Titanium and titanium alloys Josef Stráský. Lecture 2: Fundamentals of Ti alloys – Polymorphism...
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Transcript of Titanium and titanium alloys Josef Stráský. Lecture 2: Fundamentals of Ti alloys – Polymorphism...
![Page 2: Titanium and titanium alloys Josef Stráský. Lecture 2: Fundamentals of Ti alloys – Polymorphism Alpha phase Beta phase – Pure titanium – Titanium alloys.](https://reader035.fdocument.org/reader035/viewer/2022081416/56649d095503460f949dafb3/html5/thumbnails/2.jpg)
Lecture 2: Fundamentals of Ti alloys– Polymorphism• Alpha phase• Beta phase
– Pure titanium– Titanium alloys• a alloys• + a b alloys• b alloys
– Phase transformation β α– -w phase– Hardening mechanisms in Ti• Hardening of α+β alloys • Hardening of metastable β alloys
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Polymorphism• Pure titanium is polymorphic – it exists in more crystallographic
structures (phases)• Similarly to iron
• Above 882°C the titanium is body-centered cubic material (bcc) – b – phase
• Upon cooling below 882°C – (so-called -b transus temperature) – b – phase martensitically transforms to hexagonal close-packed structure - a – phase
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Stable phases – pure titanium• b – phase – stable above 882°C (b-transus temperature)• Under room temperature, only a – phase is stable
Hexagonal close-packed lattice (hcp)Phase a
Body-centered cubic lattice (bcc)Phase b
6 atoms / 0.106 mm3= 56,6 2 atoms / 0.0366mm3= 54,6
This difference allows dilatometry studies.
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HCP – hexagonal closed packed - a phase
c/a(Ti) = 1.588 < 1.633 (optimal)c/a(Cd) = 1.886 c/a(Zn) = 1.856c/a(Mg) = 1.624c/a(Co) = 1.623c/a(Zr) = 1.593
Slip systems:Principal: Prismatic {1 0 -1 0} <1 1-2 0>Secondary: Basal {0 0 0 1} <1 1 -1 0>Other: Pyramidal {1 0 -1 1} <1 1 -2 0> and {1 1 -2 2} <1 1 -2 3>
Plane - (hkl)Set of planes - {hkl}Direction - [UVW]Set of directions – <UVW>
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HCP – hexagonal closed packedSlip systems Principal: Prismatic {1 0 -1 0} <1 1-2 0>Secondary: Basal {0 0 0 1} <1 1 -1 0>Other: Pyramidal {1 0 -1 1} <1 1 -2 0> and {1 1 -2 2} <1 1 -2 3>
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The effect of alloying elements on phase composition
• Alloying elements affect the beta-transus temperature• a – stabilizing elements – increase beta-transus temperature• b – stabilizing elements – increase beta-transus temperature– Isomorphous – completely soluble in solid solution– Eutectoid – intermetallic particles are created
Neutral a - stabilizing b - stabilizing(izomorphous)
b - stabilizing(eutectoid)
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• Neutral elements– Zr, Sn• a - stabilizing elements – Al, O, N, C• b - stabilizing elements – isomorphous – Mo, V, Ta, Nb• b - stabilizing elements – eutectoid – Fe, Mn, Cr, Co, Ni, Cu, H
Neutral a - stabilizing b - stabilizing(isomophous)
b - stabilizing(eutectoid)
The effect of alloying elements on phase composition
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Titanium alloys• Pure Ti at room temperature only a – phase• Low-content of b-stabilizing elements (and/or
outweighted by a -stabilizing elements) only a – phase is stable at RT so-called a – alloys
• Increased content of b-stabilizing elements• - b phase
becomes stable at room-temperature
• + a b alloys
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Titanium alloys• Further increased content of b-stabilizing elements
The temperature „martenzite start“ of phase transition b a is decreased below room temperature
Phase a is not formed during quenching Metastable b-alloys
During annealing below b-transus temperature, the a phase is created until the balance composition is achieved
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Titanium alloys• Even more increased content of b-stabilizing
elements Beta transus temperature can be decreased below RT Afer cooling to room temperature, b phase remain
stable Stable b-alloys
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Molybdenum equivalence• : Mo one of the most important b – stabilizing elements• Comparison of b – stabilizing effect of different elements
so-called molybdenum equivalence
• [Mo]eq = [Mo] + 0,67 [V] + 0,44 [W] + 0,28 [Nb] + 0,22 [Ta] +
+ 2,9 [Fe] + 1,6 [Cr] + 1,25 [Ni] + 1,7 [Mn] + 1,7 [Co] - 1,0 [Al]
• i.e. Vanadium content must be 1.5 times higher then Molybdenum to achieve the same effect on stability of beta phase
• i.e. Iron is three times stronger beta stabilizer than Mo a 4x than V
• Molybdenum equivalence is only empirical rule based on analysis of binary alloys
• Molybdenum equivalence cannot be used quantitatively to compute beta transus temperature or equilibrium phase composition– Especially in the case of ternary and more complicated alloys
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Aluminium equivalence• Less used analogy of molybdenum equivalence for a -
stabilizers• Al: one of the most important a – stabilizing elements
[Al]eq = [Al] + 0,33 [Sn] + 0,17 [Zr] + 10 [O + C +2N]• In the case that Al equivalence is higher than approx 9%
(some sources say 5%), then Ti3X intermetalic particles are formed
• The effect of Zr remains unknown and depends strongly on the content of other alloying elements
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Phase transformation b a • Occurs below b-transus temperature• In pure Ti, a-alloys and + a b alloys
– martensitic transformation– But not until equilibrium phase composition– Followed by growth of particles (lamellae)
• In metastable -b alloys • Precipitation of -a particles
• Homogeneous precipitation• Heterogeneous precipitation
• Grain boundaries• Particles of other phases• Chemical inhomogeneities
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Phase transformations in metastable b alloys1. Phase separation: b blean + brich také b b´ + b
• Occurs in strongly stabilized b-alloys
• b-stabilization elements form clusters via spinodal decomposition brich regions
2. Formation of -w phase• Less stabilized b-alloys• Formation of particles w• Small coherent particles, size: 1-20 nm
3. Precipitation of -a phase from -w phase• Small w phase particles serve as
precursors of a phase precipitation
precipitates of a phase are tiny and homogeneously distrbiuted
significant strengthening
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w-phase• Hexagonal (but not hcp)
phase• Nanometer sized particles• Created after quenching (w-
athermal) • Grow during ageing
Transmission electron microscopy
Devaraj et al., Acta Mat 2012Ti-9Mo a,b) – quenched from b; c)-e) – 475°C/30 mins; f)-h) - 475°C/48 h
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bw a phase transformations• Volume fraction of phases in Ti-LCB alloy studied by X-ray
diffraction• + b w transforms to a with increasing ageing temperature and
ageing time (isothermal annealing)
Smilauerova et al., unpublished research
Ageing at 450°CAgeing at 400°C
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bw a phase transformations• Identified by in-situ methods
– Differential scanning calorimetry– Measurements of electrical resistivity
• bw a transformations identified during heating 5°C/min
I. Dissolution of athermal w phase, reversible diffusionless process
II. Stabilization and growth of isothermal w phase – diffuse prodcess
III. Dissolution of w phase
IV. Precipitation of a phase
V. Dissolution of a phase
VI. Above b-transus temperature – b phase
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w observed by 3D atom probe• Observation is based on chemical differences
Deveraj et al., Scripta Mat 61 (2009)
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Hardening of Titanium1. Inerstitial oxygen atoms
– If oxygen content in pure Ti is increased from 0.18 to 0.4 wt. % – the strength is increased from 180 MPa to 480 MPa (!)– Typical oxygen content in commercial Ti alloys is 0.08 – 0.20 hm. %– Higher oxygen content often causes embrittelment– Positions of oxygen atoms are correlated to vacancies
• positron annihilation spectroscopy study
2. Solid solution strengthening– a-stabilizing substitutional elements (Al, Sn) strengthen -a phase (Ti-5Al-2.5Sn
800 MPa)– Some fully soluble b-stabilizing elements strengthen -b phase (Mo, Fe, Ta),
others have negligible effect (Nb)– Size of the atoms and electron structure is decisive for this effect
• Some atoms serve as obstacles for dislocation motion
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Hardening of Titanium3. Intermetallic particles (precipitation hardening)
– Eg. Aluminides nitrides, carbides, silicide – and many others– Size of the particles and their distribution are crucial for the strengthening effect
(Orowan strengthening)
4. Dislocation density and grain refinement– Forming/working (e.g. extrusion, forging, etc.) can increase dislocation density
and cause grain refinement– Dislocations cause obstacles to movement of other dislocations causing increase
strength– Grain and sub-grain boundaries may also act as dislocation obstacles– Working must be done at sufficiently low temperatures to suppress extensive
grain growth and dislocation annihilation
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Hardening of + a b alloys– Sufficient content of Al (or Sn) leads to formation of Ti3Al
particles (when content of Al is above 9 wt. %) • Solvus of Ti3Al particles is approx. 550°C, final annealing temperature must be
below this temperature
– Ti3Al particles are hexagonal and coherent and block the dislocation movement causing precipitation hardening• In + a b alloys occurs separation of elements during annealing – a-
stabilizers diffuse to a-phase, whereas b–stabilizers diffuse to b-phase
• Intermetallic particles (Ti3Al) can be created in a-phase due to sufficient amount of a-stabilizer despite overall (average) chemical composition does not allow such precipitation
– Phase boundaries serve as dislocation motion obstacles
– The smaller are morphological features of respective phases, the bigger is strengthening effect
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Hardening of metastable -b alloys
• Interstitial, solid solution and dislocation density strengthening • Precipitation hardening/phase boundary hardening caused by
phase transformation of b-matrix– Phase separation – blean + brich
– Formation of -w phase particles– Precipitation of -a phase particles
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Lecture 2: Conclusion• Titanium is polymorphous material – two stable phases
– a-phase– hexagonal close-packed (hcp)– b-phase– body-centered cubic (bcc)
• Below b-transus temperature - a phase is formed• Pure Ti – at room temperature only a-phase• + a b alloys
– At room temperature mix of phases a + b– a phase formation cannot be suppressed
• Metastable b-alloys– After quenching - only b-phase– Upon annealing -a phase is formed
• b a transformation – Martensitic followed by growth ( + a b alloys)– Precipitation followed by growth (b alloys)
• w phase– Nano-sized particles, precursor for a-phase particles precipitation
• Hardening of Ti and Ti alloys– Pure ti is hardenend mainly by oxygen content– Alloys are hardened by solid solution strengthening, intermetalic particles and
particles of other phases
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Thank you!
Titanium and titanium alloysJosef Stráský
Project FRVŠ 559/2013 is gratefully acknowledged for providing financial support.