Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure ... more Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475 • C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes in Volta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility.
Micro-scale damage with a topographical contrast has been observed in cold-worked Nb-stabilised 2... more Micro-scale damage with a topographical contrast has been observed in cold-worked Nb-stabilised 20%Cr-25% Ni stainless steel, following irradiation with 2.2 MeV protons at 400°C and a dose rate of ∼10 −5 dpa/s. After an irradiation dose of 3 and 5 dpa, microstructural changes were found to a depth of 22 μm below the irradiated surface, coincident with the estimated mean range of protons in the material as predicted by the TRIM code. Large variations of intragranular mis-orientations, in combination with a depletion of chromium at grain boundaries, were observed in the proton-irradiated layer. This was accompanied by an increase in nano-hardness, which was linked to a partial transformation of tangled dislocation networks into dislocation loops. Such visible microstructural changes have not been reported before in the absence of post-irradiation annealing treatments.
Pitting corrosion kinetics of type 304L stainless steel have been obtained using a quasi in-situ ... more Pitting corrosion kinetics of type 304L stainless steel have been obtained using a quasi in-situ X-ray computed tomography (X-ray CT) approach. A miniature electro-chemical cell was constructed to allow imaging during potentio-dynamic polarization of a wire specimen. The formation of three discrete pits was observed, allowing comparison between real pit geometry and different geometrical assumptions to estimate pit growth kinetics. The pit volumes obtained by X-ray CT showed good fit with the volume of metal dissolution calculated using Faraday's law. Large fluctuations of the mean current density were observed during the pit nucleation stage, followed by pit growth with mean current densities of 1-3 A.cm −2. Stability products associated with these pits were on the order of 0.3-0.6 A.m-1 , with diffusivity parameter (D. C) of 1.68-3.04 × 10 −8 mol.cm −1 .s −1. Diffusion coefficients for stable pit growth of 0.83-0.96 × 10 −5 cm 2 .s −1 were estimated for metal ion concentrations of 4.2 M.
Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-ch... more Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-chemical polarisation has been observed in a type 304L stainless steel using X-ray computed tomography. The pit reactivation process was associated with the formation of a new pit, directly adjacent to a pre-existing pit. Pit growth kinetics were estimated, revealing an increase of the diffusivity parameter ( D eff Δ C ) from 3.0 × 10 −8 mol cm −1 s −1 to 4.5 × 10 −8 mol cm −1 s −1 with the application of strain, indicating higher metal dissolution rates. Applied strain resulted in fractured lacy metal covers, and its effect on pit growth kinetics is discussed.
Abstract Bipolar electrochemistry was applied to determine the full spectrum of anodic-to-cathodi... more Abstract Bipolar electrochemistry was applied to determine the full spectrum of anodic-to-cathodic polarisation behaviour of stainless steel. The potential and current distributions at the sample surface were determined using a segmented array bipolar electrode (BPE). The measured potential shows a quasi-linear gradient along the centre of the BPE, with the current following an exponential Butler–Volmer-type relationship. Bipolar electrochemistry is compared to conventional 3-electrode polarisation testing, allowing determination of the critical pitting potential, general corrosion rates and the cathodic current response. The application of bipolar electrochemistry is demonstrated via measurement of pit growth kinetics in ferritic stainless steel.
Abstract Meso-structural models are presently in common use for analysis of the mechanical behavi... more Abstract Meso-structural models are presently in common use for analysis of the mechanical behaviour, damage evolution and failure of concrete. These are constructed either from XCT images, or in silico, using statistical information about concrete’s meso-scale constituents. As a minimum, such models include mortar and aggregates as separate phases, while more detailed versions consider the interfacial transition zones (ITZ) between mortar and aggregates, and voids. Analyses of given meso-structures vary further by different constitutive modelling of constituents, with past works focusing on parameters’ calibration using experiments with one loading condition - either tension or compression. Using a detailed meso-structure representation, this work proposes a novel combination of constitutive relations, involving concrete damage plasticity (CDP) for mortar and cohesive zone behaviour for ITZ. CDP parameters are calibrated using both compression and tension experiments with mortar samples. ITZ parameters are calibrated by comparing simulated stress–strain curves and failure patterns with data from compression and tension experiments with concrete samples. This process leads to constitutive relations, applicable to both loading conditions, which has not been demonstrated previously, but is essential to extending the modelling approach to complex stress states encountered in real engineering structures. After establishing the realism of the approach, parametric studies are conducted to investigate the effects of friction between loading plate and specimen, the mortar dilation angle, the ITZ cohesive stiffness, critical stresses, fracture energy and mix mode ratio. The results show that mortar plasticity is the dominant energy dissipation mechanism in both compression and tension, and its rate governs the localisation of damage. The effect of ITZ parameters on the tensile behaviour is found to be negligible. Their effect on the compressive behaviour is found to be limited, but sufficient to propose a set of parameters working for both conditions. Importantly, under both loadings the ITZ is found to control failure localisation into a macroscopic crack in combination with mortar plasticity and damage. Predicted stress-stain curves, damage evolution and macro-crack propagation are shown to be in very good agreement with the experimental observations. This justifies the use of the proposed experimental-modelling strategy for developing models for analysis of complex loading conditions.
Modelling of concrete at the meso-scale provides an effective way to analyse the effects of its c... more Modelling of concrete at the meso-scale provides an effective way to analyse the effects of its constituents on damage initiation and evolution, leading to better understanding and predicting structural integrity. Majority of works to date focus on models calibration and validation with experiments in either tension or compression, leaving open the question of how such models perform under complex stress states. This work presents a modelling approach that includes all key constituents of the concrete meso-structure: coarse aggregates, represented by inclusions with elastic-brittle behaviour, mortar (including cement, sand and fine aggregates), represented with plastic-damage behaviour, interfacial transition zones (ITZ) between aggregates and mortar, represented by zerothickness cohesive interfaces, and air voids or pores. Tension and compression experiments with mortar specimens are conducted to obtain its plastic-damage constitutive law. Similar experiments with concrete with several aggregate volume fractions are conducted to obtain stress-strain behaviours for further calibration of cohesive laws and model validation. Numerical simulations show that the proposed approach with pre-calibration of mortar behaviour leads to very good agreements between the predictions of the concrete meso-structural models and the experimental results under both tension and compression. The calibration of ITZ cohesive laws is performed by a parametric study of the effects of critical stress and fracture energy on the predicted stress-strain curves and fracture patterns. The results are used to propose a practical set of ITZ cohesive parameters.
Corrosion rates of strained grade UNS S32202 (2202) and UNS S32205 (2205) duplex stainless steel ... more Corrosion rates of strained grade UNS S32202 (2202) and UNS S32205 (2205) duplex stainless steel wires have been measured, in situ, using time-lapse X-ray computed tomography. Exposures to chloride-containing (MgCl2) atmospheric environments at 50 °C (12–15 M Cl− and pH ~5) with different mechanical elastic and elastic/plastic loads were carried out over a period of 21 months. The corrosion rates for grade 2202 increased over time, showing selective dissolution with shallow corrosion sites, coalescing along the surface of the wire. Corrosion rates of grade 2205 decreased over time, showing both selective and pitting corrosion with more localised attack, growing preferentially in depth. The nucleation of stress corrosion cracking was observed in both wires.
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-ch... more Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-chemical polarisation has been observed in a type 304L stainless steel using X-ray computed tomography. The pit reactivation process was associated with the formation of a new pit, directly adjacent to a pre-existing pit. Pit growth kinetics were estimated, revealing an increase of the diffusivity parameter ( D eff Δ C ) from 3.0 × 10 −8 mol cm −1 s −1 to 4.5 × 10 −8 mol cm −1 s −1 with the application of strain, indicating higher metal dissolution rates. Applied strain resulted in fractured lacy metal covers, and its effect on pit growth kinetics is discussed.
Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure ... more Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475 • C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes in Volta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility.
This paper reports on the distribution and precipitation properties of interfaces in two austenit... more This paper reports on the distribution and precipitation properties of interfaces in two austenitic stainless steels as a function of thermomechanical processing. Specimens of type 316 stainless steel were aged at 750° for one week, and the ensuing intergranular precipitation was assessed. Specimens of type 304 stainless steel were processed by a single step of 5% tensile strain followed by annealing at temperatures between 950°C and 1050°C. Selected data sets from both the 316 and 304 specimens were investigated by the new ‘five-parameter’ analysis method, which allows all five parameters of a grain boundary, i.e. both the misorientation and the boundary plane, to be determined. The distributions obtained showed significant differences between the two specimens, which were explained and discussed with reference to the processing routes. Some of the data were compared with a parallel set of experiments where the deformation had been applied by cold rolling. There were differences in...
opened a general discussion of the paper by Roger Newman: Should the passive lm not be signican... more opened a general discussion of the paper by Roger Newman: Should the passive lm not be signicantly involved in pit initiation (as you suggest), how would you explain that the time to initiation is very much dependent on the nature of the passive lm? Roger Newman responded: I never said that the passive lm is not involved in pit initiation. I understand (of course) that longer passivation gives a longer induction time for pitting. What I say is that the effects of parameters like alloy composition, environment composition, potential, and temperature, are not easily accommodated (at least not predictively) within a passive lm breakdown model, but fall out naturally from a modied Galvele type of model that uses pitting potential data (or, if one has the time, lower extremes of pitting potential distributions). Now I don't know whether or not the nest details of lm breakdown, detectable at the pA level or lower in electrochemical experiments (not nA to mAthose are already pits), and/or on very pure, at alloy surfaces, follow the same rules that we nd using pitting potentials on industrial or semi-industrial alloys. Those measurements have not been done. Actually I don't think stainless steel is necessarily the best model system for such studies. Under certain conditions, as shown by Bardwell many years ago, iron shows blizzards of pits that are clearly not impurity-particle-related; probably aluminum too. In stainless steel we really don't know whether pit initiation ever occurs without a microcrevice and/or an impurity particle. From the viewpoint of practical utility, the Galvele type of approach clearly has the advantage. The Critical Pitting Temperature (CPT) is a propagation-related transition below which metastable pits never become stable at any potential. My group has published extensively on that. One can make a foolproof, if expensive, † Electronic supplementary information (ESI) available: SVET scans movie. See
The distribution of grain boundaries of particular crystallographic character can provide descrip... more The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (.S29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of S3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing.
X-ray computed tomography (X-ray CT) has been applied to nondestructively characterise changes in... more X-ray computed tomography (X-ray CT) has been applied to nondestructively characterise changes in the microstructure of a concrete used in the pressure vessel structure of Advanced Gas-cooled Reactors (AGR) in the UK. Concrete specimens were conditioned at temperatures of 105 o C and 250 o C, to simulate the maximum thermal load expected to occur during a loss of coolant accident (LOCA). Following thermal treatment, these specimens along with an unconditioned control sample were characterised using micro-focus X-ray CT with a spatial resolution of 14.6 microns. The results indicate that the air void pore structure of the specimens experienced significant volume changes as a result of the increasing temperature. The increase in the porous volume was more prevalent at 250 o C. Alterations in air void size distributions were characterized with respect to the unconditioned control specimen. These findings appear to correlate with changes in the uni-axial compressive strength of the conditioned concrete.
ABSTRACT The influence of cold roll reduction history on grain boundary network development in an... more ABSTRACT The influence of cold roll reduction history on grain boundary network development in an austenitic stainless steel has been investigated. A dominant effect of the final thermomechanical process cycles on grain boundary character development was observed. The application of low-strain processing cycles with 5% cold reduction was found to increase the fraction of Sigma 3 and Sigma 3-related variants on solution annealing. Two-step processing treatments with final reductions between 15% and 82% showed no significant differences to their single-step processed equivalents. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
SummaryTwo different grain boundary engineering processing routes for type 304 austenitic stainle... more SummaryTwo different grain boundary engineering processing routes for type 304 austenitic stainless steel have been compared. The processing routes involve the application of a small level of strain (5%) through either cold rolling or uni‐axial tensile straining followed by high‐temperature annealing. Electron backscatter diffraction and orientation mapping have been used to measure the proportions of Σ3n boundary types (in coincidence site lattice notation) and degree of random boundary break‐up, in order to gain a measure of the success of the two types of grain boundary engineering treatments. The distribution of grain boundary plane crystallography has also been measured and analyzed in detail using the five‐parameter stereological method. There were significant differences between the grain boundary population profiles depending on the type of deformation applied.
Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure ... more Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475 • C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes in Volta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility.
Micro-scale damage with a topographical contrast has been observed in cold-worked Nb-stabilised 2... more Micro-scale damage with a topographical contrast has been observed in cold-worked Nb-stabilised 20%Cr-25% Ni stainless steel, following irradiation with 2.2 MeV protons at 400°C and a dose rate of ∼10 −5 dpa/s. After an irradiation dose of 3 and 5 dpa, microstructural changes were found to a depth of 22 μm below the irradiated surface, coincident with the estimated mean range of protons in the material as predicted by the TRIM code. Large variations of intragranular mis-orientations, in combination with a depletion of chromium at grain boundaries, were observed in the proton-irradiated layer. This was accompanied by an increase in nano-hardness, which was linked to a partial transformation of tangled dislocation networks into dislocation loops. Such visible microstructural changes have not been reported before in the absence of post-irradiation annealing treatments.
Pitting corrosion kinetics of type 304L stainless steel have been obtained using a quasi in-situ ... more Pitting corrosion kinetics of type 304L stainless steel have been obtained using a quasi in-situ X-ray computed tomography (X-ray CT) approach. A miniature electro-chemical cell was constructed to allow imaging during potentio-dynamic polarization of a wire specimen. The formation of three discrete pits was observed, allowing comparison between real pit geometry and different geometrical assumptions to estimate pit growth kinetics. The pit volumes obtained by X-ray CT showed good fit with the volume of metal dissolution calculated using Faraday's law. Large fluctuations of the mean current density were observed during the pit nucleation stage, followed by pit growth with mean current densities of 1-3 A.cm −2. Stability products associated with these pits were on the order of 0.3-0.6 A.m-1 , with diffusivity parameter (D. C) of 1.68-3.04 × 10 −8 mol.cm −1 .s −1. Diffusion coefficients for stable pit growth of 0.83-0.96 × 10 −5 cm 2 .s −1 were estimated for metal ion concentrations of 4.2 M.
Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-ch... more Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-chemical polarisation has been observed in a type 304L stainless steel using X-ray computed tomography. The pit reactivation process was associated with the formation of a new pit, directly adjacent to a pre-existing pit. Pit growth kinetics were estimated, revealing an increase of the diffusivity parameter ( D eff Δ C ) from 3.0 × 10 −8 mol cm −1 s −1 to 4.5 × 10 −8 mol cm −1 s −1 with the application of strain, indicating higher metal dissolution rates. Applied strain resulted in fractured lacy metal covers, and its effect on pit growth kinetics is discussed.
Abstract Bipolar electrochemistry was applied to determine the full spectrum of anodic-to-cathodi... more Abstract Bipolar electrochemistry was applied to determine the full spectrum of anodic-to-cathodic polarisation behaviour of stainless steel. The potential and current distributions at the sample surface were determined using a segmented array bipolar electrode (BPE). The measured potential shows a quasi-linear gradient along the centre of the BPE, with the current following an exponential Butler–Volmer-type relationship. Bipolar electrochemistry is compared to conventional 3-electrode polarisation testing, allowing determination of the critical pitting potential, general corrosion rates and the cathodic current response. The application of bipolar electrochemistry is demonstrated via measurement of pit growth kinetics in ferritic stainless steel.
Abstract Meso-structural models are presently in common use for analysis of the mechanical behavi... more Abstract Meso-structural models are presently in common use for analysis of the mechanical behaviour, damage evolution and failure of concrete. These are constructed either from XCT images, or in silico, using statistical information about concrete’s meso-scale constituents. As a minimum, such models include mortar and aggregates as separate phases, while more detailed versions consider the interfacial transition zones (ITZ) between mortar and aggregates, and voids. Analyses of given meso-structures vary further by different constitutive modelling of constituents, with past works focusing on parameters’ calibration using experiments with one loading condition - either tension or compression. Using a detailed meso-structure representation, this work proposes a novel combination of constitutive relations, involving concrete damage plasticity (CDP) for mortar and cohesive zone behaviour for ITZ. CDP parameters are calibrated using both compression and tension experiments with mortar samples. ITZ parameters are calibrated by comparing simulated stress–strain curves and failure patterns with data from compression and tension experiments with concrete samples. This process leads to constitutive relations, applicable to both loading conditions, which has not been demonstrated previously, but is essential to extending the modelling approach to complex stress states encountered in real engineering structures. After establishing the realism of the approach, parametric studies are conducted to investigate the effects of friction between loading plate and specimen, the mortar dilation angle, the ITZ cohesive stiffness, critical stresses, fracture energy and mix mode ratio. The results show that mortar plasticity is the dominant energy dissipation mechanism in both compression and tension, and its rate governs the localisation of damage. The effect of ITZ parameters on the tensile behaviour is found to be negligible. Their effect on the compressive behaviour is found to be limited, but sufficient to propose a set of parameters working for both conditions. Importantly, under both loadings the ITZ is found to control failure localisation into a macroscopic crack in combination with mortar plasticity and damage. Predicted stress-stain curves, damage evolution and macro-crack propagation are shown to be in very good agreement with the experimental observations. This justifies the use of the proposed experimental-modelling strategy for developing models for analysis of complex loading conditions.
Modelling of concrete at the meso-scale provides an effective way to analyse the effects of its c... more Modelling of concrete at the meso-scale provides an effective way to analyse the effects of its constituents on damage initiation and evolution, leading to better understanding and predicting structural integrity. Majority of works to date focus on models calibration and validation with experiments in either tension or compression, leaving open the question of how such models perform under complex stress states. This work presents a modelling approach that includes all key constituents of the concrete meso-structure: coarse aggregates, represented by inclusions with elastic-brittle behaviour, mortar (including cement, sand and fine aggregates), represented with plastic-damage behaviour, interfacial transition zones (ITZ) between aggregates and mortar, represented by zerothickness cohesive interfaces, and air voids or pores. Tension and compression experiments with mortar specimens are conducted to obtain its plastic-damage constitutive law. Similar experiments with concrete with several aggregate volume fractions are conducted to obtain stress-strain behaviours for further calibration of cohesive laws and model validation. Numerical simulations show that the proposed approach with pre-calibration of mortar behaviour leads to very good agreements between the predictions of the concrete meso-structural models and the experimental results under both tension and compression. The calibration of ITZ cohesive laws is performed by a parametric study of the effects of critical stress and fracture energy on the predicted stress-strain curves and fracture patterns. The results are used to propose a practical set of ITZ cohesive parameters.
Corrosion rates of strained grade UNS S32202 (2202) and UNS S32205 (2205) duplex stainless steel ... more Corrosion rates of strained grade UNS S32202 (2202) and UNS S32205 (2205) duplex stainless steel wires have been measured, in situ, using time-lapse X-ray computed tomography. Exposures to chloride-containing (MgCl2) atmospheric environments at 50 °C (12–15 M Cl− and pH ~5) with different mechanical elastic and elastic/plastic loads were carried out over a period of 21 months. The corrosion rates for grade 2202 increased over time, showing selective dissolution with shallow corrosion sites, coalescing along the surface of the wire. Corrosion rates of grade 2205 decreased over time, showing both selective and pitting corrosion with more localised attack, growing preferentially in depth. The nucleation of stress corrosion cracking was observed in both wires.
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-ch... more Abstract The reactivation of a corrosion pit under the synergetic effect of strain and electro-chemical polarisation has been observed in a type 304L stainless steel using X-ray computed tomography. The pit reactivation process was associated with the formation of a new pit, directly adjacent to a pre-existing pit. Pit growth kinetics were estimated, revealing an increase of the diffusivity parameter ( D eff Δ C ) from 3.0 × 10 −8 mol cm −1 s −1 to 4.5 × 10 −8 mol cm −1 s −1 with the application of strain, indicating higher metal dissolution rates. Applied strain resulted in fractured lacy metal covers, and its effect on pit growth kinetics is discussed.
Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure ... more Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475 • C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes in Volta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility.
This paper reports on the distribution and precipitation properties of interfaces in two austenit... more This paper reports on the distribution and precipitation properties of interfaces in two austenitic stainless steels as a function of thermomechanical processing. Specimens of type 316 stainless steel were aged at 750° for one week, and the ensuing intergranular precipitation was assessed. Specimens of type 304 stainless steel were processed by a single step of 5% tensile strain followed by annealing at temperatures between 950°C and 1050°C. Selected data sets from both the 316 and 304 specimens were investigated by the new ‘five-parameter’ analysis method, which allows all five parameters of a grain boundary, i.e. both the misorientation and the boundary plane, to be determined. The distributions obtained showed significant differences between the two specimens, which were explained and discussed with reference to the processing routes. Some of the data were compared with a parallel set of experiments where the deformation had been applied by cold rolling. There were differences in...
opened a general discussion of the paper by Roger Newman: Should the passive lm not be signican... more opened a general discussion of the paper by Roger Newman: Should the passive lm not be signicantly involved in pit initiation (as you suggest), how would you explain that the time to initiation is very much dependent on the nature of the passive lm? Roger Newman responded: I never said that the passive lm is not involved in pit initiation. I understand (of course) that longer passivation gives a longer induction time for pitting. What I say is that the effects of parameters like alloy composition, environment composition, potential, and temperature, are not easily accommodated (at least not predictively) within a passive lm breakdown model, but fall out naturally from a modied Galvele type of model that uses pitting potential data (or, if one has the time, lower extremes of pitting potential distributions). Now I don't know whether or not the nest details of lm breakdown, detectable at the pA level or lower in electrochemical experiments (not nA to mAthose are already pits), and/or on very pure, at alloy surfaces, follow the same rules that we nd using pitting potentials on industrial or semi-industrial alloys. Those measurements have not been done. Actually I don't think stainless steel is necessarily the best model system for such studies. Under certain conditions, as shown by Bardwell many years ago, iron shows blizzards of pits that are clearly not impurity-particle-related; probably aluminum too. In stainless steel we really don't know whether pit initiation ever occurs without a microcrevice and/or an impurity particle. From the viewpoint of practical utility, the Galvele type of approach clearly has the advantage. The Critical Pitting Temperature (CPT) is a propagation-related transition below which metastable pits never become stable at any potential. My group has published extensively on that. One can make a foolproof, if expensive, † Electronic supplementary information (ESI) available: SVET scans movie. See
The distribution of grain boundaries of particular crystallographic character can provide descrip... more The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (.S29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of S3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing.
X-ray computed tomography (X-ray CT) has been applied to nondestructively characterise changes in... more X-ray computed tomography (X-ray CT) has been applied to nondestructively characterise changes in the microstructure of a concrete used in the pressure vessel structure of Advanced Gas-cooled Reactors (AGR) in the UK. Concrete specimens were conditioned at temperatures of 105 o C and 250 o C, to simulate the maximum thermal load expected to occur during a loss of coolant accident (LOCA). Following thermal treatment, these specimens along with an unconditioned control sample were characterised using micro-focus X-ray CT with a spatial resolution of 14.6 microns. The results indicate that the air void pore structure of the specimens experienced significant volume changes as a result of the increasing temperature. The increase in the porous volume was more prevalent at 250 o C. Alterations in air void size distributions were characterized with respect to the unconditioned control specimen. These findings appear to correlate with changes in the uni-axial compressive strength of the conditioned concrete.
ABSTRACT The influence of cold roll reduction history on grain boundary network development in an... more ABSTRACT The influence of cold roll reduction history on grain boundary network development in an austenitic stainless steel has been investigated. A dominant effect of the final thermomechanical process cycles on grain boundary character development was observed. The application of low-strain processing cycles with 5% cold reduction was found to increase the fraction of Sigma 3 and Sigma 3-related variants on solution annealing. Two-step processing treatments with final reductions between 15% and 82% showed no significant differences to their single-step processed equivalents. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
SummaryTwo different grain boundary engineering processing routes for type 304 austenitic stainle... more SummaryTwo different grain boundary engineering processing routes for type 304 austenitic stainless steel have been compared. The processing routes involve the application of a small level of strain (5%) through either cold rolling or uni‐axial tensile straining followed by high‐temperature annealing. Electron backscatter diffraction and orientation mapping have been used to measure the proportions of Σ3n boundary types (in coincidence site lattice notation) and degree of random boundary break‐up, in order to gain a measure of the success of the two types of grain boundary engineering treatments. The distribution of grain boundary plane crystallography has also been measured and analyzed in detail using the five‐parameter stereological method. There were significant differences between the grain boundary population profiles depending on the type of deformation applied.
Intergranular stress corrosion cracking (SCC) of austenitic stainless steel components is a life-... more Intergranular stress corrosion cracking (SCC) of austenitic stainless steel components is a life-limiting factor in nuclear power plant, in which failure of structural components presents a substantial hazard to both safety and economic performance.
This paper reports an on-going research programme into the mechanisms of intergranular stress corrosion cracking in austenitic stainless steels in simulated light water environments. These make use of new analytical and modelling techniques to develop an improved mechanistic understanding of the influence of materials, environment and stress on SCC. The work aims to establish new approaches for the engineering of microstructures and surfaces to develop stainless steels with improved SCC resistance.
17th International Corrosion Congress Corrosion Control in the Service of Society
Electron Backscatter Diffraction (EBSD) and Image Analysis (IA) techniques have been coupled with... more Electron Backscatter Diffraction (EBSD) and Image Analysis (IA) techniques have been coupled with Double Loop-Electrochemical Potentiokinetic Reactivation (DL-EPR) testing to characterize the development of sensitized grain boundary clusters in Type 304 stainless steel. DL-EPR testing revealed differences in the sensitization response of thermo-mechanically processed microstructures, despite similar grain size and grain boundary character distributions (GBCD). The same sensitization treatment produced different distributions of sensitized grain boundary clusters, and susceptible boundary clusters percolated through all microstructures after sensitization treatments of 4 hrs at 650°C. Assessment of the connectivity of Σ3n (1≤n≤3) grain boundaries in EBSD maps showed a trend to longer clusters with increasing Σ3n (1≤n≤3) fractions. A comparison to DL-EPR data showed the attacked grain boundary networks were generally in excess of 80% of the potentially susceptible grain boundary fraction.
This paper reports on the distribution and precipitation properties of interfaces in two austenit... more This paper reports on the distribution and precipitation properties of interfaces in two austenitic stainless steels as a function of thermomechanical processing. Specimens of type 316 stainless steel were aged at 750° for one week, and the ensuing intergranular precipitation was assessed. Specimens of type 304 stainless steel were processed by a single step of 5% tensile strain followed by annealing at temperatures between 950°C and 1050°C. Selected data sets from both the 316 and 304 specimens were investigated by the new ‘five-parameter’ analysis method, which allows all five parameters of a grain boundary, i.e. both the misorientation and the boundary plane, to be determined. The distributions obtained showed significant differences between the two specimens, which were explained and discussed with reference to the processing routes. Some of the data were compared with a parallel set of experiments where the deformation had been applied by cold rolling. There were differences in the two series of results. After processing which included rolling deformation, the Σ3 boundaries tended to break up the random boundary network, whereas after processing by tensile deformation the grain boundary network had been affected in other ways. The consequences of these findings for grain boundary engineering are briefly discussed.
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Papers by Dirk Engelberg
This paper reports an on-going research programme into the mechanisms of intergranular stress corrosion cracking in austenitic stainless steels in simulated light water environments. These make use of new analytical and modelling techniques to develop an improved mechanistic understanding of the influence of materials, environment and stress on SCC. The work aims to establish new approaches for the engineering of microstructures and surfaces to develop stainless steels with improved SCC resistance.