Titanium and titanium alloys

Report
Titanium and titanium alloys
Josef Stráský
Lecture 4: Production technologies,
experimental investigation, modern problems
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Technology
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Experimental methods
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Casting, forming
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
Casting
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Liquid titanium is extremely reactive
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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
Forming
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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 βTi 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)
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Stress-relief treatment is required (sometimes interferes with phase
transitions)
Other technologies
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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
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(Near)-net shape casting
– Continuously increasing percentage of casted final products
– Reduction of costly machining and welding
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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)
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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
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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
Experimental techniques –
metallography (light microscopy)
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Sample preparation
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Standard grinding using series of grinding papers
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Alumina is preferred to diamond pastes for polishing
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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
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
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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)
Light microscopy
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Ti-35Nb-7Zr-5Ta and Ti-35Nb-7Zr-5Ta-1Si alloys
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Biocompatible, low-modulus alloy
Differential interference contrast (Nomarski contrast)
Částice (Ti,Zr)5Si3 a relikty z broušení
Scanning electron microscopy
• The main requirement is flat, smooth and clean
surface
1. Similar procedure to light microscopy might be used
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Usually sufficient for Ti-6Al-4V, but not for b-alloys
2. Vibratory polishing
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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)
3. Electrolytic polishing
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Electrolyte: 5% H2SO4; 1,25% HF, in methanol
Room temperature, 30 s
Results are mixed, strongly depending on alloy
Scanning electron microscopy
• a + b alloy (Ti-6Al-4V)
• Metastable b-alloy (Ti LCB)
Scanning electron microscopy – Z-contrast
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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
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)
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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
Transmission electron microscopy
• Thin foil preparation is essential
1. Electrolytic polishing
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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
2. Ion polishing
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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)
3. FIB – focused ion beam
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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
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.
In-situ observations
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In-situ methods
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Diferential scanning calorimetry
Electric resistivity measurements
Microscopic and diffraction methods in in-situ arrangement
Ti LCB – electric resistivity
measurements and differential
scanning calorimetry (DSC)
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Dissolution of athermal w fáze –
diffusion-ess process
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Stabilization and growth of
isothermal w phase – diffusion
process
III. Dissolution of w phase
IV. Precipitation of a phase
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Dissolution of a phase
VI. Above b-transus temperature –
pure b phase
Mechanical properties and fatigue
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Standard tensile tests
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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
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Ti and Ti alloys are extremely notch sensitive (!)
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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
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Fatigue performance can be significantly improved by surface
treatment processes
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Shot-peeining, sand blasting, laser shock processing, ball burnishing etc.
Surface nitridation (hard surface layer TiN),…
Main areas of current research and development
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Isolation of Ti
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Casting and alloying
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Complicated Kroll‘s process causes high-price of titanium
Strong incentives for developing new process
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
Main areas of current research and development
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New alloys
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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
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Thermo-mechanic treatment determines
phase composition and microstructure
that significantly affect mechanical
properties; however these relationships
are still not completely qualitatively and
quantitatively understood
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‚Complete history‘ matters –
annealing/forming/ageing temperatures
and times, and also all heating and
cooling rates during production
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Requires precise production control
The effect of ageing on microhardness of LCB alloy
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
Biocompatible alloys
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a + b alloys
– The most used is Ti-6Al-4V
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But: vanadium is toxic (but: it is probably not dissolved from the implant)
– Biocompatible alternative Ti-6Al-7Nb
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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)
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Possible improvements
– Strengthening by intermetallic particles (Ti,Zr)5Si3, TiC, TiN,…
– Employing. pseudoelasticity (martensitic transformation during deformation – SIM – stressinduced martensite) – decreased elastic modulus
– Employing of ultra-fine grained material – increased strength and biocompatibility
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
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.)
Ultra-fine grained materials
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Employing severe plastic deformation (SPD) methods for manufacturing
material with high concentration of defects and grain size below 100 nm
Sever plastic deformation
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Deformation of material that does not reduce size of the product (contrary to
forging, extrusion, rolling etc.)
 Material can be deformed repetitively
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ECAP - equal channel angular pressing
HPT - high pressure torsion
ECAP-Conform – continuous ECAP
ECAP
HPT
ECAP-Conform
Ultra-fine grained materials
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Advantages
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Disadvantages
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Increased strength thanks to reduced grain size and increased
defect concentration
Small grains allow superplastic forming (!)
Increased biocompatibility
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 cuttingedge applications
CP-Ti is currently used for dental implants (stents)
Lecture 4: Summary
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Casting, forming are complicated and expensive
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Precise production control is required for phase composition and
microstructure control
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Microscopic methods require precise sample preparation
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Shape memory alloys (SMA)
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Titanium aluminides
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Shape memory effect due to martensitic transformation
Ti-Ni (nitinol), used in medicine
High strength at elevated temperatures, creep resistance (up to 750°C)
Low formability, developing field
Ultra-fine grained materials
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Severe plastic deformation  grain size < 100 nm
Increased strength and biocompatibility; modern, fast developing field
Titanium and titanium alloys
Josef Stráský
Thank you!
Project FRVŠ 559/2013 is gratefully acknowledged for providing financial support.

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