Heat Treatment of Titanium and Titanium Alloys

Many alloys, including some titanium alloys, do not have the optimal properties in their processed condition. Heat Treatment is a process whereby controlled heating and cooling of metals is performed under very precise environmental conditions in order to alter the physical or mechanical characteristics of the metal without changing the product’s shape. If heat treatment is not done correctly, the metal may not achieve the desired properties needed to meet the engineers design specification.
Heat Treatment is typically associated with increasing the strength of material, but it also is frequently used to improve machinability, improve formability, increase ductility or increase corrosion resistance. Therefore, it is a critical process that ensures that the specified characteristics of the metal are achieved.

Titanium Alloys are heat treated in order to:

Reduce residual stresses developed during fabrication (stress relieving)
Produce an optimal combination of ductility, machinability, and dimensional and structural stability (annealing)
Increase strength (solution treating and aging)

Optimize special properties such as fracture toughness, fatigue strength, and high-temperature creep strength

Stress relieving of titanium

Titanium and titanium alloys can be stress relieved without adversely affecting strength or ductility.
Stress-relieving treatments decrease the undesirable residual stresses that result from first, nonuniform hot forging or deformation from cold forming and straightening, second, asymmetric machining of plate or forgings, and, third, welding and cooling of castings. The removal of such stresses helps maintain shape stability and eliminates unfavorable conditions, such as the loss of compressive yield strength commonly known as the Bauschinger effect.
Stress relieving is probably the most common heat treatment given to titanium and titanium alloys. It is used to decrease the undesirable residual stresses that result from nonuniform hot forging deformation, nonuniform cold forming and straightening, asymmetric machining of plate (hogouts) or forgings, welding of wrought, cast, or powder metallurgy (P/M) parts, and cooling of castings.

Stress relieving helps maintain shape stability and also can eliminate unfavorable conditions such as loss of compressive yield strength — the Bauschinger effect — that can be particularly severe in titanium alloys. Stress relieving can be performed without adversely affecting strength or ductility.

Annealing

The annealing of titanium and titanium alloys serves primarily to increase fracture toughness, ductility at room temperature, dimensional and thermal stability, and creep resistance. Many titanium alloys are placed in service in the annealed state. Because improvement in one or more properties is generally obtained at the expense of some other property, the annealing cycle should be selected according to the objective of the treatment.
Common annealing treatments are:

Mill annealing is a general-purpose treatment given to all mill products. It is not a full anneal and may leave traces of cold or warm working in the microstructures of heavily worked products, particularly sheet.

Duplex annealing alters the shapes, sizes, and distributions of phases to those required for improved creep resistance or fracture toughness. In the duplex anneal of the Corona 5 alloy, for example, the first anneal is near the β transus to globularize the deformed α and to minimize its volume fraction. This is followed by a second, lower-temperature anneal to precipitate new lenticular (acicular) α between the globular α particles. This formation of acicular α is associated with improvements in creep strength and fracture toughness.

Recrystallization annealing and β annealing are used to improve fracture toughness. In recrystallization annealing, the alloy is heated into the upper end of the α-β range, held for a time, and then cooled very slowly. In recent years, recrystallization annealing has replaced β annealing for fracture critical airframe components.

β (Beta) Annealing. Like recrystallization annealing, β annealing improves fracture toughness. Beta annealing is done at temperatures above the β transus of the alloy being annealed. To prevent excessive grain growth, the temperature for β annealing should be only slightly higher than the β transus. Annealing times are dependent on section thickness and should be sufficient for complete transformation. Time at temperature after transformation should be held to a minimum to control β grain growth. Larger sections should be fan cooled or water quenched to prevent the formation of a phase at the β grain boundaries.

Solution Treating and Aging

A wide range of strength levels can be obtained in α-β or β alloys by solution treating and aging. With the exception of the unique Ti-2.5Cu alloy, the origin of heat-treating responses of titanium alloys lies in the instability of the high-temperature β phase at lower temperatures.
Heating an α-β alloy to the solution-treating temperature produces a higher ratio of β phase. This partitioning of phases is maintained by quenching; on subsequent aging, decomposition of the unstable β phase occurs, providing high strength. Commercial β alloys generally supplied in the solution-treated condition, and need only to be aged. Solution treating of titanium alloys generally involves heating to temperatures either slightly above or slightly below the β transus temperature.
β (Beta) alloys are normally obtained from producers in the solution-treated condition. If reheating is required, soak times should be only as long as necessary to obtain complete solutioning. Solution-treating temperatures for β alloys are above the β transus; because no second phase is present, grain growth can proceed rapidly.
α-β (Alpha-beta) alloys. Selection of a solution-treatment temperature for α-β alloys is based on the combination of mechanical properties desired after aging. A change in the solution-treating temperature of α-β alloys alters the amounts of β phase and consequently changes the response to aging.
To obtain high strength with adequate ductility, it is necessary to solution treat at a temperature high in the α-β field, normally 25 to 85°C (50 to 150°F) below the β transus of the alloy. If high fracture toughness or improved resistance to stress corrosion is required, β annealing or β solution treating may be desirable. However, heat treating α-β alloys in the β range causes a significant loss in ductility. These alloys are usually solution heat treated below the β transus to obtain an optimum balance of ductility, fracture toughness, creep, and stress rupture properties.

Quenching

If alloys are rapidly cooled by water quenching from the all beta region, the tendency of the alpha phase to form is suppressed, and the beta phase is retained. Certain alloy compositions, however, exhibit a peculiar transformation on quenching. This mechanism of martensitic or shear-like transformation is not completely understood. The formation of this structure, the so-called alpha prime, causes some distortion of the lattice. This distortion and the resulting strain produce a material, which is hard and tough, and possesses better fatigue properties than alpha. This quenching process is also the initial point for tempering.

Tempering

When titanium is quenched from an elevated temperature, reheated to a temperature below the beta transus, held for a length of time and again quenched, it is said to have been tempered. Three variables exist in tempering: the phases present, the time held, and the tempering temperature.

When the initial structure contains alpha prime, two changes occur: the alpha prime transforms to alpha, and at longer times the alpha becomes serrated. The result is a loss of hardness and strength and an increase in ductility and impact. Alpha-beta structures, however, do not follow this pattern. The alpha primarily remains unchanged; the beta decomposes to form more alpha at the expense of the beta phase. At low temperatures more alpha, will be formed; thus, low tempering temperatures result in a greater decrease in strength and hardness and a larger increase in ductility than the high temperature tempering over identical time intervals.

Isothermal Transformation

On hot-quenching an alloy from the all beta region to temperatures in the alpha-beta field and holding for a period of time and then further quenching to room temperature, the material is transformed isothermally. Treatment in this way causes precipitation of the alpha phase from the beta. At high temperatures the alpha precipitates first at grain boundaries and later within the beta grains themselves.
This treatment, when holding at temperatures just below the transformation temperature, at first gives a very hard material due to formation of beta prime. If the time of holding is extended, the hardness and strength decrease with an accompanying increase in ductility and toughness. At lower temperatures a gradual rise in hardness and brittleness takes place, and at prolonged times a higher hardness may be obtained than by short time high temperature treatments.

Alloy Types and Response to Heat Treatment

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.