The influence of peak cyclic stress on elevated temperature ratcheting response of a low alloy steel is investigated. For this purpose, ratcheting experiments with similar cyclic amplitude to mean stress ratio, are carried out on grade 20MnMoNi55 steel for reactor pressure vessel applications at 400 °C for two peak cyclic stress levels above its yield stress. Cycling of 20MnMoNi55 at the lower peak cyclic stress resulted in similar failure strains at ambient and 400 °C but with significantly longer elevated temperature ratcheting life. The trends are consistent with the inverse dependence of the ratio between peak cyclic stress to maximum tensile strength with room temperature ratcheting life, reported for the first time in a low alloy steel at elevated temperature. Analysis of hysteresis obtained during ratcheting provided evolutionary trends of strain energy and prevailing cyclic stresses in tensile and compressive parts of cycle from which new parameters were devised that represented cyclic damage. Quantitative power law relation of the damage parameters derived from the hysteresis analysis rationalised the increased elevated ratcheting life obtained by lowering the peak cyclic stress in low alloy steel 20MnMoNi55 to the smaller value of the applied number of cycles exponent.

The deformation and recrystallization behaviour with associated microstructural and textural evolutions, precipitation formation and dissolution behaviour, and structure-property correlations have been investigated in V-Ti-Ta alloy system. Recrystallization temperature in this system was found to be in the range of 1250-1300 degrees C, 200 degrees C higher than reference V-4Ti-4Cr alloy. Substitutional solid solution strengthening by Ta was found to be the dominant strengthening mechanism. In addition, Ta was also found to influence the precipitation behaviour of V-Ti-Ta alloys. Synchrotron XRD and TEM-EDS results indicated the composition of precipitate to be (Ti,Ta) CON in contrast to V-Ti-Cr system where no Cr is found in the precipitates. Theoretical calculations based on thermodynamics and experimental evidence are presented which indicate desirable enhanced precipitate stability in this system due to presence of Ta. Deformation texture of the alloy was found to exhibit prominent gamma-fibre, whereas the recrystallized texture had shown predominant.-fibre texture.

The tensile flow behaviour of mechanically milled iron and a low-alloy steel are analysed using power law models that describe the evolution of stress–strain during a tensile test. A new analytical power law equation is presented that is shown to be superior to the Hollomon and Ludwigson equation to describe the room temperature flow properties of the low-alloy steel over a wide strain rate range. It not only provides a better fit to the homogenous flow curve of the low-alloy steel but also predicts the maximum strength without the prior knowledge of the uniform strain. The new equation also fitted the flow curves of mechanically milled iron with grain sizes in nano to micrometer range.

Quantitative tomography is carried out on datasets derived from tensile fracture sample of electrochemically precharged Al-Zn-Mg-Cu alloy in the underaged condition and its uncharged counterpart. It is shown that precharging which induces a transition of tensile fracture mode from ductile to brittle, results in a significant increase in micro-damage content in the regions near the fracture surfaces. Using quantitative tomography analysis based on spatial mapping of morphologically segmented micro-damage content of the datasets it is found that the precharged sample contains an inhomogenous distribution of micro-pores near grain boundaries. It is also shown that the spatial architecture of micro-pores in the dataset is not influenced by the plastic zone of the intergranular cracks lying along the grain boundaries. Contrastingly the micro-pores in the tomographic dataset of the uncharged sample are shown to be present near intermetallic particles. It is therefore rationalized that the spatial architecture of micro-pores in the datasets from uncharged sample originate from particle cracking during ductile fracture, and from the tendency for damage enhancement by the synergism of hydrogen exposure near grain boundaries and localization of deformation in the precharged sample dataset.

A comparison of the state of 3-D micro-damage induced by hydrogen redistribution due to thermal treatment and deformation near an intergranular fracture surface is carried out in high strength aluminium alloy using synchrotron microtomography. The mapping of various types of micro-damage provides evidence that hydrogen redistribution occurs along with a change in content and shape of micro-pores. The presence of intergranular cracks and an extended agglomerated micro-pore damage state are found to be in close proximity to each other, suggesting a mechanistic relationship for both formation of the unique damage architecture and insidious manifestation of embrittlement in high strength aluminium alloys. (C) 2016 Elsevier Ltd. All rights reserved.

Cavitation is usually thought to be associated with the final stages of failure in structural materials. Since this view pervades the approach to component design properties and performance in serv...

The need for a new paradigm to estimate remaining creep life of service exposed steels is critically assessed. New approaches to residual life assessment are proposed, in the light of a decade's experience of the use of micro-tomography to characterise the three-dimensional (3D) nature of cavitation damage in structural materials. Imaging of conventional structural materials such as steels with high absorption to X-rays has been realised by synchrotron micro-tomography (SR-μCT), providing new insights into phenomena such as creep failure. The unique feature of SR-μCT studies is the direct imaging in 3D of cavities (hundreds of micrometres in size) present in the bulk, revealing the spatial characteristics and morphology of the creep voids. Quantitative analyses of the cavitation characteristics revealed by 3D datasets, when scaled with respect to time, stress and temperature, provide functional information suitable for developing constitutive equations for creep. The application of SR-μCT, a non-destructive technique providing high fidelity data, significantly reduces the ambiguity in developing functional relationships to predict creep failure. The explicit use of such constitutive equations to estimate the residual life of components in creep, and the consequent assessment of structural integrity, would prove invaluable. Micro-tomography studies related to creep in materials are reviewed, with special emphasis on a 10·86%Cr heat resistant steel, to demonstrate the type of data available for life assessment and design against creep failure. A brief discussion of current methods to estimate residual life in the light of recent 3D micro-tomography data follows. Finally, the possibility of new approaches, using micro-tomography data in conjunction with destructive 3D approaches such as serial sectioning, to formulate advanced residual life estimates, is briefly considered.

In recent years, a design concept for the stabilisation of the microstructure by addition of boron and nitrogen was developed. This so called martensitic boron–nitrogen strengthened steel (MARBN) combines boron strengthening by solid solution with precipitation strengthening by finely dispersed nitrides. Welded joints of MARBN steels showed no formation of a uniform fine grained region in the heat affected zone (HAZ) which is in general highly susceptible to Type IV cracking. In this work, the crossweld creep strength of a newly developed MARBN steel was analysed and the evolution of damage was investigated using synchrotron microtomography supported by electron microscopy. Three-dimensional (3D) reconstructions of the tested samples together with electron backscatter diffraction investigations revealed an intense void formation in a restricted area along small grains at prior austenite grain boundaries in the HAZ as the main reason for premature creep failures in the HAZ of welded joints.

A porous NiTi specimen prepared by self-propagating high temperature synthesis for bone implant applications has been characterised by a laboratory X-ray microtomography instrument to reveal the three-dimensional (3D) structure of the internal porosity. The reconstructed slices obtained from the tomography scan showed open porosity as well as micropores in the matrix. The 3D renderings revealed that the open porosity is highly interconnected, tortuous structure. 3D quantitative estimates of the micropores in terms of their volume fraction, number density and size have been evaluated. Metrics have been developed based on skeletonised idealisation of the complex architecture of interconnected pores that reflect its structural properties.

The serrated flow behaviour in a low alloy steel has been analysed with new metrics that captures the signatures of its typical temporal characteristics. It has been found that the temporal manifestation of serrated flow could be decomposed into a slow and fast kinetics framework. The relationship of tensile ductility with the evolving temporal characteristic of in-homogenous flow in the temperature–strain rate regime displaying PLC phenomenon has been explored. The evolutionary behaviour of these descriptors along the flow curve has been related to the unit processes of PLC phenomenon within the framework of the unified model of dynamic strain aging proposed by Beukel and Kocks.

The deformation and fracture behaviour of AISI 403, a tempered martensitic stainless steel for end fitting application of Pressurised heavy water reactor is being reported. The deformation behaviour studies entailed characterisation of tensile behaviour in the temperature range 77 - 873 K for the as recieved and the fine grained Nb modified variant of AISI 403. the study of elevated tensile behaviour in the two steels has been undertaken with the purpose of characterising the strain rate - temperature domain of the occurrence of dynamic strain aging (DSA) phenomenon. In both steels, while the temperature range for the manifestations of characteristic anomalies in the tensile curve due to DSA was observed within 523 - 673K, the strain domain for the fine grained Nb modfied variety was significantly higher as comapred with the as recievied variety. The low temperature tensile tests for the as recieved AISI 403 revealed the presence of Pseudo=alloy softening in the temperature range 273 - 193 K. The effect of high DBTT of the AISI 403 steel was shown by the fracture toughness tests in the J-integral format at room temperature that displayed significant scatter in smaples with high in-plane and out of plane constraint. Smaples with lower constraint showing stable crack growth were further tested at high temperature to obtain the temperature dependence of initiation fracture toughness and propagation touhgness. Within the DSA tempertaures a sharp decline in the fracture properties were observed. A mechanistic interpretation for the manifestations of the various observed phenomena is presented.

The paper provides an insight into the development of the micro-pore damage phenomenon during the hydrogen ingress and egress in a high strength aluminum alloy. High resolution micro tomography scans on samples subjected to combinations of cathodic hydrogen charging and de-sorption heat treatment were carried out. The quantitative analysis, spatial mapping, and probabilistic evaluation of the 3D datasets revealed that the development of micro-pore damage during hydrogen ingress–egress had a non-trivial dependence on the particle size distribution. This provides a rationale to mitigate the deleterious effects of hydrogen in aluminum alloys by controlling particle size characteristics.

Two three-dimensional (3-D) techniques, namely X-ray microtomography and serial sectioning, have been applied for characterization of creep cavitation behavior in tempered martensitic steel. For this purpose samples have been extracted from a series of specimens that were subjected to creep tests over the stress range of 120–180MPa at 600°C. The presence of creep voids in the series of samples was un-ambiguously detected in a non-destructive manner using synchrotron X-ray microtomography with a resolution of 1μm. The 3-D visualization of the datasets provided an assessment of the spatial distribution and morphology of the creep voids as a function of creep stress and high temperature exposure time. The quantitative analyses of the image datasets enabled the development of functional relationships between the macroscopic creep parameters (such as rupture ductility, applied stress, creep life) and cavitation characteristics (such as volume fraction, and number density). The quantitative analyses also provided an evaluation of manifestations of growth and coalescence processes in the respective datasets. A transition of cavitation behavior of the steel has been found to occur in the stress range of 120–150MPa at 600°C. The evolution in the pattern of cavitation and its relation to the prior-austenite boundary was explored by combining micro-tomography and serial sectioning techniques, which revealed a new possibility in the progress of cavitation in the long term creep exposed specimens of 9–12% Cr heat resistant steels.

Synchrotron X-ray microtomography(SR-μCT) scans have been carried out on sample coupons extracted from the fracture specimens of a 10.86% Cr heat resistant steel exposed to crep deformation at 873K over stresses of 120, 150, and 180 MPa. The 3D cavitation characteristics in terms of void volume fraction, numbwer density and size distribution as a function of the applied stress has been determined by quantitative analysis of the reconstructed tomograohy slice datasets. The relationship between heterogenous spatial distribution of creep voids and variation in rupture life has been exploited in terms of microstructural sites during the onset of creep embrittlement.

Due to accelerated creep damage, martensitic 9%Cr welded joints fail prematurely in the fine-grained heat affected zone (FGHAZ). These failures in the FGHAZ are categorized as "Type IV cracking". In recent years, boron containing 9% Cr steels have been developed that suppress the FGHAZ formation in order to avoid Type IV cracking and that have increased crossweld creep strength.The advanced alloying concept is based on the balanced addition of boron and nitrogen. Creep tests of 9Cr3W3CoVNbBN steel crosswelds have been carried out up to 17000 hours at 650 degrees C. The crossweld creep strength was analyzed and the damage evolution in crosswelds was investigated using synchrotron micro-tomography of creep exposed welded joints. Additionally, the microstructure of the crossweld specimens was investigated using optical and scanning electron microscopy and electron backscatter diffraction. Creep void formation along the original prior austenite grain boundaries, at a certain distance to the weld fusion line, in the heat affected zone was observed. Electron backscafter diffraction images revealed the incomplete suppression of grain refinement along the prior austenite grain boundaries. These newly formed small grains have been identified as the main reason for the increased creep void formation in this area.

Smart materials such as shape memory alloys (SMAs) are recently being used in earthquake engineering applications to control the response of structures. In this paper, a shape-memory alloy damper device made up of austenite wires (e.g. Nickel Titanium wires) is used as a passive energy absorber. NiTi wires are very attractive for passive vibration control as they have a pseudo-elastic property and can sustain large amounts (upto 10% strain) of inelastic deformation. Moreover, in contrast to regular metallic materials they can recover that deformation. The damper device is designed, fabricated and tested. Validity study is made using a thermo-mechanical model of SMA taking into account the residual martensite accumulation irreversibly due to cyclic forward/reverse martensitic transformation. Further an analytical study of a supporting steel structure of dump tanks with and without SMA dampers subjected to design wave earthquake loading is carried out. Performance of the structure with SMA dampers is compared with that of the same structure with yielding dampers. Copyright (C) 2010 John Wiley & Sons, Ltd.

Two steels with bainitic and martensitic microstructures have been investigated with respect to the changes occurring in their uniaxial tensile properties in the temperature regime where they display dynamic strain aging (DSA) behaviour. The variations in yield strength, ultimate tensile strength, ductility and strain rate sensitivity in the case of martensitic steel have been found to vary in an embrittling manner as compared with those observed in the case of the bainitic steel within the DSA temperature regime. An attempt has been made to provide an insight into the difference in manifestations of DSA on the uniaxial behaviour of the two steels.

A new generation nuclear reactor pressure vessel steel (CrMoV type) having compositional similarities with thick section 3Cr-Mo class of low alloy steels and adapted for nuclear applications was investigated for various manifestations of dynamic strain aging (DSA) using uniaxial tests. The steel investigated herein has undergone quenched and tempered treatment such that a tempered bainite microstructure with Cr-rich carbides was formed. The scope of the uniaxial experiments included tensile tests over a temperature range of 298 K to 873 K (25 °C to 600 °C) at two strain rates (10−3 and 10−4 s−1), as well as suitably designed transient strain rate change tests. The flow behavior displayed serrated flow, negative strain rate sensitivity, plateau behavior of yield, negative temperature (T), and strain rate \( \left( {\dot{\varepsilon }} \right) \) dependence of flow stress over the temperature range of 523 K to 673 K (250 °C to 400 °C) and strain rate range of 5 × 10−3 s−1 to 3 × 10−6 s−1, respectively. While these trends attested to the presence of DSA, a lack of work hardening and near negligible impairment of ductility point to the fact that manifestations of embrittling features of DSA were significantly enervated in the new generation pressure vessel steel. In order to provide a mechanistic understanding of these unique combinations of manifestations of DSA in the steel, a new approach for evaluation of responsible solutes from strain rate change tests was adopted. From these experiments and calculation of activation energy by application of vacancy-based models, the solutes responsible for DSA were identified as carbon/nitrogen. The lack of embrittling features of DSA in the steel was rationalized as being due to the beneficial effects arising from the presence of dynamic recovery effects, presence of alloy carbides in the tempered bainitic structure, and formation of solute clusters, all of which hinder the possibilities for strong aging of dislocations.

A study of the tensile flow behavior in the microstrain region of a martensitic stainless steel within the temperature range 77–273 K has been carried out. The ductility and work hardening in the low‐temperature region were found to be significantly larger than those of the ambient at temperatures where alloy softening was taking place. Thermal‐activation analysis of data has attributed the low‐temperature deformation behavior to the operation of the Peierls mechanism. While the Dorn–Rajnak model was found to fit the data, the observed alloy softening was attributed to the operation of an intrinsic mechanism that promoted the double‐kink formation on dislocations within a limited temperature range below ambient. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A micro-focus X-ray beam source along with computed tomography technique has been used to image the voids developed during tensile deformation in alloy steel used as structural material for nuclear power plant components. The technique was successful to characterize the internal void structure in the necked region of CrMoV type steel in terms of the size, distribution and volume fraction. The void volume fraction was found to be about 0.03% in a volume of 1.4mm3 of the necked region reduced in diameter by 32% as a result of tensile deformation.