Tribological contact often leads to surface deformation, resulting in a substantial increase in dislocation density and a considerable refinement in the microstructural scale. The extensive work hardening associated with this results in significant changes in the mechanical properties of the surface. It is not only the mechanical properties that change, but also the corrosion potential. In some cases, the surface changes enhance the wear resistance of the material. However, in other cases, higher wear rates are found with surface deformation that results in ultra-fine surface structures. Despite the importance of surface deformation, much is unknown about the mechanical properties of the wear induced surface layers. Nanoindentation provides useful information but does not give a good indicator of the ductility. The challenge is to test the mechanical properties of such a fine scale deformed structure. In this work, the micromechanical properties of Ti-6Al-4V worn surfaces after tribocorrosion testing were measured using an in-situ micropillar compression method in the chamber of a scanning electron microscope. Reciprocating tribocorrosion testing was undertaken in 25 vol. % Bovine Serum Albumin (BSA) in phosphate-buffered saline (PBS) solution against an alumina counterface, with a load of 0.5N and a speed of 20mm/s. Tests were conducted under Open Circuit Potential (OCP) conditions and at cathodic and anodic surface potentials, namely at +0.5V and -0.95V. The different test conditions resulted in different extents of surface deformation. This resulted in significant differences in the stress strain curves from the micropillar tests, both in terms of strength and ductility. The microstructure observed by subsequent TEM of the tested micropillars is correlated with the mechanical properties and the reasons for the different mechanical properties are discussed.

It is well known that the thermomechanical processing parameters affect the mechanical properties of dual-phase (DP) steels, but optimization still remains a key challenge. This work studied two interphase precipitation-strengthened DP steels based on V, Mo, and V, Mo, Cr microalloy additions. Hot-rolling was performed with a strain per pass of 0.2 and 0.4 before isothermal transformation at temperatures between 600 and 690 °C to determine the effect of these process parameters on microstructure, particularly interphase precipitation. The microstructure was carefully correlated with the mechanical properties. It was found that a higher strain during hot rolling increases the transformation kinetics of austenite to ferrite and also increases the volume fraction of the interphase precipitation within it, leading to higher strength values. The reaustenitization temperature before the isothermal transformation also plays an important role, with increased temperature reducing the segregation banding effect and increasing the amount of ferrite. The optimal reaustenitization temperature depends on the composition, being 1250 °C for the microalloyed steel with Cr additions and 1150 °C for the Cr-free microalloyed steel. A new method was used to calculate the yield strength of these DP microalloyed steels, including the strengthening contributors of each phase and the banding effect brought into one single expression that matches the experimental results.

There has been much research on steels strengthened by interphase precipitation, but the role of Cr remains unclear. Isothermal transformations have been performed at different temperatures to investigate the effect of Cr and Cr + Nb additions on the interphase precipitation resulting from the austenite-to-ferrite transformations in V-Mo based dual phase steels. Optical microscopy (OM) plus transmission electron microscopy (TEM) and tensile testing were used to characterize and correlate microstructural evolution to the tensile properties and the interphase precipitation formed during the austenite to ferrite (gamma ->alpha) transformation. It was found that an addition of 0.5 wt% Cr to V-Mo microalloyed steel accelerates the transformation rate of ferrite, producing a higher volume fraction of ferrite. Importantly, the addition of Cr reduces the diameter of the interphase precipitates giving an important contribution to the yield strength. A further addition of 0.03 wt% Nb to the Cr-V-Mo reduces the contribution made by the interphase precipitation and increased the grain refined contribution. A new method was used to calculate the volume fraction of interphase precipitation with TEM images from extraction replicas is presented and included in a root-mean-square model which includes the precipitation hardening made by IP on microalloyed DP steels to effectively predict their yield strength.

Steel will be an essential part of any commercial fusion reactor design. Applications in this area involve extreme conditions, imposing particular performance requirements, such as high operational temperature and creep resistance, and also a limitation on the elements that can be used due to the activation that occurs on interaction with irradiation. This work begins with a steel developed for conventional power plant applications, the IBN1 grade developed by IMPACT (a UK consortium of industrial and academic research organisations). This grade has shown excellent properties at high temperature due to high temperature-stable precipitate phases, but contains several elements that would become radiologically active to a degree that is incompatible with the required disposal routes after exposure to the fusion reactor environment. In this study, modifications of the composition are made to remove these elements, and thermodynamic modelling and experimental assessment of the phases that form are undertaken. In this, we have paid particular attention to the prediction of transformation temperatures (to understand if normalisation and tempering can be applied successfully) and the precipitates, to see if suitable phases that are likely to impart creep strength and other desirable properties would be formed. The modifications made include the removal of Nb, Mo, Ni, Co, Cu and Al from the starting alloy, and the substitution of Ta (intended to form carbides, replacing the effect of Nb). Modifications of the amount of retained elemental components, such as C, Mn and Cr, have been made with Thermo-Calc modelling, to ensure preservation of comparable phase transformation temperatures and microstructures. The predicted changes to the alloy are compared to the observations from experimental investigation, finding that tantalum can substitute for niobium in these systems and form similar carbides with similar distribution in the material, and that reduction of Cr to 8 wt% and increase of C to 0.12 wt% raises the Ae4 temperature to allow a high-temperature heat treatment without δ-ferrite formation. While assessment of the mechanical properties of this alloy would be required, the perspectives for these alloys to perform at high temperature that can be inferred from the microstructure are discussed.

The current study reports in situ TEM observations of the growth of bainitic ferrite in an Fe-0.3C-3Mn-1.5Si-0.15Mo steel held isothermally at 300 degrees C with a higher spatio-temporal resolution than in previous studies. Significant variations were found in the lengthening rate, with the highest being in excess of 30,000 nm.s(-1) while more common lengthening rates of 10-1000 nm.s(-1) provided the highest quality observations. Both sheaves with visible sub units and individual laths were observed during growth with the lengthening behaviour of sheaves found to be discontinuous - in the most favourably oriented sheave this could be linked to sub unit behaviour. The transformation behaviour was comparable to that of HT-LSCM observations of bainitic ferrite growth for the most comparable steel compositions and to 'textbook' descriptions of the formation of bainite sheaves. In addition, other relevant phenomena were recorded, including the generation and movement of dislocations in the austenite during transformation, the interaction of laths with twin boundaries and the initially slow growth of bainitic ferrite laths.

This article investigates the microstructure evolution, phase formation, and magnetic properties of Co2FeCr0.5Alx (x = 0.9, 1.2) complex component alloys, as a function of heat treatment temperatures (at 500, 600, 700, and 1150 degrees C), using XRD, optical microscopy, electron microscopy and vibrating sample magnetometry (VSM). The alloy with 20.4 at% Al (x= 0.9), identified here as C1, consisted of microscale BCC1 phase and BCC nanoscale particles containing mainly Fe and Cr, and B2 matrix with mainly Al and Co. Partial transformation of the BCC1 phase to an FCC phase was observed at 700 degrees C and full transformation at 1150 degrees C, through twinning. For the sample with 25.5 at% Al (x= 1.2), identified as sample C2, there were only nanoscale BCC particles in the B2 matrix with the same element segregation between the phases as C1. This increase in Al (from x= 0.9 to 1.2) content stabilised the B2 matrix phase, reduced the grain size, and increased both saturation magnetisation (Ms) and coercivity (Hc). Moreover, increasing the heat treatment temperature resulted in an increase in grain size of the B2 matrix, volume fraction and average size of the micro BCC 1 and nanoscale BCC phases for both C1 and C2, which also modified the soft magnetic properties, with Ms and Hc increasing up to 600 degrees C followed by a decrease until 1150 degrees C. Using the structural information as inputs for density functional theory calculations of Hc and Ms, it has been found that the Hc is influenced by the grain size of the matrix, and the volume fraction and size of the BCC1 phase at temperature higher than 600 degrees C for C1 and 700 degrees C for C2, but is controlled by nanoscale BCC particles below these temperatures. The Ms is controlled by the elemental diffusion and segregation. Thus, the best combination of Hc and Ms was seen with antiferromagnetic Cr segregated and partitioning in the microscale BCC1 phase, and a B2 matrix with less Cr rich precipitation, formed at 500 degrees C, where the misfit strain between B2 matrix and nanoscale BCC was low.

Twin boundaries have been shown to deviate from the twinning planes in hcp metals, and facets have often been observed in twin interfaces. This study presents a twinning disconnection-based model for faceting in single, double and triple twin boundaries in magnesium. Primary twinning disconnections predicted via symmetry arguments are shown to produce commensurate facets in single twin boundaries, which are then transformed into commensurate facets in double twin boundaries via the action of secondary twinning disconnections. In contrast, it is shown that for triple twin boundaries with tension-compression-tension twinning sequence, no commensurate facets can be produced by the action of tertiary twinning disconnections. The effect of facets on the macroscopic orientation of twin interfaces is discussed. Theoretical findings are validated by a transmission electron microscopy study of a hot rolled Mg-1.18wt%Al-1.77wt%Nd alloy. Single and double twins are observed, as well as rare triple twins, and the interface between the matrix and a triple twin is captured for the first time. Facets consistent with theoretical predictions are imaged via high-resolution TEM and macroscopic deviations of the boundaries from the primary twinning planes are measured.

Valves and pumps in nuclear plants operate in a particularly aggressive environment of a high neutron flux, high contact loads and in superheated water up to 300 degrees C. Traditionally there has only been one material that can survive these conditions: cobalt based hardfacing alloys called Stellites. There is an urgent need to replace cobalt-based alloys because of the formation of cobalt-60 radioisotope, which causes radioactive exposure. In this study an iron-based hardfacing alloy, Tristelle 5183 was compared with Stellite 6. The hardfacings were manufactured via laser cladding. Wear testing was performed using a pin-on-flat configuration, with like-on-like couples, from room temperature up to 300 degrees C with loads of 50-500 N. The friction coefficient for the Tristelle was higher (typically 0.45) than the Stellite (typically 0.35) across all temperatures and loads. The wear rate of the Tristelle was higher than the Stellite, but the exact difference varied with both load and temperature. The specific wear rate increased with temperature, but only gradually. The operative wear mechanism was a combination of adhesive and oxidational wear, but there were significant differences found between the two alloys. For the Tristelle, the oxide became mechanically mixed with the heavily deformed surface metal. In contrast, the oxide on the Stellite was detached from the surface without any mechanical mixing. The extent of surface deformation was much greater for the Tristelle than the Stellite. The large (5-20 mm) NbC in the Tristelle fractured a considerable distance (up to 40 mm) below the worn surface and became increasingly fine as the worn surface was approached as a result of high strain deformation in the matrix. In the Stellite no carbide fracture was observed. The reasons for the marked difference in behaviour between the two alloys, each with the same mechanical properties, are discussed in detail.

Two new processes occurring during nanorod annealing of ceria are disclosed, both belonging to the wider “oriented attachment” (OA) scheme of crystal growth, which commonly drives nanoparticles growing into nanorods. The first new process is an inversion of the standard OA, where the usual solid particles are replaced by well-facetted shape-equilibrated voids inside larger single crystals. The internal facetted voids are then found to aggregate during dry heat treatment into rod-shaped elongated voids growing eventually towards nanotubes. For the case of CeO2, a perfect equivalence is found between positive OA, involving cuboctahedral {111}/{100} nanoparticles turning into {110}/{100} nanorods, and negative or void space OA, where cuboctahedral voids turn into negative tubular rods of same indexing. The second OA process (“secondary OA”) concerns aggregation of small nanorod segments into larger, double, or quadruple sized nanorods, with perfection of alignment and bonding exceeding simple van der Waals forces. Eventually, the new rods merge into single crystal grain boundary–free larger rods, but with an external shape, including double-ended rods, indicating their origin from several rods. Both processes are found on identical samples and occur in parallel.

Ti-6Al-4V has been used as a surgical implant material for a long time because of its combination of strength, corrosion resistance and biocompatibility. However, there remains much that is not understood about how the surface reacts with the environment under tribocorrosion conditions. In particular, the conditions under which tribofilms form and their role on friction and wear are not clear. To evaluate the complicated nature of the dynamic surface microstructural changes on the wear track, high resolution transmission electron microscopy (TEM), scanning transmission electron microscope (STEM) and electron energy loss spectroscopy (EELS) have been used to characterise the structure and chemical composition of the tribofilm. Detailed analysis of the formation and structure of the tribofilm and the metal surface deformation behaviour were studied as a function of applied potential and the role of proteins in the lubricant. For the first time, graphitic and onion-like carbon structures from wear debris were found in the testing solution. The presence of carbon nanostructures in the tribocorrosion process and the formation of the tribofilm leads to an improved tribocorrosion behaviour of the system, in particular a reduction in wear and friction. A detailed, quantitative, analysis of surface deformation was undertaken, in particular, the geometrically necessary dislocation (GND) density was quantified using precession electron diffraction (PET). A clear correlation between applied potential, tribofilm formation and the surface strain was established. STATEMENT OF SIGNIFICANCE: The formation of tribofilm and microstructure modification of the Ti-6Al-4V surface during tribocorrosion in a physiological environment is not fully understood. In particular, the correlation between microstructural changes and electrochemical conditions is not clear. This study presents a detailed investigation of the structure and chemical composition of tribofilms at the nanoscale during tribocorrosion tests in simulated body fluid and gives a detailed and quantitative description of the evolved surface structure. A clear correlation between applied potential, tribofilm formation and the surface strain was established. Moreover, particular attention is paid to the wear debris particles captured from the lubricating solution, including nanocarbon onion structures. The implications for tribocorrosion of the alloy in its performance as an implant are discussed.

Titanium alloys are widely used in the field of orthopaedics, however, poor tribological performance arising from the low shear resistance and weak protection of the oxide layer limits their applicability. In this work, the tribocorrosion behaviour of novel beta and alpha + beta biomedical titanium alloys were studied in bovine serum solution under open circuit potential. Ti-5Mo-Fe-3Sn and Ti-6Al-4V ELI alloys were found to have the capability to repassivate when testing at a slower sliding speed. Scanning and transmission electron microscopy results revealed that the repassivation was mainly associated with the greater tribofilm formation on the worn surface, rather than an oxide film. The tribolayer performed as a solid lubricating film, which stabilised the coefficient of friction.

The influence of hydrogen on dislocation mobility in pure single crystal iron subjected to micro- and nanoindentation testing perpendicular to the (100) plane has been analysed using electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) and simulated using the self-consistent kinetic Monte-Carlo (SCkMC) method. The nanoindentation load curve for the hydrogen free sample has two major constant load pop-ins, and multiple smaller pop-ins, whereas the hydrogen charged material has only one pop-in but with a longer loading release. A well-defined cell structure was observed below the indenter in the hydrogen free condition, but the dislocations were homogenously distributed in the hydrogen charged sample. The SCkMC simulations showed that it is difficult for dislocations to glide out of the primary slip plane after hydrogen charging compared with the hydrogen free sample under the present loading conditions.

The nitriding process plays an essential role in the production of grain‐oriented silicon steel (GOSS) via the acquired‐inhibitor process. It is surprising at first glance to find that the increase of temperature from 840 to 930 °C retards the nitriding of GOSS rather than accelerating it. By both extensive microstructural characterization and thermodynamic calculations, this is attributed to the formation of austenite at high temperature in the oxidization layer, in which N atoms diffuse at a much lower rate than in ferrite. It is concluded that there exists an upper‐limit temperature for the efficient nitriding of GOSS, which is determined by both the preceding decarburization process and the nitriding atmosphere.

A novel medium manganese steel with composition Fe–8.3Mn–3.8Al–1.8Si–0.5C–0.06V–0.05Sn was developed and thermomechanically processed through hot rolling and intercritical annealing. The steel possessed a yield strength of 1 GPa, tensile strength of 1.13 GPa and ductility of 41 pct. In order to study the effect of cold rolling after intercritical annealing on subsequent tensile properties, the steel was further cold rolled up to 20 pct reduction. After cold rolling, it was observed that the strain hardening rate increased continuously with increasing cold rolling reduction but without a significant drop in ductility during subsequent tensile tests. The microstructural evolution with cold rolling reduction was analysed to understand the mechanisms behind this phenomena. It was found that cold rolling activated additional twinning systems which provided a large number of potent nucleation sites for strain induced martensite to form during subsequent tensile tests in what can be described as an enhanced TRIP effect.

Ti6Al4V is used in a wide variety of engineering applications and extensively in orthopaedics implants because of the excellent combination of the high specific strength and superior corrosion resistance. However, Ti6Al4V possesses poor tribological properties. The present work investigated the effect of reciprocating sliding speed on tribocorrosion properties of Ti6Al4V alloy under cathodic and open circuit potentials in simulated body fluid. The results showed a strong influence of the sliding speed on the microstructure of the worn surfaces and consequently on the wear rate under tested conditions. The wear rate at low sliding speeds was an order of magnitude lower compared to that at the faster sliding speed. Synergistic and antagonistic behaviour was strongly associated with the sliding speed.

Porosity in functional oxide nanorods is a recently discovered new type of microstructure, which is not yet fully understood and still under evaluation for its impact on applications in catalysis and gas/ion storage. Here we explore the shape and distribution of pores in ceria in three dimensions using a modified algorithm of geometric tomography as a reliable tool for reconstructing defective and strained nanoobjects. The pores are confirmed as "negative-particle" or "inverse-particle" cuboctahedral shapes located exclusively beneath the flat surface of the rods separated via a sub-5 nm thin ceria wall from the outside. New findings also comprise elongated "negative-rod" defects, seen as embryonic nanotubes, and pores in cube-shaped ceria. Furthermore, we report near-sintering secondary heat treatment of nanorods and cubes, confirming persistence of pores beyond external surface rounding. We support our experiments with molecular modeling and predict that the growth history of voids is via diffusion and aggregation of atomic point defects. In addition, we use density functional theory to show that the relative stability of pore (shape) increases in the order "cuboidal" < "hexagonal-prismatic" < "octahedral". The results indicate that by engineering voids into nanorods, via a high-temperature postsynthetic heat treatment, a potential future alternative route of tuning catalytic activities might become possible.

Hydrogen embrittlement is increasingly important in advanced high strength steels (AHHS) as strength levels increase well above 10 0 0 MPa. This work developed a detailed understanding of the embrittling mechanism in model AHHS steels based on Fe-Ti-Mo and Fe-V-Mo, both strengthened through interphase precipitation. Hydrogen charging led to an increase in the dislocation density and an enlarged strain field around precipitates, resulting in an increase in residual stress. This was much greater for the Ti-Mo steel compared to the V-Mo. Important differences in the hydrogen trapping behaviour was seen between the two steels, with hydrogen believed to be trapped at the matrix/precipitate interface for the Ti-Mo steel, but within the precipitate for the V-Mo steel. The effects of hydrogen were investigated in detail for slow strain rate tensile tests and double notched tensile samples. Hydrogen charging resulted in a loss in strength and ductility, with the Ti-Mo steel failing at yield, while the V-Mo steel exhibited a -13% loss in strength and a - 35% loss of ductility. Crack initiation in tensile samples occurred at high strain gradient dislocation boundaries. However, crack propagation rapidly became quasi-cleavage, along the {100} plane in ferrite, and also along the martensite/ferrite grain boundaries on the {110} plane in the martensite. Minimal plasticity was observed associated with the crack tip, which was believed to be a result of the suppression of dislocation emission at the crack tip by the hydrogen. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Promoting the martensitic transformation through optimum microalloying with Fe and/or Mn was observed to be an effective method to enhance the wear resistance of the Cu50Zr50 at% shape memory alloy (SMA). Among all the potential microelements and concentrations, partial replacement of Cu by up to 1 at% Fe and Mn is of interest since from density functional-based calculations, large minimization of the stacking fault energy (SFE) of the B2 CuZr phase is predicted. For this reason, an effective martensitic transformation is expected. The largest decrease of the SFE from 0.36 J/m(2) to 0.26 J/m(2) is achieved with partial replacement of Cu by 0.5 at% Fe. This results in the highest martensitic transformation upon wear testing, especially at highest load (15 N) for which the mass loss is 0.0123 g compared to 0.0177 g for Cu50Zr50 and a specific wear-rate of 5.9 mm(3)/Nm, compared to 8.5 for mm(3)/Nm for Cu50Zr50. This agrees with the low coefficient of friction of 0.48 +/- 0.05 and low roughness of 0.200 +/- 0.013 mu m of the Fe-containing alloy compared to that for Cu50Zr50, 0.55 and 0.415 +/- 0.026 mu m, respectively. All the worn surfaces show the formation of abrasive grooves, being shallowest for the more wear resistant 0.5 at% Fe alloy. The second more wear resistant alloy contains 0.5 at% Mn. Wear mechanisms of abrasion, adhesion, and delamination have been identified.

Tensile stress relaxation is combined with transmission electron microscopy to reveal dramatic changes in dislocation structure and sub structure in pure α-Fe as a result of the effects of dissolved hydrogen. We find that hydrogen charged specimens after plastic deformation display a very characteristic pattern of trailing dipoles and prismatic loops which are absent in uncharged pure metal. We explain these observations by use of a new self consistent kinetic Monte Carlo model, which in fact was initially used to predict the now observed microstructure. The results of this combined theory and experimental study is to shed light on the fundamental mechanism of hydrogen enhanced localised plasticity.

We investigate AlGaAs/GaAs superlattices as well as InGaAs/GaAs quantum wells and epitaxial quantum dots (QDs) where during the molecular beam epitaxy of InGaAs QDs the aluminium flux cell was opened briefly to incorporate fractional monolayers of Al into the InGaAs. We show that x-ray mapping with a large collection angle is capable of detecting 0.3–0.4 fractional Al monolayers with a resolution of just under 1 nm.