Micromechanical modeling of the effect of elastic and plastic anisotropies on the mechanical behavior of β-Ti alloys

International audienceNear β-titanium alloys like Ti-5553 or Ti-1023 often exhibit bimodal phase constituents embedded in a retained β-phase matrix, which represents up to 40% of the volume. The highly elastic anisotropic β-phase may strongly influence the mechanical behavior of these alloys. The present work models the effect of the coupled role of β-phase elastic and plastic anisotropies on the local and overall responses of a fully β-phase polycrystalline aggregate like the Ti-17 alloy. The model is based on an advanced elasto-viscoplastic self-consistent (EVPSC) homogenization scheme solved by the "translated field" method together with an affine linearization of the viscoplastic flow rule. The effects of elastic anisotropy, crystallographic texture and grain morphology are theoretically studied during uniaxial tensile tests, tension-compression tests as well as multiaxial plastic yielding. First, it is shown that different sets of elastic constants taken from literature give rise to similar effective responses but to widely scattered incompatibility stresses. During uniaxial tensile loading, the highest local incompatibility stresses are achieved in oriented grains at the end of the elastic regime. Likewise, the effect of the β-grain morphology for realistic grain aspect ratios is seen to be weak on the overall behavior but strong on incompatibility stresses. In addition, the elastic anisotropy can have a significant influence on yield surfaces for β-forged textured polycrystals. Finally, the simulated Bauschinger stress monotonically increases with the elastic anisotropy coefficient for a random texture while it may be reduced in case of β-forged texture due to a competition between elastic and plastic sources of incompatibility stresses

Criteria for fast and selective α precipitation at β grain boundaries in Ti-alloys Consequence for in-service microstructures

International audienceThe potential for β grain boundaries (GBs) to give rise to large Widmanstätten colonies was determined through the observation of a large amount of precipitates at β GBs in a β-metastable titanium alloy using electron backscatter imaging and diffraction. The most critical boundaries are those which transform early and where a type of Variant Selection (VS) called double Burgers VS occurs. This mechanisms take place at 'special' β boundaries misoriented so that an α precipitate can be related to both grains through the Burgers relation. It was shown that the most critical GBs have a disorientation at an angle of less than 10° from 49.5°/<110> or 60°/<110>. A simulation study allowed those boundaries to be quantifed in crystallographic textures typical of industrial products. Those texture have then been discussed as a function of their potential to form large Widmanstätten colonies

Criteria for fast and selective α precipitation at β grain boundaries in Ti-alloys Consequence for in-service microstructures

International audienceThe potential for β grain boundaries (GBs) to give rise to large Widmanstätten colonies was determined through the observation of a large amount of precipitates at β GBs in a β-metastable titanium alloy using electron backscatter imaging and diffraction. The most critical boundaries are those which transform early and where a type of Variant Selection (VS) called double Burgers VS occurs. This mechanisms take place at 'special' β boundaries misoriented so that an α precipitate can be related to both grains through the Burgers relation. It was shown that the most critical GBs have a disorientation at an angle of less than 10° from 49.5°/ or 60°/. A simulation study allowed those boundaries to be quantifed in crystallographic textures typical of industrial products. Those texture have then been discussed as a function of their potential to form large Widmanstätten colonies

Stress partitioning in a near-β Titanium alloy induced by elastic and plastic phase anisotropies: experimental and modeling

International audienceThe load transfer induced by the elas c and plas c phase anisotropies of a Ti-10V-2Fe-3Al tanium alloy is studied. The microstructure consists in α nodules embedded in elongated β grains. EBSD performed on the alloy shows no crystallographic texture neither for α nor β phase. Tensile tests along the elonga on direc on, at a strain rate of 2 x 10-3 s-1 give a yield stress of 830 MPa with 13% duc lity. Simula ons based on an advanced two-phase polycrystalline elasto-viscoplas c self-consistent (EVPSC) model predict that the β phase first plas fies with a sequen al onset of plas city star ng from oriented β grains, then and finally oriented β grains. This leads to a strong load transfer from the β grains to the α nodules whose average behavior remains elas c up to high stresses (~940 MPa). However, addi onal simula ons considering exclusively β grains of specific orienta on show that the behavior of α nodules is strongly dependent on the β texture in which they are embedded. Especially, in β grains, which plas fy the latest, the model predicts the onset of plas city in favorably orientated α nodules. Moreover, the orienta on spread within the β grains can modify the average plas c behavior of α phase. In future, these results will be compared to data obtained from in-situ High Energy XRD and SEM/EBSD experiments

Study of crack propagation mechanisms during Charpy impact toughness tests on both equiaxed and lamellar microstructures of Ti–6Al–4V titanium alloy

The impact toughness of two highly textured rolled plates of Ti–6Al–4V alloy with an α equiaxed and an α lamellar microstructures has been investigated. The results show a strong anisotropy of the fracture energy for both materials and underline that a coincidence of the prismatic planes with the shear bands at the notch tip is favorable for higher fracture energies. Moreover, it is pointed out, as it was already done by previous studies, that the α lamellar microstructure presents higher fracture energy than the α equiaxed one. Thanks to electron back scattering diffraction, and tensile tests, local microstructure heterogeneities, called macrozones, have been observed and characterized. Their size depends on microstructure element and is larger for α lamellar microstructure than for the α equiaxed. High strain is localized on the macrozones favorably oriented for prismatic slip with respect to the direction of impact and leads to a particular dimple free zone on the fracture surface. The contribution of these macrozones in the fracture behavior, and more precisely on the crack propagation rate was evaluated; thus the effects of the macroscopic texture and of the microstructure element on the impact toughness are discussed separately

EBSD-assisted Laue microdiffraction for microstrain analysis

The X-ray Laue microdiffraction (mLaue) technique has been establishing itself as a reliable means for microstrain analysis for the past few decades. One problem with this technique is that when the crystal size is significantly smaller than the probed volume and when the diffracting crystals are closely oriented, a large number of individual mLaue patterns are superimposed in a complex way on the recorded diffraction images. In that case, because of the difficulty of isolating unambiguously a single-grain mLaue pattern, a reliable analysis of strains is tedious manually and hardly achievable with current automated methods. This issue is even more severe for low-symmetry crystals or when highenergy X-rays are used, since each single-crystal mLaue pattern already contains a large number of spots. This paper proposes overcoming this challenge through the development of a combined approach coupling mLaue and electron backscatter diffraction (EBSD). The capabilities of this 'EBSD-assisted mLaue' automated method are illustrated on a monoclinic zirconia-based specimen and mLaue diffraction patterns are analysed with the crystal orientation input from EBSD. The obtained results are statistically reliable, reproducible and provide a physical insight into the micromechanical characteristics of the material

Huge local elastic strains in bulk nanostructured pure zirconia materials

From the liquid state to room temperature, two successive solid-state phase transitions occur in pure zirconia. It is well-known that the last one (tetragonal to monoclinic) is martensitic and induces large volume variations and shear strains. Elastic and inelastic behaviors of zirconia-based materials are strongly influenced by this transition and the associated strain fields that it induces. Knowledge of strain and stress at the crystal scale is thus a crucial point. Using fully dense pure zirconia polycrystals obtained by a fuse casting process, we have determined at a sub-micrometric scale, by X-ray Laue microdiffraction, the strains map at room temperature in as-cast specimens and after a post elaboration high temperature thermal treatment. We observed that the fluctuation of deviatoric elastic strain is huge, the standard deviation of normal component being in the range of 1–2%. The heat treatment tends to even further increase this range of fluctuation, despite the development of a multiscale crack network formed during the cooling. Correspondingly, the associated stress level is also huge. It lies in the 5 GPa range with stress gradient amounting 1 GPa μm−1.This work was done in the frame of the ASZTECH research program funded by the ANR (ANR-12-RMNP-0007). We acknowledge the ESRF and the French Collaborating Research Group (F-CRG) for provision of synchrotron radiation facilities beamtimes and beamline staff for their assistance. The authors are thankful to I. Cabodi and O. Bories (Saint- Gobain CREE) for the supply of the bulk zirconia-based materials

Estimating single-crystal elastic constants of polycrystalline β metastable titanium alloy: A Bayesian inference analysis based on high energy X-ray diffraction and micromechanical modeling

The authors also thank the Laboratoire Léon Brillouin (France) for beamtime allocation and Sebastien GAUTROT (LLB, France) for his help during the experiments. Vincent Jacquemain, Dr. Jean-Baptiste Marijon, and Dr. Stefan Michalik are acknowledged for their help during the synchrotron campaign at Diamond.A two-phase near- beta titanium alloy (Ti–10V–2Fe–3Al, or Ti-1023) in its as-forged state is employed to illustrate the feasibility of a Bayesian framework to identify single-crystal elastic constants (SEC). High Energy X-ray diffraction (HE-XRD) obtained at the Diamond synchrotron source are used to character- ize the evolution of lattice strains for various grain orientations during in situ specimen loading in the elastic regime. On the other hand, specimen behavior and grain deformation are estimated using the elastic self-consistent (ELSC) homogenization scheme. The XRD data and micromechanical modelling are revisited with a Bayesian framework. The effect of different material parameters (crystallographic and morphological textures, phase volume fraction) of the micromechanical model and the biases intro- duced by the XRD data on the identification of the SEC of the βphase are systematically investigated. In this respect, all the three cubic elastic constants of the βphase ( C11(beta) , C12(beta) , C44(beta) ) in the Ti-1023 alloy have been derived with their uncertainties. The grain aspect ratio in the ELSC model, which is often not considered in the literature, is found to be an important parameter in affecting the identified SEC. The Bayesian inference suggests a high probability for non-spherical grains (aspect ratio of ∼3 . 8+/-0 . 8 ) : C11(beta) = 92 . 6+/-19 . 1 GPa , C12(beta) = 82 . 5+/-16 . 3 GPa , C44(beta) = 43 . 5+/-7 . 1 GPa . The uncertainty obtained by Bayesian approach lies in the range of ~1-3 GPa for the shear modulus mu’ = (C11(beta) −C12(beta) )/2 , and ~7 GPa for the shear modulus mu’’ = C44(beta) , while it is significantly larger in the case of the bulk modulus (C11(beta) +2C12(beta))/3 (~17-24 GPa).This research is supported by the “Région Grand Est” and by the French State through two programs operated by the National Research Agency (ANR), (1) “Investment in the future” referenced by ANR-11-LABX-0 0 08-01 (Laboratory of Excellence “DAMAS”: De- sign of Alloy Metals for low-mAss Structures) and (2) “Plan d’Investissement d’Avenir” (PIA) in the frame of a research pro- gram managed by "Institut de Recherche Technologique Matériaux, Métallurgie, Procédés" (IRT M2P). We acknowledge DIAMOND for provision of synchrotron radiation beamtime at beamline I12-JEEP

Chinese Script vs Plate-Like Precipitation of Beta-Al9Fe2Si2 Phase in an Al-6.5Si-1Fe Alloy

The microstructure of a high-purity Al-6.5Si-1Fe(wt pct) alloy after solidification at various cooling rates was investigated. In most of the cases, the monoclinic beta-Al9Fe2Si2 phase was observed as long and thin lamellae. However, at a very slow cooling rate, Febearing precipitates with Chinese script morphology appeared together with lamellae. Further analysis showed all these Chinese script precipitates correspond also to the monoclinic beta phase. This finding stresses that differentiating second phases according to their shape may be misleading

Etude des changements de textures par transformation de phase beta-alpha dans des produits TA6V laminés à chaud

In the present investigation, the texture transformations induced by alpha beta phase transformation in hot rolled TA6V products were studied. Different samles were rolled at 1050°C (beta field), 900 °C (alpha - beta field). The alpha textures, inherited by phase transformation, were determined by X-ray diffraction or by EBSP and analysed according to the volume proportion of both alpha beta phases and their respective deformation mechanisms. After a 30% deformation at 1050 °C, the inherited alpha texture were characterised by a fibre type texture whereas a 75% deformation favoured the (0°, 90°, 0°) orientation. The originality of this work lies in the determination of the corresponding high temperature beta textures by specific methods. The beta texture, 30% deformed at 1050°C, was characterised by the 100 and 111 fibres. A 75% deformation promoted the 100 component in the 100 fibre whereas the 111 one was still observed. This texture evolution were analysed and compared to the results simulated by a rate sensitive Taylor model. The high temperature beta textures were then introduced in a texture transformation model using the Burgers relationship, in order to simulate the inherited alpha textures. The modelling without variant selection allowed to reproduce the fibre texture characteristic of the 30% deformation at 1050°C. After a 75% deformation, the same model made clear that a sharp variant selection mechanisms occured in the beta alpha transformation, favouring the orientations of the alpha phase centred around (0°, 90°, 0°). A transformation texture modelling were proposed in which the alpha variants, associated to the most active slip systems at the end of rolling, appeared preferentially in the transformation. This model allowed to simulate an alpha texture in agreement with the experimental oneCe travail porte sur l'étude des changements de textures de la phase, induits par transformation de phase, dans des produits TA6V laminés à chaud. Différents échantillons ont été laminés à 1050°C (domaine beta), 960°C et 900°C (domaine alpha - beta). Les textures de la phase alpha, héritées par changement de phase, ont été déterminées par diffraction de rayons X ou par EBSP et analysées en fonction des volumes respectifs des phases alpha/beta en présence à la température de déformation et de leurs mécanismes de déformation respectifs. Après une déformation de 30% à 1050°C, la texture alpha héritée correspond à une texture de fibre alors qu'une déformation plus importante de 75% à 1050°C a pour effet de favoriser la composante (0°, 90°, 0°). L'originalité du travail repose sur la détermination, par des méthodes spécifiques, de la texture beta haute température correspondante. Celle-ci se caractérise, après 30% de déformation, par les fibres 100 et 111. Une déformation de 75% renforce la composante 100 de la fibre 100 et conserve la fibre 111. L'évolution de cette texture a été analysée à l'aide d'un modèle de déformation viscoplastique de type Taylor. Ces textures de la phase beta haute température ont ensuite été utilisées pour simuler les textures de la phase alpha héritées, à l'aide de modèles de transformation de texture utilisant les relations de burgers. La modélisation sans sélection de variantes a permis de reproduire la texture de fibre caractéristique de la déformation à chaud de 30% et a mis en évidence, suite à une déformation de la phase beta de 75%, un mécanisme de sélection de variantes très marque, favorisant les variantes proches de l'orientation (0°, 90°, 0°) dans la transformation beta - alpha. Un modèle de transformation de texture avec sélection de variantes dans lequel les variantes favorisées correspondent aux systèmes de glissement les plus actifs de la phase beta, a permis de calculer une texture simulée en bon accord avec la texture expérimental