1. Prepared By: Sania Amjad Sania Amjad ROLL NO: 21 21 Prepared By: Sania Amjad Sania Amjad ROLL NO: 21 21

2. INTRODUCTION Creep is phenomenological term, which is responsible for plastic deformation. When a metal or alloy is under a constant load or stress, it may undergo progressive plastic deformation This time dependent strain is called CREEP. Creep predominates at temperature above 0.5Tm.

3. CREEP MECHANISM Stress and temperature are the two important variables, which not only affect the creep rate, but also the mechanism operative. Three kinds of mechanisms are operative in creep:  dislocation related,  diffusional,  grain boundary sliding.

4. CREEP MECHANISM

5. DISLOCATION CLIMB:- At high temperature the appreciate atomic movement causes the dislocation to climb up or down. It happens at higher strength and higher temperature. Strong dependence on applied stress

6. DISLOCATION CLIMB:-

7. Bulk Diffusion (Nabarro-Herring) The diffusion could occur predominantly via the lattice at high temperatures known as Nabarro-Herring creep. The diffusion of vacancies control creep rate. Grain boundary acts as a source and sinks for vacancies. The mechanism depends on the migration of vacancies from one side of a grain to another.

9. GRAIN BOUNDARY SLIDING:-  At low temperatures the grain boundaries are ‘stronger’ than the crystal interior.  Being a higher energy region, the grain boundaries may pre-melt before the crystal interior.  The relative motion of grain boundaries can lead to wedge cracks.  The main importance of grain-boundary sliding is that it initiates grain boundary fracture.

10. Wedge Crack

11. Grain Boundary Diffusion (Coble creep)  Diffusion on the grain boundaries at lower strength and lower temperature.  Dislocations climbs up to boundaries and transitions to grain boundaries area, then moves with the grain.  The diffusion via grain boundaries (at low temperatures) is known as Coble creep.

12. Phenomenological descriptions of creep One of the important descriptions of creep is using the power-law formula. The shear strain rate is a power function of the shear stress. Clearly this formula is not based on a mechanism operative, but a fit of data.

13. MECHANISMS OF CREEP DEFORMATION Chief creep formation grouped as: ▪ Dislocation glide: Stress range –σ/G >10- 2. ▪ Dislocation creep: Stress range 10- 4 < σ/G <10- 2. ▪ Diffusion creep: Stress range σ/G < 10- 4. ▪ Grain boundary sliding : Sliding of grains past each other

14. DEFORMATION MECHANISM MAPS