Plastic deformation throughout strain-induced phase transformation in additively manufactured maraging steels

A comprehensive study was intended to show the microstructural features of additively manufactured (AM) 18Ni-300 maraging steel. Uniaxial tensile tests were conducted on specimens built using laser powder bed fusion (LPBF) technique for two different powder layer thicknesses. The specimens were built to have the lowest possible porosity, and tensile tests showed two stages of strain hardening. In stage I, the dislocation density increased, leading to a positive strain hardening rate. A negative strain hardening rate due to the necking effect was then followed in stage 11. X-ray diffraction (XRD) analyses revealed a phase transformation through the deformation. Various analyses via electron backscattered diffraction (EBSD) technique was then conducted with large scans over three different zones representing undeformed, deformed, and severely deformed close to the fracture area. The pole figures and orientation distribution functions (ODF) revealed a texture evolving through the deformation process in agreement with the kernel average misorientation (KAM) and grain boundary maps. Transmission electron microscopy (TEM) was used to detect the inclusions and segregated alloying elements adjacent to the fractured surfaces. Results indicated that the deformation led to diminishing the austenite (gamma) phase, while the transformed austenite sourced the high dislocation density area at cell boundaries. Crown Copyright (C) 2020 Published by Elsevier Ltd

Grain boundary character distribution derived from three-dimensional microstructure reconstruction

Manual serial sectioning which includes consecutive steps of sample preparation and Electron Back Scattering Diffraction (EBSD) measurement was employed to extract the twodimensional (2D) sections of a pure nickel sample and to reconstruct the three-dimensional (3D) microstructure. A general alignment algorithm based on the minimization of misorientation between two adjacent sections has been developed to accurately align the sections. By employing this alignment algorithm, any in-plane (translational) and rotational misalignment as well as the planparallelity can be corrected. Surface triangulation technique was used to reconstruct the grain boundary surfaces. The Grain Boundary Character Distribution (GBCD) was derived from reconstructed grain boundaries. The results show that a smoother grain boundary plane can be obtained after precise translational and rotational alignment and correction of planparallelity. The relative grain boundary energy was computed as a function of the five grain boundary parameters based on equilibrium at triple lines. The results show that the grain boundary planes carrying a ?3 type misorientation are dominantly parallel to the {111} crystal plane, which indicates the presence of coherent twin boundaries. It was observed that coherent ?3 type boundaries exhibit the minimum relative grain boundary energy, which is approximately 57% smaller than the average of all ?3 boundaries, including also incoherent twin boundaries.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

Quantitative correlation between slip patterning and microstructure during tensile elongation in 6xxx series aluminum alloy

To the purpose of evaluating the effect of deformation on the microstructure, aluminum structures were analyzed on tensile strained samples extended to 25% elongation. In the substructure of these deformed samples linear slip patterns were observed, generally confined to the bulk of the grain. In order to study the crystallographic aspect of these slip patterns, two methods were applied based on orientation contrast microscopy (EBSD). The first method is the statistical analysis of stereological nature, which allows us to determine the incidence of certain crystallographic planes with the slip patterns. In other to corroborate the statistical method, also a 3D analysis was carried out on two perpendicular planes of observation (TD and ND sections). The results of both methods were in a very good agreement. It was found that the linear features are predominantly parallel to the {111} crystal planes, although the frequency of {111} planes was not exclusive; also other crystal planes such as {112} and {110} are involved. These observations give a stronger statistical basis for similar observations earlier made by TEM on much smaller fields of observation.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

Indentation-derived creep response of cast and laser powder bed fused AlSi10Mg alloy: Air temperature

Compared with time-consuming conventional uniaxial tensile/compressive creep experiments, depth-sensing indentation testing is considered a reliable, and convenient testing technique to assess the time-dependent plastic deformation of materials in a reasonable time scale. In the present study, we report the ambient (room) temperature indentation creep properties of additively manufactured (i.e., laser-powder bed fused) and cast AlSi10Mg alloy at as-fabricated and different post-fabrication heat treatment states. The indentation creep testing parameters (i.e., dwell time, peak indentation load, and indenter shape) were optimized to adequately represent the creep response (time-displacement) variations for different material conditions. To this end, dual-stage, constant loading rate followed by constant-load holding, pyramidal indentation experiments were performed at a loading rate of 10 mN/s, a peak load of 200 mN, and a dwell time of 400 s. Besides, electron backscattered diffraction was performed to evaluate the manufacturing process (selective laser melted versus cast)/ post-fabrication heat treatment/ texture/ creep properties relationships for the studied AlSi10Mg alloy. Also, the indentation hardness, indentation strain rate sensitivity, indentation creep exponent, and activation volume were analyzed to study and confirm the mechanism of indentation creep. The calculated high values of creep stress exponents (i.e., \u3e10) are attributed to dislocation-reinforcing particle interaction as the controlling mechanism of the creep which agrees with this assumption that AlSi10Mg is indeed an in-situ metal matrix composite with eutectic silicon as the reinforcing particles

Three-dimensional characterization of grain boundaries in pure nickel by serial sectioning via mechanical polishing

Five macroscopic boundary parameters can be extracted from three-dimensional orientation maps. Serial sectioning, which includes consecutive steps of material removal, and electron backscatter diffraction (EBSD) measurement were employed to extract a stack of two-dimensional sections of a pure nickel sample. The EBSD patterns were collected from large millimetre scale areas and mechanical polishing was applied to prepare the sections. The three-dimensional microstructure was then reconstructed from these sections. A new alignment algorithm based on the minimization of misorientation between two adjacent sections has been developed to accurately align the sections. Differently from the conventional alignment methods, the new algorithm corrects not only the translational misalignment but also rotational and plane parallelity misalignments. The aligned three-dimensional microstructure exhibits smooth grain boundary planes and continuous orientation gradients inside the grains as experimental scatter induced by misalignment was largely removed. Grain boundaries were reconstructed from the aligned three-dimensional map, and the distribution of boundaries in the domain of five acroscopic boundary parameters was computed using kernel density estimation. Methods for estimating the reliability of the distributions are demonstrated. This distribution is compared with the distributions obtained previously for other face-centered cubic materials, including a different pure nickel sample.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

On the solidification characteristics, deformation, and functionally graded interfaces in additively manufactured hybrid aluminum alloys

AlSi10Mg powder is deposited on top of a cast AA2618 substrate via laser powder bed fusion technique to form hybrid aluminum parts for additive repair purposes. The characterization of the additively manufactured (AM) hybrid aluminum alloys is conducted through advanced electron microscopy techniques across the interface, mechanical testing, validation of elasticity models, and crystallographic orientations analysis. In addition, the fracture behavior of the hybrid samples is investigated through electron microscopy analysis and the results are compared with those of AlSi10Mg and cast AA2618. The microstructural and physical properties of the hybrid bi-material are also discussed based on the solidification and diffusion phenomena. Finally, the dependency of elastic and shear moduli on the distance from the interface and solute concentration are separately evaluated via elastic-field mathematical equations. It is observed that an integrated bond is formed at the interface of the two dissimilar alloys showing suitable mechanical properties and shear strength, as well as a modified microstructure compared to the substrate material. The outcome of this work can be employed as a first step to use hybrid aluminum alloys in the blow molding industry and for repair and maintenance purposes