Titanium is a lustrous transition metal with a silver color, low density, and high strength. Titanium is resistant to corrosion in seawater, aqua regia, and chlorine. In power plants, titanium can be used in surface condensers. The Kroll and Hunter processes extract the metal from its principal mineral ores. Kroll’s process involved a reduction of titanium tetrachloride (TiCl4), first with sodium and calcium and later with magnesium, under an inert gas atmosphere. Pure titanium is stronger than common, low-carbon steels but 45% lighter. It is also twice as strong as weak aluminium alloys but only 60% heavier. The two most useful properties of the metal are corrosion resistance and strength-to-density ratio, the highest of any metallic element. The corrosion resistance of titanium alloys at normal temperatures is unusually high. Titanium’s corrosion resistance is based on forming a stable, protective oxide layer. Although “commercially pure” titanium has acceptable mechanical properties and has been used for orthopedic and dental implants, titanium is alloyed with small amounts of aluminium and vanadium, typically 6% and 4%, respectively, for most applications by weight. This mixture has a solid solubility that varies dramatically with temperature, allowing it to undergo precipitation strengthening.
Titanium alloys are metals that contain a mixture of titanium and other chemical elements. Such alloys have high tensile strength and toughness (even at extreme temperatures). They are light in weight, have extraordinary corrosion resistance, and can withstand extreme temperatures.
Types of Titanium Alloys
Titanium exists in two crystallographic forms. At room temperature, unalloyed (commercially pure) titanium has a hexagonal close-packed (hcp) crystal structure referred to as the alpha (α) phase. When the temperature of pure titanium reaches 885 °C (called the β transit temperature of titanium), the crystal structure changes to a bcc structure known as the beta (β) phase. Alloying elements either raise or lower the temperature for the α-to- β transformation, so alloying elements in titanium are classified as either α stabilizers or β stabilizers. For example, vanadium, niobium, and molybdenum decrease the α-to-β transformation temperature and promote the formation of the β phase.
- Alpha Alloys. Alpha alloys contain elements such as aluminum and tin and are preferred for high-temperature applications because of their superior creep characteristics. These α-stabilizing elements work by either inhibiting change in the phase transformation temperature or by causing it to increase. The absence of a ductile-to-brittle transition, a feature of β alloys, makes α alloys suitable for cryogenic applications. On the other hand, cannot be strengthened by heat treatment because alpha is the stable phase, and thus they are not as strength as beta alloys.
- Beta Alloys. Beta alloys contain transition elements such as vanadium, niobium, and molybdenum, which decrease the temperature of the α to β phase transition. Beta alloys have excellent hardenability and respond readily to heat treatment. These materials are highly forgeable and exhibit high fracture toughnesses. For example, the ultimate tensile strength of high-strength titanium alloy – TI-10V-2Fe-3Al is about 1200 MPa.
- Alpha + Beta Alloy. Alpha + beta alloys have compositions that support a mixture of α and β phases and may contain between 10 and 50% β phase at room temperature. The most common α + β alloy is Ti-6Al-4V. The strength of these alloys may be improved and controlled by heat treatment. Examples include: Ti-6Al-4V, Ti-6Al-4V-ELI, Ti-6Al-6V-2Sn, Ti-6Al-7Nb.
Pure titanium and its alloys are commonly defined by their grades defined by ASTM Internation standards. In general, there are almost 40 grades of titanium and its alloys. Following is an overview of the most frequently encountered titanium alloys and pure grades, their properties, benefits, and industrial applications.
- Grade 1. Commercially pure titanium grade 1 is the most ductile and softest titanium alloy. It is a good solution for cold-forming and corrosive environments. It possesses the greatest formability, excellent corrosion resistance, and high impact toughness. Due to its formability, it is commonly available as a titanium plate and tubing. These include:
- Chemical processing
- Chlorate manufacturing
- Medical industry
- Marine industry
- Automotive parts
- Airframe structure
- Grade 2. Commercially pure titanium grade 2 is very similar to grade 1, but it has higher strength than grade 1 and excellent cold forming properties. It provides excellent welding properties and has excellent resistance to oxidation and corrosion. This titanium grade is the most common grade in the commercially pure titanium industry, and it is the prime choice for many fields of applications:
- Chemical Processing & Chlorate Manufacturing,
- Power generation
- Grade 5 – Ti-6Al-4V. Grade 5 is the most commonly used alloy and is an alpha + beta alloy. Grade 5 alloy accounts for 50% of total titanium usage worldwide. It has a chemical composition of 6% aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen, and the remainder titanium. Generally, Ti-6Al-4V is used in applications up to 400 degrees Celsius. It has a density of roughly 4420 kg/m3. It is significantly stronger than commercially pure titanium (grades 1-4) due to its possibility to be heat treated. This grade is an excellent combination of strength, corrosion resistance, weld, and fabricability. It is the prime choice for many fields of applications:
- Aircraft turbines
- Engine components
- Aircraft structural components
- Aerospace fasteners
- High-performance automatic parts
- Marine applications
- Grade 23 – Ti-6Al-4V-ELI. Ti-6Al-4V-ELI or TAV-ELI is the higher purity version of Ti-6Al-4V. ELI stands for Extra Low Interstitial. The essential difference between Ti6Al4V ELI (grade 23) and Ti6Al4V (grade 5) is the reduction of oxygen content to 0.13% (maximum) in grade 23. Reduced interstitial elements oxygen and iron improve ductility and fracture toughness with some reduction in strength. It’s the top choice for any situation where a combination of high strength, lightweight, good corrosion resistance, and high toughness are required. This grade of titanium, medical grade of titanium, is used in biomedical applications such as implantable components due to its biocompatibility, good fatigue strength, and low modulus.
Application of Titanium Alloys – Uses
The two most useful properties of the metal are corrosion resistance and strength-to-density ratio, the highest of any metallic element. The corrosion resistance of titanium alloys at normal temperatures is unusually high. These properties determine the application of titanium and its alloys. The earliest production application of titanium was in 1952 for the nacelles and firewalls of the Douglas DC-7 airliner. High specific strength, good fatigue resistance and creep life, and good fracture toughness are characteristics that make titanium a preferred metal for aerospace applications. Aerospace applications, including use in both structural (airframe) components and jet engines, still account for the largest share of titanium alloy used. On the supersonic aircraft SR-71, titanium was used for 85% of the structure. Due to its very high inertness, titanium has many biomedical applications based on its inertness in the human body, that is, resistance to corrosion by body fluids.
Commercially Pure Titanium – Grade 1 in Steam Condensers
In nuclear power plants, the main steam condenser (MC) system is designed to condense and deaerate the exhaust steam from the main turbine and provide a heat sink for the turbine bypass system. The exhaust steam from the LP turbines is condensed by passing over tubes containing water from the cooling system. These tubes are usually made of stainless steel, copper alloys, or titanium, depending on several selection criteria (such as thermal conductivity or corrosion resistance). Titanium condenser tubes are usually the best technical choice. However, titanium is a very expensive material, and titanium condenser tubes are associated with very high initial costs. Titanium, in particular, can bring major improvements, such as higher water velocities promoting better heat coefficients and excellent resistance to abrasion, erosion, and corrosion, thereby improving resistance to fouling. Tubes are mostly welded tubes from ASTM SB 338 grade 1 made on a continuous manufacturing line. This commercially pure titanium is the softest titanium and has the highest ductility. It has a good cold forming characteristics and provides excellent corrosion resistance. It also has excellent welding properties and high impact toughness. All manufacturing operations (welding, annealing, non-destructive testing) are fully automated to produce high-quality tubes in large quantities.
Properties of Titanium Alloys
Material properties are intensive properties, which means they are independent of the amount of mass and may vary from place to place within the system at any moment. Materials science involves studying materials’ structure and relating them to their properties (mechanical, electrical, etc.). Once materials scientist knows about this structure-property correlation, they can then go on to study the relative performance of a material in a given application. The major determinants of a material’s structure and, thus, its properties are its constituent chemical elements and how it has been processed into its final form.
The density of Titanium Alloys
The density of typical titanium alloy is 4.43 g/cm3 (Ti-6Al-4V).
Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:
ρ = m/V
In words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).
Since the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance, it is obvious that the density of a substance strongly depends on its atomic mass and also on the atomic number density (N; atoms/cm3),
- Atomic Weight. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the atom’s total volume or less, but it contains all the positive charge and at least 99.95% of the atom’s total mass. Therefore it is determined by the mass number (number of protons and neutrons).
- Atomic Number Density. The atomic number density (N; atoms/cm3), which is associated with atomic radii, is the number of atoms of a given type per unit volume (V; cm3) of the material. The atomic number density (N; atoms/cm3) of a pure material having an atomic or molecular weight (M; grams/mol) and the material density (⍴; gram/cm3) is easily computed from the following equation using Avogadro’s number (NA = 6.022×1023 atoms or molecules per mole):
- Crystal Structure. The density of a crystalline substance is significantly affected by its crystal structure. FCC structure, along with its hexagonal relative (hcp), has the most efficient packing factor (74%). Metals containing FCC structures include austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium.
Mechanical Properties of Titanium Alloys
Materials are frequently chosen for various applications because they have desirable combinations of mechanical characteristics. For structural applications, material properties are crucial, and engineers must consider them.
Strength of Titanium Alloys
In the mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. The strength of materials considers the relationship between the external loads applied to a material and the resulting deformation or change in material dimensions. The strength of a material is its ability to withstand this applied load without failure or plastic deformation.
Ultimate Tensile Strength
The ultimate tensile strength of commercially pure titanium – Grade 2 is about 340 MPa.
The ultimate tensile strength of Ti-6Al-4V – Grade 5 titanium alloy is about 1170 MPa.
The ultimate tensile strength is the maximum on the engineering stress-strain curve. This corresponds to the maximum stress sustained by a structure in tension. Ultimate tensile strength is often shortened to “tensile strength” or “the ultimate.” If this stress is applied and maintained, a fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after achieving the ultimate strength. It is an intensive property; therefore, its value does not depend on the size of the test specimen. However, it depends on other factors, such as the specimen preparation, the presence or otherwise of surface defects, and the temperature of the test environment and material. Ultimate tensile strengths vary from 50 MPa for aluminum to as high as 3000 MPa for very high-strength steels.
The yield strength of commercially pure titanium – Grade 2 is about 300 MPa.
The yield strength of Ti-6Al-4V – Grade 5 titanium alloy is about 1100 MPa.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically. In contrast, the yield point is where nonlinear (elastic + plastic) deformation begins. Before the yield point, the material will deform elastically and return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behavior termed a yield point phenomenon. Yield strengths vary from 35 MPa for low-strength aluminum to greater than 1400 MPa for high-strength steel.
Young’s Modulus of Elasticity
Young’s modulus of elasticity of commercially pure titanium – Grade 2 is about 105 GPa.
Young’s modulus of elasticity of Ti-6Al-4V – Grade 5 titanium alloy is about 114 GPa.
Young’s modulus of elasticity is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to limiting stress, a body will be able to recover its dimensions on the removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position, and all the atoms are displaced the same amount and maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions, and no permanent deformation occurs. According to Hooke’s law, the stress is proportional to the strain (in the elastic region), and the slope is Young’s modulus. Young’s modulus is equal to the longitudinal stress divided by the strain.
The hardness of Titanium Alloys
Rockwell hardness of commercially pure titanium – Grade 2 is approximately 80 HRB.
Rockwell hardness of Ti-6Al-4V – Grade 5 titanium alloy is approximately 41 HRC.
Rockwell hardness test is one of the most common indentation hardness tests, that has been developed for hardness testing. In contrast to the Brinell test, the Rockwell tester measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). The minor load establishes the zero position, and the major load is applied and removed while maintaining the minor load. The difference between the penetration depth before and after applying the major load is used to calculate the Rockwell hardness number. That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale.
The Rockwell C test is performed with a Brale penetrator (120°diamond cone) and a major load of 150kg.
Thermal Properties of Titanium Alloys
Thermal properties of materials refer to the response of materials to changes in their temperature and the application of heat. As a solid absorbs energy in the form of heat, its temperature rises, and its dimensions increase. But different materials react to the application of heat differently.
Melting Point of Titanium Alloys
The melting point of commercially pure titanium – Grade 2 is around 1660°C.
The melting point of Ti-6Al-4V – Grade 5 titanium alloy is around 1660°C.
In general, melting is a phase change of a substance from the solid to the liquid phase. The melting point of a substance is the temperature at which this phase change occurs. The melting point also defines a condition where the solid and liquid can exist in equilibrium.
Thermal Conductivity of Titanium Alloys
The thermal conductivity of commercially pure titanium – Grade 2 is 16 W/(m. K).
The thermal conductivity of Ti-6Al-4V – Grade 5 titanium alloy is 6.7 W/(m. K).
The heat transfer characteristics of solid material are measured by a property called the thermal conductivity, k (or λ), measured in W/m.K. It measures a substance’s ability to transfer heat through a material by conduction. Note that Fourier’s law applies to all matter, regardless of its state (solid, liquid, or gas). Therefore, it is also defined as liquids and gases.
The thermal conductivity of most liquids and solids varies with temperature, and for vapors, it also depends upon pressure. In general:
Most materials are nearly homogeneous. Therefore we can usually write k = k (T). Similar definitions are associated with thermal conductivities in the y- and z-directions (ky, kz). However, for an isotropic material, the thermal conductivity is independent of the transfer direction, kx = ky = kz = k.