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We address the effect of elastic inhomogeneity on elastic modulus and hardness determinations made by depth-sensing indentations performed on individual particles embedded within a matrix of different elastic modulus. Finite element simulations and nanoindentation experiments are used to quantify the consequences of particle/matrix elastic inhomogeneity and we propose an adaptation of the Oliver–Pharr method that gives access to particle properties knowing those of the matrix. The method is suitable for any combination of matrix and particle elastic modulus and for any type of indenter, provided that the area of the tested particles along the surface of the sample is measured and that a large number of particles are probed. Further conditions for the implementation of the method are that testing conditions be such (i) that permanent deformation of the matrix is avoided, and (ii) that permanent deformation in each probed particle under the indenter is not affected by the matrix. Graphical abstract

Oxide inclusions are ubiquitous in steel and are known to affect, generally in a negative sense, the mechanical performance of steel products. Micromechanical properties of those phases are therefore important, yet they have remained largely unexplored. We present strength measurements performed on individual silicon oxide inclusions in iron. The inclusions are produced in laboratory-fabricated samples by silicon deoxidation of high-purity iron melts containing dissolved oxygen. Spherical silica inclusions of diameter ∼3 µm thus produced are notched using ion milling and then tested in bending for their strength, by loading under displacement control in-situ within a scanning electron microscope. Results show that silicon oxide inclusions thus precipitated within iron are amorphous in structure, exhibit a smooth outer surface, and have fracture strains in the range from 8 to 17%, corresponding to fracture stress levels on the order of 10 GPa. The strength of silicon oxide inclusions precipitated in iron can hence approach the highest values so far measured in dry silica. The presence within iron and its alloys of precipitated silica thus need not be deleterious, and might perhaps even be exploited to produce novel high-strength materials., Acta Materialia, 242, ISSN:1359-6454

Diatom algae are active and efficient photosynthesizing single cell organisms responsible for a quarter of biomass and a quarter of oxygen release on the Earth surface. Diatoms form an enormously diverse class of microorganisms possessing stiff and strong, durable exoskeletons made from hydrated amorphous silica forming neat 3D nanostructures, and displaying hierarchical organization up to the scale of tens of micrometres. In our ongoing research into the structure and properties of diatoms we observed their colonization patterns on various surfaces, including polymers (PE, PP, PC and PETF) and silicon. This process can be guided by purposeful surface patterning to introduce pits, grooves, and ledges. Guided colonization opens the prospect of assembly and harvesting diatom frustules for applications in micro- and nano-electro-mechanical systems (MEMS and NEMS). Additionally, the wide range and specificity of diatoms opens the possibility of using them as tags and markers that are below the level of visibility by naked eye, but present specific spectroscopic fingerprints in visible light and UV ranges. Diatom ‘tags’ can also be read using high magnification imaging using Scanning Electron Microscopy (SEM). As a contributory guidance to morphological diversity of diatoms we present a mini-atlas of diatom in the form of high resolution SEM images.

Microprobe tips are commonly used to perform in-situ micromechanical tests within an electron microscope. In service, such tips have a tendency to accumulate along their surface a layer of deposited material. Tip cleanliness is crucial in order to obtain reliable and reproducible data; however, cleaning of such tips can be arduous, due to their fragility. The literature on appropriate tip cleaning methods is relatively sparse; we aim in this study to fill this gap by presenting an effective way to clean flat punch diamond microprobe tips within an electron microscope, based on mechanical scraping. Initial attempts to remove deposits from a contaminated diamond tip using two micro-brush samples, one containing silica needles and the other containing cementite lamellae, were unsuccessful, due to the adherence of the deposit to the surface of the tip and its apparently high hardness. The successful cleaning method consists of milling a silicon ridge by means of a focused ion beam, and then using this ridge to effectively scrape the deposits off the tip surface in a controlled and complete manner. This method avoids potential damage to the microprobe and can be implemented easily to clean flat punch tips rapidly within a scanning electron microscope. ispartof: Micron vol:155 ispartof: location:England status: Published online

This report provides the first quantitative evaluation and prediction of residual stress arising from a single particle deposition in the metal cold spray of Ti-6Al-4V. Micro-ring-core Focused Ion Beam–Digital Image Correlation (FIB-DIC) technique is employed to determine the residual stress variation experimentally, while finite element simulation with Johnson-Cook plasticity and dynamic failure model is employed to numerically predict the residual stress distribution within single particle, and they show good agreement with each other for different impact velocities. This provides a tight link between the validated description of microscopic phenomena and the ensuing macroscopic properties and processes of the deposit.

As-received and cold-worked 55Ni–45Ti wt% Nitinol wire samples were subjected to various ageing treatments. Experiments show that the properties of as-received Nitinol, including microstructure, critical temperatures in thermally induced phase transformation, and critical stresses/strains in mechanically induced phase transformation, change significantly after ageing process at 773 K for different durations ranging from 10 min to 5 h. BSE and EBSD/EDS analysis shows that the predominant microstructural evolution is a large increase in Ni4Ti3 precipitate volume fraction, while grain growth is negligible. DSC results show that as-received material exhibits little to no phase transformation during heating-cooling cycling at 5K/min, while various ageing durations lead to the emergence of phase transformation that occurs at different critical temperatures. Tensile loading experiments conducted using a test rig equipped with a heater revealed the evolution of mechanically induced phase transformation behaviour. Mathematical models are proposed to predict the dependence of critical temperatures and critical stresses/strains on the ageing treatment conditions and alloy composition.

The Tungsten Inert Gas (TIG) welding technique is extensively used to join various automobile and aerospace components, such as control arms, rotating blades, and vanes. Highly localized heating followed by rapid cooling during welding exert complex thermal and mechanical loading on the components and give rise to significant residual stress fields which may increase the likelihood of time-dependent failure by promoting crack initiation. In the context of engineering design for structural integrity and reliability of operation, quantitative residual stress evaluation in the finished parts needs to be carried out in a reproducible manner. Samples investigated in this study were TIG fill-in weldments in single crystal superalloy components with nearly cylindrical geometry. The present research employed Focused Ion Beam – Digital Image Correlation (FIB-DIC) micro-ring-core technique for stress evaluation, and a sequentially coupled thermo-mechanical finite element model to assess the residual stress state near the weldment surface in the radial and hoop directions. Good agreement was obtained between experimentally evaluated residual stresses and the refined numerical predictions. The highest hoop and radial tensile residual stresses were both observed near the boundary between the filler metal and base metal, whilst a compressive region was found for hoop stress in the parent metal at the component edge. These observations were discussed in conjunction with the temperature history and residual stress self-equilibration. This research provides the foundation for further investigations of Post Weld Heat Treatment (PWHT) and surface treatment to improve the fatigue performance of weldments.

Nickel-based superalloys are frequently used under operating conditions that lead to the formation of an oxide layer. In this study, the mechanical properties of such a submicron oxide scale formed on a nickel-based superalloy after oxidation at 650 °C were evaluated. Poisson’s ratio and Young’s modulus of the layered oxide structure were determined to be respectively 0.29, and 259 GPa. Depth-resolved residual stress profiling with nanoscale resolution showed large compressive residual stresses up to 3000 MPa in the oxide, with magnitude correlated to the oxide scale thickness. The relevance of the results to the crack initiation and growth behaviour is discussed.

A new methodology was developed in order to obtain single crystal elastic coefficients from nanoindentation experiments on a cubic polycrystal. The method consists of locating grains that are oriented with a 〈100〉, 〈110〉 or 〈111〉 direction near-parallel to the sample surface normal by means of electron backscattering diffraction. The reduced Young's moduli of the selected grains are then determined by nanoindentation. Finally, the average reduced modulus and Euler angles of each grain are used as input for a least-squares optimisation to calculate the three independent stiffness coefficients, which can then be used to obtain Young's modulus in any crystallographic direction. This technique, which was validated on a single crystal nickel-based superalloy (CMSX-4) with known elastic coefficients, was applied to a polycrystalline nickel-based superalloy (RR1000) with unknown elastic coefficients, resulting in a correct prediction of the general trend of increasing Young's modulus from the 〈100〉 to the 〈110〉 to the 〈111〉 direction. The stiffness coefficients C11, C12 and C44 were found to be 282, 121 and 108 GPa, respectively. These results, which are representative of the γ/γ’ structure as a whole, are in good agreement with literature data on similar superalloys. By constructing a visual representation of the elastic anisotropy based on the crystallographic factor, it is shown that the observed anisotropy is lower compared to other alloys.

Dwell sensitivity in titanium alloys is generally attributed to the phenomenon of load shedding, which is a time dependent redistribution of stress from weak grains, with their c-axis perpendicular to the loading direction, to strong grains, with their c-axis approximately parallel to the loading direction. This leads to the formation of internal quasi-cleavage facets on the basal planes of strong grains. In this paper, the effect of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires is investigated, because these wires do not contain strong α grains. It has been found that this leads to a different dwell fatigue behaviour compared to what has been described in literature. In the case of drawn wires, introducing load holds promoted crack initiation at the surface, through the formation of a facet on a prismatic plane of a surface grain that was oriented for very easy prismatic slip. This was confirmed by sectioning facets, using focused ion beam milling, and electron backscatter diffraction measurements. Because of the specific crystallographic texture of drawn wires, the phenomenon of load shedding is less pronounced, and there is no formation of internal facets. The amount of cycles to failure was reduced by two to three orders of magnitude compared to fatigue tests without load holds. The time to failure remained similar, and was even higher for some dwell fatigue tests. There was a significant amount of strain accumulation during dwell fatigue tests. The maximum strain increased more rapidly in tests with 30 s load holds compared to tests with 120 s load holds, due to creep recovery during the periods in between load holds. During these periods, the strain decreased, even though a small tensile load was still applied. publisher: Elsevier articletitle: The influence of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires journaltitle: International Journal of Fatigue articlelink: http://dx.doi.org/10.1016/j.ijfatigue.2017.01.043 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved. ispartof: International Journal of Fatigue vol:98 pages:203-211 status: published

Summary In the very high cycle fatigue regime, internal crack initiation can occur in Ti-6Al-4V because of the formation of facets, which are α grains that have fractured in a transcrystalline and planar manner. Because this crack initiation phase occupies most of the fatigue life, it is essential to understand which mechanisms lead to facet formation. Fatigue tests have been performed on drawn and heat-treated Ti-6Al-4V wires, and the facets at internal crack initiation sites have been analysed in detail in terms of their appearance, their spatial orientation and their crystallographic orientation. The facets were not smooth, but showed surface markings at the nanoscale. In nearly all cases, these markings followed a linear pattern. One anomalous facet, in a sample with the largest grain size, contained a fan-shaped pattern. The facets were at relatively steep angles, mostly between 50° and 70°. Cross-sections of the fracture surfaces have been made by focused ion beam milling and were used to measure the crystallographic orientation of facets by electron backscatter diffraction. Most facet planes coincided with a prismatic lattice plane, and the linear markings were parallel to the prismatic slip direction, which is a strong indication that prismatic slip and slip band formation led to crack initiation. However, the anomalous facet had a near-basal orientation, which points to a possible cleavage mechanism. The cross-sections also exposed secondary cracks, which had formed on prismatic lattice planes, and in some cases early stage facet formation and short crack growth phenomena. The latter observations show that facets can extend through more than one grain, and that there is crack coalescence between facets. The fact that drawn wires have a specific crystallographic texture has led to a different facet formation behaviour compared to what has been suggested in the literature.

Friction stir welding (FSW) since its invention has been attracting relevant interest for joining aluminium alloys. Due to the nature of this process, the materials can be joint without melting. Thanks to this peculiar characteristic, the issues associated with the cooling from liquid phase are avoided or considerably reduced, such as cracking, porosity, and defects. However, as well as other well-established welding techniques, the FSW process gives rise to formation of residual stress in the welding region and surrounding volume: heat and thermo-mechanical affected zones. Presence of residual stress in a mechanical component is well-known to affect its performance, particularly regarding fatigue at high number of cycles. Another aspect that influences the fatigue life is the underlying microstructure. In this work, we firstly study the residual stress field and the underlying microstructural features arising in FSW butt joints and their effect on the fatigue performance of this type of weldments. The evaluation of residual stress field is carried out by means of modern experimental techniques. In the first instance, synchrotron X-ray powder diffraction was employed for two-dimensional full field maps of residual stress. Corroboration of these measurements was done by exploiting the capability of focused ion beam and digital image correlation (FIB-DIC), which is able to deliver pointwise absolute measurement of residual stress. A set of FSW samples were then tested under uniaxial fatigue loading at several loading ranges, in the high cycle fatigue regime, in order to understand whether the severity of loads affects the crack path and life endurance. Fractographic and electron backscattered diffraction (EBSD) analysis then revealed crack nucleation site and propagation mechanisms with the respect of the underlying microstructure. Outcome of these experimental studies is then thoroughly discussed.

Cold spray is an emerging additive manufacturing technique with potential applications in surface functionalization, bulk component production and restoration/repair. During the cold spray process, metallic powders are accelerated to supersonic velocities by the carrier gas of high pressure and temperature and impact on the substrate to form layers of coating through deformation-induced bonding. However, the coating fabricated by this process suffers from low cohesive strength and weak interfacial bonding. Therefore, process optimization through numerical simulation is much needed. Here we employ finite element simulation with Johnson-Cook plasticity and dynamic failure model to numerically predict the temperature distribution within single particle, and they show good agreement with experimental observation using SEM. This provides a validated description of microscopic phenomena using numerical simulation, hence it can be employed further to study the bonding strength of the metal cold spray coating. Through microstructural analysis, we propose a semi-empirical relationship between the nodal temperature profile and local bonding strength, hence identified that the increase of the localized bonding area in a single splat is the determining factor for the increase of the bonding strength.

Fatigue cracks in titanium alloys are often found to initiate at faceted alpha grains. In the very high cycle fatigue regime, crack initiation tends to shift from the surface towards the interior of the material, and more initiation facets can be found on the fracture surface. In this study, fatigue tests were performed on drawn and heat-treated Ti-6Al-4V wires. Only a few samples fractured due to interior initiation. The facets at the initiation sites of these samples were not flat, but had markings on the nano-scale, and were highly inclined. A possible explanation for these aspects is the crystallographic texture of the wire, and a reflection is made on the suggested mechanisms of facet formation. peerreview_statement: The publishing and review policy for this title is described in its Aims & Scope. aims_and_scope_url: http://www.tandfonline.com/action/journalInformation?show=aimsScope&journalCode=ymst20 ispartof: Materials Science and Technology vol:32 issue:16 pages:1639-1645 status: published

The residual stresses in a laser shock-peened (LSP) plate of titanium alloy were measured by means of the ring-core Focused Ion Beam–Digital Image Correlation (FIB-DIC) technique using pillar diameters of 10 μm and 5 μm. A cross-section of the Ti-6Al-4V laser shock peened plate was electrochemically polished to avoid induced residual stress modification due to grinding. FIB-DIC measurements with pillar diameters of 10 μm show that a near-surface compressive residual stress of approximately 350 MPa is present in the direction parallel to the peened surface. This changes to a tensile residual stress of approximately 100 MPa at the depth of 2 mm, and then changes back to a small compressive residual stress on the opposite face. These results are in good agreement with the previously obtained set of measurements using high-energy synchrotron X-ray diffraction. However, FIB-DIC measurements with pillar diameters of 5 μm show strong deviations from this general macroscopic trend that corresponds to the macroscopic average (Type I) residual stresses. The reason for the apparent discrepancy lies in the smaller gauge volume used in the second set of measurements, which causes the evaluated residual stress values to be dominated by the microscopic (Type II + III) residual stress components. These Type II + III residual stresses are the result of the local microstructure and intra- and intergranular interactions. The results demonstrate that the FIB-DIC ring-core technique can be tailored to determine either macro- or microscopic residual stress, by tuning the core diameter with respect to the grain size. Type II + III stresses have significant magnitude sufficient to produce local tensile stress in the region that is subjected to macroscopic compression due to LSP treatment, and vice versa. The implications of this finding are discussed.

A novel approach to separating macroscopic (Type I) from microscopic (Type II + III) residual stress is presented, based on Focused Ion Beam – Digital Image Correlation (FIB-DIC) ring-core stress evaluation and eigenstrain modelling. This approach was applied to study the residual stresses for a titanium alloy bar following plastic four-point bending. It was found that electrochemical polishing is a surface preparation technique that is very well suited for FIB-DIC ring-core measurements, in the sense that it removes the influence of prior sample grinding and polishing, leads to a stress profile that satisfies force and moment equilibrium, and thus enables the evaluation of absolute values of total residual stress. The obtained relief strain profile across the bar width is asymmetric, highlighting the difference in the alloy's response to tension and compression. Total experimental residual stress values were calculated using (i) the assumption of material elastic isotropy, with an average Young's modulus, and (ii) under the assumption of elastic anisotropy, taking into account the crystallographic orientation of each investigated grain. Based on the measured relief strain values, the eigenstrain distribution in the bar was reconstructed and used to obtain the macroscopic (Type I) residual stress profile. The differences in the residual stress between the eigenstrain reconstruction values and the individual experimental results were ascribed to the local microscopic (Type II + III) residual stresses. This conclusion was substantiated by revealing the correlation between the residual stress values in individual grains in the elastic zone and their respective Young's moduli in the loading direction, as well as the correlation between the residual stress values in grains located in the plastic zone and their respective Schmid factors for basal slip.

In the very high cycle fatigue regime, the location of crack initiation in titanium alloys is known to shift from the surface towards the bulk of the material. This internal fatigue crack initiation results in faceted features on the fracture surface. These facets are in fact alpha grains that have been broken in a planar manner. Typically, a cluster of many facets is observed either just below the surface or deeper inside the bulk. In this study, uniaxial tension-tension fatigue tests are performed on Ti-6Al-4V wires which have been subjected to different heat treatments in order to vary the alpha grain size. Four different microstructures are obtained, with average alpha grain sizes of approximately 1, 2, 5 and 10 µm. The fatigue life is found to decrease with increasing grain size. Electrochemical polishing of the wires prior to fatigue testing is applied in order to promote internal crack initiation at higher stresses and consequently shorter testing durations. Four samples broke due to an internal crack: three samples with average alpha grain size 5 µm, which failed after 2.6 x 10^7, 5.7 x 10^7 and 9.6 x 10^7 cycles, and one sample with average alpha grain size 10 µm, which failed after only 7.6 x 10^6 cycles. The threshold stress intensity factor range, which is calculated from the size of the facet-containing area, is between 5 and 6 MPa.m^1/2 for all four samples. Fractographic examination of the facets reveals that they are not smooth, but show roughness at the nanoscale. This roughness has a linear appearance for nearly all facets, except for one anomalous facet in the sample with the largest grain size, which shows a fan-shaped pattern. From electron backscatter diffraction measurements on cross-sections of the fracture surfaces obtained by focused ion beam milling, it is also found that nearly all the facets coincide with a prismatic plane, and the linear markings are parallel to the prismatic slip direction. Only the anomalous facet has a near-basal orientation. These observations suggest the possibility that facets are formed by either a slip-based mechanism or a cleavage-based mechanism, and that the alpha grain size is one of the parameters that controls which mechanism occurs. publisher: Elsevier articletitle: The influence of the alpha grain size on internal fatigue crack initiation in drawn Ti-6Al-4V wires journaltitle: Procedia Structural Integrity articlelink: http://dx.doi.org/10.1016/j.prostr.2016.06.135 content_type: article copyright: © 2016 The Author(s). Published by Elsevier B.V. ispartof: pages:1055-1062 ispartof: Procedia Structural Integrity vol:2 pages:1055-1062 ispartof: ECF21, European Conference on Fracture location:Catania, Italy date:20 Jun - 24 Jun 2016 status: published

Mechanical polishing is commonly used for both surface finishing and metallographic sample preparation for a broad range of materials. However, polishing causes local deformation and induces residual stress, which has an important effect on many surface phenomena. Until recently, it has not been possible to quantify the nanoscale depth variation of polishing‐induced plastic deformation (eigenstrain) and the associated residual stress. In this study, the magnitude and depth of polishing‐induced residual stress are evaluated directly by focused ion beam milling and digital image correlation using the micro‐ring‐core geometry method. Depth‐resolved residual stress profiles are obtained with sub‐micrometer resolution at the surface of a titanium alloy sample that is subjected to various polishing steps. It is found that electrochemical polishing and polishing with colloidal silica do not induce any significant residual stress. However, polishing with diamond slurry leads to the formation of compressive residual stresses of up to 300 MPa, which extend deeper into the material when larger diamond particles are used. This study paves the way for further research on polishing and its effect on surface properties.