Technical characteristics of titanium between different grades: 
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Basic Physical Properties
Notable physical characteristics of titanium:
- Its density is approximately 60% of that of steel.
- Its resistance to corrosion is exceptional in many environments such as seawater or the human body.
- Its mechanical characteristics remain high up to a temperature of approximately 600°C and remain excellent up to cryogenic temperatures.
- Its transformation into semi-finished products and parts of different shapes by the usual techniques (drilling, stamping, spinning, casting, welding, machining, etc.) is reasonably easy.
- It is available in a wide variety of forms and product types: ingots, billets, bars , wires, tubes , slabs, sheets , strip .
- It is non-magnetizable.
- Its coefficient of thermal expansion, slightly lower than that of steel, is half that of aluminum. An average coefficient of thermal expansion of 10.5 × 10⁻⁶ K⁻¹ will be taken as the standard. Its Young's modulus is very close to that of bone structures.
Crystallographic properties
Pure titanium undergoes a martensitic-type allotropic transformation in the vicinity of 882 °C.
Below this temperature, the structure is hexagonal pseudo-close-packed (a=0.295nm; c = 0.468 nm: c/a = 1.633) and is called Ti α (space group 194 / P63/mmc).
Above this temperature the structure is body-centered cubic (a=0.332 nm) and is called Ti β.
The α→β transition temperature is called the β transus.
The exact transformation temperature is largely influenced by substitutional and interstitial elements. It therefore depends strongly on the purity of the metal. Crystallographic structure of the α and β unit cells of titanium.
Isotopes
Titanium is found in nature in the form of five isotopes: 46Ti, 47Ti, 48Ti, 49Ti, and 50Ti. 48Ti is the most abundant isotope, with a natural concentration of 73.8%. Eleven radioisotopes have been observed; the most stable, 44Ti, has a half-life of 63 years.
The photocatalytic activity of TiO2 is strongly affected by its crystallinity and particle size (Pecchi et al., 2001).
Anatase modification is only sufficiently active in photocatalysis with a bandspace energy Ebg of 3.2 eV.
Hombikat UV-100 TiO2 consists of pure modified anatase, and its particles have a PARI surface area of approximately 186 m²/g (applying the Brunauer-Emmett-Teller theory of gas adsorption for determining the adsorption isotherm).
However, the majority of investigations were carried out using Degussa P-25 TiO2. This material consists of approximately 80% anatase and 20% rutile and has a BET specific surface area of approximately 55 m²/g.
The diameter of its particles is usually between 25 nm and 35 nm.
Oxides
Titanium monoxide (TiO₂) Titanium trioxide (Ti₂O₃) Titanium dioxide (TiO₂) Titanium trioxide (TiO₃)
Mechanical Properties
Erosion
The highly adherent and hard oxide layer explains the longevity of titanium parts subjected to impacts from particles suspended in fluids. This effect is amplified by the layer's ability to regenerate. Erosion in seawater is increased by higher flow rates or smaller particle sizes.
Strength and ductility
Titanium is considered a metal with high mechanical strength and good ductility under standard temperature conditions. Its specific strength (tensile strength/density ratio) surpasses that of aluminum and steel.
Its strength is inversely proportional to temperature, with a plateau between -25°C and 400°C.
Below -50°C, in cryogenic temperature ranges, the increase in strength is dramatic; however, this is accompanied by very low ductility.
Above 400°C, mechanical strength begins to decrease.
Without any theoretical basis, fatigue endurance is about 70% of tensile strength.
Wear and tear
To date, no satisfactory solution has yet been developed. Oxidation, nitriding, boriding, and carburizing have been the main methods tried. Numerous technological difficulties in implementation and adhesion have been encountered. Furthermore, surface treatments of titanium, which modify the nature or structure of the surface, should only be used with the utmost caution and after a thorough study of their effects; they generally have a more or less pronounced detrimental effect on strength and fatigue resistance.
Biocompatibility
Titanium is one of the most biocompatible metals, along with gold and platinum, meaning it is completely resistant to bodily fluids. Furthermore, it possesses high mechanical strength and a very low modulus of elasticity, making it compatible with bone structures. The International Agency for Research on Cancer (IARC) has classified titanium dioxide in Group 2B, "possibly carcinogenic to humans": studies conducted have not yet reached a definitive conclusion.
Fire resistance
Its resistance to fire, particularly hydrocarbons, is excellent. It has been demonstrated that a 2 mm thick tube can withstand a pressure of ten atmospheres while being subjected to a hydrocarbon fire at a temperature of 600°C without damage, deformation, or explosion. This is primarily due to the resistance of the oxide layer, which prevents hydrogen from penetrating the material. Furthermore, titanium's low thermal conductivity provides longer-lasting protection for the internal components against temperature increases.
Chemical Properties
Classic titanium corrosion
Titanium is an extremely oxidizable metal. In the series of standard electrochemical potentials, it is located near aluminum, between magnesium and zinc. It is therefore not a noble metal; its thermodynamic stability range does not, in fact, share any part with the thermodynamic stability range of water and is situated well below it. One of the reasons for titanium's corrosion resistance is the development of a protective, passivating layer a few fractions of a micrometer thick, composed mainly of TiO2 oxide, although it is recognized that it can contain other forms of titanium. This layer is intact and very adherent. If the surface is scratched, the oxide reforms spontaneously in the presence of air or water. Thus, titanium is virtually unaffected by air, water, and seawater.
Furthermore, this layer is stable over a wide range of pH, potential, and temperature. Highly reducing conditions, highly oxidizing environments, or the presence of fluorine ions (a complexing agent) diminish the protective nature of this oxide layer; the etching reagents used to record micrographs are most often hydrofluoric acid-based. During a reaction with this acid, titanium(II) and(III) cations are formed. The reactivity of acidic solutions can nevertheless be reduced by the addition of oxidizing agents and/or heavy metal ions. Chromic or nitric acid and salts of iron, nickel, copper, or chromium are excellent inhibitors.
This explains why titanium can be used in industrial processes and environments where conventional materials would corrode. Electrochemical equilibria can, of course, be modified by adding alloying elements that reduce the anodic activity of titanium; this leads to improved corrosion resistance.
Depending on the desired modifications, specific elements are added. A non-exhaustive list of some common additives is provided below: shifting the corrosion potential and enhancing the cathode properties: addition of platinum, palladium, or rhodium; increasing thermodynamic stability and reducing the propensity for anodic dissolution: addition of nickel, molybdenum, or tungsten; increasing the passivation tendency: addition of zirconium, tantalum, chromium, or molybdenum. These three methods can be combined.
Specific corrosion of titanium
Titanium is very resistant to specific types of corrosion such as pitting corrosion or crevice corrosion. These phenomena are only observed when used in environments close to the practical limits of general corrosion resistance. Stress corrosion cracking occurs under the following conditions: in cold seawater (only in the presence of sharp cuts); in certain specific media such as anhydrous methanol; and in hot environments, in the presence of molten NaCl. The two allotropic structures differ in their resistance to this last type of corrosion; α-titanium is highly susceptible, while β-titanium is virtually unaffected.
The Kroll Process and the Production of High-Purity Titanium
The Kroll process
The first step involves carbochlorinating titanium dioxide. The product is obtained by reacting gaseous chlorine with the oxide at approximately 800°C in a fluidized bed according to the reaction: TiO2(s) + 2 C(s) + 2 Cl2(g) → TiCl4(g) + 2 CO(g). The titanium tetrachloride, which has a boiling point of 136°C, is recovered by condensation, decanted, filtered, and purified by fractional distillation. The subsequent reduction process involves reacting this tetrachloride in the gas phase with liquid magnesium according to the reaction: TiCl4(g) + 2 Mg(l) → 2MgCl2(l) + Ti(s). The reaction is carried out under vacuum or inert gas (argon). Magnesium chloride is separated by decantation, then, in a second step, by vacuum distillation at around 900-950°C, or by acid washing. The resulting titanium is a porous solid resembling a sponge, hence its name titanium sponge. Since its initial industrial application in 1945, the Kroll process has not undergone any significant changes in its physicochemical principle, except for the reaction yield.
Production of high-purity titanium
Once the sponge is obtained, it is ground to produce titanium shavings. This batch is then homogenized in a mixer, either under inert gas or high-pressure suction, to prevent any ignition of the titanium fines (particles of about one hundred micrometers) which could lead to the formation of titanium oxynitride, a brittle compound insoluble in the liquid bath. The homogenized batch is then introduced into the die of a press where it is cold-compressed into a dense cylinder called a compact. The relative density of the compact allows for handling to create an electrode by stacking these compacts, layer by layer, and welding them together using plasma or electron beam. This produces a primary electrode. Subsequently, the titanium electrodes are fused by vacuum arc remelting (VAR). This process involves creating a low-voltage, high-current electric arc (30 to 40 V; 20,000 to 40,000 A) between the bottom of the titanium electrode and a water-cooled copper crucible. The bottom of the electrode heats up, and its temperature rises above the liquidus; the molten metal then falls into a liquid well contained within a metal sheath known as the ingot's skin. The ingot is remelted several times in this way, depending on the desired purity. With each remelting, the diameter of the ingots is increased; these typically weigh between 1 and 10 tons and have a diameter of 0.5 to 1 meter.
Compounds
Although metallic titanium is quite rare due to its price, titanium dioxide is inexpensive and widely used as a white pigment in paints and plastics. TiO2 powder is chemically inert, resistant to sunlight, and highly opaque. Pure titanium dioxide has a very high refractive index and greater optical dispersion than diamond.
Precautions, toxicology
In its divided metallic form, titanium is highly flammable, but titanium salts are generally considered safe. Chlorinated compounds such as TiCl4 and TiCl3 are corrosive. Titanium can accumulate in living tissues containing silicon, but it has no known biological role.
Conclusion
Titanium boasts an extremely diverse range of properties. Not only is it corrosion-resistant, often combined with resistance to erosion and fire, and biocompatibility, but also possesses excellent mechanical properties (strength, ductility, fatigue resistance, etc.) that allow for the design of thinner and lighter components. This attractive array of properties explains its growing applications in the aeronautical, aerospace, chemical, and medical fields. Furthermore, thanks to improved production efficiency, titanium is increasingly used in everyday applications such as the sporting goods and automotive industries.