Additively manufactured (AM) alloys have heterogeneous microstructures with broad grain size distributions and highly anisotropic and/or non-convex grain shapes. AM components can have complex geometries and porosity which may affect the local microstructure. Currently there is no electron backscatter diffraction (EBSD)-based grain size measurement standard suitable for typical AM materials. An interlaboratory comparison study was conducted to find out what grain size metrics and summary statistics are currently used to describe average grain size. Participants were asked to measure and report the average grain size from the same EBSD map dataset. Detailed reports have been published in Reference [1]. Based on these results, we have tested and propose recommendations for a new standard for measuring average grain size in AM materials. The present work demonstrates the suitability and limitations of the proposal across several different Ni and Al AM components.

During abrasive wear of hardmetals, preferential removal of the binder phase is a leading cause of detachment of tungsten carbide grains and increased wear. This work tests the use of a metastable austenitic binder phase that can form deformation induced martensite as a way to improve the abrasion resistance without loss of fracture toughness. The samples are subjected to tests of different severity and the results are evaluated with respect to wear and wear mechanisms. It was found that the wear mechanisms were very similar to those of a cobalt binder reference material and the sample with best performance varied from test to test. Signs of increased resistance to fracture and binder phase removal was seen for the new material, which could explain an increased resistance to wear in the most severe test.

The formulation of paediatric medicines faces significant challenges to meet the requirements for safe and accurate administration, while maintaining a suitable taste. Multiparticulate formulations have a strong potential to address these challenges because they combine dose flexibility with ease of administration. Understanding the stability of multiparticulate formulations over storage as a function of time and environmental parameters, such as humidity and temperature, is important to manage their commercialisation and use. In this work, we have expanded the toolkit of available techniques for studying multiparticulates beyond those such as scanning electron microscopy (SEM) and confocal laser scanning microscopy. We include advanced methods of environmentally-controlled SEM to monitor temperature- and humidity-induced changes in-situ, and a variety of Raman spectroscopies including stimulated Raman scattering microscopy to identify and localise the different ingredients at the surface and inside the multiparticulates. These techniques allowed unprecedented monitoring of specific changes to the particulate structure and distribution of individual ingredients due to product aging. These methods should be considered as valuable novel tools for in-depth characterisation of multiparticulate formulations to further understand chemical changes occurring during their development, manufacturing and long-term storage. We envisage these techniques to be useful in furthering the development of future medicine formulations.

Hard materials, particularly tooling compositions based on WC/Co structures, are predominantly designed for applications that take advantage of their extremely good properties in compression. There is a standard test method for obtaining compression properties (strength and ductility), ISO 4506:1979, but it is rather expensive to use, needing high capacity testing machines and, in some cases, quite complex testpieces to avoid problems with premature failure from parasitic tensile stresses at the platen/testpiece interface. Consequently, for the most part, the compressive properties of hardmetals are estimated through measurements of hardness; using techniques such as Rockwell (ISO 3738: parts 1-1982 and 2-1988) or Vickers (ISO 3878:1983). This paper discusses the correlation of hardness values with compression data and reviews the testing background, advantages and disadvantages of the test methods, together with comments on the underlying science and comparative studies by complementary researchers. The adoption of stiff compressive testing rigs at NPL for the measurement of almost 40 grades of hardmetals with a comprehensive spread of microstructures is used to comment on good testing practice.

Metrology provides the underpinning science that supports the practical use of testing standards; ensuring that different users, be it academia, government or industry, develop a common language regarding the capabilities of many materials sectors. For the hardmetal community, after a century of fundamental and effective applications, it isn't surprising that there are already a considerable number of formal International Standards. For example, ISO and ASTM are well known [1], and over time these have evolved, with international consensus, from metrological studies. Currently important properties of hardmetals such as Stiffness, Density, Hardness, Toughness, Abrasion Resistance, Macroscopic Strength and Phase and Magnetic properties are comprehensively supported by these international standards. However, there are other testing strategies where improved knowledge could contribute to enhanced carbide functionality. These might include high resolution microscopy, indentation strength measurements, micro-tribology, thermal fatigue testing and modelling in all its diverse approaches which are not yet addressed through standards, though they often feature strongly in published academic papers. The hardmetal research community is international, diverse and productive in addressing these many facets and frequently embed metrological studies that are relevant to the practical characterisation of hardmetals. In the UK the Materials Group at the National Physical Laboratory, NPL, has a prime remit to focus on metrology and our contribution to the WC-Co 100 year anniversary publication provides some specifics of typical metrological issues that need further attention before addressing the formal stage of standardisation. In this celebratory paper we have chosen examples of recent metrological activities that fall under this remit and which have only partly been contributed to the scientific published domain. These five examples are High Hardness measurements, Dislocation Analysis, Micro-Tribology, Nanoindentation and Structural Characterisation.

Grain microstructures are important for a whole range of materials properties. With the widespread use of additive manufacturing (AM), current standards for grain boundary evaluation, such as ASTM E2627-13, may not be fit for purpose due to their microstructural complexity. They are notoriously anisotropic across a range of length scales due to the non-equilibrium nature of the solidification process and the tracking of the heat source. In this work we examine the grain microstructure of a nickel superalloy produced by laser powder bed AM by electron back scatter diffraction. We find that the guidelines provided by the ASTM E2627-13 are not suitable for such heterogeneous and anisotropic materials and some modified guidelines are provided on the field of view and EBSD step size that should be used to characterise such materials.

WC/Co hardmetals are used in many applications where resistance to abrasive wear is important. To provide further information on the mechanisms of damage that can occur in abrasion, a series of micro-tribology experiments were carried out on a number of WC/Co hardmetal samples. Samples were traversed under a diamond indenter with a sequence of controlled applied loads. The resultant damage was examined by scanning electron microscopy (SEM). Electron back scattered diffraction (EBSD) analysis of the damage was also carried out, both of the test surface, and also at some distance below the surface. To ensure that no damage was introduced to the structure of the materials, broad ion beam polishing was used to polish the original test surfaces and form the sub-surface section. It was found that plastic deformation of the WC grains and the Co binder phase occurred at all test loads. The visibility of slip increased with increasing test load, with fracture to the WC grains starting to occur at higher loads. EBSD analysis confirmed the trend for increasing plastic deformation with increasing load, but also showed that the degree of deformation had a strong relationship with orientation of the WC grains. Some transformation of the normal fcc Co phase to hcp was also detected close to the scratch path.

The mechanical response of tungsten carbide (WC) single crystals under extreme conditions was investigated by means of pendulum-based nano-impact testing, from ambient to high temperatures. Dynamic hardness mea-surements and damage accumulation observations were conducted by single and multiple impact tests, respectively, at high strain rates (similar to 10(3)-10(4) s(-1)). Large WC single crystals with different orientations, namely, (0001) basal and (10 (1) over bar0) prismatic crystals, were studied. The results obtained from impact testing were compared to results acquired by high temperature nanoindentation in the quasi-static regime (10(-2) s(-1)), at a similar length scale, up to 600 degrees C. Whilst basal WC crystals were shown to exhibit a higher dynamic hardness at high strain rates across the entire range of temperatures, the hardness of prismatic WC crystals at high strain rates were only found higher at temperatures below 200 degrees C and comparable at elevated temperatures. Post-mortem topographical scans revealed the generation of substantial plastic deformation during impact and large pile up formation in the vicinity of the impact imprints in prismatic crystals. Electron Channelling Contrast Imaging (ECCI) of the repetitive impacts suggested preferential planes for the accommodation of stress through plastic flow at high strain rates, as previously reported in the quasi-static regime. The results shed light on the mechanical behaviour of WC in extreme conditions, evidencing anisotropic mechanical behaviours.

Tungsten carbide cobalt hardmetals are commonly used as cutting tools subject to high operation temperature and pressures, where the mechanical performance of the tungsten carbide phase affects the wear and lifetime of the material. In this study, the mechanical behaviour of the isolated tungsten carbide (WC) phase was investigated using single crystal micropillar compression. Micropillars in two crystal orientations, 1-5 mu m in diameter, were fabricated using focused ion beam (FIB) machining and subsequently compressed between room temperature and 600 degrees C. The activated plastic deformation mechanisms were strongly anisotropic and weakly temperature dependent. The flow stresses of basal-oriented pillars were about three times higher than the prismatic pillars, and pillars of both orientations soften slightly with increasing temperature. The basal pillars tended to deform by either unstable cracking or unstable yield, whereas the prismatic pillars deformed by slip-mediated cracking. However, the active deformation mechanisms were also sensitive to pillar size and shape. Slip trace analysis of the deformed pillars showed that { 1010} prismatic planes were the dominant slip plane in WC. Basal slip was also activated as a secondary slip system at high temperatures.

The plastic deformation mechanisms of tungsten carbide at room and elevated temperatures influence the wear and fracture properties of WC-Co hardmetal composite materials. The relationship between residual defect structures, including glissile and sessile dislocations and stacking faults, and the slip deformation activity, which produce slip traces, is not clear. Part 1 of this study showed that {10 (1) over bar0} was the primary slip plane at all measured temperatures and orientations, but secondary slip on the basal plane was activated at 600 degrees C, which suggests that < a > dislocations can cross-slip onto the basal plane at 600 degrees C. In the present work, Part 2, lattice rotation axis analysis of deformed WC micropillar mid-sections has been used to discriminate < a > prismatic slip from multiple < c + a > prismatic slip in WC, which has enabled the dislocation types contributing to plastic slip to be distinguished, independently of TEM residual defect analysis. Prismatic-oriented micropillars deformed primarily by multiple < c + a > prismatic slip at room temperature, but by < a > prismatic slip at 600 degrees C. Deformation in the near-basal oriented pillar at 600 degrees C can be modelled as prismatic slip along < c > constrained by the indenter face and pillar base. Secondary < a > basal slip, which was observed near the top of the pillar, was activated to maintain deformation compatibility with the indenter face. The experimentally observed lattice rotations, buckled pillar shape, mechanical data, and slip traces are all consistent with this model.

Nanoindentation was carried out on pure tungsten carbide (WC) on the basal (0001) and prismatic (1010) planes, using Berkovich and spherical indenters, in both single load and multi-load testing. The work focuses on correlating the load-displacement curves, including elastic to plastic deformation, size effect and hysteresis with the deformation behaviour of WC. With different specimen preparation processes, the elastic to plastic deformation started at different threshold loads: This observation was found to be due to the variation in surface dislocation density. Staircase deformation was observed thought to be caused by dislocation motion and the formation of slip bands; sudden displacement discontinuities in the load-displacement response - associated with dislocation loop nucleation - occurred at, or near the theoretical shear strength. Furthermore, discontinuities in load-displacement curves were also used to confirm that hysteresis loops were a result of plastic deformation, as they when the loading was purely elastic.

The functional performance of coatings can be transformed using coatings to form an engineered surface system where resistance to wear and friction are improved relative to the uncoated substrate. Understanding the failure mechanisms of coatings in tribological contacts is difficult to achieve. This paper shows results from real time in situ measurements of tribological behaviour of coated surfaces. Two different test systems have been used. These are a microtribometer fitted inside an SEM which provides sequences of high resolution images of the deformation processes occurring in coatings, and a ball on disc tribometer fitted with imaging and profilometric systems giving real-time information on the tribological response of coatings. DLC and TiN coatings on tool steel substrates were tested in the experiments reported here. Delamination failure of the DLC coatings was observed with both test system. A new rotating scratch test is also described that can provide real-time in situ information on the response of the coatings to repeated abrasion. The in situ results are supported by post-test surface examination.

High-temperature properties of hardmetals are critical to their use in many applications but a challenge to measure accurately. Creep behaviour is not well understood so this work has studied uniaxial tensile testing of small simple geometry samples to look at how modifications to the microstructure can affect creep behaviour at temperatures between 800 and 900 degrees C. In particular, a carbon-ladder series with high, medium and low carbon contents in the 10wt-%Co binder has been investigated. Significant differences between the stress-strain curves of the different carbon contents have been observed, but the underlying microstructural mechanisms appear to be similar in detailed large area examination of samples after failure. Penetration of Co along WC-WC boundaries with 'precipitation' of discrete islands is seen as well as the formation of continuous thin lamellae while void formation tends to occur at WC-Co boundaries. EBSD mapping suggests Co penetration varies as a function of WC-WC misorientation.

There is an increasing requirement for the acquisition of large two (2D) or three (3D) dimensional electron back scattered diffraction (EBSD) maps. It is a well-known, but largely neglected fact, that EBSD maps may contain distortions. These include long-range distortions, which can be caused by the interaction of the electron beam with the sample geometry and it can also arise from sample or beam drift. In addition there are shorter range artefacts arising from topographical features, such as curtaining. The geometrical distortions can be minimised by careful SEM calibrations and sample alignment. However, the long-range distortions become increasingly prevalent when acquiring large area 2D EBSD maps which take a long time to acquire and thus are especially prone to drift. These distortions are especially evident in serial section tomography (SST) when 2D maps are stacked on top of one another to produce 3D maps. Here we quantify these distortions for large area EBSD data by referencing them to secondary electron (SE) images for 3D-EBSD data acquired on a WC-Co hardmetal. Longrange distortions (due to drift) equating to around 10 mu m across a 200 mu m x 175 mu m area map, and short-range distortions (due to topographical effects) as large as 3 mu m over a distance of 40 & micro;m were observed. Methods for correcting these distortions are then proposed. This study illustrates the benefits and necessity of such corrections if morphological features are to be properly interpreted when collecting large 3D EBSD datasets, for example by mechanical sectioning, serial block face SEM ultramicrotomy, laser sectioning, FIB-SEM tomography, PFIB spin milling, etc.

The dependence of the mechanical behaviour of individual phases in WC-Co on microstructural parameters such as grain size and orientation were investigated by means of nanoindentation and electron microscopy. A broad range of WC grain dimensions, from about 1 to 1000 µm2, were selected and subsequently indented to investigate any size effect. A decrease in hardness as a function of grain dimensions was observed, due to an increase in dislocation mobility in larger grains. Whilst the binder phase only exhibits a hardness of about 11 GPa, the hardness of WC grains was measured about 29 and 53 GPa for the prismatic and basal orientations, respectively, in ambient conditions. All WC orientations exhibited a similar decrease in hardness with temperature, up to 700 ˚C. Damage mechanisms occurring in WC-Co during nanoindentation were investigated for the different grain orientations at various temperatures. The damage was visualised using electron microscopy near the residual indent coupled with Focused Ion Beam (FIB) sectioning across the indent. The three-dimensional distribution of plastic deformation across WC grains in the vicinity of an indent was examined using Electron Channelling Contrast Imaging (ECCI). ECCI micrographs enabled the observation of crystal defects, especially dislocations, and slip lines as well as the entire plastic zone. The defect density and spatial distribution in the deformed WC grains were compared to that of untested WC grains to identify the type of deformation originating from spherical indentation. The work provides important information on the relationship between WC-Co microstructure and performance at operating temperatures.

The diffraction based scanning electron microscopy (SEM) technique of electron channeling contrast imaging (ECCI) provides rapid and non-destructive information on defects on length scales from tens of nanometres to tens of micrometres. ECCI may be complemented by electron backscatter diffraction (EBSD) and hyperspectral cathodoluminescence imaging (CL). EBSD provides orientation, phase, polarity and strain information, whilst CL reveals the influence of phase, composition, strain and defects on luminescence. I will discuss our recent investigations of phase, composition and polarity, the type, density and distribution of defects and the distribution of strain in a range of nitride semiconductor structures.

In this article we describe the scanning electron microscopy (SEM) techniques of electron channelling contrast imaging and electron backscatter diffraction. These techniques provide information on crystal structure, crystal misorientation, grain boundaries, strain and structural defects on length scales from tens of nanometres to tens of micrometres. Here we report on the imaging and analysis of dislocations and sub-grains in nitride semiconductor thin films (GaN and AlN) and tungsten carbide-cobalt (WC-Co) hard metals. Our aim is to illustrate the capability of these techniques for investigating structural defects in the SEM and the benefits of combining these diffraction-based imaging techniques.