Jawaharlal Nehru Technological University Hyderabad B.Tech IV SUPER ALLOYS Previous Question Paper (December - 2020 - R16)

The aerospace industry is the largest consumer of superalloys. This dominance is due to the extreme operating conditions found in gas turbine engines, specifically in the high-pressure turbine section.

Superalloys are essential here because they maintain high mechanical strength, creep resistance, and oxidation resistance at temperatures approaching their melting points. Components such as turbine blades, vanes, and disks must withstand high centrifugal stresses and thermal fatigue while operating in corrosive combustion environments. No other class of materials provides the necessary combination of high-temperature structural integrity and surface stability required for modern jet propulsion efficiency.

The processing route for superalloys is determined by their microstructural stability, alloy chemistry, and intended application. Fe-Ni base superalloys are typically wrought because they rely on solid solution strengthening and the precipitation of the gamma-prime phase, which is generally less complex than in Ni-base alloys. Their chemical composition is designed to be highly ductile at hot-working temperatures, making them amenable to rolling, forging, and extrusion processes to produce uniform, fine-grained microstructures that enhance fatigue life.

In contrast, Ni-base and Co-base superalloys are often designed with high concentrations of refractory elements (like W, Ta, Re) and carbon to facilitate carbide formation and extreme creep resistance. These additions significantly reduce hot ductility, making them difficult to work plastically. Consequently, they are frequently cast into complex shapes, such as single-crystal or directionally solidified turbine blades, to eliminate grain boundaries perpendicular to the stress axis. While some Ni-base alloys are wrought for disks, the high-performance requirements of stationary or rotating components in the hottest engine zones necessitate casting techniques to achieve the desired coarse-grained or single-crystal architectures that cannot be produced through deformation processing.