Smith chapter 1 problem 12, Nickel-base superalloys
for aircraft turbine engines
Note: The efficiency of heat engines increases as
the operating temperature (ΔT above ambient) increases, so engineers like
to run these engines as hot as possible to produce the greatest thrust, gas
mileage, etc.
Properties of this metal which
make it suitable for this application include:
- Ni superalloys can operate at fairly high temperatures
without failing due to melting or creep.
Creep is deformation which increases with time at a constant load,
usually above ½ the melting point of the material.
- At
those elevated temperatures these alloys still have decent strength to
withstand the centripetal forces as blades and rotor spin at many
thousands of RPM and have decent strength to withstand impact loading due to
birds, etc. The way impact
strength is tested is rather interesting.
In a test stand on the ground, the turbine is spun up to full
operating speed and a large air cannon is used to
shoot frozen chickens into the spinning turbine at a speed of a few
hundred miles per hour.
- The Ni
superalloys are fairly resistant to fatigue
fracture which can result from many cyclec of
loading and unloading.
- The
metal does not react much with air or hot exhaust gases CO2, CO, H2O,
etc. No carburization or
de-carburization of the metals in the exhaust section of the turbine can occur via the
reversible reaction MC + CO2 =
2CO .
- Cost
is tolerable.
- Turbine
blades for certain applications are grown as single crystals rather than
cast into a mold to form a polycrystalline material. The absence of grain boundaries in the
single crystal blades improves creep and other properties. This single-crystal growth can be done with
certain Ni superalloys.