arXiv subjects

David Dye

Publications and source records attributed to David Dye.

At least 19 recordsLinked to original sources

arXiv2024

Mechanisms of Elevated Temperature Galling in Hardfacings

The galling mechanism of Tristelle 5183, an Fe-based hardfacing alloy, was investigated at elevated temperature. The test was performed using a bespoke galling rig. Adhesive transfer and galling were found to occur, as a result of shear at the adhesion boundary and the activation of an internal shear plane within one of the tribosurfaces. During deformation, carbides were observed to have fractured, as a result of the shear train they were exposed to and their lack of ductility. In the case of niobium carbides, their fracture resulted in the formation of voids, which were found to coalesce and led to cracking and adhesive transfer. A tribologically affected zone (TAZ) was found to form, which contained nanocrystalline austenite, as a result of the shear exerted within 30{\mu}m of the adhesion boundaries. The galling of Tristelle 5183 initiated from the formation of an adhesive boundary, followed by sub-surface shear in only one tribosurface, Following further sub-surface shear, an internal shear plane is activated. internal shear and shear at the adhesion boundary continues until fracture occur, resulting in adhesive transfer.

physics.app-ph↗

arXiv2023

Slip intermittency and dwell fatigue in titanium alloys: a discrete dislocation plasticity analysis

Slip intermittency and stress oscillations in titanium alloy Ti-7Al-O that were observed using in-situ far-field high energy X-ray diffraction microscopy (ff-HEDM) are investigated using a discrete dislocation plasticity (DDP) model. The mechanistic foundation of slip intermittency and stress oscillations are shown to be dislocation escape from obstacles during stress holds, governed by a thermal activation constitutive law. The stress drop events due to -basal slip are larger in magnitude than those along -prism, which is a consequence of their differing rate sensitivities, previously found from micropillar testing. It is suggested that interstitial oxygen suppresses stress oscillations by inhibiting the thermal activation process. Understanding of these mechanisms is of benefit to the design and safety assessment of jet engine titanium alloys subjected to dwell fatigue.

cond-mat.mtrl-sci↗

arXiv2022

Crystallographic ordering of Al and Sn in {\alpha}-Ti

Increasing attention is being paid to $\alpha$$_2$ Ti$_3$(Al,Sn) precipitation from the $\alpha$ phase of titanium alloys owing to its effect on slip band formation, localisation and the implications for fatigue performance in jet engine titanium. However, the early stages of $\alpha$$_2$ precipitation have historically been difficult to observe in TEM, neutron diffraction or atom probe analysis. Here, small angle X-ray scattering is used to reexamine the phase boundary in binary Ti-Al and Ti-Sn alloys with around 500 ppmw O. It is found that the phase boundaries in the literature are approximately correct, at 6.2 wt.% Al and 16.9 wt.% Sn, and that this favours the use of Al as a solid solution strengthener over Sn for ambient temperature applications. However, once O content and phase partitioning in $\alpha$+$\beta$ alloys are taken into account, this implies that Al$_{eq}$ limits for future alloy design of critical rotating parts should be lowered substantially.

cond-mat.mtrl-sci↗

arXiv2022

Adhesive Transfer operates during Galling

In order to reduce cobalt within the primary circuit of pressurised water reactors (PWRs), wear-resistant steels are being researched and developed. In particular interest is the understanding of galling mechanisms, an adhesive wear mechanism which is particularly prevalent in PWR valves. Here we show that large shear stresses and adhesive transfer occur during galling by exploiting the 2 wt per cent manganese difference between 304L and 316L stainless steels, even at relatively low compressive stresses of 50 MPa. Through these findings, the galling mechanisms of stainless steels can be better understood, which may help with the development of galling resistant stainless steels.

physics.app-ph↗

arXiv2022

The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium

Dwell fatigue, the reduction in fatigue life experienced by titanium alloys due to holds at stresses as low as 60% of yield, has been implicated in several uncontained jet engine failures. Dislocation slip has long been observed to be an intermittent, scale-bridging phenomenon, similar to that seen in earthquakes but at the nanoscale, leading to the speculation that large stress bursts might promote the initial opening of a crack. Here we observe such stress bursts at the scale of individual grains in situ, using high energy X-ray diffraction microscopy in Ti-7Al-O alloys. This shows that the detrimental effect of precipitation of ordered Ti_3Al is to increase the magnitude of rare pri and bas slip bursts associated with slip localisation. In contrast, the addition of trace O interstitials is beneficial, reducing the magnitude of bas slip bursts and increasing the homogeneity between basal and prismatic slip. This is further evidence that the formation of long paths for easy basal plane slip localisation should be avoided when engineering titanium alloys against dwell fatigue.

cond-mat.mtrl-sci↗

arXiv2022

The role of \b{eta} pockets resulting from Fe impurities in hydride formation in titanium

The corrosion potential of commercially pure titanium in NaCl solutions is dramatically affected by trace Fe additions, which cause the appearance of submicron pockets of \b{eta} phase at grain boundary triple points. Furthermore, the low solubility of hydrogen in hexagonal close-packed {\alpha}-Ti makes titanium alloys prone to subsequent hydride-associated failures due to stress corrosion cracking. We analyzed {\alpha}-{\alpha} and {\alpha}-\b{eta} sections of the abutting grain boundary of a \b{eta} pocket in a Grade 2 CP-Ti, and the {\alpha}-\b{eta} phase boundary. Fe and H partition to \b{eta} and segregate at the grain boundary, but no segregation is seen at the {\alpha}-\b{eta} phase boundary. In contrast, a significant Ni (>1 at%) accumulation is observed at the {\alpha}-\b{eta} phase boundary. We propose that the \b{eta}-pockets act as hydrogen traps and facilitate the nucleation and growth of hydrides along grain boundaries in CP-Ti.

cond-mat.mtrl-sci↗

arXiv2021

Precipitate dissolution during deformation induced twin thickening in a CoNi-base superalloy subject to creep

The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal gamma prime distribution has been examined at 800C and 300MPa. The rupture life of the alloy is comparable to that of RR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealed the presence of continuous gamma prime precipitates and M23C6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary gamma prime precipitates occurred at the expense of the fine tertiary gamma prime. Long planar deformation bands, free of gamma prime, were also observed to traverse individual grains ending in steps at the grain boundaries. Examination of the deformation bands confirmed that they were microtwins. Long sections of the microtwins examined were depleted of gamma prime stabilising elements across their entire width, suggesting that certain alloy compositions are susceptible to precipitate dissolution during twin thickening. A mechanism for the dissolution of the precipitates is suggested based on the Kolbe reordering mechanism.

cond-mat.mtrl-sci↗

arXiv2021

Microstructure evolution and tensile behaviour of a cold rolled 8 wt\% Mn medium manganese steel

A novel medium manganese steel named Novalloy with composition Fe-8.3Mn-3.8Al-1.8Si-0.5C-0.06V-0.05Sn was developed and thermomechanically processed through hot rolling and intercritical annealing. The steel possessed a yield strength of 1 GPa, tensile strength of 1.13 GPa and ductility of 41\%. In order to study the effect of cold rolling after intercritical annealing on subsequent tensile properties, the steel was further cold rolled up to 20\% reduction. After cold rolling, it was observed that the strain hardening rate increased continuously with increasing cold rolling reduction but without a significant drop in ductility during subsequent tensile tests. The microstructural evolution with cold rolling reduction was analysed to understand the mechanisms behind this enhanced TRIP effect. It was found that cold rolling activated additional twinning systems which provided a large number of potent nucleation sites for strain induced martensite to form during subsequent tensile tests.

cond-mat.mtrl-sci↗

arXiv2020

AgCl-induced hot salt stress corrosion cracking in a titanium alloy

The mechanism of AgCl-induced stress corrosion cracking of Ti-6246 was examined at \SI{500}{\mega\pascal} and \SI{380}{\celsius} for \SI{24}{\hour} exposures. SEM and STEM-EDX examination of a FIB-sectioned blister and crack showed that metallic Ag was formed and migrated along the crack. TEM analysis also revealed the presence of \ce{SnO2} and \ce{Al2O3} corrosion products mixed into \ce{TiO2}. The fracture surface has a transgranular nature with a brittle appearance in the primary $\alpha$ phase. Long, straight and non-interacting dislocations were observed in a cleavage-fractured primary $\alpha$ grain, with basal and pyramidal traces. This is consistent with a dislocation emission view of the the cracking mechanism.

cond-mat.mtrl-sci↗

arXiv2020

Mechanisms of Ti3Al precipitation in hcp alpha-Ti

Nucleation and growth of Ti$_3$Al \textalpha{}$_2$ ordered domains in \textalpha{}-Ti--Al--X alloys were characterised using a combination of transmission electron microscopy, atom probe tomography and small angle X-ray scattering. Model alloys based on Ti--7Al~(wt.\%) and containing O, V and Mo were aged at \SI{550}{\celsius} for times up to \SI{120}{\day} and the resulting precipitate dispersions were observed at intermediate points. Precipitates grew to around \SI{30}{\nano\metre} in size, with a volume fraction of 6--10\% depending on tertiary solutes. Interstitial O was found to increase the equilibrium volume fraction of \textalpha{}$_2$, while V and Mo showed relatively little influence. Addition of any of the solutes in this study, but most prominently Mo, was found to increase nucleation density and decrease precipitate size and possibly coarsening rate. Coarsening can be described by the Lifshitz-Slyozov-Wagner model, suggesting a matrix diffusion-controlled coarsening mechanism (rather than control by interfacial coherency). Solutionising temperature was found to affect nucleation number density with an activation energy of $E_{\mathrm{f}} = 1.5\pm{}0.4$~eV, supporting the hypothesis that vacancy concentration affects \textalpha{}$_2$ nucleation. The observation that all solutes increase nucleation number density is also consistent with a vacancy-controlled nucleation mechanism.

cond-mat.mtrl-sci↗

arXiv2020

A cracking oxygen story: a new view of stress corrosion cracking in titanium alloys

Titanium alloys can suffer from halide-associated stress corrosion cracking at elevated temperatures e.g., in jet engines, where chlorides and Ti-oxide promote the cracking of water vapour in the gas stream, depositing embrittling species at the crack tip. Here we report, using isotopically-labelled experiments, that crack tips in an industrial Ti-6Al-2Sn-4Zr-6Mo alloy are strongly enriched (>5 at.%) in oxygen from the water vapour, far greater than the amounts (0.25 at.%) required to embrittle the material. Surprisingly, relatively little hydrogen (deuterium) is measured, despite careful preparation and analysis. Therefore, we suggest that a combined effect of O and H leads to cracking, with O playing a vital role, since it is well-known to cause embrittlement of the alloy. In contrast it appears that in alpha+beta Ti alloys, it may be that H may drain away into the bulk owing to its high solubility in beta-Ti, rather than being retained in the stress field of the crack tip. Therefore, whilst hydrides may form on the fracture surface, hydrogen ingress might not be the only plausible mechanism of embrittlement of the underlying matrix. This possibility challenges decades of understanding of stress-corrosion cracking as being related solely to the hydrogen enhanced localised plasticity (HELP) mechanism, which explains why H-doped Ti alloys are embrittled. This would change the perspective on stress corrosion embrittlement away from a focus purely on hydrogen to also consider the ingress of O originating from the water vapour, insights critical for designing corrosion resistant materials.

cond-mat.mtrl-sci↗

arXiv2020

Quantitative precipitate classification and grain boundary property control in Co/Ni-base superalloys

A correlative approach is employed to simultaneously assess structure and chemistry of (carbide and boride) precipitates in a set of novel Co/Ni-base superalloys. Structure is derived from electron backscatter diffraction (EBSD) with pattern template matching, and chemistry obtained with energy dispersive X-ray spectroscopy (EDS). It is found that the principal carbide in these alloys is Mo and W rich with the M6C structure. An M2B boride, also exhibiting Mo and W segregation is observed at B levels above approximately 0.085 at.%. These phases are challenging to distinguish in an SEM with chemical information (EDS or backscatter Z-contrast) alone, without the structural information provided by EBSD. Only correlative chemical and structural fingerprinting is necessary and sufficient to fully define a phase. The identified phases are dissimilar to those predicted using ThermoCalc. We additionally perform an assessment of the grain boundary serratability in these alloys, and observe that significant amplitude is only obtained in the absence of pinning intergranular precipitates.

cond-mat.mtrl-sci↗

arXiv2020

Fatigue cracking in gamma titanium aluminide

Cast and HIP'ed \textgamma-TiAl 4522XD is being developed for use in jet engine low pressure turbine blades, where temperature variations occur through the flight cycle. The effects of temperature variations on fatigue cracking were therefore examined in this study. It was found that fatigue crack growth rates were higher at 750C than 400C, but that $\Delta K_\mathrm{th}$ was also higher. Temperature excursions between 400 and 750C during fatigue crack growth resulted in retardation of the crack growth rate, both on heating and cooling. It was also found that for notches $0.6$~mm in length and smaller, initiation from the microstructure could instead be observed at stresses similar to the material failure stress; a microstructural initiation site exists. A change from trans- to mixed trans-, inter- and intra-lamellar cracking could be observed where the estimated size of the crack tip plastic zone exceeded the colony size.

cond-mat.mtrl-sci↗

arXiv2020

Spherical-angular dark field imaging and sensitive microstructural phase clustering with unsupervised machine learning

Electron backscatter diffraction is a widely used technique for nano- to micro-scale analysis of crystal structure and orientation. Backscatter patterns produced by an alloy solid solution matrix and its ordered superlattice exhibit only extremely subtle differences, due to the inelastic scattering that precedes coherent diffraction. We show that unsupervised machine learning (with PCA, NMF, and an autoencoder neural network) is well suited to fine feature extraction and superlattice/matrix classification. Remapping cluster average patterns onto the diffraction sphere lets us compare Kikuchi band profiles to dynamical simulations, confirm the superlattice stoichiometry, and facilitate virtual imaging with a spherical solid angle aperture. This pipeline now enables unparalleled mapping of exquisite crystallographic detail from a wide range of materials within the scanning electron microscope.

cond-mat.mtrl-sci↗

arXiv2019

The Interaction of Galling and Oxidation in 316L Stainless Steel

The galling behaviour of 316L stainless steel was investigated in both the unoxidised and oxidised states, after exposure in simulated PWR water for 850 hours. Galling testing was performed according to ASTM G196 in ambient conditions. 316L was found to gall by the wedge growth and flow mechanism in both conditions. This resulted in folds ahead of the prow and adhesive junction, forming a heavily sheared multilayered prow. The galling trough was seen to have failed through successive shear failure during wedge flow. Immediately beneath the surface a highly sheared nanocrystalline layer was seen, termed the tribologically affected zone (TAZ). It was observed that strain-induced martensite formed within the TAZ. Galling damage was quantified using Rt (maximum height - maximum depth) and galling area (the proportion of the sample which is considered galled), and it was shown that both damage measures decreased significantly on the oxidised samples. At an applied normal stress of 4.2 MPa the galled area was 14 % vs. 1.2 % and the Rt was 780 um vs. 26 um for the unoxidised and oxidised sample respectively. This trend was present at higher applied normal stresses, although less prominent. This difference in galling behaviour is likely to be a result of a reduction in adhesion in the case of the oxidised surface.

physics.app-ph↗

arXiv2019

Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?

A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x>1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed.

cond-mat.mtrl-sci↗

arXiv2019

The role of dwell hold on the dislocation mechanisms of fatigue in a near alpha titanium alloy

The dislocation structures appearing in highly mis-oriented soft/hard grain pairs in near-alpha titanium alloy Ti6242Si were examined with and without the application of load holds (dwell) during fatigue. Dislocation pile-up in a soft grain resulted in internal stresses in an adjacent hard grain which could be relaxed by dislocation multiplication at localized Frank-Read sources, a process assisted by the provision of a relaxation time during a load hold. The rate of this process is suggested to be controlled by pyramidal cross-slip and basal junction formation. A high density of prism pile-ups is observed by dual slip, together with the nucleation of edge dislocations in the soft grain of a highly mis-oriented grain pair, increasing the possibility of cracking. The stress concentration developed by such pile-ups is found to be higher in dwell fatigue (single-ended pile-ups) than in LCF (double ended). Analytical modelling shows that the maximum normal stress produced on the hard grain in dwell fatigue by this pile-up would be near-basal, ~2.5 degree to (0002). This provides support for the dominant hypothesis for the rationalisation of dwell fatigue crack nucleation in Ti alloys, which derives from the Stroh pile-up model, and elaboration of the underlying dislocation phenomena that result from load shedding and lead to basal faceting.

cond-mat.mtrl-sci↗

arXiv2018

Rapid Electron Backscatter Diffraction Mapping: Painting by Numbers

Microstructure characterisation has been greatly enhanced through the use of electron backscatter diffraction (EBSD), where rich maps are generated through analysis of the crystal phase and orientation in the scanning electron microscope (SEM). Conventional EBSD analysis involves raster scanning of the electron beam and the serial analysis of each diffraction pattern in turn. For grain shape, crystallographic texture, and microstructure analysis this can be inefficient. In this work, we present Rapid EBSD, a data fusion approach combining forescatter electron (FSE) imaging with static sparse sampling of EBSD patterns. We segment the FSE image into regions of similar colour (i.e. phase and crystal orientation) and then collect representative EBSD data for each segmented region. This enables microstructural assessment to be performed at the spatial resolution of the (fast) FSE imaging whilst including orientation and phase information from EBSD analysis of representative points. We demonstrate the Rapid EBSD technique on samples of a cobalt based superalloy and a strained dual phase titanium alloy, comparing the results with conventional analysis. Rapid EBSD is advantageous for assessing grain size distributions in time-limited experiments.

cond-mat.mtrl-sci↗