Nickel supperalloys

The driving force for the development of Ni-based superalloys originated from the construction of gas turbines for aircraft and power-generating stations. In order to increase the efficiency of these machines, the combustion temperature must be raised. For this application, the components must simultaneously withstand high temperatures and high mechanical stress in sustained operation. However, these materials have also become firmly established in the manufacture of hot-forming tools for some applications, for instance, in extruding presses for heavy metals, especially extrusion dies, where operating temperatures up to 1000 °C can be attained. Commercially available Ni superalloys offer decided advantages, both technological and economical, in comparison with conventional hot-working steels.

However, optimal properties are achieved only by an appropriate solution heat treatment and subsequent artificial ageing. These heat treatments are of paramount importance for attaining the maximal hardness and strength of the material.

The excellent fatigue behaviour of the nickel alloys is affected by five essential parameters:

• Solid-solution hardening
• Precipitation hardening
• Carbide hardening
• Microstructure
• Trace elements

An exact mutual adjustment of these parameters is a prerequisite for achieving the intended properties for a particular application.

Especially the elements Mo, W, Cr, and Co are involved in solid-solution hardening; after addition to the nickel-rich solid solution, these elements hinder the motion of the dislocations by distortion of the atomic lattice. At high temperatures above 0.6 TS, the high-temperature strength properties, especially the time-dependent creep, are controlled predominantly by diffusion. For this case, the diffusion-inert elements Mo and W are especially well suited for solid-solution hardening. For this purpose, Mo is preferred because of its lower atomic mass.

The addition of Fe to the alloy results from purely economic considerations; that is, the alloys can be produced more economically if iron is present. However, the resistance to oxidation is thus lowered, and the σ-phase, which impairs the properties, is also formed. A considerable increase in the creep strength of Ni superalloys is achieved by precipitation hardening. With the addition of the elements Ti, Al, and Nb, the finely distributed intermetallic γ’ phase Ni₃(Al, Ti) can be precipitated coherently from a supersaturated solid solution by an appropriate annealing treatment. Above 4 per cent (mass content), Nb forms the intermetallic γ’’ phase Ni₃Nb. Below this limit, Nb is substituted for Al and Ti in the γ’ phase. The hindrance to the motion of dislocations in a matrix with finely distributed precipitates is most pronounced if the grain size of the precipitates is between 20 and 50 nm, /Decker72/. At a carbon content between 0.05 and 0.2 per cent (mass contents), various carbide types, such as MC, M₆C, and M₂₃C₆, are already formed during solidification, and during the heat treatment at the latest; these carbides depend on the composition (table 1). Small, globular, non-coherent carbides, especially the primary precipitated carbides of types MC and M₆C, are well suited for stabilising the grain boundaries. Because of the high Cr content of the Ni superalloys, the formation of M₂₃C₆ carbides is unavoidable. Furthermore, carbides of types MC and M₆C tend to transform to carbide of the M₂₃C₆ type during sustained annealing. This carbide has a tendency to form coherent grain-boundary precipitates and thus increases the susceptibility toward brittle fracture. Hafnium is a grain-boundary-active element and prevents brittle fracture by the early formation of carbides which are highly stable and finely distributed in the structure. Boron and zirconium segregate at the grain boundaries because of the great difference between their atomic diameters and that of nickel. Consequently, they occupy vacant sites there and thus hinder the diffusion of other elements. As a result, not only the sliding of grains is hindered; the creation of γ’-depleted grain-boundary edges as well as continuous carbide films are also prevented in this manner.

Protection against high-temperature corrosion is provided by Al and Cr, which form firmly bonded oxide films. The protective action of Al₂O₃ is superior to that of Cr₂O₃. Up to about 1000 °C, Cr₂O₃ is insoluble; above 1100 °C, however, Al₂O₃ alone must assume the protective function. The ratios of the alloying elements employed in Ni superalloys are summarised in figure 1.

Besides the three classes of γ-, γ’-stabilising and grain-boundary-active elements, two subclasses must be distinguished: carbide-forming elements and those which form a protective film. The strength is enhanced by the microstructures located in the interior of the grains and at the grain boundaries; these microstructures in turn depend on the grain geometry, of course. However, it is important to realise that the mechanical properties also depend on the grain geometry, especially at high temperature. The creep strength is affected by strain processes at the grain boundaries. With a coarse-grained structure, considerably fewer possibilities are available for sliding processes than with a fine-grained structure. With an appropriate increase in grain size, the creep rupture strength can be substantially increased, especially at very high temperatures.

Figure 1: Metallurgical effects of the main alloying elements of Ni superalloys