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David Collins

University of Birmingham, Metallurgy and Materials, Faculty Member

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• Micromechanics of materials during deformation and solid state phase transformations
• Microstructural evolution of high temperature and strength nickel-base superalloys
• Sheet forming processes of high strength steels for automotive applications
• Observation of material behaviour using synchrotron X-ray and neutron diffraction, SANS, SAXS, TEM and high resolution EBSD techniques.
Phone: +44 (0) 1865 273768
Address: Department of Materials
University of Oxford
Parks Road
Oxford
OX1 3PH

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Chinnapat Panwisawas

Queen Mary, University of London

Rutherford Appleton Laboratory

Queen's University at Kingston

Haroldo C . Pinto

USP

Max Planck Institute for Iron Research

Marie-Hélène Mathon

CEA

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Papers by David Collins

$Research paper thumbnail of In-situ neutron diffraction during stress relaxation of a single crystal nickel-base superalloy$

In-situ neutron diffraction during stress relaxation of a single crystal nickel-base superalloy

by David Collins and Chinnapat Panwisawas

The stress relaxation behaviour of a single crystal nickel-base superalloy has been quantified us... more The stress relaxation behaviour of a single crystal nickel-base superalloy has been quantified using time-of-flight neutron diffraction analysis for a range of temperatures relevant to casting. A new iterative analysis
methodology is described to isolate the lattice strain behaviour of the
gamma matrix and gamma' precipitate phases from data obtained sufficiently rapidly to help elucidate the microscopic effect of macroscopic stress relaxation. The independent response of gamma and gamma' is revealed, showing the temperature sensitivity of lattice strain relaxation. The gamma/gamma' response is discussed in the context of thermo-mechanical conditions that may affect the propensity for recrystallisation.

$Research paper thumbnail of Dynamic contact strain measurement by time-resolved stroboscopic energy dispersive synchrotron X-ray diffraction$

Dynamic contact strain measurement by time-resolved stroboscopic energy dispersive synchrotron X-ray diffraction

by David Collins and Selim Barhli

Recent developments of synchrotron X-ray sources and dedicated high-energy beamlines are now enab... more Recent developments of synchrotron X-ray sources and dedicated high-energy beamlines are now enabling strain measurements from large volumes of industrially relevant metallic materials. Such capability is allowing the validation of novel and alternative non-destructive experimental methods of strain measurement or computational models of complex deformation processes. This study describes the first dynamic contact strain measurement of a ball bearing using stroboscopic Energy Dispersive X-ray Diffraction (EDXD). The experiment probed the dynamic contact strain in the outer raceway of a test bearing. The inner raceway of the bearing was attached to a shaft rotating at 150 revolutions per minute and the outer raceway, where the measurements were made, was fixed in a stationary bearing housing. A triggering system was used to synchronise the data acquisition of the EDXD detector with the bearing rotation. Specifically, diffraction data was acquired, stroboscopically, from the material volume within the raceway, in a known location, when the ball was positioned directly below it. A total of 20 seconds of accumulated diffraction signal was recorded, acquiring 2 milliseconds of data per revolution, providing diffraction patterns of sufficient quality for the dynamic contact strain to be measured. Macromechanical stress field was calculated from the micromechanical strains measured from five lattice planes. This allowed a comparison of the experimentally measured stress field and that of finite element (FE) simulations. Good agreement was observed between the FE results and experimental measurements indicating the applicability of this novel dynamic strain measurement technique for tribological systems.

Environmentally-assisted grain boundary attack as a mechanism of embrittlement in a nickel-based superalloy

The loss of ductility in the high strength polycrystalline superalloy 720Li is studied in air bet... more The loss of ductility in the high strength polycrystalline superalloy 720Li is studied in air between room temperature and 1000 C. Tensile ductility is influenced profoundly by the environment, leading to a pronounced minimum at 750 C. A relationship between tensile ductility and oxidation kinetics is identified. The physical factors responsible for the ductility dip are established using energy-dispersive X-ray spectroscopy, nanoscale secondary ion mass spectrometry and the analysis of electron back-scatter diffraction patterns. Embrittlement results from internal intergranular oxidation along the g-grain boundaries, and in particular, at incoherent interfaces of the primary g 0 precipitates with the matrix phase. These fail under local microstresses arising from the accumulation of dislocations during slip-assisted grain boundary sliding. Above 850 C, ductility is restored because the accumulation of dislo-cations at grain boundaries is no longer prevalent.

On the role of boron on improving ductility in a new polycrystalline superalloy

The role of boron in promoting ductility at high temperature in a prototype nickel-based superall... more The role of boron in promoting ductility at high temperature in a prototype nickel-based superalloy designed for industrial gas turbines is studied. Both a boron-containing and boron-free variant are tested in tension at 750 C, with further in-situ tests carried out using scanning electron microscopy (SEM), to clarify the mechanism of ductility improvement. The improvement in ductility is observed to be greater at the lowest investigated strain rate, where the grain boundary character plays a significant role on the mechanical properties; no ductility improvement was observed at the highest investigated strain rate. The in-situ tests were also performed at 750 C and revealed directly the greater susceptibility of the grain boundary morphology in the boron-free case to fracture and e in the boron-containing case e the mechanism of ductility enhancement. The findings are supported further by high-resolution electron backscattered diffraction (HR-EBSD) strain mapping which confirms that the distribution of elastic strain and geometrically necessary dislocation (GND) content are influenced markedly by boron addition. The mechanism through which boron indirectly enhances the mechanical properties at elevated temperatures is discussed.

Strengthening mechanisms in an Al-Fe-Cr-Ti nano-quasicrystalline alloy and composites

by David Collins, Fernando Audebert, and M. Galano

We report a study of the structure-processing-property relationships in a high strength Al 93 Fe ... more We report a study of the structure-processing-property relationships in a high strength Al 93 Fe 3 Cr 2 Ti 2 nano-quasicrystalline alloy and composites containing 10 and 20 vol% ductilising pure Al fibres. The superimposed contributions of several different strengthening mechanisms have been modelled analytically using data obtained from systematic characterisation of the monolithic alloy bar. An observed yield strength of 544 MPa has been substantiated from a combination of solid solution strengthening, work hardening, precipitation hardening and Hall-Petch grain size dependent effects. These materials have been shown by other authors in previous published work to be highly sensitive to the size distribution of particles in the powder from which they are made, and the subsequent thermomechanical processing conditions. The processing condition employed in this study provided micron-sized grains with a strong [111] preferential orientation along the extrusion direction and a bimodal size distribution of the icosahedral nano-quasicrystalline precipitates. Both were deemed to be a significant contributor to the high yield strength observed. The addition of pure Al fibres was found to decrease the yield strength linearly with increasing Al content, and to augment the ductility of the composites.

$Research paper thumbnail of A synchrotron X-ray diffraction study of non-proportional strain-path effects$

A synchrotron X-ray diffraction study of non-proportional strain-path effects

by David Collins and Angus Wilkinson

Common alloys used in sheet form can display a significant ductility benefit when they are subjec... more Common alloys used in sheet form can display a significant ductility benefit when they are subjected to certain multiaxial strain paths. This effect has been studied here for a polycrystalline ferritic steel using a combination of Nakajima bulge testing, X-ray diffraction during biaxial testing of cruciform samples and crystal plasticity finite element (CPFE) modelling. Greatest gains in strain to failure were found when subjecting sheets to uniaxial loading followed by balanced biaxial deformation, resulting in a total deformation close to plane-strain. A combined strain of approximately double that of proportional loading was achieved. The evolution of macrostrain, microstrain and texture during non-proportional loading were evaluated by in-situ high energy synchrotron diffraction. The results have demonstrated that the inhomogeneous strain accumulation from non-proportional deformation is strongly dependent on texture and the applied strain-ratio of the first deformation pass. Experimental diffraction evidence is supported by results produced by a novel method of CPFE-derived diffraction simulation. Using constitutive laws selected on the basis of good agreement with measured lattice strain development, the CPFE model demonstrated the capability to replicate ductility gains measured experimentally.

$Research paper thumbnail of Assessment of X-ray diffraction and crystal plasticity lattice strain evolutions under biaxial loading$

Assessment of X-ray diffraction and crystal plasticity lattice strain evolutions under biaxial loading

A methodology to simulate X-ray diffraction lattice strains using crystal plasticity, replicating... more A methodology to simulate X-ray diffraction lattice strains using crystal plasticity, replicating in-situ synchrotron experimental measurements during the deformation of a low-carbon steel, has been developed. Uniquely, the model calculated lattice strains for full Debye–Scherrer diffraction rings, providing the in-plane lattice strain distributions determined from crystal plasticity. Thus, a direct method of comparison between experimental and crystal plasticity results becomes possible. The model considered two forms of hardening whilst subjecting the material to two dissimilar proportional strain-paths; uniaxial and balanced biaxial deformation. Both deformation paths showed influence on resulting lattice strain distributions which were also found to depend upon texture. Biaxial straining led to a stronger dependence on the material's hardening behaviour and this was attributed to the higher rate of work hardening seen under biaxial compared to uniaxial straining. However, biaxial deformation showed quite isotropic lattice strains distribution, irrespective of initial texture or hardening. Quantitatively, good agreement between the computed and experimentally determined lattice strain distributions was obtained for each strain path. This success demonstrates the possibility of calibrating crystal plasticity model parameters using such methodologies, or simply to provide insight into the governing mechanisms in polycrystal deformation.

$Research paper thumbnail of Analysis of Dislocation Densities using High Resolution Electron Backscatter Diffraction$

Analysis of Dislocation Densities using High Resolution Electron Backscatter Diffraction

by David Collins, Arantxa Vilalta-Clemente, and Jun Jiang

In situ measurement of the strains within a mechanically loaded polygranular graphite

by Dong Liu, James Marrow, Selim Barhli, Yelena Vertyagina, David Collins, and Peter E J Flewitt

Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in ... more Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in situ, the mechanical response to tensile and bending loading of polygranular Gilsocarbon nuclear grade near-isotropic graphite (grade IM1-24). Digital image correlation of X-ray radiographs and digital volume correlation of tomographs allow measurement of bulk elastic moduli and examination of the heterogeneity of deformation in the microstructure. Both the neutron and X-ray studies show the application of tensile strain reduces the bulk elastic modulus. A permanent set is observed to develop with applied tensile strain. The elastic strains within the graphite crystals were measured by diffraction; a cross-correlation analysis method has been applied for greater speed, robustness and improved precision in the measurement of the change in basal plane separation distance. In compression, a linear relation is observed between the elastic strains in the graphite crystals and the applied strain. In tension, this relationship is non-linear. The results are discussed with respect to the distribution of elastic and inelastic strain within the graphite microstructure. It is deduced that the significant residual elastic strains in the as- manufactured graphite are relaxed by microcracking as tensile strain is applied.

Measurements of stress fields near a grain boundary: Exploring blocked arrays of dislocations in 3D

The interaction between dislocation pile-ups and grain boundaries gives rise to heterogeneous str... more The interaction between dislocation pile-ups and grain boundaries gives rise to heterogeneous stress distributions when a structural metal is subjected to mechanical loading. Such stress heterogeneity leads to preferential sites for damage nucleation and therefore is intrinsically linked to the strength and ductility of polycrystalline metals. To date the majority of conclusions have been drawn from 2D experimental investigations at the sample surface, allowing only incomplete observations. Our purpose here is to significantly advance the understanding of such problems by providing quantitative measurements of the effects of dislocation pile up and grain boundary interactions in 3D. This is accomplished through the application of differential aperture X-ray Laue micro-diffraction (DAXM) and high angular resolution electron backscatter diffraction (HR-EBSD) techniques. Our analysis demonstrates a similar strain characterization capability between DAXM and HR-EBSD and the variation of stress intensity in 3D reveals that different parts of the same grain boundary may have different strengths in resisting slip transfer, likely due to the local grain boundary curvature.

$Research paper thumbnail of Time-resolved synchrotron diffractometry of phase transformations in high strength nickel-based superalloys$

Time-resolved synchrotron diffractometry of phase transformations in high strength nickel-based superalloys

Phase transformations in prototype high strength polycrystalline nickel-based superalloys of vary... more Phase transformations in prototype high strength polycrystalline nickel-based superalloys of varying Ti/Nb ratio are studied using time-resolved, high resolution X-ray synchrotron diffractometry. The dissolution kinetics of the ordered phase Ni3(Al, Ti, Nb, Ta) upon heating to the solutioning temperature of ∼∼1200 °C and its reprecipitation on cooling are deduced; effects of varying Nb and Ti alloy composition on the reaction kinetics are identified. Heating to 800 °C does not alter substantially the fraction of the strengthening phase Ni3(Al, Ti, Nb, Ta) but further heating causes its rapid dissolution. At higher temperatures, evidence is provided for the formation of further ordered phases; Ni3(Ti, Ta) is proposed and possibly Ni0.45Ta0.55; cooling causes their dissolution and reprecipitation of Ni3(Al, Ti, Nb, Ta), so that it seems probable that the reactions are coupled. The unforeseen high temperature precipitation of further ordering by phases other than Ni3(Al, Ti, Nb, Ta) implies the possibility of a contribution by them to the high temperature mechanical behaviour of these materials, which until now has been thought to be solely due to Ni3(Al, Ti, Nb, Ta). The MC carbide, probably TiC, is stable even at the solution heat treatment temperature; no evidence of reactions involving other carbides such as M23C6 is found.

$Research paper thumbnail of A synchrotron X-ray diffraction study of in situ biaxial deformation$

A synchrotron X-ray diffraction study of in situ biaxial deformation

The biaxial deformation of a ferritic sheet steel has been examined using high energy in situ X-r... more The biaxial deformation of a ferritic sheet steel has been examined using high energy in situ X-ray diffraction. A purpose built biaxial loading mechanism was constructed to enable deformation across a wide range of strain ratios. Three nominal deformation conditions were compared: (1) uniaxial loading, ∊TD/∊RD=-ν∊TD/∊RD=-ν, (2) biaxial deformation where ∊TD/∊RD=0.4∊TD/∊RD=0.4, and (3) approximately balanced biaxial deformation, with ∊TD/∊RD=1.5∊TD/∊RD=1.5. This novel setup allowed the full Debye–Scherrer diffraction rings to be acquired during arbitrary selected strain-paths, permitting lattice strains and reflection intensities to be measured across an unrivalled grain orientation range for such deformation conditions. This experiment reveals that the accumulation of lattice strain during deformation, as a function of azimuthal angle, is highly sensitive to strain path. For the ∊TD/∊RD=1.5∊TD/∊RD=1.5 strain path, whilst lattice strain accumulates most rapidly in the ∊TD∊TD direction during early stages of plastic deformation, the lattice strain is shown to distribute almost perfectly isotropically for the observed orientations when plastic strain is high. This was found to be in contrast to strain paths where ∊TD/∊RD≪1.5∊TD/∊RD≪1.5, demonstrating that lattice strain magnitudes remain highest in the direction parallel to the tensile axis with the highest applied load. Furthermore, the technique provides the capability to observe the evolution of texture fibres via changes in reflection intensity during different applied strain ratios.

Measurement of probability distributions for internal stresses in dislocated crystals

Here, we analyse residual stress distributions obtained from various crystal systems using high r... more Here, we analyse residual stress distributions obtained from various crystal systems using high resolution electron backscatter diffraction (EBSD) measurements. Histograms showing stress probability distributions exhibit tails extending to very high stress levels. We demonstrate that these extreme stress values are consistent with the functional form that should be expected for dislocated crystals. Analysis initially developed by Groma and co-workers for X-ray line profile analysis and based on the so-called “restricted second moment of the probability distribution” can be used to estimate the total dislocation density. The generality of the results are illustrated by application to three quite different systems, namely, face centred cubic Cu deformed in uniaxial tension, a body centred cubic steel deformed to larger strain by cold rolling, and hexagonal InAlN layers grown on misfitting sapphire and silicon carbide substrates.

Yield behavior beneath hardness indentations in ductile metals, measured by three-dimensional computed X-ray tomography and digital volume correlation

Plastic deformation in the vicinity of hardness indentations in two ductile metals, an aluminium–... more Plastic deformation in the vicinity of hardness indentations in two ductile metals, an aluminium–silicon carbide composite (Al–SiC) and a cast magnesium alloy (WE43), was characterized. Using in situ and ex situ X-ray computed tomography with digital volume correlation, the three-dimensional full-field displacements beneath the indentations were measured; these show strong agreement with simulated predictions using finite element models. It is demonstrated that plastic material properties, i.e. yield stress and hardening exponent, may be extracted via analysis of the deformation field.

$Research paper thumbnail of Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading$

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

Electron backscattered diffraction (EBSD) has been employed to study a polycrystalline nickel sup... more Electron backscattered diffraction (EBSD) has been employed to study a polycrystalline nickel superalloy containing a complex non-metallic agglomerate under thermal loading. Heterogeneous distributions of elastic strains are observed near the inclusion due to its complex geometry and these have been quantified. Lattice rotations were also related to geometrically necessary dislocation (GND) density (View the MathML source∼1014m-2), indicating the development of localized plasticity arising from the mismatch in thermal expansivity between the Ni polycrystal and the inclusion. A crystal plasticity finite-element (CPFE) model which explicitly represents the full detail of the complex microstructure was developed to interpret the experimental measurements, and good quantitative and qualitative agreement has been obtained. However, a limitation of the EBSD technique when investigating polycrystal systems is that full-field, transgranular strain measurement remains difficult due to the necessity to reference a lattice spacing within a grain for strain calculation. An inverse reference shifting methodology has been developed using CPFE modeling to overcome this problem, thereby allowing like-for-like and grain-by-grain strain comparisons to be made. The method, in conjunction with high-resolution EBSD, shows promise for the determination of full-field strains and rotations in polycrystalline materials, and provides key information for fatigue nucleation in these material systems.

3D Studies of Damage by Combined X-ray Tomography and Digital Volume Correlation

The Effect of Cooling Rate from Solution on the Lattice Misfit During Isothermal Aging of a Ni-Base Superalloy

The evolution of lattice misfit in the polycrystalline nickel-base superalloy, RR1000, has been i... more The evolution of lattice misfit in the polycrystalline nickel-base superalloy, RR1000, has been investigated using high resolution neutron diffraction at interrupted time intervals during an aging heat treatment. Samples were subjected to a super-solvus heat treatment followed by either a 100 or a 1 K min−1 cooling rate prior to aging. Irrespective of cooling rate, the lattice misfit remained unchanged at approximately 0.1 pct throughout the aging cycle, indicating the microstructure remained stable. Microstructural observations validated this result for samples cooled at 1 K min−1. However, for the faster, 100 K min−1, cooling rate, whilst the secondary γ′ remained unchanged, the tertiary γ′ showed significant coarsening. Simulated diffraction patterns were used to investigate the influence of volume fraction, particle size, and lattice parameter of individual γ′ distributions on the measured lattice misfit. The results obtained indicate that conventional methods of measuring lattice misfit will be dominated by the γ′ distribution with the highest volume fraction, and may therefore obscure subtle changes in the γ′ distributions with lower a volume fraction.

Lattice misfit during ageing of a polycrystalline nickel-base superalloy

A modelling approach to yield strength optimisation in a nickel-base superalloy

Grain growth behaviour during near-γ′ solvus thermal exposures in a polycrystalline nickel-base superalloy

$Research paper thumbnail of In-situ neutron diffraction during stress relaxation of a single crystal nickel-base superalloy$

In-situ neutron diffraction during stress relaxation of a single crystal nickel-base superalloy

by David Collins and Chinnapat Panwisawas

The stress relaxation behaviour of a single crystal nickel-base superalloy has been quantified us... more The stress relaxation behaviour of a single crystal nickel-base superalloy has been quantified using time-of-flight neutron diffraction analysis for a range of temperatures relevant to casting. A new iterative analysis
methodology is described to isolate the lattice strain behaviour of the
gamma matrix and gamma' precipitate phases from data obtained sufficiently rapidly to help elucidate the microscopic effect of macroscopic stress relaxation. The independent response of gamma and gamma' is revealed, showing the temperature sensitivity of lattice strain relaxation. The gamma/gamma' response is discussed in the context of thermo-mechanical conditions that may affect the propensity for recrystallisation.

$Research paper thumbnail of Dynamic contact strain measurement by time-resolved stroboscopic energy dispersive synchrotron X-ray diffraction$

Dynamic contact strain measurement by time-resolved stroboscopic energy dispersive synchrotron X-ray diffraction

by David Collins and Selim Barhli

Recent developments of synchrotron X-ray sources and dedicated high-energy beamlines are now enab... more Recent developments of synchrotron X-ray sources and dedicated high-energy beamlines are now enabling strain measurements from large volumes of industrially relevant metallic materials. Such capability is allowing the validation of novel and alternative non-destructive experimental methods of strain measurement or computational models of complex deformation processes. This study describes the first dynamic contact strain measurement of a ball bearing using stroboscopic Energy Dispersive X-ray Diffraction (EDXD). The experiment probed the dynamic contact strain in the outer raceway of a test bearing. The inner raceway of the bearing was attached to a shaft rotating at 150 revolutions per minute and the outer raceway, where the measurements were made, was fixed in a stationary bearing housing. A triggering system was used to synchronise the data acquisition of the EDXD detector with the bearing rotation. Specifically, diffraction data was acquired, stroboscopically, from the material volume within the raceway, in a known location, when the ball was positioned directly below it. A total of 20 seconds of accumulated diffraction signal was recorded, acquiring 2 milliseconds of data per revolution, providing diffraction patterns of sufficient quality for the dynamic contact strain to be measured. Macromechanical stress field was calculated from the micromechanical strains measured from five lattice planes. This allowed a comparison of the experimentally measured stress field and that of finite element (FE) simulations. Good agreement was observed between the FE results and experimental measurements indicating the applicability of this novel dynamic strain measurement technique for tribological systems.

Environmentally-assisted grain boundary attack as a mechanism of embrittlement in a nickel-based superalloy

The loss of ductility in the high strength polycrystalline superalloy 720Li is studied in air bet... more The loss of ductility in the high strength polycrystalline superalloy 720Li is studied in air between room temperature and 1000 C. Tensile ductility is influenced profoundly by the environment, leading to a pronounced minimum at 750 C. A relationship between tensile ductility and oxidation kinetics is identified. The physical factors responsible for the ductility dip are established using energy-dispersive X-ray spectroscopy, nanoscale secondary ion mass spectrometry and the analysis of electron back-scatter diffraction patterns. Embrittlement results from internal intergranular oxidation along the g-grain boundaries, and in particular, at incoherent interfaces of the primary g 0 precipitates with the matrix phase. These fail under local microstresses arising from the accumulation of dislocations during slip-assisted grain boundary sliding. Above 850 C, ductility is restored because the accumulation of dislo-cations at grain boundaries is no longer prevalent.

On the role of boron on improving ductility in a new polycrystalline superalloy

The role of boron in promoting ductility at high temperature in a prototype nickel-based superall... more The role of boron in promoting ductility at high temperature in a prototype nickel-based superalloy designed for industrial gas turbines is studied. Both a boron-containing and boron-free variant are tested in tension at 750 C, with further in-situ tests carried out using scanning electron microscopy (SEM), to clarify the mechanism of ductility improvement. The improvement in ductility is observed to be greater at the lowest investigated strain rate, where the grain boundary character plays a significant role on the mechanical properties; no ductility improvement was observed at the highest investigated strain rate. The in-situ tests were also performed at 750 C and revealed directly the greater susceptibility of the grain boundary morphology in the boron-free case to fracture and e in the boron-containing case e the mechanism of ductility enhancement. The findings are supported further by high-resolution electron backscattered diffraction (HR-EBSD) strain mapping which confirms that the distribution of elastic strain and geometrically necessary dislocation (GND) content are influenced markedly by boron addition. The mechanism through which boron indirectly enhances the mechanical properties at elevated temperatures is discussed.

Strengthening mechanisms in an Al-Fe-Cr-Ti nano-quasicrystalline alloy and composites

by David Collins, Fernando Audebert, and M. Galano

We report a study of the structure-processing-property relationships in a high strength Al 93 Fe ... more We report a study of the structure-processing-property relationships in a high strength Al 93 Fe 3 Cr 2 Ti 2 nano-quasicrystalline alloy and composites containing 10 and 20 vol% ductilising pure Al fibres. The superimposed contributions of several different strengthening mechanisms have been modelled analytically using data obtained from systematic characterisation of the monolithic alloy bar. An observed yield strength of 544 MPa has been substantiated from a combination of solid solution strengthening, work hardening, precipitation hardening and Hall-Petch grain size dependent effects. These materials have been shown by other authors in previous published work to be highly sensitive to the size distribution of particles in the powder from which they are made, and the subsequent thermomechanical processing conditions. The processing condition employed in this study provided micron-sized grains with a strong [111] preferential orientation along the extrusion direction and a bimodal size distribution of the icosahedral nano-quasicrystalline precipitates. Both were deemed to be a significant contributor to the high yield strength observed. The addition of pure Al fibres was found to decrease the yield strength linearly with increasing Al content, and to augment the ductility of the composites.

$Research paper thumbnail of A synchrotron X-ray diffraction study of non-proportional strain-path effects$

A synchrotron X-ray diffraction study of non-proportional strain-path effects

by David Collins and Angus Wilkinson

Common alloys used in sheet form can display a significant ductility benefit when they are subjec... more Common alloys used in sheet form can display a significant ductility benefit when they are subjected to certain multiaxial strain paths. This effect has been studied here for a polycrystalline ferritic steel using a combination of Nakajima bulge testing, X-ray diffraction during biaxial testing of cruciform samples and crystal plasticity finite element (CPFE) modelling. Greatest gains in strain to failure were found when subjecting sheets to uniaxial loading followed by balanced biaxial deformation, resulting in a total deformation close to plane-strain. A combined strain of approximately double that of proportional loading was achieved. The evolution of macrostrain, microstrain and texture during non-proportional loading were evaluated by in-situ high energy synchrotron diffraction. The results have demonstrated that the inhomogeneous strain accumulation from non-proportional deformation is strongly dependent on texture and the applied strain-ratio of the first deformation pass. Experimental diffraction evidence is supported by results produced by a novel method of CPFE-derived diffraction simulation. Using constitutive laws selected on the basis of good agreement with measured lattice strain development, the CPFE model demonstrated the capability to replicate ductility gains measured experimentally.

$Research paper thumbnail of Assessment of X-ray diffraction and crystal plasticity lattice strain evolutions under biaxial loading$

Assessment of X-ray diffraction and crystal plasticity lattice strain evolutions under biaxial loading

A methodology to simulate X-ray diffraction lattice strains using crystal plasticity, replicating... more A methodology to simulate X-ray diffraction lattice strains using crystal plasticity, replicating in-situ synchrotron experimental measurements during the deformation of a low-carbon steel, has been developed. Uniquely, the model calculated lattice strains for full Debye–Scherrer diffraction rings, providing the in-plane lattice strain distributions determined from crystal plasticity. Thus, a direct method of comparison between experimental and crystal plasticity results becomes possible. The model considered two forms of hardening whilst subjecting the material to two dissimilar proportional strain-paths; uniaxial and balanced biaxial deformation. Both deformation paths showed influence on resulting lattice strain distributions which were also found to depend upon texture. Biaxial straining led to a stronger dependence on the material's hardening behaviour and this was attributed to the higher rate of work hardening seen under biaxial compared to uniaxial straining. However, biaxial deformation showed quite isotropic lattice strains distribution, irrespective of initial texture or hardening. Quantitatively, good agreement between the computed and experimentally determined lattice strain distributions was obtained for each strain path. This success demonstrates the possibility of calibrating crystal plasticity model parameters using such methodologies, or simply to provide insight into the governing mechanisms in polycrystal deformation.

$Research paper thumbnail of Analysis of Dislocation Densities using High Resolution Electron Backscatter Diffraction$

Analysis of Dislocation Densities using High Resolution Electron Backscatter Diffraction

by David Collins, Arantxa Vilalta-Clemente, and Jun Jiang

In situ measurement of the strains within a mechanically loaded polygranular graphite

by Dong Liu, James Marrow, Selim Barhli, Yelena Vertyagina, David Collins, and Peter E J Flewitt

Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in ... more Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in situ, the mechanical response to tensile and bending loading of polygranular Gilsocarbon nuclear grade near-isotropic graphite (grade IM1-24). Digital image correlation of X-ray radiographs and digital volume correlation of tomographs allow measurement of bulk elastic moduli and examination of the heterogeneity of deformation in the microstructure. Both the neutron and X-ray studies show the application of tensile strain reduces the bulk elastic modulus. A permanent set is observed to develop with applied tensile strain. The elastic strains within the graphite crystals were measured by diffraction; a cross-correlation analysis method has been applied for greater speed, robustness and improved precision in the measurement of the change in basal plane separation distance. In compression, a linear relation is observed between the elastic strains in the graphite crystals and the applied strain. In tension, this relationship is non-linear. The results are discussed with respect to the distribution of elastic and inelastic strain within the graphite microstructure. It is deduced that the significant residual elastic strains in the as- manufactured graphite are relaxed by microcracking as tensile strain is applied.

Measurements of stress fields near a grain boundary: Exploring blocked arrays of dislocations in 3D

The interaction between dislocation pile-ups and grain boundaries gives rise to heterogeneous str... more The interaction between dislocation pile-ups and grain boundaries gives rise to heterogeneous stress distributions when a structural metal is subjected to mechanical loading. Such stress heterogeneity leads to preferential sites for damage nucleation and therefore is intrinsically linked to the strength and ductility of polycrystalline metals. To date the majority of conclusions have been drawn from 2D experimental investigations at the sample surface, allowing only incomplete observations. Our purpose here is to significantly advance the understanding of such problems by providing quantitative measurements of the effects of dislocation pile up and grain boundary interactions in 3D. This is accomplished through the application of differential aperture X-ray Laue micro-diffraction (DAXM) and high angular resolution electron backscatter diffraction (HR-EBSD) techniques. Our analysis demonstrates a similar strain characterization capability between DAXM and HR-EBSD and the variation of stress intensity in 3D reveals that different parts of the same grain boundary may have different strengths in resisting slip transfer, likely due to the local grain boundary curvature.

$Research paper thumbnail of Time-resolved synchrotron diffractometry of phase transformations in high strength nickel-based superalloys$

Time-resolved synchrotron diffractometry of phase transformations in high strength nickel-based superalloys

Phase transformations in prototype high strength polycrystalline nickel-based superalloys of vary... more Phase transformations in prototype high strength polycrystalline nickel-based superalloys of varying Ti/Nb ratio are studied using time-resolved, high resolution X-ray synchrotron diffractometry. The dissolution kinetics of the ordered phase Ni3(Al, Ti, Nb, Ta) upon heating to the solutioning temperature of ∼∼1200 °C and its reprecipitation on cooling are deduced; effects of varying Nb and Ti alloy composition on the reaction kinetics are identified. Heating to 800 °C does not alter substantially the fraction of the strengthening phase Ni3(Al, Ti, Nb, Ta) but further heating causes its rapid dissolution. At higher temperatures, evidence is provided for the formation of further ordered phases; Ni3(Ti, Ta) is proposed and possibly Ni0.45Ta0.55; cooling causes their dissolution and reprecipitation of Ni3(Al, Ti, Nb, Ta), so that it seems probable that the reactions are coupled. The unforeseen high temperature precipitation of further ordering by phases other than Ni3(Al, Ti, Nb, Ta) implies the possibility of a contribution by them to the high temperature mechanical behaviour of these materials, which until now has been thought to be solely due to Ni3(Al, Ti, Nb, Ta). The MC carbide, probably TiC, is stable even at the solution heat treatment temperature; no evidence of reactions involving other carbides such as M23C6 is found.

$Research paper thumbnail of A synchrotron X-ray diffraction study of in situ biaxial deformation$

A synchrotron X-ray diffraction study of in situ biaxial deformation

The biaxial deformation of a ferritic sheet steel has been examined using high energy in situ X-r... more The biaxial deformation of a ferritic sheet steel has been examined using high energy in situ X-ray diffraction. A purpose built biaxial loading mechanism was constructed to enable deformation across a wide range of strain ratios. Three nominal deformation conditions were compared: (1) uniaxial loading, ∊TD/∊RD=-ν∊TD/∊RD=-ν, (2) biaxial deformation where ∊TD/∊RD=0.4∊TD/∊RD=0.4, and (3) approximately balanced biaxial deformation, with ∊TD/∊RD=1.5∊TD/∊RD=1.5. This novel setup allowed the full Debye–Scherrer diffraction rings to be acquired during arbitrary selected strain-paths, permitting lattice strains and reflection intensities to be measured across an unrivalled grain orientation range for such deformation conditions. This experiment reveals that the accumulation of lattice strain during deformation, as a function of azimuthal angle, is highly sensitive to strain path. For the ∊TD/∊RD=1.5∊TD/∊RD=1.5 strain path, whilst lattice strain accumulates most rapidly in the ∊TD∊TD direction during early stages of plastic deformation, the lattice strain is shown to distribute almost perfectly isotropically for the observed orientations when plastic strain is high. This was found to be in contrast to strain paths where ∊TD/∊RD≪1.5∊TD/∊RD≪1.5, demonstrating that lattice strain magnitudes remain highest in the direction parallel to the tensile axis with the highest applied load. Furthermore, the technique provides the capability to observe the evolution of texture fibres via changes in reflection intensity during different applied strain ratios.

Measurement of probability distributions for internal stresses in dislocated crystals

Here, we analyse residual stress distributions obtained from various crystal systems using high r... more Here, we analyse residual stress distributions obtained from various crystal systems using high resolution electron backscatter diffraction (EBSD) measurements. Histograms showing stress probability distributions exhibit tails extending to very high stress levels. We demonstrate that these extreme stress values are consistent with the functional form that should be expected for dislocated crystals. Analysis initially developed by Groma and co-workers for X-ray line profile analysis and based on the so-called “restricted second moment of the probability distribution” can be used to estimate the total dislocation density. The generality of the results are illustrated by application to three quite different systems, namely, face centred cubic Cu deformed in uniaxial tension, a body centred cubic steel deformed to larger strain by cold rolling, and hexagonal InAlN layers grown on misfitting sapphire and silicon carbide substrates.

Yield behavior beneath hardness indentations in ductile metals, measured by three-dimensional computed X-ray tomography and digital volume correlation

Plastic deformation in the vicinity of hardness indentations in two ductile metals, an aluminium–... more Plastic deformation in the vicinity of hardness indentations in two ductile metals, an aluminium–silicon carbide composite (Al–SiC) and a cast magnesium alloy (WE43), was characterized. Using in situ and ex situ X-ray computed tomography with digital volume correlation, the three-dimensional full-field displacements beneath the indentations were measured; these show strong agreement with simulated predictions using finite element models. It is demonstrated that plastic material properties, i.e. yield stress and hardening exponent, may be extracted via analysis of the deformation field.

$Research paper thumbnail of Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading$

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

Electron backscattered diffraction (EBSD) has been employed to study a polycrystalline nickel sup... more Electron backscattered diffraction (EBSD) has been employed to study a polycrystalline nickel superalloy containing a complex non-metallic agglomerate under thermal loading. Heterogeneous distributions of elastic strains are observed near the inclusion due to its complex geometry and these have been quantified. Lattice rotations were also related to geometrically necessary dislocation (GND) density (View the MathML source∼1014m-2), indicating the development of localized plasticity arising from the mismatch in thermal expansivity between the Ni polycrystal and the inclusion. A crystal plasticity finite-element (CPFE) model which explicitly represents the full detail of the complex microstructure was developed to interpret the experimental measurements, and good quantitative and qualitative agreement has been obtained. However, a limitation of the EBSD technique when investigating polycrystal systems is that full-field, transgranular strain measurement remains difficult due to the necessity to reference a lattice spacing within a grain for strain calculation. An inverse reference shifting methodology has been developed using CPFE modeling to overcome this problem, thereby allowing like-for-like and grain-by-grain strain comparisons to be made. The method, in conjunction with high-resolution EBSD, shows promise for the determination of full-field strains and rotations in polycrystalline materials, and provides key information for fatigue nucleation in these material systems.

3D Studies of Damage by Combined X-ray Tomography and Digital Volume Correlation

The Effect of Cooling Rate from Solution on the Lattice Misfit During Isothermal Aging of a Ni-Base Superalloy

The evolution of lattice misfit in the polycrystalline nickel-base superalloy, RR1000, has been i... more The evolution of lattice misfit in the polycrystalline nickel-base superalloy, RR1000, has been investigated using high resolution neutron diffraction at interrupted time intervals during an aging heat treatment. Samples were subjected to a super-solvus heat treatment followed by either a 100 or a 1 K min−1 cooling rate prior to aging. Irrespective of cooling rate, the lattice misfit remained unchanged at approximately 0.1 pct throughout the aging cycle, indicating the microstructure remained stable. Microstructural observations validated this result for samples cooled at 1 K min−1. However, for the faster, 100 K min−1, cooling rate, whilst the secondary γ′ remained unchanged, the tertiary γ′ showed significant coarsening. Simulated diffraction patterns were used to investigate the influence of volume fraction, particle size, and lattice parameter of individual γ′ distributions on the measured lattice misfit. The results obtained indicate that conventional methods of measuring lattice misfit will be dominated by the γ′ distribution with the highest volume fraction, and may therefore obscure subtle changes in the γ′ distributions with lower a volume fraction.

Lattice misfit during ageing of a polycrystalline nickel-base superalloy

A modelling approach to yield strength optimisation in a nickel-base superalloy

Grain growth behaviour during near-γ′ solvus thermal exposures in a polycrystalline nickel-base superalloy

$Research paper thumbnail of Studying the fracture process zone in a heterogeneous brittle material (polygranular nuclear graphite) by synchrotron computed tomography, image correlation and X-ray diffraction$

Studying the fracture process zone in a heterogeneous brittle material (polygranular nuclear graphite) by synchrotron computed tomography, image correlation and X-ray diffraction

by Selim Barhli, James Marrow, David Collins, and Yelena Vertyagina

Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, howe... more Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, however, a heterogeneous quasi-brittle material, with non-linear mechanical behavior and the development of a micro-cracked fracture process zone, which can cause strength to vary with size. Small test specimens from nuclear graphite, which are extracted either from operating reactors or used in material test reactor (MTR) accelerated experiments, provide the data to predict the performance of structural components; it is necessary to have confidence that such small specimen tests are representative and conservative. The objective of this work is to better understand how the microstructure of a coarse grained polygranular graphite accommodates applied strain, and the effect of this applied strain on its mechanical properties. To study this, it is necessary to be able to observe, in situ, the relationship between the applied strains, the total strains in the material’s microstructure and the elastic strains in the crystals.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of graphite. Microcracked fracture process zones are common to quasi-brittle materials as diverse as high toughness monolithic ceramics, polymeric and natural biological composites, geological minerals and even volcanic structures. Experimental methods that support the study and modeling of damage development are thus important to a wide range of problems, beyond nuclear graphite.