Titanium and titanium alloys

1. Titanium and titanium alloys Josef Stráský

2. Lecture 4: Production technologies, experimental investigation, modern problems • Technology • Casting, forming • Experimental methods • Metalography • Scanning and transmission electron microscopy • X-ray diffraction • Phase transformations investigation • Mechanical properties and fatigue • Titanium aluminides • Shape memory effect • Biocompatible alloys • Ultra-fine grained Ti

3. Casting • Liquid titanium is extremely reactive • Casting must be done at vacuum of very clean inert atmosphere (He, Ar) • Currently, there is no continuous process • The bigger batch (size of primer ingot) the better economic efficiency  typical ingot size: 10 – 15 tones • Other option is electric arc melting (feasible only for research and development) • Typical casting defects are segregation of either a or b stabilizing elements • Homogenizationtreatment: 200 – 450°C above b-transus temperature; 20 – 30 hrs in industrial process; in laboratory usually 2-4 hrs

4. Forming • Forging and rolling are commonly use to produce rods and sheets (sheets constitute 40% of production) • First forming steps are done usually above b-transus temperature due to increased formability • The temperature of future forming depends on thy particular alloy (and its type), formability and required properties • The temperature control is essential due to undergoing phase transformations that may cause degradation of mechanical properties or even embrittlement during forming • Precise time and temperature control allows wider use of metastable β-Ti alloys since material properties depend on ‘complete history‘ of material processing • Hot working refer to forming above recrystallization temperature (usually it refers to forming above b-transus temperature) • Cold working is forming below recrystallization temperature (well-bellow b-transus temperature, only rarely at room temperature) • Stress-relief treatment is required (sometimes interferes with phase transitions)

5. Other technologies • Machining, cutting • Machining and cutting of Ti alloys is complicated, time demanding and costly • Ti alloys are extremely tough causing extreme heat generation and increased tool wear • Processes are costly due to low machining/cutting rate, cooling requirements and frequent tool replacements • Generated heat may also cause a) contamination, b) microstructural/phase transtion causing deterioration of mechanical properties • (Near)-net shape casting • Continuously increasing percentage of casted final products • Reduction of costly machining and welding • Powder metallurgy • Advanced technology for net-shape processing; small-scale production and chemical composition variability • However, industrial application is limited due to extreme titanium reactivity (requiring vacuum/inert atmosphere) • Continuous improvement in available technologies (e.g. field assisted sintering technique) • Superplastic forming • Ti alloys can be formed superplastically, but usually at high temperatures (>900°C) and using low strain rate (10-4 s-1), which significantly limits industrial use • Superplasticity depends on grain-size, therefore advances in grain size control (e.g. severe plastic deformation) can make superplastic forming feasible • Welding • The main issue is atmospheric contamination of welds that are extremely heated • Other issue are undergoing phase/microstructural transformations that significantly affect mechanical properties of the weld

6. Experimental techniques – metallography (light microscopy) • Sample preparation • Standard grinding using series of grinding papers • Abrasives of finer papers (starting from 800 mesh) tend to dig into the material • This can be avoided by short etching by weak Kroll‘s etchant after each grinding/polishing step • Alumina is preferred to diamond pastes for polishing • Polishing should be done down to 0.05 μm fineness • Hydrogen peroxide and/or very weak solution of hydrofluoric acid can be added during polishing • Grinding and polishing is generally much more complicated for β-Ti alloys (especially when in β solution treated condition) • Etching • Kroll‘s etchant is used to image different phases (i.e. α and β – typically to show microstructure of Ti6Al4V alloy) • Oxalic acid shall be used to show grains (e.g. in pure Ti or solution treated β alloys)

7. Light microscopy • Ti-35Nb-7Zr-5Ta and Ti-35Nb-7Zr-5Ta-1Si alloys • Biocompatible, low-modulusalloy • Differential interference contrast (Nomarskicontrast) • Částice (Ti,Zr)5Si3 a relikty z broušení

8. Scanning electron microscopy • The main requirement is flat, smooth and clean surface • Similar procedure to light microscopy might be used • Usually sufficient for Ti-6Al-4V, but not for b-alloys • Vibratory polishing • Excellent labor-saving polishing method • Vibratory polisher is currently produced only by Buehler • Three steps polishing: Alumina 0,3 mm, Alumina 0,05 mm and colloidal silica • 8 hours (or more) each step • Precise cleaning of samples and holders required between steps (or even short etching by diluted hydrofluoric acid) • Electrolytic polishing • Electrolyte: 5% H2SO4; 1,25% HF, in methanol • Room temperature, 30 s • Results are mixed, strongly depending on alloy

9. Scanning electron microscopy • Metastable b-alloy (Ti LCB) • a + b alloy (Ti-6Al-4V) Scanning electron microscopy – Z-contrast • a and b phase can be distinguished by back-scattered electrons thanks to different chemical composition (Z-contrast) • b-phase contains more heavier elements  it appears lighter

10. SEM observationsof Ti-6Al-7Nb alloy • Biomedicalalternative to Ti-6Al-4V alloy • SEM image of duplex structure • Z contrast • dark – alpha • interior– Al enriched, • Nbdepleted • edge –equilibriumcomposition • bright– beta • caused by double step annealing (970° + 750°C)

11. EBSD observations Ti-6Al-7Nb alloy • LEFT: EBSD image (orientation) image map) • RIGHT: grain boundaries with misorientation 55 – 65 ° • UP: distribution of grain boundaries • Alpha lamellae are not created randomly but follows Burgers relationship between b and a some misorientations are preferred

12. Transmission electron microscopy • Thin foil preparation is essential • Electrolytic polishing • Electrolyte: 300 ml methanol, 175 ml butanol, 30 ml perchloric acid • As low temperature as possible (-50°C) • Different phases are polished with different rate • Tricky for β-alloys • Ion polishing • Material removal by Ar ions under small-angle (PIPS) • Uniform removal, but smaller area for TEM observations, alost material independent, but time demanding (24 hrs and more) • FIB – focused ion beam • Usually part of scanning electron microscope • Area for manufacturing TEM foil can be selected by SEM observations • Multi-step process with decreasing energy of FIB leads to high quality foil

13. Transmission electron microscopy • Essential for ω-phase observations Devaraj et al., Acta Mat 2012 Ti-9Mo a,b) –water quenched from b-field; c)-e) – 475°C/30 min.; f)-h) - 475°C/48 hod.

14. In-situobservations • In-situmethods • Diferentialscanning calorimetry • Electric resistivity measurements • Microscopic and diffractionmethods in in-situ arrangement • Ti LCB – electricresistivity measurementsand differentialscanning calorimetry (DSC) • Dissolutionofathermalw fáze – diffusion-essprocess • Stabilization and growthofisothermalwphase – diffusionprocess • Dissolutionofwphase • Precipitationofa phase • Dissolutionofa phase • Aboveb-transustemperature – pureb phase

15. Mechanicalproperties and fatigue • Standard tensiletests • Ti alloysundergo necking, moreover many β-alloys show worksoftening samplesshouldbemanufacturedaccording to appropriatestandards (e.g A5 standard) • Increasedworkhardeningparadoxicallyincreaseelongationdue to avoiding necking • Ti and Ti alloys are extremelynotch sensitive (!) • Unevensurface (groove, scratch) serve as fatiguecrackinitiationsitewithfataleffect on fatigue performance • Samplesmustcarefullypolished (prefereably in longitudialdirection) orelectropolished to obtaincomparableresults • Fatigue performance canbesignificantlyimproved by surfacetreatmentprocesses • Shot-peeining, sandblasting, laser shockprocessing, ballburnishingetc. • Surfacenitridation (hard surfacelayerTiN),…

16. Main areas of current research and development • Isolation of Ti • Complicated Kroll‘s process causes high-price of titanium • Strong incentives for developing new process • Casting and alloying • No continuous process available; typical batch size: 10 – 15 tones – low production flexibility • New processes (powder metallurgy, magnetic levitation casting) are being developed, however still more expensive • Significant improvement would decrease price of titanium leading to massive use in automobile industry

17. Main areas of current research and development • New alloys • Rapid development mainly in the field of metastable b-alloys • Elimination of expensive alloying elements • Minimization of segregation problems – simplifying casting procedure • Tailored composition to intended properties/use • Thermo-mechanic treatment optimization • Thermo-mechanic treatment determines phase composition and microstructure that significantly affect mechanical properties; however these relationships are still not completely qualitatively and quantitatively understood • ‚Complete history‘ matters – annealing/forming/ageing temperatures and times, and also all heating and cooling rates during production • Requires precise production control • The effect of ageing on microhardness of LCB alloy

18. Biocompatiblealloys • Requirements • Using biocompatible elements (Ti, Nb, Zr, Ta, Mo) • Elimination of toxic elements (V, Sn) • Sufficient strength and formability • Lowering the elastic modulus • Stiff implant causes stress shielding • Elastic modulus of bone – 30 GPa • Elastic modulus Ti-6Al-4V 120 GPa • Achievable elastic modulus in metastable beta alloys: 50 – 80 GPa • Cost is not the key factor due to specialized application that require low amount of material with outstanding properties

19. Biocompatible alloys • a + b alloys • The most used is Ti-6Al-4V • But: vanadium is toxic (but: it is probably not dissolved from the implant) • Biocompatible alternative Ti-6Al-7Nb • Equivalent properties to Ti-6Al-4V alloy • No toxic vanadium • Metastable b-alloys • Commercial alloys TMZF® and TNTZ® • Comparatively low-strength in beta-solution condition, further ageing increases elastic modulus • TMZF® (Ti-12Mo-6Zr-2Fe) • Yield stress: 965-1060 MPa; Ultimate tensile strength (UTS): 1000-1100 MPa • Elastic modulus: 74-85 GPa • TiOstalloy® - TNZT (Ti-35Nb-7Zr-5Ta) • Yield stress 530-793 MPa; UTS: 590-827 MPa; • Elastic modulus: 55 GPa (after quenching from b region) • Possible improvements • Strengthening by intermetallic particles (Ti,Zr)5Si3, TiC, TiN,… • Employing. pseudoelasticity (martensitic transformation during deformation – SIM – stress-induced martensite) – decreased elastic modulus • Employing of ultra-fine grained material – increased strength and biocompatibility

20. Shape memory alloys • Shape memory effect • Martensite transformation • Diffusionless, reversible • Martensite is created upon cooling • Can be deformed • Upon heating transforms back to austenite and the original shape is restored • Nitinol (Ti-Ni) • Martensitic transformation temperature is around room temperature (or 37°C) • Temperature can be fine-tuned by Ni content • Applicable in blood vessels‘ stents • Stent is cooled and deformed, then it is moved to correct position; after heating exactly to 37°C it restore its shape • serve as blood vessel reinforcements

21. Titaniumaluminides • Intermetallic compounds • Ti3Al (a2), TiAl(g) • High strength at elevated temperatures • Excellent creep resistance up to 750°C • Currently not widely applied in industry • Promising potential to replace nickel superalloys in some parts of airplane engines • Cost and weight saving • But: very limited formability • Can be partly increased by alloying (so-called gamma alloys) • Can be improved by sophisticated manufacturing (powder metallurgy, sintering etc.)

22. Ultra-fine grainedmaterials • Employing severe plastic deformation (SPD) methods for manufacturing material with high concentration of defects and grain size below 100 nm • Sever plastic deformation • Deformation of material that does not reduce size of the product (contrary to forging, extrusion, rolling etc.) •  Material can be deformed repetitively • ECAP - equal channel angular pressing • HPT - high pressure torsion • ECAP-Conform – continuous ECAP ECAP ECAP-Conform HPT

23. Ultra-fine grainedmaterials • Advantages • Increased strength thanks to reduced grain size and increased defect concentration • Small grains allow superplastic forming (!) • Increased biocompatibility • Disadvantages • Limited size of final products • Technology is currently developed only for CP-Ti and Ti-6Al-4V • Manufacturing is expensive and must be rationalized by cutting-edge applications • CP-Ti is currently used for dental implants (stents)

24. Lecture 4: Summary • Casting, forming are complicated and expensive • Precise production control is required forphasecomposition and microstructurecontrol • Microscopic methods require precise sample preparation • Shape memory alloys (SMA) • Shape memory effect due to martensitic transformation • Ti-Ni (nitinol), used in medicine • Titanium aluminides • High strength at elevated temperatures, creep resistance (up to 750°C) • Low formability, developing field • Ultra-fine grained materials • Severe plastic deformation  grain size < 100 nm • Increased strength and biocompatibility; modern, fast developing field

25. Titanium and titanium alloys Josef Stráský Thankyou! Project FRVŠ 559/2013 is gratefully acknowledged for providing financial support.