Environmental Engineering Reference
In-Depth Information
nickel-based alloys. In the process, Nimonic-80, the initial
-hardened creep-
resistant Ni-Cr alloy, was invented and proved to be the forerunner of the exten-
sive family of nickel-based superalloys which now provide the vast majority of
materials used for the highest temperature gas turbine blades.
The nickel- and cobalt-based superalloys are no doubt capable of developing
higher strength at higher temperatures than the iron-based alloys, but such high
strength is achieved only by reduction of their chromium content. The lower
chromium content is required not only to maintain microstructural stability but
to allow additions of alloying elements such as aluminum, titanium, and niobium
for optimum creep properties. The influence of chromium content on the oxida-
tion resistance and creep rupture strength of some of the currently used super-
alloys is shown in Fig. 6.42a and b, respectively [65]. Gradually, iron-, nickel-,
and cobalt-based superalloys found applications in most of the high-temperature
components of the aircraft jet engine, such as turbine blades, disks, combustion
systems, tail pipes, and so on, and provided a major step forward in the develop-
ment of shaft power gas turbines for stationary and transport purposes (particu-
larly for naval ships). There is no doubt that the development of aircraft gas
turbine engines has been spectacular over the past five decades; the thrust-to-
weight ratio of about 2:1 for the earliest engine has been increased to about 7:
1 in a modern military aircraft. Such gain has been achieved predominantly by
enhanced thermodynamic efficiency as a consequence of the increased tempera-
ture of the inlet gas to the tubing. However, the major credit for such advances
must go to the design and development engineers engaged in superior materials
development, for whom the permissible temperature of operation for a given
lifetime under specified mechanical stresses could be substantially increased.
However, such significant developments in materials are reaching an asymptote
because these alloys can be used at a maximum metal temperature of 1323 K
as their melting points are of the order of 1523-1573 K. Another significant
development is that despite the fact that use of the superalloys has allowed the
gas turbine operating temperature to increase by 423 K since the 1960s, develop-
ments in blade cooling technology over the same period have allowed an addi-
tional increase in temperature of 573 K, as illustrated in Fig. 6.43 [65].
The successful performance of most high-temperature metallic materials in
service conditions is often dependent on the formation and maintenance of an
impervious, protective oxide scale. In the selection of high-temperature material
for a specific application, it is generally a design requirement that the mechanical
properties of the alloy substrate (e.g., strength, creep, fatigue) remain unaltered
by the compositional and structural changes that may result from the degradation
process caused by the environment during the service period of the component.
But the alloy compositions and microstructures that provide optimum mechanical
properties often do not provide the satisfactory high-temperature corrosion resis-
tance property. It has already been pointed out that the development of alloy
γ′
