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[Display omitted] • An excellent combination of strength and ductility was achieved in LPBF TA15 alloy by tailoring the microstructure, which is better than that of forged counterpart. • In situ tensile tests revealed that activation of pyramidal slip effectively alleviated the cracking tendency caused by strain localization, which contributes to the excellent mechanical property at high temperatures. The microstructure of Ti-6.5Al-2Zr-1Mo-1 V (TA15) alloy fabricated by laser powder bed fusion (LPBF) is characterized by the ultrafine α′ martensite with a high density of dislocations and twins, which is distinct to that of conventionally forged TA15 alloy. The α′ martensite in the as-built alloy transforms to stable α phase when annealing below the β transus, and meanwhile the dislocation density decreases and the α lamellae coarsen with increasing of annealing temperature. An excellent synergy of strength and ductility was achieved in the sample after designated heat treatments, which is better than that of the forged counterpart at both room temperature and 500 °C. In-situ tensile test at 500 °C demonstrated that the plasticity of as-built TA15 alloy was dominated by the formation of microscale shear bands (MSBs) between the α′ lamellae, which results in pronounced strain localization and limited tensile ductility. In the sample with coarsened α lamellae, it was found that the activation of pyramidal < c + a > slip system at 500 °C effectively alleviated the strain localization and cracking tendency during mechanical loading, which is essential to achieve a desired balance of strength and ductility. These findings are helpful to promote the practical applications of LPBF near-α titanium alloys. [ABSTRACT FROM AUTHOR]

Ni-based superalloys fabricated by laser powder bed fusion (LPBF) have shown great potential in various industrial sectors with demanding environments, but limited understanding of the microstructure-property relationship of the LPBF fabricated alloys hinders their practical applications. In this study, Ni-based superalloy Hastelloy X (HX) was manufactured by LPBF, and the resulting microstructure was characterized using multiscale correlative microscopy and its relationships with mechanical behaviors were investigated. A typical cellular structure formed during LPBF, and cell walls enriched with Cr and Mo and contained a high density of dislocations. Most of cell boundaries had very low misorientation angles, and misorientation accumulation across a bundle of cells was observed. Correlative microscopic characterizations demonstrated that epitaxial cell growth across molten pool boundaries between adjacent layers and side-branching between neighboring tracks resulted in the formation of a strong Goss texture. Mechanical testing showed that the fabricated HX alloy exhibited a lower strength and a higher tensile ductility along the building direction, compared to those along transverse directions. The governing mechanism for the strength anisotropy was the anisotropic columnar grain structure, and the better tensile ductility was caused by the higher tendency in forming mechanical twins of Goss-oriented grains and lower vulnerability in microcrack propagation during tensioning. More importantly, a very pronounced anisotropy in plasticity was found, where macroscopic shape change from circular to highly elliptical was observed in the sample after tensioning along the building direction. A model based on the orientation-dependent dislocation activity that resulted in a planar strain during tensioning was proposed to interpret the anisotropic plastic deformation. [ABSTRACT FROM AUTHOR]

A metastable titanium alloy Ti–3Al–6Fe–6V–2Zr (wt. %, Ti3662) was fabricated by laser powder bed fusion (LPBF), and high resolution digital image correlation (HRDIC) coupled with multiscale characterizations were employed, in order to uncover the effect of microstructural characteristics on the mechanical behavior of LPBF Ti3662 alloy. Results show that the LPBF Ti3662 alloy exhibits near equiaxial microstructure with fine β grain size and weak texture, due to the strong growth restriction effect from Fe addition. Mechanical tests and HRDIC results demonstrate that microstructural heterogeneity with a chessboard-like pattern has a pronounced influence on the microstrain evolution and mechanical properties of LPBF Ti3662. Strain localization is observed at the interfacial zones between the fine and coarse grains, and the irregular prior β grain morphology with rugged grain boundaries is significant in mitigating the adverse effect of grain boundary α phase. This is attributed to the successful suppression of strain localization and cracking at precipitate-free zones near the irregular grain boundaries. This study demonstrates that the irregular grain morphology is beneficial for the mechanical properties of metastable titanium alloy, whereas the heterostructure formed during LPBF causes strain localization. [ABSTRACT FROM AUTHOR]

Maintaining sufficient uniformity and stability of a ceramic slurry throughout the printing cycle is key to ensuring the performance of ceramic parts in ceramic-slurry stereolithography. In this study, a stable three-dimensional network structure was introduced into a slurry to achieve slurry uniformity and stability based on rheology theory. The effects of the particle size, solid loading, dispersant content, and gelling agent content on the stability of the three-dimensional network structure in the slurry were analyzed. Relatively stable three-dimensional network structures were constructed to achieve 4-week stability of micron- and submicron-size particle slurries by adjusting the contents of the dispersant and gelling agent. Stabilization ideas for different particle size ranges are provided. In addition, an empirical stability model was obtained based on the experimental results. When the predicted empirical stability factor of A was less than 0.035, the slurry exhibited good stability. [ABSTRACT FROM AUTHOR]

Cellular structure with a high density of dislocations and elemental segregation at the cell boundaries is the characteristic microstructure of the laser powder bed fusion (LPBF) CoCrMo alloy. This structure is known to be merit to the alloy's strength, but this structure and elemental heterogeneity are likely to be detrimental to the ductility. In this study, the effect of solution treatment and aging on the microstructure, phase fraction, and mechanical behaviors of a CoCrMo alloy fabricated by laser powder bed fusion (LPBF) was investigated, in order to achieve a microstructure state with excellent synergy in strength and ductility. The results showed that solution treatment at 1150 °C for 1 h was sufficient to remove the cellular structure and eliminated the elemental segregation, but it resulted in a dramatic reduction in yield strength. Direct aging at 750 °C for 2 h led to a boost in the yield strength due to the precipitation of Cr-rich carbides, but the alloy ductility was significantly reduced and the elemental segregation was maintained. An excellent combination of strength and ductility was achieved via a two-step solution and aging treatment, by which both the strength and ductility were improved compared to that of the as-built CoCrMo alloy. Multiscale characterizations demonstrated that the synergetic strength and ductility were mainly attributed to the strengthening effect of the ε phase formed during aging and stress-induced γ to ε phase transformation under mechanical loading. In addition, the enhanced ductility was also evidently benefited from the eliminated cellular structure and elemental segregation, which reduces the tendency in crack nucleation. [ABSTRACT FROM AUTHOR]

[Display omitted] • This paper present an attention-based method that uses contrastive learning. • The loss function is a combination of CE loss and contrastive learning loss. • This paper shown to exhibit state-of-the-art performances on well-known datasets. Image captioning is a significant step toward achieving automatic interactions between humans and computers, in which a textual sequence of the content of an image is generated. Recently, the transformer-based encoder–decoder paradigm has made great achievements in image captioning. This method is usually trained with a cross-entropy loss function. However, for various captions of images with the same meaning, the computed losses may be different. The result is that the descriptions of images tend to be consistent, which limits the diversity of image captioning. In this paper, we present an attention-reinforced transformer, a transformer-based architecture for image captioning. The architecture improves the image encoding stage, which exploits the relationships between image regions by integrating a feature attention block (FAB). During the training phase, we trained the model with a combination of cross-entropy loss and contrastive loss. We experimentally explored the performance of ArCo and other fully attentive models. We also validated the baseline of the transformer for image captioning with different pre-trained models. Our proposed approach was demonstrated to achieve a new state-of-the-art performance on the offline 'Karpathy' test split and online test server. [ABSTRACT FROM AUTHOR]

[Display omitted] • Comparison between micro LPBF and conventional LPBF in the fabrication of IN718 specimens were investigated. • Increase in solidification velocity was notably more pronounced than the change in temperature gradient during μ-LPBF. • μ-LPBF demonstrated success in minimizing property anisotropy and enhancing the yield strength. • A higher volume fraction of Laves phase due to μ-LPBF indeed lead to a lower ultimate tensile strength. Compared to conventional laser powder bed fusion (c-LPBF), micro laser powder bed fusion (μ-LPBF) offers enhanced capabilities for fabricating complex components with exceptional accuracy. This is achieved through the utilization of a finer laser beam and smaller metal powder, leading to distinct differences in microstructure when compared to c-LPBF. Despite promising outcomes, a comprehensive understanding of these distinctions remains incomplete. One notable observation in the μ-LPBF samples in this study was the presence of a weaker 〈1 0 0〉 texture along the build direction. Additionally, these samples exhibited smaller grain sizes, a higher density of low-angle grain boundaries, and the presence of geometrically necessary dislocations. Significantly, μ-LPBF demonstrates the potential to produce IN718 with exceptional isotropic mechanical properties, primarily attributed to its capacity to generate refined microstructures and facilitate distinct solidification behaviors. Of particular significance was the remarkable 39.1 % increase in yield strength observed along the building direction by μ-LPBF. Furthermore, the differences in yield strength and ultimate tensile strength between various directions are notably reduced to 7 and 41 MPa, respectively, with a minimal total elongation variation of only 1.8 %. To gain deeper insights into the solidification behavior, Computational fluid dynamics were conducted in conjunction with the discrete element method. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (L -PBF) is an additive manufacturing method with potential applications in the aerospace industry. Currently, IN738LC has been a challenging engineering material to work with as it is inherently brittle due to its high γ′ volume fraction. In this study, crack-free IN738LC was manufactured on a customized L -PBF system capable of preheating to 700 °C. A wide processing parameter range (58–83 J/mm3) could be selected to achieve solid bulk fabrication with a density of 99.95%. The microstructure and mechanical behavior difference between preheating at 200 °C and 700 °C were assessed. In addition to achieving cracking suppression, the high preheating temperature resulted in a strong textured columnar microstructure with finely dispersed carbide particles along the grain boundaries of as-built parts, and superior room temperature ultimate tensile strength of 1612 MPa associated with good ductility of 18% was achieved. Work hardening rate evolution that differed from 200 °C preheating was thought to cause abnormal anisotropic tensile behavior. The influence of preheating on the melt pool morphology, the development of fluid dynamics, and the thermal behavior were discussed according to simulations with the discrete element model (DEM) and volume of fluid (VOF) method. [Display omitted] • A superior crack-free IN738LC was fabricated by laser powder bed fusion at a preheating temperature of 700 °C. • Texture in the as-built samples changed from Rotated Cube to a Cube type in the XY plane; and from a weak Rotated Goss type to a strong Rotated Goss type in the XZ plane with the increasing preheating. • A large amount of MC particles and small-size γ′ precipitates were obtained in the as-built state by preheating to 700 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • The increase on depth of molten pool is more sensitive to preheating than width during SLM Inconel 738; • Preheating is not always a positive factor, especially in the pores generation with high energy density condition. • By preheating, the variations of cooling rates distribution and temperature gradients within the molten pool makes the solidification process more stable. • By preheating, the reduction of vectorial temperature gradient through X-axis helps to eliminate cracks though center-line; • Higher preheating temperature(＞1000℃) further improves positive impacts. Due to the poor weldability of Inconel 738 alloy, defects (cracks, pores) always exist in the selective laser melting(SLM) process. In this paper, the molten pool evolution of the single track, including the morphology, porosity and crack formation, was systematically investigated by numerical simulation, and the effects of different preheating temperature and process parameters were verified by experimental analysis. The detailed temperature-dependent thermal physical properties were applied into the Volume of Fluid (VOF) method to simulate a powder bed computational domain which was established by Discrete Element Model (DEM). The simulation results show that the change of molten pool width by different laser power and preheating was not as significant as that of depth. The results also demonstrate that higher energy density and preheating temperature induced the generation of pores at the bottom of molten pool, which can be explained by the variation in cooling rates. Moreover, the change of temperature gradients in three-dimensional directions with the increase of preheating temperature was also investigated, which explained the reason for crack forming and extending through the center line. Finally, the evolution of molten pool at a higher preheating temperature 1000℃ was simulated to further underpin the positive effect of preheating on preventing cracks formation. [ABSTRACT FROM AUTHOR]

Abstract: As a surface-sensitive optical method, the quasi-Brewster angle technique is extended to evaluate the subsurface qualities of quartz crystal and fused silica. By measuring the ellipsometry parameters at λ =900nm with variable incidence angle, a fitting process is executed using a proper subsurface nanostructure with an exponential porosity distribution. The fitting results predict the surface and subsurface information which are in good agreement with that of the Zygo interference microscope measurements and etching methods. In addition, the shift characteristics of phase change at the Brewster angle of such crystal and non-crystal material are compared and demonstrated. [Copyright &y& Elsevier]