DoITPoMS

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!

1. The gradient of which of these plots will give the stress exponent?

   	a	ln (strain rate) / ln (stress)
   	b	ln (strain rate) / (1/T)
   	c	strain rate / stress
   	d	strain rate / (1/T)

2. Which plot would you draw to calculate the activation energy?

   	a	ln (strain rate) / ln (stress)
   	b	ln (strain rate) / T
   	c	strain rate / (1/T)
   	d	ln (strain rate) / (1/T)

3. Which of these grain structures would exhibit the highest resistance to diffusional creep?

   	a	Equiaxed
   	b	Directionally solidified, with the grain boundaries aligned parallel to the major stress axis
   	c	Directionally solidified, with the grain boundaries aligned perpendicular to the major stress axis
   	d	Single Crystal

Deeper questions

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

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

   • Fine grain structure
   • Single crystal
   • High dislocation density
   • Lattice strain from epitaxy at γ/γ '
   • Thermal barrier coatings
   • Air cooling channels
   • Aerofoil shape
   • High purity

5. Which of these components do you think is likely to be most susceptible to creep?

   • An old drainpipe of Pb (MPt~600 K) at room temperature
   • An Al alloy (MPt~900 K) crankshaft, operating at 350°C
   • A Ni superalloy (MPt~1750 K) turbine blade at its peak operating T
   • A mild steel (MPt~1750 K) support structure in a furnace, reaching 500°C