12.9: Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

Which of the following could be reasons tertiary creep is observed?

a	Rising true stress can cause the strain rate to rise
b	Microstructural damage such as cavitation can occur
c	Rising true stress can exceed the yield stress, causing conventional plastic deformation
d	All of these"

Answer

D

In the γ/γ' structure of Ni-based superalloys, why is the existence of a coherent interface important?

a	It leads to lattice strains that can inhibit dislocation motion
b	The ordered structure of the γ' phase inhibits diffusion
c	The ordered structure of the γ' phase inhibits dislocation motion
d	All of these

Answer

A

What is the function of the recess in Indentation Creep Plastometry?

a	It prevents tertiary creep
b	It allows the stresses created during the early stages of the process to be kept relatively low (below the yield stress)
c	It ensures that the indenter is located properly before indentation starts
d	It removes the near-surface region, which could have different properties from the bulk
e	It eliminates primary creep, so that the steady state regime is immediately established

Answer

B

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

Why is creep deformation different from conventional deformation?

a	Strain develops at a much slower rate in the material
b	Strain produced depends on the time for which the stress is applied
c	Creep cannot occur at room temperature
d	The strain produced doesn’t depend on the applied stress

Answer

A. Incorrect. For conventional deformation any strain is produced instantaneously as a result of a change in applied stress.

B. Correct. Creep differs from conventional plastic deformation because it is time-dependent deformation.

C. Incorrect. Creep can occur, regardless of absolute temperature, provided the homologous temperature is above (roughly) 0.4

D. Incorrect. The creep strain rate is proportional to applied stress raised to the stress exponent.

Which of the following statements is true?

a	Only Coble creep is controlled by diffusion
b	Only Nabarro-Herring creep is controlled by diffusion
c	Only dislocation creep is controlled by diffusion
d	All of the above mechanisms are controlled by diffusion

Answer

D. All creep mechanisms tend to be controlled by diffusional processes, which explains why creep deformation is time-dependent – it takes time for atoms to diffuse a given distance."

Which of the following is not an assumption that we make during the creeping coil experiment?

a	Assume constant true stress when we have constant load
b	Assume that the stress is uniform throughout each turn
c	Assume that only steady state creep occurs throughout
d	Assume that the creep regime is the same throughout the coil

Answer

A. Because the deformation occurs via torsion there is no reduction in cross sectional area and hence, although we are under constant load, we are also under constant true stress conditions

Which of the following features of Ni superalloy turbine blades helps to make them resistant to creep? (There may be more than one)

Yes	No	a	Fine grain structure
Yes	No	b	Single crystal
Yes	No	c	High dislocation density
Yes	No	d	High purity
Yes	No	e	Order hardening due to γ' precipitates in γ matrix
Yes	No	f	Low inherent diffusivity of FCC structure

Answer

A. No

B. Yes

C. No

D. No

E. Yes

F. Yes