Basic alloying and heat-treatment features

8.5.1 Basic alloying and heat-treatment features

Since the emergence of titanium as a ‘wonder metal’ in the 1950s, the titanium industry has developed

a wide range of alloys with different compositions (see Figure 8.11). 2 These alloys rely on the high strength/weight ratio, good resistance to corrosion, combined low thermal conductivity and thermal expansion of titanium, properties which make it attractive for aerospace applications in both engine and airframe components.

C and then as bcc β to its melting point. Alloying additions change the temperature at which the α to β transition takes place, solutes that raise the transus are termed α-stabilizers and those that lower the β-transus temperature are termed β-stabilizers

Titanium exists in the cph α form up to 882 ◦

2 The Larson–Miller parameter φ is given by φ = T (A + log 10 t), where T is the temperature in degrees Kelvin, t the time in hours and A a constant, and defines the conditions to produce a given amount of plastic strain (e.g. 0.2%).

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10 h specific rupture strength (MN

900 1000 10 1200 Temperature (°C)

Figure 8.10 Comparison of MA6000 with other high-strength nickel alloys: (A) TD-Nickel, (B) MA6000, (C) directionally solidified Mar M200 and (D) single-crystal PWA 454.

6-2-4-6

500 IMI 834

IMI 685

400 Ti–4Al–4Mn 2 )

⫺ m 300 IMI 829

Ti–8Mn Stress (MN 200

6Al–4V

Near-a alloys

a ⫹ b alloys

550°C 600°C 100

Larson–Miller parameter f ⫻ 10 ⫺ 3 TEMPERATURE FOR TPS ⫽ 0.2% IN 100 h

Figure 8.11 Plot of stress versus Larson–Miller parameter φ for a range of titanium alloys.

464 Physical Metallurgy and Advanced Materials

1200 b ⫹ Liquid 1600

b Liquid °C)

3 Liquid 1000 b⫹ Ti Cu

a ⫹ Ti 2 Cu TiCu

(c) Figure 8.12 Representative phase diagrams for Ti alloys. (a) Ti–V. (b) Ti–Al. (c) Ti–Cu (after

(a)

(b)

E. A. Brandes and G. B. Brook, Smithells Metals Reference Book, 1998).

(Figure 8.12). The predominant α-stabilizer is aluminum. It is also an effective α-strengthening element at ambient and elevated temperatures up to 550 ◦

C, and thus a major constituent of most commercial alloys. The low density of aluminum is an important additional advantage. α-Phase strengthening is also achieved by additions of tin and zirconium. These metals exhibit extensive solubility in both α and β titanium but have little influence on the β-transus and are thus regarded as neutral additions. β-Stabilizers may be either β-isomorphous (i.e. have the bcc structure like β–Ti) or β -eutectoid elements. β-isomorphous elements have a limited α-solubility and are completely soluble in β-titanium, typical additions being molybdenum, vanadium and niobium. In contrast, β-eutectoid elements have a restricted solubility in β-titanium and form intermetallic compounds by eutectoid decomposition of the β-phase. In some alloy systems containing β-eutectoid elements, such as silicon

or copper, the compound formation (i.e. respectively Ti 5 Si 3 and Ti 2 Cu) leads to an improvement in mechanical properties. Titanium will also take interstitial solutes in solid solution, hydrogen being a β -stabilizer while carbon, nitrogen and oxygen are strong α-stabilizers. To minimize gas in Ti leads to a high cost of manufacture and heat treatment requires vacuum or inert gas conditions and freedom from refractory contact.

In describing titanium alloys it is conventional to classify them in terms of the microstructural phase (α alloys, β alloys, (α + β) alloys or near-α alloys, i.e. predominantly α-phase but with a small volume of β-phase). Commercial alloys are usually heat treated to optimize the mechanical properties by controlling the transformation of the β- to α phase, the extent of which is governed by the alloy composition and the cooling rate. The α alloys can transform completely from the β- to α phase no matter what the cooling rate. Such treatments have a negligible effect on properties and α alloys tend to be used in the annealed state.

Rapidly cooled alloys containing β-stabilizers form martensitic α from the β-phase, whereas slower cooling rates favor α formation by a nucleation and growth process. Several morphologies of α can

be produced by controlling the nucleation and growth mechanism; slow cooling, for example, tends to produce similarly aligned α platelets in colonies, combined with primary α at the grain boundaries. Faster cooling and higher α-stabilizer contents result in a basket-weave microstructure. Metastable-β,

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0 0.1mm

0 0.1mm

(b) Figure 8.13 Microstructure of near-α titanium alloy (IMI 829) initially β heat-treated at 1050 ◦ C

(a)

for 1 h. (a) Oil quenched. (b) Air cooled (after Woodfield, Loretto and Smallman, 1988). when aged, precipitates fine α, giving increased strength. The less stable the β, the more α can be

precipitated and hence the higher the strength attained. β -Stabilizing elements improve strength by strengthening the β-phase. The microstructure consists of primary α combined with the β-phase, which can be strengthened by an ageing treatment to precipitate acicular-α. Further strengthening is achieved by the limited solubility of the β-stabilizing element. Generally, these alloys have poor ductility properties.

The most important alloys contain both α- and β-stabilizers which, after working and annealing, give good strength and fabrication properties. For good creep strength an α-titanium base, strength- ened as much as possible by solute elements, is required. To meet this requirement the near-α alloys have been developed. These alloys combine the high α stability with sufficient β-stabilizer to give adequate strength. By β heat treating, the (α + β) microstructure changes to a totally transformed β structure containing basket-weave α. These alloys have good creep resistance and reasonable room- temperature properties. The basket-weave morphology is effective in inhibiting crack growth, and near-α alloys exhibit lower crack propagation rates than the α + β microstructures. Most of the com- mercial alloys which have been developed recently are of this type. The major factor influencing the post-forging microstructure is the cooling rate; an oil quench results in a basket-weave structure (see Figure 8.13a) and an air cool gives a typically aligned microstructure (see Figure 8.13b). The alloy is usually stress-relieved by annealing for 2 hours at 625 ◦

C or above.