Not all heat treatments are applicable to titanium alloys because of the differences in composition and microstructure. Alpha alloys generally are not heat treatable, having medium strength, good notch toughness and good creep resistance.
The response of titanium alloys to heat treatment depends on their composition and the effect of heat treatment on the alpha-beta phase balance. Strength of annealed alloys increases gradually and linearly with increasing alloy contents.
Alloys of the beta type respond to heat treatment, are characterized by higher density than pure titanium and are more easily fabricated. The purpose of alloying to promote the beta phase is either to form an all-beta-phase alloy having commercially useful qualities, to form alloys that have duplex alpha and beta structure to enhance heat-treatment response (i.e. changing the alpha and beta volume ratio) or to use beta eutectoid elements for intermetallic hardening.
Quenching from the beta-phase field gives a martensitic transformation with improved strength (depending on composition). Rapid quenching of titanium with relatively few alloying elements from the beta-phase field gives maximum strength at Mf. For highly alloyed titanium, rapid quenching from beta-phase field gives lowest strength, but the maximum strength is obtained after aging. The most important beta alloying element is vanadium.
Beta and alpha-beta alloys are heat treated to enhance specific properties. The general classifications for these heat treatments include:
• Stress relief – to reduce residual stress due to fabrication (e.g., forming, machining, welding) or heat treatment
• Process annealing – to optimize microstructure, manufacturability, dimensional stability and service life
• Solution treat and age – for mechanical-property development (e.g., strength, ductility, fracture toughness, creep resistance, fatigue strength)