Heat Treatment of Titanium and Titanium Alloys (2)
(Continued)<br/>
Alloy Types and Response to Heat Treatment<br/>
The response of titanium and titanium alloys to heat treatment depends on the composition of the metal and the effects of alloying elements on the α-β crystal transformation of titanium. In addition, not all heat treating cycles are applicable to all titanium alloys, because the various alloys are designed for different purposes.
Based on the types and amounts of alloying elements they contain, titanium alloys are classified as α, near-α, α-β, or β alloys. Alpha and near-alpha titanium alloys can be stress relieved and annealed, but high strength cannot be developed in these alloys by any type of heat treatment (such as aging after a solution beta treatment and quenching).
The basic alpha, near-alpha, alpha-beta, and beta alloys have heat treat-ment responses attuned to the microstructure (phases and distribution) that can be produced, which is a function of chemical composition.
Alpha, near-alpha: Because alpha alloys undergo little in the way of phase change, their microstructure cannot be manipulated much by heat treatment. Consequently, high strength cannot be developed in the alpha alloys by heat treatment. However, some near-alpha alloys, such as Ti-8Al-1Mo-1V, can be solution treated and aged to develop higher strengths. Both alpha and near-alpha titanium alloys can be stress relieved and annealed.
Alpha-beta: The alpha-beta alloys make up the largest class of titanium alloys. Microstructures can be substantially altered by working (forging) and/or heat treating them below or above the beta transus. Compositions, sizes, and distributions of phases in these two-phase alloys can be manipulated within certain limits. As a result, alpha-beta alloys can be hardened by heat treatment, and solution treating plus aging is used to produce maximum strengths. Other heat treatments, including stress relieving, also may be applied to these alloys.
Beta alloys: In commercial (meta-stable) beta alloys, stress relieving and aging treatments can be combined. Also, annealing and solution treating can be identical operations.
With respect to their effects on the allotropic transformation, alloying elements in titanium are classified as α stabilizers or β stabilizers. Alpha stabilizers, such as oxygen and aluminum, raise the α-to-β transformation temperature. Nitrogen and carbon are also stabilizers, but these elements usually are not added intentionally in alloy formulation. Beta stabilizers, such as manganese, chromium, iron, molybdenum, vanadium, and niobium, lower the α-to-β transformation temperature and, depending on the amount added, may result in the retention of some β phase at room temperature.
Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr and Ti-6Al-2Sn-4Zr-6Mo are designed for strength in heavy sections.
Alloys Ti- 6Al-2Sn-4Zr-2Mo and Ti-6Al-5Zr-0.5Mo-0.2Si for creep resistance.
Alloys Ti-6Al-2Nb-1 Ta-1Mo and Ti-6Al-4V, for resistance to stress corrosion in aqueous salt solutions and for high fracture toughness.
Alloys Ti-5Al-2.5Sn and Ti-2.5Cu for weldability
Alloys Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high strength at low-to-moderate temperatures.
