Papers by Keyword: Aluminide

Abstract: One of the surface modification processes for high-temperature oxidation resistance is slurry aluminizing process, forming protective layer of alumina (Al₂O₃). However, several important parameters such as annealing times and temperatures should be intensively considered. The objective of this study is to improve the process of slurry aluminide coating of ferritic stainless steels type AISI430 (16%Cr) combat to high-temperature oxidation. The specimens were cut, then ground, and finally sprayed with slurry mixture (Al powder + polyvinyl alcohol (PVA)). They were annealed in Ar at 1100°C for 15 minutes in order to eliminate PVA and form aluminide on their suface. The protective layer Al₂O₃ was finally formed in the temperature range of 900-1100 °C for 15-60 minutes. The cyclic oxidation tests were performed at 1000 °C for 24 hours. The surface morphology were then examined by XRD, SEM equipped EDS. The results showed that all oxidation kinetics of coated specimens were parabolic. The oxidation rate of uncoated specimens was apparently higher than that of coated specimens. Comparing with all coated specimens, the oxidation rate decreased with the increasing temperature and annealing time. In this study, the coating process at 1100°C for 60 minutes exhibited the lowest oxidation rate due to the most complete layer of Al₂O₃. The surface morphology showed the formation of continuous layer of Fe₂Al₅ and Al₂O₃, acting as barrier layer to oxide growth. Effect of temperature and time on oxidation resistance were discussed in this study.

Abstract: Alloys based on TiAl and FeAl aluminides are modern materials for high-temperature applications in automotive or aerospace industry due to low density combined with good high-temperature mechanical properties and oxidation resistance. Previous works proved that the addition of silicon to these alloys improves the oxidation resistance as well as the thermal stability. In this work, the mechanism of the silicon effect was investigated by observing the microstructure of the oxide layer and the near-surface area of the Ti-Al-Si and Fe-Al-Si alloys prepared by reactive sintering powder metallurgy. It was found that silicon improves the compactness of the oxide layers. The oxide layers on Fe-Al-Si alloys are formed by Al₂O₃ and small amount of iron oxide (Fe₂O₃) while Ti-Al-Si alloys cover by TiO₂ and Al₂O₃ during the oxidation. Due to aluminium depletion of the alloy, a layer of silicides is formed under the oxide layer, thus acting as the additional protection against high-temperature oxidation.

Abstract: This work presents the degradation mechanism of the platinum modified aluminide diffusion coating of the GTD 111 SC Ni-base superalloy turbine blades after 16000 h of exposition at different thermal cycles (critical heating temperatures reported ~1000°C and 1120°C). The initial coating condition and the evolution of degradation were characterized applying conventional microscopy and backscatter scanning electron microscopy. The initial microstructure condition consisted of a two phase coating (intermetallics PtAl₂ dispersed in a matrix β-(Ni,Pt)Al). The major microstructure degradation was associated with: intermediate and interdiffusion zones growing, partial transformation of β-(Ni,Pt)Al to γ´-Ni₃Al, and the dissolution of the intermetallic PtAl₂ resulting in a more brittle single phase β-(Ni,Pt)Al coating. The degradation facilitates spallation and crack initiation, resulting in the loss of the coating and by consequence the blade failure.

Abstract: In the present work, the formation of an aluminide coating prior to Hot Press Forming (HPF) was investigated. It was found that the formation of Fe3Al phases could suppress coating degradation during the HPF process. This new method was studied as a way to simultaneously improve the coating ductility and achieve a good hot corrosion resistance. In this new method, the conventional type1 aluminized coating was transformed into an aluminide coating. This aluminide coating protects the steel from high temperature oxidation and enables its plastic deformation at high temperature. The effect of this new solution on the mechanical properties of HPF steel is reviewed in detail.

Abstract: The densification behavior of WC composites based on iron aluminide binder was investigated using laser scanning confocal mi¬croscopy (LSCM). Doped Fe60Al40 alloys with boron levels ranging from 0 to 0.1 wt% were used as the aluminide binders. The aluminide binders were prepared using controlled atmosphere ring grinding and then blended with WC powder. The composite powder compacted in an alumina crucible and held in a platinum holder in the confocal microscope. The temperature increased from ambient temperature up to 1500 °C under high purity argon. The presence of boron was found to facilitate compaction of the composites and improve the wetting between WC and FeAl binder during liquid phase sintering. Increasing the amount of boron in the binder resulted in the melting of binder at lower temperature and increasing of the compacting of the intermetallic tungsten carbide composites.

Abstract: This paper presents the results of an investigation aimed at understanding microstructure formation of Al-Fe-Mn-Si intermetallics during pressure-assisted reactive sintering of elemental powders. The proportion of elements was selected such that the composition of the product was 55 wt % Al, 17 wt % Si, 14 wt % Mn, and 14 wt % Fe. Experiments were conducted at temperatures between 600 and 800°C, using compaction stresses of up to 20 MPa. Rietveld analysis of x-ray diffraction patterns of fully processed samples showed that the powders were transformed into a mixture of Al9FeMnSi and Al9FeMn2Si phases. However, as temperature and pressure were increased, the Al9FeMnSi phase was transformed into the Al9FeMn2Si phase. Differential Thermal Analysis, as well as microstructural characterization by scanning electron microscopy and x-ray diffraction, showed that these intermetallics do not form directly from the powder mixtures. Rather, they are the result of metallurgical reactions between a molten Al-Si solution and various intermediate phases formed during reactive sintering.

Abstract: The diffusion aluminide coatings are widely used in the air-cooling passages to protect their surfaces against high temperature corrosion. In this study plain and Si-modified aluminide coatings were applied by slurry technique on internal surfaces of Ni-base GTD-111 superalloy cylindrical specimens derived from a gas turbine air-cooled blade. The slurries containing Al or Al plus Si powders were applied on internal surfaces by injection method. Then, the samples were heated to high temperature (800-1000°C) to form the coatings. Optical, SEM-EDS, and XRD were utilized for characterizing microstructures and phase compositions of the coatings. The thicknesses of applied coatings on internal surfaces were in the range of 30-50 μm that meets specifications for diffusion coatings in such application (i.e. 25-756m). The examinations demonstrated that both coating types were contained β-NiAl phase as the matrix. The uniformities of coatings applied on different surface positions of passageway were determined. In addition, the effects of time and temperature of coating process as well as mass of dried slurry on the coating thickness were also discussed.

Abstract: The present work, performed on nickel aluminides deposited on a single Ni-based superalloy AM1, focuses on the effect of the following several parameters on the microstructural and chemical changes occurring during isothermal heat treatment at 1100°C for 50h : -oxygen pressure by comparing heat treatment under ambient air (PO2 = 0.2 bar) and under secondary vacuum (PO2 = 0.2x10-6 bar). -cooling rate after isothermal heat treatment by comparing furnace cooling (3°C/min) and water quenching (500°C/min). -Pt addition in the coating by comparing NiAl and NiPtAl coatings. Characterizations were performed using SEM, analytical TEM and electron microprobe analyses. The results show that these parameters have a strong influence on both the microstructural evolution and the oxidation of the thermal barrier coating (TBC) system. Appropriate heat treatments are essential to improve interfacial resistance and increase the durability of TBC systems.

Abstract: The effect of boron on the WC morphology and on the grain size of binders in sub micron WC composites containing Fe60Al40 and Ni3Al binders was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composites were prepared under uniaxial hot pressing of milled powder samples at 1500 °C in inert argon atmosphere. Doped aluminides with boron levels ranging from 0 to 0.1 wt% were used as the binders. It was found that the microstructural characteristics of boron doped aluminide WC composites were similar to those of hot pressed WC-Co and commercial grade WC-10wt%Co (H10F) hardmetals. The contiguity of WC particles (WC/WC contact) and the grain sizes of aluminides decreased and the extent of faceting of tungsten carbide increased in the aluminide tungsten carbide composites in presence of boron.

Abstract: Isothermal oxidation behavior of a 4th generation Ni-base single crystal superalloy with Pt-modified and Ru-modified aluminide coating was examined in a temperature range 1223 to 1373 K in air. Both Pt and Ru modification improve the oxidation resistance of a simple aluminide coating, especially above 1273 K. They allow thin protective and continuous Al2O3 scales to be intact for at least 500 h at temperatures up to 1323 K. However, the Pt modification drastically accelerates the formation of a secondary reaction zone (SRZ). This suggests that Pt promotes the formation of a topologically close-packed phase by lowering the solubility of refractory elements in γ-Ni. In contrast, the Ru modification reduces the SRZ, and is expected to enhance the phase stability under the coating by preventing the depletion of Ru due to its outward diffusion.