Ahmad Keyvani

The present work investigates the influence of build orientation–induced anisotropy on the microstructure and corrosion behavior of selective laser melted (SLM) Inconel 718 (IN718) superalloy in 1 M H ₂ SO ₄ solution. Microstructural characterization using field emission scanning electron microscopy, electron backscatter diffraction, energy dispersive x-ray spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy revealed distinct orientation-dependent features. The top surface of SLMed alloy exhibited a refined cellular microstructure (grain size ≈8.4 μ m) with a low-angle grain boundary fraction of 40.7% and a moderate crystallographic texture, resulting in limited Nb segregation and a Cr ₂ O ₃ -dominated passive film (11.54 at.%). In contrast, the cross section of alloy displayed columnar grains (grain size ≈ 7.7 μ m) aligned with the build direction, pronounced melt-pool boundaries, and the strongest texture, promoting extensive interdendritic Nb segregation (Nb ⁵⁺ /Nb ₂ O ₅ : 1.98 at.%) and lower chromium oxide content (Cr ₂ O ₃ : 10.30 at.%). These heterogeneities promoted the formation of secondary phases, including Laves or MC-type carbides, particularly along interdendritic regions. Electrochemical measurements, including electrochemical impedance spectroscopy and potentiodynamic polarization, demonstrated significant anisotropy in corrosion behavior. The top surface of SLMed IN718 (the surface perpendicular to the build direction, i.e., the XY-plane) exhibited the lowest corrosion current density ( i _corr = 0.1 μ A cm ⁻² ) and highest polarization resistance ( R _p = 2097 kΩ cm ² ) after 1 h immersion, retaining R _p = 712 kΩ cm ² after 30 d. The cross-section of SLMed IN718 (the surface parallel to the build direction, i.e. the XZ-plane) showed the poorest performance, with i _corr = 4.5 μ A cm ⁻² after 1 h rising to 9.9 μ A cm ⁻² after 30 d, and R _p collapsing from 39 to 1.96 kΩ cm ² . Long-term immersion tests confirmed the persistence of orientation-dependent corrosion performance. The results demonstrate that corrosion behavior in SLM IN718 is strongly governed by microstructural anisotropy, highlighting the importance of controlling solidification conditions and build orientation to enhance corrosion resistance of additively manufactured nickel-based superalloys.

Cyclic oxidation in thermal barrier coatings (TBCs) based on ZrO₂ with MoSi₂ + Al₂O₃ has gained increasing attention. The present study the atmospheric plasma spray (APS) process was used for applying a new dual-layer TBC (CSZ: ZrO₂-25 wt.% CeO₂-2.5 wt.% Y₂O₃ / MAC: MoSi₂ + Al₂O₃ + CSZ) with enhanced oxidation resistance compared to the single-layer CSZ. CSZ/MAC and CSZ TBCs on NiCrAlY (bond coat) to nickel-based superalloy (IN738LC). The self-healing effects of the MAC overlayer were studied at 720°C for 5 and 10 h. The high-temperature cyclic oxidation tests were performed at 1100°C for 4 h in each cycle in an air-electric furnace and the samples were cooled during each cycle in the furnace. The role of MAC over layer in the improvement of the oxidation behaviour of CSZ was investigated through field emission scanning electron microscopy (FE-SEM/EDS), and X-ray diffraction (XRD) analyses. The coating was healed during the oxidation process due to the reaction of healing particles (MoSi₂) on the surface of the sample with oxygen through the cracks at temperatures above 720°C. The MAC coatings showed improved oxygen diffusion resistance due to the filling and sealing effect. Thus, there is a low partial pressure of oxygen at the interface of BC-CSZ in the CSZ/MAC samples than in the CSZ samples. The low partial pressure of oxygen resulted in a slow growth rate of TGO, resulting in the improved oxidation and spallation resistance of the CSZ/MAC coatings. The results showed the higher oxidation resistance of dual-layer CSZ/MAC compared to the single-layer CSZ coating. Results of the study indicate that the MAC layer over the CSZ layer increased the coatings’ oxidation resistance for extended periods by preventing oxygen infiltration into the CSZ layer.

Advanced thermal barrier coatings (TBCs) developed by incorporating multicomponent rare earth oxide dopants into zirconia are promising alternative to replace yttria-stabilized zirconia (YSZ) thermal barrier coatings. In this study, a zirconia-based nanopowder coating doped by multiple rare earth oxides (YLaYbZr: 5.86 Mol.%Y₂O₃-1.99 Mol.%La₂O₃-1.98 Mol.%Yb₂O₃-ZrO₂) was synthesized using co-precipitation technique which is advantageous in terms of simplicity and cost-effectiveness. The product was then characterized using X-ray diffraction (XRD) method and field emission scanning electron microscopy (FESEM). The stability of YLaYbZr compound was studied after a heat treating the product at 1300°C for 50 h. The results indicated that the initially obtained powder was a metastable tetragonal (t-prime) zirconia. Rietveld refinement of the XRD data from YLaYbZr powder after 50 h of heat treatment at 1300°C confirmed stabilization of zirconia in the t-prime phase with around 15 wt.% monoclinic impurity. Furthermore, FESEM results (before and after heat treatment) indicated orderly particles of uniform shape and size with a small tendency toward agglomeration.