156 results found
Poole B, Barzdajn B, Dini D, et al., 2019, The roles of adhesion, internal heat generation and elevated temperatures in normally loaded, sliding rough surfaces, International Journal of Solids and Structures, ISSN: 0020-7683
The thermal effects of plastic and frictional heat generation and elevated temperature were examined along with the role of adhesion in the context of galling wear, using a representative crystal plasticity, normally loaded, sliding surface model. Galling frequency behaviour was predicted for 316L steel. Deformation of the surfaces was dominated by the surface geometry, with no significant effect due to variations in frictional models. Plastic and frictional heating were found to have a minimal effect on the deformation of the surface, with the rapid conduction of heat preventing any highly localised heating. There was no corresponding effect on the predicted galling frequency response.Isothermal, elevated temperature conditions caused a decrease in galling resistance, driven by the temperature sensitivity of the critical resolved shear stress. The extent of deformation, as quantified by the area of plastically deformed material and plastic reach, increased with temperature. Comparisons were made with literature results for several surface amplitude and wavelength conditions. Model results compared favourably with those in the literature. However, the reduction in predicted galling resistance with elevated temperature for a fixed surface was not as severe as observations in the literature, suggesting other mechanisms (e.g. phase transformations, surface coatings and oxides) are likely important.
Zheng Z, Dunne FPE, 2019, Effects of grain size, orientation, and source density on dislocation configurational energy density, JOM, Vol: 71, Pages: 2576-2585, ISSN: 1047-4838
The effects of grain size, source density, and misorientations on the dislocation configurational energy area density are investigated using two-dimensional discrete dislocation plasticity. Grain boundaries are modeled as impenetrable to dislocations. The considered grain size ranges from 0.4μm2 to 8.0μm2 . The configurational energy area density displays a strong size dependence, similar to the stress response. Two sets of materials are considered, with low and high source/obstacle density. The high-source-density specimens exhibit negative configurational energy, implying that the dislocation structure is more stable than for isolated dislocations . The contribution of misorientation to the configurational energy density is analyzed using specimens with a single orientation or a checkerboard arrangement. The configurational energy density is found not only to depend on the dislocation spacing but also to be related to the local stress states. Low source densities lead to higher (positive) configurational energy densities.
Wilson D, Wan W, Dunne FPE, 2019, Microstructurally-sensitive fatigue crack growth in HCP, BCC and FCC polycrystals, Journal of the Mechanics and Physics of Solids, Vol: 126, Pages: 204-225, ISSN: 0022-5096
Microstructurally sensitive fatigue crack growth in four material systems with BCC, FCC and HCP crystallography was investigated through integrated crystal plasticity eXtended Finite Element (XFEM) modelling and experiment. The mechanistic drivers for crack path tortuosity and propagation rate have been investigated and crack propagation found to be controlled by crack tip stored energy and the crack direction by anisotropic crystallographic slip at the crack tip. Experimentally observed microstructurally-sensitive fatigue crack path tortuosities and growth rates in titanium alloy (Ti-6Al-4V), ferritic steel, nickel superalloy and zirconium alloy (zircaloy 4) have been shown to be captured, supporting the underpinning mechanistic arguments. Very short crack growth is dominated by local slip, but with increasing length, crack tip stresses begins to predominate, increasing the availability of slip systems and giving smaller amplitude oscillations between slip systems. This leads to overall crack paths which are in fact crystallographic but which appear not to be. Key features of crack retardation at grain boundaries, changes in rate resulting from crystallography, and intragranular crack path deflections have been experimentally observed and captured.
Zheng Z, Prastiti NG, Balint DS, et al., 2019, The dislocation configurational energy density in discrete dislocation plasticity, Journal of the Mechanics and Physics of Solids, ISSN: 0022-5096
Dislocation configurational energy is the term assigned to describe the elastically-stored energy associated with the interaction of dislocations and their structures. It is the energy which is over and above that from the summation of the dislocation line energies when considered isolated and non-interacting. It is therefore different to the free energy and the stored energy. This paper presents a formulation for its determination utilising discrete dislocation plasticity. The total geometrically necessary (GND) and statistically stored dislocation density mean free distance allows the configurational energy density to be determined, thus providing a length scale over which the configurational energy is stored. This quantity is assessed in polycrystals undergoing fatigue loading showing that clear microstructural locations, often associated with high GND density, become established at which the progressive, cyclic, increasing configurational energy occurs. A higher length scale crystal plasticity stored energy density has recently been introduced which attempts to capture local dislocation configurational energy density as an indicator of fatigue crack nucleation and growth. The former is compared and assessed against the dislocation configurational energy density in this paper.
Chen L, James Edwards TE, Di Gioacchino F, et al., 2019, Crystal plasticity analysis of deformation anisotropy of lamellar TiAl alloy: 3D microstructure-based modelling and in-situ micro-compression, International Journal of Plasticity, ISSN: 0749-6419
Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales. This study focuses on two specific configurations of lamellar microstructure with the interfaces being aligned and to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the micropillar while the anisotropy of the micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the micropillar less localised than that in the micropillar. Moreover, the CP-FEM model successfully predicted substantial build-up of internal stresses at interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins.
Wilson D, Dunne FPE, 2019, A mechanistic modelling methodology for microstructure-sensitive fatigue crack growth, Journal of the Mechanics and Physics of Solids, Vol: 124, Pages: 827-848, ISSN: 0022-5096
A mechanistic methodology for simulating microstructurally-sensitive (tortuosity and propagation rate) fatigue crack growth in ductile metals is introduced which utilises the recently introduced dislocation configurational stored energy as the measure of the driving force. The model implements crystal plasticity finite element simulations using the eXtended Finite Element Method (XFEM) to represent the crack. Two methods of predicting the direction of growth (based on the crystallographic slip or the maximum principal stress) are compared. The crystallographic slip based direction model is shown to predict microstructurally-sensitive fatigue crack growth in single crystals which displays many features of path tortuosity that have been observed experimentally. By introducing a grain boundary, the crystallographic model is shown to capture behaviour similar to that observed experimentally including crack deflection and retardation at the grain boundaries. Finally, two experimental examples of fatigue cracks growing across three grains are analysed, and the model is shown to capture the correct crystallographic growth paths in both cases.
Waheed S, Zheng Z, Balint D, et al., 2019, Microstructural effects on strain rate and dwell sensitivity in dual-phase titanium alloys, Acta Materialia, Vol: 162, Pages: 136-148, ISSN: 1359-6454
In this study, stress relaxation tests are performed to determine and compare the strain rate sensitivity of different titanium alloy microstructures using discrete dislocation plasticity (DDP) and crystal plasticity finite element (CPFE) simulations. The anisotropic α and β phase properties of alloy Ti-6242 are explicitly included in both the thermally-activated DDP and CPFE models together with direct dislocation penetration across material-interfaces in the DDP model. Equiaxed pure α, colony, Widmanstatten and basketweave microstructures are simulated together with an analysis of the effect of α grain size and dislocation penetration on rate sensitivity. It is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity in agreement with experimental evidence in the literature, whereas dislocation penetration is found not to be as significant as previously considered for small deformations. The mechanistic cause of these effects is argued to be changes in dislocation mean free path and the total propensity for plastic slip in the specimen. Comparing DDP results with corresponding CPFE simulations, it is shown that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. Finally, the dwell sensitivity in a polycrystalline dual-phase titanium alloy specimen is shown to be strongly dependent on its microstructure.
Wilson D, Zheng Z, Dunne F, 2018, A microstructure-sensitive driving force for crack growth, Journal of the Mechanics and Physics of Solids, Vol: 121, Pages: 147-174, ISSN: 0022-5096
A stored energy based methodology for calculating the driving force for crack growth is introduced which can capture the highly local microstructural sensitivity. This has been implemented in the context of crystal plasticity finite element simulations with explicit representation of the crack with the eXtended Finite Element Method (XFEM), with non-local approaches for both stored energy and J-integral calculation. The model is shown to have good agreement with discrete dislocation plasticity (DDP) models in terms of the crack-tip dislocation configurational energy, and with experimental observations of long and very short (microstructurally-sensitive) cracks for both fracture toughness and crack growth rate data. The method is shown to capture the microstructural sensitivity, in contrast with the widely used J-Integral method. By modelling different crack lengths, the diminution of the microstructural sensitivity with increasing crack length is quantified and a critical length defined above which the microstructural sensitivity is insignificant.
Barzdajn B, Paxton AT, Stewart D, et al., 2018, A crystal plasticity assessment of normally-loaded sliding contact in rough surfaces and galling, Journal of the Mechanics and Physics of Solids, Vol: 121, Pages: 517-542, ISSN: 0022-5096
An investigation of rough metal to metal contacting surfaces under normal load and undergoing sliding has been carried out with explicit representation of measured surface profiles within a crystal plasticity finite element formulation in which grain size, texture and slip properties are incorporated. A new metric called plastic reach has been introduced for contacting surfaces which reflects both the magnitude of the local surface asperity plasticity and its spatial reach. This quantity has been shown to obey a power law relationship with the applied normal load for sliding contact which in turn has been related to a hazard function. In this way, a new methodology to predict the galling frequency that follows a Weibull distribution has been established. Additionally, a quantitative definition of galling for the class of metal on metal contacting surfaces is considered. The predicted galling frequency distribution for a 316 stainless steel has been compared with independently experimentally measured galling frequencies showing qualitative agreement of the distributions. An assessment of confidence limits has also therefore been provided for the modelling methodology.
Zheng Z, Stapleton A, Fox K, et al., 2018, Understanding thermal alleviation in cold dwell fatigue in titanium alloys, International Journal of Plasticity, Vol: 111, Pages: 234-252, ISSN: 0749-6419
Dwell fatigue facet nucleation has been investigated in isothermal rig disc spin tests and under anisothermal in-service engine conditions in titanium alloy IMI834 using α-HCP homogenised and faithful α-β lamellar microstructure crystal plasticity representations. The empirically observed facet nucleation and disc failure at low stress in the isothermal spin tests has been explained and originates from the material rate sensitivity giving rise to soft grain creep accumulation and hard grain basal stresses which increase with fatigue cycling until facet nucleation. The α-HCP homogenised model is not able to capture this observed behaviour at sensible applied stresses. In contrast to the isothermal spin tests, anisothermal in-service disc loading conditions generate soft grain slip accumulation predominantly in the first loading cycle after which no further load shedding nor soft grain creep accumulation is observed, such that the behaviour is stable, with no further increase in hard grain basal stress so that facet nucleation does not occur, as observed empirically. The thermal alleviation, which derives from in-service loading conditions and gives the insensitivity to dwell fatigue dependent on the temperature excursions, has been explained. A stress-temperature map for IMI834 alloy has been established to demarcate the ranges for which the propensity for dwell fatigue facet nucleation is high, threatening or low.
Lan B, Carpenter MA, Gan W, et al., 2018, Rapid measurement of volumetric texture using resonant ultrasound spectroscopy, Scripta Materialia, Vol: 157, ISSN: 1359-6462
This paper presents a non-destructive evaluation method of volumetric texture using resonant ultrasound spectroscopy (RUS). It is based on a general theoretical platform that links the directional wave speeds of a polycrystalline aggregate to its texture through a simple convolution relationship, and RUS is employed to obtain the speeds by measuring the elastic constants, where well-established experimental and post-processing procedures are followed. Important lower-truncation-order textures of representative hexagonal and cubic metal samples with orthorhombic sample symmetries are extracted, and are validated against independent immersion ultrasound and neutron tests. The successful deployment of RUS indicates broader applications of the general methodology.
Zhao C, Stewart D, Jiang J, et al., 2018, A comparative assessment of iron and cobalt-based hard-facing alloy deformation using HR-EBSD and HR-DIC, Acta Materialia, Vol: 159, Pages: 173-186, ISSN: 1359-6454
Three iron-based alloys (Nitronic 60, Tristelle 5183 and RR2450) and a cobalt alloy (Stellite 6) are studied using bend-testing to induce progressive straining and both high resolution DIC and EBSD are utilized to provide quantitative characterization of the deformation mechanisms. The roles of austenite, ferrite and carbide/silicide phases are investigated, together with how each contributes to slip activation and localisation, GND development and hardening through to particle pull-out and fracture. The observed mechanisms are discussed in the context of galling performance.The results suggest that a distribution of fine precipitates, both intra-granular and at grain/phase boundaries, promote more homogeneous and distributed slip, and the development of distributed higher densities of GNDs. The latter promotes hardening which in turn also facilitates homogeneity of deformation and potentially better galling resistance. A uniform size of fine precipitates is also helpful; large silicides lead to particle fracture and pull-out, likely highly damaging under conditions of sliding contact and galling.
Lan B, Britton TB, Jun T-S, et al., 2018, Direct volumetric measurement of crystallographic texture using acoustic waves, Acta Materialia, Vol: 159, Pages: 384-394, ISSN: 1359-6454
Crystallographic texture in polycrystalline materials is often developed as preferred orientation distribution of grains during thermo-mechanical processes. Texture dominates many macroscopic physical properties and reflects the histories of structural evolution, hence its measurement and control are vital for performance optimisation and deformation history interogation in engineering and geological materials. However, exploitations of texture are hampered by state-of-the-art characterisation techniques, none of which can routinely deliver the desirable non-destructive, volumetric measurements, especially at larger lengthscales. Here we report a direct and general methodology retrieving important lower-truncation-order texture and phase information from acoustic (compressional elastic) wave speed measurements in different directions through the material volume (avoiding the need for forward modelling). We demonstrate its deployment with ultrasound in the laboratory, where the results from seven representative samples are successfully validated against measurements performed using neutron diffraction. The acoustic method we have developed includes both fundamental wave propagation and texture inversion theories which are free from diffraction limits, they are arbitrarily scalable in dimension, and can be rapidly deployed to measure the texture of large objects. This opens up volumetric texture characterisation capabilities in the areas of material science and beyond, for both scientific and industrial applications.
Erinosho TO, Collins DM, Todd RI, et al., 2018, Statistical effects in X-ray diffraction lattice strain measurements of ferritic steel using crystal plasticity, Materials and Design, Vol: 153, Pages: 159-165, ISSN: 0264-1275
The influence of statistics on calculated lattice strains has been studied by comparing crystal plasticity finite element (CPFE) calculations with strains measured experimentally. Experimentally, when Bragg's law is obeyed, a plane normal must lie within a narrow orientation range (∼ 0.02° for synchrotron diffraction), or Bragg tolerance. However, CPFE models consider only a small number of grains compared to experiments, necessitating a justification of the statistically representative volume. It also becomes necessary to assess the threshold of Bragg tolerance allowable for the determined statistically representative volume. In this study, an 8 × 8 × 8 model was deemed as statistically representative such that only small benefits are obtained in terms of lattice strain calculations by adopting larger models such as 10 × 10 × 10. Based on the selected model, an allowable Bragg tolerance of approximately 5° was calculated. Also highlighted was the coupling between lattice strain, texture, hardening and applied boundary condition which are discriminators that will affect the choice of model size and Bragg tolerance threshold.
Zhang Z, Wilkinson A, Preuss M, et al., 2018, Quantitative investigation of micro slip and localization in polycrystalline materials under uniaxial tension, International Journal of Plasticity, Vol: 108, Pages: 88-106, ISSN: 0749-6419
Micro slip activation and localization inTi-6Al-4Vdeformed in tension havebeen examinedquantitatively using high-resolution (HR) digital image correlation (DIC),HR-electron backscatter diffraction(EBSD)and crystal plasticityfinite element modelling. The measured polycrystal slip, strain, lattice rotation and geometrically necessary dislocation (GND) densitydistributionsare generally well capturedby thea prioricrystal plasticity model based on the rate-sensitive properties of α-titanium. An overall slip trace analysis showed over 80% agreement betweenHR-DIC and crystal plasticity modellingofthe primary slip activation. Thetexture beneath the characterised free-surface has been found toaffectthe local slip, stress distribution, lattice curvature and GND density andthreetexturevariationshave been considered.Grain-levelslip trace analysis shows that the crystal plasticity modelling can capture single (straight)slip, multiple slip activation andcomplex wavyslip.The latterhas been found to result from the interactionof independently activated basal andprismatic slip systems with commonslip direction. Initial inter-granular misorientations greater than about 5ohave been shown to influence the subsequent micromechanical grain behaviour including slip, lattice rotation and GND density. This work contributes to the understanding of slip localization and load shedding in dwell fatigue in polycrystalline hexagonal materials.
Zhang Z, FPE D, 2018, Phase morphology, variants and crystallography of alloy microstructures in cold dwell fatigue, International Journal of Fatigue, Vol: 113, Pages: 324-334, ISSN: 0142-1123
This paper examines microscale crystal slip accumulation, cold creep, and stress redistribution (load shedding) related to dwell fatigue in a range of α–β Ti alloy microstructures. The role of basal slip and prism slip is evaluated in load shedding in a rogue grain combination. The results enrich the Stroh dislocation pile up interpretation of dwell by accounting for the anisotropic rate dependence of differing slip systems together with morphology.Microstructural morphology has been found to play an essential role in cold creep and load shedding in dwell fatigue. Basketweave structures with multiple α variants have been shown to give the lowest load shedding for which the mechanistic explanation is that the β lath structures provide multiple, small-scale α variants which inhibit creep and hence stress relaxation, thus producing more uniform, diffuse stress distributions across the microstructure through microscale kinematic confinement, imposed by multi (α)-to-single (β) BOR relations (i.e. multiple α variants sharing the same parent β grain). The critical consequence of this is that alloys typically having multi-variant basketweave structure (e.g. Ti-6246), remain free of dwell fatigue debit whereas those alloys associated with globular colony structures (e.g. Ti-6242) suffer significant dwell debit. This understanding is important in microstructural design of titanium alloys for resisting cold dwell fatigue.
Zheng Z, Waheed S, Balint D, et al., 2018, Slip transfer across phase boundaries in dual phase titanium alloys and the effect on strain rate sensitivity, International Journal of Plasticity, Vol: 104, Pages: 23-38, ISSN: 0749-6419
Dislocation transmission through α/β phase boundaries in titanium alloys is studied using integrated crystal plasticity (CP) and discrete dislocation plasticity (DDP) modelling techniques, combined with experimental micro-pillar compression test results. Direct dislocation transmission together with the nucleation of new dislocations ahead of a pile-up at an α/β interface, termed indirect slip transfer, are both assessed and their role in controlling microstructure-dependent strain rate sensitivity considered. A critical shear stress criterion for direct slip transfer across an α/β interface in Ti-6242 has been established by capturing the local slip penetration through the phase boundary using CP and DDP comparisons with experimental two phase micro-pillar compression. The competition between direct and indirect slip transfer has been investigated using a single Frank-Read source DDP model. Direct slip transfer is found to occur only under specific conditions which have been quantified. The strain rate sensitivity of dual phase titanium alloys is demonstrated to depend on average pile-up size which is significantly influenced by α/β morphology.
Chen B, Jiang J, Dunne FPE, 2017, Is stored energy density the primary meso-scale mechanistic driver for fatigue crack nucleation?, International Journal of Plasticity, Vol: 101, Pages: 213-229, ISSN: 0749-6419
Fatigue crack nucleation in a powder metallurgy produced nickel alloy containing a non-metallic inclusion has been investigated through integrated small-scale bend testing, quantitative characterisation (HR-DIC and HR-EBSD) and computational crystal plasticity which replicated the polycrystal morphology, texture and loading. Multiple crack nucleations occurred at the nickel matrix-inclusion interface and both nucleation and growth were found to be crystallographic with highest slip system activation driving crack direction. Local slip accumulation was found to be a necessary condition for crack nucleation, and that in addition, local stress and density of geometrically necessary dislocations are involved. Fatemi-Socie and dissipated energy were also assessed against the experimental data, showing generally good, but not complete agreement. However, the local stored energy density (of a Griffith-Stroh kind) identified all the crack nucleation sites as those giving the highest magnitudes of stored energy.
Zheng Z, Balint D, Dunne F, 2017, Mechanistic basis of temperature-dependent dwell fatigue in titanium alloys, Journal of the Mechanics and Physics of Solids, Vol: 107, Pages: 185-203, ISSN: 0022-5096
The temperature-dependent dwell sensitivity of Ti-6242 and Ti-6246 alloys has been assessed over a temperature range from −50∘C to 390 °C using discrete dislocation plasticity which incorporates both thermal activation of dislocation escape from obstacles and slip transfer across grain boundaries. The worst-case load shedding in Ti-6242 alloy is found to be at or close to 120 °C under dwell fatigue loading, which diminishes and vanishes at temperatures lower than −50∘C or higher than 230 °C. Load shedding behaviour is predicted to occur in alloy Ti-6246 also but over a range of higher temperatures which are outside those relevant to in-service conditions. The key controlling dislocation mechanism with respect to load shedding in titanium alloys, and its temperature sensitivity, is shown to be the time constant associated with the thermal activation of dislocation escape from obstacles, with respect to the stress dwell time. The mechanistic basis of load shedding and dwell sensitivity in dwell fatigue loading is presented and discussed in the context of experimental observations.
Chen B, Jiang J, Dunne F, 2017, Microstructurally-sensitive fatigue crack nucleation in Ni-based single and oligo crystals, Journal of the Mechanics and Physics of Solids, Vol: 106, Pages: 15-33, ISSN: 1873-4782
An integrated experimental, characterisation and computational crystal plasticity study of cyclic plastic beam loading has been carried out for nickel single crystal (CMSX4) and oligocrystal (MAR002) alloys in order to assess quantitatively the mechanistic drivers for fatigue crack nucleation.The experimentally validated modelling provides knowledge of key microstructural quantities (accumulated slip, stress and GND density) at experimentally observed fatigue crack nucleation sites and it is shown that while each of these quantities is potentially important in crack nucleation, none of them in its own right is sufficient to be predictive. However, the local (elastic) stored energy density, measured over a length scale determined by the density of SSDs and GNDs, has been shown to predict crack nucleation sites in the single and oligocrystals tests. In addition, once primary nucleated cracks develop and are represented in the crystal model using XFEM, the stored energy correctly identifies where secondary fatigue cracks are observed to nucleate in experiments. This (Griffith-Stroh type) quantity also correctly differentiates and explains intergranular and transgranular fatigue crack nucleation.
Jiang J, Dunne F, Britton T, 2017, Toward predictive understanding of fatigue crack nucleation in Ni-based Superalloys, JOM, Vol: 69, Pages: 863-871, ISSN: 1047-4838
Predicting when and where materials fail is a holy grail for structural materials engineering. Development of a predictive capability in this domain will optimize the employment of existing materials, as well as rapidly enhance the uptake of new materials, especially in high-risk, high-value applications, such as aeroengines. In this article, we review and outline recent efforts within our research groups that focus on utilizing full-field measurement and calculation of micromechanical deformation in Ni-based superalloys. In paticular, we employ high spatial resolution digital image correlation (HR-DIC) to measure surface strains and a high-angular resolution electron backscatter diffraction technique (HR-EBSD) to measure elastic distortion, and we combine these with crystal plasticity finite element (CPFE) modeling. We target our studies within a system of samples that includes single, oligo, and polycrystals where the boundary conditions, microstructure, and loading configuration are precisely controlled. Coupling of experiment and simulation in this manner enables enhanced understanding of crystal plasticity, as demonstrated with case studies in deformation compatibility; spatial distributions of slip evolution; deformation patterning around microstructural defects; and ultimately development of predictive capability that probes the location of microstructurally sensitive fatigue cracks. We believe that these studies present a careful calibration and validation of our experimental and simulation-based approaches and pave the way toward new understanding of crack formation in engineering alloys.
Ashton P, Jun TS, Britton TB, et al., 2017, The effect of the beta phase on the micromechanical response of dual-phase titanium alloys, International Journal of Fatigue, Vol: 100, Pages: 377-387, ISSN: 1879-3452
This paper investigates the role of beta phase on the micro-mechanical behaviour of dual-phase titanium alloys, with particular emphasis on the phenomenon of cold dwell fatigue, which occurs in such alloys under room temperature conditions. A strain gradient crystal plasticity model is developed and calibrated against micro-pillar compression test data for a dual-phase alpha-beta specimen. The effects of key microstructural variables, such as relative beta lath orientation, on the micromechanical response of idealised alpha-beta colony microstructures are shown to be consistent with previously-published test data. A polycrystal study on the effects of the calibrated alpha-beta crystal plasticity model on the local micromechanical variables controlling cold dwell fatigue is presented. The presence of the alpha-beta phase is predicted to increase dwell fatigue resistance compared to a pure alpha phase microstructure.
Zhang Z, Dunne FPE, 2017, Microstructural heterogeneity in rate-dependent plasticity of multiphase titanium alloys, Journal of the Mechanics and Physics of Solids, Vol: 103, Pages: 199-220, ISSN: 1873-4782
Polycrystalline rate-dependent plasticity is found to originate from heterogeneous slip system/phase rate response. Micro-mechanism under low stress and low temperature (T < 0.3Tm) has been shown to be different from conventional rate sensitivity expectations. Hence the constitutive framework developed is dependent on the crystallographic orientation, properly capturing micro-scale anisotropic rate behaviour.The intrinsic rate anisotropy of the HCP α prism and basal and BCC β phase slip systems in Ti-6242, recently determined from micro-pillar and crystal plasticity modelling, have been utilised to investigate the structural strain rate sensitivities of colonies, polycrystals, bimodal and basket weave microstructures.The rate sensitivity of colony structures is dominated by the HCP α phase behaviour, at least for alloys containing up to ∼20% volume fraction β phase, and is largely independent of β-lath orientation. The apparent anisotropy of a1, a2 and a3 basal resolved shear stresses in Ti-6242 colonies is shown to originate from the local crystal stress states established as opposed to the α−β interfaces.Texture and α−β morphology are shown to affect rate dependence and to corroborate that the basal rate sensitivity is stronger than that for prism slip in Ti-6242. Morphological effects are shown to affect rate dependence but not strongly, but the number of HCP α phase variants in basketweave structures is found to have a significant effect with higher numbers of variants leading to lower strain rate sensitivities. This is potentially important in designing alloys to resist cold dwell fatigue.
Zheng Z, Balint D, Dunne F, 2017, Investigation of slip transfer across HCP grain boundaries with application to cold dwell facet fatigue, Acta Materialia, Vol: 127, Pages: 43-53, ISSN: 1359-6454
This paper addresses the role of grain boundary slip transfer and thermally-activated discretedislocation plasticity in the redistribution of grain boundary stresses during cold dwell fatigue intitanium alloys. Atomistic simulations have been utilised to calculate the grain boundary energies fortitanium with respect to the misorientation angles. The grain boundary energies are utilised within athermally-activated discrete dislocation plasticity model incorporating slip transfer controlled byenergetic and grain boundary geometrical criteria. The model predicts the grain size effect on the flowstrength in Ti alloys. Cold dwell fatigue behaviour in Ti-6242 alloy is investigated and it is shown thatsignificant stress redistribution from soft to hard grains occurs during the stress dwell, which isobserved both for grain boundaries for which slip transfer is permitted and inhibited. However, thegrain boundary slip penetration is shown to lead to significantly higher hard-grain basal stresses nearthe grain boundary after dwell, thus exacerbating the load shedding stress compared to animpenetrable grain boundary. The key property controlling the dwell fatigue response is argued toremain the time constant associated with the thermal activation process for dislocation escape, but theslip penetrability is also important and exacerbates the load shedding. The inclusion of a macrozonedoes not significantly change the conclusions but does potentially lead to the possibility of a largerinitial facet.
Cuddihy MA, Stapleton A, Williams SJ, et al., 2016, On cold dwell facet fatigue in titanium alloy aero-engine components, International Journal of Fatigue, Vol: 97, Pages: 177-189, ISSN: 0142-1123
This paper investigates the mechanisms of facet nucleation through combining aero-engine manufacturer disc component test data with microstructure-sensitive crystal plasticity finite element (CPFE) models. Full-scale component testing has been carried out in a manner representative of in-service conditions. Elastic FE analyses of discs under these conditions and fully accounting for thermal and residual processing strains have also been carried out. Disc facet nucleation sites have been identified and the local stress states evaluated in order to establish crystal plasticity oligocrystal sub-models. The oligocrystal RVE models provide knowledge of hard-soft grain stresses under dwell loading, and the consequent load shedding in order to provide stresses required for the facet nucleation.The disc component facet observations together with the crystal plasticity sub-model oligocrystal approach provide persuasive evidence that a hard-soft grain combination is required for facet formation, that the remote stress state influences the resolved shear stress on the soft grain initiating slip (with tensile uniaxial stress state more damaging than a tension-tension biaxial stress state), and that the load shedding which results is essential in pushing up the hard-grain basal stress to nucleate facets.
Collins DM, Erinosho T, Dunne FPE, et al., 2016, A synchrotron X-ray diffraction study of non-proportional strain-path effects., Acta Materialia, Vol: 124, Pages: 290-304, ISSN: 1359-6454
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.
Dunne FPE, Guan Y, Britton TB, et al., 2016, Crystal Plasticity Modelling and HR-DIC Measurement of Slip Activation and Strain Localisation in Single and Oligo-crystal Ni Alloys under Fatigue, International Journal of Plasticity, Vol: 88, Pages: 70-88, ISSN: 0749-6419
Single crystal (CMSX4) and oligocrystal (MAR002) nickel have been studied using three-point beambending under conditions of cyclic loading. SEM images have enabled identification of slip activation,and high resolution digital image correlation has been utilized to quantify the developing strain fieldsand the strain localization in both single and oligocrystals in fatigue. The single and oligocrystalmicrostructures have been replicated within crystal plasticity finite element models and the fatigueloading analysed such that grain-by-grain comparisons of slip may be carried out. Single and multipleslip activation, slip localisation and microstructure-sensitive stress evolution have been examined.Single crystal bend fatigue gives rise to non-symmetric slip fields and localisation depending oncrystallographic orientation. Modelling correctly captures slip activation and the developing nonsymmetricslip fields. Oligocrystal slip is markedly heterogeneous, with grain misorientations drivingstrong variations, also reasonably captured by the model. Microstructure behaviour is found to varyspatially and include elastic-plastic hysteresis which is stable, and which undergoes mean stressrelaxation so that plastic shakedown occurs. Remarkable variations occur between locations eitherside of grain boundaries, providing appropriate opportunities for fatigue crack nucleation.
Zheng Z, Balint D, Dunne F, 2016, Discrete dislocation and crystal plasticity analyses of load shedding in polycrystalline titanium alloys, International Journal of Plasticity, Vol: 87, Pages: 15-31, ISSN: 0749-6419
The focus of this paper is the mechanistic basis of the load shedding phenomenon that occurs under the dwell fatigue loading scenario. A systematic study was carried out using a discrete dislocation plasticity (DDP) model to investigate the effect of crystallographic orientations, localised dislocation behaviour and grain combinations on the phenomenon. Rate sensitivity in the model arises from a thermal activation process at low strain rates, which is accounted for by associating a stress- and temperature-dependent release time with obstacles; the activation energy was determined by calibrating an equivalent crystal plasticity model to experimental data. First, the application of Stroh's dislocation pile-up model of crack nucleation to facet fracture was quantitatively assessed using the DDP model. Then a polycrystalline model with grains generated using a controlled Poisson Voronoi tessellation was used to investigate the soft-hard-soft rogue grain combination commonly associated with load shedding. Dislocation density and peak stress at the soft/hard grain boundary increased significantly during the stress dwell period, effects that were enhanced by dislocations escaping from pile-ups at obstacles. The residual stress after dwell fatigue loading was also found to be much higher compared to standard fatigue loading. Taylor (uniform strain) and Sachs (uniform stress) type assumptions in a soft-hard grain combination have been assessed with a simple bicrystal DDP model. Basal slip nucleation in the hard grain was found to be initiated by high stresses generated by strong pile ups in the soft grain, and both basal and pyramidal slip nucleation was observed in the hard grain when the grain boundary orientation aligned with that of an active slip system in the soft grain. The findings of this study give new insight into the mechanisms of load shedding and faceting associated with cold dwell fatigue in Ti alloys used in aircraft engines.
Peng Z, Tian G, Jiang J, et al., 2016, Mechanistic behaviour and modelling of creep in powder metallurgy FGH96 nickel superalloy, Materials Science and Engineering A, Vol: 676, Pages: 441-449, ISSN: 0921-5093
The creep properties of a nickel-based superalloy at 700 °C and 690 MPa resulting from differing aging heat treatments have been investigated. The heat treatments gave rise to significantly different tertiary γ′ precipitate distributions which in turn influence the propensity for precipitate shearing. The creep life was found to decrease with an increase of volume fraction of tertiary γ′. It is shown that the γ′ precipitates undergo shearing by matrix dislocations resulting in residual stacking faults which have been identified from TEM studies.A physically-based crystal slip model for creep deformation in FGH96 has been developed. A critical γ′ precipitate size is found to exist above which precipitate shearing occurs by strongly-coupled dislocations pairs. The critical size for FGH96 superalloy is less than 15 nm, which is smaller than most of the γ′ precipitates in the aged treated samples. A bimodal precipitate hardening model has been presented from which the slip strength resulting from differing heat treatments may be determined. Creep strain rate is found to decrease with increasing slip strength. The crystal slip model successfully captures the effect of heat treatment (in terms of γ′ precipitate distributions) on resulting creep behaviour in alloy FGH96.
Zheng Z, Balint D, Dunne F, 2016, Dwell fatigue in two Ti alloys: An integrated crystal plasticity and discrete dislocation study, Journal of the Mechanics and Physics of Solids, Vol: 96, Pages: 411-427, ISSN: 1873-4782
It is a well known and important problem in the aircraft engine industry that alloy Ti-6242shows a significant reduction in fatigue life, termed dwell debit, if a stress dwell is includedin the fatigue cycle, whereas Ti-6246 does not; the mechanistic explanation for thediffering dwell debit of these alloys has remained elusive for decades. In this work, crystalplasticity modelling has been utilised to extract the thermal activation energies for pinneddislocation escape for both Ti alloys based on independent experimental data. This thenallows the markedly different cold creep responses of the two alloys to be captured accuratelyand demonstrates why the observed near-identical rate sensitivity under nondwellloading is entirely consistent with the dwell behaviour. The activation energiesdetermined are then utilised within a recently developed thermally-activated discretedislocation plasticity model to predict the strain rate sensitivities of the two alloys associatedwith nano-indentation into basal and prism planes. It is shown that Ti-6242 experiencesa strong crystallographic orientation-dependent rate sensitivity while Ti-6246does not which is shown to agree with recently published independent measurements;the dependence of rate sensitivity on indentation slip plane is also well captured. Thethermally-activated discrete dislocation plasticity model shows that the incorporation of astress dwell in fatigue loading leads to remarkable stress redistribution from soft to hardgrains in the classical cold dwell fatigue rogue grain combination in alloy Ti-6242, but thatno such load shedding occurs in alloy Ti-6246. The key property controlling the behaviouris the time constant of the thermal activation process relative to that of the loading. Thiswork provides the first mechanistic basis to explain why alloy Ti-6242 shows a dwell debitbut Ti-6246 does not.
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