STRUCTURE AND PHASE COMPOSITION OF COATING ON EP718 ALLOY AFTER HOT-DIP ALUMINIZING

The results of studies of the structure, chemical and phase composition of the aluminide coating formed on the surface of EP718 alloy after its hot-dip aluminizing are presented. It is shown that the inner layer of the coating consists of a continuous layer of a solid solution based on chromium aluminide CrAl with a monoclinic lattice, in which some of the Cr atoms are replaced by atoms of elements from the EP718 alloy, and the outer layer, which has a pronounced heterogeneous structure, is an aluminum matrix with inclusions of CrAl aluminide. with a lower aluminum content.

Oxidation Behavior and Oxide Transformation of a Pt-Modified Aluminide Coating at Moderate High Temperature

The oxidation performance of a single-phase Pt-modified aluminide coating was assessed in oxidation test at 980 °C in comparison with the single crystal superalloy. The results suggested that the Pt-modified aluminide coating exhibited superior oxidation resistance. During oxidation, the oxide scale formed on bare alloy changed constantly followed by the constitution of the multi-layer scale structure: An outer scale mainly consisted of Cr2O3 + NiCr2O4 + TiO2 with scarce protection, and an internal scale mainly consisted of Al2O3. The thickness of the outer oxide scale increased with time, where the scale became looser and more porous. Meanwhile, the internal scale was discontinuous. Oxygen and nitrogen inwardly diffused into substrate, forming Ta2O5 and TiN particles. In contrast to the complex constitution of oxide scale, a uniform and continuous Al2O3 scale formed on Pt-modified aluminide-coated samples after oxidation at 980 °C for 1500 h, which showed no spallation and cracking. Interestingly, θ-Al2O3 and α-Al2O3 phases remained after such a long oxidation time. It is the relatively lower temperature and the presence of Pt retarded θ-α transformation. The degradation rate from β-NiAl to γ′-Ni3Al was very slow in the coating. The various development of oxide scale on the coating and substrate was individually studied.

Titanium Aluminide Coating: Structural and Elastic Properties by DFT Approach

The stability, elastic and electronic properties of titanium aluminide compounds have been systematically studied by the first-principles calculation. The calculated lattice parameters are consistent with the results found in the literature. The three Ti-Al binary compounds are thermodynamically stable intermetallics depending on their negative formation enthalpy. It has been found that the Ti-Al binary compounds are composed of both metallic and covalent bonds. Elastic properties revealed that these alloys are more resistant to deformation along the a- and c-axis. Besides, the (001)[100] deformation would be easier than (010)[100] deformation for these alloys. The results found in this chapter give a reliable reference for the design of novel Ti-Al binary alloys.

Influence of Pre-Oxidation on High Temperature Oxidation and Corrosion Behavior of Ni-Based Aluminide Coating in Na2SO4 Salt at 1050°C

Aluminide coating is prepared on K438 Ni-based superalloy by pack cementation. The hot corrosion test of as-received and pre-oxidation aluminide coating in Na2SO4 salt are carried out at 1050°C. The coating morphologies, phase composition, and corrosion products are characterized by XRD, SEM, EDS, and TEM. The conclusion is that the pre-oxidation aluminide coating has excellent hot corrosion properties. After hot corrosion, the oxide scale of pre-oxidation aluminide coating is the thinnest and maintains good integrity. However, the oxide scale of as-received aluminide coating fluctuates greatly and there are corrosion cavities. CrS is formed in the alloy immediately below the oxide scale for the two coating systems.