Plain Aluminide Research Articles

Hot corrosion behaviour of single-phase platinum-modified aluminide coatings: Effect of Pt content and pre-oxidation

Hot corrosion behaviour of nickel-based aluminide coatings with different thicknesses of initial Pt plating was evaluated in the Na2SO4/NaCl (75:25, wt./wt.) salt mixture at 900°C. Compared to plain aluminide, the Pt-modified coatings showed better hot corrosion resistance by hindering the outward diffusion of undesirable elements from the substrate alloy. Pre-oxidation was confirmed to be capable of preventing molten salt from accessing the alloy surface and reversibly dissolving the oxide scale. Oxide growth and dissolution, acting as two competitive processes, occurred simultaneously and had an important impact on the entire performance of the Pt-modified aluminide coatings in hot corrosion.

On the oxidation behavior of titanium within coated nickel-based superalloys

Rutile precipitation within alumina scales grown on coated nickel-based superalloy CMSX-4 has been found to occur preferentially at grain boundaries within the scale. Misorientation analysis using Rodrigues–Frank space has revealed clustering of the misorientation between neighboring grains of corundum and rutile about the established 〈0001〉c{112¯0}c//〈010〉r{101}r orientation relationship observed in Ti-containing sapphire crystals. The fraction of interfaces found to exist in this configuration is sufficient to explain the nucleation of rutile from a single corundum grain abutting the rutile grain. The diffusive behavior of Ti has been observed to vary considerably within three commercially used coatings, a plain aluminide coating, a plat-aluminide coating and a diffused platinum coating. Titanium diffusion is enhanced by the presence of Pt. However this did not lead to the precipitation of more rutile, which although observed in all three coatings, was present in sufficient quantity to be detected using XRD only within the plain aluminide coated samples.

Aluminide Coatings on Titanium-based Alloy IMI-834 for High Temperature Oxidation Protection

Microstructural aspects and cyclic oxidation behaviour of plain aluminide and Pt-aluminide coatings on the near-a Ti-alloy IMI-834 have been studied. Both the coatings provide good oxidation resistance to the above alloy at 650 °C, 750 °C, and 850 °C. However, significant cracking develops in the coatings during coating formation as well as during cyclic oxidation exposure. The extent of cracking during oxidation is found to increase with the exposure temperature. Presence of through-thickness cracks in the coatings leads to localised oxidation damage of the underlying substrate at all the three temperatures. At 850 °C, such localised oxidation generates enough TiO2 so that this oxide phase grows through the cracks and emerges at the sample surface forming a clearly identifiable mud-crack pattern. The extent of such oxidation damage is comparatively much lower at 750 °C and 650 °C. Defence Science Journal, 2011, 61(2), pp.180-190 , DOI:http://dx.doi.org/10.14429/dsj.61.477

Microstructural aspects of plain aluminide and Pt-aluminide coatings on Ti-base alloy IMI-834

One of the major limitations of the near-α titanium alloys such as IMI-834 and IMI-829 that is currently restricting their use at high temperatures is their poor oxidation resistance. Several protective coatings including diffusion aluminide coatings are currently being examined to enhance the oxidation performance of these alloys in the temperature range of 600–750 °C. In the present study, various microstructural aspects of plain aluminide and Pt-aluminide coatings on Ti-base alloy IMI-834 have been studied. Plain aluminide coating shows a single layer consisting of only Al 3Ti phase in the as-aluminized state. Pt-aluminide coating consists of three layers, namely an outer platinum-rich layer, an intermediate Al 3Ti layer, and the inner interdiffusion layer. The above structure of Pt-aluminide coating on IMI-834 alloy is very similar to that reported on Ni-base superalloys. It has further been found in the present study that the structure of Pt-aluminide coating depends on its Pt and Al contents. The nature of such dependence is similar to that reported for Pt-aluminide coatings on Ni-base superalloy substrates.

Microstructure of diffusion aluminide coatings on Ti-base alloy IMI-834 and their cyclic oxidation behaviour at 650 °C

The present study describes the microstructural details of plain pack aluminide and Pt-aluminide coatings on Ti-base alloy IMI-834 and their cyclic oxidation performance at 650 °C in air. While the plain aluminide coating consists of a single layer of Al 3Ti phase, the Pt-aluminide coating is constituted of two layers, with the outer layer consisting of PtAl 2 and Al 3Ti phases and the inner layer of Al 3Ti phase. These two coatings display excellent cyclic oxidation resistance on IMI-834 alloy although the Pt-aluminide coating is found to be somewhat superior to its plain aluminide counterpart. Virtually no coating degradation occurs even after 500 h of oxidation exposure at 650 °C. In this study, a layer of pure Pt deposited by electroplating is also found to offer limited protection against oxidation to the above alloy. TiO 2 is found as the primary oxide phase in the oxidation of bare IMI-834 alloy.

Isothermal Oxidation of Conventional and Pt Modified Aluminide Diffusion Coatings on a Ni Based Superalloy

The microstructure and microchemistry of two Pt modified and one plain aluminide diffusion coating (RT22, SS82A and PWA73) in the as-coated condition and after heat treatment at 1 050 °C for times up to 5 000 h have been investigated by means of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS). It was found that high concentrations of Pt in the outer zone of the coating causes precipitation of α-W by lowering the solubility of W in the β-NiAl matrix. Low concentrations of Pt (up to about 4 at%) may have a direct or indirect stabilising effect on β-NiAl on the Al-deficient side. The interdiffusion zone between the coating and the substrate was observed to grow during heat treatment, due to inward diffusion of Al. A model where the growth of the interdiffusion zone is connected to the inward flux of Al is proposed.

Development of oxidation resistant coatings for titanium alloys

This work addresses the development of a coating to protect the titanium alloy designated IMI834 from oxidation and alpha case formation. This is to enable the safe use of the alloy at relatively high operating temperatures and thereby significantly improve the efficiency of aeroengines. Two different types of surface modification techniques have been employed to coat the titanium alloy: electrodeposition and pack aluminising. Cyclic isothermal oxidation studies revealed that platinum aluminide coating obtained by electrodeposition followed by pack aluminising exhibits good oxidation resistance compared with that exhibited by a plain aluminide coating. Based on results obtained by different techniques, it is suggested that platinum aluminide is a prospective coating material for preventing alpha case formation and protecting against oxidation in components fabricated from titanium alloy IMI834.

Effect of aluminizing on the oxidation behavior of the titanium alloy, IMI 834

This article presents the development of a simple and cost-effective coating to protect the titanium alloy, IMI 834, from oxidation and alpha-case formation so that the alloy can be used safely, even at relatively high operating temperatures. A surface-modification technique, pack aluminizing, was employed to coat the alloy. Cyclic-isothermal oxidation studies at 800°C revealed that the plain aluminide coating obtained by pack aluminizing exhibits good oxidation resistance compared to the uncoated alloy. Based on the results obtained by different techniques, it is suggested that the plain aluminide is an economical, simple, and efficient coating for prevention of alpha-case formation and protection against oxidation of components fabricated from IMI 834.

Microstructural degradation of plain and platinum aluminide coatings on superalloy CM247 during isothermal oxidation

Isothermal oxidation at 1100°C of a high activity plain aluminide coating and a platinum aluminide coating, developed by the pack cementation technique, on cast nickel base superalloy CM247 has been carried out with the primary objective of systematically understanding the coating degradation process during oxidation. While the weight gains during oxidation for both plain aluminide and platinum aluminide coatings follow parabolic kinetics from the very beginning of oxidation exposure, the bare alloy was seen to exhibit a considerably long initial transient oxidation period (∼20 h), beyond which the parabolic law was followed. The parabolic rate constant for the platinum aluminide coating was found to be nearly two orders of magnitude lower than that for the plain aluminide coating. Alumina was identified as the only oxide phase that formed on both plain aluminide and platinum aluminide coatings during most of the oxidation exposure, although NiAl2 O4 was also found in the case of the plain aluminide coating beyond ∼200 h. The oxide layer on the bare alloy, however, was found to consist of Al2 O3 , Cr2 O3 , and NiAl2 O4 . The microstructural degradation of both the plain aluminide and platinum aluminide coatings during oxidation was seen to occur in three distinct stages which, however, differed for each coating. This stagewise degradation, which involves final obliteration of the interdiffusion layer in each case, is discussed in detail.

Effects of pretreatments on structure and oxidation behaviour of d.c.-sputtered platinum aluminide coatings

Pt aluminide coatings exhibit better high temperature oxidation and hot corrosion resistance than aluminide coatings. In this study the Pt layer was deposited by d.c. sputtering on Hastelloy-X superalloy instead of by an electroplating method. Laser remelting and diffusion heat treatment were applied to eliminate defects such as leaders existing in the as-deposited Pt layer and to effect the bonding between the Pt layer and the substrate before aluminization. The results of this study showed that the Pt aluminide coating pretreated by diffusion heat treatment and laser surface remelting consisted of a PtAl 2 + NiAl duplex-phase outer layer, an (Ni 2Pt)Al single-phase layer and an interdiffusion zone. The outer duplex-phase layer of the Pt aluminide coating pretreated by diffusion heat treatment consisted of a continuous PtAl 2 phase with NiAl as a second phase. In the laser-remelted Pt aluminide coating, by contrast, a minor PtAl 2 phase was embedded in NiAl. The results of oxidation tests at 1100°C showed that the diffusion-heat-treated Pt aluminide coating exhibited excellent oxidation resistance whereas the laser-remelted Pt aluminide coating failed to provide protection against high temperature oxidation. The latter is deemed to be inferior to a plain aluminide coating.