Creep

Question 1

When does creep happen?

Answer 1

0.3 -> 0.4*Tm (and higher) if enough load and time is present

Question 2

What are the three phases of creep? (In the creep curve)

Answer 2

1. Instantaneous increasing
2. Constant period
3. Increase again, leads to failure

Question 3

Why does the creep curve have a constant period?

Answer 3

It is a result of both an increase in dislocation density as a cause of strengthening (deformation related hardening) and a decrease in dislocation density due to thermally related softening.

Question 4

Each metal-alloy features its own creep deformation. How can we visualise the mechanisms for each alloy?

Answer 4

Ashby-mechanism map. Plotting Normalized shear stress [Sigma/G] VS Homologus temperature [T/Tm]

Question 5

What is creep?

Answer 5

“The time-dependent plastic deformation of metals and alloys caused by a constant load applied even for long periods at temperatures generally higher than room temperature”.

Question 6

What happens towards the third phase of the creep curve?

Answer 6

Towards the end of the primary creep stage, voids start to form as a result of diffusion at the grain boundaries. Voids gradually coalesce (merge together) so that microcracks with a length of around one grain diameter start to appear towards the end of the secondary stage.

This internal damage as well as external necking increase the creep rate in the tertiary (third) stage and the material ultimately fails.

Question 7

What are the different mechanisms that can occur so that a solid polycrystalline metal becomes plastically deformed?

Answer 7

1. Plasticity ruled by tangential stress exceeding the ideal shear stress of the lattice (defect-less flow)
2. Plasticity by glide of dislocations (glide motion)
3. Plasticity by twinning (see very last slide to recall this mechanism)
4. Plasticity at high temperature, due to glide and climb of dislocations (dislocation creep)
5. Plasticity induced by vacancy flow (diffusion) within the crystal lattice (Nabarro- Herring creep)
6. Plasticity induced by vacancy flow at grain boundary (Coble creep)

Question 8

Explain GB creep damage.

Answer 8

GBs are sliding in their preferential directions and form of a cavities due do differences in the geometry (orientation/angle) of the grains.

When the strain increases, then imperfections like cracks or cavities can nucleate and grow.

GB creep damage usually starts once the tertiary creep is reached and a significant fraction of the microstructure is already affected by damage.