Page 66 - The Jet Engine

27. In the past, turbine discs have been made in
ferritic and austenitic steels but nickel based alloys
are currently used. Increasing the alloying elements
in nickel extend the life limits of a disc by increasing
fatigue resistance. Alternatively, expensive powder
metallurgy discs, which offer an additional 10% in
strength, allow faster rotational speeds to be
achieved.
Turbine blades
28. A brief mention of some of the points to be
considered in connection with turbine blade design
will give an idea of the importance of the correct
choice of blade material. The blades, while glowing
red-hot, must be strong enough to carry the
centrifugal loads due to rotation at high speed. A
small turbine blade weighing only two ounces may
exert a load of over two tons at top speed and it must
withstand the high bending loads applied by the gas
to produce the many thousands of turbine horse-
power necessary to drive the compressor. Turbine
blades must also be resistant to fatigue and thermal Fig. 5-13 Comparison of turbine blade life
shock, so that they will not fail under the influence of properties.
high frequency fluctuations in the gas conditions, and
they must also be resistant to corrosion and
oxidization. In spite of all these demands, the blades
must be made in a material that can be accurately
formed and machined by current manufacturing
methods.

29. From the foregoing, it follows that for a
particular blade material and an acceptable safe life
there is an associated maximum permissible turbine
entry temperature and a corresponding maximum
engine power. It is not surprising, therefore, that met-
allurgists and designers are constantly searching for
better turbine blade materials and improved methods
of blade cooling.
30. Over a period of operational time the turbine
blades slowly grow in length. This phenomenon is
known as 'creep' and there is a finite useful life limit
before failure occurs.

31. The early materials used were high temperature
steel forgings, but these were rapidly replaced by
cast nickel base alloys which give better creep and
fatigue properties.
32. Close examination of a conventional turbine
blade reveals a myriad of crystals that lie in all
directions (equi-axed). Improved service life can be
obtained by aligning the crystals to form columns
along the blade length, produced by a method known
as 'Directional Solidification'. A further advance of
this technique is to make the blade out of a single Fig. 5-14 Ceramic turbine blades.

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