PhD Theses by James Marrow
The new generation of highly alloyed super duplex stainless steels such as Zeron 100 are preferab... more The new generation of highly alloyed super duplex stainless steels such as Zeron 100 are preferable materials for industrial applications demanding high strength, toughness and superior corrosion resistance, especially against stress corrosion cracking (SCC). SCC is an environmentally assisted failure mechanism that occurs due to exposure to an aggressive environment while under a tensile stress. The mechanism by which SCC of duplex stainless steel is expected to suffer depends on the combination of electrochemical and the mechanical interaction between austenite and ferrite in the duplex alloys. The main aims of this work are to study the suitability of digital image correlation (DIC) to monitor the initiation and propagation of SCC and to understand how the microstructure of duplex stainless steel influences the kinetics of crack initiation and growth. The combined analysis of DIC, SEM and EBSD was used to study the relative crack propagation and the effect of interphase boundaries on crack growth as well.
Cracking was initiated beneath saturated MgCl2 droplets in an atmospheric environment at 80°C and relative humidity of 30-33%. As-received and 10% cold rolled samples (with two orientations transverse and longitudinal to the loading direction) were subjected to an applied strain of 0.03 under displacement controlled tests. Regular optical observations were recorded of the droplets and their surrounding area. DIC analyses used the differentiation of the displacement fields to obtain the apparent surface strains used to detect crack initiation and propagation, and to measure crack opening displacements.
It was found that DIC was efficiently observed the strain developments and the displacements in observed surfaces outside of the droplets but it could not identify or quantify the initiation of the cracks inside the droplets because of the mobility of the salt film and the high amount of the corrosion products formed which obscure the vision under the droplets. In addition, results showed that early stage microcracks were initiated in α phase and α/γ interfaces and propagated preferentially in the ferrite phase. Also, SCC initiation and propagation was accelerated by cold rolling and the grains orientations were of major effects on the retardation of crack propagation which was more severe in the transverse rolling direction. Also, there was no relation established between the strain level and the density of pitting in either phase.
Irradiated Graphite Waste - Stored Energy
The cores of early UK graphite moderated research and production nuclear fission reactors operate... more The cores of early UK graphite moderated research and production nuclear fission reactors operated at temperatures below 150°C. Due to this low temperature their core graphite contains significant amounts of stored (Wigner) energy that may be released by heating the graphite above the irradiation temperature. This exothermic behavior has lead to a number of decommissioning issues which are related to long term "safe-storage", reactor core dismantling, graphite waste packaging and the final disposal of this irradiated graphite waste. The release of stored energy can be modeled using kinetic models. These models rely on empirical data obtained either from graphite samples irradiated in Material Test Reactors (MTR) or data obtained from small samples obtained from the reactors themselves. Data from these experiments is used to derive activation energies and characteristic functions used in kinetic models. This present research involved the development of an understanding of the different grades of graphite, relating the accumulation of stored energy to reactor irradiation history and an investigation of historic stored energy data. The release of stored energy under various conditions applicable to decommissioning has been conducted using thermal analysis techniques such as Differential Scanning Calorimetry (DSC). Kinetic models were developed, validated and applied, suitable for the study of stored energy release in irradiated graphite components. A potentially valid method was developed, for determining the stored energy content of graphite components and the kinetics of energy release.Another parameter investigated in this study was dedicated in the simulation of irradiation damage using ion irradiation. Ion bombardment of small graphite samples is a convenient method of simulating fast neutron irradiation damage. In order to gain confidence that irradiation damage due to ion irradiation is a good model for neutron irradiation damage the properties and microstructure of various grades of ion irradiated nuclear graphite were also investigated. Raman Spectroscopy was employed to compare the effects of ion bombardment with the reported effects of neutron irradiation on the content of the defects. The changes of the of defect content with thermal annealing of the ion irradiated graphite have been compared with the annealing of neutron irradiated nuclear graphite
Residual Stress and Fracture in High Temperature Ceramics
Recrystallised alumina is used as a high performance crucible material. Its thermal shock resista... more Recrystallised alumina is used as a high performance crucible material. Its thermal shock resistance is known to be affected by component shape, and also by processing variables, since the defects and internal stress at both the microscale (i.e. between grains due to anisotropic crystal properties) and macroscale (i.e. due to differential shrinkage during sintering) influence the fracture strength. The aim of this thesis is to study the nucleation and growth of defects in pure alumina and Cr-doped alumina, and to investigate how their behavior is affected by residual stresses, such those introduce by thermal expansion of the crystal grains.In this thesis, digital image correlation is applied to polycrystalline aluminas (i.e. Cr-doped alumina and pure alumina with average grain 3.6 µm and 1.5 µm respectively) that are stressed in an optical microscope. The defect size and the surface crack opening displacement were measured using digital image correlation. The distribution and population of crack nucleating defects were obtained by in-situ observation of the stressed surface and by analysis with digital image correlation. These data are then compared with independent measurements of the defect population using Hertzian indentation, from which defect populations are derived for the pure and Cr-doped alumina samples.Grain boundary plane and grain orientations in the vicinity of crack nuclei were characterised by electron microscopy. Crack nuclei were shown to develop at boundaries predicted to have high tensile thermal strains, caused by the orientation of the grain boundary plane relative to the adjacent grains, such as basal plane grain facets. The techniques of focused ion beam (FIB) milling and electron backscatter diffraction (EBSD) characterization of the crystallographic orientations and structure of cracked grain boundaries were used to provide data for a model to explain the cracking of these boundaries as a result of the thermal strains and the anisotropic thermal expansion behaviour of alumina.
The effect of surface finish on fatigue limit of two types of austenitic stainless steels (AISI 3... more The effect of surface finish on fatigue limit of two types of austenitic stainless steels (AISI 304L and AISI 316L) has been investigated. Fatigue specimens having two different surface conditions were obtained by changing the final cutting condition; annealing was performed to separate the residual stress effects from surface roughness. Electropolished samples were tested as a reference for each material.
A generic mechanistic model for short fatigue crack propagation proposed by Navarro and Rios (N-R model) was implemented to assess its suitability for predicting the fatigue behaviour of specimens with various controlled surface conditions, obtained by machining. The surface/material properties required to implement this model were obtained by electron backscatter diffraction (EBSD), surface profilometry, hardness testing and X-ray diffraction residual stress measurement. The fatigue limits were determined using rotating-bending by means of the staircase method.
The fatigue limits predicted by the N-R fatigue model were compared with the results of the fatigue tests. There was no agreement between the prediction and observations, indicating that the original form of the N-R model is not appropriate for austenitic stainless steels.
In AISI 304L, the surface residual stresses are the dominant parameter, allowing prediction of the effects of machining on fatigue resistance while, the surface roughness developed by machining has no significant effect. In AISI 316L, the effect of surface roughness is found to be negligible, with a weaker effect of surface residual stress than has been observed for AISI 304L.
Crack nuclei in run-out (>107 cycles) fatigue tests were observed to arrest at twins and martensite packets, developed by fatigue in AISI 316L and AISI 304L, respectively. Good agreement with experiments was achieved by using a modification to the fatigue model, which takes account of the observed effect of the plastic deformation on the microstructure.
Austenitic stainless steels with their good weldability, superior corrosion resistance and excell... more Austenitic stainless steels with their good weldability, superior corrosion resistance and excellent performances in higher temperatures are an important material for engineering applications in industrial plants. Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels is a critical failure mechanism where cracking can result from sensitisation of certain grain boundaries after heat treatment (e.g. post-weld stress relief) or fast neutron irradiation in nuclear plant. Sensitisation is a decrease in the local resistance to stress corrosion, to a degree that depends on the grain boundary structure. Developments of predictive models for stress corrosion crack nucleation require more information about the effect of several external parameters (e.g. stress and time) on the likely extent of crack growth. Understanding is also required about how the grain boundary crystallography and the orientations of grain boundary plane and its surrounding grains affect crack propagation.
In this PhD thesis, the effects of time, applied stress and microstructure on populations of short crack nuclei have been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical evaluation, using the Gumbel extreme value distributions enables analysis of the growth rate of the population of short crack nuclei. This methodology has been developed, in order to quantitatively evaluate the influence of grain boundary control on crack development. These investigations showed an increase in the expected crack length with increasing time and grain size. Although the crack length tends to increase with stress, the effect is not strong. The grain boundary controlled microstructures exhibited significantly higher resistance to intergranular crack propagation. Direct observations of intergranular crack initiation and propagation, using digital image correlation (DIC) along with electron back scatter diffraction (EBSD), in various microstructures has been used to study the crack nucleation sites and crack interactions with grain boundaries of different characteristics. The effect of microstructural modification on crack growth kinetics has also been investigated. A significantly longer incubation period for crack initiation and lower crack growth rate were observed in thermo-mechanically treated microstructure.
New methods have been developed to assess the clustering characteristics of grain boundaries of particular properties. The network properties of boundaries classified by EBSD data have been compared with the network of corroded grain boundaries in electro-chemically tested samples. Image analyses (IA) was employed to evaluate the geometrical properties of susceptible boundaries clusters in a range of microstructures produced by sequential thermo-mechanical processing (TMP).
DL-EPR testing method of sensitisation assessment has been augmented by large area image analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. This approach determines the degree of sensitisation of the susceptible grain boundaries in the microstructure, and is used to explain IGSCC behaviour. A new method of degree of sensitisation determination is proposed, based on normalisation by a cluster parameter for the network of susceptible grain boundaries.
Effect of residual stress and surface condition on the stress corrosion cracking of austenitic stainless steel
Stress corrosion cracking (SCC) is a spontaneous failure of a material due to the com- bined effe... more Stress corrosion cracking (SCC) is a spontaneous failure of a material due to the com- bined effects of tensile stress (residual and/or applied) and a corrosive environment. Surface condition is thought to influence SCC, it was the aim of this project to examine the effects of surface condition, and moreover, the nature of the residual stress fields imposed by the surface preparation technique used, on the development and nature of stress corrosion cracks in austenitic stainless steels.
Six different surface machining operations on two different sample geometries, varied by milling cut depth and feed rate, were applied to cylindrical and rectangular-prismed 316Ti steel samples cut from plate. Two of the faces from rectangular samples were left ’as-received’, i.e. these surfaces corresponded to the upper and lower faces of the original plate the samples were cut from. One face was ground using a grinding wheel such that no roughness was visible to the naked eye. The remaining (end) faces were coarse-cut using a bandsaw. Sets of these samples (containing one of each of the milled profiles) were boiled in a magnesium chloride solution for two weeks, a similar (refer- ence) set of six samples were stress relieved by annealing in argon at 1100oC for 30 minutes. Subsequently, the resulting cracks and pits were examined by means of optical microscopy and scanning electron microscopy. Also, x-ray diffraction (XRD) residual stress characterisation work was performed on the samples, including electropolished ones (in order to characterise residual stress with depth). Finally, optical profilometry was employed to acquire single line and maps of the roughness.
The results indicate that machined samples retain significant and relatively high tensile stresses that are present over several 10’s of microns into the material from the surfaces. Beneath the tensile layer, a compressive sub-layer was found. SCC cracks were generally observed to grow into the tensile layer and were then deflected close to the compressive layer. However, many cracks were able to extend into the compressive layer, growing to over 100 microns in length. The dominant feature affecting cracking appeared to be the residual tensile layer; the surface profile had no clear affect.
Influence of grain boundary structure in proton irradiated stainless steel on susceptibility to irradiation assisted stress corrosion cracking
Irradiation Assisted Stress Corrosion Cracking (IASCC) is a form of Stress Corrosion Cracking (SC... more Irradiation Assisted Stress Corrosion Cracking (IASCC) is a form of Stress Corrosion Cracking (SCC) that operates under a complex mix of stress, environment and neutron irradiation. It is of concern not only in current nuclear fission reactors where a number of component failures related to it have occurred but also in proposed water cooled nuclear fusion reactors
Grain Boundary Engineering has been used to produce austenitic metals with a high increased fraction of ‘special’ boundaries that have increased resistance to SCC. Crack bridging at these ‘special’ boundaries has been shown in thermally sensitised stainless steels and modelling work predicts that crack bridging slows down the propagation of Intergranular Stress Corrosion Cracking. ‘Special’ boundaries have been defined as those with a low Sigma Coincident Site Lattice (CSL) structure but work has also shown that grain boundaries whose plane lies on a low index plane of the adjoining grains can also exhibit special properties.
An austenitic stainless steel was irradiated with protons to produce a microstructure similar to that obtained in nuclear reactors. Autoclave testing to initiate SCC was performed and grain boundary elemental composition measured. Evidence of crack bridging was observed in the autoclave sample and differences in segregation behaviour were observed. In particular it was noted that while segregation occurred at some random boundaries, no segregation was found at a random boundary which had a plane that lay on the {221} planes of the adjacent grains.
Grain boundary engineering for intergranular stress corrosion in austenitic stainless steel
Austenitic stainless steels are frequently used for engineering applications in aggressive enviro... more Austenitic stainless steels are frequently used for engineering applications in aggressive environments. Typical sources of component failures are associated with localized attack at grain boundaries, such as intergranular corrosion and stress corrosion cracking. To prevent premature failures, structural integrity assessments are carried out, with the aim of predicting the maximum likelihood of cracking that may develop. For accurate predictions it is of great importance to know the interaction of parameters involved in life-determining processes.
This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in- situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip.
Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models.
Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,
(i) a thorough examination of the microstructure and analysis parameters employed,
(ii) the determination of the degree of sensitisation with subsequent crack path
investigations,
(iii) the identification of a suitable test system with associated grain boundary
susceptibility criteria,
(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,
(v) statistical crack length assessments of calibrated IGSCC test specimens.
The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.
Stress corrosion cracking in duplex stainless steels
Stress corrosion cracking (SCC) in duplex stainless steel was investigated. Wedge open loaded (W... more Stress corrosion cracking (SCC) in duplex stainless steel was investigated. Wedge open loaded (WOL) specimens of age hardened Zeron 100 were tested in 3.5wt% NaCl solution with cathodic polarization applied at -9OOmV/SCE. The interaction between microstructure and mechanism of stress corrosion cracking was studied. The fracture mechanism was investigated using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The threshold stress intensity factor for SCC (KISCC) was determined. Material was found to crack by ferrite cleavage and austenite ductile tearing. The cracking mode was transgranular through the ferrite. No intergranular cracking was observed. Ferrite cleavage took place along (100) planes and { 112) twin habit planes. Ferrite cleavage decreased with decreasing ageing time. A transition exists in ferrite cracking mode, to less cleavage, based on its hardness. The austenite not appear to crack due to SCC and acted as a crack arrestor. The environment may assist ferrite fracture by hydrogen diffusion into the material, produced due to cathodic polarization. The threshold stress intensity factor (K1SCC) increased with amount of crack growth giving R-curve behaviour. This was due to unbroken ligements and austenite, which exerted a bridging effect on the crack tip. This caused an increase in the threshold stress intensity factor for stress corrosion cracking. The K1SCC value was scattered due to the variable effect of crack shielding. The bridging effect of unbroken ligaments/austenite on SCC mechanism was modelled for the tested material.
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PhD Theses by James Marrow
Cracking was initiated beneath saturated MgCl2 droplets in an atmospheric environment at 80°C and relative humidity of 30-33%. As-received and 10% cold rolled samples (with two orientations transverse and longitudinal to the loading direction) were subjected to an applied strain of 0.03 under displacement controlled tests. Regular optical observations were recorded of the droplets and their surrounding area. DIC analyses used the differentiation of the displacement fields to obtain the apparent surface strains used to detect crack initiation and propagation, and to measure crack opening displacements.
It was found that DIC was efficiently observed the strain developments and the displacements in observed surfaces outside of the droplets but it could not identify or quantify the initiation of the cracks inside the droplets because of the mobility of the salt film and the high amount of the corrosion products formed which obscure the vision under the droplets. In addition, results showed that early stage microcracks were initiated in α phase and α/γ interfaces and propagated preferentially in the ferrite phase. Also, SCC initiation and propagation was accelerated by cold rolling and the grains orientations were of major effects on the retardation of crack propagation which was more severe in the transverse rolling direction. Also, there was no relation established between the strain level and the density of pitting in either phase.
A generic mechanistic model for short fatigue crack propagation proposed by Navarro and Rios (N-R model) was implemented to assess its suitability for predicting the fatigue behaviour of specimens with various controlled surface conditions, obtained by machining. The surface/material properties required to implement this model were obtained by electron backscatter diffraction (EBSD), surface profilometry, hardness testing and X-ray diffraction residual stress measurement. The fatigue limits were determined using rotating-bending by means of the staircase method.
The fatigue limits predicted by the N-R fatigue model were compared with the results of the fatigue tests. There was no agreement between the prediction and observations, indicating that the original form of the N-R model is not appropriate for austenitic stainless steels.
In AISI 304L, the surface residual stresses are the dominant parameter, allowing prediction of the effects of machining on fatigue resistance while, the surface roughness developed by machining has no significant effect. In AISI 316L, the effect of surface roughness is found to be negligible, with a weaker effect of surface residual stress than has been observed for AISI 304L.
Crack nuclei in run-out (>107 cycles) fatigue tests were observed to arrest at twins and martensite packets, developed by fatigue in AISI 316L and AISI 304L, respectively. Good agreement with experiments was achieved by using a modification to the fatigue model, which takes account of the observed effect of the plastic deformation on the microstructure.
In this PhD thesis, the effects of time, applied stress and microstructure on populations of short crack nuclei have been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical evaluation, using the Gumbel extreme value distributions enables analysis of the growth rate of the population of short crack nuclei. This methodology has been developed, in order to quantitatively evaluate the influence of grain boundary control on crack development. These investigations showed an increase in the expected crack length with increasing time and grain size. Although the crack length tends to increase with stress, the effect is not strong. The grain boundary controlled microstructures exhibited significantly higher resistance to intergranular crack propagation. Direct observations of intergranular crack initiation and propagation, using digital image correlation (DIC) along with electron back scatter diffraction (EBSD), in various microstructures has been used to study the crack nucleation sites and crack interactions with grain boundaries of different characteristics. The effect of microstructural modification on crack growth kinetics has also been investigated. A significantly longer incubation period for crack initiation and lower crack growth rate were observed in thermo-mechanically treated microstructure.
New methods have been developed to assess the clustering characteristics of grain boundaries of particular properties. The network properties of boundaries classified by EBSD data have been compared with the network of corroded grain boundaries in electro-chemically tested samples. Image analyses (IA) was employed to evaluate the geometrical properties of susceptible boundaries clusters in a range of microstructures produced by sequential thermo-mechanical processing (TMP).
DL-EPR testing method of sensitisation assessment has been augmented by large area image analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. This approach determines the degree of sensitisation of the susceptible grain boundaries in the microstructure, and is used to explain IGSCC behaviour. A new method of degree of sensitisation determination is proposed, based on normalisation by a cluster parameter for the network of susceptible grain boundaries.
Six different surface machining operations on two different sample geometries, varied by milling cut depth and feed rate, were applied to cylindrical and rectangular-prismed 316Ti steel samples cut from plate. Two of the faces from rectangular samples were left ’as-received’, i.e. these surfaces corresponded to the upper and lower faces of the original plate the samples were cut from. One face was ground using a grinding wheel such that no roughness was visible to the naked eye. The remaining (end) faces were coarse-cut using a bandsaw. Sets of these samples (containing one of each of the milled profiles) were boiled in a magnesium chloride solution for two weeks, a similar (refer- ence) set of six samples were stress relieved by annealing in argon at 1100oC for 30 minutes. Subsequently, the resulting cracks and pits were examined by means of optical microscopy and scanning electron microscopy. Also, x-ray diffraction (XRD) residual stress characterisation work was performed on the samples, including electropolished ones (in order to characterise residual stress with depth). Finally, optical profilometry was employed to acquire single line and maps of the roughness.
The results indicate that machined samples retain significant and relatively high tensile stresses that are present over several 10’s of microns into the material from the surfaces. Beneath the tensile layer, a compressive sub-layer was found. SCC cracks were generally observed to grow into the tensile layer and were then deflected close to the compressive layer. However, many cracks were able to extend into the compressive layer, growing to over 100 microns in length. The dominant feature affecting cracking appeared to be the residual tensile layer; the surface profile had no clear affect.
Grain Boundary Engineering has been used to produce austenitic metals with a high increased fraction of ‘special’ boundaries that have increased resistance to SCC. Crack bridging at these ‘special’ boundaries has been shown in thermally sensitised stainless steels and modelling work predicts that crack bridging slows down the propagation of Intergranular Stress Corrosion Cracking. ‘Special’ boundaries have been defined as those with a low Sigma Coincident Site Lattice (CSL) structure but work has also shown that grain boundaries whose plane lies on a low index plane of the adjoining grains can also exhibit special properties.
An austenitic stainless steel was irradiated with protons to produce a microstructure similar to that obtained in nuclear reactors. Autoclave testing to initiate SCC was performed and grain boundary elemental composition measured. Evidence of crack bridging was observed in the autoclave sample and differences in segregation behaviour were observed. In particular it was noted that while segregation occurred at some random boundaries, no segregation was found at a random boundary which had a plane that lay on the {221} planes of the adjacent grains.
This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in- situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip.
Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models.
Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,
(i) a thorough examination of the microstructure and analysis parameters employed,
(ii) the determination of the degree of sensitisation with subsequent crack path
investigations,
(iii) the identification of a suitable test system with associated grain boundary
susceptibility criteria,
(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,
(v) statistical crack length assessments of calibrated IGSCC test specimens.
The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.