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 metastable beta (Fig. 2). 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 titanium alloys are used primarily for corrosion resistance. They are also useful in applications requiring high ductility for fabrication but relatively low strength in service. Yield strengths range from 170 to 520 MPa (25 to 75 ksi). Basically, oxygen and iron contents determine the strength levels of commercially pure titanium. In the higher-strength grades, oxygen and iron are intentionally added to the residual amounts already in the sponge to provide extra strength.
Alpha and near-alpha alloys contain aluminum as the principal alloying element. Aluminum provides solid-solution strengthening, oxidation resistance, and reduces density. Other additions include the neutral elements tin and zirconium, along with small amounts of beta stabilizers. Alpha and near-alpha alloys 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.
Ti-5Al-2.5Sn is the only true alpha alloy that is commercially produced. The remainder of the commercially available alpha and near-alpha alloys are near-alpha alloys. Ti-5Al-2.5Sn is quite ductile, and the extra-low interstitial grade retains ductility and toughness at cryogenic temperatures. Because Ti-5Al-2.5Sn is a single-phase alloy containing only alpha, it cannot be strengthened by heat treatment.
Near-alpha alloys contain small amounts of beta phase dispersed in an otherwise all-alpha matrix. The near-alpha alloys generally contain 5 to 8 wt% Al. The near-alpha alloys retain their strength to high temperatures and have good creep resistance in the range of 320 to 590 °C (600 to 1100 °F).
Alpha-beta alloys contain both the alpha and beta phases. 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 (STA). Alpha-beta alloys have a good combination of mechanical properties, rather wide processing windows, and can be used at temperatures up to approximately 320 to 400 °C (600 to 750 °F). The alpha-beta alloys include Ti-6Al-4V, which is the workhorse of the aerospace industry. It accounts for approximately 60 wt% of the titanium used in aerospace and up to 80 to 90 wt% of that used for airframes.
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.
Commercially pure titanium alloys are used primarily for corrosion resistance. They are also useful in applications requiring high ductility for fabrication but relatively low strength in service. Yield strengths range from 170 to 520 MPa (25 to 75 ksi). Basically, oxygen and iron contents determine the strength levels of commercially pure titanium. In the higher-strength grades, oxygen and iron are intentionally added to the residual amounts already in the sponge to provide extra strength.
Alpha and near-alpha alloys contain aluminum as the principal alloying element. Aluminum provides solid-solution strengthening, oxidation resistance, and reduces density. Other additions include the neutral elements tin and zirconium, along with small amounts of beta stabilizers. Alpha and near-alpha alloys 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.
Ti-5Al-2.5Sn is the only true alpha alloy that is commercially produced. The remainder of the commercially available alpha and near-alpha alloys are near-alpha alloys. Ti-5Al-2.5Sn is quite ductile, and the extra-low interstitial grade retains ductility and toughness at cryogenic temperatures. Because Ti-5Al-2.5Sn is a single-phase alloy containing only alpha, it cannot be strengthened by heat treatment.
Near-alpha alloys contain small amounts of beta phase dispersed in an otherwise all-alpha matrix. The near-alpha alloys generally contain 5 to 8 wt% Al. The near-alpha alloys retain their strength to high temperatures and have good creep resistance in the range of 320 to 590 °C (600 to 1100 °F).
Alpha-beta alloys contain both the alpha and beta phases. 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 (STA). Alpha-beta alloys have a good combination of mechanical properties, rather wide processing windows, and can be used at temperatures up to approximately 320 to 400 °C (600 to 750 °F). The alpha-beta alloys include Ti-6Al-4V, which is the workhorse of the aerospace industry. It accounts for approximately 60 wt% of the titanium used in aerospace and up to 80 to 90 wt% of that used for airframes.
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.
