Superalloys are simply defined as "alloys developed for elevated temperature service, usually based on group VIIA elements, where relatively severe mechanical stressing is encountered and high surface stability is frequently required". Three classes of alloys have appeared - cobalt-base, nickel-base, iron-base - to meet this definition.
The driving force behind their development has been the jet engine which has required ever higher operating temperatures. The use of the alloys has, however, extended into many other fields - all types of turbines, space vehicles, rocket motors, nuclear reactors, power plants, chemical equipment and possibly 20% of alloys have arisen for corrosion resistant applications.
Nickel-based alloys which form the bulk of alloys produced, are basically nickel-chrome alloys with a fcc solid solution matrix containing carbides and the coherent intermetallic precipitate Y(Ni3(Al1Ti). This latter precipitate provides most of the alloy strengthening and results in useful operating temperatures up to 90% of the start of melting. Further additions of aluminium, titanium, niobium and tantalum are made to combine with nickel in the Y' phase and of molybdenum, tungsten and chromium which strengthen the solid solution matrix.
The role of cobalt is not completely understood but it certainly increases the useful temperature range of nickel-based alloys. Y’ also occurs as Y’’ which has a body centred tetragonal structure (i.e. two cubes stacked). Cobalt is thought to raise the melting point of this phase thus enhancing high temperature strength.
As well as structure, processing has been responsible for enhancement of these alloys.
The cobalt-base superalloys have their origins in the Stellite® alloys patented in the early 1900’s by Elwood Haynes.
Although in terms of properties the (Y’) hardened nickel-based alloys have taken the lion’s share of the superalloy market, cast and wrought cobalt alloys continue to be used because:
Cobalt alloys have higher melting points than nickel (or iron) alloys. This gives them the ability to absorb stress to a higher absolute temperature.
Cobalt alloys give superior hot corrosion resistance to gas turbine atmospheres, this is due to their high chromium content.
Cobalt alloys show superior thermal fatigue resistance and weldability over nickel alloys.
Composition and Structure
Cobalt alloys are termed austenitic in that the high temperature “Face Centred Cubic” phase is stabilised at room temperature.
Processing is of course vital and whilst the above metals are helpful, others such as dissolved oxygen are not. Vacuum melting is therefore becoming the norm to give close alloy control. It is also critical that the specified compositions are adhered to, as excess of the soluble metals, W, Mo, Cr, will tend to form unwanted and deleterious phases similar to the nickel alloys s and Laves (Co3Ti – tetragonal close packed TCP phases).
They are hardened by carbide precipitation, thus carbon content is critical. Chromium provides hot corrosion resistance and other refractory metals are added to give solid solution strengthening – tungsten and molybdenum – and carbide formation – tantalum, niobium, zirconium, hafnium.
Powder metallurgical alloys, giving a finer carbide dispersion and smaller grain size have superior properties to cast alloys.
Further process development by hot isostatic pressing (HIP) has even further improved the properties by removal of possible failure sites.
Compared to nickel alloys, the stress rupture curve for cobalt alloys is flatter and shows lower strength up to about 930°C. The greater stability of the carbides, which provide strengthening of cobalt alloys, then asserts itself.
This factor is the primary reason cobalt alloys are used in the lower stress, higher temperature stationary vanes for gas turbines.
Casting is important for cobalt-based alloys and directionally solidified alloys (DS) have led to increased rupture strength and thermal fatigue resistance.
Even further improvements in strength and temperature resistance have been achieved by the development of single crystal alloys. Both these trends have allowed the development of higher thrust jet engines which operate at even higher temperatures.