Superalloys classification, properties and Applications

Superalloys 

 Superalloys are complex, high-performance alloys, which have a high tolerance of oxidising environments and high temperatures. They are typically classified according to their predominant matrix element; nickel, cobalt, or iron, and they contain multiple alloying elements including the refractory metal (Nb, Mo, W, Ta), chromium, and titanium. They exhibit high mechanical strength, creep resistance and corrosion resistance, especially at high temperatures.  These properties make them more challenging to produce and costlier than other alloys, but they are also critical for components in the aerospace industries. 

Properties and classification of superalloys

Superalloys are intended for use in high-temperature applications, which means they need to maintain their shape at elevated temperatures close to their melting points (above 650 °C or 1200°F). When alloyed with certain elements, at extreme temperatures superalloys can maintain high strength, stability, and corrosion and oxidation resistance.

Superalloys are classified into three main categories :- 

Nickel-based superalloys 

• High strength
• High thermal resistance
• High corrosion resistance
• Machinability
• Shape memory
• Low coefficient of thermal expansion

Cobalt-based superalloys

• Higher melting point compared to nickel- or iron-based alloys
• Superior hot corrosion resistance compared to nickel- or iron-based alloys
• Higher thermal fatigue resistance and weldability compared to nickel-based alloys.

Iron-based superalloys

• High strength at room temperature
• High resistance to creep, oxidation, corrosion.

The iron-based grades, which are less expensive than cobalt or nickel-based grades, are of three types: alloys that can be strengthened by a martensitic type of transformation, alloys that are austenitic and are strengthened by a sequence of hot and cold working (usually, forging at 2,000 to 2,100°F followed by finishing at 1,200 to 1,600°F), and austenitic alloys strengthened by precipitation hardening.

Some metallurgists consider the last group only as superalloys, the others being categorized as high-temperature, high-strength alloys. In general, the martensitic types are used at temperatures below 1,000°F; the austenitic types, above 1,000°F.

The AISI 600 series of superalloys consists of six subclasses of iron-based alloys:

• 601 through 604: Martensitic low-alloy steels.
  
• 610 through 613: Martensitic secondary hardening steels.
  
• 614 through 619: Martensitic chromium steels.
  
• 630 through 635: Semiaustenitic and martensitic precipitation-hardening stainless steels.
  
• 650 through 653: Austenitic steels strengthened by hot/cold work.
  
• 660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation.

Iron-based superalloys are characterized by high temperature as well as room-temperature strength and resistance to creep, oxidation, corrosion, and wear. Wear resistance increases with carbon content. Maximum wear resistance is obtained in alloys 611, 612, and 613, which are used in high-temperature aircraft bearings and machinery parts subjected to sliding contact. Oxidation resistance increases with chromium content. The martensitic chromium steels, particularly alloy 616, are used for steam-turbine blades.

The superalloys are available in all conventional mill forms -- billet, bar, sheet, and forgings -- and special shapes are available for most alloys. In general, austenitic alloys are more difficult to machine than martensitic types, which machine best in the annealed condition. Austenitic alloys are usually "gummy" in the solution-treated condition and machine best after being partially aged or fully hardened.

Crack sensitivity makes most of the martensitic steels difficult to weld by conventional methods. These alloys should be annealed or tempered prior to welding; even then, preheating and postheating are recommended. Welding drastically lowers the mechanical properties of alloys that depend on hot/cold work for strength.

All of the martensitic low-alloy steels machine satisfactorily and are readily fabricated by hot working and cold working. The martensitic secondary-hardening and chromium alloys are all hot worked by preheating and hot forging. Austenitic alloys are more difficult to forge than the martensitic grades.

Applications of Superalloys

Superalloys have many applications. These mainly include aircraft components, chemical plant equipment and petrochemical equipment. The below shows some of the applications of superalloys

 1. Aircraft gas turbine components :-  Disks, bolts, shafts, cases, blades. 

2. Nuclear power components :- Control-rod drive mechanisms, Valve stems , Springs, Ducting.

3.Chemical products :- Bolts, valves, Reaction vessels, Springs, Ducting.

4.Power plant components for steam turbines :- Bolts and blades 

5.Heat treating equipment :- Trays and fixtures , Conveyor belt furnaces.

Future Trends of Superalloys

The demand for superalloys is continuously increasing, primarily driven by the aerospace industry. One challenge is the high cost of production for unique and complex parts. This is partly being met by the printing of complex parts using additive manufacturing.

Another interesting focus of superalloy research is the synthesis of nanoparticles. This has been performed via the radiolysis process, a radiation method in which the molecular structure of substances is broken down to form nanoparticles. This approach is a flexible and versatile method to manufacture large quantities of superalloy nanoparticles that can't be easily created by other methods.

Sources 

[1] Donachie, M.J. and Donachie, S.J., 2002, ASM International, Superalloys: A Technical Guide, 2nd Edition, Materials Park, Ohio, USA

[2] El-Bagoury, N. 2016, “Ni Based superalloys : Casting Technology, metallurgy, Development, properties And Applications ”, International Journal of Engineering Sciences and Research Technology, Vol. 5, No. 108.

[3] Encyclopedia Britannica, “Superalloy”, https://www.britannica.com/technology/superalloy.

[4] Nickel Institute, “Nickel based alloys”, https://www.nickelinstitute.org/about-nickel/nickel-alloys.

[5] Cobalt Institute, “Superalloys”, [Online] https://www.cobaltinstitute.org/superalloys.html.

[6] Kracke, A., 2010, "Superalloys, The most successfull system of modern time - time, present and future", 7th International Symposium on Superalloy 718 and Derivatives, TMS.

[7] Reed, R.C., 2008, The Superalloys, Fundamentals and Applications, Cambridge University Press

[8] Materials Technology TMS, “Processing of Superalloys”, [Online]