The superplastic performance of the dual-phase Ti-6242S titanium alloy makes it a good material for aerospace application to produce structural components using the advanced superplastic forming (SPF) process. The need to optimize the SPF process demands the understanding and quantifications of the influence of the different phase constituents - α and β on the global superplastic behavior. Numerical modelling has been useful to predict mechanical behavior for both one-level and multiscale approach. Multiscale approach: bottom-up (microscale to macroscale) has enabled to understand how the different microstructural parameters influence global material/structural mechanical response; which by large means the modelling approach depends on the material local properties. The identification of these local properties is non-trivial in polycrystal materials, particularly at superplastic (elevated) temperatures. We have developed a methodology that permit us to quantify the microstructural parameters of each of the constitutive phases of a polycrystal at a superplastic temperature using genetic algorithm optimization method on the data from in-situ high energy X-ray diffraction (synchrotron radiation), coupled with SEM (scanning electron microscope) and EBSD (electron backscattered diffraction). These identified local microstructural parameters were directly used in the finite strain crystal plasticity model to simulate the material global response. This approach enabled the quantification of the phase influences on global behavior with much accuracy. It was found that α phase planes have high critical resolved shearing stress (CRSS) at 730°C which is similar to its behaviours at room temperatures, while β phase slip planes have low CRSS that encourage slip shearing at low stress. However, more applied load is partitioned in β phase than in α phase, despite that β phase fraction is about 15% at 730°C. Keyword: Multiscale modelling, CPFE, optimization, HEXRD, dual-phase titanium alloy, superplasticity

The determination of single-crystal elastic constants (SECs) of individual phases in multi-phase materials is essential for high-fidelity multiscale modelling. Conventional experimental approaches and atomistic simulations remain limited by temperature range, computational cost, and applicability to complex polycrystalline alloys. This study proposes a computationally efficient methodology that combines in-situ high energy X-ray diffraction (HEXRD), lattice-strain tensor reconstruction, micromechanical homogenisation, and genetic-algorithm optimisation to identify phase-specific SECs directly from diffraction measurements. The method was applied to the dual-phase Ti‐6242S alloy at 730 °C and 840 °C. The identified elastic constants satisfy crystal elastic stability requirements and were validated through finite-element simulations of the macroscopic elastic response. Excellent agreement between simulated and experimental stress–strain curves demonstrate the capability of the proposed framework to determine temperature-dependent SECs in complex multi-phase materials.

Laser powder bed fusion (LPBF) printing of Ti-6Al-4V often suffers from surface defects such as weld traces, unmelted particles, spatter, and porosity that degrade surface integrity and induce significant residual stresses due to a higher thermal gradient. This study aims to elucidate the evolution of surface topography and residual stresses as a function of build height and process energy input during LPBF fabrication. Ti-6Al-4V samples were produced at volumetric energy densities ranging from 21.8 to 65.4 J·mm⁻³ by varying laser power and scanning speed. Focus variation microscopy measurements show propagation of surface irregularities extending over several tens of layers. Higher energy densities resulted in stable melt pools and reduced surface roughness. The computed and measured residual stresses agreed well for the sound builds. In contrast, notable deviation was observed for the conditions that exhibited high porosity. Overall, the findings show that residual stress evolution in LPBF significantly influences surface morphology, which further supports the importance of process energy optimization to achieve a sound part.

This work is devoted to studying the coupled effect of grain size and crystallographic texture on the mechanical behavior at room temperature of Ti-6Al-4V alloy with an equiaxed microstructure. To this end, experimental data was collected on a broad range of mechanical solicitation conditions through monotonic and cyclic tests, followed by macroscopic elasto-viscoplastic modeling. The experimental results show that the mechanical behavior of Ti-6Al-4V is mainly influenced by both the grain size and the crystallographic texture. The microstructure with the finest grain size exhibits the highest flow stress. The weakly textured alloy presents the highest ductility. Moreover, all the Ti-6Al-4V microstructures present cyclic softening behavior. In addition, at room temperature, no microstructure was found to exhibit significant strain rate sensitivity. The results also show that the grain size affects the yield strength of the Ti-6Al-4V alloy, as well as its ductility and its kinematic hardening. The proposed model formulation accurately predicts the effect of the microstructural features of the Ti-6Al-4V alloy. Isotropic and kinematic hardening laws are modified by introducing the grain size effects via the Hall-Petch relationship.

To develop a behavior model, properties such as Young's modulus, viscous stress, kinematic hardening, isotropic hardening, yield strength and transformation-induced plasticity parameter (TRIP) for austenite and martensite were determined using a specially developed experimental set-up.

Duplex martensitic microstructures have recently attracted significant attention due to their unique ability to combine high strength, excellent ductility, and superior work-hardening properties, making them ideal for structural applications. This study explores the micromechanisms underlying the tensile properties of a Ti-6Al2Sn-4Zr-2Mo-Si alloy with hot-rolled T-split textures and various microstructures, tested along two tensile directions: TensileD//RD (referred to as 0 degrees) and TensileD perpendicular to RD (referred to as 90 degrees). A comparative analysis highlights the superior work-hardening capacity and isotropic behavior of duplex (alpha+alpha ') and (alpha+alpha") microstructures containing martensite, compared to equiaxed (alpha+(3) microstructures. The enhanced work-hardening observed in the duplex microstructures is attributed to mechanisms such as variant reorientation in the martensitic phases, the pronounced mechanical contrast between the harder alpha phase and the softer alpha'/alpha" phases, and the interactions between alpha slipping and alpha'/alpha" twinning. Macroscopic Hall-Petch relations further clarify how microstructures influence strength and ductility. Specifically, duplex (alpha+alpha ') microstructures exhibit improved ductility due to lower Hall-Petch constants and diminished grain boundary effects. Interestingly, reverse Hall-Petch behavior is observed in the duplex (alpha+alpha") microstructure at 90 degrees, which is associated with the presence of alpha" martensite. Slip trace analysis is conducted to determine the experimental Critical Resolved Shear Stress (CRSS) ratios and qualitatively assess the impact of grain size and tensile direction on the activation of slip systems. Multiscale simulations are then utilized to calculate CRSS values and investigate the roles of deformation modes and crystallographic texture in shaping the macroscopic behavior of duplex (alpha+alpha") microstructures. At 0 degrees, the T-split texture and the facilitation of prismatic slip between adjacent prismatic grains result in a low Hall-Petch constant and minimal grain boundary effects, acting as soft grains. In contrast, basal and pyramidal systems exhibit much higher Hall-Petch constants, behaving as hard grains and significantly contributing to work hardening. At 90 degrees, basal slips uniquely display reverse Hall-Petch behavior, which is linked to the macroscopic reverse Hall-Petch phenomenon. This behavior is thought to stem from the presence of alpha" martensite and the hot-rolled texture, combined with the tensile direction of 90 degrees, which triggers a shift in the dominant mechanism around basal grains from intragrain dislocation movement to grain boundary sliding.

Surface effects were investigated using ultrathin specimens with thicknesses in the order of the grain size of the material. The candidate material was a polycrystalline Ni-based superalloy (Alloy 718) purposely heat treated to document both the effects of the grain size and the metallurgical state, i.e., solid solution and precipitation hardened state, on the polycrystalline-tomulticrystalline behavior. Ultrathin tensile specimens were prepared with a dedicated technique to obtain specimens with thicknesses ranging between 20 and 550 mu m, then tensile tested at room temperature. The polycrystalline-to-multicrystalline transition (PMT) was found to depend on the material grain size relative to the specimen thickness and to impair severely the tensile strength of the material. The yield strength, ultimate tensile strength (maximal stress on the stress-strain curve) and strain-to-failure severely dropped for specimens thinner than approximately two times the grain size of the material regardless of the metallurgical state. Such a decrease in tensile properties is mainly attributed to free-surface effects acting as an escape sink of dislocations, thus leading to a significant decrease of the primary dislocations density within the surface grains in comparison with the core grains. Interestingly, difference in work-hardening behavior with size reduction was found between both precipitation states, the solid solution state being more sensitive with the size reduction. The decrease in tensile properties was not found as expected from the commonly reported "thickness/grain size (t/D)"ratio. Therefore, a numerical approach using a modified Berveiller-Zaoui self-consistent model based on a continuum crystal plasticity approach was conducted in the present paper to distinguish microstructural features acting as strengthening (dislocation accumulation) and softening (dislocation escape at the free-surface) features. 3D numerical materials were produced using Voronoi tessellation methods to represent the fraction of "core grains" versus "surface grains". These fractions were then used as microstructural parameters for the identification of a crystal plasticity model using mean-field homogenization with different populations of grains, i.e., core versus surface features. The present work aimed at distinguishing the mechanical behavior of surface grains from core grains in Alloy 718 Ni-based superalloys using various thicknesses of specimens and different microstructure and metallurgical state variants.

This study investigates the mechanisms behind the great mechanical properties observed at room temperature for a dual-phase Ti-6Al-2Sn-4Zr-2Mo-Si titanium alloy with equiaxed (α + β) microstructures. More precisely, analyzing the material deformation modes and the possible effects of β fraction and grain size was done to better understand such micromechanisms. With this idea in mind, uniaxial tensile deformation tests were performed at room temperature, and the resulting mechanical behaviors were analyzed. It was observed that increasing β fraction would enhance the overall ductility and work hardening while conversely decreasing the material resistance. Additionally, the material strengthening due to grain size effect, quantified by the Hall-Petch parameter, was also found to be dependent on β fraction. Slip trace analysis was conducted to understand the effects of grain size and β fraction on the activation of the basal , prismatic , and pyramidal slip systems and their critical resolved shear stress (CRSS) ratios were established. The qualitative study of CRSS ratios revealed that at smaller grain sizes, the basal slip systems were dominant (e.g. basal/prismatic CRSS ratio of 0.86 for d=2.98 µm) whereas the prismatic slip systems were prevalent and more easily activated for coarser grains (e.g. basal/prismatic CRSS ratio of 1.19 for d=4.21 µm). Such CRSS ratios were then used to identify the material parameters of a self-consistent multiscale model employed to reproduce the tensile behaviors. For a more quantitative analysis, the CRSS values were evaluated and correlated to grain sizes with Hall-Petch relations. Clear correlations regarding grain size and β fraction were found for the CRSS of prismatic and pyramidal systems. However, special attention was given to the ambiguous results regarding basal slip systems because of the potential manifestation of the compatibility stresses and grain boundary sliding mechanisms due to the higher density of grain boundaries at small grain sizes.

This paper develops a new micro-mechanical damage model for fatigue crack propagation in tensile (Mode I) cyclic loading. The double-scale fatigue damage model is entirely obtained through small-scale yielding descriptions of micro-crack propagation. Based on the Paris fatigue crack growth law, the stress intensity factor range is expressed thanks to an appropriate micro-mechanical energy release rate analysis coupled with asymptotic homogenization developments. The macroscopic fatigue crack evolution law is established. Numerical simulations are presented. The reliability of the model to reproduce stress-strain shielding is demonstrated. The influence of the microstructural length on fatigue damage evolution is illustrated. The numerical results are compared with fatigue crack propagation tests performed on Ti-6Al-4V titanium fashioned by additive layer manufacturing (ALM). Through the calibration process, the model agrees with the experimental results.

This research focuses on the systematic study of a Ti-6Al-2Sn-4Zr-2Mo-Si titanium alloy and the characterization of alpha+(3 ( equiaxed and bimodal) and alpha+alpha' ( duplex ) microstructures. It provides more insights on the outstanding advantages of the duplex ( alpha + alpha ' ) microstructure, especially on its exceptional work hardening and strength-ductility balance. The heat treatment conditions required to form equiaxed, bimodal and duplex microstructures and their effects on the grain size and the phase proportion are discussed. It shows how the microstructural parameters can be controlled thanks to the heat treatment temperatures, the holding times and possible aging processes. The influence of such microstructural factors on the tensile properties of each alloy is investigated, especially on strength ( proof stress, ultimate tensile strength), ductility ( plastic elongation) and work hardening properties. The duplex ( alpha + alpha ' ) microstructure is compared with the equiaxed and bimodal microstructures and its advantages are displayed, highlighting the better strength-ductility balance and superior work hardening properties of the duplex microstructure. Indeed, the deformed microstructure of the duplex ( alpha + alpha ' ) microstructure reveals more homogeneous strain partitioning than that of the bimodal ( alpha + (3 ) microstructure. Thus, this work proved the potential of an optimized duplex ( alpha + alpha ' ) microstructure for the enhanced tensile properties at room temperature. Finally, a machine learning model using gradient boosting regression trees is used to quantify the importance of the microstructural factors ( type of microstructure, grain size and phase ratio) on the mechanical properties. [ doi:10.2320 / jinstmet.JC202410 ]

Ti-6Al-4V is a widely used titanium alloy in superplastic forming process which requires high temperatures (T approximate to 900 degrees C) and low strain rates ((epsilon)over dot approximate to 10(-3) s(-1)). One way to reduce the costs of the process is to use a lower forming temperature and/or higher strain rate, up to 10(-2) s(-1). However, the behavior of Ti-6Al-4V alloy still need to be modeled at such forming conditions and for different initial microstructure. In order to characterize the mechanical behavior of Ti-6Al-4V at temperature between 400 degrees C and 700 degrees C, relaxation and tensile tests performed under small and large deformation conditions were conducted. Depending on the test, the deformations were evaluated through two kinds of measurements, respectively with an extensometer and with a digital image correlation technique (DIC). Similar experimental results were obtained, validating the use of DIC at high temperature to evaluate high strain levels. Two different microstructures of Ti-6Al-4V alloy were tested to study the impact of the initial microstructure on the mechanical behavior. For similar conditions of strain rate and temperature, representative of the forming process, a fine-grained microstructure exhibits an enhancement of mechanical behavior in comparison with the classical coarse-grained microstructure used in the current industrial process. Finally, an elasto-viscoplastic has been identified for each microstructure.

: Due to high solubility of oxygen and nitrogen in titanium alloys, the influence of the diffusion zone on the macroscopic tensile properties of pre-oxidized annealed Ti-6Al-4V tensile specimens was examined at room temperature. Thin microtensile specimens were prepared with different thicknesses ranging from 100 µm to 500 µm and then exposed at 750°C for durations between 5 and 200h. A dedicated gripping technique was developed in the present study to investigate the brittleness of such pre-oxidized and ultrathin specimens at room temperature. Tensile testing was paired with digital image correlation techniques to assess both macroscopic deformation and full-field strain maps. High temperature pre-oxidation treatments significantly decreased the ductility of the specimen and the tensile strength of the materials (yield strength and ultimate tensile strength). Fractographic examinations revealed typical brittle fracture features in the oxygen/nitrogen-affected diffusion zone in the periphery of the cross-section while the fracture remained ductile in the core of the specimen for most of the specimens. Some specimens fully failed in a brittle manner for “(pre-ox. duration) 1/2 /thickness” configurations with ratio equal or higher than 0.45 h 1/2 .µm -1 .

Understanding the intertwining of biology and mechanics in tissue architecture is a challenging issue, especially when it comes to the 3D tissue organization. Addressing this challenge requires both a biological model allowing multiscale observations from the cell to the tissue, and theoretical and computational approaches allowing the generation of a synthetic model, relevant to the biological model, and allowing access to the mechanical constraints experienced by the tissue. Here, using human colon epithelium monolayer organoid as biological model, and combining vertex and FEM approaches, we generated a comprehensive elastic finite element model of the human colon organoid and demonstrated its flexibility. This FEM model provides a basis for relating cell shape, tissue deformation, and strain at the cellular level due to imposed stresses. In conclusion, we demonstrated that the combination of vertex and FEM approaches allows for better modeling of the alteration of organoid morphology over time and better assessment of the mechanical cues involved in establishing the architecture of the human colon epithelium.

Superplastic forming is an effective way to manufacture complex-shaped parts of titanium-based alloys. This paper studies the influence of the initial microstructure and its strain-induced evolution on superplastic deformation behavior and the formability of a titanium-based alloy. Two types of Ti-Al-V-Mo alloy samples having a different fraction of recrystallized grains before the start of the superplastic deformation were studied. The deformation behavior, including strain hardening and strain rate sensitivity of the flow stress, was analyzed in a temperature range of 775 degrees C-900 degrees C and a strain rate range of 10(-5) to 10(-2) s(-1). Strain-induced changes of the microstructure within the bulk of the samples and on the surface of the pre-polished samples were studied during superplastic deformation with a constant strain rate. The dynamic recrystallization and dynamic grain growth in the volume of the samples and the multiple slip bands on the samples' surface were revealed after superplastic deformation. The grain structure evolution and slip bands localization depended on the samples' initial microstructure. The results showed that the samples with an increased fraction of recrystallized grains exhibited better superplasticity and higher quality of the formed parts with a more uniform thickness distribution across the section than the samples with a lower initial recrystallized fraction.

Taking into account the interaction between the engine oil and the crankshaft to model crankshaft thermomechanical behavior under dynamic loading is very important. In particular, when the crankshaft is working in severe conditions. This paper deal with an air cooled direct injection-type engine crankshaft thermomechanical FEM modelling account for engine oil-cranks half interaction in severe working conditions. As case of application we consider the diesel engine Deutz F8L413. The model takes into account 2 forced convectives heat flux: engine oil and crankcase air. The severe mechanical and thermal characteristics of engine are experimentally measured on a bench test equipped with a hydraulic brake. The temperature distribution inside the crankshaft was computed using the measured temperature as boundary conditions. The most thermo-mechanical stressed zones of the crankshaft have been determined. The fatigue resistance of the crankshaft under thermo-mechanical conditions was examined using Dang-Van multi-axial fatigue criteria. To prove our model efficiency, we have compared crankshaft damage in service to the numerical simulation results. It was found the breakage occurred in an area where the numerical simulations give the highest stresses.

The aim of this paper is to investigate the coupled effects of grain size and crystallographic texture on the mechanical behavior induced by gliding mechanism in Ti-6Al-4V alloy. Thus, four microstructures of Ti-6Al-4V alloy whose grain size and crystallographic texture are different were examined by tensile tests along the rolling direction at room temperature. In this study, the contribution of gliding on basal < a >, prismatic < a > and pyramidal < c + a > plans in the accommodation of plastic strain was estimated by means of a slip trace analysis. The role of the individual grain size and the crystallographic texture was then statistically evaluated. Based on the results of slip trace analysis, numerical optimizations of the Critical Resolved Shear Stress < CRSS > of basal < a >, prismatic < a > and pyramidal < c + a > in the four microstructures were then carried out, using transition scale rules and a local behavior model. The results suggest that the mechanical behavior of Ti-6Al-4V is controlled by the activation of slip systems that depend not only on their CRSS but also the initial orientation and size of each individual grain. The low CRSS of prismatic < a > slip systems can lead to early activation of these systems in favorably oriented coarse grains. Therefore, a local plastic deformation can be shown. At high levels of loading, increasing the grain size can minimize the crystallographic texture effects by deforming the unfavorably oriented coarse grains. Moreover, based on the results of the numerical optimization, it can be also suggested that the CRSS can decrease with the increase in grain size according to the local Hall Petch relationship.

In this work, the hot deformation characteristics of a near-α Ti-Al-2SnZr-2Mo alloy (Ti6242 alloy) with a Fine-Grained (FG) microstructure (dα = 2.86 μm) were investigated at two levels of temperature, T = 730 ∘C and T = 840 ∘C. The initial microstructure consists of equiaxed nodules of the α phase as well as some α lamellae sparsely distributed and separated by thin layers of the BCC β phase. For both temperatures, three strain rates (10−4,10−3,10−2s−1) were analysed during loading. Moreover, the microstructural evolution (α size and morphology) was also evaluated by conducting interrupted tensile tests. The different tensile testing conditions greatly influence the stress-strain response of the material as well as the microstructure evolution. Indeed, various phenomena can take place such as elongation of the grain structure, globularization, dynamic recrystallization and grain growth of the equiaxed areas depending on the temperature, the strain rate and the strain level. The FG Ti6242 alloy exhibits interesting superplastic ductility at T = 840 ∘C. At this temperature either a very gradual flow softening (at higher strain rate) or flow hardening (at lower strain rate) can be observed and are related respectively to one or more of the following mechanisms: lamellae globularization, DRX and grain growth. At the intermediate strain rate, both mechanisms, strain hardening and softening, coexist. At T = 730 ∘C, the onset of the α lamellae globularization was only promoted at low strain rate. A mechanical behavior model was developed in the temperature range of 730–840 ∘C, which was able to take into account all the observed phenomena: viscosity, softened behavior and strain hardening. Constitutive equations were calibrated from the stress-strain responses and microstructural observations, and the computed results were in good agreement with the experiments.

Due to high solubility of oxygen and nitrogen in titanium alloys, the influence of the diffusion zone on the macroscopic tensile properties of pre-oxidized annealed Ti-6Al-4V tensile specimens was examined at room temperature. Thin microtensile specimens were prepared with different thicknesses ranging from 100 µm to 500 µm and then exposed at 750°C for durations between 5 and 200h. A dedicated gripping technique was developed in the present study to investigate the brittleness of such pre-oxidized and ultrathin specimens at room temperature. Tensile testing was paired with digital image correlation techniques to assess both macroscopic deformation and full-field strain maps. High temperature pre-oxidation treatments significantly decreased the ductility of the specimen and the tensile strength of the materials (yield strength and ultimate tensile strength). Fractographic examinations revealed typical brittle fracture features in the oxygen/nitrogen-affected diffusion zone in the periphery of the cross-section while the fracture remained ductile in the core of the specimen for most of the specimens. Some specimens fully failed in a brittle manner for “(pre-ox. duration)1/2/thickness” configurations with ratio equal or higher than 0.45 h1/2.µm-1.