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Introduction to Selection of Titanium Alloys

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© 2000 ASM International. All Rights Reserved. www.asminternational.org Titanium: A Technical Guide, 2nd Edition (#06112G) Chapter 2 Introduction to Selection of Titanium Alloys General Background composition. Titanium aluminide alloys show • Bar promise for applications at temperatures up to • Sheet 760 °C (1400 °F). • Strip TITANIUM is a low-density element (ap- Titanium and titanium alloys are produced in • Tube proximately 60% of the density of steel and a wide variety of product forms, with some ex- • Plate superalloys) that can be strengthened greatly by amples shown in Fig. 2.1. Titanium can be alloying and deformation processing. (Charac- wrought, cast, or made by P/M techniques. It Nonmill products teristic properties of elemental titanium are may be joined by means of fusion welding, given in Table 2.1.) Titanium is nonmagnetic brazing, adhesives, diffusion bonding, or fas- • Sponge and has good heat-transfer properties. Its coef- teners. Titanium and its alloys are formable and • Powder ficient of thermal expansion is somewhat lower readily machinable, assuming reasonable care than that of steel and less than half that of alu- is taken. Customized product forms minum. Titanium and its alloys have melting Some specific examples of product forms are: points higher than those of steels, but maxi- • Forgings mum useful temperatures for structural applica- Mill products • P/M items tions generally range from as low as 427 °C • Castings (800 °F) to the region of approximately 538 °C • Ingot to 595 °C (1000 °F to 1100 °F), dependent on • Billet One of many different types of investment cast titanium parts now produced is shown in Fig. 2.2. Figure 2.3 shows a large forged titanium part. This part weighs approximately 1400 kg Table 2.1 Physical and mechanical properties of elemental titanium (3000 lb). Titanium has the ability to passivate and Property Description or value thereby exhibit a high degree of immunity Atomic number 22 against attack by most mineral acids and chlo- Atomic weight 47.90 Atomic volume 10.6 W/D rides. Pure titanium is nontoxic; commercially Covalent radius 1.32 Å pure titanium and some titanium alloys gener- Ionization potential 6.8282 V ally are biologically compatible with human Thermal neutron absorption cross section 5.6 barns/atom Crystal structure tissues and bones. Alpha (≤882.5 °C, or 1620 °F) Close-packed hexagonal The excellent corrosion resistance and Beta ( ≥882.5 °C, or 1620 °F) Body-centered cubic biocompatibility coupled with good strengths Color Dark gray make titanium and its alloys useful in chemical Density 4.51 g/cm3 (0.163 lb/in.3) Melting point 1668 ± 10 °C (3035 °F) and petrochemical applications, marine envi- Solidus/liquidus 1725 °C (3135 °F) ronments, and biomaterials applications. The Boiling point 3260 °C (5900 °F) combination of high strength, stiffness, good Specific heat (at 25 °C) 0.5223 kJ/kg ⋅ K toughness, low density, and good corrosion re- Thermal conductivity 11.4 W/m ⋅ K Heat of fusion 440 kJ/kg (estimated) sistance provided by various titanium alloys at Heat of vaporization 9.83 MJ/kg very low to elevated temperatures allows Specific gravity 4.5 weight savings in aerospace structures and Hardness 70 to 74 HRB Tensile strength 240 MPa (35 ksi) min other high-performance applications. Young’s modulus 120 GPa (17 × 106 psi) Poisson’s ratio 0.361 Coefficient of friction At 40 m/min (125 ft/min) 0.8 Selection of At 300 m/min (1000 ft/min) 0.68 Coefficient of linear thermal expansion 8.41 µm/m ⋅ K Titanium Alloys for Service Electrical conductivity 3% IACS (where copper = 100% IACS) Electrical resistivity (at 20 °C) 420 nΩ⋅m Electronegativity 1.5 Pauling’s Primary Aspects. Titanium and its alloys Temperature coefficient of electrical resistance 0.0026/°C are used primarily in two areas of application Magnetic susceptibility (volume, at room temperature) 180 ( ±1.7) × 10–6 mks where the unique characteristics of these metals © 2000 ASM International. All Rights Reserved. www.asminternational.org Titanium: A Technical Guide, 2nd Edition (#06112G) 6 / Titanium: A Technical Guide (d) (a) (b) (c) (e) (f) (g) Fig. 2.1 Some titanium and titanium alloys product forms. (a) Strip. (b) Slab. (c) Billet. (d) Wire. (e) Sponge. (f) Tube. (g) Plate. Courtesy of Teledyne Wah Chang Albany Fig. 2.2 Investment cast titanium transmission case for Osprey vertical take-off and landing aircraft Fig. 2.3 Forged titanium landing gear beam for Boeing 757 aircraft © 2000 ASM International. All Rights Reserved. www.asminternational.org Titanium: A Technical Guide, 2nd Edition (#06112G) Introduction to Selection of Titanium Alloys / 7 justify their selection: corrosion-resistant ser- Ti-6V-2Sn-2Zr-2Cr-2Mo+Si are used or Selection for Corrosion Resistance. Eco- vice and strength-efficient structures. For these planned for use in aircraft or in gas turbine en- nomic considerations normally determine two diverse areas, selection criteria differ gines for aerospace applications. whether titanium alloys will be used for corro- markedly. Corrosion applications normally use Desired mechanical properties such as yield sion service. Capital expenditures for titanium lower-strength “unalloyed” titanium mill prod- or ultimate strength to density (strength effi- equipment generally are higher than for equip- ucts fabricated into tanks, heat exchangers, or ciency), fatigue crack growth rate, and fracture ment fabricated from competing materials such reactor vessels for chemical-processing, desali- toughness, as well as manufacturing consider- as stainless steel, brass, bronze, copper nickel, nation, or power-generation plants. In contrast, ations such as welding and forming require- or carbon steel. As a result, titanium equipment high-performance applications such as gas tur- ments, are extremely important. These factors must yield lower operating costs, longer life, or bines, aircraft structures, drilling equipment, normally provide the criteria that determine the reduced maintenance to justify selection, which and submersibles, or even applications such as alloy composition, structure (alpha, alpha-beta, most frequently is made on a lower total- biomedical implants, bicycle frames, and so or beta), heat treatment (some variant of either life-cycle cost basis. on, typically use higher-strength titanium al- annealing or solution treating and aging), and Commercially pure (CP) titanium satisfies loys. However, this use is in a very selective level of process control selected or prescribed the basic requirements for corrosion service. manner that depends on factors such as thermal for structural titanium alloy applications. A Unalloyed titanium normally is produced to environment, loading parameters, corrosion en- summary of some commercial and semi- requirements such as those of ASTM standard vironment, available product forms, fabrica- commercial titanium grades and alloys is given specifications B 265, B 338, or B 367 in tion characteristics, and inspection and/or reli- in Table 2.2. grades 1, 2, 3, and 4 in the United States. These ability requirements (Fig. 2.4). Alloys for For lightly loaded structures, where titanium grades vary in oxygen and iron content, which high-performance applications in strength-effi- normally is selected because it offers greater re- control strength level and corrosion behavior, cient structures normally are processed to more sistance to the effects of temperature than alu- respectively. For certain corrosion applica- stringent and costly requirements than “unal- minum offers, commercial availability of re- tions, Ti-0.2Pd (ASTM grades 7, 8, and 11) loyed” titanium for corrosion service. As exam- quired mill products, along with ease of may be preferred over unalloyed grades 1, 2, ples of use, alloys such as Ti-6Al-4V and fabrication, may dictate selection. Here, one of 3, and 4. Ti-3Al-8V-6Cr-4Mo-4Zr are being used for the grades of unalloyed titanium usually is cho- Selection for Strength and Corrosion Re- offshore drilling applications and geothermal sen. In some cases, corrosion resistance, not sistance. Due to its unique corrosion behavior, piping, while alloys such as Ti-6Al-4V, strength or temperature resistance, may be the titanium is used extensively in prosthetic de- Ti-6Al-2Sn-4Zr-2Mo+Si, Ti-10V-2Fe-3Al, and major factor in selection of a titanium alloy. vices such as heart-valve parts and load-bearing (a) (b) (c) (d) Fig. 2.4 A few typical areas of application for high-performance titanium parts. (a) Offshore drilling rig components. (b) Subsea equipment and submersibles requiring ultrastrength. (c) Aircraft. (d) Components for marine and chemical processing operations. © 2000 ASM International. All Rights Reserved. www.asminternational.org Titanium: A Technical Guide, 2nd Edition (#06112G) 8 / Titanium: A Technical Guide hip and other bone replacements. In general, and at the same time to have sufficient total weight of all titanium alloys shipped. Dur- body fluids are chloride brines that have pH workability to be fabricated into mill products ing the life of the titanium industry, various values from 7.4 into the acidic range and also suitable for a specific application. compositions have had transient usage; contain a variety of organic acids and other Selection for Other Property Reasons. Ti-4A1-3Mo-1V, Ti-7A1-4Mo, and Ti-8Mn components—media to which titanium is to- Optic-system support structures arealit- are a few examples. Many alloys have been in- tally immune. Ti-6Al-4V normally is employed tle-known but very important structural appli- vented but have never seen significant commer- for applications requiring higher strength, but cation for titanium. Complex castings are used cial use. Ti-6Al-4V alloy is unique in that it other titanium alloys are used as well. Moder- in surveillance and guidance systems for air- combines attractive properties with inherent ately high strength is important in the applica- craft and missiles to support the optics where workability (which allows it to be produced in tion of titanium to prosthetics, but strength effi- wide temperature variations are encountered in all types of mill products, in both large and ciency (strength to density) is not the prime service.
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