Titanium Alloys and Their Characteristics: Part One

Sažetak:

Due to its unprecedented strength, lightness, stable market and non-corrosive characteristics, titanium has emerged as the metal of choice for aerospace, industry and medical, leisure and consumer products, notably golf clubs and bicycle frames. Furthermore, the automobile industry has found that the use of titanium for connecting rods and moving parts has resulted in significant fuel efficiency.
Titanium alloys can be divided into three main groups: corrosion resistant alloys, high strength alloys, and high temperature alloys.

Titanium is a light metal (4.5 g/cm3), strong and highly resistant to corrosion. Titanium is highly resistant to heat with a melting temperature as high as 1668°C; its melting point is higher than that of steel. Although heat conductivity of titanium is almost the same as that of stainless steel, its weight is almost half of stainless steel. Titanium is also non-toxic and non-allergenic, often used in piercing jewelry.

Due to its unprecedented strength, lightness, stable market and non-corrosive characteristics, titanium has emerged as the metal of choice for aerospace, industry and medical, leisure and consumer products, notably golf clubs and bicycle frames. Furthermore, due to its strength and lightness, titanium is currently being tested in the automobile industry which has found that the use of titanium for connecting rods and moving parts has resulted in significant fuel efficiency.

The major titanium alloy which is the workhorse of the industry is the titanium-aluminum-vanadium alloy Ti-6Al-4V which is priced at a factor of 1.5 or 2 over commercially pure titanium. The relatively small size of the titanium industry and the considerable use of alloying metals can leave the industry vulnerable to price changes.

Commercially pure titanium, grades 1, 2, 3 and 4 is available in bar and billet and the following alloys of titanium also commonly available in bar and billet form: Ti-6Al-4V (grades 5 and 24), Ti-5Al-2.5Sn (grade 6), Ti-0.2Pd (grades 7 and 11), Ti-3Al-8V-6Cr-4Zr-4Mo (also known as BETA-C), Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al- 1Mo-1V, Ti-6Al-6V-2Sn, Ti-10V-2Fe-3Al (TIMETAL 10.2.3), Ti-4Al-4Mo-2Sn (IMI 550), Ti-4Al-4Mo-4Sn-0.5Si-0.1C (IMI 551), Ti-15Mo (IMI 205) and TIMETAL 100.

Alloys available in casting form include commercially pure grades 2,3,4, Ti-6Al-4V (grades 5 and 24), Ti-0.2Pd (grade 7), Ti-5Al-2.5Sn (grade 6), Ti-15V-3Cr-3Sn-3Al (TIMETAL 15.3), TIMETAL 1100 and Ti-5.8Al-4Sn-3.5Zr-0.7Nb (IMI 834).

Titanium alloys can be divided into three main groups:
1) Corrosion resistant alloys, such as Commercially Pure grades 1,2,3,4, Ti-Pd (grade 7 and 16), Ti-3Al-2.5V (grade 9 and 18), Ti-Pd (grade 11 and 17), Ti-0.3Mo-0.8Ni (grade 12), BETAC (grade19 and 20).
2) High strength alloys are Ti-6Al-4V (grade 5), Ti-5Al-2.5Sn (grade 6), Ti-6Al-6V-2Sn, Ti-10V-2Fe-3Al, Ti-15V-3Cr-3Sn-3Al, Ti-5Al-2Sn-4Mo-2Zr-4Cr, Ti-4Al-4Mo-2Sn (Ti550), Ti-8Al-1Mo-1V.
3) High temperature alloys like Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-2Sn-4Zr-6Mo, Ti-11Sn-5Zr-2.5Al-1Mo-0.2Si (IMI679), Ti-6Al-5Zr-0.5Mo-Si (IMI685), Ti-5.5Al-3.5Sn-3Zr-1Nb (IMI829), Ti-5.8Al-4Sn-3.5Zr-0.7Nb (IMI834), TIMETAL 1100 and the titanium alumnides.

Ti-6Al-4V is the most widely used of the titanium alloys as it can be heat-treated to different strength levels, is readily weldable and is relatively easy to machine. The many uses of Ti-6Al-4V include blades and discs for aircraft turbines and compressors, rocket motor cases, marine components, steam turbine blades, structural forgings and fasteners. To enhance durability a range of thermal, mechanical, chemical and other treatments have been developed to modify the surface characteristics and a considerable body of data on its use and properties is available.

Three structural types of titanium alloys are:
1. Alpha alloys are non-heat treatable and are generally very weldable. They have low to medium strength, good notch toughness, reasonably good ductility and possess excellent mechanical properties at cryogenic temperatures. The more highly alloyed alpha and near-alpha alloys offer optimum high temperature creep strength and oxidation resistance as well.
2. Alpha-Beta alloys are heat treatable and most are weldable. Their strength levels are medium to high. Their hot-forming qualities are good, but the high temperature creep strength is not as good as in most alpha alloys.
3. Beta or near-beta alloys are readily heat treatable, generally weldable, and capable of high strengths and good creep resistance to intermediate temperatures. Excellent formability can be expected of the beta alloys in the solution treated condition. Beta-type alloys have good combinations of properties in sheet, heavy sections, fasteners and spring applications.

Alpha alloys

Alpha alloys (α-alloys) are easily welded and are relatively tough even at cryogenic temperatures. Aluminum is the main alloying element apart from Zr and Sn. The combined effect is expressed as:

aluminium equivalent, wt% = Al + (1/3) Sn + (1/6) Zr + 10 (O + C + 2N)

If this exceeds about 9 wt% then there may be detrimental precipitation reactions.

The presence of a small amount of the more ductile β-phase in nearly α alloys is advantageous for heat treatment and the ability to forge. The alloys may therefore contain some 1wt% of Mo e.g.

Ti - 6Al - 2Sn - 4Zr - 2Mo

where the Zr and Sn give solid solution strengthening.

Ti-5Al-2.5Sn is an α alloy which is available commercially in many forms. Because it is stable in the α condition, it cannot be hardened by heat treatment. It is therefore not particularly strong, but can easily be welded. The toughness at cryogenic temperatures increases when the oxygen, carbon and nitrogen concentrations are reduced to produce a variant designated ELI, standing for extra low interstitials. The fact that the strength increases at low temperatures, without any deterioration in toughness, makes the alloy particularly suitable for the manufacture of cryogenic storage vessels, for example to contain liquid hydrogen.

Near-α alloys

A near-α alloy has been developed, with good elevated temperature properties (T<590°C):

Ti - 6Al - 4Sn - 3.5Zr - 0.5Mo - 0.35Si - 0.7Nb - 0.06C

The niobium is added for oxidation resistance and the carbon to allow a greater temperature range over which the alloy is a mixture of α+β, in order to facilitate thermomechanical processing. This particular alloy is used in the manufacture of aero engine discs and has replaced discs made from much heavier nickel base super alloys. The final microstructure of the alloy consists of equiaxed primary-α grains, Widmanstätten α plates separated by the β-phase.

Alpha-Beta Alloys (α+β Alloys)

Most α+β alloys have high-strength and formability, and contain 4-6 wt% of β-stabilizers which allow substantial amounts of β to be retained on quenching from the β→α+β phase fields, e.g. Ti-6Al-4V.

Al reduces density, stabilizes and strengthens α while vanadium provides a greater amount of the more ductile β phase for hot-working. This alloy, which accounts for about half of all the titanium that is produced, is popular because of its strength (1100 MPa), creep resistance at 300°C, fatigue resistance and castability.

One difficulty with the β phase, which has a body-centered cubic crystal structure, is that like ferritic iron, it has a ductile-brittle transition temperature. The transition temperature tends to be above room temperature, with cleavage fracture dominating at ambient temperatures.

A powder metallurgical variant of Ti-6Al-4V, containing small concentrations of boron and carbon has been developed with an approximately 25% higher strength and modulus, but significantly lower ductility. The alloy contains stable TiB precipitates which prevent grain growth during the hot-processing operations.

Burn-resistant β-alloys

Titanium fires can occasionally occur in aero engines or in titanium-based heat exchangers used in the chemical industries. The addition of chromium in concentrations exceeding 10 wt% helps to improve the burn-resistance of titanium alloys.

The alloy Ti-35V-15Cr wt%, has sufficient chromium to resist burning in an aero engine environment to temperatures up to about 510°C. The chromium is not found to be effective in binary Ti-Cr alloys where it does not encourage the formation of a continuous film of protective oxide.

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