Titanium Alloys

• Titanium is the 4 th abundant metal on earth crust (~ 0.86%) after aluminium, iron and magnesium. ◼ Not found in its free, pure metal form in nature but as oxides, i.e., ilmenite (FeTiO3 ) and rutile (TiO2 ). ◼ Have similar strength as steel but with a weight nearly half of steel.
• Titanium can be extracted from rutile TiO2 ore by chemical reduction, using either the Kroll process, which uses magnesium, or by the Hunter process, which uses sodium.
• Titanium Alloys have a low density of 4430 kg/m3, making them about half as heavy as Steel and Nickel based Superalloys
• The use of Titanium Alloys is restricted to temperatures below 400°C
• Primary reasons for using titanium alloys in aerospace applications (80% of Ti usage) include:

  • Weight savings due to high strength-to-weight ratio, allowing them to replace Steel in applications requiring high strength and fracture toughness
  • Fatigue strength superior to Aluminium alloys, often used in highly loaded bulkheads and frames in fighter aircraft
  • Operating temperature capability exceeding 270 ºF (133 ºC), where aluminium alloys lose strength and titanium alloys are needed
  • Corrosion resistance superior to both aluminium and steel alloys
• • Space savings, used for landing gear components on commercial aircraft where aluminium components would not fit within the landing gear space envelope
• Pure Titanium exists in two crystalline forms : – one stable at room temperature called α (HCP structure), – one stable at elevated temperature called β (BCC structure)
• Elements that increase the β transus temperature through stabilizing the α phase are called α stabilizers (aluminium, oxygen, nitrogen, carbon)
• Elements that decrease the β transus temperature are called β stabilizers. They are further subdivided into: – β isomorphous elements, which have a high solubility in titanium (vanadium, molybdenum, niobium, tantalum). – β eutectoid elements, which have only limited solubility and tend to form intermetallic compounds (manganese, chromium, cobalt, iron, nickel, copper, silicon).
• Elements that neither raise nor lower β transus temperature are considered neutral (Tin and zirconium )
• Commercially pure grades are available with yield strengths ranging from 25-70 ksi (172–483 MPa) , with the higher strength grades containing more oxygen. ◼ The CP grades have good formability, are readily weldable, and have excellent corrosion resistance. ◼ The CP grades are supplied in the mill annealed condition which permits extensive forming at room temperature, while severe forming operations can be conducted at 300–900 ºF (150–482 ºC). ◼ Property degradation can be experienced after severe forming if the material is not stress relieved.
• The alpha and near-alpha alloys are not heat treatable, have medium formability, are weldable, and have medium strength, good notch toughness, and good creep resistance in the range of 600–1100 ºF (315–593 ºC). ◼ Aluminum is the most important alloying element in titanium, because it is a potent strengthener and also reduces density. ◼ The aluminum content is usually restricted to about 6% because higher contents run the risk of forming the brittle intermetallic compound Ti3Al.
• Near-alpha alloys are those which contain some beta phase dispersed in an otherwise all alpha matrix. ◼ The near-alpha alloys generally contain 5–8% aluminum, some zirconium, and tin, along with some beta stabilizer elements. ◼ Because these alloys retain their properties at elevated temperature and posses good creep strength, they are often specified for elevated temperature applications. ◼ Silicon in the range of 0.10–0.25% enhances the creep strength. ◼ High temperature near-alpha alloys
• The alpha–beta alloys are heat treatable to moderate strength levels but do not have as good elevated temperature properties as the near-alpha alloys. ◼ Their weldability is not as good as the near-alpha alloys, but their formability is better. ◼ The alpha–beta alloys, which include Ti-6Al-4V, Ti-6Al-6V-2Sn, and Ti-6Al2Sn-4Zr-6Mo, are all capable of higher strengths than the near-alpha alloys. ◼ They have a good combination of mechanical properties, rather wide processing windows, and can be used in the range of about 600–750 ºF
• Beta alloys contain high percentages of the BCC beta phase that greatly increases their response to heat treatment, provides higher ductility in the annealed condition, and provides much better formability than the alpha or alpha–beta alloys. ◼ They exhibit good weldability, high fracture toughness, and a good fatigue crack growth rate; however, they are limited to about 700 ºF (370 ºC) due to creep. ◼ The beta alloys (can be heat treated to high tensile strength levels 200 ksi
• Heat treatments for titanium alloys include stress relieving, annealing, and solution treating and aging (STA)
• All titanium alloys can be stress relieved and annealed
• Only the alpha–beta and beta alloys can be STA to increase their strength
• The response to STA is determined by the amount of beta phase
• Beta alloys, with higher percentages of beta phase, are heat treatable to higher strength levels than the alpha–beta alloys
• One disadvantage of heat treating titanium alloys is the lack of a nondestructive test method to determine the actual response to heat treatment
• Stress relief is used to remove residual stresses that result from mechanical working, welding, cooling of castings, machining, and heat treatment. ◼ All titanium alloys can be stress relieved without affecting their strength or ductility
• Annealing is similar to stress relief but is done at higher temperatures to remove almost all residual stresses and the effects of cold work.
• Mill annealing ➔ conducted at the mill. For near-alpha and alpha–beta alloys
• Recrystallization annealing ➔ improve the fracture toughness. The part is heated into the upper range of the alpha + beta field, held for a period of time and then slowly cooled.
• Duplex annealing ➔ provide better creep resistance for high temperature alloys such as Ti6242S. It is a two-stage annealing process that starts with an anneal high in the alpha + beta field followed by air cooling.
• Beta annealing ➔ conducted by annealing at temperatures above the beta transus followed by air cooling or water quenching to avoid the formation of grain boundary alpha. This treatment maximizes fracture toughness at the expense of a substantial decrease in fatigue strength
• The purpose of the solution treatment is to transform a portion of the alpha phase into beta and then to cool rapidly enough to retain the beta phase at room temperature. ◼ During aging, alpha precipitates from the retained beta.
• STA is used with both alpha–beta and beta alloys to achieve higher strength levels than can be obtained by annealing. ◼ Solution treatments for alpha–beta alloys are conducted by heating to slightly below the beta transus. ◼ During aging, the unstable retained beta transforms into fine alpha phase which increases the strength.
• Solution treatment for beta alloys is carried out above the beta transus
• Commercial beta alloys are usually supplied in the solution treated condition with a 100% beta structure to provide maximum formability
• Commercial beta alloys only need to be aged to achieve high strength levels after forming
• Beta processed alloys have:

  • Improved fracture toughness
  • Better creep strength
  • More resistance to stress corrosion cracking
• There is a considerable loss in ductility and fatigue strength in beta processed alloys
• Beta alloys are usually air cooled from the solution treating temperature
• The best practice is to conduct heat treatments for beta alloys in a vacuum furnace to protect them from oxygen, hydrogen, and nitrogen