ENG31 1000 Engineering Materials I
Assignment I: Innovative Application of Metal Alloys
Application: Aerospace Engineering (Jet Engine Propulsion System)
• Jet Engine Propulsion System
Jet propulsion is the propulsion of an object in one direction, produced by ejecting
a jet of fluid in the opposite direction. By Newton's third law, the moving body is
propelled in the opposite direction to the jet. Reaction engines operating on the
principle of jet propulsion include the jet engine used for aircraft propulsion, the
pump-jet used for marine propulsion, and the rocket engine and plasma thruster used
for spacecraft propulsion. Underwater jet propulsion is also used by several marine
animals, including cephalopods and slaps, with the flying squid even displaying the
only known instance of jet-powered aerial flight in the animal kingdom
 Metal Component Selection: High-Pressure Turbine Blade
1. Innovative Application and Functions
o Application: Aerospace Engineering (Jet Engine Propulsion System).
o Component: High-Pressure Turbine (HPT) Blade.
o Function: The turbine blade is a high-performance component located immediately downstream from the combustion
chamber. Its primary role is to extract energy from the high-velocity, high-temperature gas stream. This extracted energy
rotates the turbine shaft, which in turn drives the compressor and the front fan to generate aircraft thrust.
o Operating Conditions: HPT blades operate under some of the most punishing conditions in engineering. They experience
centrifugal forces from rotational speeds exceeding 10,000 RPM and face gas temperatures ranging from 1,200°C to 1,500°C—
temperatures that often exceed the melting point of the unprotected base metal.
2. Suggested Metal Alloys
o Selected Material: Nickel-based Superalloy (e.g., CMSX-4 or Inconel 718).
o Reason for Selection: Nickel-based superalloys are chosen because they maintain high mechanical strength, excellent surface
stability, and resistance to creep, oxidation, and corrosion at elevated temperatures where standard structural steels would
lose their strength or melt.
 Engineering application
1. Engineering Function and Operating Principle
The primary function of a jet engine propulsion system is to generate thrust by accelerating a mass of air rearward. This is
achieved through the gas turbine cycle (the Brayton Cycle), which consists of four continuous stages:
o Intake: Air is pulled into the engine by a large front fan.
o Compression: High-efficiency compressor blades compress the incoming air, raising its pressure and temperature.
o Combustion: The compressed air is mixed with aviation fuel and ignited in the combustion chamber, creating a continuous,
ultra-high-temperature gas expansion.
o Exhaust / Turbine: The expanding gas rushes backward, spinning the turbine blades to power the compressor and fan, and
then exits through a nozzle at high velocity to create forward thrust (following Newton's Third Law of Motion).
2. Extreme Engineering Environment
Designing materials for a jet engine propulsion system is one of the most demanding tasks in engineering because different zones
experience vastly different stressors:
o The Front (Fan & Compressor): Subjected to intense aerodynamic forces, high rotational stress, and potential foreign object
damage (e.g., bird strikes) at relatively low to moderate temperatures.
o The Core (Combustion & Turbine): Operates under severe mechanical loads while exposed to highly corrosive, oxygen-rich
combustion gases at temperatures ranging from 1,200°C to 1,500°C.
3. Materials Application across the System
To balance weight, strength, and thermal resistance, a modern jet engine utilizes a diverse palette of advanced materials:
Engine Section
Material Class Used
Key Engineering Reason
Fan Blades
Titanium Alloys (e.g., Ti-6Al-4V) or Carbon
Fiber Composites
Titanium Alloys (front) & Nickel-based
Superalloys (back)
High strength-to-weight ratio, high fracture
toughness, and excellent impact resistance.
Titanium handles the cooler stages with
low weight; Nickel handles the higher
temperatures as air compresses.
Combustion Chamber
Cobalt-based Superalloys or Ceramic Matrix
Composites (CMCs)
Extreme thermal resistance and resistance
to thermal shock/warping
Turbine Blades
Single-Crystal Nickel-based Superalloys
(e.g., CMSX-4, Inconel)
Exceptional creep resistance at
temperatures approaching or exceeding the
metal's melting point.
Compressor
4. Advanced Engineering Sub-Systems
To allow the metal parts, especially the turbine blades—to survive gas temperatures that actually exceed their melting points,
engineers use a combination of active thermal technologies:
o Thermal Barrier Coatings (TBCs): Sub-millimeter layers of advanced ceramics (like
Yttria-Stabilized Zirconia) are plasma-sprayed onto metal components to provide a
thermal insulation barrier, dropping the metal's surface temperature by up to 200°C.
o Internal Film Cooling: Turbine blades are manufactured with intricate, hollow
internal cooling passages. Cooler air from the compressor is bypassed through these
channels and bled out of tiny laser-drilled holes on the blade surface, creating a
protective "blanket" of cool air over the metal.
 Manufacturing process
o Detailed: Conventional casting results in a polycrystalline structure with numerous grain boundaries. Under the combination of
high temperatures and extreme centrifugal stresses inside a jet engine, these grain boundaries become the weakest links,
susceptible to grain boundary sliding and subsequent creep rupture.
To eliminate this, Single Crystal technology is utilized. During the investment casting process, a helical "Grain Selector" (or
pigtail selector) is integrated at the base of the mold. As the mold is withdrawn slowly from the heating zone (Directional
Solidification), solidification initiates from the chilled base. The helical geometry of the selector acts as a physical filter, allowing
only a single, favorably oriented grain to propagate upward and fill the entire blade cavity. This results in a component entirely
free of grain boundaries, drastically improving high-temperature structural integrity.
 Heat treatment
o Detailed: As-cast single-crystal superalloys exhibit micro segregation and non-uniform distribution of alloying elements. To
fully develop their high-temperature mechanical properties, a precise heat treatment sequence is mandatory:
o Solution Treatment: The blade is heated to an elevated temperature close to its solidus line (approximately 1,300°C). This
high-temperature holding homogenizes the chemical composition and dissolves the course, ascast eutectic phases back into the Nickel solid solution (matrix). The component is then rapidly
cooled (quenched) to lock in this supersaturated solid solution.
o Two-step Aging Treatment: The supersaturated matrix is then reheated in two distinct stages.
The first aging step (around 1,000°C) promotes the nucleation and growth of the primary
strengthening precipitate phase, Gamma prime . The second aging step (around 850°C)
stabilizes the microstructure and refines the morphology of these precipitates into a perfectly
ordered, cuboidal nanostructure. This dense array of coherent precipitates effectively pins and
restricts dislocation motion, giving the alloy its extraordinary high-temperature creep strength.
 Microstructure & Phases
o Bimodal (Duplex) Microstructure Configuration: The fabricated Ti-6Al-4V alloy exhibits a highly optimized bimodal (or
duplex) microstructure, which is specifically engineered through precise thermomechanical processing and subsequent
Solution Treating and Aging (STA). This distinctive microstructure is characterized by a balanced combination of two distinct
constituents: primary alpha (\alpha) grains and a transformed beta (beta) matrix.
o Phase Characteristics & Morphology: * Primary Alpha (\alpha) Phase: This phase appears as globular or equiaxed grains that
are uniformly distributed throughout the material. The primary \alpha phase is a closely-packed hexagonal (hcp) structure
stabilized by the aluminum content, providing the alloy with excellent structural stability and high resistance to crack
initiation.
o Transformed Beta (\beta) Matrix: Surrounding the equiaxed \alpha grains is the transformed \beta matrix, which undergoes a
phase transformation during the cooling and aging processes. This matrix consists of a very fine lamellar (platelet-like)
\alpha+\beta colony structure. The \beta phase itself is a body-centered cubic (bcc) structure stabilized by the vanadium
content.
o Mechanical Significance of the Structure: The dual nature of this bimodal configuration is critical for high-performance
applications. The globular primary \alpha grains contribute to high ductility and superior low-cycle fatigue strength, while the
fine lamellar \alpha+\beta structure within the transformed \beta matrix effectively hinders dislocation movement and
impedes crack propagation, resulting in exceptional fracture toughness and creep resistance.
 Properties
o Mechanical Properties (High Specific Strength & Fatigue Resistance): * High Specific Strength (Strength-to-Weight Ratio): Ti6Al-4V possesses an outstanding ultimate tensile strength (typically \get 950\text{ MPa}) and yield strength (\get 880\text{
MPa}) while maintaining a relatively low density of approximately 4.43\text{ g/cm}^3. This high specific strength is paramount
for compressor blades, as it minimizes the rotational mass and reduces the severe centrifugal stresses generated at high
rotational speeds.
o Excellent Fatigue & Creep Resistance: Due to its refined bimodal microstructure, the alloy exhibits superior resistance to highcycle fatigue, preventing structural failure under the continuous, high-frequency cyclic aerodynamic loading experienced
during flight operations. It also retains high mechanical strength and resists time-dependent deformation (creep) at elevated
operating temperatures.
o Physical & Chemical Properties (Thermal Stability & Corrosion Resistance): * Thermal and Structural Stability: The alloy
maintains its mechanical integrity and structural stability at elevated working temperatures up to approximately 350–400°C,
which is the typical environment for high-pressure compressor stages before the combustion chamber.
o Outstanding Corrosion and Oxidation Resistance: In harsh atmospheric environments, titanium spontaneously reacts with
ambient oxygen to form a continuous, highly adherent, and chemically inert titanium dioxide (TiO_2) passive oxide film on its
surface. This micro-scale protective layer provides exceptional resistance to pitting, crevice corrosion, and high-temperature
oxidation, protecting the compressor blade from degradation caused by moisture, salt air, and ingested atmospheric
contaminants.
 Group Member
1. Puchida Udomthanakornkul B6815370
2. Napatsorn Phiwaram B6830229
3. Sumita Changkrathok B6830250