Nickel base superalloys are used within the industrial gas turbine (IGT) engine leading to the need of the high temperature path components. These materials can be exposed to very severe operating conditions where high tensile strength, good fatigue and creep resistance as well as oxidation and hot corrosion resistances are required. A range of nickel base superalloys, from dilute, solid solution strengthened alloys to the highly alloyed precipitation hardened materials are available for high temperature loading performance and environmental resistance. For optimal performance, high pressure/high temperature turbine blades are generally made of nickel base superalloys. Due to mechanical properties these turbine blades are closely related to the microstructures.
In addition to use of these superalloys for turbine blades, nickel-based alloys, chromium-based alloys and others are also in use mechanical design purposes, such as: aerospace engine designs, ship engines designs, and other industries like, glass and petroleum industries. Other superalloys which are being used in turbine blade casting are cobalt-based and nickel-chromium-based too. In answer to the question: why superalloys are being used?.. Is that Superalloys are the preference for investment casting turbine blade for their capabilities to tolerate high heat and opposition to oxidation and deterioration through rusting and other factors.
Advancements in gas
turbine materials have always played a prime role – higher the capability of
the materials to withstand elevated temperature service, more the engine
efficiency; materials with high elevated temperature strength to weight ratio
help in weight reduction. A wide spectrum of high performance materials -
special steels, titanium alloys and superalloys - is used for construction of
gas turbines. Manufacture of these materials often involves advanced processing
techniques. Other material groups like ceramics, composites and inter-metallic
have been the focus of intense research and development; aim is to exploit the
superior features of these materials for improving the performance of gas
turbine engines.
Cast super alloys
Cast graded nickel-base
super alloys are used in manufacturing of turbine engine blades. There have been developments with regard to
rupture durability and low cycle fatigue strength over the years. Increasing predictive techniques is being
used for the purpose to ensure phase stability during service. Conventional
methods of equiaxed investment process are use for the designing and casting of
most of the nozzle and blade.
Directional solidification (DS) is being deployed for the production of
advanced technology nozzles and blades. Enhanced
creep and rupture strength levels could be achieved by this way of eliminating
transverse grain boundaries. Single
crystal blade technologies were consequently developed, through eradication of
all crumb boundaries, even though higher rupture strengths and low cycle
fatigue life could be achieved.
(Advanced Gas turbine
materials and Coatings, P.W.Scilke, GE Energy, 2004, pp19)
Single crystal super alloys (SC)
The logical progression to
grain-boundary reduction is the total elimination thereof. Thus, single-crystal
turbine blade/vane casting technology soon designed, on condition that further viewpoint
for nickel-base alloy design improvement. The furthermost advancement in the
metal temperature potential of turbine blades has been the single-crystal super
alloy and process technology. Tantalum substitute titanium to a noteworthy degree
in single crystal super alloys, since it both strengthens g¢ and raises the solidus temperature. Modern single crystal alloys
can contain between 70 and 80% ofg¢. This characterizes a useful limit, since further
boost to near 100% g¢ lead to a trivial crash in strength. Since no grain boundary
strengthening is essential in single crystal compositions, grain-boundary strengthening elements (boron, hafnium,
zirconium, and carbon) could be eliminated from the composition, resulting in a
substantial increase in the incipient melting temperature. Subsequently higher
solution treatment temperatures can be adopted to enable increased solutioning
of the fortification elements, devoid of aggravating incipient melting of the
alloy. Some single crystal alloys introduced in late 1980’s contain additions
of the heavy element rhenium, which has been found to retard constituent part
coarsening. Enhancements in creep strength were noticed predominantly at very
high temperatures (1000-1150 oC). [Materials and
manufacturing of advanced gas turbine components, M.Konter, M.Thumann, Journal
of Material Processing Technology 117, 2001, pp386-390]
Advanced Ceramics
In ceramic gas turbine, highly developed ceramic
objects are used in high temperature apparatus such as turbine blades and
nozzles which result in rise of the turbine inlet temperature up to 1300-1400°C
which in turn effect in high thermal efficiency. Highly
developed ceramics, such as Alumina, Silicon carbide and Aluminium Nitride are presently
being used to create critical aerospace machinery, for the reason that they
have some beneficial physical properties. The inert, non-metallic materials
preserve dimensional strength through a variety of high temperatures and reveal
very high mechanical strength. It put forward tremendous chemical confrontation
and stiffness-to-weight fraction, thus providing manufacturers with the capacity
to develop components that intend best possible performance in their proposed
application. [Experimental Analysis of High
temperature reliability of advanced ceramics in gas turbine by Tatsuki Ohji National Industrial Research Institute of Nagoya,
Nagoya 463-8687, Japan- March 2001- Volume 123, Issue 1,pp64]
By adopting the developed silicon
nitride to the components including turbine blades, nozzles,
combustor liners, and nose cones, turbine inlet temperature can be
increased without cooling; this leads to high thermal efficiency of
about 40 percent.
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