This study aims analyzing the grain-scale mechanical behavior of a High Entropy Shape Memory Alloy (HESMA) using synchrotron-based diffraction techniques. The alloy Ti_30Hf_19Zr_25Nb_13Ta_13 was subjected to in-situ tensile tests at ambient temperature and grain-resolved data were acquired using Diffraction Contrast Tomography (DCT) and Three-Dimensional X-ray Diffraction (3DXRD), enabling simultaneous mapping of grain morphology, orientation, and elastic strain tensors. Both techniques showed consistent strain measurements, with 3DXRD exhibiting lower dispersions in the inelastic regime. Indexed grain number decreased with increasing load, indicating high lattice strains and/or a phase transformation. Partial recovery during unloading suggested reversible martensitic transformation. An inverse method was used to extract intrinsic single-crystal elastic constants via a least-squares fitting, showing correlation with valence electron concentration, consistent with β -Ti alloy trends. Combined with DCT and 3DXRD, a digital twin of the microstructure with local grain properties is therefore enabled.

The superelasticity in shape memory alloys is a useful property for many applications. It is due to a martensitic transformation induced by applied stress and can reach up to about 10

Laser Powder Bed Fusion (LPBF) process, is becoming more and more widespread in industry. The possibilities of microstructural control offered by this process are an opportunity to study the contribution of the different length scales of microstructure to the fatigue behavior. This paper is devoted to the understanding of this fatigue behavior resulting from the interactions between the process induced defects and the different polycrystal length scales. Two distinct defect - microstructure competition regimes have been identified. The first concerns microstructures containing large Lack of Fusion (LoF) defects. These LoFs drastically reduce the fatigue life, while microstructure has no influence on the fatigue strength. The second regime concerns microstructures containing small defects. A limited effect of the polycrystalline microstructure was revealed. Furthermore, this paper demonstrates that the ratio between damage initiation defect size and grain size, used in literature to describe the defect and microstructure sensitivity of fatigue strength, is not applicable over a wide range of defects and microstructures, such as obtained by additive manufacturing processes. Finally, the comparison between the fatigue behavior of different microstructure and defect features shows that producing a finer microstructure improves fatigue strength despite the presence of a significant defect population.

This study investigates the decomposition of retained austenite (RA) in tool steels for plastic molding in correlation with the alloy chemical composition and the tempering parameters. Two grades differing in their silicon content with initial mixed bainitic/martensitic microstructures were investigated using in situ synchrotron high-energy X-ray diffraction (HEXRD) during tempering in the 550 °C to 600 °C temperature range for one-hour holding time. Results indicated carbide formation during heating or isothermal holding; however, retained austenite remained untransformed up to the end of the tempering holding time in all investigated conditions for both grades. In situ HEXRD provides direct evidence of the transformation of retained austenite into fresh martensite on cooling from the tempering stage. This behavior is correlated to the evolution of carbon enrichment of retained austenite and the effect of silicon is discussed.

This work explores the advantages and disadvantages of a methodology using high-energy X-ray diffraction to determine residual stresses in multilayer structures produced by atmospheric plasma spraying. These structures comprise a titanium alloy substrate (Ti64), a bonding layer (Ni-Al), and an abrasive coating (Al2O3). This study focuses on analyzing the residual stress gradients within these layers. The presented method is used to determine stresses across the entire thickness of multilayer structures. Experiments were carried out using a high-energy rectangular beam, operating in transmission mode, on the cross-section of the sample. The results indicate variable stresses throughout the depth of the sample, particularly near the layer interfaces. The semi-automatic methodology presented here enables us to follow stress evolution within the different layers, providing indications of the load transfer between them and at their interfaces. The sin2ψ method was used to analyze the diffraction data and to determine the stresses in each phase along the sample depth. However, interpreting results near the interfaces is complex due to the geometric and chemical effects. We present a discussion of the main advantages and disadvantages of the methodology for this kind of industrial sample.

The precipitation hardening mechanisms and their dependence on precipitate size (rp) r p ) and precipitate/matrix lattice misfit (delta) delta ) in gamma'/gamma / gamma systems were studied in nickel-base superalloy using micro-indentations experiments and theoretical models. Metallurgical states were characterized by TEM, SEM-EBSD, XRD and APT. For the precipitate size range studied (radius rp p smaller than 110 nm), gamma' precipitates are ordered spherical Ni3(Al,Ti) 3 (Al,Ti) phase, the volume fraction remains relatively constant and equal to 20 % and lattice misfit delta is lower than 0.45 %. Prevalence of order strengthening by anti-phase boundary formation and Orowan bypassing was observed in various rp p range, whereas chemistry and coherency strengthening were found insignificant in the nickel-base superalloy studied (Waspaloy (R)). (R)). Shearing by weakly coupled dislocations was dominant for fine precipitates, i.e. rp p lower than 18 nm, shearing by strongly-coupled dislocations for an intermediate rp p range, i.e. from 18 nm to 98 nm and Orowan bypassing for coarse precipitates with a rp p larger than 98 nm.

Based on new experimental observations, a comprehensive analysis of factors influencing microstructure refinement in laser powder bed fusion additive manufacturing of 316L stainless steel components is presented. In contrast to existing hypotheses, the study reveals that neither the solidification mode nor the mere presence of nano-oxides in powders suffices to fully elucidate the observed grain refinement. Instead, this research highlights the intricate interplay between a strongly ferrite forming composition and the simultaneous presence of Mn-Si nano-oxides as essential contributors to the microstructure refinement process. The study explores the role of heterogeneous nucleation mechanism involving nano-oxides and provides fresh insights into the solidification mechanisms in laser powder bed fusion process, enhancing our understanding of microstructure control in laser powder bed fusion processes and offering novel perspectives for advanced materials engineering.

Additive manufacturing (AM) offers distinct advantages in terms of complexity, reduced material waste, and shorter production times. However, the microstructures of AM alloys differ significantly from conventionally manufactured ones, and their impact on grain behavior remains uncertain. This study employs neutron diffraction, coupled with Crystal Plasticity Finite Element (CPFE) modeling, to investigate microstructural effects on cyclic behavior in 316L LPBF alloys. Neutron diffraction provides high-resolution strain and stress data at the polycrystalline level during loading, offering valuable insights into grain behavior. The CPFE model is initially validated at the grain scale using neutron diffraction, demonstrating its ability to predict local mechanical behavior in additively manufactured materials. This methodology holds promise for broader applications in various AM alloys, particularly when macroscopic identification of mechanical behavior is insufficient. Furthermore, the comparison between neutron diffraction and CPFE predictions allows to identify the influence of dendrites and dislocation substructures on the mechanical behavior at the grain scale. Notably, the micromechanical anisotropy resulting from dislocation organization along dendritic structures with specific orientations is highlighted. This study shows that despite the observed anisotropy, experimental measurement of intragranular strain distribution are correctly captured with a quantification of the deviation observed on some specific orientations due to the above cited microstructure sub-structures.

Abradable systems are used in the aeronautical industry to improve the efficiency of gas turbine. Those materials are exposed in service to temperature up to 450 °C. The increase in gas turbine efficiency requires to increase the operating temperature and therefore the service temperature of abradable coating. The present study focuses on the isothermal and cyclic thermal aging of the Al–Si abradable coating system in a laboratory air at high temperature up to 500 °C. The investigation encompasses the microstructural evolution, phase transformation, and the formation of cracks, along with their interrelated effects. During aging, silicon particles precipitate in the abradable top coat. In addition, coarsening of those particles is observed and the coarsening kinetics appears to be faster in cyclic thermal aging conditions compared to isothermal aging. During cyclic and isothermal aging, brittle aluminides develope at the abradable top–coat/bond–coat interface, due to the interdiffusion of Al and Ni species. During cyclic aging, thermal cycles create thermomechanical stress at the top‐coat/bont‐coat interface due to coefficient of thermal expansion mismatch between the Al‐Si deposit and intermetallic phases. The stress generated results in the formation of cracks and porosities at the top–coat/bond–coat interface resulting in a dramatic failure of the system.

The reliability and failure mechanisms of silicide-based thermoelectric modules (n-type Mg2(Si,Sn)/p-type HMS) were investigated thanks to two types of thermal tests with either a fixed or a cycling thermal gradient, under different atmospheres. The hot interfaces of the thermoelectric modules were analyzed by scanning electron microscopy and X-ray diffraction after the reliability tests. The current thermoelectric modules do not exhibit any failure mechanism under ambient air for a hot side temperature of 250 °C for tests conducted either during 500 h at a fixed temperature gradient or after 1000 thermal cycles. However, when the temperature was increased to 350 °C, pesting phenomena were detected at the hot side of the n-type Mg2(Si,Sn) legs caused by the decomposition/oxidation of the material. These phenomena are strongly slowed down for thermoelectric modules tested under a primary vacuum, highlighting the predominant role of oxygen in the degradation mechanism. Interdiffusion phenomena are the most pronounced at the interface of the hot side of the n-type thermoelectric materials. The formation of a MgO layer, which is an electrical and thermal insulator, has decreased the thermoelectric modules' performances. For thermal cycling tests, cracks are observed on the hot side of the n-type legs. The presence of these cracks drastically increases the thermal and electrical resistances, leading to an overheating of the system and limiting its efficiency and failure by breaking electrical continuity. The interfaces at the hot side temperature of the p-type HMS legs remained intact whatever the test conditions were, indicating a high chemical stability and a good mechanical strength.

Duralumin-type alloys have been employed for structural and non-structural parts of aircraft since the early 1920s up to now. In the European project “PROtection and Conservation of Heritage AirCRAFT” (PROCRAFT), up to 34 aircraft wrecks from World War II from different nations (United States of America, United Kingdom, France, Germany, Italy) were identified. On most of them, the collection of numerous fragments was possible, thus constituting a large sampling group. A metallographic analysis was carried out, and elemental composition, microstructure and hardness were measured. This article focuses firstly on the elemental composition of this group of historical Al alloys. From these data, some fine characteristics specific to the manufacturing countries will be investigated through principal component analysis. Then, on a restricted group constituted of German alloys, we will discuss how some manufacturing parameters, such as micro-alloying and thermo-mechanical treatments, influence the mechanical properties of the assessed aircraft components. Other influencing parameters, linked to the archaeological nature of the parts, are considered and addressed based on a specific study of an alloy collected on a crashed JU88 aircraft originating from the German company Junkers.

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The main subject of this study is to investigate the correlations between the evolution of mechanical behavior and the multiscale microstructure of 316 L stainless steel obtained by laser powder bed fusion process (LPBF) after various post-manufacturing heat treatments across a wide temperature range. The microstructure of 316 L LPBF parts exhibits a hierarchical microstructure based on unique grain structures, chemical segregations, dislocation arrangements at the microscopic scale and fine nano-oxides at the nanoscopic scale. These microstructural elements play a crucial role in determining the material's mechanical and corrosion properties. To understand how different microstructural features contribute to the material's behavior, the researchers conduct post-manufacturing heat treatments to isolate and study these components. The results show that dislocation and/or micro-segregation networks significantly influence the high tensile properties of 316 L LPBF steel in its asbuilt state and after heat treatments above 900 circle C. Despite their disappearance during heat treatments, the material maintains high tensile strength due to an increase in strain-hardening capabilities. The study also examines the impact of nano-oxides and the sigma phase on the material's properties. The contribution of nano-oxides to yield strength diminishes with increasing temperature. Interestingly, the sigma phase does not necessarily lead to a detrimental effect on failure elongation for 316 L LPBF steel. Overall, this research provides insights into the relationship between microstructure and mechanical properties for additively manufactured stainless steel. By understanding these relationships, it becomes possible to tailor the microstructure to achieve desired mechanical properties for specific applications and extend the use of 316 L LPBF to higher temperatures compared to conventional methods.

The particular roles of grain morphology and defects, controlled using laser-scan strategies, on the mechanical properties and the fatigue behavior of 316L stainless steel are investigated. Microstructural characterization and X-ray tomography analysis was performed to understand the genesis of polycrystalline microstructure and defects. Tensile and fatigue tests were performed to analyze the effect of defect population and microstructural properties on plasticity and damage mechanisms during monotonic and cyclic loading. The effect of the grain-size and shape and type of defect was carefully investigated to evaluate the mechanisms driving the mechanical behavior under quasi-static and fatigue loading. It is shown that the laser-scan strategy determines the anisotropy in the plane perpendicular to the building direction. Moreover, contrary to the existing literature, for 316L obtained by AM, the grain size and shape does not affect the mechanical properties, and LoF defects drive the fatigue life, independent of the defect/grain size ratio.

The sequential-coupled thermo-mechanical model and direct cyclic technique are employed to investigate laser welding and low-cycle fatigue residual stress, respectively. The effects of residual stress on fatigue properties and the relaxation behaviour are analyzed. The simulation results highlight the strong dependence of laser welding residual stress on constitutive models and how low-cycle fatigue residual stress is influenced by the initial laser welding residual stress. Furthermore, the simulation indicates that residual stress redistributes and relaxes in the weld and heat-affected zone after low-cycle fatigue. To validate the accuracy of the simulation, the neutron diffraction experiment is carried out, and the experimental data are consistent with the simulation results.

Laser-welded structures are often subjected to dynamic service loads ranging from cyclic fluctuations to completely random ones. The laser-welded lap joints suffer from defects resulting in notch effects, surface cracks, residual strains and stresses. The fatigue strength of the laser-welded lap joints is reduced significantly because of the presence of these defects, and the size of the welded joints is small. Therefore, the mechanical strength of laser-welded structures must be defined in terms of the fatigue strength and residual stress of the obtained joints or assemblies. To analyze the above-discussed effects, this paper proposed two approaches: numerical and experimental methods. The originality of the work is to weld the rolled sheet in three directions (0°, 45°, 90°). The residual stresses before and after low cyclic tensile tests of assemblies obtained from overlapped thin DP600 steel sheets were calculated by ABAQUS. The obtained results were compared to the experimental data by neutron diffraction. The presented results in terms of residual stresses curves, spatial distributions of residual stresses obtained at the end of laser welding and low-cycle fatigue. It showed the relaxation effect of residual stresses and the direction effect of welding. These results have been explained by several factors.

The superelastic behavior of a Cu-Al-Be alloy was studied in situ during tensile tests combining two high-energy synchrotron techniques. The initial microstructure was reconstructed using diffraction contrast tomography; the elastic strain and stress tensors of each individual grain were then determined using a 3D X-ray diffraction microscopy technique. The alloy was heat-treated until coarse grains formed, and the probed volume fraction was limited to -200 grains. The mean grain size, as estimated from both techniques, agreed well with that determined by optical microscopy, i.e., approximately 130 mu m. During the early stage of the martensitic transformation (MT), 187 grains with strong stress heterogeneities were detected in the elastic domain; in particular, the stress values along the tensile direction varied by a factor of three between different grains. The reconstructed 3D microstructure served as input data for finite element modeling, wherein a micromechanical approach factoring in the martensitic transformation was used to simulate the in situ tensile tests. The coupled numerical and experimental tensors of the 187 grains confirmed strong stress heterogeneities between them. The influence of the position and crystallographic orientation of the neighboring grains were also examined.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Recent developments in additive manufacturing technologies allows generating different type of microstructure by modifying the additive manufacturing parameters. Indeed, with the laser powder bed fusion process, the laser power, energy density or laser scan strategies are such of parameters which affect size, morphology and orientations of grains. This allows to build microstructures which were no accessible with conventional processes. Furthermore, the control of laser scan strategies allows to significantly reducing the size and density of defects inherent to the process (gas pores, lack of fusion). These advances make it possible to study the role of microstructure and defect on fatigue crack initiation behaviour for additive manufactured materials. This paper aims to take this opportunity for studying the 316L stainless steel built with the laser powder bed fusion process.

The goal of this study is to investigate the mechanical and elastic characteristics of the Mn15Si26 compound via experimental nanoindentation measurements and ab-initio calculations. The mechanical properties such as Young's modulus (E) and nanohardness are important inputs for improving the design and mechanical reliability of thermoelectric modules. The high-energy X-ray diffraction pattern of Mn15Si26 has been indexed with the Miller indices of a tetragonal crystalline structure whose cell parameters are the following: a = b = 5.535(3) angstrom and c = 65.552(4) angstrom. Nanoindentation measurements, with a Berkovich indenter tip have been performed on higher manganese silicide (HMS) compound mainly composed of Mn15Si26 grains. For the first time ever, it has been evidenced that both elastic modulus and nanohardness of the latter varied significantly depending on their crystallographic orientations provided by electron backscatter diffraction. Nanohardness and Young's modulus along the < 001 > orientations are higher than the < 100 > ones. The nanohardness value of Mn15Si26 ranges from 16 GPa to 20 GPa and the Young's modulus measured varies between 234 GPa and 300 GPa. The stiffness tensor (S-ij = (C-ij)(-1) of Mn15Si26 has been deduced from these experimental measurements as well as calculated using Ab-initio calculations. The macroscopic elastic modulus (E, G, B) and Poisson's coefficient have been examined and discussed and their 3D-representation has been plotted. The mechanical anisotropy hereby evidenced as the existence of anisotropy of the thermoelectric properties could be a significant factor for the mechanical reliability of thermoelectric modules which consisted of Mn15Si26 legs with a possible preferred crystallographic orientation induced during their fabrication. (C) 2021 Elsevier B.V. All rights reserved.