A steel structural component is operating at 750°C (approximately 0.6 Tm) under a constant stress that is 40% of its room-temperature yield strength. After 8,000 hours, the component has permanently elongated by 1.2%. What phenomenon explains this?
AElastic deformation — the component is below yield strength, so any strain must be recoverable
BCreep — thermally activated vacancy diffusion and dislocation climb enable permanent strain accumulation at stresses far below the room-temperature yield strength
CFatigue — cyclic thermal expansion and contraction causes progressive damage even under constant mechanical load
DStrain hardening — dislocation multiplication under sustained load eventually produces permanent deformation
Creep is permanent, time-dependent deformation occurring at elevated temperature (above ~0.3–0.4 Tm) even when stress is below the room-temperature yield strength. At 0.6 Tm, thermal energy activates vacancy diffusion and dislocation climb — mechanisms that are negligible at room temperature — allowing dislocations to bypass obstacles continuously. The room-temperature yield criterion simply does not apply at elevated temperature over long timescales. Elastic deformation is recoverable by definition; fatigue involves cyclic loading; strain hardening increases yield strength, it doesn't produce this kind of slow ongoing elongation.
Question 2 Multiple Choice
Two identical metal components are tested for creep resistance at the same stress and temperature (both in the diffusion creep regime). One has fine grain size, the other has coarse grain size. Which shows higher creep rate, and why?
AThe fine-grained specimen — smaller grains mean shorter diffusion distances for vacancies, and more grain boundaries provide fast diffusion paths
BThe coarse-grained specimen — larger grains have more internal volume where vacancies can accumulate
CBoth creep at the same rate — grain size only affects room-temperature yield strength, not elevated-temperature creep
DThe fine-grained specimen — small grains distribute stress more evenly, increasing the driving force for diffusion
Diffusion creep operates through directional vacancy migration along stress gradients. Grain boundaries have significantly enhanced atomic diffusivity compared to grain interiors (Coble creep). A fine-grained material has more grain boundary area per unit volume and shorter diffusion paths — both of which accelerate diffusion creep. This is why fine-grained materials are actually *worse* for creep resistance in the diffusion creep regime, and why single-crystal turbine blades (zero grain boundaries) are used in the hottest engine stages.
Question 3 True / False
The secondary (steady-state) stage of creep is the most dangerous stage because the strain rate is highest during this period.
TTrue
FFalse
Answer: False
Secondary (steady-state) creep has the *minimum* strain rate — the rates of work hardening and thermal recovery are in balance. It is the longest-lasting stage and is the primary design concern because it determines how much the component deforms over service life. Tertiary creep has an accelerating strain rate due to void nucleation and microcrack coalescence, and it leads to rupture. Secondary creep is 'dangerous' in the sense that it determines life, but the strain rate is not at its maximum — that occurs in tertiary creep just before fracture.
Question 4 True / False
Single-crystal nickel superalloy turbine blades are designed to eliminate grain boundaries specifically to suppress grain boundary sliding and diffusion creep at high operating temperatures.
TTrue
FFalse
Answer: True
Grain boundaries are pathways for enhanced atomic diffusion (Coble creep) and sites for grain boundary sliding — both of which contribute to creep deformation and crack initiation at high temperature. By growing the blade as a single crystal, these mechanisms are eliminated entirely, dramatically extending creep life at turbine inlet temperatures that would cause rapid failure in polycrystalline blades. This is why single-crystal casting is one of the key materials engineering innovations that enables modern high-bypass turbofan engines.
Question 5 Short Answer
Explain why a component can fail by creep at a stress well below its room-temperature yield strength, and what material property determines the temperature threshold above which creep becomes significant.
Think about your answer, then reveal below.
Model answer: At room temperature, dislocation motion requires stress to exceed the yield strength because dislocations cannot bypass obstacles by glide alone. At elevated temperature, thermal energy activates vacancy diffusion, enabling dislocations to climb over obstacles perpendicular to the slip plane — a process that accumulates permanent strain continuously even at low stresses. The relevant material property is the absolute melting temperature Tm (in Kelvin): creep becomes significant above approximately 0.3–0.4 Tm. Materials with high Tm (e.g., tungsten at 3695 K, nickel at 1728 K) have creep thresholds at higher absolute temperatures, which is why refractory metals and nickel superalloys are used in high-temperature applications.
The key insight is that yield strength is a rate-independent property measured at room temperature and short timescales. Creep is inherently time- and temperature-dependent: the same stress that causes no measurable deformation in one second can cause 1% strain over 10,000 hours at high enough temperature. Homologous temperature (T/Tm) is the unifying variable that determines whether a material is in the creep regime, regardless of absolute temperature.