Titanium Aluminum Alloys

Overview

Titanium Aluminum Alloys are a class of metallic materials that contain a mixture of titanium and aluminum. They are lightweight, have high strength, and excellent corrosion and oxidation resistance at high temperatures.

TiAl alloys are considered an important high-temperature structural material for aerospace and automotive applications due to their unique combination of properties. Their low density makes them lighter than nickel-based superalloys, while still retaining strength and stability at temperatures up to 750°C.

Key Properties of Titanium Aluminum Alloys

Property	Description
Density	3.7 – 4.1 g/cm3, much lower than nickel alloys
Strength	Retain high strength at temperatures up to 750°C
Stiffness	High elastic modulus of about 160 GPa
Ductility	Brittle at room temperature but becomes more ductile at high temperatures
Corrosion Resistance	Excellent corrosion resistance due to presence of titanium
Oxidation Resistance	Form protective oxide layer resulting in good oxidation resistance up to 750°C
Cost	More expensive than titanium alloys but cheaper than nickel alloys

Types of Titanium Aluminum Alloys

There are two primary types of titanium aluminum alloys:

Gamma TiAl Alloys

Gamma TiAl alloys have a lamellar microstructure and contain about 45-48% titanium, with the remainder aluminum. Small additions of elements like niobium, carbon, boron and chromium are also made to enhance properties.

The gamma phase TiAl alloys offer a good balance of low density, strength, ductility and oxidation resistance. They are the most widely used TiAl alloys.

Alpha-2 Ti3Al Alloys

Alpha-2 Ti3Al alloys contain about 25% aluminum and have a hexagonal crystal structure. They offer very high tensile strength but have lower ductility and fracture toughness compared to gamma TiAl alloys.

Alpha-2 alloys are typically used in very high temperature applications above 800°C such as in turbochargers.

Composition of Titanium Aluminum Alloys

Titanium aluminum alloys contain titanium as the major component, with aluminum and small amounts of other elements. Here is the typical composition range:

Alloy Element	Composition Range	Role
Titanium (Ti)	52-56%	Primary base element
Aluminum (Al)	44-48%	Main alloying element with Ti
Niobium (Nb)	Up to 2%	Increases strength and creep resistance
Chromium (Cr)	Up to 2%	Increases oxidation resistance
Boron (B)	Up to 0.2%	Improves ductility
Carbon (C)	Up to 0.1%	Increases strength
Silicon (Si)	0.1-1%	Improves oxidation resistance
Tungsten (W)	0.1-1%	Refines grain size
Molybdenum (Mo)	0.1-1%	Increases strength

The percentages of alloying elements are precisely controlled to achieve the right microstructure and properties in the alloy.

Key Properties of Titanium Aluminum Alloys

Titanium Aluminum Alloy Strength Properties

Property	Value	Description
Tensile Strength	500 – 1100 MPa	Very high strength compared to titanium alloys
Yield Strength (0.2% offset)	400 – 1000 MPa	Measure of elastic strength in alloy
Compressive Strength	600 – 1500 MPa	Excellent compressive strength
Creep Strength	100 – 350 MPa	Ability to withstand loads at high temperatures
Fracture Toughness	15 – 35 MPa√m	Resistance to crack propagation is lower than nickel alloys

Physical Properties

Property	Value
Density	3.7 – 4.1 g/cm3
Melting Point	1360°C – 1460°C
Thermal Conductivity	6 – 25 W/mK
Electrical Resistivity	150 – 250 μΩ.cm
Coefficient of Thermal Expansion	11 – 13 x 10<sup>-6</sup> /K

Mechanical Properties at Room Temperature

Property	Value	Description
Hardness	300 – 400 HV	Measure of resistance to indentation
Young’s Modulus	150 – 160 GPa	Measure of stiffness
Shear Modulus	60 – 65 GPa	Measure of rigidity
Poisson’s Ratio	0.25 – 0.34	Ratio relating strain in directions perpendicular and parallel to applied load
Machinability	Difficult	Challenging to machine compared to steels

Applications and Uses of Titanium Aluminum Alloys

Titanium aluminum alloys are used in wide range of high performance engineering applications. Some key uses are:

Uses in Aerospace Industry

• Aircraft engine components like blades, discs, air inlet cowls
• Airframe and wing structures in high-speed aircraft
• Space vehicle parts due to combination of low weight and temperature resistance

Automotive Industry Uses

• Turbocharger turbine wheels and housings
• Connecting rods, valves, springs and fasteners in high performance engines
• Motorsport components like conrods and valves

Other Applications

• Gas turbine engine parts, power generation and marine applications
• Biomedical implants like artificial hip joints
• Sporting goods like bicycle frames, golf clubs

Here is a comparison of the use of titanium aluminum alloys versus alternatives:

Application	TiAl Alloys	Alternative Materials
Aircraft Engines	✅ Excellent strength-to-weight ratio up to 750°C makes it suitable for blades, vanes, shafts	Nickel superalloys have higher temperature capability but are heavier
Automotive Turbochargers	✅ Good balance of high strength, temperature resistance and lower density than nickel alloys	Nickel alloys can withstand higher peak temperatures
Airframes	✅ 20-35% lighter than titanium alloys with equivalent strength for plane wings, tails and fuselage	Titanium alloys offer higher fracture toughness
Biomedical Implants	✅ Contains titanium which allows natural bonding to human bone	Stainless steel, cobalt chrome alloys also commonly used

Industry Standards and Specifications

Some widely used industry standards for titanium aluminum alloys are:

Standard	Description
AMS 4928	Standard specification for gamma titanium aluminide alloy sheet, strip and plate
AMS 4965	Standard for gamma titanium aluminide alloys processed by powder metallurgy
AMS 4972	Standard specification for alpha-beta or beta titanium aluminides bars, rods and wire
ISO 21365	Specification for structural gamma TiAl alloys
ASTM B381	Standard classification for titanium-aluminum-vanadium alloys for surgical implants

Alloy products are offered in variety of grades that meet different standards for chemistry, microstructure, and mechanical properties.

Some common titanium aluminum grades are:

• Ti-48Al-2W-0.5Si (AMS 4928)
• Ti-47Al-2Cr-2Nb (ISO 21365 Grade 5)
• Ti-45Al-5Nb-0.2C-0.2B (AMS 4965 Grade 5)

Suppliers and Costs

Some leading global suppliers of titanium aluminum alloys include:

Supplier	Grades Offered	Production Methods
VSMPO	Ti-47Al-2Cr-2Nb<br>Ti-48Al-2Cr-2Nb-1Ta-0.7W	Investment casting<br>Forging
ATI	Ti-48Al-2W-0.5Si<br>Ti-47Al-2Cr-2Nb	Precision casting<br>Powder metallurgy
Precision Castparts Corp	Custom alloys	Investment casting
Plansee	TiAl gamma alloys	Powder metallurgy

Titanium aluminum alloys are more expensive than titanium alloys but cheaper than nickel-based superalloys. Some typical pricing estimates are:

Grade	Pricing Estimate
Ti-48Al-2Cr-2Nb	$85 – $125 per kg
Ti-47Al-2W-0.5Si	$100 – $150 per kg
Custom TiAl alloys	$150 – $250 per kg

Pricing varies based on order volume, size specifications, certification requirements and other customizations.

Advantages and Limitations of Titanium Aluminum Alloys

Benefits and Advantages

• Very high specific strength – high strength-to-weight ratio
• Excellent strength retention up to 750°C
• Good environmental resistance – oxidation, burning and corrosion
• Lower cost than nickel and cobalt superalloys
• Some hot workability for forging, rolling

Shortcomings and Limitations

• Processing difficulties – hot working as well as machining
• Brittle behavior at room temperature
• Relatively low fracture toughness
• Maximum use temperature limited to 750°C
• Subject to hydrogen and moisture absorption

Here is a comparison of the advantages and disadvantages relative to alternatives:

Parameter	TiAl Alloys	Nickel Superalloys	Titanium Alloys
High Temperature Strength	Good up to 750°C	✅ Excellent above 900°C	Poor above 500°C
Density	✅ Lowest	Higher	Comparable
Oxidation Resistance	Good up to 750°C	✅ Best above 800°C	Poor above 550°C
Cost	✅ Lower	Highest	Higher
Workability	Poor	Good	✅ Best
Damage Tolerance	Poor	Good	✅ Excellent

FAQs

Q: What are gamma titanium aluminides?

A: Gamma TiAl aluminides are intermetallic alloys containing titanium (Ti) and aluminum (Al) with a gamma (γ) phase crystal structure. They have an ordered lamellar arrangement of Ti and Al atoms. Gamma TiAl is the most commonly used alloy type.

Q: Why are TiAl alloys considered for aerospace applications?

A: TiAl alloys offer an excellent combination of low density and good mechanical properties up to 750°C. This allows lighter and more efficient aero-engine components to be designed using TiAl instead of much heavier nickel alloys.

Q: What are some examples of TiAl turbocharger components?

A: TiAl alloys are increasingly used to make turbocharger wheels and housings in high performance diesel and gasoline car engines. The low density and temperature resistance provide higher power density and efficiency.

Q: What are the main challenges in using TiAl alloys?

A: Difficulty in processing via casting, forging and machining along with intrinsic brittleness at room temperature, and lower damage tolerance than competing alloys creates barriers for adoption. However, processing methods and alloy development continue to advance.

Q: What is the typical oxygen content limit for TiAl alloys?

A: Oxygen is limited to less than 0.2% in TiAl alloys. Higher oxygen levels negatively impact ductility. Advanced melting and casting methods are used to control oxygen pickup.

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