Publications
203 results found
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.
Dye D, 2015, SHAPE MEMORY ALLOYS Towards practical actuators, NATURE MATERIALS, Vol: 14, Pages: 760-761, ISSN: 1476-1122
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 number of alpha/beta 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 number of specimens were examined to determine the cause and formation mechanism of these “blue spots.” This feature was associated with elevated oxygen and chloride levels and the presence of sodium. A mechanism based on hot-salt stress-corrosion cracking is proposed and the implications for service components are discussed.
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.
Béreš M, Abreu HFG, Santos LPM, et al., 2015, Effect of variant transformations in fusion zones of gas metal arc welds, Science and Technology of Welding and Joining, Vol: 20, Pages: 353-360, ISSN: 1362-1718
The macro- and microtextures of gas metal arc welds fabricated using both conventional ferritic and low transformation temperature (LTT) filler metals were examined. Both welds were largely composed of acicular ferrite. The weld textures were found to be distinct, which is observed to be due to variant selection during the austenite to ferrite transformation. In situ electron backscatter diffraction performed during thermal cycling of the LTT fusion zone into the austenite phase field confirmed that these transformations satisfy the expected crystallographic relationships. Implications for welding consumable design and finite element process modelling are drawn.
Weekes HE, Vorontsov VA, Dolbnya IP, et al., 2015, In situ micropillar deformation of hydrides in Zircaloy-4, Acta Materialia, Vol: 92, Pages: 81-96, ISSN: 1873-2453
Deformation of hydrided Zircaloy-4 has been examined using in situ loading of hydrided micropillars in the scanning electron microscope and using synchrotron X-ray Laue microbeam diffraction. Results suggest that both the matrix and hydride can co-deform, with storage of deformation defects observed within the hydrides, which were twinned. Hydrides placed at the plane of maximum shear stress showed deformation within the hydride packet, whilst packets in other pillars arrested the propagation of shear bands. X-ray Laue peak broadening, prior to deformation, was associated with the precipitation of hydrides, and during deformation plastic rotation and broadening of both the matrix and hydride peaks were observed. Post-mortem TEM of the deformed pillars has indicated a greater density of dislocations associated with the precipitated hydride packets, while the observed broadening of the hydride electron diffraction spots further suggests that plastic strain gradients were induced in the hydrides by compression.
Rahman KM, Jones NG, Dye D, 2015, Micromechanics of twinning in a TWIP steel, Materials Science and Engineering A - Structural Materials Properties Microstructure and Processing, Vol: 635, Pages: 133-142, ISSN: 0921-5093
The deformation behaviour of a TWinning Induced Plasticity (TWIP) steel was studied at quasi-static strain rates using synchrotron X-ray diffraction. A {111} RD and {200} RD texture developed from the earliest stages of deformation, which could be reproduced using an elasto-plastic self consistent (EPSC) model. Evidence is found from multiple sources to suggest that twinning was occurring before macroscopic yielding. This included small deviations in the lattice strains, {111} intensity changes and peak width broadening all occurring below the macroscopic yield point. The accumulation of permanent deformation on sub-yield mechanical cycling of the material was found, which further supports the diffraction data. TEM revealed that fine deformation twins similar to those observed in heavily deformed samples formed during sub-yield cycling. It is concluded that twinning had occurred before macroscopic plastic deformation began, unlike the behaviour traditionally expected from hexagonal metals such as Mg.
Rahman KM, Vorontsov VA, Dye D, 2015, The effect of grain size on the twin initiation stress in a TWIP steel, Acta Materialia, Vol: 89, Pages: 247-257, ISSN: 1873-2453
The influence of grain size on the twinning stress of an Fe–15Mn–2Al–2Si–0.7C Twinning Induced Plasticity (TWIP) steel has been investigated. Five grain sizes were obtained using a combination of cold rolling and annealing. Electron backscatter diffraction (EBSD) analysis revealed that the material exhibited a typical cold rolled and annealed texture. Tensile testing showed the yield stress to increase with decreasing grain size, however, the ductility of the material was not substantially affected by a reduction in grain size. Cyclic tensile testing at sub-yield stresses indicated the accumulation of plastic strain with each cycle, consequently the nucleation stress for twinning was determined. The twin stress was found to increase with decreasing grain size. Furthermore, the amount of strain accumulated was greater in the coarser grain material. It is believed that this is due to a difference in the twin thickness, which is influenced by the initial grain size of the material. SEM and TEM analysis of the material deformed to 5% strain revealed thinner primary twins in the fine grain material compared to the coarse grain. TEM examination also showed the dislocation arrangement is affected by the grain size. Furthermore, a larger fraction of stacking faults was observed in the coarse-grained material. It is concluded that the twin nucleation stress and also the thickness of the deformation twins in a TWIP steel, is influenced by the initial grain size of the material. In addition grain refinement results in a boost in strength and energy absorption capabilities in the material.
Coakley J, Vorontsov VA, Littrell KC, et al., 2015, Nanoprecipitation in a beta-titanium alloy, Journal of Alloys and Compounds, Vol: 623, Pages: 146-156, ISSN: 0925-8388
Vorontsov VA, Jones NG, Rahman KM, et al., 2015, Superelastic load cycling of Gum Metal, Acta Materialia, Vol: 88, Pages: 323-333, ISSN: 1873-2453
The superelastic beta titanium alloy, Gum Metal, has been found to accumulate plastic strain during tensile load cycling in the superelastic regime. This is evident from the positive drift of the macroscopic stress vs. strain hysteresis curve parallel to the strain axis and the change in its geometry subsequent to every load–unload cycle. In addition, there is a progressive reduction in the hysteresis loop width and in the stress at which the superelastic transition occurs. In situ synchrotron X-ray diffraction has shown that the lattice strain exhibited the same behaviour as that observed in macroscopic measurements and identified further evidence of plastic strain accumulation. The mechanisms responsible for the observed behaviour have been evaluated using transmission electron microscopy, which revealed a range of different defects that formed during load cycling. The formation of these defects is consistent with the classical mathematical theory for the bcc to orthorhombic martensitic transformation. It is the accumulation of these defects over time that alters its superelastic behaviour.
Davies CM, Ahn J, Tsunori M, et al., 2015, The Influence of Pre-existing Deformation on GMA Welding Distortion in Thin Steel Plates, JOURNAL OF MATERIALS ENGINEERING AND PERFORMANCE, Vol: 24, Pages: 261-273, ISSN: 1059-9495
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- Citations: 8
Knop M, Mulvey P, Ismail F, et al., 2014, A New Polycrystalline Co-Ni Superalloy, JOM, Vol: 66, Pages: 2495-2501, ISSN: 1047-4838
Yan H-Y, Coakley J, Vorontsov VA, et al., 2014, Alloying and the micromechanics of Co-Al-W-X quaternary alloys, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 613, Pages: 201-208, ISSN: 0921-5093
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- Citations: 39
Azeem MA, Dye D, 2014, Lattice instability during the martensitic transformation in the high temperature shape memory alloy Zr(Cu0.5Co0.25Ni0.25), Journal of Alloys and Compounds, Vol: 618, Pages: 469-474, ISSN: 1873-4669
The martensitic transformation on thermal cycling of the polycrystalline Zr(Cu0.5Co0.25Ni0.25) high temperature shape memory alloy was examined using in situ synchrotron X-ray diffraction. A single step B2 ↔ B19′ transformation was observed with a thermal hysteresis of 161 °C and a martensite finish temperature of 294 °C. Large anisotropic lattice constrictions and dilations, up to 1.3%, were observed during the course of transformation in the B19′ martensite, but corresponding changes in the B2 austenite were not observed. The coefficients of thermal expansion in both phases were found to be quite large, 33×10-6°C-1 in B2 and 39, 88 and 29 × 10−6 °C−1 for aB19′, bB19′ and cB19′ respectively. Possible mechanisms governing the transformation behaviour are discussed.
Yan H-Y, Vorontsov VA, Dye D, 2014, Effect of alloying on the oxidation behaviour of Co-Al-W superalloys, CORROSION SCIENCE, Vol: 83, Pages: 382-395, ISSN: 0010-938X
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- Citations: 103
Yan H-Y, Vorontsov VA, Dye D, 2014, Alloying effects in polycrystalline γ′ strengthened Co-Al-W base alloys, INTERMETALLICS, Vol: 48, Pages: 44-53, ISSN: 0966-9795
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- Citations: 147
Dye D, Stone HJ, Watson M, et al., 2014, Characterization of Phase Transformations and Stresses During the Welding of a Ferritic Mild Steel, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 45A, Pages: 2038-2045, ISSN: 1073-5623
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- Citations: 5
Azeem MA, Dye D, 2014, In situ evaluation of the transformation behaviour of NiTi-based high temperature shape memory alloys, INTERMETALLICS, Vol: 46, Pages: 222-230, ISSN: 0966-9795
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- Citations: 16
Rahman KM, Vorontsov VA, Dye D, 2014, The dynamic behaviour of a twinning induced plasticity steel, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 589, Pages: 252-261, ISSN: 0921-5093
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- Citations: 34
Knop M, Vorontsov VA, Hardy MC, et al., 2014, High-temperature γ (FCC)/γ′ (L1<sub>2</sub>) Co-Al-W based superalloys, 2nd European Symposium on Superalloys and their Applications, Publisher: E D P SCIENCES, ISSN: 2261-236X
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- Citations: 14
Shastry VV, Divya VD, Azeem MA, et al., 2013, Combining indentation and diffusion couple techniques for combinatorial discovery of high temperature shape memory alloys, ACTA MATERIALIA, Vol: 61, Pages: 5735-5742, ISSN: 1359-6454
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- Citations: 27
Warwick JLW, Jones NG, Bantounas I, et al., 2013, In situ observation of texture and microstructure evolution during rolling and globularization of Ti-6Al-4V, ACTA MATERIALIA, Vol: 61, Pages: 1603-1615, ISSN: 1359-6454
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- Citations: 92
Jones NG, Dye D, 2013, Influence of applied stress on the transformation behaviour and martensite evolution of a Ti-Ni-Cu shape memory alloy, INTERMETALLICS, Vol: 32, Pages: 239-249, ISSN: 0966-9795
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- Citations: 13
Warwick JLW, Jones NG, Rahman KM, et al., 2012, Lattice strain evolution during tensile and compressive loading of CP Ti, ACTA MATERIALIA, Vol: 60, Pages: 6720-6731, ISSN: 1359-6454
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- Citations: 70
Meher S, Yan H-Y, Nag S, et al., 2012, Solute partitioning and site preference in γ/γ′ cobalt-base alloys, SCRIPTA MATERIALIA, Vol: 67, Pages: 850-853, ISSN: 1359-6462
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- Citations: 81
Yan HY, Vorontsov VA, Coakley J, et al., 2012, Quaternary Alloying Effects and the Prospects for a New Generation of Co‐Base Superalloys, Superalloys 2012, Publisher: John Wiley & Sons, Inc., Pages: 705-714
Yan HY, Vorontsov VA, Coakley J, et al., 2012, Quaternary Alloying Effects and the Prospects for a New Generation of Co-Base Superalloys, Superalloys 2012, Pages: 705-714, ISBN: 9780470943205
Polycrystalline Co-Al-W alloys with quaternary alloying substitutions have been studied to investigate whether this alloy system might be attractive as an alternative to Ni-base superalloys in high temperature applications. A process route for the alloys was developed and an initial aging heat treatment established. The initial findings indicate that the addition of Ti, Ta and Ni increases the solvus temperature, whereas the addition of Mo lowers it slightly. The addition of Ti, Ta and Ni to the base alloy increased the γ' precipitate size, whereas the addition of Mo had a negligible effect. Diffraction measurements were performed using both X-ray and neutron scattering. Positive lattice misfit values were obtained at both room and elevated temperatures; moreover, the lattice misfit increased with temperature. The thermal expansion coefficient of the γ phase was similar to reference data for Co, whereas the γ' phase had a greater thermal expansion coefficient than the γ phase. Finally, cyclic oxidation experiments have been carried out in air at 800 C°. The CoO scales grown were not protective, and the alloys suffered from spallation during cooling. © 2012 The Minerals, Metals, & Materials Society. All rights reserved.
Coakley J, Dye D, 2012, Lattice strain evolution in a high volume fraction polycrystal nickel superalloy, SCRIPTA MATERIALIA, Vol: 67, Pages: 435-438, ISSN: 1359-6462
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- Citations: 29
Warwick JLW, Coakley J, Raghunathan SL, et al., 2012, Effect of texture on load partitioning in Ti-6Al-4V, ACTA MATERIALIA, Vol: 60, Pages: 4117-4127, ISSN: 1359-6454
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- Citations: 58
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
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- Citations: 16
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