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Titanium

A Technical Guide

Second Edition

Matthew J. Donachie, Jr.

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Copyright2000 by

No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or other-wise, without the written permission of the copyright owner.

First printing, December 2000

Great care is taken in the compilation and production of this book, but it should be made clear that NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PAR-TICULAR PURPOSE, ARE GIVEN IN CONNECTION WITH THIS PUBLICATION. Although this information is believed to be accurate by ASM, ASM cannot guarantee that favorable results will be obtained from the use of this publication alone. This publi-cation is intended for use by persons having technical skill, at their sole discretion and risk. Since the conditions of product or material use are outside of ASM’s control, ASM assumes no liability or obligation in connection with any use of this information. No claim of any kind, whether as to products or information in this publication, and whether or not based on negligence, shall be greater in amount than the purchase price of this product or publication in respect of which damages are claimed. THE REMEDY HEREBY PROVIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under end-use conditions prior to specification is essential. Therefore, specific testing under actual conditions is recommended.

Nothing contained in this book shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters patent, copyright, or trade-mark, and nothing contained in this book shall be construed as a defense against any al-leged infringement of letters patent, copyright, or trademark, or as a defense against lia-bility for such infringement.

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Library of Congress Cataloging-in-Publication Data

Titamium: a technical guide / Matthew J. Donachie, Jr.—2nd ed. p. cm. Includes bibliographical references and index.

1. Titanium. 2. Titanium alloys. I. Title. TA480.T54 D66 2000 669’.7322—dc21 00-033134 ISBN: 0-87170-686-5 SAN: 204-7586 ASM International® Materials Park, OH 44073-0002 www.asminternational.org

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I wish to dedicate this book to my wife, Martha. She has been with me through many an adventure in this life and has put up with uncounted hours of my toil-ing on books, lectures and the like.

My life is a homing bird that flies Through the starry dusk and dew Home to the heaven of your true eyes Home, dear heart, to you.

from the poem My Life is a Bowl by May Riley Smith

When my hair shall shade the snowdrift, And mine eyes shall dimmer grow, I would lean upon some loved one, Through the valley as I go. I would claim of you a promise, Worth to me a world of gold: It is only this, my darling, That you’ll love me when I’m old.

from the poem Will You Love Me When I’m Old author unknown

Sing, for faith and hope are high--None so true as you and I--Sing the Lover’s Litany: “Love like ours can never die!”

from the poem Lovers Litany by Rudyard Kipling

Matt

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iv Sunniva R. Collins (Chair)

Swagelok/Nupro Company Eugen Abramovici

Bombardier Aerospace (Canadair) A.S. Brar

Seagate Technology Inc. Ngai Mun Chow

Det Norske Veritas Pte Ltd. Seetharama C. Deevi

Philip Morris, USA Bradley J. Diak

Queen’s University James C. Foley

Ames Laboratory Dov B. Goldman

Precision World Products James F.R. Grochmal

Metallurgical Perspectives Nguyen P. Hung

Nanyang Technological University

Serope Kalpakjian

Illinois Institute of Technology Gordon Lippa

North Star Casteel Jacques Masounave

Université du Québec Charles A. Parker

AlliedSignal Aircraft Landing Systems

K. Bhanu Sankara Rao

Indira Gandhi Centre for Atomic Research

Mel M. Schwartz

Sikorsky Aircraft Corporation (retired)

Peter F. Timmins

University College of the Fraser Valley

George F. Vander Voort Buehler Ltd.

ASM International Technical

Books Committee (1999-2000)

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Preface

Titanium and its alloys continue to provide excellent service in a variety of industries. As we progress into the twenty-first century, in the sixth decade of titanium’s commercial and industrial use, it appears that the industry has matured, but new technology and applications for the metal continue to develop. Despite the utility of titanium and its alloys, the number of books dealing with the metal has been limited. A series of

International Conferences on Titanium, held periodically since

1968, have provided a focus for research reports while other occasional symposia and articles have contributed to the industrial literature on titanium. ASM International has been a leader in providing coverage of titanium and its alloys and has issued several books, including the first edition of Titanium: A

Technical Guide.

Titanium: A Technical Guide, Second Edition, is meant to

provide the most complete introduction possible to the metal and its alloys through the use of 14 chapters and 11 appendices. The aim has been to condense and review the significant features of the metallurgy and application of titanium and its alloys. The text has been revised and expanded from that of the first edition with many additional figures and new and revised tables. The second edition of the Guide not only contains more information than the previous edition, but the book also has been modified to a larger page size to better accommodate the tables provided. All technical aspects of the use of titanium are covered with sufficient metals property data for most users. The

Guide has been reviewed for accuracy, but it is possible that

errors will have occurred. The editor would appreciate receiving either corrections or suggestions from readers.

If you are new to the use of titanium, I would strongly recommend starting with Chapter 1: A Primer on Titanium and Its Alloys. This executive summary of the metal and its uses

should suit the needs of readers who require a brief introduction to titanium and who do not have time to devote to more intense study of the subject. If you are knowledgeable in metallurgy and/or materials engineering, or wish more in-depth information, you may prefer to choose from one of the chapter topics or the appendices that is more relevant to your immediate needs. For additional property data, see the ASM book

Materials Property Handbook: Titanium Alloys.

The editor wishes to thank not only those who contributed to the first edition of Titanium: A Technical Guide, but also the many contributors to other ASM books and the ASM Handbook series. This book is a product of the editor’s experience and personal bias, as well as his technical files. Most of all, however, it is a product of the resources available in the ASM International system. The editor especially would like to thank Veronica Flint of ASM International for her perseverance with him as the material made its way into electronic form. Veronica and I worked together on the first edition of Titanium: A

Technical Guide, and it has been a pleasure to work with her

again on this significant update. Its successful publication is a tribute to the dedication of ASM International to providing access to materials information for the widest possible audience.

M. J. D.

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Chapter 1 A Primer on Titanium and Its Alloys

ReadMe.First

IN THE BUSINESS WORLD OF TODAY, the extended treatment offered by many refer-ence books may pose an obstacle to a manager or other person needing to find information on a specific topic in a reasonable time. This is es-pecially true when only an operational under-standing of a subject is required.Titanium: A Technical Guide, Second Edition addresses the

need for a concise printed summary of the most useful information required to understand tita-nium and its alloys.

Even in a summary volume, there is a need for an overview of the technical aspects of a metal. This primer supports the needs of engi-neering, management, and other professionals for information on titanium by providing a brief overview of the major topics that are discussed more thoroughly throughout the book. The in-formation in this chapter will maximize the reader’s ability to use the volume in the most efficient way and, at the same time, help the reader to glean enough information to satisfy his or her immediate requirements.

After reading the primer, the reader might wish to refer to the Contents and the Index to locate more information about specific topics. Helpful information can also be found in the Glossary (Appendix J) and the list of Symbols (Appendix I).

Why Use Titanium and Its Alloys?

Titanium was discovered in 1790 but not pu-rified until the early 1900s. Moreover, the metal did not become widely used until the sec-ond half of the twentieth century. However, ti-tanium now has the accumulated experience of some 50 years of modern industrial practice and design application to support its use. Much of this use has come in military applications in aircraft such as the SR71 (Fig. 1.1) or gas tur-bine engines (Fig. 1.2). More recent uses have featured such items as golf clubs and bicycles. Titanium has found its niche in many indus-tries, owing to its unique density, corrosion re-sistance, and relative strength advantages over competing materials such as aluminum, steels, and superalloys. Some significant facts and/or

important benefits offered by titanium alloys il-lustrate the basis for the widespread use of titanium today:

•

The density of titanium is only about 60% of that of steel or nickel-base superalloys.

•

The tensile strength (as an alloy) of titanium

can be comparable to that of lower-strength martensitic stainless and is better than that of austenitic or ferritic stainless. Alloys can have ultimate strengths comparable to iron-base superalloys, such as A286, or cobalt-base alloys, such as L605.

•

The commercial alloys of titanium are useful at temperatures to about 538°C to 595 °C (1000°F to 1100 °F), dependent on composi-tion. Some alloy systems (titanium alumi-nides) may have useful strengths above this temperature.

•

The cost of titanium, while approximately four times that of stainless steel, is compara-ble to that of superalloys.

•

Titanium is exceptionally corrosion resistant. It often exceeds the resistance of stainless steel in most environments, and it has outstanding corrosion resistance in the human body.

Fig. 1.1 SR71 aircraft: first use of beta alloys in aerospace structures. Courtesy of

Lockheed California Co. Fig. 1.2 F119 engine by Pratt & Whitney powering the F22 Raptor aircraft

Titanium: A Technical Guide

Matthew J. Donachie, Jr., p1-3 DOI:10.1361/tatg2000p001

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•

Titanium may be forged or wrought by stan-dard techniques.

•

Titanium is castable, with investment cast-ing the preferred method. (Investment cast titanium alloy structures have a lower cost than conventional forged/wrought fabri-cated titanium alloy structures.)

•

Titanium may be processed by means of P/M technology. (Powder may cost more, yet P/M may offer property and processing im-provements as well as an overall cost-sav-ings potential.)

•

Titanium may be joined by means of fusion welding, brazing, adhesives, diffusion bond-ing, and fasteners.

•

Titanium is formable and readily machin-able, assuming reasonable care is taken.

•

Titanium is available in a wide variety of

types and forms.

Titanium Metallurgy—

A Short Course

Structures in General

The melting point of titanium is in excess of 1660°C (3000 °F), although most commercial alloys operate at or below 538°C (1000 °F). Ti-tanium has two elemental crystal structures: in one, the atoms are arranged in a body-centered cubic (bcc) array; in the other, the atoms are ar-ranged in a close-packed hexagonal array (Fig. 1.3). The cubic structure is found only at high temperatures, unless the titanium is alloyed with other elements to maintain the cubic struc-ture at lower temperastruc-tures.

The two crystal structures of titanium are commonly known as alpha and beta. Alpha ac-tually refers to any hexagonal titanium, pure or alloyed, while beta denotes any cubic titanium, pure or alloyed. The alpha and beta “struc-tures”—sometimes called systems or types— are the basis for the generally accepted four classes of titanium alloys: alpha, near-alpha, al-pha-beta, and beta.

Figure 1.4 schematically shows some effects of alloying elements on structure for represen-tative alloys and classes or subclasses of tita-nium alloys. The figure also indicates the ef-fects that structures have on some selected properties. The alloy compositions indicated

are not meant to be all inclusive but rather to suggest some of the alloys used in titanium al-loy design.

More on Structure

Commercially pure (CP) titanium is alpha in structure. Additions of alloying elements to pure titanium produce the range of possible microstructures in titanium alloys.

With sufficient beta-favoring alloy element level, beta phase is produced on heating and transformed during the cooling following high processing. The resulting structures are repre-sentative of the alpha-beta alloys.

A variation of alpha alloys recognizes the wide range of alloy chemistry and structure possible within the essentially alpha range. This variation is termednear-alpha.

Beta structures generally should be referred to as metastable beta. These are alloys that re-tain an essentially beta structure on cooling to room temperature.

Titanium aluminides are intermetallic com-pounds of titanium and aluminum (with one or

more additional alloy element provided as well).

Titanium and

Titanium Alloy Characteristics

Commercially pure titanium and the alpha and near-alpha titanium alloys generally dem-onstrate the best general corrosion-resistance qualities. They are the most weldable of the ti-tanium/titanium alloy family.

Pure titanium usually has some amount of oxygen alloyed with it. The strength of CP tita-nium is affected by the interstitial (oxygen and nitrogen) element content.

Alpha alloys usually have high amounts of aluminum that contribute to oxidation resis-tance at high temperatures. (Alpha-beta alloys also contain, as the principal element, high amounts of aluminum, but the primary reason is to stabilize the alpha phase.)

Alpha alloys cannot be heat treated to de-velop higher mechanical properties because they are single-phase alloys. The addition of certain alloying elements to pure titanium

en-2 / Titanium: A Technical Guide

(a) (b)

Fig. 1.3 Appearance of crystal structures of titanium at

the atomic level. (a) Hexagonal, close packed.

(b) Cubic, body centered Fig. 1.4

Schematic showing effects of alloy elements on structure and some selected proper-ties (representative alloys noted)

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ables the resultant alloys to be heat treated or processed in the temperature range where the alloy is two phase (alpha and beta). The two-phase condition permits the structure to be refined and, by permitting some beta to be re-tained temporarily at lower temperature, en-ables optimum control of the microstructure during subsequent transformation when the al-loys are “aged” after cooling from the forging or solution heat treatment temperature.

The alpha-beta alloys, when properly treated, have an excellent combination of strength and ductility. They are stronger than the alpha or the beta alloys.

The beta alloys are metastable; that is, they tend to transform to an equilibrium, or balance of structures. The beta alloys generate strength from the intrinsic strength of the beta structure and the precipitation of alpha and other phases from the alloy through heat treatment after pro-cessing.

The most significant benefit provided by a beta structure is the increased formability of such alloys relative to the hexagonal crystal structure types (alpha and alpha-beta).

Titanium aluminides differ from conven-tional titanium alloys in that they are princi-pally chemical compounds alloyed to enhance strength, formability, and so on. The alumin-ides have higher operational temperatures than conventional titanium, but at higher cost, and generally have lower ductility and formability.

Getting the Most

Out of Titanium Alloys

The greatest potential that titanium and tita-nium alloys can provide in a specific applica-tion is realized if a few simple rules of thumb are kept in mind initially before a design is ac-tually begun. Some of the more important guidelines are as follows:

•

Wrought titanium alloy products are the more readily available, but castings are close be-hind. Wrought alloys also have the greatest experience factor. Castings, however, are useful for savings in weight and cost. Cast-plus-HIP (hot isostatic pressed) mate-rial can attain comparable operating strength levels to wrought products for most alloys.

•

Powder alloys are becoming more accepted.

Also, powder processing allows more exotic titanium alloys to be mixed. However, be-cause of the interaction of titanium with inter-stitial gases such as oxygen and nitrogen, complex powder production techniques are necessary. Consequently, titanium alloy powder may be too expensive for many ap-plications. Furthermore, property levels for powder-processed conventional alloy com-positions may not reach expectations. Nev-ertheless, with powder, there is the built-in, and possibly cost-offsetting, near-net shape (NNS) capability that powder offers. This

implies at least a potential for overall lower costs when amortized over the entire project.

•

Cast or powder titanium alloys always should be possible candidate materials for structural applications. However, planning for such use should begin during the initial design stage rather than waiting and trying to fit the cast or powder-processed material into a wrought alloy design late in the devel-opmental stages.

•

It is wise when making a titanium alloy se-lection to use the more common alloys un-less uncommon properties are absolutely needed. (Ti-6A1-4V clearly has widespread advantages, or else it would not be so com-monly used.)

•

Handbooks, reference material, and so on all are valuable in design. Numerous handbooks are available (Appendix K provides a se-lected references list), but there is no substi-tute for personal contact with a supplier or fabricator. (A partial list of titanium trade or-ganizations, suppliers, and primary metal fabricators appears in Appendix E.)

•

Properties that assume unusual forming

con-ditions and/or unrealistic casting or powder processing yields should not be depended on, nor should unusual cooling or heating practices for properties. Cast and powder al-loy properties may fall short of the best of wrought alloy properties. Typical properties may be roughly comparable, but data scatter in cast (and possibly in powder) products could result in lower design minimums. If a design admits of no flexibility with respect to property level realization, the design may be irreversibly compromised later.

•

Aerospace specifications provide for the best properties and performance. When us-ing titanium in noncritical applications, less stringent specifications should be chosen, where possible, to save money and time.

Some Thoughts about the Future

The dynamic nature of industry as well as developments of a political nature can and will continue to affect the future of the titanium in-dustry. For up-to-date information on business aspects of titanium, trade groups such as those listed in Appendix E can be contacted. How-ever, some projections about the technical as-pects of titanium use can be made:

•

Titanium alloy compositions available and used in the near future will remain substan-tially the same as those available at the end of the twentieth century, although the rela-tive mix of alloys may change. Aerospace product volume is declining; fewer funds are available for research. A result is that new ti-tanium alloy composition development will diminish. Furthermore, nonaerospace appli-cations are consuming more titanium than in the early years of titanium development. Most of these applications use existing

al-loys that are available with limited added de-velopment costs.

•

Greater emphasis will continue to be placed on the use of cast alloys.

•

Textured alloys may be accepted for selected applications. (While these are technically feasible, there still is no real driving factor behind the concept.)

•

Superplastic forming in conjunction with bonding should increase in favor, although it may remain largely a process for the aero-space industry.

•

Advanced P/M processed materials will con-tinue to be worked, but extensive cost-effec-tive applications are unlikely in the near fu-ture. Much development work will be needed before P/M techniques can effec-tively be applied to an application. A good property base does not yet exist.

•

Rapid solidification rate (RSR) processing is comparable to P/M in application and is not likely to be useful for most commercial service.

•

Aluminides will continue to be developed

and tested for applications requiring higher-temperature capability, but economic appli-cation for industrial and commercial use is going to be limited for many years.

A Few Facts about

Titanium and Its Production

Titanium is the ninth most-abundant element on the planet and the fourth most-abundant structural metal. Mineral sources of titanium are rutile, ilmenite, and leucoxene, an alteration product of ilmenite.

Principal world producers of ilmenite and ti-tanium slag made from ilmenite are Australia, Canada, Norway, the Republic of South Africa, the United States, and Russia. Main producers of rutile are Australia, Sierra Leone, and the Republic of South Africa. Titanium sponge is produced mainly by Russia, Kazakhstan, the United States, Japan, the United Kingdom, and China. Titanium sponge and ingot are available worldwide.

The titanium business was in a state of flux during the 1990s. Consolidations and closures modified not only the business names but also the delivery of titanium services in the world. Since titanium operations start with the avail-ability of sponge and then ingot for remelt, casting, or for subsequent working, it is desir-able that some players in the titanium market be identified. The primary producers of titanium sponge and ingot in the United States at the end of the twentieth century were Timet, RMI, and Allegheny-Teledyne-Oremet.

In view of the fluidity of business operations, no other listing of titanium-related organizations is practical. When information is required, the appropriate trade organizations should be con-tacted as a start in locating titanium producers, fabricators, and other information for any tita-nium or titatita-nium alloy application (Appendix E provides a listing of such organizations).

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Chapter 2 Introduction to Selection of Titanium Alloys

General Background

TITANIUM is a low-density element (ap-proximately 60% of the density of steel and superalloys) that can be strengthened greatly by alloying and deformation processing. (Charac-teristic properties of elemental titanium are given in Table 2.1.) Titanium is nonmagnetic and has good heat-transfer properties. Its coef-ficient of thermal expansion is somewhat lower than that of steel and less than half that of alu-minum. Titanium and its alloys have melting points higher than those of steels, but maxi-mum useful temperatures for structural applica-tions generally range from as low as 427 °C (800°F) to the region of approximately 538 °C to 595°C (1000 °F to 1100 °F), dependent on

composition. Titanium aluminide alloys show promise for applications at temperatures up to 760°C (1400 °F).

Titanium and titanium alloys are produced in a wide variety of product forms, with some ex-amples shown in Fig. 2.1. Titanium can be wrought, cast, or made by P/M techniques. It may be joined by means of fusion welding, brazing, adhesives, diffusion bonding, or fas-teners. Titanium and its alloys are formable and readily machinable, assuming reasonable care is taken.

Some specific examples of product forms are: Mill products

•

Ingot

•

Billet

•

Bar

•

Sheet

•

Strip

•

Tube

•

Plate Nonmill products

•

Sponge

•

Powder

Customized product forms

•

Forgings

•

P/M items

•

Castings

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 (3000 lb).

Titanium has the ability to passivate and thereby exhibit a high degree of immunity against attack by most mineral acids and chlo-rides. Pure titanium is nontoxic; commercially pure titanium and some titanium alloys gener-ally are biologicgener-ally compatible with human tissues and bones.

The excellent corrosion resistance and biocompatibility coupled with good strengths make titanium and its alloys useful in chemical and petrochemical applications, marine envi-ronments, and biomaterials applications. The combination of high strength, stiffness, good toughness, low density, and good corrosion re-sistance provided by various titanium alloys at very low to elevated temperatures allows weight savings in aerospace structures and other high-performance applications.

Selection of

Titanium Alloys for Service

Primary Aspects. Titanium and its alloys are used primarily in two areas of application where the unique characteristics of these metals Table 2.1 Physical and mechanical properties of elemental titanium

Property Description or value

Atomic number 22

Atomic weight 47.90

Atomic volume 10.6W/D

Covalent radius 1.32 Å

Ionization potential 6.8282 V

Thermal neutron absorption cross section 5.6 barns/atom Crystal structure

Alpha (≤882.5 °C, or 1620 °F) Close-packed hexagonal

Beta (≥882.5 °C, or 1620 °F) Body-centered cubic

Color Dark gray

Density 4.51 g/cm3_{(0.163 lb/in.}3₎

Melting point 1668 ± 10 °C (3035 °F)

Solidus/liquidus 1725 °C (3135 °F)

Boiling point 3260 °C (5900 °F)

Specific heat (at 25 °C) 0.5223 kJ/kg⋅ K

Thermal conductivity 11.4 W/m⋅ K

Heat of fusion 440 kJ/kg (estimated)

Heat of vaporization 9.83 MJ/kg

Specific gravity 4.5

Hardness 70 to 74 HRB

Tensile strength 240 MPa (35 ksi) min

Young’s modulus 120 GPa (17 × 106_psi)

Poisson’s ratio 0.361 Coefficient of friction At 40 m/min (125 ft/min) At 300 m/min (1000 ft/min) 0.8 0.68 Coefficient of linear thermal expansion 8.41μm/m ⋅ K

Electrical conductivity 3% IACS (where copper = 100% IACS)

Electrical resistivity (at 20 °C) 420 nΩ ⋅ m

Electronegativity 1.5 Pauling’s

Temperature coefficient of electrical resistance 0.0026/°C Magnetic susceptibility (volume, at room temperature) 180 ( ±1.7) × 10–6_mks

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6 / Titanium: A Technical Guide

(a) (b) (c) (d)

(e) (f) (g)

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

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justify their selection: corrosion-resistant ser-vice and strength-efficient structures. For these two diverse areas, selection criteria differ markedly. Corrosion applications normally use lower-strength “unalloyed” titanium mill prod-ucts fabricated into tanks, heat exchangers, or reactor vessels for chemical-processing, desali-nation, or power-generation plants. In contrast, high-performance applications such as gas tur-bines, aircraft structures, drilling equipment, and submersibles, or even applications such as biomedical implants, bicycle frames, and so on, typically use higher-strength titanium al-loys. However, this use is in a very selective manner that depends on factors such as thermal environment, loading parameters, corrosion en-vironment, available product forms, fabrica-tion characteristics, and inspecfabrica-tion and/or reli-ability requirements (Fig. 2.4). Alloys for high-performance applications in strength-effi-cient structures normally are processed to more stringent and costly requirements than “unal-loyed” titanium for corrosion service. As exam-ples of use, alloys such as Ti-6Al-4V and Ti-3Al-8V-6Cr-4Mo-4Zr are being used for offshore drilling applications and geothermal piping, while alloys such as Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo+Si, Ti-10V-2Fe-3Al, and

Ti-6V-2Sn-2Zr-2Cr-2Mo+Si are used or planned for use in aircraft or in gas turbine en-gines for aerospace applications.

Desired mechanical properties such as yield or ultimate strength to density (strength effi-ciency), fatigue crack growth rate, and fracture toughness, as well as manufacturing consider-ations such as welding and forming require-ments, are extremely important. These factors normally provide the criteria that determine the alloy composition, structure (alpha, alpha-beta, or beta), heat treatment (some variant of either annealing or solution treating and aging), and level of process control selected or prescribed for structural titanium alloy applications. A summary of some commercial and semi-commercial titanium grades and alloys is given in Table 2.2.

For lightly loaded structures, where titanium normally is selected because it offers greater re-sistance to the effects of temperature than alu-minum offers, commercial availability of re-quired mill products, along with ease of fabrication, may dictate selection. Here, one of the grades of unalloyed titanium usually is cho-sen. In some cases, corrosion resistance, not strength or temperature resistance, may be the major factor in selection of a titanium alloy.

Selection for Corrosion Resistance. Eco-nomic considerations normally determine whether titanium alloys will be used for corro-sion service. Capital expenditures for titanium equipment generally are higher than for equip-ment fabricated from competing materials such as stainless steel, brass, bronze, copper nickel, or carbon steel. As a result, titanium equipment must yield lower operating costs, longer life, or reduced maintenance to justify selection, which most frequently is made on a lower total-life-cycle cost basis.

Commercially pure (CP) titanium satisfies the basic requirements for corrosion service. Unalloyed titanium normally is produced to requirements such as those of ASTM standard specifications B 265, B 338, or B 367 in grades 1, 2, 3, and 4 in the United States. These grades vary in oxygen and iron content, which control strength level and corrosion behavior, respectively. For certain corrosion applica-tions, Ti-0.2Pd (ASTM grades 7, 8, and 11) may be preferred over unalloyed grades 1, 2, 3, and 4.

Selection for Strength and Corrosion Re-sistance. Due to its unique corrosion behavior, titanium is used extensively in prosthetic de-vices such as heart-valve parts and load-bearing

Introduction to Selection of Titanium Alloys / 7

(a) (b)

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.

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hip and other bone replacements. In general, body fluids are chloride brines that have pH values from 7.4 into the acidic range and also contain a variety of organic acids and other components—media to which titanium is to-tally immune. Ti-6Al-4V normally is employed for applications requiring higher strength, but other titanium alloys are used as well. Moder-ately high strength is important in the applica-tion of titanium to prosthetics, but strength effi-ciency (strength to density) is not the prime criterion, assuming that biocompatibility con-cerns are addressed. However, while strength efficiency is not the defining factor, it has been suggested that the lesser weight of titanium al-loy implants plays a noticeable role in patient perception of the efficacy of the device im-planted in the body.

Selection for Strength Efficiency. His-torically, wrought titanium alloys have been used widely instead of iron or nickel alloys in aerospace applications because titanium saves weight in highly loaded components that oper-ate at low-to-moderoper-ately elevoper-ated temperatures. Many titanium alloys have been custom de-signed to have optimum tensile, compressive, and/or creep strength at selected temperatures,

and at the same time to have sufficient workability to be fabricated into mill products suitable for a specific application.

Selection for Other Property Reasons.

Optic-system support structures are a lit-tle-known but very important structural appli-cation for titanium. Complex castings are used in surveillance and guidance systems for air-craft and missiles to support the optics where wide temperature variations are encountered in service. The chief reason for selecting titanium for this application is that the thermal-expan-sion coefficient of titanium most closely matches that of the optics.

Although prosthetic applications for titanium alloys are made for biocompatibility and strength reasons, there is a benefit for structural implants such as hip stems because the lower modulus (than cobalt alloys and stainless) al-lows more load transfer to the bone and the po-tential for longer-lasting implant performance.

The Titanium Alloys

For most of the last half of the twentieth cen-tury, Ti-6Al-4V accounted for about 45% of the

total weight of all titanium alloys shipped. Dur-ing the life of the titanium industry, various compositions have had transient usage; Ti-4A1-3Mo-1V, Ti-7A1-4Mo, and Ti-8Mn are a few examples. Many alloys have been in-vented but have never seen significant commer-cial use. Ti-6Al-4V alloy is unique in that it combines attractive properties with inherent workability (which allows it to be produced in all types of mill products, in both large and small sizes), good shop fabricability (which al-lows the mill products to be made into complex hardware), and the production experience and commercial availability that lead to reliable and economic usage. Consequently, wrought Ti-6Al-4V became the standard alloy against which other alloys must be compared when se-lecting a titanium alloy (or custom designing one) for a specific application. Ti-6Al-4V also is the standard alloy selected for castings that must exhibit superior strength. It even has been evaluated in P/M processing. Ti-6Al-4V will continue to be the most-used titanium alloy for many years in the future.

Ti-6Al-4V has temperature limitations that restrict its use to approximately 400 °C (750 °F). For elevated-temperature applications, the

Table 2.2 Some commercial and semicommercial grades and alloys of titanium

Tensile strength (min) 0.2% yield strength (min) Impurity limits, wt% (max) Nominal composition, wt%

Designation MPa ksi MPa ksi N C H Fe O Al Sn Zr Mo Others

Unalloyed grades ASTM grade 1 240 35 170 25 0.03 0.08 0.015 0.20 0.18 … … … … … ASTM grade 2 340 50 280 40 0.03 0.08 0.015 0.30 0.25 … … … … … ASTM grade 3 450 65 380 55 0.05 0.08 0.015 0.30 0.35 … … … … … ASTM grade 4 550 80 480 70 0.05 0.08 0.015 0.50 0.40 … … … … … ASTM grade 7 340 50 280 40 0.03 0.08 0.015 0.30 0.25 … … … … 0.2Pd ASTM grade 11 240 35 170 25 0.03 0.08 0.015 0.20 0.18 … … … … 0.2Pd

α and near-α alloys

Ti-0.3Mo-0.8Ni 480 70 380 55 0.03 0.10 0.015 0.30 0.25 … … … 0.3 0.8Ni Ti-5Al-2.5Sn 790 115 760 110 0.05 0.08 0.02 0.50 0.20 5 2.5 … … … Ti-5Al-2.5Sn-ELI 690 100 620 90 0.07 0.08 0.0125 0.25 0.12 5 2.5 … … … Ti-8Al-1Mo-1V 900 130 830 120 0.05 0.08 0.015 0.30 0.12 8 … … 1 1V Ti-6Al-2Sn-4Zr-2Mo 900 130 830 120 0.05 0.05 0.0125 0.25 0.15 6 2 4 2 0.08Si Ti-6Al-2Nb-1Ta-0.8Mo 790 115 690 100 0.02 0.03 0.0125 0.12 0.10 6 … … 1 2Nb, 1Ta Ti-2.25Al-11Sn-5Zr-1Mo 1000 145 900 130 0.04 0.04 0.008 0.12 0.17 2.25 11 5 1 0.2Si Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si 1030 149 910 132 0.03 0.08 0.006 0.05 0.15 5.8 4 3.5 0.5 0.7Nb, 0.35Si α-β alloys Ti-6Al-4V(a) 900 130 830 120 0.05 0.10 0.0125 0.30 0.20 6 … … … 4V Ti-6Al-4V-ELI(a) 830 120 760 110 0.05 0.08 0.0125 0.25 0.13 6 … … … 4V Ti-6Al-6V-2Sn(a) 1030 150 970 140 0.04 0.05 0.015 1.0 0.20 6 2 … … 0.75Cu, 6V Ti-8Mn(a) 860 125 760 110 0.05 0.08 0.015 0.50 0.20 … … … … 8.0Mn Ti-7Al-4Mo(a) 1030 150 970 140 0.05 0.10 0.013 0.30 0.20 7.0 … … 4.0 … Ti-6Al-2Sn-4Zr-6Mo(b) 1170 170 1100 160 0.04 0.04 0.0125 0.15 0.15 6 2 4 6 … Ti-5Al-2Sn-2Zr-4Mo-4Cr(b)(c) 1125 163 1055 153 0.04 0.05 0.0125 0.30 0.13 5 2 2 4 4Cr Ti-6Al-2Sn-2Zr-2Mo-2Cr(c) 1030 150 970 140 0.03 0.05 0.0125 0.25 0.14 5.7 2 2 2 2Cr, 0.25Si Ti-3Al-2.5V(d) 620 90 520 75 0.015 0.05 0.015 0.30 0.12 3 … … … 2.5V

Ti-4Al-4Mo-2Sn-0.5Si 1100 160 960 139 (e) 0.02 0.0125 0.20 (e) 4 2 … 4 0.5Si

β alloys Ti-10V-2Fe-3Al(a)(c) 1170 170 1100 160 0.05 0.05 0.015 2.5 0.16 3 … … … 10V Ti-13V-11Cr-3Al(b) 1170 170 1100 160 0.05 0.05 0.025 0.35 0.17 3 … … … 11.0Cr, 13.0V Ti-8Mo-8V-2Fe-3Al(b)(c) 1170 170 1100 160 0.03 0.05 0.015 2.5 0.17 3 … … 8.0 8.0V Ti-3Al-8V-6Cr-4Mo-4Zr(a)(c) 900 130 830 120 0.03 0.05 0.20 0.25 0.12 3 … 4 4 6Cr, 8V Ti-11.5Mo-6Zr-4.5Sn(a) 690 100 620 90 0.05 0.10 0.020 0.35 0.18 … 4.5 6.0 11.5 … Ti-15V-3Cr-3Al-3Sn 1000(b) 145(b) 965(b) 140(b) 0.05 0.05 0.015 0.25 0.13 3 3 … … 15V, 3Cr 1241(f) 180(f) 1172(f) 170(f) Ti-15Mo-3Al-2.7Nb-0.2Si 862 125 793 115 0.05 0.05 0.015 0.25 0.13 3 … … 15 2.7Nb, 0.2Si

(a) Mechanical properties given for the annealed condition; may be solution treated and aged to increase strength. (b) Mechanical properties given for the solution-treated-and-aged condition; alloy not normally applied in an-nealed condition. (c) Semicommercial alloy; mechanical properties and composition limits subject to negotiation with suppliers. (d) Primarily a tubing alloy; may be cold drawn to increase strength. (e) Combined O2+ 2N2= 0.27%. (f) Also solution treated and aged using an alternative aging temperature (480 °C, or 900 °F)

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most commonly used alloy is Ti-6Al-2Sn-4Zr-2Mo + Si. This alloy is primarily used for turbine components and in sheet form for after-burner structures and various “hot” airframe applications. Titanium aluminides may dis-place the latter alloy but not for commercial ap-plications in the foreseeable future.

During the approximately 50 years that tita-nium has been commercially available, many other alloys have been developed, but none match the almost 50% market share that Ti-6Al-4V enjoys. In addition to the use of 6Al-4V, Pratt & Whitney has used Ti-8Al-1Mo-1V, Ti-5A1-2.5Sn, Ti-6Al-2Sn-4Zr-2Mo, and Ti-6Al-2Sn-4Zr-6Mo in its gas turbine en-gines. General Electric has used Ti-4A1-4Mn, Ti-l.5Fe-2.7Cr, and Ti-17 among other alloys in addition to the Ti-6Al-4V alloy. Rolls Royce has used IMI 550, IMI 679, IMI 685, IMI 829, and IMI 834 alloys as well as Ti-6Al-4V (IMI 318) in its engines. (IMI Titanium, Ltd. was a British producer-manufacturer that now oper-ates as Timet UK.) Some of these mentioned al-loys have found use in airframes. Other alal-loys used or evaluated extensively in aerospace, missile and space, and other high-performance applications have included Ti-6V-2Sn-2Zr-2Cr-2Mo + Si, Ti-6Al-6V-2Sn, Ti-10V-2Fe-3A1, and Ti-13V-11Cr-3A1. The latter alloy also is called BI2OVCA. It was the first of a line of metastable beta alloys, although it is now considered somewhat obsolete when com-pared with most contemporary alloys.

Chemical processing operations have been concerned principally with the unalloyed grades, palladium-containing pure grades, and Ti-6Al-4V. Ti-3A1-8V-6Cr-4Zr-4Mo (also called beta C) was approved for use in deep, sour-well technology. Other alloys are in various stages of use.

The reader may wish to refer to Appendix A (“Summary Table of Titanium Alloys”) and/or Appendix B (“Titanium Alloy Datasheets”) for more specific information on the types of alloys available and their possible applications.

Application and

Control of Titanium Alloys

Rotating components such as jet-engine blades and gas turbine parts require titanium al-loys that maximize strength efficiency and met-allurgical stability at elevated temperatures. These alloys also must exhibit low creep rates along with predictable behavior with respect to stress rupture and low-cycle fatigue. To reproducibly provide these properties, stringent user requirements are specified to ensure con-trolled, homogeneous microstructures and total freedom from melting imperfections such as al-pha segregation, high-density or low-density tramp inclusions, and unhealed ingot porosity or pipe. The greater the control is, however, the greater the cost will be.

Aerospace pressure vessels similarly require optimized strength efficiency, although at

lower temperatures. Required auxiliary proper-ties include weldability and predictable fracture toughness at cryogenic-to-moderately elevated temperatures. To provide this combination of properties, stringent user specifications require controlled microstructures and freedom from melting imperfections. For cryogenic applica-tions, the interstitial elements oxygen, nitrogen, and carbon are carefully controlled to improve ductility and fracture toughness. Alloys with such controlled interstitial element levels are designated ELI (extra-low interstitial), for ex-ample, Ti-6Al-4V-ELI.

Aircraft structural applications, along with high-performance automotive and marine ap-plications, also require high-strength effi-ciency, which normally is achieved by judi-cious alloy selection combined with close control of mill processing. However, when the design includes redundant structures, when op-erating environments are not severe, when there are constraints on the fabrication methods that can be used for specific components, or when there are low operational risks, selection of the appropriate alloy and process must take these factors into account.

There are instances of less highly loaded structures in which titanium normally is se-lected because it offers greater resistance to temperature effects than aluminum does or greater corrosion resistance than brass, bronze, and stainless steel alloys provide. In such cases, commercial availability of required mill prod-ucts and ease of fabrication customarily dictate selection. Here, one of the grades of unalloyed titanium usually is chosen. Formability (as with tubes) frequently is a characteristic required of this class of applications.

Titanium Alloy Systems Availability

In the United States, 70 to 80% of the de-mand for titanium was from the aerospace in-dustries during most of the first 50 years that ti-tanium alloys were available commercially. About 20 to 30% was from industrial applica-tions. In the last decade of the twentieth cen-tury, demand from nonaerospace industries se-verely impacted the availability of titanium and its alloys for more traditional high-performance applications at times. For a while, titanium golf clubs were in great demand. Bicycles with tita-nium frames became quite popular. The golf club market proved to be less durable than ex-pected, and demand is driven by the aerospace applications once again. In view of the fluidity of the market, any speculation or report about titanium application volume would best be got-ten from sources such as trade associations, trade journals, or specialized reports prepared by consulting firms.

Several dozen common titanium alloys are readily available. However, as is the case in many industries, there are often significant variations in the specifications to which a given organization purchases, or designs with, tita-nium alloys. To a large extent, aerospace

appli-cations are the prime cause of titanium alloy and process development and, thus, material availability.

The industry has been cyclical in nature and has operated at peak capacity only a few times in the approximately five decades since titanium was introduced as a commer-cial material. The business conditions of the last decade of the twentieth century led inexorably to a consolidation of the produc-ers of titanium alloys. Further consolida-tion may be expected in the alloy specifica-tions that govern the use of titanium. Common specification agreements are in the works whereby a single specification may serve as a buying guide for a given composi-tion.

Single specification requirements for a given alloy should not be considered to grant a com-mon design data base for a material, however. Actual design data will continue to be within the purview of titanium users such as gas tur-bine engine and airframe manufacturers. Com-monality of purchasing requirements via com-mon specifications should eventually drive design data to a more common framework. The data provided in this book and most handbooks (examples can be found in Appen-dix K) are meant to be typical data, not design data.

Evolution of

Casting and Precision Forging

While total titanium availability has re-mained relatively flat for many years, the avail-ability of castings has risen remarkably. In ad-dition to intricate castings, precision forgings, including near-net shape (NNS) forgings, and superplastic forming/forging have shown promise for extending the application of tita-nium alloys. Figure 2.5 illustrates schemati-cally the areas of titanium usage in an advanced fighter airframe, that of the F-22 Raptor. Only the areas of titanium usage are shown. In the F-22, some 42% of all structural weight will be of titanium. In the aft fuselage alone, almost two-thirds of the weight is titanium.

Titanium castings (Fig. 2.6) represented only 6% of the weight of aircraft gas turbines in the 1980s, but casting usage was expanded in the 1990s, especially when casting vendors moved to reduce costs to engine manufacturers. Pow-der parts may be available in limited quantities, but they are currently and principally restricted to somewhat more exotic alloys and/or applica-tions.

Titanium usage may increase for advanced gas turbines, but there are not that many new turbines in the works, and there is a tendency to look for “low-cost” materials/components for newer designs. Airframes represent a large-volume application for titanium, and titanium usage for airframes increased steadily through the latter decades of the twentieth century, as seen in Fig. 2.7. Military applications remain

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the largest volume uses for titanium, and Ta-bles 2.3 and 2.4 show the airframe and/or en-gine titanium requirements as well as the buy weights for some commercial and military ap-plications.

It was not until about 1965 that nonaerospace usage accounted for a significant fraction of the titanium production. Continued modest growth has been taking place since then in many areas, including biomedical engineering, marine and chemical applications, automotive, and sport-ing goods. Table 2.5 provides a list of some ti-tanium applications.

The Role of Processing

Titanium alloys are particularly sensitive to the processing conditions that precede their use in service applications. Processing denotes the wrought, cast, or powder methods used to pro-duce the alloy in the appropriate condition for the intended application, as well as the heat treatments that are applied to the alloy. Heat treatment of alpha-beta alloys seems to produce microstructures that are substantially the same as structures produced, for example, by forging

the same alloy in the same general temperature region of the phase diagram as that where the heat treatment is carried out. However, the

Wings

• Side of body fitting: titanium HIP casting • Spars:

Front, titanium

Intermediate, resin transfer molded composite and titanium Rear, composite and titanium

Aft fuselage

• Forward boom: titanium welded • Bulkheads/Frame: titanium • Upper skins: titanium and composite

Mid fuselage

• Skins: composite and titanium • Bulkheads and frames: titanium aluminum, composite

Fig. 2.5 Some areas of titanium use in the F-22 Raptor advanced fighter aircraft

Fig. 2.6 Typical titanium alloy casting for aircraft gas

turbine use. Courtesy of Precision Castparts Corp.

Fig. 2.7 Titanium usage in Boeing aircraft from the first

commercial jet to the Boeing 757

Table 2.3 Military aircraft (including engines) titanium requirements

Titanium buy weight

Aircraft/engine(a) kg lb A-10/(2) TF-34 1,814 4,000 F-5E/(1) J85 635 1,400 F-5G/(1) F404 1,089 2,400 F-14/(2) TF-30 24,630 54,300 F-15/(2) F-100 29,030 64,000 F-16/(1) F-100 3,085 6,800 F-18/(2) F-404 7,620 16,800 C-130/(4) T-56 499 1,100 C-5B/(4) TF-39 24,812 54,700 B-1B/(4)F101-GE-102 90,402 199,300 KG-10/CF-6-50 32,206 71,000 CH-53E/(3) T-64 8,800 19,400 CH-60/(2) T-700 2,041 4,500 S-76/(2) A11.250 544 1,200 AH-64/(2) T-700 635 1,400

(a) Typical uses are A-10 ballistic armament; structural forgings and wing skins for F-14 and F-15 aircraft; rotor parts for helicopter blade systems; B-1B fracture-critical forgings and wing carry-through section; and rotor discs, blades, and compressor cases on various engines.

Table 2.4 Titanium buy weights for commercial and military aircraft Titanium buy weight

Aircraft/engine(a) kg lb Fairchild A-10 862 1,900 Northrop F-5 408 900 Grumman F-14 18,870 41,600 McDonnell Douglas F-15 24,494 54,000 General Dynamics F-16 861 1,800 McDonnell Douglas F-18 6,214 13,700 Lockheed C-130 454 1,000 Lockheed C-5B 6,804 15,000 Rockwell B-1B 82,646 182,200 707/(4) JT3 4,445 9,800 727/(3) JT8 4,309 9,500 737-200/(2) JT8 3,810 8,400 737/300/(3) CFM-56 3,810 8,400 747/(4) JT-9 42,593 93,900 757/(2) PW2037 12,746 28,100 757/(2) RB211/535 12,973 28,600 767/(2) JT-9 17,554 38,700 767/(2) CF-6 11,703 25,800 MD-80 (2) JT8-217 6,260 13,800 DC-10/(3) CF-6 32,387 71,400 A300/(2) CF-6 6,350 14,000 A310/(2) CF-6 6,350 14,000

(a) Airframe only; slight variations by specific model. Product forms purchased include sheet, plate, bar, billet, and extrusions.

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Introduction to Selection of Titanium Alloys / 11

properties of wrought stock produced by defor-mation of the alloy at a high temperature gener-ally seem to be better than those produced by heat treatment alone to effect the desired struc-ture. Furthermore, the degree of work placed into the alloy seems to be a controlling factor in the attainment of optimum properties. (Bar stock does not have the same properties as a forged disk.)

Once the alloy composition is selected, the properties of titanium alloys are linked

inextri-cably to the nature of the processing applied to them. One of the more considerable recent pro-cessing challenges was to develop satisfactory heat treatment procedures for optimizing the properties and the microstructure of cast tita-nium alloys after they have been hot isostati-cally pressed. Heat treatments and fabrication conditions to consolidate titanium powder or to make components from titanium aluminides represent ongoing challenges to the process technology involving titanium.

Property Data

Properties of commercially pure and alloyed titanium may vary from the data presented in Table 2.1. For specific information on many of the commonly used Ti CP grades and alloys, re-fer to Materials Properties Handbook: Tita-nium Alloys, published by ASM International

(Appendix K provides a listing of references for additional information).

Aerospace

Gas turbine engines Aircraft structures Spacecraft Helicopter rotors Power generation Gas turbines Steam turbines Piping systems Heat exchangers

Flue gas desulphurization systems

Chemical processing industries

Pressure and reaction vessels Heat exchangers Pipe and fittings Liners Tubing Pumps Condensers

Valves, ducting, and filters Agitators

Automotive

Body panels Connecting rods Valves and valve springs Rocker arms

Marine

Surface ship hulls Deep-sea submersibles Pleasure boat components Racing yacht components Shipboard cooling systems Ship propellers Service water systems Ducting

Fire pumps

Water jet propulsion systems

Fashion and apparel

Eyeglasses Jewelry Watches Writing instruments

Oil, gas, and petroleum processing

Tubing and pipe Liners Springs Valves Risers

Biomedical

Artificial joint prostheses Bone plates, intramedullary rods, etc. Heart valves Pacemakers Dental implants Attachment wire Surgical instruments Wheelchairs Architectural Roofing Window frames Eaves and gables Railings Ventilators

Sports

Golf clubs

Bicycle frames, gears, etc. Lacrosse sticks Racing wheelchairs Horseshoes Tennis rackets Scuba gas cylinders Skis

Pool cues

Miscellaneous

Shape memory alloys Pollution control systems Hand tools

Desalination systems Military vehicle armor Hunting knives Backpack cookware Table 2.5 Some titanium applications

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Chapter 3 Understanding the Metallurgy of Titanium

Crystal Structure and Alloy Types

METALS generally have simple atomic ar-rangements compared to ceramics and plastics. Metal atoms, which can be pictured as hard spheres for convenience, are arranged on crys-tal lattices. A grain is formed by the aggregate of a group of similar crystals of a given metal (or alloy). The orientation of lattice aggregates generally differs over distance and so a metal such as titanium is composed of many grains unless deliberately grown as a single crystal. Within each grain the orientation of the lattice structure is the same with distance but across a grain boundary the next grain will have a dif-ferent spatial orientation.

In addition to the existence of grains and concurrent grain boundaries, titanium is an al-lotropic element; that is, it exists in more than one crystallographic form. At room tempera-ture, titanium has a hexagonal close-packed (hcp) crystal structure, which is referred to as “alpha” phase. This structure transforms to a body-centered cubic (bcc) crystal structure, called “beta” phase, at 888°C (l621 °F). Beta phase and alpha phase hard-sphere models are shown in Fig. 1.1.

It is common to separate the alloys into four categories, referring to the phases normally present. The alloy categories generally are called:

•

Alpha

•

Near-alpha

•

Alpha-beta (alpha-plus-beta)

•

Beta

Sometimes a category of near-beta is also con-sidered.

These categories denote the general type of microstructure after processing. (Micro-structure refers to the phases and grain struc-ture present in a metallic component.) The cate-gories listed describe the origin of the microstructure in terms of the basic crystal structure favored by an alloy composition. Thus, an alloy with only alpha phase present

becomes an alpha alloy. Crystal strucure and grain structure (i.e., microstructure) are not synonymous terms. Both must be specified to completely identify the alloy and its expected mechanical, physical, and corrosion behavior. The important fact to keep in mind is that, while grain shape and size affect behavior, the crystal structure changes (from alpha to beta and back again) that occur during processing play a major role in defining titanium proper-ties. Chapter 12 covers this subject in detail.

Phase Diagrams—Road Maps for Alloy Relationships. The phase relationships in alloy systems can be represented by phase diagrams. When more than two elements are present, it is difficult to show the quantitative relationships. Pseudobinary phase diagrams, however, are a useful way to show behavior, especially on a comparative basis.

Figure 3.1 shows the compositions of some U.S. alloys marked on such a road map, a pseudobinary phase diagram where the

compo-Fig. 3.1Some U.S. alloy compositions relative to a pseudobinary titanium phase diagram

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sition axis represents the amount of beta phase stabilizing element. The diagram clearly shows that alloys such as Ti-6Al-2Sn-4Zr-2Mo are “near-alpha” alloys because they are barely into the alpha-plus-beta region of the phase dia-gram. Alloys such as Ti-13V-11Cr-3Al, how-ever, are clearly in the high end of the alpha-plus-beta region and, owing to slow transfor-mation kinetics, will remain beta on cooling from higher temperatures. The M_fand M_slines introduced into the diagram refer to non-equilibrium martensitic phases introduced

dur-ing “rapid” cooldur-ing, as in steel. Martensitic phases are discussed shortly.

Crystal Structure Behavior. An alpha al-loy (so described because its chemistry favors alpha phase) does not normally form beta phase on heating. A near-alpha (sometimes called “superalpha”) alloy forms only limited beta phase on heating, and so it may appear micro-structurally similar to an alpha alloy when viewed at lower temperatures. An alpha-beta alloy is one for which the composition permits complete transformation to beta on heating but transforms back to alpha plus retained and/or transformed beta at lower temperatures. A near-beta or beta alloy composition is one that tends to retain, indefinitely at lower tures, the beta phase formed at high tempera-tures. However, the beta that forms on initial cooling to room temperature is metastable. De-pendent on chemistry, it may precipitate sec-ondary phases during heat treatment.

Microstructures show variations in the mor-phological development of the alpha phase and

the beta phase, which are dependent on alloy chemistry, prior work, temperature from which cooled, and rate of cooling. Coarse and fine acicular structures can be produced, but equiaxed structures also are possible. (This topic is discussed later in this chapter.) Typical titanium microstructures are shown in Fig. 3.2. The microstructures shown are intended to be representative but definitely not all-inclusive because the actual microstructure depends on chemistry and processing. Figures 3.3 and 3.4 give some additional illustrations of the effect of prior temperature (and cooling rate) on microstructure of a near-alpha and an alpha-beta titanium alloy.

Effects of Alloying Elements

Alloying elements generally can be classi-fied as alpha stabilizers or beta stabilizers. Al-pha stabilizers, such as aluminum, oxygen, and

(a)

(b)

(c)

(d)

Fig. 3.2 Typical microstructures of alpha,

alpha-plus-beta, and beta titanium alloys. (a) Equiaxedα in unalloyed Ti after 1 h at 699 °C (1290 °F). (b) Equiaxed α+β. (c) Acicular α+β in Ti-6Al-4V. (d) Equiaxed β in

Ti-13V-11Cr-3Al Fig. 3.3

Microstructures of an annealed near-alpha alloy (Ti-8Al-1Mo) after cooling from different areas of the phase field. (a) Acicular alpha. (b) Equiaxed alpha and intergranular beta. (c) Fine alpha-beta structure

(c) (b) (a)