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The crystal structure of pure titanium at ambient temperature and pressure is HCP (close-packed hexagonal) α phase with a c/a ratio of 1.587. At about 890°C, the titanium undergoes an allotropic transformation to a BCC (body-centered cubic) β phase which remains stable to the melting temperature.
The selective addition of alloying elements to titanium enables a wide range of physical and mechanical properties to be obtained. Some alloying elements raise the alpha-to-beta transition temperature (alpha stabilizers) while others lower the transition temperature (beta stabilizers). Aluminum, gallium, germanium, carbon, oxygen and nitrogen are alpha stabilizers. Aluminum, the principal alpha stabilizer, increases tensile strength, creep strength, and elastic module. Molybdenum, vanadium, tantalum, niobium, manganese, iron, chromium, cobalt, nickel, copper and silicon are beta stabilizers. Some elements notably tin and zirconium behave as neutral solutes in titanium and have little effect on the transformation temperature. Tin has extensive solid solubility in both the alpha and beta phases and is often used as a solid-solution strengthener in conjunction with aluminum to achieve higher strength without embrittlement. Zirconium forms a continuous solid solution with titanium and increases strength at low and intermediate temperatures.
Titanium alloys are classified according to the amount of alpha and beta retained in their structures at room temperature. Classifications include commercially pure, alpha and near-alpha, alpha-beta, and beta. The commercially pure and alpha alloys have essentially all-alpha microstructures. Beta alloys have largely all-beta microstructures after air cooling from the solution treating temperature above the beta transus. Alpha-beta alloys contain a mixture of alpha and beta phases at room temperature.
Commercially pure (CP) or unalloyed titanium typically contains between 99%-99.5% titanium, with the balance being made up of iron (as a main element) and the interstitial impurity elements hydrogen, nitrogen, carbon, and oxygen. In the alloys Fe and O content determine the grade and strength and C, N and H present as impurities and in the higher-strength grades, oxygen and iron are intentionally added to the residual amounts already in the sponge to provide extra strength. The microstructure of unalloyed titanium consists of grains of alpha phase with HCP structure and the possibility of small amounts of beta phase.
CP titanium is less expensive, generally more corrosion resistant and lower in strength than its alloys, and is not heat-treatable. It is highly weldable and formable, with a good creep resistance at high temperature and used primarily in applications requiring corrosion resistance and high ductility, where strength is not a prime consideration.
Alpha alloys have a fully alpha HCP structure only if they contain alpha stabilizers such as aluminum, tin, and oxygen. These elements also act as solid solution strengtheners. O and N present as impurities and give interstitial hardening. In the alloy Zr is added in small amount to stabilize α phase and gives strength and Sn is also added to improve ductility. The amount of α stabilizer should not exceed 9% in the Aluminum equivalent to prevent embrittlement due to ordering. They are slightly less corrosion resistant but higher in strength than unalloyed titanium. They develop moderate strengths and have good notch toughness. They have medium formability and are weldable and non-heat treatable. The more highly alloyed alpha offer optimum high temperature creep strength and oxidation resistance as well.
Near-alpha alloys contain small amount of ductile beta-phase. Besides alpha-phase stabilizers, near-alpha alloys are alloyed with 1-2% of beta phase stabilizers such as molybdenum, silicon or vanadium. Small amount of Mo and V giving a microstructure of β phase dispersed in α phase structure and improve performance and efficiency. In the alloy Sn and Zr are added to compensate Al contents while maintaining strength and ductility. Near–alpha alloys show greater creep strength than fully alpha alloys up to 400°C and are moderately high strength at room temperature and relatively good ductility (≈15%), high toughness and good creep strength at high temperature, good weldability and good resistance to salt water environment.
Alpha & Beta Alloys are metastable and contain both alpha stabilizers and beta stabilizers. α stabilizers are used to give strength with 4-6% and β stabilizers are used to allow the β phase to retain at room temperature after quenching from β and α+β phase field. Again, aluminum is the principal alpha stabilizer that strengthens the alpha phase. Beta stabilizers, such as vanadium, also provide strengthening and allow these to be hardened by solution heat treating and aging. In the alloy microstructure depends on chemical composition, processing history and heat treatment. Heat treatment can be done in corporation with thermo-mechanical processes to achieve desired microstructure/properties. These alloys are heat treatable and most are weldable. Their strength levels are medium to high. Their hot forming qualities are good, but the high temperature creep strength is not as good as most alpha alloys. Alpha-beta alloys are capable of excellent combinations of strength, toughness and corrosion resistance.
Beta alloys are sufficiently rich in beta stabilizers and lean in alpha stabilizers that the beta phase can be completely retained with appropriate cooling rates. Beta alloys are metastable, and precipitation of alpha phase in the metastable beta is a method used to strengthen the alloys. Beta alloys contain small amounts of alpha-stabilizing elements as strengthening agents. As a class, beta and near-beta alloys offer increased fracture toughness over alpha-beta alloys at a given strength level. Beta alloys also exhibit better room-temperature forming and shaping characteristics than alpha-beta alloys, higher strength than alpha-beta alloys at temperatures where yield strength instead of creep strength is the requirement, and better response to STA in heavier sections than the alpha-beta alloys. They are limited to approximately 370 °C (700 °F) due to creep. The beta alloys are readily heat treatable and weldable. Excellent formability can be expected in the solution treated condition. Beta-type alloys also have good combinations of properties in sheet and heavy sections
Titanium and titanium alloys are attractive structural materials due to their high strength, low density, and excellent corrosion resistance. However, even though titanium is the fourth most abundant element in the Earth’s crust, the cost of titanium is high due to its high melting point and extreme reactivity. The high cost includes both the mill operations (extraction, ingot melting, and primary working) as well as many of the secondary operations conducted by the user. The advantages of titanium include:
In addition to above properties different crystal structures of titanium allow manipulation of heat treatments to produce different types of alloy microstructures to suit the required mechanical properties.
As mentioned titanium and titanium-based alloys feature unique physico-chemical and physico-mechanical properties. These peculiar properties of titanium determine its wide application in various industries:
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