72 results found
Joseph S, Lindley T, Dye D, 2018, Dislocation interactions and crack nucleation in a fatigued near-alpha titanium alloy, International Journal of Plasticity, Vol: 110, Pages: 38-56, ISSN: 0749-6419
Dislocation interactions at the crack nucleation site were investigated in near-alpha titanium alloy Ti-6242Si subjected to low cycle fatigue. Cyclic plastic strain in the alloy resulted in dislocation pile-ups in the primary alpha grains, nucleated at the boundaries between the primary alpha and the two-phase regions. These two phase regions provided a barrier to slip transfer between primary alpha grains. We suggest that crack nucleation occurred near the basal plane of primary alpha grains by the subsurface double-ended pile-up mechanism first conceived by Tanaka and Mura. Superjogs on the basal dislocations were observed near the crack nucleation location. The two phase regions showed direct transmission of dislocations between secondary alpha plates, transmitted through the beta ligaments as , which then decompose into dislocation networks in the beta. The beta ligaments themselves do not appear to form an especially impenetrable barrier to slip, in agreement with the micropillar and crystal plasticity investigations of Zhang et al.
Joseph S, Lindley TC, Dye D, et al., 2018, The mechanisms of hot salt stress corrosion cracking in titanium alloy Ti-6Al-2Sn-4Zr-6Mo, Corrosion Science, Vol: 134, Pages: 169-178, ISSN: 0010-938X
Hot salt stress corrosion cracking in Ti 6246 alloy has been investigated to elucidate the chemical mechanisms that occur. Cracking was found to initiate beneath salt particles in the presence of oxidation. The observed transgranular fracture was suspected to be due to hydrogen charging; XRD and high-resolution transmission electron microscopy detected the presence of hydrides that were precipitated on cooling. SEM-EDS showed oxygen enrichment near salt particles, alongside chlorine and sodium. Aluminium and zirconium were also involved in the oxidation reactions. The role of intermediate corrosion products such as Na2TiO3, Al2O3, ZrO2, TiCl2 and TiH are discussed.
Joseph S, Bantounas, Lindley, et al., 2017, Slip Transfer and Deformation Structures Resulting from the Low Cycle Fatigue of Near-alpha Titanium Alloy Ti-6242Si, International Journal of Plasticity, Vol: 100, Pages: 90-103, ISSN: 0749-6419
Near-alpha titanium alloy Ti6242Si, widely used in aero-engine compressor discs, was subjected to low cycle fatigue loading at room temperature. Fracture initiated by facet formation, followed by striated fatigue crack growth prior to final failure. Deformation occurred primarily by planar slip, localized into slip bands in the primary alpha. Within soft-oriented grains in a microtextured region, pile-up of a slip band within one grain resulted in the direct transfer of slip into an adjacent similarly oriented grain. In contrast, pile up of dislocations in a soft grain with a ‘hard’ oriented neighbour resulted in the activation of few non-connected dislocations in the hard grain, with <a>-type dislocations being activated and the observation of cross-slip. Whilst a high density of dislocations was present from precipitation of secondary alpha in the retained beta ligaments, a little dislocation interaction was observed between the transformed beta and the primary alpha grains.
Ismail FB, Vorontsov VA, Lindley TC, et al., 2016, Alloying effects on oxidation mechanisms in polycrystalline Co-Ni base superalloys, Corrosion Science, Vol: 116, Pages: 44-52, ISSN: 0010-938X
Oxidation mechanisms in polycrystalline Co-Ni-Cr-Al-W-Ta alloys were investigated using 16O/18O isotopic tracer anal-ysis in the focused ion-beam secondary ion mass spectrometer (FIB-SIMS). It was found that Al additions favour theformation of a continuous alumina-rich layer, that Cr indirectly improved oxidation resistance and that increasing theCo fraction resulted in poorer oxidation performance. In the alloy containing 15 at.% Cr and 10 at.% Al, the outer scaleformed after 200 h oxidation at 800 ⁰C comprised oxides less than 1 µm thick. It is concluded that protective oxide scalescan be formed in Co-Ni base superalloys.
Weekes HE, Lindley TC, Jones NG, et al., 2016, Hydride Reorientation in Zircaloy-4 Examined by In Situ Synchrotron X-ray Diffraction, Journal of Nuclear Materials, Vol: 478, Pages: 32-41, ISSN: 1873-4820
The phenomenon of stress-reorientation has been investigated using in situ X-ray diffraction during the thermomechanicalcycling of hydrided Zircaloy-4 tensile specimens. Results have shown that loading along a sample’s transverse direction(TD) leads to a greater degree of hydride reorientation when compared to rolling direction (RD)-aligned samples. Theelastic lattice micro-strains associated with radially oriented hydrides have been revealed to be greater than those orientedcircumferentially, a consequence of strain accommodation. Evidence of hydride redistribution after cycling, to α-Zr grainsoriented in a more favourable orientation when under an applied stress, has also been observed and its behaviour hasbeen found to be highly dependent on the loading axis. Finally, thermomechanical loading across multiple cycles hasbeen shown to reduce the difference in terminal solid solubility of hydrogen during dissolution (TSSD,H) and precipitation(TSSP,H).
Abolghasemi S, Williamson J, Lindley TC, et al., 2016, Embrittlement of alloy 625 and effect of remedial treatments, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART L-JOURNAL OF MATERIALS-DESIGN AND APPLICATIONS, Vol: 230, Pages: 328-331, ISSN: 1464-4207
Chapman TP, Vorontsov VA, Sankaran A, et al., 2016, The Dislocation Mechanism of Stress Corrosion Embrittlement in Ti-6Al-2Sn-4Zr-6Mo, Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science, Vol: 47, Pages: 282-292, ISSN: 1543-1940
An observation of the dislocation mechanisms operating below a naturally initiated hot-salt stress corrosion crack is presented, suggesting how hydrogen may contribute to embrittlement. The observations are consistent with the hydrogen-enhanced localized plasticity mechanism. Dislocation activity has been investigated through post-mortem examination of thin foils prepared by focused ion beam milling, lifted directly from the fracture surface. The results are in agreement with the existing studies, suggesting that hydrogen enhances dislocation motion. It is found that the presence of hydrogen in (solid) solution results in dislocation motion on slip systems that would not normally be expected to be active. A rationale is presented regarding the interplay of dislocation density and the hydrogen diffusion length.
Tympel PO, Lindley TC, Saunders EA, et al., 2015, Influence of complex LCF and dwell load regimes on fatigue of Ti–6Al–4V, Acta Materialia, Vol: 103, Pages: 77-88, ISSN: 1359-6454
Real components are usually subjected to variable amplitude fatigue, and yet the deformation micromechanisms that occur due to such load changes have barely been the subject of study. Here, unidirectionally rolled equiaxed Ti–6Al–4V plate was subjected to mixed dwell and variable amplitude low cycle fatigue (LCF), with the finding that overloads near the yield stress were found to retard subsequent fatigue crack growth, whilst elastic underloads were found to accelerate subsequent growth. Dwell intervals were found to be especially damaging, to a far greater extent than either dwell or LCF alone. Dwell facets were found to initiate subsurface and to be smoother than LCF facets, but were otherwise similar in orientation (∼30° to the loading axis) and crystallographic plane, 2–13° from (0002). However, no alteration of the slip bands underlying striations was observed at the point of load changes using TEM. In failure investigation, striation counting is an important tool; the loading changes used were not found to affect the number of striations formed. Dislocation networks were found between similarly oriented grains in the as-received material, which disintegrated under dwell loading and at high stresses.
Radecka A, Coakley J, Jones IP, et al., 2015, Ordering and the micromechanics of Ti-7Al, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 650, Pages: 28-37, ISSN: 1873-4936
The evolution of intergranular lattice strain in the α titanium alloy Ti–7Al wt% was characterised using in situ time-of-flight (TOF) neutron diffraction during room temperature tensile loading. Samples were aged to promote ordering and the formation of nanometre-scale α2 (Ti3Al). On ageing, at 550 °C and 625 °C, dislocations were observed to travel in pairs, and in planar arrays, which has been attributed to the presence of ordering. A slight change in c/a was observed, from 1.6949 to 1.6945, and a slight increase in the macroscopic modulus. However, no changes were observed in the residual lattice strains, which are the grain-orientation average elastic strains produced by plasticity. Therefore it is inferred that the changes in deformation mechanisms caused by ordering that result in an enhanced vulnerability to dwell fatigue affect primarily the extent of slip localisation. The overall strain distributions between grains in different orientations are not changed.
Chapman TP, Kareh KM, Knop M, et al., 2015, Characterisation of short fatigue cracks in titanium alloy IMI 834 using X-ray microtomography, Acta Materialia, Vol: 99, Pages: 49-62, ISSN: 1873-2453
A first attempt at the three-dimensional evaluation of naturally initiated surface connected and internal fatigue cracks is presented. Fatigue crack initiation and growth in air and vacuum environments have been investigated through X-ray microtomography in air and vacuum environments at elevated temperatures (350 °C), accompanied by post-mortem electron microscopy of the fracture surfaces. In vacuum (<10⁻⁵ mbar), multiple internal and surface-connected crack initiation was observed, but only the surface-connected cracks grew. In contrast, fewer cracks formed in air, these were mostly surface-connected and all were observed to grow. In all instances the initiation features were associated with globular primary α. An improved fatigue life was found in vacuum, which was mostly a consequence of delayed initiation, but was also due to slower fatigue crack propagation. The non-propagation of internal cracks was taken to imply that even the good laboratory vacuum obtained here was insufficient to simulate the conditions obtained for an internal crack in a component. The crack shape evolved towards a semi-circular shape a/c=1 in air during fatigue crack growth, whilst the vacuum cracks remained semi-elliptical (a/c≃1.4). This was taken to imply that oxide-induced crack closure played a role in fatigue crack growth in air.
Saunders EA, Chapman TP, Walker ARM, et al., 2015, Understanding the “blue spot” sodium chloride hot salt stress-corrosion cracking in titanium-6246 during fatigue testing at low pressure, Engineering Failure Analysis, Vol: 61, Pages: 2-20, ISSN: 1873-1961
During hot component fatigue tests there have been two cases of low life crack initiation of gas turbine rotating parts manufactured from the Titanium alloy Ti-6246. Both exhibited a small (~ 0.1 mm) elliptical ‘blue spot’ at the origin. Through validated striation count work and fracture mechanics it was established that fatigue had propagated with a near-nil initiation life. Early investigation suggested that the ‘blue spot’ was possibly a region of stage 1 fatigue growth, and was therefore a material behaviour concern with potential implications for service. During an investigation of a later cracking incident in this alloy, subsequently shown to have resulted from stress-corrosion cracking (SCC), near-identical fractographic characteristics to that seen in the “blue spot” were found that subtly differentiated it from stage 1 fatigue. Also, similar ‘blue spots’ have since been identified on Ti6246 Laboratory hot LCF test specimens and found to have been due to contamination by NaCl, through the application of focussed long-term EDX examination and other novel chemical analyses techniques. By the application of those techniques, fractography, and comparison against these specimens, Rolls-Royce and Imperial College London have collaborated to show that the original two component ‘blue spots’ were subtle examples of NaCl-related Hot Salt Stress-Corrosion Cracking (HSSCC). Such cracking has not been found to occur in service components, due to air pressure within the engine, and the effect is therefore confined to Laboratory and component tests at near-atmospheric pressure or below.
Chapman TP, Chater RJ, Saunders EA, et al., 2015, Environmentally assisted fatigue crack nucleation in Ti-6Al-2Sn-4Zr-6Mo, Corrosion Science, Vol: 96, Pages: 87-101, ISSN: 0010-938X
An unexplained feature was observed at the fatigue crack origin of a numberof α/β titanium specimens tested at 450 ◦C in the low cycle fatigue regime.The origin was discoloured blue but this was not a result of temper colouration;this feature sometimes resulted in large reductions in fatigue lives. A numberof specimens were examined to determine the cause and formation mechanismof these “blue spots.” This feature was associated with elevated oxygen andchloride levels and the presence of sodium. A mechanism based on hot-saltstress-corrosion cracking is proposed and the implications for service componentsare discussed.
Abolghasemi S, Williamson J, Lindley TC, et al., 2013, Thermal imaging and stress analysis for predicting the behaviour and long-term performance of flare tips, JOURNAL OF STRAIN ANALYSIS FOR ENGINEERING DESIGN, Vol: 48, Pages: 103-111, ISSN: 0309-3247
Thomas M, Lindley T, Rugg D, et al., 2012, The effect of shot peening on the microstructure and properties of a near-alpha titanium alloy following high temperature exposure, ACTA MATERIALIA, Vol: 60, Pages: 5040-5048, ISSN: 1359-6454
Evans C, Jones NG, Rugg D, et al., 2012, The effect of deformation mechanisms on the high temperature plasticity of Zircaloy-4, JOURNAL OF NUCLEAR MATERIALS, Vol: 424, Pages: 123-131, ISSN: 0022-3115
Jones NG, Tailing RJ, Lindley TC, et al., 2012, Failure Behaviour and Energy Adsorption in Gum Metal (Ti-36Nb-2Ta-3Zr-O), 12th World Conference on Titanium (Ti-2011), Publisher: SCIENCE PRESS BEIJING, Pages: 1169-1173
Thomas M, Lindley T, Rugg D, et al., 2012, High Cycle Fatigue Properties of Ti-834 Following Prolonged Thermal Exposure in air, 12th World Conference on Titanium (Ti-2011), Publisher: SCIENCE PRESS BEIJING, Pages: 1872-1876
Singh R, Lee PD, Jones JR, et al., 2010, Hierarchically structured titanium foams for tissue scaffold applications, ACTA BIOMATERIALIA, Vol: 6, Pages: 4596-4604, ISSN: 1742-7061
Singh R, Lee PD, Dashwood RJ, et al., 2010, Titanium foams for biomedical applications: a review, MATERIALS TECHNOLOGY, Vol: 25, Pages: 127-136, ISSN: 1066-7857
Singh R, Lee PD, Lindley TC, et al., 2010, Characterization of the deformation behavior of intermediate porosity interconnected Ti foams using micro-computed tomography and direct finite element modeling, ACTA BIOMATERIALIA, Vol: 6, Pages: 2342-2351, ISSN: 1742-7061
Bantounas I, Dye D, Lindley TC, 2010, The role of microtexture on the faceted fracture morphology in Ti-6Al-4V subjected to high-cycle fatigue, ACTA MATERIALIA, Vol: 58, Pages: 3908-3918, ISSN: 1359-6454
Li P, Lee PD, Maijer DM, et al., 2009, Quantification of the interaction within defect populations on fatigue behavior in an aluminum alloy, ACTA MATERIALIA, Vol: 57, Pages: 3539-3548, ISSN: 1359-6454
Bantounas I, Dye D, Lindley TC, 2009, The effect of grain orientation on fracture morphology during high-cycle fatigue of Ti-6Al-4V, ACTA MATERIALIA, Vol: 57, Pages: 3584-3595, ISSN: 1359-6454
Zhang Q, Lee PD, Singh R, et al., 2009, Micro-CT characterization of structural features and deformation behavior of fly ash/aluminum syntactic foam, ACTA MATERIALIA, Vol: 57, Pages: 3003-3011, ISSN: 1359-6454
Singh R, Lee PD, Lindley TC, et al., 2009, Characterization of the structure and permeability of titanium foams for spinal fusion devices, ACTA BIOMATERIALIA, Vol: 5, Pages: 477-487, ISSN: 1742-7061
Thomas M, Lindley T, Jackson M, 2009, The microstructural response of a peened near-alpha titanium alloy to thermal exposure, SCRIPTA MATERIALIA, Vol: 60, Pages: 108-111, ISSN: 1359-6462
Thomas M, Jackson M, Lindley T C, 2009, The microstructural response of a peened near-alpha titanium alloy to thermal exposure, Scripta Materialia, Vol: 60, Pages: 108-111
Stapleton AM, Raghunathan SL, Bantounas I, et al., 2008, Evolution of lattice strain in Ti-6Al-4V during tensile loading at room temperature, ACTA MATERIALIA, Vol: 56, Pages: 6186-6196, ISSN: 1359-6454
Fox AB, Huang YY, Rafik D, et al., 2008, Increasing the life of flare tips, Pages: 88-95
Flare tips are essential for safety. Maintenance is difficult and costly. Flare tips are subjected to high combustion temperatures, thermal cycling, oxidation and marine corrosion. Following a number of flare tip failures an in depth study by Imperial College was carried out into the failure of a flare tip from a UK platform, looking for service life improvement. Materials selection and design solutions were considered. The study considered alternative materials and concluded that materials selection was the smaller part of the answer; design changes can double service life. This study used failure investigation, high temperature experimental and thermo-mechanical modelling analysis. The modelling process simulated two common flaring conditions and correctly predicted the observed failure of initiation and crack propagation from holes used to bolt on flame stabilizing plates to the top of the flare. The calculated thermal stress and strains enabled the low cycle fatigue life and minimum creep life to be predicted. It was concluded that service life could be improved by replacing Incoloy alloy 800HT (UNS N08800) with Inconel alloy 625 (UNS N06625), an alloy with attractive mechanical properties and improved high temperature corrosion resistance. Repositioning or eliminating bolt holes can double service life. Copyright 2008, Society of Petroleum Engineers.
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