CREEP CREEP. Mechanical Metallurgy George E Dieter McGraw-Hill Book Company, London (1988)

Transcription

1 CREEP CREEP Mechanical Metallurgy George E Dieter McGraw-Hill Book Company, London (1988)

2 Review If failure is considered as change in desired performance*- which could involve changes in properties and/or shape; then failure can occur by many mechanisms as below. Mechanisms / Methods by which a can Material can FAIL Elastic deformation Plastic deformation Twinning Slip Fracture Fatigue Creep Twinning Phase transformations Grain growth Microstructural changes Chemical / Electro-chemical degradation Corrosion Oxidation Wear Erosion Physical degradation Particle coarsening * Beyond a certain limit

3 Review Though plasticity by slip is the most important mechanism of plastic deformation, there are other mechanisms as well (plastic deformation here means permanent deformation in the absence of external constraints): Plastic Deformation in Crystalline Materials Slip (Dislocation motion) Twinning Phase Transformation Creep Mechanisms + Other Mechanisms Grain rotation Grain boundary sliding Vacancy diffusion Dislocation climb Note: Plastic deformation in amorphous materials occur by other mechanisms including flow (~viscous fluid) and shear banding

4 High-temperature behaviour of materials Designing materials for high temperature applications is one of the most challenging tasks for a material scientist. Various thermodynamic and kinetic factors tend to deteriorate the desirable microstructure. (kinetics of processes are an exponential function of temperature). Strength decreases and material damage (void formation, creep oxidation ) tends to accumulate. Cycling between high and low temperature will cause thermal fatigue.

5 High temperature effects (many of the effects described below are coupled) Increased vacancy concentration at high temperatures more vacancies are thermodynamically stabilized. Thermal expansion material will expand and in multiphase materials/hybrids thermal stresses will develop due to differential thermal expansion of the components. High diffusion rate diffusion controlled processes become important. Phase transformations can occur this not only can give rise to undesirable microstructure, but lead to generation of internal stresses. Precipitates may dissolve. Grain related: Grain boundary weakening may lead to grain boundary sliding and wedge cracking. Grain boundary migration Recrystallization / grain growth decrease in strength Dislocation related these factors will lead to decrease in strength Climb New slip systems can become active Change of slip system Decrease in dislocation density Overaging of precipitates and precipitate coarsening decrease in strength The material may creep (time dependent elongation at constant load/stress). Enhanced oxidation and intergranular penetration of oxygen Etc.

6 Creep In some sense creep and superplasticity are related phenomena: in creep we can think of damage accumulation leading to failure of sample; while in superplasticity extended plastic deformation may be achieved (i.e. damage accumulation leading to failure is delayed). Creep is permanent deformation of a material under constant load (or constant stress) as a function of time. (Usually at high temperatures lead creeps at RT). Normally, increased plastic deformation takes place with increasing load (or stress) In creep plastic strain increases at constant load (or stress) Usually appreciable only at T > 0.4 T m High temperature phenomenon. Mechanisms of creep in crystalline materials is different from that in amorphous materials. Amorphous materials can creep by flow. At temperatures where creep is appreciable various other material processes may also active (e.g. recrystallization, precipitate coarsening, oxidation etc.- as considered before). Creep experiments are done either at constant load or constant stress. Creep can be classified based on Phenomenology Mechanism Harper-Dorn creep Power Law creep

7 Strain ( ) Constant load creep curve Constant load creep curve I II III 0 0 Initial instantaneous strain t The distinguishability of the three stages strongly depends on T and

8 Strain ( ) Constant Stress creep curve I II III t

9 Stages of creep I II III Creep rate decreases with time Effect of work hardening more than recovery Stage of minimum creep rate constant Work hardening and recovery balanced Absent (/delayed very much) in constant stress tests Necking of specimen start specimen failure processes set in

10 0 increases Strain ( ) Effect of stress Elastic strains Increasing stress 0 ' 0 '' 0 0 Effect of stress t

11 0 increases Strain ( ) Effect of temperature E as T Increasing T 0 ' 0 '' 0 0 Effect of temperature t As decrease in E with temperature is usually small the 0 increase is also small

12 Creep Mechanisms of crystalline materials Cross-slip Dislocation related Climb Glide Harper-Dorn creep Creep Coble creep Grain boundary diffusion controlled Diffusional Nabarro-Herring creep Lattice diffusion controlled Dislocation core diffusion creep Diffusion rate through core of edge dislocation more Interface-reaction controlled diffusional flow Grain boundary sliding Accompanying mechanisms: creep with dynamic recrystallization

13 Creep can be classified based on Phenomenology Mechanism Harper-Dorn creep Power Law creep

14 Cross-slip In the low temperature of creep screw dislocations can cross-slip (by thermal activation) and can give rise to plastic strain [as f(t)]

15 Dislocation climb Edge dislocations piled up against an obstacle can climb to another slip plane and cause plastic deformation [as f(t), in response to stress] Rate controlling step is the diffusion of vacancies

16 Diffusional creep Nabarro-Herring creep high T lattice diffusion Coble creep low T Due to GB diffusion In 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 causing elongation. This process like dislocation creep is controlled by the diffusion of vacancies but diffusional does not require dislocations to operate. Flow of vacancies

17 Grain boundary sliding At 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 melt before the crystal interior Above the equicohesive temperature grain boundaries are weaker than grain and slide past one another to cause plastic deformation

18 Creep Resistant Materials Higher operating temperatures gives better efficiency for a heat engine. Hence, there is a need to design materials which can withstand high temperatures. High melting point E.g. Ceramics Creep resistance Dispersion hardening ThO 2 dispersed Ni (~0.9 T m ) Solid solution strengthening Single crystal / aligned (oriented) grains

19 Cost, fabrication ease, density etc. are other factors which determine the final choice of a material Commonly used materials Fe, Ni (including superalloys), Co base alloys Precipitation hardening (instead of dispersion hardening) is not a good method as particles coarsen (smaller particles dissolve and larger particles grow interparticle separation ) Ni-base superalloys have Ni 3 (Ti,Al) precipitates which form a low energy interface with the matrix low driving force for coarsening 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 grain boundary sliding can cause creep elongation / cavitation Single crystals (single crystal Ti turbine blades in gas turbine engine have been used) Aligned / oriented polycrystals

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