Work hardening, annealing, tempering and alloying are commonly used processes to modify the crystalline structure of a metal or the way the metal atoms interact with each other. Alloying and work hardening introduce obstacles within the metal structure, making it harder to deform. Tempering and annealing refine these obstacles.

The most common work hardening method is the application of force to compress the grains through cold rolling or stamping. The most common hot working or heat treatment methods include annealing, normalizing and tempering. 

Annealing increases ductility and reduces hardness. This makes the metal more ‘workable’. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a certain amount of time and then cooling. The cooling off non-ferrous metals is done by immersing the material in a quenching medium, usually an oil, for (rapid) cooling. The cooling of ferrous metals is done slower and more controlled in a furnace. Metals like wires, sheets, and tubes are often shaped through processes like rolling, drawing, or bending. These processes work-harden the metal, making it harder and less ductile (bendable). Annealing is used to soften the metal and restore its ductility, allowing for further shaping or fabrication without the risk of cracking.

Normalizing is the annealing process applied to ferrous alloys, but allowing the material to cool in air. Normalizing aims for a finer grain structure than annealing. This improves overall strength and uniformity of the metal. Machine gears require a balance of strength and wear resistance. Normalizing can provide this balance without excessive hardness that might make machining difficult

Tempering is used to increase the toughness (resistance to shattering) of ferrous alloys. Tempering is usually performed after hardening and is done by heating the metal to a temperature below the recrystallization point for some time and then allowing it to cool in still air. The exact temperature determines the amount of hardness removed and depends on both the specific composition of the alloy and the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

Alloying is the mixing of one metal with another metal or non-metal. The combination creates a material that has different properties from the original pure metal. Alloying is often used to increase hardness and strength. At the same time, it often reduces malleability and ductility. Common alloys are steel, bronze, and brass. We can see these materials used in many daily objects. Pure iron, for example, is quite soft. By alloying it with carbon, carbon steel can be produced which has much greater strength and thus is more useful for construction and large mechanical engineering applications.

Super alloys are designed to excel in harsh environments. They exhibit high degrees of mechanical strength, resistance to corrosion, and surface stability. Design criteria for such alloys could be:

• High-Temperature Strength: This is the most crucial factor. Superalloys need to retain significant mechanical strength even at temperatures exceeding 600°C (1112°F). This allows them to function in applications like jet engines and turbines.

  • Nickel is the most common base element for superalloys due to its inherent strength at high temperatures. Other elements like cobalt and tungsten can also be added to further enhance strength.

• Creep Resistance:  Creep is the tendency of a material to slowly deform under constant stress. At high temperatures, even small stresses can cause superalloys to creep over time. Superalloys are designed to resist creep by incorporating elements that form stable microstructures at high temperatures.

  • Elements like molybdenum, tantalum, and rhenium form stable carbides and intermetallic compounds that prevent grain boundaries from sliding and promote creep resistance.

• Oxidation Resistance:  Superalloys are often exposed to hot, oxidizing environments. They need to resist the formation of oxide layers that can weaken the material and degrade performance. 

  • Elements like chromium and aluminium are added to form a protective oxide layer that inhibits further oxidation.

• Low Density: In some applications, weight reduction might be a factor. Superalloys can be designed with lower-density elements like titanium or niobium to achieve a balance between strength and weight.

Superalloy development has relied heavily on both chemical and process innovations and are a good example of design requirements driving the development of materials. Certain elements used in superalloys might have environmental concerns though.