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
Homogenization treatment: 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 (Near)-net shape casting
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-modulus alloy
Differential interference contrast (Nomarski contrast)
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
a + b alloy (Ti-6Al-4V)
Metastable b-alloy (Ti LCB)
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 observations of Ti-6Al-7Nb alloy
Biomedical alternative to Ti-6Al-4V alloy
SEM image of duplex structure
Z contrast
dark – alpha
interior – Al enriched,
Nb depleted
edge – equilibrium composition
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-situ observations In-situ methods Diferential scanning calorimetry
Electric resistivity measurements
Microscopic and diffraction methods in in-situ arrangement
Ti LCB – electric resistivity measurements and differential scanning calorimetry (DSC)
Dissolution of athermal w fáze – diffusion-ess process
Stabilization and growth of isothermal w phase – diffusion process
Dissolution of w phase
Precipitation of a phase
Dissolution of a phase
Above b-transus temperature – pure b phase

15. Mechanical properties and fatigue
Standard tensile tests
Ti alloys undergo necking, moreover many β-alloys show work softening  samples should be manufactured according to appropriate standards (e.g A5 standard)
Increased work hardening paradoxically increase elongation due to avoiding necking
Ti and Ti alloys are extremely notch sensitive (!)
Uneven surface (groove, scratch) serve as fatigue crack initiation site with fatal effect on fatigue performance
Samples must carefully polished (prefereably in longitudial direction) or electro polished to obtain comparable results
Fatigue performance can be significantly improved by surface treatment processes
Shot-peeining, sand blasting, laser shock processing, ball burnishing etc.
Surface nitridation (hard surface layer TiN),…

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. Biocompatible alloys 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 Metastable 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: MPa; Ultimate tensile strength (UTS): MPa
Elastic modulus: GPa
TiOstalloy® - TNZT (Ti-35Nb-7Zr-5Ta)
Yield stress MPa; UTS: 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 Nitinol (Ti-Ni)
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. Titanium aluminides 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 grained materials
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
HPT
ECAP-Conform

23. Ultra-fine grained materials
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 for phase composition and microstructure control
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
Thank you!
Project FRVŠ 559/2013 is gratefully acknowledged for providing financial support.