Ordered intermetallic alloys have been known for a very long time, but their impact on the metallurgical community has been relatively unremarkable, however that is now changing. The principal reason for this lack of attention has been the often immeasurably low ductility at ambient temperatures, and the very great difficulty of manufacturing anything useful from them. Their greatest impact has been in precipitates in many age hardening alloys, (the best known example probably being γ’ precipitates in nickel based alloys), and as the cause of low ductility in numerous metallurgical systems, for example the σ phase in stainless steels.
The main properties of interest for these materials are:
• High temperature strength
• High temperature corrosion resistance
However, intermetallic alloys also possess many remarkable properties. Their high temperature strength is often very attractive (some alloys even get stronger as they get hotter over certain temperature ranges), and they frequently exhibit very good resistance to high temperature corrosion, due to the formation of A1203 or Si02 rich oxide films. These properties have been the driving force behind an increasing research effort in many countries to develop alloys based on intermetallic compounds which have mechanical properties attractive to the engineering industries.
Considerable success has now been achieved, and many alloys are now emerging. At the forefront are the titanium aluminides, based on TiAl, which are of intense interest to the gas turbine and aircraft engine industries, where their high temperature properties and low density offer prospects for tremendous weight savings. However, alloys based on iron and nickel aluminides are also developing rapidly, and it is these alloys which may be expected to be of great interest in corrosive environments. Nickel aluminides, based on Ni3Al or NiAl, have considerable potential for hot corrosive environments containing chlorine, and their use has been suggested for example in waste incinerators. Iron aluminides, based on Fe3Al or FeAl can perform well in sulphur bearing atmospheres, and are therefore attractive in several power generation and petrochemical applications.
At present it is not feasible to make large components from these alloys, although small components can be cast or extruded from the more ductile alloys. For large structures the alloys are more likely to make an impact as a corrosion resistant cladding, which will avoid the considerable technical challenges which remain in fabricating components.
TWI (UK) have run trials to investigate the feasibility of using iron and nickel aluminides as a surfacing medium. Results have demonstrated considerable success, although it is recognised that a substantial research effort is required to develop the technologies to a point where industry could benefit from the coatings. Initial trials have concentrated on high velocity oxy-fuel (HVOF) spraying, friction surfacing with powders and plasma transferred arc cladding. These are three very different processes, but they all require the coating (ie the intermetallic alloy) to be in a powder form. At present powder of various compositions and morphologies is relatively easy to manufacture and procure.
HVOF is particularly attractive for coating large areas with relatively thin coatings. It is possible to obtain very high densities in the coating, but as with all spraying processes full density is virtually impossible to obtain. Some oxidation of the coating is also inevitable. Adhesion between the coating and the substrate can also be problematical in some systems if the coefficients of thermal expansion differ too widely; HVOF performs better in this respect than some other spraying processes, due to the high impact velocities.
A brief investigation has indicated that HVOF coatings of intermetallic alloys can be remelted by several processes, eg TIG and electron beam, and this leads to fully dense, oxide-free coatings which are fully bonded to the substrate. Careful control of the remelting process can also limit dilution effects, although dilution is not seen as a major problem when the substrate is steel.
Friction surfacing using powders as a consumable is a new process being developed at TWI. It has considerable potential for a number of niche markets and, since it uses materials in powder form, can also be used to deposit composite coatings. It has been demonstrated that intermetallic alloys can be deposited onto steel substrates very successfully using this process. Although the alloys may have restricted ductilities at ambient temperatures, they are in general quite ductile at the high temperatures at which a friction surfacing layer is deposited.