Commercial titanium alloys

8.5.2 Commercial titanium alloys

α alloys transform entirely to α on cooling from the β-phase, regardless of cooling rate. Commercially pure titanium with a nominal oxygen content of 500, 1000, 1550 and 2700 ppm respectively gives tensile properties which range from 450 to 640 MN m −2 (see Figure 8.14). These are used mainly in sheet form. Solid-solution strengthening by aluminum, tin or zirconium increases tensile and creep strength and IMI 317 (Ti–5Al–2.5Sn) is typical.

Increasing the α-stabilizing composition increases creep strength but makes fabrication more dif- ficult and can lead to embrittlement during prolonged exposure at temperature in service due to the formation of the coherent ordered phase α 2 (Ti 3 Al). To avoid this it was empirically established that the aluminum equivalent Al ∗ must be no greater than 9, where Al ∗ (in wt%) is given by:

Al ∗ = Al +

Sn

Zr

3 + 6 + (10 × O 2 ).

466 Physical Metallurgy and Advanced Materials

Near-a alloys

IMI 551 (a ⫹ b)

CPTi (high O grade)

Tensile strength (MN m

Near-b/b alloys

CPTi

(low O 2 grade)

6AI/4V (a ⫹ b)

Figure 8.14 Variation of tensile strength with temperature for a range of commercial titanium alloys.

IMI 317 is difficult to fabricate and is often replaced by an IMI 230, which is an α-phase alloy containing the precipitation-hardening phase Ti 2 Cu; it can be fabricated and welded, and has good strength and ductility up to 400 ◦ C. β alloys contain enough β-stabilizing elements to maintain the bcc β-phase to room temperature. Unfortunately, bcc β–Ti alloys are prone to embrittlement. The binary alloys, titanium with Fe, Cr, Mn, Nb, Mo, Cr or V, all precipitate the embrittling ω-phase. More complex β–Ti alloys containing

Cr also suffer embrittlement from TiCr 2 . More stable alloys have been developed (e.g. (Ti–11.5Mo– 6Zr–4.5Sn)) but are little used. However, β-phase alloys, such as Ti–10V–2Fe–3Al, have potential as airframe construction materials offering high strength (1250 MN m −2 ) and toughness (45 MN m −3/2 ) in relatively thick cross-sections (90 mm) and superior hot-working characteristics which are attractive in expensive forging operations.

Highly stabilized β alloys have been developed for burn resistance. Ti alloys can burn when they rub together in gas turbine engines and steel stators are commonly used to separate the blades in compressors. Replacement of steel components by Ti alloys is a considerable weight saving. Pratt and Whitney developed a composition (wt%) Ti–35V–15Cr where the burn resistance is associated

with the high V-content which produces a volatile V 2 O 5 to shield the component from oxygen. A cheaper version with lower V-content has been developed by Rolls-Royce with composition Ti–25V– 15Cr–2Al–0.2C. This is made from a low-cost V/Cr/Al master alloy readily available in the steel industry. Unfortunately, the introduction of Al results in a B2 structure which embrittles the alloy at the operating temperature of 550 ◦

C when α is precipitated at the grain boundaries. Alpha is very much influenced by the presence of oxygen which is an α-stabilizer. The addition of carbon forms Ti(CO) carbides, which increases the oxygen concentration in the β-matrix and prevents oxygen segregation at the grain boundaries, with a corresponding reduction in grain boundary precipitation of α.

(α + β) alloys are probably the most widely used titanium alloys and contain alloying additions which strengthen both phases. These alloys are thermomechanically processed to control the size, shape and distribution of both α and β. The most versatile (α + β) alloy is IMI 318, which contains 6% Al and 4% V; it can be used at temperatures up to 350 ◦

C and has good forging and fabrication properties. It initially replaced steel as a disk material in jet engines, leading to 20% weight saving. Another important (α + β) alloy is IMI 550 (Ti–6Al–2Sn–4Mo–0.5Si), which has higher strength and

good creep resistance up to 400 ◦ + β) alloys remain the principal materials for fan disks and C. (α

Advanced alloys 467 blades, and for low- and intermediate-pressure compressor disks and blades of current gas turbine

engines. (α + β) alloys with extra low interstitial (ELI) content are attractive as ‘damage-tolerant’ materials for critical airframe components. Ti–6Al–4V with low oxygen has a tensile strength 8% lower than the standard alloy but, more importantly, the minimum fracture toughness is 60 MN m −3/2 .

Near-α alloys have increased the strength and the volume of the more creep-resistant α-phase at the expense of the bcc β-phase, which imports good low-temperature strength and forgeability. IMI 685 was the first titanium alloy to operate above 500 ◦

C. It contains Ti–6Al–5Zr–0.5Mo–0.2Si with Al and Zr instead of Sn as α-stabilizers, reduced Mo, the β-stabilizer, to minimize β at the α -needles and Si to improve creep resistance. These alloys are worked and heat treated in the β -range, have a tensile strength of about 1000 MN m −2 and give less than 0.1% creep strain in 100 h under a stress of 310 MN m −2 at 520 ◦

C. IMI 829 (Ti–5.6Al–3.5Sn–3Zr–1Nb–0.25Mo–0.3Si) has been derived from IMI 685 by replacing some of the Zr with the more potent strengthener Sn. It is β heat treated and has sufficient higher-temperature capability to be used in the hotter regions of engines. IMI 834 (Ti–5.8Al–4Sn–3.5Zr–0.7Nb–0.5Mo–0.35Si–0.06C) has been developed for use up to 600 ◦

C and combines the high fracture toughness and crack propagation resistance of a transformed β -structure with the typical equiaxed structure of the α + β alloys, providing good fatigue resistance and ductility. The small addition of carbon allows a controlled high α/β heat treatment. Hot working is carried out in the α + β field and heat treatment involves solution treatment for 2 hours at 1025 ◦ C, consistent with about 15% primary α, followed by oil quenching prior to ageing for 2 hours at 700 ◦ C, then air cooling. With such good high-temperature properties the alloy is being specified for engine compressor applications.