Question

Draw a typical creep curve and show the 1st, 2nd and 3rd Creep zones on them and explain the creep mechanisms that occur in these zones.

Answer 1

There are basically 3 zones in the creep curve.

Primary zone- Region of decreasing creep rate.In this zone creep resistance of material increases by virtue if its deformation.At loe temperatures primary creeo is the predominant creep process.

Secondary zone- Region of nearly constant creep rate.So there is a balance between Strain hardening and its competing process of i.e Recovery.

Tertiary zone- Region of increasing creep rate.This zone is often associated with metallurgical cganges such as recrystallization, diffusion or coarsening of precipitate particles.

Mechanisms of Creep Deformation-

1.) Dislocation Glide- It involves dislocations moving along slip planes and overcoming barriers by thermal activation.This mechanism is generally observed at high stress levels compared to those considered in creep deformation.The creep rate here is established by resistance offered to movement of dislocations by the solut atoms, precipitates or by other dislocations.

2.) Dislocation Creep- It takes places due to combination of dislocation glide and vacancy diffusion.This mechanism closely explains the steady state creep rate zone of creep.

3.) Diffusion Creep- This mechanism is dominant at high temperature and at relatively low stress values.It has been proposed that creep process is controlled by stress directed stomic diffusion.Stress cganges the chemical potential of the atoms on the surfaces of grains in polycrystal in such a way that there is a flow of vacancies frim grain boundaries experiencing tensule stress to thise which have compressive stresses. At the same time there is corresponding flow of atoms in the opposite direction, and this leads to elongation of the garin.

This mechanism is often referred as Nabbari- Harring creep. And a similar mechanism which is predominant at comparatively lower temperatures is called Coble type creep ( grain boundry diffusion).

4.) Grain- Boundry Sliding- This mechanism dies not contribute significantly to steady state creeo but it is important in initiating intergranular fracture. However it must noted that grain -boundry sliding is essential to maintain grain continuity during diffusional flow mechanisms.