicated a greater density of dislocations associated with the precipitated hydride packets, while the observed broadening of the hydride electron diffraction spots further suggests that plastic strain gradients were induced in the hydrides by compression

The role of β-titanium ligaments in the deformation of dual phase titanium alloys

Multiphase titanium alloys are critical materials in high value engineering components, for instance in aero engines. Microstructural complexity is exploited through interface engineering during mechanical processing to realise significant improvements in fatigue and fracture resistance and strength. In this work, we explore the role of select interfaces using in-situ micromechanical testing with concurrent observations from high angular resolution electron backscatter diffraction (HR-EBSD). Our results are supported with post mortem transmission electron microscopy (TEM). Using micro-pillar compression, we performed in-depth analysis of the role of select {\beta}-titanium (body centred cubic) ligaments which separate neighbouring {\alpha}-titanium (hexagonal close packed) regions and inhibit the dislocation motion and impact strength during mechanical deformation. These results shed light on the strengthening mechanisms and those that can lead to strain localisation during fatigue and failure