Creep is the tendency of a solid material to slowly deform permanently under the influence - PowerPoint Presentation

1. Creep is the tendency of a solid material to slowly deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature.The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade.

2. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. Moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.

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6. CreepDislocation relatedDiffusionalGrain boundary slidingCreep Mechanisms of crystalline materialsNabarro-Herring creepCoble creepLattice diffusion controlledGrain boundary diffusion controlledDislocation core diffusion creepClimbCross-slipGlideDiffusion rate through core of edge dislocation moreInterface-reaction controlled diffusional flowAccompanying mechanisms: creep with dynamic recrystallization

7. Cross-slipThis kind of creep is observed at relatively low temperatures. Herein screw dislocations cross-slip by thermal activation and give rise to plastic strain as a function of time.Dislocation related mechanismsDislocation climbEdge dislocations piled up against an obstacle can climb to another slip plane and cause plastic deformation. In response to stress this gives rise to strain as a function of time. It is to be noted that at low temperatures these dislocations (being pinned) are sessile and become glissile only at high temperatures. Rate controlling step is the diffusion of vacancies.Two roles can be differentiated with respect to of dislocations activity: (i) it is the primary source of strain, (ii) it plays a secondary role to accommodate local strain (while the major source of strain is another mechanism (e.g. grain boundary sliding).

8. Diffusional creepIn response to the applied stress vacancies preferentially move from surfaces/interfaces (GB) of specimen transverse to the stress axis to surfaces/interfaces parallel to the stress axis→ thus causing elongation.Diffusion of vacancies in one direction can be thought of as flow of matter in the opposite direction.This process like dislocation creep (involving climb) is controlled by the diffusion of vacancies (but diffusional creep does not require dislocations to operate).The diffusion could occur predominantly via the lattice (at high temperatures) or via grain boundaries (at low temperatures). The former is known as Nabarro-Herring creep, while the later is known as Coble creep. Diffusion through edge dislocation cores (pipe diffusion) could play an important role in creep.Flow of vacanciesCoble creep → low T → Due to GB diffusion Nabarro-Herring creep → high T → lattice diffusion

9. Grain boundary slidingAt low temperatures the grain boundaries are ‘stronger’ than the crystal interior and impede the motion of dislocations.Being a higher energy region, the grain boundaries may pre-melt before the crystal interior.Above the equicohesive temperature, due to shear stress at the ‘local scale’, grain boundaries slide past one another to cause plastic deformation. The relative motion of grain boundaries can lead to wedge cracks at triple lines (junction of three grains). If these wedge cracks are not healed by diffusion (or slip), microstructural damage will accumulate and will lead to failure of the specimen.GrainsWedge crack due to grain boundary sliding

10. The Creep Test: a typical creep curve showing the strain produced as a function of time for a constant stress and temperature.Apply stress to a material at an elevated temperatureCreep: Plastic deformation at high temperature

11. The Creep Test:

12. Creep Failure• Failure: along grain boundaries.appliedstressg.b. cavities12

13. Creep Resistant MaterialsThe is a growing need for materials to operate at high temperatures (and in some applications for long times). For example, higher operating temperatures gives better efficiency for a heat engine. Hence, there is a need to design materials which can withstand high temperatures.It is to be noted that material should also be good in other properties for high temperature applications (like it should possess good oxidation resistance). Factors like cost, ease of fabrication, density, etc. play an important role in determining the final choice of a material.Some of the material design strategies, which work at low temperature are not useful at high temperatures (e.g. work hardening, precipitation hardening with precipitates which coarsen, grain size reduction, etc.).Some strategies which work are: (i) having grain boundaries aligned along the primary loading axis, (ii) produce single crystal components (like turbine blades), (iii) use precipitates with low interfacial energy for strengthen (which will not coarsen easily), (iv) use dispersoids for strengthening.Creep resistanceDispersion hardening → ThO2 dispersed Ni (~0.9 Tm)Solid solution strengtheningHigh melting point → E.g. CeramicsSingle crystal / aligned (oriented) grains

14. Commonly used materials → Fe, Ni (including superalloys), Co base alloys.Precipitation hardening involving ‘usual precipitates*’ is not a good method as precipitates coarsen (smaller particles dissolve and larger particles grow  interparticle separation ↑ thus lowering the strength)Ni-base superalloys have Ni3(Ti,Al) precipitates, which form a low energy interface with the matrix. This reduces the driving force for coarsening. (Note: other phenomena like rafting may lead to the deterioration of the properties of such materials).Cold work cannot be used for increasing creep resistance, as recrystallization can occur which will produced strain free crystals.Fine grain size is not desirable for creep resistance (this is contrary to what is usually practiced for increasing the low temperature strength)→ grain boundary sliding can cause creep elongation/cavitation. Hence, the following two strategies can be used:► Use single crystals (single crystal Ti turbine blades in gas turbine engine have been used though they are very costly).► Aligned/oriented polycrystals → as all the grain boundaries are aligned along the primary tensile axis, they experience no shear stress and creep is negated.Creep Resistant Materials, cotd..* Which coarsen at high temperatures due to high interfacial energy.

15. 15 Evaluation of Creep BehaviorCreep test - Measures the resistance of a material to deformation and failure when subjected to a static load below the yield strength at an elevated temperature.Climb - Movement of a dislocation perpendicular to its slip plane by the diffusion of atoms to or from the dislocation line.Creep rate - The rate at which a material deforms when a stress is applied at a high temperature.Rupture time - The time required for a specimen to fail by creep at a particular temperature and stress.