JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year HIGH TEMPERATURE MATERIALS Previous Question Paper (July/August - 2022 - R18)

Creep is the time-dependent, permanent deformation of a material subjected to a constant load or stress at elevated temperatures, typically above 0.4 times the absolute melting temperature of the material. The mechanism occurs in three distinct stages:

1. Primary (Transient) Creep: The strain rate is initially high but decreases over time due to strain hardening as dislocations multiply and interact.
2. Secondary (Steady-State) Creep: A balance is achieved between strain hardening and recovery processes (such as dislocation climb and cross-slip), resulting in a constant, minimum creep rate. This is the most critical stage for engineering design.
3. Tertiary Creep: The creep rate accelerates rapidly due to the formation of internal voids, micro-cracks, and necking, eventually leading to rupture.

The fundamental mechanisms driving these stages include dislocation climb (assisted by vacancy diffusion), grain boundary sliding, and Nabarro-Herring or Coble diffusion creep, where atoms migrate through the lattice or along grain boundaries under stress.

Properties required for creep-resistant materials include:

• High melting point to minimize thermal activation of diffusion.
• High elastic modulus and stable microstructure at service temperatures.
• Low stacking fault energy to inhibit dislocation cross-slip.
• Large grain size or single-crystal structures to reduce grain boundary sliding.
• Presence of fine, stable precipitates (dispersion strengthening) to pin dislocations and prevent their movement.

Examples of creep-resistant materials include Nickel-based superalloys (e.g., Inconel, Nimonic), Cobalt-based alloys, Refractory metals (e.g., Tungsten, Molybdenum), and Ceramics like Silicon Carbide or Silicon Nitride.