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We present an electron backscattered diffraction (EBSD)-trained deep learning (DL) method integrating traditional material characterization informatics and artificial intelligence for a more accurate classification and quantification of complex microstructures using only regular scanning electron microscope (SEM) images. In this method, EBSD analysis is applied to produce accurate ground truth data for guiding the DL model training. An U-Net architecture is used to establish the correlation between SEM input images and EBSD ground truth data using only small experimental datasets. The proposed method is successfully applied to two engineering steels with complex microstructures, i.e., a dual-phase (DP) steel and a quenching and partitioning (Q&P) steel, to segment different phases and quantify phase content and grain size. Alternatively, once properly trained the method can also produce quasi-EBSD maps by inputting regular SEM images. The good generality of the trained models is demonstrated by using DP and Q&P steels not associated with the model training. Finally, the method is applied to SEM images with various states, i.e., different imaging modes, image qualities and magnifications, demonstrating its good robustness and strong application ability. Furthermore, the visualization of feature maps during the segmenting process is utilised to explain the mechanism of this method's good performance.

The effect of Luders banding on hydrogen embrittlement (HE) susceptibility of medium Mn steels was investigated by tuning the degree of yield point elongation (YPE). It was found that the steel with a larger YPE was more susceptible to HE. A larger YPE led to a higher local flow stress and a higher fraction of strain-induced martensite, which in turns resulted in more H-induced cracks in the Luders banding area and the consequent catastrophic failure. It thus suggests that Luders banding and the associated localized deformation play a key role in influencing the overall HE susceptibility of medium Mn steels.

Abstract The precipitation of supersaturated solutes at free surfaces in ternary Fe–3Au–4W and binary Fe–3Au and Fe–4W alloys (composition in weight percentage) for different ageing times was investigated at a temperature of 700 °C. The time evolution of the surface precipitation is compared among the three alloys to investigate the interplay between the Au and W solutes in the ternary system. The Au-rich grain-interior surface precipitates show a similar size and kinetics in the Fe–Au–W and Fe–Au alloys, while the W-rich grain-interior surface precipitates show a smaller size and a higher number density in the Fe–Au–W alloy compared to the Fe–W alloy. The kinetics of the precipitation on the external free surface for the ternary Fe–Au–W alloy is compared to the previously studied precipitation on the internal surfaces of the grain-boundary cavities during creep loading of the same alloy. Graphical abstract

Autonomous healing of creep-induced grain boundary cavities by Au-rich and W-rich precipitates was studied in a Fe-3Au-4W (wt pct) alloy at a fixed temperature of 823 K (550 °C) with different applied stresses. The ternary alloy, with two supersaturated healing solutes, serves as a model system to study the interplay between two separate healing agents. The creep properties are evaluated and compared with those of the previously studied Fe-Au and Fe-W binary systems. The microstructures of the creep-failed samples are studied by electron microscopy to investigate the cavity filling behavior and the mass transfer of supersaturated solute to the defect sites. Compared to the Fe-Au and Fe-W alloys, the new Fe-Au-W alloy has the lowest steady-state strain rate and the longest lifetime. The site-selective filling of the creep-induced cavities is attributed to two different categories of precipitates: micron-sized Au-rich precipitates and nano-sized W-rich precipitates. The Au-rich precipitates are found capable to fully heal the cavities, while the W-rich precipitates show only a limited degree of healing. The two types of precipitates show a reluctance to coexistence, and the formation of W-rich precipitates is suppressed strongly. A model is proposed to describe the competitive healing behavior of the Au-rich and W-rich precipitates.

The current study reports the detailed analysis of an observation of the local pinning of a slowly moving austenite-ferrite interface by a single nanosized oxidic particle. The observations were made during an in situ cyclic partial phase transformation experiment on a Fe-0.1C-1.0Mn alloy close to the inversion stage at which the interface migrates at a rather low velocity. The low velocity allowed capturing the interface pinning effect over a period of no less than 16 seconds. From our observations, it was possible to follow the progression of the pinning effect from the initial stages all the way through to the release of the interface. The pinning force exerted by the individual particle having a diameter of 140 nm on the austenite-ferrite interface was estimated as 175 nJ m−1, while the maximum pinning length was approximately 750 nm to either side of the particle, leading to an interface line tension of 170 nJ m−1. The observed pinning behavior is compared with the most relevant models in the literature.

The influence of the cohesive zone length on the crack driving force is quantified and analyzed in a representative system of particles dispersed in a matrix of a composite material. For heterogeneous material systems, e.g. particulate composites, it is known that as a crack approaches the particles, the crack driving force may increase (shielding) or decrease (anti-shielding) depending on the relative stiffness of the particles. These results have been established in numerous studies using the classical linear elastic fracture mechanics approach (LEFM). The cohesive zone method (CZM) introduces a length scale parameter, referred to as the cohesive zone (or fracture process zone) length scale, into the formulation of fracture mechanics. It is generally established that fracture mechanics predictions using the CZM are similar to those obtained using LEFM in the limit case where the process zone is very small relative to a suitable characteristic dimension of the problem. However, the influence of the length scale parameter has not been clearly demonstrated for crack propagation in a heterogeneous material system, especially when the cohesive zone length is not negligible. By considering a simple crack-particle-matrix system, it is shown that, in addition to the elastic properties, the process zone length scale parameter exhibits a critical influence on the crack driving force. For this study, the concept of configurational forces is utilized and the eXtended Finite Element Method (XFEM) is employed as a tool to simulate crack propagation. Through numerical simulations, it is shown that (i) the magnitude of the driving force vector directly depends on the length scale parameter and (ii) the direction of the driving force is largely influenced by the presence of a cohesive zone. This, in turn, alters the crack trajectory in the particulate system if the criterion for the direction of crack propagation depends on the orientation of the driving force vector. Towards this end, two different criteria for direction of crack propagation, namely maximum principal stress and maximum energy dissipation, are compared in the presence of a cohesive zone and the results are reported. The study reveals the crucial influence of the inherent length scale associated with the cohesive zone method when applied to crack propagation in particulate composite systems and elucidates important differences when comparing predictions from distinct theories of fracture mechanics.

Two piezoelectric series bimorph sensors were embedded below the skin of a NACA 0012 symmetrical airfoil to detect the local state of the boundary layer during wind tunnel testing. Small vanes piercing the airfoil skin were glued onto the bimorphs providing a mechanical coupling to the local mechanical force fluctuations imparted by the local unsteady boundary layer flow. The state of the boundary layer at the sensor sites was varied by changing the angle of attack. The objective of this work was to establish the ability of this sensor concept to accurately distinguish among typical boundary layer states such as attached laminar flow, turbulent flow and separated flow. The output of the sensor was compared to concurrent time-resolved particle image velocimetry measurements, which served as a validation technique. Using the developed sensor response envelope, a single data point time series of the piezo electrical signal was proven to be sufficient to accurately detect the boundary layer state on classical airfoils in the low Reynolds number regime. In projected future applications, single or arrays of bimorph sensors can be used to map the boundary layer of more complex or morphing shape airfoils. The fast response of the sensor can in principle be utilised in closed-loop flow control systems, aimed at drag reduction or lift enhancement.

A universal numerical model based on the particle size distribution (PSD) approach has been developed for the simulation of precipitation kinetics in multicomponent alloys during isothermal ageing. Nucleation was implemented utilizing the classical nucleation theory (CNT). Growth and coarsening were modeled by a single growth kinetics equation, which is constructed based on the interfacial diffusion flux balance and the capillarity effect. Only partial off-diagonal terms in the diffusion matrix (diffusion of individual components in the matrix) were taken into account in the calculations to minimize the computational cost while coupling with CALPHAD to extract thermodynamics equilibrium around the interface. A new feature of the model is the incorporation of a more realistic spatial site distribution via a Voronoi construction in the characteristic cell, for the purpose of modifying the diffusion distance. Computational predictions of the precipitate dimensions and the precipitation kinetics were compared with the atom probe tomography (APT) measurements on ternary Ni-Al-Cr alloys isothermally aged at 873 K. It is found that the temporal evolution of the dimensions and composition of the precipitates is well captured, as is the dependence on changes in the alloy composition. The new modification with Voronoi construction demonstrates that the overall precipitation kinetics depends on the density and the spatial site distribution of precipitates. The ability to handle sophisticated alloy chemistries by quantitative equations, the compositional sensitivity of microstructural characteristics emerging from the simulation results, and the ability to visualize the spatial distribution of precipitates make the work very promising for multicomponent alloy design and optimization.

A set of numerical and analytical models is presented to predict the growth and contraction of grain-boundary creep cavities in binary self-healing alloys. In such alloys, the healing is realised by preferential precipitation of supersaturated solutes at the free surface of the cavity. The cavity grows due to the diffusional flux of vacancies towards the cavity, which is driven by the stress gradient along the grain boundary. Upon deposition of healing solute atoms on the cavity wall, effectively vacancies are removed from the cavity due to the inverse Kirkendall effect. The competition between the inward and outward vacancy fluxes results in a time-dependent filling ratio (i.e. the fraction of the vacancies removed from the original cavity) of the creep cavity. It is found that for stress levels lower than a critical stress σcr, the filling ratio can proceed to unity, i.e. to complete filling and annihilation of the pore. For applied stresses higher than σcr, complete filling is not achieved and the open volume of the creep cavity will continue to grow once a maximum filling ratio is reached at the critical time tcr. The critical stress σcr, critical time tcr, and time for complete filling th (if fully filling is achievable) are derived from the models for different combinations of parameters. The results from the analytical model and from previous nanotomography experiments are compared and are found to be in good agreement. Graphical abstract

Computationally-efficient surrogate models based on a Polynomial Chaos Expansion (PCE) are developed to quantify the uncertainties in the fracture behavior and lifetime of a self-healing thermal barrier coating system (SH-TBC) and a benchmark conventional TBC system. The surrogate models are built using deterministic information from micromechanical finite element simulations of thermal cycling of the systems, which are conducted until failure by spallation. Fracture and healing events are simulated using a cohesive-zone based crack healing model. The thermally-grown oxide layer (TGO) interface amplitude and its growth rate, the diameter and volume fraction of healing particles, and the mean distance of particles from the interface are used as training variables. Statistical characteristics and sensitivity indices are obtained from the trained models. It is found that the interface amplitude is the most significant contributor to the variance in the TBC lifetime, with other parameters displaying a relatively minor influence. Healing particles extend the expected value of TBC lifetime, however they also increase the uncertainty of thermal fatigue life. The analysis of self-healing TBCs exemplifies how PCE-based surrogate models can serve as a powerful tool for deriving design insights in complex material systems.

The thermal cyclic behavior of self-healing thermal barrier coatings (SH-TBC) is analyzed numerically to develop a lifetime prediction model. Representative microstructures are studied adopting a unit cell based multiscale modeling approach along with a simplified evolution model for the thermally-grown oxide layer (TGO) to study the evolution of damage and healing in a self-healing TBC system. The fracture and healing process is modeled using the cohesive zone-based healing model along with a crack tracking algorithm. The microstructural model includes splat boundaries and a wavy interface between the Top Coat and the Bond Coat, typical of Air Plasma Sprayed TBCs. A particle-based self-healing mechanism is accounted for with a random distribution of healing particles subjected to a numerically accelerated thermal cyclic loading condition. Lifetime extension of the self healing TBCs is quantified by conducting thermal cyclic analyses on conventional TBCs (benchmark system without self-healing particles). Parametric analyses on healing parameters such as crack filling ratio and strength recovery of the healed crack are also conducted. The results are presented in terms of the evolution of the crack pattern and the number of cycles to failure. For self-healing TBCs with a suitable healing reaction (i.e., cracks being partially filled and a minimal local strength after healing), an improvement in TBC lifetime is observed. In contrast, if the healing mechanism is not activated, the presence of the healing particles is actually detrimental to the lifetime of the TBC. Correspondingly, in addition to superior crack filling ratio and healed strength, significant improvement in lifetime is achieved for self healing TBCs with a higher probability of crack-healing particle interaction. This highlights the importance of a robust activation mechanism and a set of key material requirements in order to achieve successful self-healing of the TBC system.

Flow-induced disturbances were applied during the synthesis of a naphthalene diimide covalent organic framework (NDI-COF), which resulted in different COF polymer networks. We discovered that a high intensity of stirring resulted in more aggregated structures on both the micro- and nano-length scale. Subsequently, these structures absorbed light over longer wavelengths due to a relatively higher contribution of intermolecular interactions between the NDI-segments.

Professor John Edwin Field passed away on October 21st, 2020 at the age of 84. Professor Field was widely regarded as a leader in high-strain rate physics and explosives. During his career in the Physics and Chemistry of Solids (PCS) Group of the Cavendish Laboratory at Cambridge University, John made major contributions into our understanding of friction and erosion, brittle fracture, explosives, impact and high strain-rate effects in solids, impact in liquids, and shock physics. The contributions made by the PCS group are recognized globally and the impact of John’s work is a lasting addition to our knowledge of the dynamic effects in materials. John graduated 84 Ph.D. students and collaborated broadly in the field. Many who knew him attribute their success to the excellent grounding in research and teaching they received from John Field.

Piezoelectric transducers which rely on oscillating cantilever-type beams to harvest mechanical energy locally available in environments have been of great interest as a substitute for batteries. Most of the research efforts focus mostly on designs which aim at resonance matching to achieve maximum energy output without taking the mechanical degradation of the piezoelectric layers into consideration. The purpose of this study is to propose an energy harvesting design which maximizes power output on the long run. Unimorph cantilevers, in which the neutral axis is located at the interface between the soft lead zirconium titanate (PZT) (PZT5A4) layer and the inert substrate (Pernifer 45), are used. An analytical model is developed to quantify the performance of the harvesters as a function of free length and tip mass. An experiment is set up to validate the theoretical model. To reduce the occurrence of cracks induced in the piezoelectric element due to the cyclic nature of the vibrational excitation, a housing acting as mechanical stroke limiter is adopted. The effect of the single-side stroke limiter on the power output and lifetime of the cantilevers is investigated. A 40 mm free length unimorph cantilever with 300 mg mass attached on the tip exhibiting an 18% increase in power output (0.1 mW) is proposed. An improved lifespan of the cantilevers is obtained by limiting the tensile deformation of the piezoelectric layer. This study opens the opportunity for more effective energy harvesting mainly through compressive operation for longer periods.

The combined effects of the initial microstructure and heating rate on ferrite recrystallization, austenite formation and transformation kinetics of a regular ferrite-martensite dual phase steel are investigated. Special attention is given to the effect of the martensite distribution on mechanical properties, particularly the bendability. Ferrite recrystallization in the cold rolled bainitic grade proceeds faster than in the ferrite-pearlite grade. The reason for this is the higher defect density leading to a lower activation energy. The junctions between the cementite and the ferrite grain boundaries are the preferred austenite nucleation sites. Those ferrite boundaries far away from the cementite can only be invaded by austenite through carbon supplied by cementite dissolution and carbon segregation. When using a high heating rate, nucleation of austenite can occur in a massive-like manner, and the austenite transformation kinetics is remarkably accelerated. The bendability of DP steel is determined by a combination of structural parameters, such as grain size, volume fraction as well as spatial distribution of martensite. A short in-line holding-step during heating can alleviate the banding severity and improve the bendability of this ferrite-martensite DP steel.

Al-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al4Ca phases distributed as a network along the grain boundary and Al3(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al4Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.

We describe the analysis of in-situ HT-XRD data of a dual phase stainless steel exposed to a complex thermal cycle of heating, holding and cooling. For the conditions used only low quality diffraction data could be collected. Peak positions, peak areas and peak broadening are modeled by the Rietveld method. The low signal-to noise ratio and the presence of artificial peaks due to tube tails complicate the data evaluation. In a first attempt the parameters are refined by a local optimization procedure (e.g. Levenberg-Marquardt). However, this procedure fails by being caught in one of several local minima. Next, a Bayesian approach with a Markov Chain Monte Carlo (MCMC) algorithm is used as a global optimization procedure to refine the simulated Rietveld diffractograms. Accurate estimates of the evolution of the phase fractions and dislocation densities in martensite and austenite during all stages of the thermal cycle are obtained by this MCMC algorithm. While an approach based on multivariate second order Taylor series completely underestimates the error, the uncertainties in the model parameters could be estimated appropriately from histograms obtained by the MCMC method.

An experimental study on broadband noise scattered by permeable trailing edges with different pore arrangements is performed. A NACA 0018 airfoil with chord c = 0.2 m is investigated at chord-based Reynolds numbers ranging from 1.4 × 105 to 3.8 × 105 and angles of attack of 0.2 and 5.4 degrees. Noise emission from five 3D-printed perforated trailing-edge inserts, with channels normal to the chord, is measured with a microphone antenna. For comparison, inserts manufactured with metallic foams, with comparable flow permeability K but more tortuous pore paths, are also analysed. All the inserts have a permeable extension s equal to 20% of the chord (s/c = 0.2). It is shown that noise mitigation ΔLp, computed as the difference between far-field noise scattering from solid and permeable edges, collapse when nondimensionalizing frequency as Strouhal number based on the chord. From the collapsed data, it is observed that the maximum noise attenuation reported for each insert ΔLp,max reaches an asymptotic value of 9.3 dB for increasing K. To parameterize such asymptotic behaviour, noise reduction levels are fitted to a newly proposed relation ΔLp,max=γ1tanh(γ2K) (where γ1 and γ2 are fitting coefficients, that depend on the type of insert and angle of attack). Following this analysis, limit permeability values for perforated and metal foam inserts of K = 3.5 × 10−9 and 1 × 10−9 m2 are found, respectively; above these thresholds, less than 1 dB additional noise mitigation is reported; below, a difference in ΔLp,max of up to 4 dB for a given K is measured depending on the pore organization. Consequently, the tortuosity of the permeable structure is identified as an additional parameter (to K) controlling noise attenuation. It is also observed that the acoustic performance of lower-permeability edges is less sensitive to changes in the angle of attack. Tests for permeable lengths equal to s/c = 0.05 and 0.1 are performed: the change of ΔLp,max with increasing s/c is also properly described with a hyperbolic tangent, evidencing equally good performance in noise reduction for all measured extents. Finally, for the most permeable insert with periodic pore arrangement, an extremely loud tonal noise caused by vortex shedding (+30 dB higher than broadband levels) from the blunt solid-permeable junction at s/c=0.8 is reported. Since applying a longer permeable surface, or increasing the permeability at the trailing edge decreases the aerodynamic performance of the blade, a permeable trailing edge with s/c = 0.05, K = 1 × 10−9 m2 and tortuosity of 1.15 is recommended to optimize broadband noise abatement and avoid shedding-related tones for the conditions explored in the current study.

The degradation of creep resistance in Nickel-based single crystal superalloys is essentially ascribed to their microstructure evolution. Yet there is a lack of work that manages to simulate the effect of alloying element concentrations on microstructure degradation. In this research, a computational model is developed to connect the rafting kinetics of Ni superalloys with their chemical composition, by combining thermodynamics calculation and an energy-based microstructure model. The isotropic coarsening rate and γ/γ misfit stresses have been selected as composition related parameter, and the effect of service temperature, time and applied stress are also taken into consideration to simulate the evolutions of microstructure parameters during creep process. The different generations of commercial Ni superalloys are selected and their chemical compositions are calculated based on this model. The simulated microstructure parameters are validated by the results from experimental results and the existing analytical model. The capability of the model in predicting the microstructure characteristics may provide instructional thought in developing a novel computational guided design approach in Ni superalloys.

A computational fracture analysis is conducted on a self-healing particulate composite employing a finite element model of an actual microstructure. The key objective is to quantify the effects of the actual morphology and the fracture properties of the healing particles on the overall mechanical behaviour of the (MoSi2) particle-dispersed Yttria Stabilised Zirconia (YSZ) composite. To simulate fracture, a cohesive zone approach is utilised whereby cohesive elements are embedded throughout the finite element mesh allowing for arbitrary crack initiation and propagation in the microstructure. The fracture behaviour in terms of the composite strength and the percentage of fractured particles is reported as a function of the mismatch in fracture properties between the healing particles and the matrix as well as a function of particle/matrix interface strength and fracture energy. The study can be used as a guiding tool for designing an extrinsic self-healing material and understanding the effect of the healing particles on the overall mechanical properties of the material.

A wedge loaded testing methodology to determine the fracture energy and strength of (semi-) brittle (metallo-)ceramics is presented. The methodology combines a tailored specimen geometry and a comprehensive finite element analysis based on cohesive zone modelling. The use of a simulation-based approach to extract both fracture strength and energy from experimental data avoids the inherent inaccuracies found in closed-form expressions that rely on a priori assumptions about the deformation field. Results from wedge splitting tests on Ti3SiC2 and Ti2AlC (MAX phase) materials are used to illustrate the procedure. The simulation-based approach is further validated by comparing the fracture strength and fracture energies predicted by the proposed method and those indicated by a conventional four-point bending fracture toughness test (ASTM standard). The new protocol offers the possibility to measure not only the fracture properties of brittle material in its pristine state but also in the healed state.

A cohesive zone-based constitutive model, originally developed to model fracture, is extended to include a healing variable to simulate crack healing processes and thus recovery of mechanical properties. The proposed cohesive relation is a composite-type material model that accounts for the properties of both the original and the healing material, which are typically different. The constitutive model is designed to capture multiple healing events, which is relevant for self-healing materials that are capable of generating repeated healing. The model can be implemented in a finite element framework through the use of cohesive elements or the extended finite element method (XFEM). The resulting numerical framework is capable of modeling both extrinsic and intrinsic self-healing materials. Salient features of the model are demonstrated through various homogeneous deformations and healing processes followed by applications of the model to a self-healing material system based on embedded healing particles under non-homogeneous deformations. It is shown that the model is suitable for analyzing and optimizing existing self-healing materials or for designing new self-healing materials with improved lifetime characteristics based on multiple healing events.

The effect of particle size on the oxidation kinetics of TiC powders is studied. Different sizes of TiC powder ranging from nanometre to submillimetre sizes are investigated. The samples are heated at different heating rates from room temperature up to 1200 °C in dry synthetic air. The Kissinger method for analysis of non-isothermal oxidation is used to estimate the activation energy for oxidation of the powders and to identify the active temperature window for efficient self-healing. The master curve plotting method is used to identify the model which best describes the oxidation of TiC powders, and the Senum and Yang method is used to approximate the value for the Arrhenius constant. The oxidation of TiC proceeds via the formation of oxycarbides, anatase and then finally the most stable form: rutile. The activation energy is found to be a strong function of the particle size for particle sizes between 50 nm and 11 µm and becomes constant at larger particle sizes. The data demonstrate how the minimal healing temperature for oxide ceramics containing TiC as healing particles can be tailored between 400 and 1000 °C by selecting the right average TiC particle size.

The transition temperature between upper bainite and lower bainite is calculated with an extended Gibbs energy balance model, which is able to quantitatively describe the evolution of carbon supersaturation within bainitic ferrite sheaves during the entire thickening process. The nucleation rate of intra-lath cementite precipitation on a dislocation is calculated based on of the degree of carbon supersaturation. Upper bainite and lower bainite are thus distinguished by the effective nucleation density and therefore a numerical criterion can be set to define the transition. The model is applied to Fe-xC-1Mn/2Mn/1Mo ternary alloys. Results show that the transition temperature increases with bulk carbon content at lower carbon concentration but decreases in the higher carbon region. This prediction agrees very well with the experimental observations in Mn and Mo alloyed systems. Moreover, the highest transition temperature and the carbon content at which it occurs in the Fe-xC-2Mn system are in good agreement with reported experimental data. The inverse “V” shaped character of the carbon concentration–transition temperature curve indicates two opposite physical mechanisms operating at the same time. An analysis is carried out to provide an explanation.

The degradation of creep resistance in Ni-based single-crystal superalloys is essentially ascribed to their microstructural evolution. Yet there is a lack of work that manages to predict (even qualitatively) the effect of alloying element concentrations on the rate of microstructural degradation. In this research, a computational model is presented to connect the rafting kinetics of Ni superalloys to their chemical composition by combining thermodynamics calculation and a modified microstructural model. To simulate the evolution of key microstructural parameters during creep, the isotropic coarsening rate and γ/γ′ misfit stress are defined as composition-related parameters, and the effect of service temperature, time, and applied stress are taken into consideration. Two commercial superalloys, for which the kinetics of the rafting process are selected as the reference alloys, and the corresponding microstructural parameters are simulated and compared with experimental observations reported in the literature. The results confirm that our physical model not requiring any fitting parameters manages to predict (semiquantitatively) the microstructural parameters for different service conditions, as well as the effects of alloying element concentrations. The model can contribute to the computational design of new Ni-based superalloys.

In the present work, three different scratch tests are compared on their ability to rank the abrasion resistance of low alloy steels for industrial applications where the abrasion play a key role, e.g., in earthmoving, agricultural and mining equipment. The first test involves single pass scratching of pristine surfaces with a relatively large rigid indenter. The second test involves multi-pass scratching along a fixed track using the same large indenter. The third test involves the creation of a multi-pass scratch track using the same large indenter followed by final scratching of the abrasion track with a sharp indenter, i.e., Multi-Pass Dual Indenter (MPDI) scratch test. The three test protocols activate different abrasion mechanisms. Five low alloy construction steel grades with different strain hardening capabilities, i.e., Interstitial-free Ferritic steel (IF steel), Fully Martensitic steel (FM steel), Dual Phase steel (DP steel), Quench Partitioning steel (Q&P steel) and TWining Induced Plasticity steel (TWIP steel), are used. The results show that for both single (large) indenter scratch test protocols, the scratch depths always increase with indenter load and the failure mechanism is pure ploughing except for the IF steel due to the nature of softness. While for the dual-indenter scratch test, the scratch depth is a more complex function of the load applied during creation of the work hardened surface layer. The conventional scratch test protocols cannot well reveal micro-cracks and defect in (sub) surface and cannot well reflect the real response of strain hardening of steels. However, the MPDI test can distinguish between different steels with different initial hardness and different strain hardening behaviour and reveal the damage and, equally important, can indicate how average loading conditions may affect the relative abrasion resistance of construction steels during steady state conditions.

Self healing of early stage radiation damage by site selective solute segregation is a promising approach to extend the lifetime of nuclear reactor components. In the present study, the creation and autonomous healing of irradiation-induced damage is investigated in pure Fe and high purity Fe–Au and Fe–Cu model alloys. To create radiation damage samples are irradiated at 550 °C by 120 keV He+ ions with fluences of 5.0 × 1015, 1.0 × 1016 and 5.0 × 1016 ions/cm2. The observed increase in the S and W parameters determined in the variable energy positron annihilation spectroscopy measurements indicates the formation of vacancy-like defects, precipitates and vacancy-solute complexes. The presence of substitutionally dissolved Au is found to reduce the formation of radiation defects more efficiently than solute Cu. Site-specific Au precipitation at defect sites is indicated, which results in damage healing with a reduced swelling, whereas Cu precipitates and radiation damage only show weak interaction. Ab initio calculations show that the binding energies of Au solutes to vacancy clusters (Au-Vn) are significantly larger than those of Cu solutes (Cu-Vn) whereas the binding energies of helium filled vacancy clusters Au-HenVn and Cu-HenVn are comparable.

Multi-pass-dual-indenter (MPDI) scratch tests with various loading conditions were performed on two newly developed titanium alloys (Ti–10V–1Fe–3Al and Ti–10V–2Cr–3Al) to investigate their abrasion resistance under repetitive local sliding contact. A technically pure titanium sample was used as the reference. Various microstructures were established by different heat treatments, such as to turn the β-phase into a stable phase or a meta-stable phase showing Stress Induced Martensite (SIM) formation. The influence of phase evolution on the scratch resistance and corresponding failure mechanisms was unravelled. It was found that the phase morphology and fraction have a significant impact on the scratch resistance and that effect is applied load dependent. The scratch behaviour is closely related to the work hardening ability of the material surface especially at high loading conditions, while the original surface hardness is more relevant at low loading conditions. The observations definitely prove that, not withstanding the modest hardness level, a microstructure showing a combination of metastable β (trigger the stress-induced martensitic transformation) and flake α (enhancing the initial surface hardness) is the best route to improve the scratch resistance for these two titanium alloys, in particular for high load conditions.

The turbulent flow over a porous trailing edge of a NACA 0018 airfoil is experimentally investigated to study the link between the hydrodynamic flow field and the acoustic scattering. Four porous trailing edges, obtained from open-cell metal foams, are tested to analyze the effects on far-field noise of the permeability of the material and of the hydrodynamic communication between the two sides of the airfoil. The latter is assessed by filling the symmetry plane of two of the porous trailing edges with a thin layer of adhesive that acts as a solid membrane. Experiments are performed at a zero degree angle of attack. Far-field noise measurements show that the most permeable metal foam reduces noise (up to 10 dB) with respect to the solid trailing edge for Strouhal numbers based on the chord below 16. At higher nondimensional frequencies, a noise increase is measured. The porous inserts with an adhesive layer show no noise abatement in the low frequency range, but only a noise increase at higher frequency. The latter is, therefore, attributed to surface-roughness noise. Flow field measurements, carried out with time-resolved planar particle image velocimetry, reveal correlation of near-wall velocity fluctuations between the two sides of the permeable trailing edges only within the frequency range where noise abatement is reported. This flow communication suggests that permeable treatments abate noise by distributing the impedance jump across the foam in the streamwise direction, promoting noise scattering from different chordwise locations along the inserts. This is further confirmed by noise source maps obtained from acoustic beamforming. For the frequency range where noise reduction is measured, the streamwise position of the main noise emission depends on the permeability of the insert. At higher frequencies, noise is scattered from upstream the trailing edge independently of the test case, in agreement with the roughness-generated noise assumption.

BiFeO3 is an interesting multiferroic material with potential use in sensors and transducers. However, the high coercive field and low dielectric strength of this material make the poling process extremely difficult. Poling becomes a lot easier if the ceramic particles are incorporated in a non-conductive polymer with comparable dielectric properties. In this work, unstructured composites consisting of BiFeO3 particles in a non-piezoactive PVDF terpolymer matrix are made with a ceramic volume fraction ranging from 20% to 60%. The highest piezoelectric charge and voltage constant values (d33 = 31 pC/N and g33 = 47 mV m/N) are obtained for a BiFeO3-PVDF terpolymer composite with a volume fraction of 60%. The Poon model is chosen to analyse the volume fraction dependence of the dielectric constant while the modified Yamada model is used to analyse the piezoelectric charge constant data. It is concluded that the maximum possible piezoelectric constant for bulk BiFeO3 can be as high as 56 pC/N.

In the literature many researchers have tried to correlate the (dry) abrasion resistance of steels to their standard mechanical properties as determined in simple tensile and hardness testing. However, many unclear or even conflicting correlations have been reported, in part due to small data sets being used, different testing methods and relatively simple data processing techniques. In the present work 40 different low alloy steels were tested using the same ASTM G65 abrasion test and standard mechanical testing. The data set consists of 20 samples having the same chemical composition yet processed to different microstructures and different mechanical properties, as well as 20 steel grades with various compositions but having mechanical properties comparable to the first data set. The results show that for steels of the same composition significantly higher correlations between the abrasion resistance and certain mechanical properties (in particular strength coefficient K , hardness and UTS) are observed than those for steels having different compositions. In case both data sets were combined and a new regression analysis was applied, the quality of the correlation dropped significantly and the correlations became barely statistically relevant. Furthermore, when taking into account the measurement errors in both independent and dependent variables, the correlation coefficient became even worse and no statistically relevant correlation was observed at all. The current study clearly casts doubts on the validity of reported correlations between material (mechanical) properties and abrasion resistance.

Over the past decades, the mechanism of interface migration during the austenite-ferrite transformation in steels has attracted significant attention from physical metallurgists. There are two challenging research questions in this field: (i) What is the effect of (substitutional) alloying elements on migrating interfaces? and (ii) How to accurately determine the value of interface mobility?. Recently, a cyclic partial phase transformation approach has been proposed to study interface migration, and new insights into the above two questions have been provided. An overview of the cyclic partial phase transformation concept is given, and pathways for future research are suggested.

An experimental investigation of the scratch and abrasive wear behaviour of a lean C–Mn construction steel in its tempered fully martensitic (TM) state is presented. The scratch resistance and the corresponding failure mechanisms as a function of the tempering temperature (200–500 °C) were evaluated using a multi-pass dual-indenter (MPDI) scratch test applying different loading conditions. Results show that the scratch resistance depends not only on the tempering temperature, but also on the load applied during scratching. The optimal tempering temperature depends on the applied load. For both low and high loading conditions, the dual phase (ferrite–martensite) variant with an optimised martensite volume fraction and morphology yields an even better combination of scratch/abrasion resistance and hardness. The scratch resistance at different loading conditions is linked to the strength coefficient K in the Hollomon equation ( σ = K e n ). The scratch behaviour in the MPDI scratch test at a low load correlates quite well with the standard ASTM G65 multi-particle abrasion test.

The present work describes a model study to explore the possibility to heal early stage erosion damage in Cr 2 AlC MAX phase when exposed to high air temperatures and erosive conditions. Such a healing reaction should lead to a reduction of the wear rate of this promising material for application in jet turbine engines. To this aim Cr 2 AlC ceramic disks were subjected to room temperature erosion for 60 min using glass microbeads accelerated to 110 m/s and impinging perpendicular to the sample surface. After the usual incubation time, the erosion rate reaches a constant rate, which is associated with the formation of network of small cracks underneath the surface. Next, the material was annealed at 1200 °C for 10 min in air resulting in filling of the network of small cracks due to the formation of well-adhering Al 2 O 3 . The subsequent erosion rate of the healed Cr 2 AlC ceramic at room temperature is drastically reduced. Once the healed zone is removed by erosion the erosion rate attained its original value. Clearly, exposure to high temperature oxidative conditions extends the lifetime of Cr 2 AlC MAX phase components subjected to erosive conditions.

Over many decades, significant efforts have been made to improve the strength-elongation product of advanced high strength steels (AHSSs) by creating tailored multi-phase microstructures. Successive solid-state phase transformations for steels with a well selected chemical composition turned out to be the key instrument in the realisation of such microstructures. In this contribution, we first provide a brief review of the desired microstructures for Transformation-induced plasticity (TRIP), Carbide-free Bainitic (CFB), Quenching & Partitioning (Q&P) and Medium Manganese steels followed by comprehensive discussions on the phase transformations to be used in their creation. The implications for the steel composition to be selected are addressed too. As the presence of the right amount and type of metastable retained austenite (RA) is of crucial importance for the mechanical performance of these AHSSs, special attention is paid to the important role of successive solid-state phase transformations in creating the desired fraction and composition of RA by suitable element partitioning (in particular C and Mn). This critical partitioning not only takes place during final cooling (austenite decomposition) but also during the back transformation (austenite reversion) during reheating. This review aims to be more than just descriptive of the various findings, but to present them from a coherent thermodynamic / thermo-kinetic perspective, such that it provides the academic and industrial community with a rather complete conceptual and theoretical framework to accelerate the further development of this important class of steels. The detailed stepwise treatment makes the review relevant not only for experts but also metallurgists entering the field.

The hydrodynamic and acoustic fields for a NACA 0018 with solid and porous trailing edge inserts are investigated. The porous inserts, covering 20% of the chord, are manufactured with metal foams with cell diameters of 450 and 800 μm and permeability values of 6 × 10−10 and 2.7 × 10−9 m2. The experiments are performed at a chord-based Reynolds number of 2.63 × 105 and an angle of attack of 0°. The porous trailing edge with higher permeability provides up to 11 dB noise attenuation with respect to the solid case for frequencies below a cross-over Strouhal number St = 0.26. Lower noise abatement (up to 7 dB) takes place below St = 0.3 for the insert with lower permeability. Conversely, noise increase with respect to the solid case is measured above the previously defined St value. A decrease in turbulence intensity is reported (up to 3% of the free-stream velocity), with lower intensity being measured for the insert with lower permeability. It is also observed that the permeability of the insert is linked to the increase of the anisotropy of highly energetic turbulent motions, being stretched in the streamwise direction, and the reduction of the eddy convection velocity (up to 20% with respect to the baseline case). In view of the results, the reduction of the velocity fluctuations is proposed as one of the mechanisms for low frequency noise abatement, being more relevant for the metal foam insert with lower permeability.

The performance of a self-healing Thermal Barrier Coating (TBC) containing dispersed healing particles depends crucially on the mismatch in thermomechanical properties between the healing particles and the TBC matrix. The present work systematically investigates this phenomenon based on numerical simulations using cohesive element-based finite element analysis. The effect of the mismatch in Coefficient of Thermal Expansion (CTE) and fracture strength between the healing particles and the matrix on the fracture characteristics is quantified in detail. Unit cell-based analyses are conducted on a representative self-healing TBC system under a thermal loading step typically experienced by TBC systems in jet turbines. Two different simulation setups are considered within the TBC unit cell namely (i) a single pair of healing particles and (ii) a randomly distributed array of healing particles. The results of the simulations are reported in terms of the fracture pattern, crack initiation temperature and crack length for various CTE mismatch ratios. Correlations are established between the results obtained from the two simulation setups essentially revealing the effect of spatial distribution and proximity of healing particles on the fracture pattern. The results obtained from the analyses can be utilised to achieve a robust design of a self-healing TBC system. Keywords: Thermal barrier coatings, Cohesive elements, Healing particles, Thermal mismatch, Fracture mechanics

In this paper a novel numerical model is proposed for qualitative simulation of the structure formation during dielectrophoresis in a medium of dielectric particles suspended in a liquid between parallel-plate electrodes. Upon application of an electric field particles reorient with respect to the imposed electric field, followed by rotational and axial displacement of particles interaction until chains are formed. The performance of the model is illustrated in a number of fundamental cases. The influence of parameters such as size, aspect ratio and heterogeneity of the particles is studied for the purpose of obtaining insight in the ideal conditions required to obtain the desired structure. In a multi-particle system, the relative particle size is shown to be a key parameter in chain evolution process. The quality of the structure evolution is investigated using a set of geometric parameters. A new topological parameter, chain perfection degree, relating the microstructure to the overall performance of the piezoelectric composite, is proposed and calculated for the fundamental case-studies.

A systematic experimental investigation concerning the effect of ferrite–martensite morphology on the scratch and abrasion resistance of ferrite–martensite dual phase (DP) steels is reported. A hot rolled 22MnB5 steel was subjected to different heat treatments to generate dual phase microstructures with different ferrite–martensite morphologies. The effects of morphology on the scratch resistance and the corresponding failure mechanisms were unravelled using a multi-pass dual-indenter (MPDI) scratch test applying different load combinations. Results show that the ferrite–martensite morphology has a significant influence on scratch resistance and that the effect is contact load dependent. The scratch behaviour is linked to the strength coefficient K in the Hollomon model ( σ = K e n ) as well as the initial indentation hardness. Results suggest that the strength coefficient K corresponds well with the scratch resistance. The optimal microstructure to yield the best combination of abrasion resistance and hardness depends on the working conditions. At a low loading condition the relative ranking of the scratch resistance of the various microstructures created is in good agreement with that of the ASTM G65 abrasion test.

A systematic experimental investigation was conducted to study the effect of the martensite volume fraction on the scratch and abrasion resistance of ferrite–martensite dual phase (DP) steels. Dual phase microstructures with relatively high martensite fractions were generated by tailored intercritical annealing treatments. The scratch resistance and corresponding failure mechanisms were evaluated using a multi-pass dual-indenter (MPDI) scratch test for different loading conditions. Results show that the scratch resistance depends on not only the volume fractions of each phase, but also the loading conditions employed. Under low load conditions, the scratch depth decreases with increasing martensite fraction, in line with the increasing hardness evolution. However, under high load conditions, the scratch depth initially decreases with increasing martensite fraction up to a critical value (optimal fraction), beyond which the scratch depth starts to rise. This optimal fraction displays the best scratch resistance despite of a lower hardness compared to that of the full martensite. The scratch behaviour was correlated to the tensile behaviour and the corresponding work hardening characteristics. A two-stage strain hardening model was applied to interpret the scratch behaviour at different loads. The scratch behaviour at low load was found to correlate well with the standard ASTM G65 abrasion test.

The classical Johnson–Mehl–Avrami–Kolmogorov equation was modified to take into account the normal local strain distribution in deformed samples. This new approach is not only able to describe the influence of the local heterogeneity of recrystallization but also to produce an average apparent Avrami exponent to characterize the entire recrystallization process. In particular, it predicts that the apparent Avrami exponent should be within a narrow range of 1 to 2 and converges to 1 when the local strain varies greatly. Moreover, the apparent Avrami exponent is predicted to be insensitive to temperature and deformation conditions. These predictions are in excellent agreement with the experimental observations on static recrystallization after hot deformation in different steels and other metallic alloys.

Multi-pass dual-indenter (MPDI) scratch tests were carried out on a wide range of steels with different microstructures to investigate the relation between the formation of subsurface deformation layers under repetitive local sliding contact and their abrasion resistance. Five steel grades with different work hardening capability, i.e. Interstitial-Free steel, Fully Martensitic steel, Dual Phase steel, Quench Partitioning steel and TWining Induced Plasticity steel were selected. The results of this well-defined local contact damage test method and those of the standard ASTM G65 dry sand rubber wheel abrasion test were correlated. Results suggest that the abrasion resistance of steels is controlled by the properties of the work hardening layer formed beneath the abraded surface and depends on the thickness of the work hardening layer. When applying the correct evaluation parameters the new scratch test MPDI methodology reproduces the material response to the ASTM G65 abrasion test rather well and provides a reproducible and quantitative method to screen the abrasion resistance of new construction steels. Moreover, the MPDI test allows to quantitatively evaluate the abrasion resistance of steels for working conditions beyond those of the standardized G65 test.

Recently, the Laves phase precipitate family has been identified as a promising alternative for Precipitation Hardening (PH) in ferritic creep resistant steels owing to its significant contribution to creep strength. A computational alloy design approach to create novel ferritic creep resistant steels strengthened by optimized Laves phase is presented. This novel approach aims at a simultaneous optimization of the steel’s chemical composition involving 9 chemical elements to obtain a ferritic steel optimally strengthened by Laves phases with an ideal combination of a high volume fraction and a reduced coarsening rate at the intended service temperature. Using this approach, three alloys possessing ideal PH contributions at their intended service temperatures, i.e. 650, 700 and 750 C, respectively, are designed. This approach is validated by analyzing PH contributions in existing 15Cr ferritic steels at different service temperatures. Good agreement between reported experimental performance and model predictions is found. The newly designed alloys are predicted to have a substantially higher PH contribution at corresponding service temperature than those of the existing alloy. While Laves phases generally precipitate slowly during service and have a more significant strengthening effect at the late stage, the precipitation of the alternative M23C6 precipitates can be used to increase the initial strength of ferritic steel. Hence, our computational approach was extended to aim for a high strengthening contribution at both initial and late stages at the intended service temperature.

The present paper focuses on the Spark Plasma Sintering (SPS) manufacturing of a new type of self-healing thermal barrier coating (TBC) and a study of its thermal cycling behaviour. The ceramic coating consists on an Yttria Partially Stabilized Zirconia (YPSZ) matrix into which healing agents made of MoSi2-Al2O3 core-shell particles are dispersed prior to sintering. The protocol used to sinter self-healing TBCs on MCrAlY (M: Ni or NiCo) pre-coated Ni-based superalloys is described and the reaction between the particles and the MCrAlY bond coating as well as the preventive solutions are determined. Thermal cycling experiments are performed on this complete multilayer system to study the crack healing behaviour. Post-mortem observations highlighted local healing of cracks due to the formation of silica and the subsequent conversion to zircon at the rims of the cracks. Keywords: Spark plasma sintering, Self-healing thermal barrier coatings, High-temperature cyclic oxidation, Intermetallic particles, Multi-layer materials, Ceramic matrix composites

Piezoelectric energy harvesters are at the front of scientific research as enablers of renewable, sustainable energy for autonomous wireless sensor networks. Crucial for this disruptive technology is the achievable output power. Here we show, analytically, that the maximum output energy per unit volume, under a single sinusoidal excitation, is equal to 1/(4 - 2k2) × 1/2dgX2, where k2 is the electromechanical coupling coefficient, d and g are the piezoelectric charge and voltage coefficient, respectively, and X is the applied stress. The expression derived is validated by the experimentally measured output energy for a variety of piezoelectric materials over an unprecedented range of more than five orders of magnitude. As the prefactor 1/(4 - 2k2) varies only between 1/2 and 1/4 the figure of merit for piezoelectric materials for energy harvesters is not k2, as commonly accepted for vibrational harvesters, but dg. The figure of merit does not depend on the compliance, or Young's modulus. Hence we argue that commonly used brittle inorganic piezoelectric ceramics can be replaced by soft, mechanically flexible polymers and composite films, comprising inorganic piezoelectric materials embedded in a polymer matrix.

Formation of the microstructural ferrite/pearlite bands in medium Mn steels is an undesirable phenomenon commonly addressed through fast cooling treatments. In this study, a novel approach using the cyclic partial phase transformation concept is applied successfully to prevent microstructural band formation in a micro-chemically banded Fe-C-Mn-Si steel. The effectiveness of the new approach is assessed using the ASTM E1268-01 standard. The cyclic intercritical treatments lead to formation of isotropic microstructures even for cooling rates far below the critical one determined in conventional continuous cooling. In contrast, isothermal intercritical experiments have no effect on the critical cooling rate to suppress microstructural band formation. The origin of the suppression of band formation either by means of fast cooling or a cyclic partial phase transformation is investigated in detail. Theoretical modeling and microstructural observations confirm that band formation is suppressed only if the intercritical annealing treatment leads to partial reversion of the austenite-ferrite interfaces. The resulting interfacial Mn enrichment is responsible for suppression of the band formation upon final cooling at low cooling rates.

While natural materials such as bone or skin are able to heal themselves in an autonomous manner after mechanical damage, traditional man-made materials generally lack this intrinsic capacity for self-healing. In 1969, self-healing properties were for the first time built-in inside polymeric materials[1] and in the following decades publications about self-healing in thermoplastic and cross-linked systems and concrete appeared.[2] Although, it was only in 2001, through an article about self-healing in polymer based materials published in Nature, that research in the field of self-healing materials research was really triggered.[3] By restoring their functional properties autonomously after damage, self-healing materials would offer tremendous advantages over traditional materials in application areas such as civil, chemical, electrical, aerospace, automotive, and biomedical engineering. Autonomous self-healing does not require an external trigger (like heat or irradiation), whereas non-autonomous self-healing only occurs in response to such external triggers.[4] Another classification of self-healing materials is based on the incorporation of separate healing agents (extrinsic self-healing) as opposed to intrinsic self-healing which proceeds without the need to include separate healing agents.[4] Irrespective of their classification, self-healing of materials requires the creation of a local mobile phase which can close and repair cracks by physical flow into the crack and subsequent reformation of chemically stable interfaces.

A 3D model has been developed to predict the average ferrite grain size and grain size distribution for an austenite-to-ferrite phase transformation during continuous cooling of an Fe-C-Mn steel. Using a Voronoi construction to represent the austenite grains, the ferrite is assumed to nucleate at the grain corners and to grow as spheres. Classical nucleation theory is used to estimate the density of ferrite nuclei. By assuming a negligible partition of manganese, the moving ferrite–austenite interface is treated with a mixed-mode model in which the soft impingement of the carbon diffusion fields is considered. The ferrite volume fraction, the average ferrite grain size, and the ferrite grain size distribution are derived as a function of temperature. The results of the present model are compared with those of a published phase-field model simulating the ferritic microstructure evolution during linear cooling of an Fe-0.10C-0.49Mn (wt pct) steel. It turns out that the present model can adequately reproduce the phase-field modeling results as well as the experimental dilatometry data. The model presented here provides a versatile tool to analyze the evolution of the ferrite grain size distribution at low computational costs.

A proper understanding of abrasion resistance and associated damage mechanisms is of vital importance in the design of improved abrasion resistant construction steels. The conventional scratch test, sliding a rigid indenter under a controlled load and speed against a smooth surface, mimics the nature of the abrasion process and can be used to evaluate the abrasion resistance of various microstructures. However, scratch tests are mostly done on the initial surface, which can be very different from that formed during the abrasion process and hence do not truly reflect its abrasion resistant response. In the present work, a new scratch test methodology is developed to approach the real abrasion condition by carrying out a multi-pass dual indenter scratch tests, in which a small indenter scratches a pre-scratched surface produced by a large indenter. Five steel grades with different work hardening capacities, i.e. Interstitial-Free Ferritic steel (IF steel), Fully Martensitic steel (FM steel), Dual Phase steel (DP steel), Quench Partitioning steel (Q&P steel) and TWining Induced Plasticity steel (TWIP steel) were selected. Systematic scratch resistance experiments were performed to investigate the damage mechanisms, the work hardening behavior and the development of subsurface deformation layers. Results suggest that the work hardening layer formed beneath the abraded surface plays the dominant role in determining the abrasion resistance. Steels grades of DP, Q&P and TWIP display superior scratch and abrasion resistances, notwithstanding their relative low hardness compared to that of a corresponding steel with a fully martensitic microstructure.

A comparative study is performed on the monitoring of delamination healing in CFRP-ionomer sandwich composites by non-destructive techniques and destructive compression testing. Artificial delaminations of various areal dimensions and nature were introduced during production of the composites. The extent of the delamination and the healing thereof was monitored in both air and water-coupled ultrasonic C-scan experiments as well as by the frequency shift of the local defect resonance (LDR). It is shown that the LDR approach can be used to detect the early stage healing of the delaminations while ultrasonic C-scanning techniques are very effective to determine the extent of healing in the final stages of the repair process. A quasi-linear relation was observed between the delaminated area measured with ultrasonic C-scan and the compressive failure strength in destructive testing. This correlation shows the beneficial effect on the compression strength of the delaminated area reduction by on-demand healing.