| 1. |
- Adegoke, Olutayo, et al.
(författare)
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Scanning electron microscopy and atom probe tomography characterization of laser powder bed fusion precipitation strengthening nickel-based superalloy
- 2023
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Ingår i: Micron. - 0968-4328 .- 1878-4291. ; 171
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Tidskriftsartikel (refereegranskat)abstract
- Atom probe tomography (APT) was utilized to supplement scanning electron microscopy (SEM) characterizationof a precipitation strengthening nickel-based superalloy, Alloy 247LC, processed by laser powder bed fusion (LPBF). It was observed that the material in the as-built condition had a relatively high strength. Using both SEMand APT, it was concluded that the high strength was not attributed to the typical precipitation strengtheningeffect of γ’. In the absence of γ’ it could be reasonably inferred that the numerous black dots observed in thecells/grains with SEM were dislocations and as such should be contributing significantly to the strengthening.Thus, the current investigation demonstrated that relatively high strengthening can be attained in L-PBF even inthe absence of precipitated γ’. Even though γ’ was not precipitated, the APT analysis displayed a nanometer scalepartitioning of Cr that could be contributing to the strengthening. After heat-treatment, γ’ was precipitated and itdemonstrated the expected high strengthening behavior. Al, Ta and Ti partitioned to γ’. The strong partitioningof Ta in γ’ is indicative that the element, together with Al and Ti, was contributing to the strain-age crackingoccurring during heat-treatment. Cr, Mo and Co partitioned to the matrix γ phase. Hf, Ta, Ti and W were found inthe carbides corroborating previous reports that they are MC.
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| 2. |
- Bates, William P., et al.
(författare)
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Properties Augmentation of Cast Hypereutectic Al-Si Alloy Through Friction Stir Processing
- 2022
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Ingår i: Metals and Materials International. - : KOREAN INST METALS MATERIALS. - 1598-9623 .- 2005-4149.
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Tidskriftsartikel (refereegranskat)abstract
- The present endeavour is to augment mechanical attributes via friction stir processing (FSP) in hypereutectic aluminium-silicon castings by the means of microstructural modifications and defects reduction. Wherein, the study proceeds with mainly two approaches namely, alteration in tool revolution (TR) and the number of FSP passes. The prepared specimens were evaluated investigating volume fraction of porosities, microstructural characterizations and microhardness. Therefrom, the specimen with highest number of passes delivered most uniform properties resulting from the reduction in casting porosities and refined silicon particle uniform distribution throughout friction stir processed zone. This endeavour may be considered as a footstep towards more industrial readied material transformation.
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| 3. |
- Hanning, Fabian, 1988, et al.
(författare)
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Investigation of the Effect of Short Exposure in the Temperature Range of 750-950 degrees C on the Ductility of Haynes (R) 282 (R) by Advanced Microstructural Characterization
- 2019
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Ingår i: Metals. - : MDPI. - 2075-4701. ; 9:12
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Tidskriftsartikel (refereegranskat)abstract
- A Gleeble-based test method has been developed to study the change in the ductility signature of Haynes (R) 282 (R) during isothermal exposure from 5 s to 1800 s. A temperature range of 750 to 950 degrees C has been used to investigate the effect of age-hardening reactions. Microstructural constituents have been analyzed and quantified using scanning and transmission electron microscopy. Carbides present in the material are identified as primary MC-type TiC carbides, Mo-rich M6C secondary carbides, and Cr-rich M23C6 secondary carbides. Gamma prime (gamma’) precipitates are present in all the material conditions with particle sizes ranging from 2.5 nm to 58 nm. Isothermal exposure causes the growth of gamma’ and development of a grain boundary carbide network. A ductility minimum is observed at 800-850 degrees C. The fracture mode is found to be dependent on the stroke rate, where a transition toward intergranular fracture is observed for stroke rates below 0.055 mm/s. Intergranular fracture is characterized by microvoids present on grain facets, while ductility did not change during ongoing age-hardening reactions for intergranularly fractured Haynes (R) 282 (R).
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| 4. |
- Jonnalagadda, Krisha Praveen, et al.
(författare)
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Hot Corrosion Mechanism in Multi-Layer Suspension Plasma Sprayed Gd2Zr2O7 /YSZ Thermal Barrier Coatings in the Presence of V2O5 + Na2SO4
- 2017
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Ingår i: Journal of thermal spray technology (Print). - New York : Springer Science and Business Media LLC. - 1059-9630 .- 1544-1016. ; 26:1-2, s. 140-149
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates the corrosion resistance of two-layer Gd2Zr2O7/YSZ, three-layer dense Gd2Zr2O7/ Gd2Zr2O7/YSZ, and a reference single-layer YSZ coating with a similar overall top coat thickness of 300-320 µm. All the coatings were manufactured by suspension plasma spraying resulting in a columnar structure except for the dense layer. Corrosion tests were conducted at 900 °C for 8 h using V2O5 and Na2SO4 as corrosive salts at a concentration of approximately 4 mg/cm2. SEM investigations after the corrosion tests show that Gd2Zr2O7-based coatings exhibited lower reactivity with the corrosive salts and the formation of gadolinium vanadate (GdVO4), accompanied by the phase transformation of zirconia was observed. It is believed that the GdVO4 formation between the columns reduced the strain tolerance of the coating and also due to the fact that Gd2Zr2O7 has a lower fracture toughness value made it more susceptible to corrosion-induced damage. Furthermore, the presence of a relatively dense layer of Gd2Zr2O7 on the top did not improve in reducing the corrosion-induced damage. For the reference YSZ coating, the observed corrosion-induced damage was lower probably due to combination of more limited salt penetration, the SPS microstructure and superior fracture toughness of YSZ.
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| 5. |
- Karimi Neghlani, Paria, 1986-, et al.
(författare)
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Microstructure Development in Track-by-Track Melting of EBM-Manufactured Alloy 718
- 2018
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Ingår i: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives. - Cham : Springer. - 9783319894799 - 9783319894805 ; , s. 643-654
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Konferensbidrag (refereegranskat)abstract
- Electron beam melting (EBM) is a powder-bed fusion process within the group of additive manufacturing (AM) technology that is used to fabricate high performance metallic parts. Nickel-Iron base superalloys, such as Alloy 718, are subjected to successive heating and cooling at temperatures in excess of 800 °C during the EBM process. Characterization of the dendritic structure, carbides, Laves and δ-phase were of particular interest in this study. These successive thermal cycles influence the microstructure of the material resulting in a heterogeneous structure, especially in the building direction. Hence, the aim of this study was to gain increased fundamental understanding of the relationship between the processing history and the microstructure formed within a single layer. Different numbers of tracks with equal heights were for this purpose produced, varying from one to ten tracks. All tracks used the same process parameters regardless of number and/or position. Microstructure characteristics (sub-grain structure, grain structure and phases) were analyzed by optical microscopy, scanning electron microscopy equipped with energy disperse spectroscopy and electron backscatter diffraction. The direction of dendrites changed in the overlap zones within the tracks due to re-melting of material in the overlap zone. The primary dendrite arm spacings slightly increased along multi-tracks owing to a slight decrease in cooling rate by addition of the next tracks. Epitaxial growth of grains were observed in all samples due to partial re-melting of grains in previous layers and surface nucleation was also found to occur in all tracks.
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| 6. |
- Karimi Neghlani, Paria, 1986-, et al.
(författare)
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Tailored grain morphology via a unique melting strategy in electron beam-powder bed fusion
- 2021
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 824
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a unique melting strategy in electron beam-powder bed fusion of Alloy 718 to tailor the grain morphology from the typical columnar to equiaxed morphology. For this transition, a specific combination of certain process parameters, including low scanning speeds (400–800 mm/s), wide line offsets (300–500 μm) and a high number of line order (#10) was selected to control local solidification conditions in each melt pool during the process. In addition, secondary melting of each layer with a 90° rotation with respect to primary melting induced more vigorous motions within the melt pools and extensive changes in thermal gradient direction, facilitating grain morphology tailoring. Four different types of microstructures were classified according to the produced grain morphology depending on the overlap zone between two adjacent melt pools, i.e., fully-columnar (overlap above 40 %), fully-equiaxed (overlap below 15 %), mixed columnar-equiaxed grains, and hemispherical melt pools containing mixed columnar-equiaxed grains (overlap ~20–25 %). The typical texture was <001>; however, the texture was reduced significantly through the transition from the columnar to equiaxed grain morphology. Along with all four different microstructures, shrinkage defects and cracks were also identified which amount of them reduced by a reduction in areal energy input. The hardness was increased through the transition, which was linked to the growth of the γʺ precipitates and high grain boundary density in the fully-equiaxed grain morphology.
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| 7. |
- Rakoczy, Lukasz, et al.
(författare)
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Investigation of the y´ Precipitates Dissolution in a Ni-Based Superalloy During Stress-Free Short-Term Annealing at High Homologous Temperatures
- 2021
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Ingår i: Metallurgical and Materials Transactions. A. - : SPRINGER. - 1073-5623 .- 1543-1940. ; 52:11, s. 4767-4784
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Tidskriftsartikel (refereegranskat)abstract
- The equiaxed Ni-based superalloy Rene 108 was subjected to short-term annealing at five temperatures between 900 degrees C and 1100 degrees C. The phase composition, phase lattice parameters, microstructure, stereological parameters, and chemical composition of y´ precipitates were investigated by thermodynamic simulations, X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Analysis of the y and y´ lattice parameters using the Nelson-Riley extrapolation function showed that the misfit parameter for temperatures 900 degrees C to 1050 degrees C is positive (decreasing from 0.32 to 0.11 pct). At 1100 degrees C, the parameter becomes negative, delta = - 0.18 pct. During the short-term annealing, gamma ‘ precipitates dissolution occurred progressing more rapidly with increasing temperatures. The surface fraction of y´ precipitates decreased with increasing temperature from 0.52 to 0.34. The dissolution of gamma ‘ precipitates did not only proceed through uninterrupted thinning of each individual precipitate, but also included more complex mechanisms, including splitting. Based on transmission electron microscopy, it was shown that after y´ precipitates dissolution, the matrix close to the y/y interface is strongly enriched in Co and Cr and depleted in Al.
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| 8. |
- Sadeghimeresht, Esmaeil, 1985-, et al.
(författare)
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Isothermal Oxidation Behavior of EBM-Additive Manufactured Alloy 718
- 2018
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Ingår i: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives. - Cham : Springer. - 9783319894799 - 9783319894805 ; , s. 219-240
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Konferensbidrag (refereegranskat)abstract
- Oxidation of Alloy 718 manufactured by electron beam melting (EBM) process has been undertaken in ambient air at 650, 700, and 800 °C for up to 168 h. At 800 °C, a continuous external chromia oxide enriched in (Cr, Ti, Mn, Ni) and an internal oxide that was branched structure of alumina formed, whereas at 650 and 700 °C, a continuous, thin and protective chromia layer was detected. The oxidation kinetics of the exposed EBM Alloy 718 followed the parabolic rate law with an effective activation energy of ~248 ± 22 kJ/mol in good agreement with values in the literature for conventionally processed chromia-forming Ni-based superalloys. The oxide scale formed on the surface perpendicular to the build direction was slightly thicker, and more adherent compared to the scale formed on the surface along the build direction, attributed to the varied grain texture in the two directions of the EBM-manufactured specimens. The increased oxygen diffusion and high Cr depletion found on the surface along the build direction were attributed to the fine grains and formation of vacancies/voids along this grain orientation.
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| 9. |
- Tu, Juei-feng, et al.
(författare)
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Nanostructure diffraction analysis of a copper/single walled carbon nanotube nanocomposite synthesized by Laser Surface Implanting
- 2017
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 113, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- A new wet process, denoted as Laser Surface Implanting (LSI), has been developed to synthesize a Copper-Single Wall Carbon NanoTube (Cu-SWCNT) metal nanocomposite by dispersing SWCNTs into molten copper, followed by rapid and non-equilibrium solidification to form the Cu-SWCNT nanocomposite such that dispersed SWCNTs could locked in positions without agglomerating into large clusters. However, the nanometer sizes of the SWCNT clusters inside this nanocomposite make it extremely difficult to obtain TEM images with discernable SWCNT clusters in the copper matrix. In this paper, TEM images and their diffraction patterns for annealed pure copper, quenched pure copper (by the same synthesis process without introducing SWCNTs), and Cu-SWCNT nanocomposite are compared. It is concluded that TEM images with discernable SWCNT clusters are rare. Therefore, diffraction patterns are better tools to identify SWCNTs within the copper matrix. The indexed diffraction patterns confirm that the copper fcc lattice is preserved. However, the Cu-SWCNT nanocomposite samples also exhibit ordered diffuse scattering, consisting of at least two polyhedra of diffuse-scattering bounded by the 110* and 200* family of reciprocal lattice planes, respectively. In addition several samples exhibit super-lattice Bragg diffraction indicative expanded unit cells. It thus appears that the SWCNTs are incorporated into the Cu matrix with precise arrangements commensurate with specific Cu lattice planes.
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| 10. |
- Zaikovska, Liene, et al.
(författare)
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Three-Dimensional Columnar Microstructure Representation Using 2D Electron Backscatter Diffraction Data for Additive-Manufactured Haynes ® 282 ®
- 2024
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 17:7
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Tidskriftsartikel (refereegranskat)abstract
- This study provides a methodology for exploring the microstructural and mechanical properties of the Haynes®282® alloy produced via the Powder Bed Fusion-Electron Beam (PBF-EB) process. Employing 2D Electron Backscatter Diffraction (EBSD) data, we have successfully generated 3D representations of columnar microstructures using the Representative Volume Element (RVE) method. This methodology allowed for the validation of elastic properties through Crystal Elasticity Finite Element (CEFE) computational homogenization, revealing critical insights into the material behavior. This study highlights the importance of accurately representing the grain morphology and crystallographic texture of the material. Our findings demonstrate that created virtual models can predict directional elastic properties with a high level of accuracy, showing a maximum error of only ~5% compared to the experimental results. This precision underscores the potential of our approach for predictive modeling in Additive Manufacturing (AM), specifically for materials with complex, non-homogeneous microstructures. It can be concluded that the results uncover the intricate link between microstructural features and mechanical properties, underscoring both the challenges encountered and the critical need for the accurate representation of grain data, as well as the significance of achieving a balance in EBSD area selection, including the presence of anomalies in strongly textured microstructures.
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