Introduction and Objectives: This study redefines the 1400 °C experiment as an over-temperature degradation test rather than a conventional service-temperature oxidation test, and evaluates the microstructural stability limit of conventional and germanium-modified aluminide coatings on Hastelloy X.Materials and Methods: Ge-containing precursor layers were produced by CVD using 600 °C/4 h and 900 °C/2 h routes, followed by CVD aluminizing at 1020 °C for 3 h. The coated specimens were exposed at 1400 °C for 55 h and then characterized by mass-change measurement, cross-sectional BSE observations, semi-quantitative EDS, elemental mapping, and XRD. Results: In the available dataset, Al and Ge900-Al showed net mass changes of -0.09687 and -0.09421 mg/cm², respectively, whereas Ge600-Al retained a positive net mass change of +0.05971 mg/cm². Ge900-Al showed clearer XRD evidence of retained β-NiAl, while Ge600-Al showed the most favorable mass-change response within the present 55 h window. Conclusion: The results should be interpreted as a short-window over-temperature degradation response. The behavior of the coatings is controlled not only by Ge addition but also by the initial coating architecture, the continuity of Ge/Cr-bearing barrier-like regions, and the relative retention of β-NiAl. Because the test temperature exceeds the nominal melting range of Hastelloy X, the results are not intended to represent normal service oxidation behavior.
Introduction and Objectives: Inconel 738 low-carbon (IN738LC) nickel-based superalloy is widely used in high-temperature turbine components due to its high strength and thermal stability. However, microstructural heterogeneity resulting from the solidification process can lead to a non-uniform distribution of strengthening precipitates and deterioration of mechanical performance. The objective of this study was to investigate the effect of increasing the solution heat treatment temperature beyond the standard cycle on the microstructural evolution and mechanical properties of the superalloy. Materials and Methods: Two heat treatment cycles, including a standard cycle and a proposed cycle, were applied to cast specimens. Microstructural changes were evaluated using optical microscopy, scanning electron microscopy, and quantitative image analysis. In addition, hardness and stress rupture tests were conducted to evaluate the mechanical behavior of the alloy. Results: The results showed that increasing the solution temperature significantly reduced microstructural heterogeneity between dendritic core and interdendritic regions. The size of coarse precipitates in interdendritic areas decreased, leading to a more uniform precipitate distribution. Furthermore, the volume fraction of strengthening precipitates increased from 39% to 45%. Carbide investigations indicated that the blocky morphology of MC carbides was preserved, and no evidence of significant decomposition was observed. Hardness increased from 363 HV to 447 HV, while stress rupture life improved from 35 h to 45.8 h. Conclusion: The findings demonstrated that a controlled increase in solution heat treatment temperature can simultaneously improve mechanical properties and high-temperature performance through enhanced microstructural homogenization, increased strengthening precipitate fraction, and preservation of MC carbide stability. These results provide useful insight for designing optimized heat treatment schedules for high-temperature turbine components.
Introduction and Objectives: Laser drilling is widely used in the aerospace, energy, and medical industries because of its high precision and ability to machine advanced materials. However, the formation of a recast layer, molten material redeposition, and deterioration of hole wall quality remain major challenges in nickel-based superalloys. This study investigated the effects of the processing environment and the application of a magnetic field on the morphological quality of laser-drilled holes in Hastelloy X superalloy using a fiber laser. Materials and Methods: Single-pulse laser drilling experiments were performed on 1-mm-thick Hastelloy X sheets under three processing conditions including argon shielding gas, argon shielding gas with distilled water, and argon shielding gas with distilled water under a magnetic field. The inlet, outlet, sidewall, and longitudinal cross-sectional morphologies of the holes were examined using scanning electron microscopy, while changes in chemical composition were analyzed by energy-dispersive X-ray spectroscopy. Results: The results showed that the processing environment significantly affected molten material behavior, recast layer formation, hole wall quality, and elemental distribution. The use of distilled water reduced molten material redeposition and improved hole wall uniformity. Furthermore, applying a magnetic field guided the molten material flow resulted in fewer surface discontinuities and a more uniform hole wall morphology. energy-dispersive X-ray spectroscopy analysis indicated that compositional changes were mainly confined to the regions adjacent to the hole, whereas the base metal exhibited no significant changes. Conclusion: The results of this research indicated that combined use of distilled water and a magnetic field improved the morphological quality of laser-drilled holes by controlling molten material behavior and reducing its redeposition. This approach can serve as an effective method for enhancing the quality of laser drilling in nickel-based superalloys for engineering applications.
Introduction and Objectives: The widespread application of NiTi alloys in biomedical implants is constrained by the release of nickel ions, which may provoke allergic responses and carcinogenic risks. Employing biocompatible hydroxyapatite composite coatings offers a promising approach to suppress nickel ion leaching and improve the electrochemical performance of NiTi alloys. This study focuses on assessing how varying the concentration of a secondary phase within the composite affects the coating’s composition, microstructure, and the overall electrochemical behavior of the substrate. Materials and Methods: Hydroxyapatite–tantalum oxide composite coatings were produced on NiTi substrates via the electrophoretic deposition method. Tantalum oxide was introduced into the hydroxyapatite nanoparticle suspension at concentrations of 10, 15, and 20 wt.%. All coated samples were sintered at 800 °C for one hour under vacuum conditions. Results: Experimental findings, based on nickel ion release measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy, clearly demonstrate that increasing the content of the secondary phase within the composite significantly enhances the corrosion resistance of the underlying NiTi substrate. The positive effect of the secondary phase on the adhesion strength of hydroxyapatite coating to NiTi was assessed using a tensile test. Conclusion: Engineering composite coatings with dense structural characteristics, designed to restrict the ingress of environmental corrosive species and to block nickel ion diffusion, presents a robust strategy for the safe and durable utilization of NiTi alloys in biomedical applications.
Introduction and Objectives: This study investigates the wear behavior of FeCrCoNiMnTix high-entropy alloy coatings deposited on AISI 431 martensitic stainless steel. Materials and Methods: The alloy powders were prepared via mechanical alloying in a planetary ball mill under an argon atmosphere. To evaluate their wear resistance, the synthesized powders were applied as coatings using the spark plasma sintering technique. Comprehensive analyses, including X-ray diffraction and field emission scanning electron microscopy, revealed that the addition of Ti induces a microstructural transition from a dominant FCC phase to a BCC-rich structure and facilitates the formation of Laves intermetallic phases. Microhardness measurements and pin-on-disk wear test were employed to assess the wear behavior. Results: The results strongly suggest that TiC formed in situ during the SPS process due to the interaction between Ti and carbon. This, combined with solid solution strengthening and the formation of intermetallic phases, resulted in an increase in hardness up to 1030 HV0.05 for the FeCrCoNiMnTi composition. Wear test results indicated that the best performance was achieved by the FeCrCoNiMnTi composition, with a wear rate of approximately 3.78 × 10⁻⁵ mm³/N.m, which is nearly four times lower than that of the AISI 431 stainless steel. Conclusion: This improvement is attributed to the synergistic strengthening effects of the BCC matrix, laves intermetallic phases, and the in-situ formed carbides. The findings of this research suggest that FeCrCoNiMnTi high-entropy alloys are promising candidates for wear-resistant coatings and underscore the critical role of chemical composition design in engineering their structural and tribological properties.
