News
17
August 2015
Strengthening of Superalloys
Metals are crystalline in nature and in their solid form, the atoms of alloys or metals exhibit different crystallographic structures, usually cubic structures. Some crystal structures are linked with enhanced properties as compare to others. Moreover, crystalline group of atoms maintain orientation relationships. Consistent crystalline clusters are known as grains, and in an alloy, there are several grains with irregular orientations. Alloys with several random grain directions are called as polycrystals that have almost equal size in all directions, however columnar shaped grains are common in cast products.
The peripheral surface of a grain is known as grain boundary. Clusters of atoms without grain boundaries are distinct. But, metals without grain boundaries or with straight boundaries like in columnar grained structure are made in superalloys using suitable production methods. The introduction of various atom kinds, new crystal phases and/ or modifications of grain boundaries allow prevention of movement across crystal lattice or grains of imperfections that results into deformation. Superalloys hence are the fundamental metals improved by changes in chemical composition,in a polycrystalline form with different phases present in grains or grain boundaries.
The superalloys comprise of austenitic face centered cubic crystal structure matrix phase, gamma and different secondary phases. Essential secondary phases are gamma fcc arranged Ni3(Al, Ti) and different MC, M23C6, M6C and M7C3 carbides in nickel and nickel-iron based superalloys where M shows different metal elements like titanium zirconium, hafnium,niobium, tungsten, molybdenum and chromium. The carbides are the principal secondary phases in alloys of cobalt. Gamma phase in tetragonal (Ni3Nb), n phase of hexagonal (Ni3Ti) and delta phase of orthorhombic (Ni3Nb) are observed in nickel and nickel-iron based superalloys.
It is crucial for an engineer who is responsible for choosing alloys to possess a deep knowledge of strengthening mechanism of superalloys since their properties can be enhanced significantly through processing to modify the resulting strengthening level. The Superalloys achieve their strength from solid-solution hardeners and from secondary precipitate phases that produce in the gamma matrix and result into precipitation hardening. The prime strengthening precipitate phases in nickel based superalloys and Nickel-iron based superalloys are gamma’ and gamma”. Carbides may provide limited strengthening directly (dispersion hardening) or usually indirectly(stabilizing grain boundaries against motion). Delta and n phases are critical in controlling grain structure of wrought superalloys while wrought processing subsequent to melting. By controlling the grain structure, strength can be essentially impacted. The degree of role of secondary phase on strengthening is based on alloy and its treatment. Remember that inadequate distribution of carbides and precipitate phases can be damaging to properties of alloys.
Besides of those elements that produce solid solution hardening and/or enhance carbide and gamma’ production, other elements like boron, zirconium, hafnium are included to improve chemical and mechanical characteristics of alloys. The superalloy microstructure and characteristic control can be complicated. Maximum 14 elements can be controlled in few superalloys. Elements causing production of carbide and gamma precipitates may enhance corrosion strength considerably. Following table provides a standard list of elements wt% in superalloys.
| Element | Range % | |
| Nickel-Iron base and Nickel based | Cobalt based | |
| Chromium | 5 to 25 | 19 to 30 |
| Molybdenum, tungsten | 0 to 12 | 0 to 11 |
| Aluminum | 0 to 6 | 0 to 4.5 |
| Titanium | 0 to 6 | 0 to 4 |
| Cobalt | 0 to 20 | - |
| Nickel | - | 0 to 22 |
| Niobium | 0 to 5 | 0 to 4 |
| Tantalum | 0 to 12 | 0 to 9 |
| Rhenium | 0 to 6 | 0 to 2 |
