194 results found
Worsnop FF, Lea SLM, Ilavsky J, et al., 2023, Crystallographic ordering of Al and Sn in alpha-Ti, SCRIPTA MATERIALIA, Vol: 226, ISSN: 1359-6462
Llewelyn SCH, Owen LR, Playford HY, et al., 2022, Influence of Ni:Co ratio and temperature on the lattice misfit of ?-?' Ni-Co-Al-Ti-Cr alloys, JOURNAL OF ALLOYS AND COMPOUNDS, Vol: 937, ISSN: 0925-8388
Rogers SR, Daure J, Shipway P, et al., 2022, Adhesive transfer operates during galling, Scripta Materialia, Vol: 221, ISSN: 1359-6462
In order to reduce cobalt within the primary circuit of pressurised water reactors (PWR’s), 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.% manganese difference between 304L and 316L stainless steels, even at relatively low compressive stresses of 50MPa. Through these findings, the galling mechanisms of stainless steels can be better understood, which may help with the development of galling resistant stainless steels.
Kwok TWJ, Gong P, Rose R, et al., 2022, The relative contributions of TWIP and TRIP to strength in fine grained medium-Mn steels, Materials Science and Engineering: A, Vol: 855, Pages: 1-13, ISSN: 0921-5093
A medium Mn steel of composition Fe-4.8Mn-2.8Al-1.5Si-0.51C (wt.%) was processed to obtain two different microstructures representing two different approaches in the hot rolling mill, resulting in equiaxed vs. mixed equiaxed and lamellar microstructures. Both were found to exhibit a simultaneous Twinning Induced Plasticity and Transformation Induced Plasticity (TWIP ＋ TRIP) mechanism where deformation twins and -martensite formed independently of twinning with strain. Interrupted tensile tests were conducted in order to investigate the differences in deformation structures between the two microstructures. A constitutive model was used to find that, surprisingly, twinning contributed relatively little to the strength of the alloy, chiefly due to the fine initial slip lengths that then gave rise to relatively little opportunity for work hardening by grain subdivision. Nevertheless, with lower high-cost alloying additions than equivalent Dual Phase steels (2–3 wt% Mn) and greater ductility, medium-Mn TWIP ＋ TRIP steels still represent an attractive area for future development.
Huang Y, Gao J, Vorontsov V, et al., 2022, Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate, JOURNAL OF MATERIALS SCIENCE & TECHNOLOGY, Vol: 124, Pages: 217-231, ISSN: 1005-0302
Worsnop FF, Lim RE, Bernier JV, et al., 2022, The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium, Nature Communications, Vol: 13, ISSN: 2041-1723
Dwell fatigue, the reduction in fatigue life experienced by titanium alloysdue to holds at stresses as low as 60% of yield, has been implicated in severaluncontained jet engine failures. Dislocation slip has long been observed to bean intermittent, scale-bridging phenomenon, similar to that seen in earthquakesbut at the nanoscale, leading to the speculation that large stress bursts mightpromote the initial opening of a crack. Here we observe such stress bursts atthe scale of individual grains in situ, using high energy X-ray diffractionmicroscopy in Ti-7Al-O alloys. This shows that the detrimental effect ofprecipitation of ordered Ti_3Al is to increase the magnitude of rare pri<a> andbas<a> slip bursts associated with slip localisation. In contrast, the additionof trace O interstitials is beneficial, reducing the magnitude of bas<a> slipbursts and increasing the homogeneity between basal and prismatic <a> slip.This is further evidence that the formation of long paths for easy basal planeslip localisation should be avoided when engineering titanium alloys againstdwell fatigue.
Vorontsov VA, McAuliffe TP, Hardy MC, et al., 2022, Precipitate dissolution during deformation induced twin thickening in a CoNi-base superalloy subject to creep, Acta Materialia, Vol: 232, Pages: 1-11, ISSN: 1359-6454
The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal γ distribution has been examined at 800◦C and 300 MPa. The rupture life of the alloy is comparable to that ofRR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealedthe presence of continuous γ precipitates and M23C6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary γ precipitates occurred at the expense of the fine tertiary γ. Longplanar deformation bands, free of γ, were also observed to traverse individual grains. Examination ofthe deformation bands confirmed that they were microtwins. Long sections of the microtwins examinedwere depleted of γ stabilising elements across their entire width, suggesting that certain alloy compositions are susceptible to precipitate dissolution during twin thickening. A mechanism for the dissolutionof the precipitates is suggested based on the Kolbe reordering mechanism.
Kwok TWJ, Rahman KM, Vorontsov VA, et al., 2022, Strengthening kappa-carbide steels using residual dislocation content, Scripta Materialia, Vol: 213, ISSN: 1359-6462
A steel with nominal composition Fe-28Mn-8Al-1.0C in mass percent was hot rolled at two temperatures, 1100 °C and 850 °C and subsequently aged at 550 °C for 24 h. The lower temperature rolling resulted in a yield strength increment of 299 MPa while still retaining an elongation to failure of 26%. The large improvement in strength was attributed to an increase in residual dislocation density which was retained even after the ageing heat treatment. A homogeneous precipitation of -carbides in both samples also showed that the high residual dislocation density did not adversely affect precipitation kinetics. These findings demonstrate that the tensile properties of this class of steel can yet be improved by optimising hot rolling process parameters.
Tan Q, Yan Z, Wang H, et al., 2022, The role of β pockets resulting from Fe impurities in hydride formation in titanium, Scripta Materialia, Vol: 213, Pages: 114640-114640, ISSN: 1359-6462
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 β phase at grain boundary triple points. Furthermore, the low solubility of hydrogen in hexagonal close-packed α-Ti makes titanium alloys prone to subsequent hydride-associated failures due to stress corrosion cracking. We analyzed α-α and α-β sections of the abutting grain boundary of a β pocket in a Grade 2 CP-Ti, and the α-β phase boundary. Fe and H partition to β and segregate at the grain boundary, but no segregation is seen at the α-β phase boundary. In contrast, a significant Ni (>1 at%) accumulation is observed at the α-β phase boundary. We propose that the β-pockets act as hydrogen traps and facilitate the nucleation and growth of hydrides along grain boundaries in CP-Ti.
Joseph S, Kontis P, Chang Y, et al., 2022, A cracking oxygen story: a new view of stress corrosion cracking in titanium alloys, Acta Materialia, Vol: 227, Pages: 117687-117687, ISSN: 1359-6454
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 α + β Ti alloys, it may be that H may drain away into the bulk owing to its high solubility in β-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.
Xu X, Kwok TWJ, Gong P, et al., 2022, Tailoring the deformation behaviour of a medium Mn steel through isothermal intercritical annealing, Materialia, Vol: 22, Pages: 101422-101422, ISSN: 2589-1529
A novel concept of varying the strain hardening rate of a medium Mn steel with 8 wt.% Mn by varying the duration of the intercritical anneal after hot rolling was explored. It was found that the stability of the austenite phase showed an inverse square root relationship with intercritical annealing duration and that the maximum strain hardening rate showed a linear relationship with austenite stability. The change in austenite stability was attributed to continuous Mn enrichment with increasing intercritical annealing duration. Twinned martensite was also found to be the most likely product of the martensitic transformation during deformation.
Kwok TWJ, Gong P, Xu X, et al., 2022, Microstructure evolution and tensile behaviour of a cold rolled 8 Wt Pct Mn medium manganese steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 53, Pages: 597-609, ISSN: 1073-5623
A novel medium manganese steel 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 pct. 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 pct 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 phenomena. 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 in what can be described as an enhanced TRIP effect.
Zhao G, Xu X, Dye D, et al., 2022, Facile route to implement transformation strengthening in titanium alloys, Scripta Materialia, Vol: 208, Pages: 1-5, ISSN: 1359-6462
Developing lighter, stronger and more ductile aerospace metallic materials is in demand for energy efficiency strategies. Alloys with twinning-induced plasticity (TWIP) and/or transformation-induced plasticity(TRIP) effects have been exploited to defeat the conflict of strength versus ductility, yet very few if anyphysically informed methods exist to address the complex interactions between the transitions. Here wereport a facile route to deploy transformation-mediated strengthening in Ti alloys, which particularly focuses on the supervised activation of TRIP and TWIP via a mechanism-driven modelling approach. Newalloys were comparatively developed and presented notable resistances to strain localisation, but interestingly through distinct mechanical characteristics. Specifically, extraordinary strain-hardening rate (dσ/dε)with a peak value of 2.4 GPa was achieved in Ti-10Mo-5Nb (wt.%), resulting from the synergetic activationof hierarchical transformations. An efficient model integrating TRIP and TWIP was applied to understandthe interplays of the transition mechanisms.
Kwok TWJ, Slater C, Xu X, et al., 2021, A scale-up study on chemical segregation and the effects on tensile properties in two medium mn steel castings, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 53, Pages: 585-596, ISSN: 1073-5623
Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250 ∘C for various times to study the degree of chemical homogeneity and δ-ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some δ-ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed δ-ferrite in the 5 kg ingot turned into coarse δ-ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed >900 MPa yield strength, >1100 MPa tensile strength, and >40 pct elongation with <10 pct difference in strength and no change in ductility when compared to a fully homogenized sample.
Ghouse S, Oosterbeek R, Tayub A, et al., 2021, Vacuum heat treatments of titanium porous structures, Additive Manufacturing, Vol: 47, ISSN: 2214-8604
Additive manufacturing (AM) of Ti-6Al-4V enables rapid fabrication of complex parts, including porous lattices which are of interest for aerospace, automotive, or biomedical applications, however currently the fatigue resistance of these materials is a critical limitation. Engineering the alloy microstructure provides a promising method for increasing fatigue strength, but conventional heat treatment procedures are known to produce atypical results for AM and porous samples, and must therefore be optimised for these materials. Using vacuum heat treatment, microstructures comparable to those observed for conventional wrought and heat treated alloys were achieved with porous AM Ti-6Al-4V. Fine lamellar microstructures were produced using sub-transus heat treatment at 920 °C, while coarse lamellar microstructures were produced using super-transus heat treatment at 1050 °C or 1200 °C. Increasing the heat treatment temperature increased the elastic modulus from 2552 ± 22 MPa to a maximum of 2968 ± 45 MPa, due to strut sintering increasing the effective strut thickness, and removal of prior β-grain orientation. Heat treatment eliminated the brittle α’ martensite phase in favour of an α + β mixture, where the phase boundaries and β-phase provide greater resistance to crack propagation. Super-transus heat treatments increased the α-lath size which typically reduces crack propagation resistance, however strut sintering reduced surface crack initiation sites, increasing the fatigue strength by 75% from 4.86 MPa for the as-built material to a maximum of 8.51 MPa after 1200 °C heat treatment. This work demonstrates that vacuum heat treatment is effective at tuning the micro- and macro-structure of porous AM Ti-6Al-4V, thereby improving the crucial fatigue resistance.
Kim J, Hall D, Yan H, et al., 2021, Roughening improves hydrogen embrittlement resistance of Ti-6Al-4V, Acta Materialia, Vol: 220, Pages: 1-12, ISSN: 1359-6454
Polished surfaces of Ti-6Al-4V, the most commonly used titanium alloy, were observed to suffer from hydride growth and associated embrittlement during hydrogen charging, whereas rough surfaces suffered no such susceptibility. Direct microscopic analyses of recombined hydrogen bubbles and thermal desorption spectroscopy (TDS) revealed that the surface roughening promotes recombination of atomic hydrogen to molecular hydrogen, in turn, reducing the relative amount of atomic hydrogen uptake. Subsurface time-of-flight secondary-ion mass spectrometry (ToF-SIMS) further revealed that the high defect density underneath the roughened surface impedes hydrogen diffusion into the bulk. These combined effects mean that, unexpectedly, roughening significantly reduces hydrogen uptake into Ti-6Al-4V and enhances its resistance against hydrogen embrittlement – all resulting from a simple surface treatment.
Shi Y, Joseph S, Saunders EA, et al., 2021, AgCl-induced hot salt stress corrosion cracking in a titanium alloy, Corrosion Science, Vol: 187, Pages: 1-11, ISSN: 0010-938X
The mechanism of AgCl-induced stress corrosion cracking of Ti-6246 was examined at 500 MPa and 380 °C for 24 h 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 SnO2 and Al2O3 corrosion products mixed into TiO2. The fracture surface has a transgranular nature with a brittle appearance in the primary α phase. Long, straight and non-interacting dislocations were observed in a brittle appearance fractured primary α grain, with basal and pyramidal traces. This is consistent with a dislocation emission view of the cracking mechanism.
Dear FF, Kontis P, Gault B, et al., 2021, Mechanisms of Ti3Al precipitation in hcp α-Ti, Acta Materialia, Vol: 212, ISSN: 1359-6454
Nucleation and growth of Ti3Alα2ordered domains inα-Ti–Al–X alloys were characterised using a combination of transmission electronmicroscopy, atom probe tomography and small angle X-ray scattering. Model alloys based on Ti–7Al (wt.%) and containing O, V and Mowere aged at 550◦C for times up to 120 d and the resulting precipitate dispersions were observed at intermediate points. Precipitates grewto around 30 nm in size, with a volume fraction of 6–10% depending on tertiary solutes. Interstitial O was found to increase the equilibriumvolume fraction ofα2, while V and Mo showed relatively little influence. Addition of any of the solutes in this study, but most prominentlyMo, was found to increase nucleation density and decrease precipitate size and possibly coarsening rate. Coarsening can be described by theLifshitz-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 ofEf=1.5±0.4 eV, supporting the hypothesisthat vacancy concentration affectsα2nucleation. The observation that all solutes increase nucleation number density is also consistent with avacancy-controlled nucleation mechanism.
Knowles A, Dye D, Dodds R, et al., 2021, Tungsten-based bcc-superalloys, Applied Materials Today, Vol: 23, Pages: 1-6, ISSN: 2352-9407
Applications from nuclear energy to rockets and jet engines are underpinned by advanced high temperature materials. Whilst state of the art, the performance of current nickel-based superalloys is fundamentally limited to Ni’s melting point, T. Here, we develop an analogous superalloy concept but with superior high temperature capability by transitioning to a bcc tungsten base, T. This strategy involves reinforcing bcc -W by TiFe intermetallic compound, which results in impressive high temperature compressive strengths of 500 MPa at. This bcc-superalloy design approach has wider applicability to other bcc alloy bases, including Mo, Ta, and Nb, as well as to refractory-metal high entropy alloys (RHEAs). By investigation of the underlying phase equilibria, thermodynamic modelling, characterisation and mechanical properties, we demonstrate the capability of ternary W-Ti-Fe tungsten-based bcc-superalloys as a new class of high temperature materials.
Cann JL, De Luca A, Dunand DC, et al., 2021, Sustainability through alloy design: Challenges and opportunities, PROGRESS IN MATERIALS SCIENCE, Vol: 117, ISSN: 0079-6425
McAuliffe TP, Bantounas I, Reynolds LR, et al., 2021, Quantitative precipitate classification and grain boundary property control in Co/Ni-base superalloys, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 52, Pages: 1649-1664, ISSN: 1073-5623
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. pct. 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.
Collins CR, Dear FF, Rugg D, et al., 2021, The effect of dissolved nitrogen on the fatigue behavior of Ti-6Al-4V, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 52, Pages: 1596-1608, ISSN: 1073-5623
The effect of nitrogen additions on fatigue behavior has been examined in near-equiaxed, rolled Ti-6Al-4V bar. This is the first-time nitrogen content that has been systematically explored with respect to monotonic and cyclic properties in a Ti-6Al-4V alloy base composition. Nitrogen additions were found to increase the β-transus temperature and strength, but they decreased ductility, even in microstructures where some β phase remained. This carried across into both the low- and high cycle fatigue behavior; even small contents of 240 and 560 ppmwN caused reductions in both low cycle fatigue life and high cycle fatigue strength. In samples containing 240 and 560 ppmwN, a conventional striated fractographic appearance was observed, but a dramatic change to a macroscopically brittle fracture surface was observed at 1800 and 3600 ppmwN, but still with substantial evidence of plasticity at the microscale. Therefore, neither microstructure or fractographic examination, nor EDX-based compositional analysis in the electron microscope are necessarily a reliable indicator of an absence of deleterious nitrogen contamination. This is significant for the investigation of potentially nitrogen-contaminated surface-initiated cracks, either due to service or processing exposures.
Coakley J, Higginbotham A, McGonegle D, et al., 2020, Femtosecond quantification of void evolution during rapid material failure, Science Advances, Vol: 6, Pages: 1-10, ISSN: 2375-2548
Understanding high velocity impact, and the subsequent high strain rate material deformation and potential catastrophic failure, is of critical importance across a range of scientific and engineering disciplines that include astrophysics, materials science and aerospace engineering. The deformation and failure mechanisms are not thoroughly understood, given the challenges of experimentally quantifying material evolution at extremely short time-scales. Here, copper foils are rapidly strained via picosecond laser ablation and probed in situ with femtosecond x-ray free electron (XFEL) pulses. Small angle x-ray scattering (SAXS) monitors the void distribution evolution while wide angle scattering (WAXS) simultaneously determines the strain evolution. The ability to quantifiably characterize the nanoscale during high strain rate failure with ultrafast-SAXS, complementing WAXS, represents a broadening in the range of science that can be performed with XFEL. It is shown that ultimate failure occurs via void nucleation, growth and coalescence, and the data agree well with molecular dynamics simulations.
McAuliffe TP, Dye D, Britton TB, 2020, Spherical-angular dark field imaging and sensitive microstructural phase clustering with unsupervised machine learning., Ultramicroscopy, Vol: 219, Pages: 1-11, ISSN: 0304-3991
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 principal component analysis, non-negative matrix factorisation, 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.
Xu Y, Joseph S, Karamched P, et al., 2020, Predicting dwell fatigue life in titanium alloys using modelling and experiment, Nature Communications, Vol: 11, ISSN: 2041-1723
Fatigue is a difficult multi-scale modelling problem nucleating from localised plasticity at the scale of dislocations and microstructure with significant engineering safety implications. Cold dwell fatigue is a phenomenon in titanium where stress holds at moderate temperatures lead to substantial reductions in cyclic life, and has been implicated in service failures. Using discrete dislocation plasticity modelling complemented by transmission electron microscopy, we successfully predict lifetimes for ‘worst case’ microstructures representative of jet engine spin tests. Fatigue loading above a threshold stress is found to produce slip in soft grains, leading to strong dislocation pile-ups at boundaries with hard grains. Pile-up stresses generated are high enough to nucleate hard grain basal dislocations, as observed experimentally. Reduction of applied cyclic load alongside a temperature excursion during the cycle lead to much lower densities of prism dislocations in soft grains and, sometimes, the elimination of basal dislocations in hard grains altogether.
Trant CF, Lindley TC, Martin N, et al., 2020, Fatigue cracking in gamma titanium aluminide, Publisher: ELSEVIER SCIENCE SA
- Author Web Link
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- Citations: 1
Joseph S, Joseph K, Lindley TC, et al., 2020, The role of dwell hold on the dislocation mechanisms of fatigue in a near alpha titanium alloy, International Journal of Plasticity, Vol: 131, ISSN: 0749-6419
The dislocation structures appearing in highly mis-oriented soft/hard grain pairs in near-alphatitanium 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 hardgrain which could be relaxed by dislocation multiplication at localised Frank–Read sources, aprocess assisted by the provision of a relaxation time during a load hold. The rate of this processis suggested to be controlled by⟨𝑐+𝑎⟩pyramidal cross-slip and⟨𝑎⟩basal junction formation.A high density of⟨𝑎⟩prism pile-ups was observed with dual slip on two prism planes, togetherwith edge dislocations on the third prism plane in the soft grain of a highly mis-oriented grainpair, increasing the pile-up stress. The stress concentration developed by such pile-ups is foundto be higher in dwell fatigue (single-ended pile-ups) than in LCF (double ended). Analyticalmodelling shows that the maximum normal stress produced on the hard grain in dwell fatigueby this pile-up would be near-basal,≈ 2.5◦to (0002). This provides support for the dominanthypothesis for the rationalisation of dwell fatigue crack nucleation in Ti alloys, which derivesfrom the Stroh pile-up model, and elaboration of the underlying dislocation phenomena thatresult from load shedding and lead to basal faceting.
Dichtl C, Zhang Z, Gardner H, et al., 2020, Element segregation and α2 formation in primary α of a near-α Ti-alloy, Materials Characterization, Vol: 164, Pages: 1-10, ISSN: 1044-5803
Alloy TIMETAL®834 is a near-α Ti-alloy typically processed to have a complex bimodal microstructure that provides a good combination of mechanical properties at temperatures in excess of 450 °C. Due to the high Al content, typical ageing procedures result in the formation of intragranular and coherent nano-scale Ti3Al precipitation (α2), which increase strength but also promotes slip planarity. The present study focuses on chemical partitioning as a result of sub-β-transus heat treatment and the consequences for the two different constituents in the bimodal microstructure. The detailed chemical and structural analysis were carried out by combining Electron Probe Micro Analysis (EPMA), Wavelength Dispersive Spectroscopy (WDS), Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) for investigating local compositional variations and their effect on the formation of α2 precipitates. Detailed microchemical analysis shows a core-shell composition arrangement of α-stabilisers with the shell composition similar to the one of secondary α. Selected area electron diffraction in the TEM and APT analysis demonstrates that those local variations in α stabilisers affect the level of α2 precipitation. In addition, EPMA maps show that while Zr and Sn are often considered to be neutral alloying elements in Ti-alloys, they do segregate to the β-phase during sub-β-transus heat treatment.
Kwok TWJ, Rahman KM, Xu X, et al., 2020, Design of a high strength, high ductility 12 wt% Mn medium manganese steel with hierarchical deformation behaviour, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 782, ISSN: 0921-5093
- Author Web Link
- Open Access Link
- Citations: 15
McAuliffe TP, Ackerman AK, Savitzky BH, et al., 2020, 4D-STEM elastic stress state characterisation of a TWIP steel nanotwin, Publisher: arXiv
We measure the stress state in and around a deformation nanotwin in atwinning-induced plasticity (TWIP) steel. Using four dimensional scanningtransmission electron microscopy (4D-STEM), we measure the elastic strain fieldin a 68.2-by-83.1 nm area of interest with a scan step of 0.36 nm and adiffraction limit resolution of 0.73 nm. The stress field in and surroundingthe twin matches the form expected from analytical theory and is on the orderof 15 GPa, close to the theoretical strength of the material. We infer that themeasured back-stress limits twin thickening, providing rationale for why TWIPsteel twins remain thin, continuously dividing grains to give substantial workhardening. Our results support modern mechanistic understanding of theinfluence of twinning on crack propagation and embrittlement in TWIP steels.
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