Mechanical properties of ultrafine-grained Ti-6Al-4V ELI alloy processed by severe plastic deformation (Mechanische Eigenschaften einer ultrafeinkörnigen TiAl6V4 ELI Legierung hergestellt mittels Hochverformung) Der Technischen Fakultät der Universität Erlangen-Nürnberg zur Erlangung des Grades DOKTOR-INGENIEUR vorgelegt von Lilia May Erlangen 2009 Als Dissertation genehmigt von der Technischen Fakultät der Universität Erlangen-Nürnberg Tag der Einreichung: 30. 06. 2009 Tag der Promotion: 01. 12. 2009 Dekan: Prof. Dr.-Ing. R. German Berichterstatter: Prof. Dr. rer. nat. M. Göken Prof. Dr. R.Z. Valiev Table of contents I Table of contents 1. INTRODUCTION AND RESEARCH OBJECTIVES 1 2. LITERATURE REVIEW 4 2.1. Biomedical metallic materials 4 2.2. Titanium alloy Ti-6Al-4V 9 2.2.1. Development of Ti-6Al-4V 9 2.2.2. Metallurgy of Ti-6Al-4V 11 2.3. Formation of ultrafine-grained microstructure by SPD 16 2.4. Microstructure of Ti-6Al-4V and its influence on mechanical properties 18 2.4.1. Microstructure and monotonic stress-strain behavior 18 2.4.2. Microstructure and monotonic stress-strain behavior under superplastic conditions 29 2.4.3. Microstructure and cyclic stress-strain behavior 36 3. MATERIAL AND EXPERIMENTAL PROCEDURES 50 3.1. Initial materials 50 3.2. Processing of Ti-6Al-4V ELI 50 3.2.1. Heat treatment 50 3.2.2. Equal channel angular pressing 51 3.2.3. Multicycle extrusion 51 3.3. Mechanical tests 52 3.3.1. Hardness and microhardness measurements 52 3.3.2. Tensile tests 52 3.3.3. Compression tests 53 3.3.4. Fatigue tests 54 3.4. Microstructural investigations 57 3.4.1. Light microscopy 57 3.4.2. Scanning electron microscopy 58 3.4.3. Transmission electron microscopy 58 3.4.4. X-ray diffraction measurements 59 3.4.5. Atomic force microscopy 60 4. RESULTS AND DISCUSSION 62 4.1. Microstructure of Ti-6Al-4V 62 II Table of contents 4.1.1. Equal channel angular pressing of VT-6 62 4.1.2. Equal channel angular pressing of Ti-6Al-4V ELI 68 4.1.3. Equal channel angular pressing and extrusion of Ti-6Al-4V ELI 70 4.1.4. Preliminary heat treatment of Ti-6Al-4V ELI 73 4.1.5. Microstructure of Ti-6Al-4V ELI for fatigue experiments 75 4.1.6. Discussion 82 4.2. Mechanical properties under monotonic loads 85 4.2.1. Tensile properties and hardness at room temperature 85 4.2.2. Mechanical properties at elevated temperatures 89 4.2.3. Microstructural studies after tensile deformation 91 4.2.4. Compression properties and strain rate sensitivity at elevated temperature 93 4.2.5. Microstructural studies after compression deformation 96 4.2.6. Discussion 98 4.3. Mechanical properties under cyclic loads 100 4.3.1. Cyclic deformation behavior 100 4.3.2. Finite life criterion (ε-N curves) 104 4.3.3. Cyclic stress – strain curves 107 4.3.4. Infinite life criterion (S-N curves) 108 4.3.5. Damage mechanisms 111 4.3.6. Discussion 121 5. CONCLUSIONS AND OUTLOOK 128 5.1. English 128 5.2. German 130 6. REFERENCES 133 7. APPENDICES 150 7.1. X-ray diffraction patterns for Ti-6Al-4V ELI 150 List of symbols and abbreviations III List of symbols and abbreviations Latin symbols and abbreviations A Initial cross-sectional area of the sample mm2 0 a Lattice parameter nm 0 AFM Atomic force microscopy / microscope b Fatigue strength exponent bcc Body-centered cubic lattice c Fatigue ductility exponent CG Conventional grain size d Grain size / diameter µm E Young’s modulus GPa e Total deformation strain % ECAP Equal channel angular pressing ELI Extra low interstitials fcc Face-centered cubic lattice FL Fatigue limit MPa HCF High cycle fatigue hcp Hexagonal close-packed lattice HPT High pressure torsion k’ Cyclic strain hardening coefficient L Grain length µm l Initial gage length of the sample mm 0 LCF Low cycle fatigue LM Light microscopy / microscope m Strain rate sensitivity of flow stress m Instantaneous strain rate sensitivity inst N Number of cycles N Number of cycles to failure f P Load N R(ε ) Plastic strain amplitude ratio pl R(σ) Stress amplitude ratio SAED Selected area electron diffraction SEM Scanning electron microscopy / microscope SPD Severe plastic deformation T Temperature TEM Transmission electron microscopy / microscope TMT Thermomechanical treatment UFG Ultrafine-grained IV List of symbols and abbreviations UTS Ultimate tensile strength MPa w Grain width µm YS Yield strength MPa Greek symbols α Hexagonal close-packed phase of titanium alloys α Primary α-phase I α Secondary α-phase II α Thermal expansion coefficient th β Body-centered cubic phase of titanium alloys ε Technical strain %; 10-3 ε Elongation to failure % f ε Uniform elongation % U Δε Total strain range %; 10-3 tot Δε /2 Total strain amplitude %; 10-3 tot ε Elastic strain %; 10-3 el Δε Elastic strain range %; 10-3 el Δε /2 Elastic strain amplitude %; 10-3 el ε Plastic strain %; 10-3 pl Δε Plastic strain range %; 10-3 pl Δε /2 Plastic strain amplitude %; 10-3 pl Δε /2 Cyclic saturation plastic strain amplitude %; 10-3 pl,s έ Strain rate s-1 ε Maximum strain %; 10-3 max ε Minimum strain %; 10-3 min ε' Fatigue ductility coefficient B ε True strain %; 10-3 true ε Uniform strain %; 10-3 u λ Total extrusion ratio σ Technical stress MPa σ Maximum stress MPa max σ Minimum stress MPa min Δσ/2 Stress amplitude MPa σ' Fatigue strength coefficient B Δσ /2 Saturation stress amplitude MPa S φ Angle of channels’ intersection in ECAP-die ° ψ Relative reduction of area % Introduction and research objectives 1 1.Introduction and research objectives Owing to the excellent combination of high strength, corrosion resistance and biocompatibility, titanium and titanium alloys are used in different areas including sea water and chlorine-bearing media; as highly loaded blades of low-pressure stages of steam turbines and parts of aircraft and spacecraft engines; for sports equipment, etc. [Zwi74; Eyl81; Boy97]. Besides, biomedical applications of titanium alloys should be particularly emphasized. Ti alloys are widely used as implant materials. Generally, metallic implants are applied for hard tissue replacement in the human body to help restore motion and support functions of the organism. The first implants were made of stainless steel and cobalt- chromium alloys. However, increasing requirements for biocompatibility of metallic materials led to the use of titanium implants. Due to successful alloying, one of the widely utilized titanium alloys is a two-phase Ti-6Al-4V alloy that exhibits a good combination of strength and ductility. This alloy is manufactured on industrial scales and is a relatively inexpensive material. Moreover, another modification of the alloy with extra low interstitials (ELI) content, which was initially developed for cryogenic applications, is nowadays the mostly used material for hard tissue replacement. High tensile and fatigue strength of titanium implants guarantee relatively long life time of implants for patients. However, medical implants undergo high loads that lead to rapid wear and revision surgery. Up to date, the ultimate tensile strength (UTS) of the Ti-6Al-4V ELI alloy reaches 950 MPa, the relative elongation to failure is at least 11% and the fatigue limit is about 530 MPa [Boy97]. Therefore, the fatigue life is limited to 107 cycles or 10 years at this stress. In order to increase the life time of the implants and avoid or postpone revision surgery, implants with increased fatigue properties are required. For instance, an increase of the fatigue limit by 50 MPa can increase the fatigue life several times. Therefore, the development of the Ti-6Al-4V alloy with high tensile and fatigue properties is of current interest. Since the first International Conference on Titanium in 1968 [Jaf70], numerous studies have been dedicated to improvement of strength of the titanium alloys, see for example [Mar70; Fop70]. The main approaches included solid solution strengthening, heat treatment, and thermomechanical treatment (TMT). However, new alloying elements and high alloyed 2 Introduction and research objectives compositions can confine the application of titanium materials in medicine because of the harmful effect of particular chemical elements on the human body. Strengthening by heat treatment due to control of morphology of phase constituents or dispersion of secondary phases is employed only for a number of titanium alloys and in many instances the possibilities of this approach are already depleted. Work hardening procedures such as rolling, drawing and forging can significantly affect the microstructure and increase the mechanical properties. However, in this case the elongation to failure, which is a very important characteristic, is reduced [Boy97]. One of the perspective methods for improvement of the physical and mechanical properties of metallic materials is the formation of nanocrystalline (NC) or ultrafine-grained (UFG) microstructures using severe plastic deformation (SPD). In this approach large strains are imposed to a material resulting in grain refinement [Val91; Val92]. The most significant results in this field were achieved on pure metals such as aluminum, copper, nickel, titanium, and also on aluminum and copper alloys [Val00]. For instance, the ultimate tensile strength (UTS) of CP-Ti processed by SPD increased by a factor of 2.5 and the fatigue limit by a factor of 1.5 [Vin01a]. These results make it possible to assume that the tensile and fatigue strength of the Ti-6Al-4V alloy processed by the above-mentioned approach will also increase. However, grain refinement in hard-to-deform titanium materials is a technological challenge. Up to the moment of the set up of this work (2003) some experiments on the development of equal channel angular pressing (ECAP, one of the SPD methods) applied to the Ti-6Al-4V alloy were already started. Therefore, it was important to determine the possibilities and effectiveness of the ECAP-method for obtaining an UFG microstructure in the alloy. Hence, the objective of this work is to investigate the influence of the UFG microstructure obtained by ECAP on the mechanical properties of the Ti-6Al-4V ELI alloy and to establish a new class of materials for a successful use in biomedical applications. According to these objectives the following problems are to be solved in the current work: 1) to study the influence of ECAP processing on the microstructure of Ti-6Al-4V of both commercial and ELI purity; 2) to investigate the influence of a combination of ECAP and thermomechanical treatment (TMT) on the microstructure of the Ti-6Al-4V ELI alloy;