Materials for engineering, 3rd Edition - (Malestrom)

102
Materials for engineering
to the presence of a tenacious oxide film), and is used in the chemical
processing industry, as well as in food processing applications. As an
electroplated coating, nickel is widely used in the electronics industry.
Nickel–copper alloys (see Fig. 1.11) possess excellent corrosion resistance,
notably in seawater. The monel (~30% Cu) series of alloys is used for turbine
blading, valve parts and for marine propeller shafts, because of their high
fatigue strength in seawater.
Nickel–chromium alloys form the basic alloys for jet engine development
– the nickel-based superalloys, e.g. the Nimonic alloys. The earliest of these,
Nimonic 80A was essentially ‘Nichrome’ (80/20 Ni/Cr) precipitation hardened
by the γ′ phase (Ni 3 Ti,Al). Alloy development proceeded by modifying the
composition in order to increase the volume fraction of the γ′ phase; this
made the alloys increasingly difficult to forge into turbine blades, the higher
γ′ volume fraction alloys had to be cast to shape. The limitation of the
conventionally cast superalloys was their lack of creep ductility due to
cavitation at the grain boundaries lying perpendicular to the maximum tensile
stress, so directionally solidified (DS) and eventually single crystal (SC)
alloys were developed. By incorporating channels into the turbine blades
through which cooling air was passed, it proved possible to use these alloys
at significantly higher engine temperatures than was possible with uncooled
blades.
Their maximum operating temperature is limited by the tendency of the γ′
phase to return into solid solution and, thus, by choosing an insoluble phase,
powder metallurgically produced oxide dispersion-strengthened (ODS)
superalloys have been developed by a technique known as mechanical alloying.
More recent increases in operating temperatures have been achieved by
deposition of thermal barrier coatings (TBCs) on high-temperature gas turbine
components. TBCs are complex films (typically 100 µm to 2 mm in thickness)
of a refractory material that protect the metal part from the extreme
temperatures. One important TBC material is yttria-stabilized zirconia, or
YSZ.
In spite of their high creep strength, the low ductility of present ceramics
limits their use in bulk for the turbine blades themselves. Figure 3.20 illustrates
how, over the years, the gas turbine entry temperature has been progressively
increased as the properties of the superalloy blades have been enhanced by
the methods referred to above. The lower series of curves illustrate the
behaviour of uncooled, uncoated blades of the wrought or cast alloys. The
series of dotted lines are data for production materials in specific Rolls
Royce aeroengines (the names of which are shown). This illustrates the
magnitude of the additional benefits obtained with cooled blades and with
cooled and coated blades. Even greater high temperature capability has been
demonstrated on a research (rather than production) basis and this is indicated
by the upper line of the diagram.