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Edited byC. Leyens and M. Peters

Titanium and Titanium Alloys

Fundamentals and Applications

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Fundamentals and Applications

Edited by

Dr. Christoph LeyensDr. Manfred PetersDLR German Aerospace CenterInstitute of Materials Research51170 KlnGermany

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In 1791 the British reverend, mineralogist and chemist, William Gregor, was thefirst to discover titanium. Four years later, Martin Klaproth, a Berlin chemist, in-dependently isolated titanium oxide. The story of the Greek mythological childrenof Uranos and Gaia, the Titans, provided him the inspiration for naming it tita-nium. The Titans, utterly hated by their father, were held in captivity in theearths crust, similar to the hard to extract ore. It took more than 100 years to iso-late the metal. The first alloys, including todays most popular Ti-6Al-4V, were de-veloped in the late 1940s in the United States. Today a large number of titaniumalloys have paved the way for light metals to vastly expand into many industrialapplications.

Titanium and its alloys stand out primarily due to their high specific strengthand excellent corrosion resistance, at just half the weight of steels and Ni-basedsuperalloys. This explains their early success in the aerospace and the chemicalindustries. But other markets such as architecture, chemical processing, medicine,power generation, marine and offshore, sports and leisure, and transportation areseeing increased application of titanium.

This book is intended for students, materials scientists, engineers, and techni-cians from research, development, production, and design departments who wantto become familiar with titanium and its alloys. Introductory chapters coveringthe metallurgical background, mechanical properties, oxidation behavior, and oxi-dation protection are followed by chapters on production and processing, and in-troductions to various traditional and new fields of application. Besides titaniumand its conventional alloys, insight is also provided on titanium aluminides and ti-tanium matrix composites. The variety of applications of titanium and its alloys inaerospace and non-aerospace markets are documented in detail. Extensive refer-ences allow further expansion on each individual subject.

Cologne, June 2003 C. Leyens and M. Peters

V

Foreword

Foreword V

List of Contributors XVII

1 Structure and Properties of Titanium and Titanium Alloys 1M. Peters, J. Hemptenmacher, J. Kumpfert and C. Leyens

1.1 Introduction 11.2 The Metallurgy of Titanium 41.2.1 Crystal Structure 41.2.2 Plastic Deformation 51.2.3 /-Transformation 61.2.4 Diffusion 81.3 The Classification of Titanium Alloys 91.4 Metallographic Preparation of the Microstructure 111.5 The Microstructure of Titanium Alloys 121.6 Property Profiles of the Titanium Alloy Classes 161.7 The Alloying Elements of Titanium 181.8 The Conventional Titanium Alloys 191.8.1 Alloys 191.8.2 Near- Alloys 221.8.3 + Alloys 221.8.4 Metastable Alloys 231.9 Textures in Titanium Alloys 231.10 Mechanical Properties of Titanium Alloys 251.10.1 Strength 271.10.2 Stiffness 271.10.3 Elevated Temperature Strength 301.10.4 Damage Tolerance and Fatigue 331.11 Referenced Literature and Further Reading 35

VII

Contents

2 Beta Titanium Alloys 37G. Terlinde and G. Fischer

2.1 Introduction 372.2 Metallurgy and Processing 392.3 Mechanical Properties 422.3.1 Tensile Properties 422.3.2 Fracture Toughness 442.3.3 Fatigue (HCF) 492.3.4 Fatigue Crack Propagation (FCP) 522.4 Applications 542.5 Referenced Literature and Further Reading 55

3 Orthorhombic Titanium Aluminides: Intermetallics with Improved DamageTolerance 59J. Kumpfert and C. Leyens

3.1 Introduction 593.2 Physical Metallurgy: Crystal Structures, Phase Equilibria, and Alloy

Chemistry 623.3 Properties of Orthorhombic Titanium Aluminides 643.3.1 Physical Properties 653.3.2 Microstructures 653.3.3 Mechanical Properties 683.3.3.1 Tensile Properties 683.3.3.2 Creep Behavior 713.3.3.3 Fatigue Strength, Crack Growth Behavior, and Fracture Toughness 723.4 Oxidation and Environmental Embrittlement 783.5 Concluding Remarks 843.6 Referenced Literature and Further Reading 85

4 -Titanium Aluminide Alloys: Alloy Design and Properties 89F. Appel and M. Oehring

4.1 Introduction 894.2 Constitution of -Titanium Aluminide Alloys 904.3 Phase Transformations and Microstructure 934.4 Micromechanisms of Deformation 954.4.1 Slip and Twinning Systems 974.4.2 Dislocation Multiplication 1004.4.3 Twin Nucleation 1044.4.4 Glide Resistance and Dislocation Mobility 1054.5 Mechanical Properties 1124.5.1 Grain Refinement 1124.5.2 Effects of Alloy Composition 1144.5.3 Solid Solution Effects due to Nb Additions 1154.5.4 Precipitation Hardening 1164.5.5 Creep Resistance 121

ContentsVIII

4.5.6 Crack Propagation and Fracture Toughness 1284.5.7 Fatigue Behavior 1314.6 Basic Aspects of Processing 1334.6.1 Manufacture of Ingots 1334.6.2 Casting 1354.6.3 Dynamic Recrystallization on Hot Working 1364.6.4 Development of Hot Working Routes 1394.7 Conclusions 1454.8 Acknowledgments 1464.9 Referenced Literature and Further Reading 146

5 Fatigue of Titanium Alloys 153L. Wagner and J.K. Bigoney

5.1 Introduction 1535.2 Influence of Microstructure 1545.2.1 Commercially Pure Titanium, Alloys 1545.2.2 Near- and + Alloys 1575.2.3 Alloys 1645.3 Influence of Crystallographic Texture on Fatigue Life 1695.4 Influence of Mean Stress on Fatigue Life 1715.5 Influence of Mechanical Surface Treatments 1715.6 Influence of Thermomechanical Surface Treatments 1755.6.1 Alloys 1755.6.2 Near- and + Alloys 1765.6.3 Alloys 1775.7 Titanium Aluminides 1785.8 Composite Materials 1805.9 Summary 1815.10 Referenced Literature and Further Reading 182

6 Oxidation and Protection of Titanium Alloys and Titanium Aluminides 187C. Leyens

6.1 Introduction 1876.2 Fundamentals of Oxidation of Metals 1886.2.1 Thermodynamics of Oxidation 1896.2.2 Oxidation Kinetics 1916.2.2.1 Disorder Features in Oxides 1926.2.2.2 Kinetics 1946.2.3 Oxidation of Alloys 1956.2.3.1 Selective Oxidation 1966.2.3.2 Internal Oxidation 1976.3 Oxidation Behavior of Titanium Alloys and Titanium Aluminides 1986.3.1 Oxide Scale Formation 1996.3.1.1 Ti-Al-O Phase Diagram 1996.3.1.2 Oxide Scale Growth 201

Contents IX

6.3.1.3 Effect of Alloying Elements 2076.3.1.4 Effect of Atmosphere 2096.3.2 Dissolution of Non-metals in the Subsurface Zone of Alloys 2106.3.2.1 Effect of Non-metal Dissolution on the Mechanical Properties 2116.4 Measures to Improve Oxidation Resistance 2136.4.1 Alloying Elements 2136.4.2 Pre-oxidation 2156.4.3 Coatings 2166.4 Summary and Outlook 2236.5 Referenced Literature and Further Reading 223

7 Titanium and Titanium Alloys From Raw Material to Semi-finished Products 231H. Sibum

7.1 Introduction 2317.2 Titanium Sponge 2317.3 From Sponge to Ingot 2347.4 Titanium, Titanium Alloys and Special Alloys 2367.5 Processing to Semi-finished Products 2397.6 Applications 2417.7 Recycling 2437.8 Summary and Outlook 244

8 Fabrication of Titanium Alloys 245M. Peters and C. Leyens

8.1 Introduction 2458.2 Machining of Titanium Alloys 2458.3 Casting 2478.4 Welding 2508.4.1 Fusion Welding 2518.4.2 Friction Welding 2528.4.3 Electron Beam Welding 2528.4.4 Laser Beam Welding 2538.4.5 Spot Welding 2538.4.6 Properties of Welded Structures 2538.5 Superplastic Forming/Diffusion Bonding 2558.6 Powder Metallurgy 2588.7 Referenced Literature and Further Reading 261

9 Investment Casting of Titanium 263H.-P. Nicolai and Chr. Liesner

9.1 Titanium 2639.2 Cast Alloys 2639.3 Melting Units 2659.4 Molding Materials 265

ContentsX

9.5 Casting Design 2669.6 Finishing 2669.6.1 Pickling (Chemical Milling) 2679.6.2 Hot Isostatic Pressing (HIP) 2679.6.3 Welding 2679.7 Examples of Cast Parts 267

10 Superplastic Forming and Diffusion Bonding of Titaniumand Titanium Alloys 273W. Beck

10.1 Introduction 27310.2 Superplasticity 27510.3 Diffusion Bonding 27810.4 The SPF Process 27910.5 SPF-Material Investigations for Parameter Definition 28110.6 SPF Tooling 28310.7 Examples of SPF Components 28310.8 SPF Forming Presses 28510.9 SPF/DB Processing 28510.10 SPF/DB Structures and Components 28610.11 Summary 28710.12 Referenced Literature and Further Reading 288

11 Forging of Titanium 289G. Terlinde, T. Witulski and G. Fischer

11.1 Introduction 28911.2 General Properties and Applications 28911.3 Thermomechanical Treatment of Titanium Alloys 29211.3.1 Processing of Forging Stock 29211.3.2 Forgings 29311.3.3 Heat Treatment 29611.4 Process Design 29611.4.1 Geometric Requirements 29611.4.2 Forged Components and Forging Equipment 29711.4.3 Processing Window for Forgings 29811.4.4 Finite Element Simulation 30011.5 Examples for Process Optimization and Applications 30111.6 Referenced Literature and Further Reading 304

12 Continuous Fiber Reinforced Titanium Matrix Composites:Fabrication, Properties and Applications 305C. Leyens, J. Hausmann and J. Kumpfert

12.1 Introduction 30512.2 Fabrication Processes 30612.3 Properties 310

Contents XI

12.3.1 Strength and Stiffness 31112.3.2 Creep Properties 31312.3.3 Fatigue Properties 31612.3.4 Anisotropy of TMCs 31712.3.5 Thermal Residual Stresses 32012.3.5.1 Influence of the Fiber Distribution on Residual Stresses 32212.3.5.2 Residual Stresses and Fatigue 32412.4 Dimensioning and Design with TMCs 32412.5 Material Modeling 32512.6 Applications 32612.7 Summary and Outlook 32912.8 Referenced Literature and Further Reading 330

13 Titanium Alloys for Aerospace Applications 333M. Peters, J. Kumpfert, C.H. Ward and C. Leyens

13.1 Introduction 33313.2 Titanium Alloys in Aerospace 33413.2.1 Airframe 33513.2.2 Gas Turbine Engines 33913.2.3 Helicopters 34613.3 Space Applications 34713.4 Referenced Literature and Further Reading 349

14 Production, Processing and Application of (TiAl)-Based Alloys 351H. Kestler and H. Clemens

14.1 Introduction 35114.2 Constitution of (TiAl)-Based Alloys 35214.3 Controlled Microstructures by Heat-Treatments 35614.4 Processing of (TiAl)-Based Alloys 36014.4.1 Ingot Production 36014.4.2 Powder Processing and Compaction 36314.4.3 Thermomechanical Processing 36714.4.4 Forging 36814.4.4.1 Forging of Large Ingots 36814.4.4.2 Forging of Components 36914.4.5 Single and Multi-Step Extrusion 37014.4.6 Rolling of Sheet and Foil 37214.4.7 Superplastic Forming 37514.5 Further Processing 37814.5.1 Joining 37814.5.2 Machining 38014.6 Requirements, Components, Tests and Applications 38014.6.1 Gas Turbine Engines 38014.6.2 Aerospace 38314.6.3 Automotive Engines 385

ContentsXII

14.7 Concluding Remarks 38614.8 Referenced Literature and Further Reading 388

15 Non-Aerospace Applications of Titanium and Titanium Alloys 393M. Peters and C. Leyens

15.1 Introduction 39315.2 Chemical, Process and Power Generation Industries 39315.2.1 Heat Exchangers and Condensers 39415.2.2 Containers and Apparatus Manufacturing 39515.2.3 Dimensionally Stable Anodes Extractive Metallurgy 39615.2.4 Petrochemical Refineries 39715.2.5 Flue Gas Desulphurization 39715.2.6 Steam Turbine Blades 39715.2.7 Other Applications 39915.3 Marine and Offshore Applications 39915.4 Automotive Industry 40115.5 Architecture 40515.6 Sports and Leisure 40715.6.1 Golf 40715.6.2 Tennis Racquets, Baseball Bats and Pool Cues 40915.6.3 Bicycles: Not only Frames 40915.6.4 Scuba Diving Equipment 41015.6.5 Expedition and Trekking 41015.6.6 Knives 41115.6.7 Winter Sport Equipment 41215.6.8 Diverse Sports Applications 41215.7 Medical Applications 41215.8 Dental Implants 41615.9 Jewelry and Fashion 41715.10 Musical Instruments 41815.11 Optical Industry 41915.12 Information Technology 42015.13 Safety and Security 42015.14 Referenced Literature and Further Reading 422

16 Titanium and its Alloys for Medical Applications 423J. Breme, E. Eisenbarth and V. Biehl

16.1 Introduction 42316.2 Comparison of the Various Groups of Metallic Biomaterials 42416.2.1 Corrosion Resistance 42416.2.2 Biocompatibility 42516.2.3 Bioadhesion (Osseointegration) 42716.2.4 Mechanical Properties, Processability, Availability 43016.3 Examples of Tailor-made Ti-based Composites 432

Contents XIII

16.3.1 Structured Surfaces on Ti Materials with Special MechanicalProperties 432

16.3.2 Ti/Ceramic Composites with Special Biological Properties 43716.3.3 Ti/Ceramic Composites with Special Physical Properties 44016.3.4 Ti/Ceramic Composite with Improved Wear Resistance 44316.4 Referenced Literature and Further Reading 449

17 Titanium in Dentistry 453J. Lindigkeit

17.1 Introduction 45317.2 Clinically Relevant Properties of Titanium and Titanium Alloys

in Dentistry 45317.2.1 Corrosion Resistance 45417.2.1.1 Resistance Against Fluorine 45517.2.2 Biocompatibility 45517.2.3 Physical properties 45617.3 Use of Titanium and Titanium Alloys in Dentistry 45817.3.1 Orthodontics 45917.3.2 Prosthetics 46017.3.3 Implantology 46217.4 Processing of Titanium in the Dental Laboratory 46317.4.1 Dental Melting and Casting Technology 46317.4.2 CAD/CAM Technique 46417.5 Summary 46517.6 Referenced Literature and Further Reading 465

18 Titanium in Automotive Production 467O. Schauerte

18.1 Introduction 46718.2 Possible Applications for Titanium in Automotive Production 46818.2.1 Properties 46818.2.2 Potential Uses 47018.2.2.1 Applications in the Powertrain 47018.2.2.2 Applications in the Chassis 47318.2.2.3 Further Applications 47418.3 Suspension Springs made from Titanium 47418.4 Exhaust Systems 47718.5 Conclusion 48018.6 Referenced Literature and Further Reading 481

19 Offshore Applications for Titanium Alloys 483L. Lunde and M. Seiersten

19.1 Introduction 48319.2 Materials and Materials Requirements 48319.2.1 Titanium Materials for Offshore Applications 483

ContentsXIV

19.2.2 Seawater Corrosion 48419.2.3 Corrosion in Oil and Gas Environments 48519.2.4 Stress Corrosion Cracking (SCC) 48519.2.5 Galvanic Corrosion 48619.2.6 Fatigue 48819.3 Fabrication 48819.3.1 Welding 48819.3.2 Cold Forming 48919.3.3 Nitriding 49019.4 Applications 49119.4.1 Seawater Systems 49119.4.2 Heat Exchangers 49219.4.3 Hypochlorite Systems 49319.4.4 Riser Pipes 49319.4.5 Riser Taper Stress Joint 49419.4.6 Sub-Sea Systems 49419.5 Availability and Cost 49419.5.1 Deliveries 49419.5.2 Cost 49519.6 Standards 49519.7 Conclusion 49619.8 Referenced Literature and Further Reading 496

Subject Index 499

Contents XV

XVII

List of Contributors

Dr. rer. nat. habil. F. AppelGKSS Research CenterInstitute for Materials ResearchMax-Planck-Strae 1D-21502 GeesthachtGermanyemail: [email protected]

Dipl.-Ing. W. BeckFormTech GmbHMittelwendung 35D-8844 Weyhe-DreyeGermanyemail: [email protected]

Priv.-Doz. Dr. V. BiehlStryker Leibinger GmbH & Co. KGBtzinger Strae 41D-79111 FreiburgGermanyemail: [email protected]

Prof. Dr.-Ing. J.K. BigoneySpringfield Metallurgical Services Inc.127 Main StreetSpringfield, VT 05156-0826USAemail: [email protected]

Prof. Dr.-Ing. J. BremeLehrstuhl fr Metallische WerkstoffeUniversitt des SaarlandesD-66041 SaarbrckenGermanyemail: [email protected]

Prof. Dr. H. ClemensThe University of LeobenDepartment of Physical Metallurgieand Materials TestingA-8700 LeobenAustriaemail:[email protected]

Dr. E. EisenbarthLehrstuhl fr Metallische WerkstoffeUniversitt des SaarlandesD-66041 SaarbrckenGermanyemail: [email protected]

Dr.-Ing. F. FischerOtto Fuchs MetallwerkeD-58528 MeinerzhagenGermanyemail: [email protected]

Dr.-Ing. J. HausmannDLR-German Aerospace CenterInstitute of Materials ResearchD-51170 KlnGermanyemail: [email protected]

Dr.-Ing. J. HemptenmacherDLR-German Aerospace CenterInstitute of Materials ResearchD-51170 KlnGermanyemail: [email protected]

List of ContributorsXVIII

Dr.-Ing. H. KestlerPlansee AGTechnology CenterA-6600 ReutteAustriaemail: [email protected]

Dr.-Ing. J. KumpfertAirbus Industrie1 Rond Point Maurice BellonteF-31707 BlagnacFranceemail: [email protected]

Dr.-Ing. C. LeyensDLR-German Aerospace CenterInstitute of Materials ResearchD-51170 KlnGermanyemail: [email protected]

Dr.-Ing. C. LiesnerTitan-Aluminium-Feingu GmbHKapellenstr. 44D-59909 BestwigGermanyemail: [email protected]

Dr.-Ing. J. LindigkeitDentaurumJ.P. Winkelstroeter KGTurnstrae 31D-75228 IspringenGermanyemail: [email protected]

Dr. L. LundeInstitute for Energy TechnologyN-2027 KjellerNorwayemail: [email protected]

H.-P. NicolaiTitan-Aluminium-Feingu GmbHKapellenstr. 44D-59909 BestwigGermanyemail: [email protected]

Dr. rer. nat. M. OehringGKSS Research CenterInstitute for Materials ResearchMax-Planck-Strae 1D-21502 GeesthachtGermanyemail: [email protected]

Dr.-Ing. M. PetersDLR-German Aerospace CenterInstitute of Materials ResearchD-51170 KlnGermanyemail: [email protected]

Dr.-Ing. O. SchauerteVolkswagen AGBrieffach 1504D-38436 WolfsburgGermanyemail: [email protected]

Dr. M. SeierstenInstitute for Energy TechnologyN-2027 KjellerNorwayemail: [email protected]

Dr.-Ing. H. SibumDTG-Deutsche Titan GmbHD-45143 EssenGermanyemail: [email protected]

Dr.-Ing. G. TerlindeOtto Fuchs MetallwerkeD-58528 MeinerzhagenGermanyemail: [email protected]

List of Contributors XIX

Prof. Dr.-Ing. L. WagnerInstitut fr Werkstoffkundeund WerkstofftechnikTU ClausthalAgricolastrae 6D-38678 Clausthal-ZellerfeldGermanyemail: [email protected]

Charles H. Ward, PhDEuropean Office of Aerospace Researchand Development223/231 Old Marylebone RoadLondon NW1 5THUKemail: [email protected]

Dr.-Ing. T. WitulskiOtto Fuchs MetallwerkeD-58528 MeinerzhagenGermanyemail: [email protected]

1.1Introduction

In 1791 William Gregor the British reverend, mineralogist, and chemist discov-ered titanium. He examined the magnetic sand from the local river, Helford, inthe Menachan Valley in Cornwall, England, and isolated black sand, now knownas ilmenite. By removing the iron with a magnet and treating the sand with hy-drochloric acid he produced the impure oxide of a new element. He named itmechanite, after the location. Four years later, the Berlin chemist Martin Hein-rich Klaproth independently isolated titanium oxide from a Hungarian mineral,now known as rutile. Greek mythology provided him a new name from the chil-dren of Uranos and Gaia, the titans. The titans were utterly hated by their fatherand so detained in captivity by him in the earths crust, similar to the hard to ex-tract ore hence he named it Titanium.

It took more than 100 years before Matthew Albert Hunter from RensselaerPolytechnic Institute in Troy, N.Y., was able to isolate the metal in 1910 by heat-ing titanium tetrachloride (TiCl4) with sodium in a steel bomb. Finally, WilhelmJustin Kroll from Luxembourg is recognized as father of the titanium industry. In1932 he produced significant quantities of titanium by combining TiCl4 with cal-cium. At the beginning of World War II he fled to the United States. At the U.S.Bureau of Mines he demonstrated that titanium could be extracted commerciallyby reducing TiCl4 by changing the reducing agent from calcium to magnesium.Today this is still the most widely used method and is known as the Kroll pro-cess. After the Second World War, titanium-based alloys were soon consideredkey materials for aircraft engines. In 1948 the DuPont Company was the first toproduce titanium commercially. Today aerospace is still the prime consumer of ti-tanium and its alloys, but other markets such as architecture, chemical process-ing, medicine, power generation, marine and offshore, sports and leisure, andtransportation are gaining increased acceptance.

Titanium is not actually a rare substance as it ranks as the ninth most plentifulelement and the fourth most abundant structural metal in the Earths crust ex-ceeded only by aluminum, iron, and magnesium. Unfortunately, it is seldomfound in high concentrations and never found in a pure state. Thus, the difficulty

1

1

Structure and Properties of Titanium and Titanium AlloysM. Peters, J. Hemptenmacher, J. Kumpfert* and C. LeyensDLR German Aerospace Center, Cologne, Germany* Airbus Industrie, Blagnac, France

in processing the metal makes it expensive. Even today it is produced only in abatch process, and no continuous process exists as for other structural metals. Ti-tanium usually occurs in mineral sands containing ilmenite (FeTiO3), found inthe Ilmen mountains of Russia, or rutile (TiO2), from the beach sands in Austra-lia, India, and Mexico. Titanium dioxide is a very versatile white pigment used inpaint, paper, and plastic, and consumes most of world production. Besides Russia,Australia, India, and Mexico, workable mineral deposits include sites in the Unit-ed States, Canada, South Africa, Sierra Leone, Ukraine, Norway, and Malaysia.

Of all the 112 chemical elements in the periodic system known today, about85% are metals or metalloids. There are various ways to classify the metals, suchas ferrous or nonferrous metals, ingot or sintered metals, light or heavy metals.Titanium is classified as a nonferrous and light metal.

The properties of metals are essentially based on the metallic bonding of theatoms in the crystal lattice. This means that the free, mobile valence electrons inthe lattice result in classic metallic properties such as electrical conductivity,plastic deformation by atomic slip in crystal lattices, and alloying by incorporationof impurity atoms into the crystal lattice with the consequence of increased hard-ness and strength as well as reduced ductility. Tab. 1.1 shows a selection of impor-tant physical properties of highly pure polycrystalline titanium.

Metals vary substantially in weight. At 0.5 g cm3 Lithium has the lowest den-sity while Osmium and Iridium are the heaviest metals with a density of22.5 g cm3. The separation point between light and heavy metals is 5 g cm3.Therefore, Titanium with a density of 4.51 g cm3 is the heaviest light metal.Although twice as heavy as the classic light metal aluminum it has only abouthalf the specific weight of iron or nickel (Fig. 1.1).

Titanium alloys primarily stand out due to two properties: high specificstrength and excellent corrosion resistance. This also explains their preferentialuse in the aerospace sector, the chemical industry, medical engineering, and the

1 Structure and Properties of Titanium and Titanium Alloys2

Tab. 1.1 Physical properties of high-purity polycrystalline titanium (> 99.9%) at 25 C.

Structure prototype MgPearson symbol hP2Space group P63/mmc (194)-transus temperature 882 CLattice parameters a = 0.295 nm

c= 0.468 nmc/a= 1.587

Thermal expansion coefficient [106K1] 8.36Thermal conductivity [W/mK] 14.99Specific heat capacity [J/kgK] 523Electrical resistance [109 m] 564.9Elastic modulus [GPa] 115Shear modulus [GPa] 44Poissons ratio 0.33

leisure sector. Only at temperatures below 300 C do carbon fiber reinforced plas-tics have a higher specific strength than titanium alloys (Fig. 1.2). At higher tem-peratures the specific strength of titanium alloys is particularly attractive. How-ever, the maximum application temperature is limited by their oxidation behavior.Since titanium aluminides partly overcome this disadvantage, they have becomethe subject of intense alloy development efforts. While conventional elevated tem-perature titanium alloys are used only up to temperatures slightly above 500 C,TiAl-based alloys directly compete with well-established high temperature steelsand Ni-base superalloys (Fig. 1.2).

1.1 Introduction 3

Fig. 1.1 Density of selected metals.

Fig. 1.2 Specific strength versus use temperature of selected structur-al materials compared with titanium alloys and aluminides.

1.2The Metallurgy of Titanium

1.2.1Crystal Structure

Like a number of other metals e.g. Ca, Fe, Co, Zr, Sn, Ce, and Hf titaniumcan crystallize in various crystal structures. However, each modification is onlystable within particular temperature ranges. The complete transformation fromone into another crystal structure is called allotropic transformation; the respectivetransformation temperature is called the transus temperature.

Pure titanium, as well as the majority of titanium alloys, crystallizes at low tem-peratures in a modified ideally hexagonal close packed structure, called tita-nium. At high temperatures, however, the body-centered cubic structure is stableand is referred to as titanium. The -transus temperature for pure titanium is8822 C. The atomic unit cells of the hexagonal close packed (hcp) titaniumand the body-centered cubic (bcc) titanium are schematically shown in Fig. 1.3with their most densely packed planes and directions highlighted.

The existence of the two different crystal structures and the corresponding allo-tropic transformation temperature is of central importance since they are the ba-sis for the large variety of properties achieved by titanium alloys.

Both plastic deformation and diffusion rate are closely connected with the re-spective crystal structure. In addition, the hexagonal crystal lattice causes a distinc-tive anisotropy of mechanical behavior for the titanium. The elastic anisotropyis particularly pronounced. The Youngs modulus of titanium single crystals con-sistently varies between 145 GPa for a load vertical to the basal plane and only100 GPa parallel to this plane.


Fig. 1.3 Crystal structure of hcp and bcc phase.

1.2.2Plastic Deformation

The essential features of the three crystal structures pertinent to metals are sum-marized in Tab. 1.2. The ease of plastic deformation increases from the hexagonalclose packed (hcp) lattice to the body-centered cubic (bcc) to the face-centered cu-bic (fcc) lattice. This phenomenon also explains the limited plastic deformabilityof the hcp titanium compared to the bcc titanium. Generally the number ofslip systems which is equivalent to the number of dislocation glide opportu-nities in a crystal lattice is only 3 for the hcp structure while it is 12 for the bcclattice. The number of slip systems is determined by the number of slip planesmultiplied by the number of slip directions. These planes and directions of highlydense packed atoms are energetically most favorable for plastic deformation.

The denser slip planes are packed with atoms, the easier dislocations can glide.Therefore, a slip plane in the hcp lattice with a packing density of 91% should besuperior to a slip plane in the bcc lattice with a packing density of only 83%.However, the energy needed for plastic deformation is also directly dependent onthe length of the minimal slip path. For hcp lattice structures this minimum slippath corresponds to bmin = 1a, while for bcc structures bmin = 0.87a, with a beingthe lattice parameter of the respective unit cell. This in turn favors the plastic de-formation of the bcc over the hcp structure.

In titanium the lattice parameters of the hexagonal close packed crystal structureare a = 0.295 nm and c= 0.468 nm, giving a c/a ratio of 1.587. For an ideally closepacked hexagonal lattice the c/a ratio is 1.633. The insertion of interstitially dis-solved atoms in the hcp lattice, e.g. C, N, or O, or the incorporation of substitutionalatoms with smaller atomic radii than titanium, e.g. Al, slightly increases the c/a ratioof the titanium. The lattice parameter of bcc titanium at 900 C is a = 0.332 nm.

1.2 The Metallurgy of Titanium 5

Tab. 1.2 Characteristic parameters of metallic structure types.

Structure type N CN P Slip planesSlip directions

Slip systemper unit cell

Atom den-sity of slipplane

bmin/a

indices numbers

hcp(c/a= 1,633)

6 12 74% {0001}1120

13

13 = 3 91% 1

bcc 2 8 68% {110}111

62

62 = 12 83% 1/23

0,87

fcc 4 12 74% {111}110

43

43 = 12 91% 1/22

0,71

N Number of atoms per unit cellCN Coordination numberP Packing densitybmin/a Minimal slip component

Compared to an ideally packed hexagonal crystal structure the reduced c/a ratioof titanium leads to a larger spacing between prism planes. This causes thepacking density of the prism planes to increase relative to the basal plane andthus favoring slip on prism planes rather than on basal planes.

Prism and basal planes have three slip systems each. However, only two are in-dependent of each other, resulting in only four independent slip systems. Slip onpyramidal planes does not increase the number further since this glide is com-posed of a prism and a basal component and therefore cannot be considered anindependent slip system. However, according to the von-Mises criterion at leastfive independent slip systems are required for homogeneous plastic deformationof metals. In fact, polycrystalline hexagonal titanium is extremely difficult to de-form. The limited ductility that is observed is the result of additional deformationon secondary slip systems as well as possible mechanical twinning. The three ac-tive slip systems in titanium are depicted in Fig. 1.4.

1.2.3/-Transformation

Upon cooling from the phase field of titanium the most densely packed planesof the bcc phase {110} transform to the basal planes {0001} of the hexagonal phase. The distance between the basal planes in is slightly larger than the corre-sponding distance between the {110} planes in (see also Tab. 1.2: bmin/a). There-fore, the / transformation causes a slight atomic distortion (Fig. 1.5). This leadsto a slight contraction of the c-axis relative to the a-axis in the hcp and reducesthe c/a-ratio below the value of ideally close packed hexagonal atomic structures.A slight increase in volume is observed macroscopically during cooling throughthe / transformation temperature.


Fig. 1.4 Slip systems of hexagonal crystal lattices.

The corresponding transformation of the slip planes of the bcc titanium intothe basal planes of the hcp titanium and the respective orientations of the slipdirections is given by the following orientation relationship:

00011101120111

Since the Burgers vectors can also describe the slip directions, the above orientationrelationship is referred to as a Burgers relationship. The six slip planes and the twoslip directions of the titanium unit cell give a maximum of 12 variants of orienta-tion to the . This variety of orientations is also reflected in the metallographicmicrostructure. Within the prior grains, which can be as large as several milli-meters, individual lamellar packets nucleate and grow according to the previouslymentioned 12 orientation relationships, with the individual lamellar packets havinga common orientation within them. The large but limited to 12 number of pos-

1.2 The Metallurgy of Titanium 7

Fig. 1.5 / transformation accord-ing to Burgers relationship.

Fig. 1.6 Lamellar microstructure of Ti-6Al-4V (basket-weave).

sible orientations results in multiple repetitions of the orientation of the lamellarpackets. Consequently, this results in a very characteristic microstructure similarin appearance to the weave pattern of a basket and are therefore referred to as bas-ket-weave structures (Fig. 1.6).

1.2.4Diffusion

Because of the densely packed atoms in hcp titanium, diffusion is considerablylower than in bcc titanium: the diffusion coefficient of titanium is orders ofmagnitude smaller than that of titanium. The following coefficients are givenfor self-diffusion of titanium at 500 C and 1000 C. The resulting diffusion paths,d, after 50 h at 500 C and 1 h at 1000 C illustrate these differences.

500 C: D-Ti1019 m2/s after 50 h: d0.8 m

D-Ti1018m2/s d0.9 m

1000 C: D-Ti1015 m2/s after 1 h: d4 m

D-Ti1013 m2/s d40 m

The different diffusion coefficients of and titanium are influenced by the mi-crostructure and thus influence the mechanical behavior of the two phases, e.g.creep performance, hot workability, and superplasticity. The limited volume diffu-sion in titanium translates into a superior creep performance of titanium and containing Ti alloys compared to titanium.

Below the -transus temperature, time- and temperature-dependent diffusion pro-cesses are substantially slower. Therefore, fast cooling leads to a very fine lamellarstructure whereas upon slow cooling a coarse lamellar structure is obtained. Theradial spread of the lamellae is parallel to the {110} planes of the phase. If suffi-cient cooling rate is provided, the individual lamellae not only nucleate at grainboundaries but also on the growth front of individual lamellar packets.

At high cooling rates from temperatures above the martensite start tempera-ture, the bcc transforms completely into the hcp by a diffusionless transfor-mation process, leaving behind a metastable fine plate-like, or acicular, martensi-tic microstructure.

The martensitic transformation does not lead to embrittlement; however,strength is slightly increased compared to titanium. The martensite can befurther split into hexagonal martensite and orthorhombic martensite, the lat-ter being observed on quenching from temperatures below about 900 C. Theorthorhombic martensite is characterized by good deformability. The hexagonal martensite has a similar orientation relationship to as that of . The marten-sitic microstructure is therefore also characterized by a very fine basket-weavestructure with needle-like character due to its diffusionless nucleation process.


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