CREEP CREEP Dr. Mohammed Abdulrazzaq Materials Engineering Department

Mechanisms / Methods by which a can Material can FAIL 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 Chemical / Electro-chemical degradation Creep Physical degradation Fatigue Plastic deformation Fracture Microstructural changes Twinning Wear Slip Twinning Corrosion Erosion Phase transformations Oxidation Grain growth Particle coarsening * Beyond a certain limit

Plastic Deformation in Crystalline Materials 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 Grain boundary sliding + Other Mechanisms Vacancy diffusion Grain rotation Dislocation climb Note: Plastic deformation in amorphous materials occur by other mechanisms including flow (~viscous fluid) and shear banding

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.

CREEP → Permanent deformation of a material under constant load (or constant stress) as a function of time 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 Tm  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.

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

Constant Stress creep curve I II Strain () → III   t →

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

Effect of stress Strain () →  → Increasing stress 0 increases 0  →  → Elastic strains Increasing stress 0 increases 0 t →

Effect of temperature E↓ as T↑ Strain () → Increasing T  → 0 increases 0  → t → As decrease in E with temperature is usually small the 0 increase is also small

Creep Mechanisms of crystalline materials Cross-slip Dislocation related Climb Glide Harper-Dorn creep Coble creep Creep Grain boundary diffusion controlled Diffusional انتشاري 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

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

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)]

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

 Flow of vacancies 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

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

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 → ThO2 dispersed Ni (~0.9 Tm) Solid solution strengthening Single crystal / aligned (oriented) grains

Cost, fabrication ease, density etc 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 Ni3(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