Superalloys are defined as "alloys developed for elevated temperature service where severe mechanical stressing is encountered and high surface stability is frequently required". There are three classes of alloys that meet this definition - cobalt-base, nickel-base and iron-base.
The driving force behind their development is the jet engine which requires ever-higher operating temperatures. Cobalt based superalloys are also used in several other applications including:
- gas turbines
- space vehicles
- rocket motors
- nuclear reactors
- power plants
- chemical equipment
The cobalt-base superalloys have their origins in the Stellite® alloys, designed for their wear resistance in the early 1900’s by Elwood Haynes.
Although in terms of properties the hardened nickel-based alloys ("Y" alloy) have taken the majority share of the superalloy market, cast and wrought cobalt alloys continue to be used because of the following characteristics:
- Higher melting points than nickel (or iron) alloys
- Superior hot corrosion resistance to gas turbine atmospheres
- Superior thermal fatigue resistance and weldability over nickel superalloys
Compared to nickel superalloys, the stress rupture curve for cobalt superalloys is flatter and shows lower strength up to 930°C. The greater stability of the carbides, which provide strengthening of cobalt superalloys, is then exhibited.
This factor is the primary reason cobalt superalloys are used in the lower stress, higher temperature stationary vanes for gas turbines.
Composition and Structure
Cobalt superalloys are termed austenitic in that the high temperature “Face Centred Cubic” phase is stabilised at room temperature.
They are hardened by carbide precipitation; carbon content is thus critical. Chromium provides hot corrosion resistance whilst other refractory metals are added to give solid solution strengthening including tungsten and molybdenum. Additional metals can also be incorporated for carbide formation such as tantalum, niobium, zirconium and hafnium.
Processing is vital and whilst the above metals are helpful, others such as oxygen can weaken the structure. Vacuum melting is therefore commonly used to give strict control over the elemental make-up of the superalloy. It is also critical that the specified compositions are adhered to, as excess soluble metals could form unwanted and deleterious phases.
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 has even further improved the properties by removal of possible failure sites.
Casting is important for cobalt-based alloys and directionally solidified alloys 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. These trends have allowed the development of higher thrust jet engines which operate at even higher temperatures.