Aluminide surfaces

The main protective method against atmospheric catastrophic attack is surface coatings of silicides, and aluminides. ... 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

There are two important titanium aluminides Tig A1 which has a hexagonal structure with a density of 4.20 g/cm and a melting point of 1600°C and Ti A1 which has a tetragonal structure with a density of 3.91 g/cm and a melting point of 1445°C. As do all aluminides, they have excellent high temperature oxidation resistance owing to the formation of a thin alumina layer on the surface. They have potential applications in aerospace structures. 

Diffusion aluminide and silicide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 106/yr in North America and about 50 x 106 worldwide. 

It is instructive to review order/segregation interplay in specific alloys in view of the diverse predictions furnished by the model calculations of the previous section. We have chosen to focus on the well-studied Cu3Au(100) surface, and to assess comparatively the segregation/order interplay in a large number of equiatomic aluminides. Pertinent theoretical and/or experimental recent findings for CusPd, Pt3Sn and Co3Pt are addressed too. 

Fig. 15. FCEM calculated variations with temperature in the average A1 concentration at the (110) siuface of bcc aluminides (B2 structure - solid lines, B32 structure - dashed lines). 1 -ScAl, RuAl, RhAl, NiAl, CoAl, 2 - FeAl, 3 - CrAl, 4 - MoAl, 5 - TcAl. The surface concentrations were calculated in accordance with data of Table 3.

The large difference in stability between NiO and A1203 makes nearly all Al-Ni alloy compositions in equilibrium with A1203, as shown by the wide A1203 stability region in Fig. 10, even though numerous very stable nickel aluminides can form. Once again, a balance between the alloy liquidus surface composition, matrix formation rate, and the diffusion of transition metal away from the reaction front must be maintained. Typically the presence of a reducible compound in the preform, especially if it is the sole constituent, results in a further refinement of the composite microstructure. 

A number of studies have introduced the reactive element into the aluminides by ion implantation. The author considers this approach to be of limited value since, as discussed by Pint and Hobbs [27], the high local concentration of reactive element can stabilize the transition aluminas and, at high temperatures, has a short-lived effect. More importantly, from a fundamental standpoint, the implantation process can have a profound effect on the nature of the exposed surface. For example, Schumann [28] has shown that Y implantation into single-crystal NiAl results in a 45 nm thick, finegrained crystalline region which is disordered. 

There has been relatively little work published on the reaction of titanium aluminides in atmospheres other than air or oxygen. Niu et al. [96] studied the reaction of Ti-25Al-llNb in a simulated combustion atmosphere (N2+1%02+ 0.5%SO2) with and without surface deposits of Na2S04-t- NaCl at temperatures between 600 and 800°C. Exposures in the absence of surface deposits resulted in reaction rates similar to those described above for simple oxidation. The rates in the presence of the deposits at 600 and 700 °C were initially rapid and then slowed markedly after 25 to 50 hours exposure. The rate at 800°C remained rapid with the kinetics being essentially linear. The major difference in the corrosion morphology at 800 °C was the presence of copious amounts of sulfides below the oxide scales. The authors postulate a mechanism of attack involving a combination of sulfidation-oxidation and scale-fluxing. 

Several interesting features were apparent when the aluminide specimens were observed in plan after exposure at 700°C (Figure 3). In all cases, there was little evidence for loss of scale by spallation. However, second-phase particles were observed on all the surfaces, particularly of the lower aluminium-containing alloys, FA 56 and FA 57, while the number and size of such particles for a given alloy were less for preoxidized specimens than for those exposed directly in the mixed gas. Detailed examination and EDX analysis of the particles (e. g. labelled, C, A and B respectively in Figures 3(c), 3(d) and 3(f)) indicated them to be rich in iron and sulphur although too small for precise analysis, they were undoubtedly sulphide nodules. These nodules were always small and discrete, with no evidence that extensive sulphidation of the substrate had... 

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

Figure 10.20 Scanning electron micrographs showing the surface of a Pt-modified aluminide coating on a nickel-base single-crystal superalloy after 1000 one-hour cycles at 1100 °C showing severe wrinkling of the coating.

Austenitic Ni-Cr-Al aUoys, and aluminide and MCrAlY (M = Ni, Co, or Fe) coatings, which form AI2O3 surface layers, are used to about 1100 °C. 

---