Showing papers on "Electron backscatter diffraction published in 2020"

Selective laser melting additive manufacturing of 7xxx series Al-Zn-Mg-Cu alloy: Cracking elimination by co-incorporation of Si and TiB2

Abstract: 7xxx Al alloys such as Al-Zn-Mg-Cu are typical lightweight materials of excellent mechanical performance. Their near-net-shape manufacturing by selective laser melting (SLM) additive manufacturing, however, remains challenging due to hot-cracking prone nature of these alloys, when subjected to rapid solidification during the SLM process. In this study, we propose that co-incorporation of submicron Si and TiB2 to an Al-Zn-Mg-Cu alloy is capable to solve the long-standing problem by reducing solidification shrinkage and, simultaneously, enhancing its fracture toughness. Results show that solidification cracks indeed have been eliminated by the co-incorporation, along with much-refined microstructure. The resultant mechanical properties are high in ultimate tensile (556 ± 12 MPa) and yield strengths (455 ± 4.3 MPa). For disclosing the underlying mechanism, analytical means including high-resolution computer tomography, transmission electron microscopy and electron backscatter diffraction, as well as finite element simulation have been employed. It is aspired that the current approach can enable SLM to process critical engineering materials such as the hard-to-weld Al-Zn-Mg-Cu alloys.

Crystal symmetry determination in electron diffraction using machine learning

Abstract: Electron backscatter diffraction (EBSD) is one of the primary tools for crystal structure determination. However, this method requires human input to select potential phases for Hough-based or dictionary pattern matching and is not well suited for phase identification. Automated phase identification is the first step in making EBSD into a high-throughput technique. We used a machine learning–based approach and developed a general methodology for rapid and autonomous identification of the crystal symmetry from EBSD patterns. We evaluated our algorithm with diffraction patterns from materials outside the training set. The neural network assigned importance to the same symmetry features that a crystallographer would use for structure identification.

Microstructure, Deformation, and Property of Wrought Magnesium Alloys

Abstract: Pure magnesium (Mg) develops a strong basal texture after conventional processing of hot rolling or extrusion. Consequently, it exhibits anisotropic mechanical properties and is difficult to form at room temperature. Adding appropriate alloying elements can weaken the basal texture or even change it, but the improvement in formability and mechanical properties is still far from expectations. Over the past 20 years, considerable efforts have been made and significant progress has been made on wrought Mg alloys at the fundamental and technological levels. At the fundamental level, textures formed in sheets and extrusions of different alloy compositions and produced under different strain paths or thermomechanical processing conditions are relatively well established, with the assistance of the advanced characterization technique of electron backscatter diffraction. At the technological level, room temperature formability of sheet has been significantly improved, and tension–compression yield asymmetry of extrusion is also remarkably reduced or eliminated. This paper starts with an overview of dislocations, stacking faults and twins, and deformation of single crystals of pure Mg along different orientations and under different loading conditions, followed by a review of microstructure (texture and grain size) and deformation of polycrystalline pure Mg with different textures, grain sizes, and loading conditions. With this information as a base, texture, grain size, and deformation of polycrystalline Mg alloy sheets and extrusions produced under different processing conditions are systematically examined and compared. Remaining and emerging scientific and technology issues are then highlighted and discussed in the context of texture and grain size. The need for better-resolution diffraction and spectroscopy techniques is also discussed in the relationship between texture change and grain boundary solute segregation.

Laser powder bed fusion of titanium-tantalum alloys: Compositions and designs for biomedical applications.

Abstract: In this study, laser powder bed fusion (L-PBF), also known as selective laser melting (SLM), was used to fabricate samples of titanium-tantalum (TiTa) alloys with 0, 10, 30 and 50 wt% of tantalum using in-situ alloying. As-fabricated samples comprised of randomly-dispersed pure tantalum particles in a titanium-tantalum matrix. Porosity and unmelted tantalum particles of the samples were revealed using an optical microscope (OM). The microstructure of the alloys were determined by combination of field emission scanning electron microscopy (FE-SEM), electron back scatter diffraction (EBSD) and X-ray diffraction (XRD). The mechanical properties of the alloys were investigated with tensile and Vickers hardness tests. To ascertain the suitability of these alloys as biomaterials, Ti50Ta scaffolds with 60% porosity were characterized biologically. This study further shows that porous TiTa scaffolds fabricated using L-PBF are biocompatible with comparable biological results and manufacturability as Ti6Al4V and commercially pure titanium, based on the results obtained from cell culture with human osteosarcoma cell line SAOS-2.

Stacking Fault Energy Analyses of Additively Manufactured Stainless Steel 316L and CrCoNi Medium Entropy Alloy Using In Situ Neutron Diffraction.

Abstract: Stacking fault energies (SFE) were determined in additively manufactured (AM) stainless steel (SS 316 L) and equiatomic CrCoNi medium-entropy alloys. AM specimens were fabricated via directed energy deposition and tensile loaded at room temperature. In situ neutron diffraction was performed to obtain a number of faulting-embedded diffraction peaks simultaneously from a set of (hkl) grains during deformation. The peak profiles diffracted from imperfect crystal structures were analyzed to correlate stacking fault probabilities and mean-square lattice strains to the SFE. The result shows that averaged SFEs are 32.8 mJ/m2 for the AM SS 316 L and 15.1 mJ/m2 for the AM CrCoNi alloys. Meanwhile, during deformation, the SFE varies from 46 to 21 mJ/m2 (AM SS 316 L) and 24 to 11 mJ/m2 (AM CrCoNi) from initial to stabilized stages, respectively. The transient SFEs are attributed to the deformation activity changes from dislocation slip to twinning as straining. The twinning deformation substructure and atomic stacking faults were confirmed by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The significant variance of the SFE suggests the critical twinning stress as 830 ± 25 MPa for the AM SS 316 L and 790 ± 40 MPa for AM CrCoNi, respectively.

Influence of sub-cell structure on the mechanical properties of AlSi10Mg manufactured by laser powder bed fusion

Abstract: AlSi10Mg is one of the most applied alloys for laser powder bed fusion (LPBF) technology, due to its great possibilities for implementing new lightweight concepts such as in automotive industries. For the component design it is necessary to know about the mechanical properties and the mechanical behaviour. The many published strength properties of LPBF processed AlSi10Mg show significant differences up to approximately 225 MPa in ultimate tensile strength (UTS) and 195 MPa in yield strength (YS). To understand these varying properties, a ring trial was carried out manufacturing specimens on 6 LPBF machines with different parameters and build-up strategies. They were studied in the as-built (AB) condition and after heat treatment at 300 °C for 30 min, respectively. For examining the mechanical properties, tensile tests and hardness measurements were carried out. The microstructure was characterized by optical light microscopy (OM), field emission scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD). The identified differences in strength properties were discussed based on the 4 strengthening mechanism known for metallic materials and at the background of material defects. It was found that the size of the typical sub-cell structure of LPBF AlSi10Mg affected substantially the mechanical properties in the AB condition, in which with decreasing sub-cell size strength increased. If heat treatment was applied, the strength properties decreased and did not differ anymore. Since annealing led to coarsened sub-cells, whereas the grains itself did not change in size, the influence of sub-cell structure on strength was further confirmed. In addition, acicular precipitates in the AB condition were observed at specimens from one LPBF machine showing the lowest tensile elongation.

Evolution of microstructures, dislocation density and arrangement during deformation of low carbon lath martensitic steels

Abstract: In this paper, the role of solute carbon on the strengthening and work hardening behavior of lath martensite was studied by analyzing the microstructures and dislocation density in the undeformed and deformed conditions. An increase in carbon content from 0.18% to 0.30% decreases the martensite start (Ms) temperature, leading to refinement of both the block and lath widths. Although reduction of the “effective grain size” is observed via Electron Backscatter Diffraction (EBSD) and Electron Channeling Contrast Imaging (ECCI) techniques, this effect is considered secondary in increasing the strength of lath martensite with increased carbon content. The higher strength is attributed mainly to the phase transformation-induced dislocation density in the high-carbon martensite. Comparing this total dislocation density calculated using a Convolutional Multiple Whole Profile (CMWP) fitting procedure with the estimated geometrically necessary dislocations (GND) from the misorientation distribution of EBSD analysis, it appears that a high fraction of the dislocations in lath martensitic steel is GND. Furthermore, the analysis of the samples strained to a different level suggests that the dislocation density shows minimal change during deformation, whereas the dislocation arrangement rapidly decreases at the beginning of the plastic deformation. Finally, the strain hardening behavior of the lath martensitic steel is quantitatively described by considering lath width, dislocation density, and dislocation arrangement parameters through the α coefficient in Taylor's equation.

Anisotropic mechanical properties and deformation behavior of low-carbon high-strength steel component fabricated by wire and arc additive manufacturing

Abstract: Wire and arc additive manufacturing (WAAM) is an efficient technique for fabricating large and complex components that are applied in the manufacturing industry. In this study, anisotropic mechanical properties of a low-carbon high-strength steel component fabricated by WAAM were investigated via mechanical testing, and the transversal and longitudinal deformation behavior of the component were studied using the digital image correlation (DIC) method. Additionally, the features of microstructure, texture, and fracture mode of the inter-layer area and deposited area were also investigated to reveal the mechanism of anisotropy. The results showed the mechanical properties of longitudinal specimens were inferior to that of the transversal specimens. Several strain concentration zones in the longitudinal specimen were relevant to the inter-layer characteristics observed from the fracture surface and macrostructure, which was confirmed by the strain evolution recorded by DIC. The inter-layer areas were proved to be the weak link in the deposited component by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) analysis results, including various phase composition, phase morphology, misorientation angle, grain size, Schmid factor, and texture. Finally, based on the fractography analysis, anisotropy resulted from inter-layer zones is also confirmed via the comparison of transversal and longitudinal fracture morphology.

Flow behavior and dynamic recrystallization mechanism of A5083 aluminum alloys with different initial microstructures during hot compression

Abstract: Hot compression tests of A5083 aluminum alloys with extruded and homogenized initial microstructures were carried out at varying temperatures (350 to 450 °C) and strain rates (0.1 to 10/s). The effects owing to nonuniform temperature and strain distributions caused by deformation were compensated by inverse analysis to determine the flow curves. The obtained flow curves with a homogenized initial microstructure were lower than those with an extruded initial microstructure. Constitutive descriptions were obtained to study the dynamic kinetics during compression, and comparisons between the predicted results from constitutive equations and the experimental results from associate microstructures verified their accuracy. Electron backscatter diffraction (EBSD) was utilized for observing microstructures. The characteristics of continuous dynamic recrystallization, particle-stimulated dynamic recrystallization and conventional dynamic recrystallization were confirmed. The dynamic recrystallization process with a homogenized initial microstructure is slower than that with an extruded initial microstructure, owing to the different amounts of subgrain structures during the initial deformation stage, which can affect the speed of the continuous dynamic recrystallization process. Microhardness tests were conducted to study the connection between microstructure and mechanical property. The microhardness after deformation with an extruded initial microstructure are higher than those with a homogenized initial microstructure.

Effects of laser additive manufacturing on microstructure and crystallographic texture of austenitic and martensitic stainless steels

Abstract: Powder-fed laser additive manufacturing (LAM) based on directed energy deposition (DED) technology is used to produce S316-L austenitic, and S410-L martensitic stainless steel structures by 3D-printing through a layer-upon-layer fashion. The microstructural features and crystallographic textural components are studied via electron backscattering diffraction (EBSD) analysis, hardness indentation and tensile testing. The results are compared with commercial rolled sheets of austenitic and martensitic stainless steels. A well-developed 200 > direction solidification texture (with a J-index of ∼11.5) is observed for the austenitic structure produced by the LAM process, compared to a J-index of ∼2.0 for the commercial austenitic rolled sheet. Such a texture in the LAM process is caused by equiaxed grain formation in the middle of each layer followed by columnar growth during layer-upon-layer deposition. A quite strong preferred orientation (J-index of 17.5) is noticed for martensitic steel developed by LAM. Large laths of martensite exhibit a dominant textural component of { 011 } 111 > in the α-phase, which is mainly controlled by transformation during layer-by-layer deposition. On the other hand, the martensitic commercial sheet consists of equiaxed grains without any preferred orientation or completely random orientations. In the case of the austenitic steel, mechanical properties such as tensile strength, hardness and ductility were severely deteriorated during the LAM deposition. A ductility loss of about 50% is recorded compared to the commercially rolled sheets that is attributed to the cast/solidified structure. However, LAM manufacturing of martensitic stainless steel structures leads to a considerably enhanced mechanical strength (more than double) at the expense of reduced ductility, because of martensitic phase transformations under higher cooling rates.

Influence of Grain Refinement and Microstructure on Fatigue Behavior for Solid-State Additively Manufactured Al-Zn-Mg-Cu Alloy

Abstract: A solid-state severe deformation-based additive manufacturing process, additive friction stir-deposition (AFS-D), offers an innovative solution to achieve wrought-like mechanical performance from metals that are susceptible to solidification cracking. In this work, the microstructural evolution and fatigue mechanisms of an Al-Zn-Mg-Cu alloy (AA7075) manufactured via a rapid solid-state deposition process are quantified for the first time. The AFS-D process deposits feedstock via frictional heat and severe plastic deformation, while avoiding the deleterious effects of solid–liquid phase transformation. A fully dense AA7075 deposit was manufactured without the need for additional alloying elements. The microstructural characterization of the as-deposited AA7075 employed optical, scanning electron microscope, and electron backscatter diffraction. The as-deposited AA7075 exhibited a refinement of the constituent particles and grains within the microstructure. Additionally, to quantify the fatigue behavior of the as-deposited AA7075, strain-life experiments were conducted, where a reduction in fatigue resistance was observed compared to the heat-treated feedstock, due to coarsening of strengthening precipitates η′ and η (MgZn2). Post-mortem analysis of the as-deposited AFS-D AA7075 revealed a change in the fatigue nucleation and growth mechanisms compared to the control feedstock. Lastly, a microstructure-sensitive fatigue life model was utilized to elucidate process-structure–property fatigue mechanism relations of the as-deposited and feedstock AA7075 materials.