Titanium and titanium alloys

1. Titanium and titanium alloys Josef Stráský

2. Lecture 2: Fundamentals of Ti alloys • Polymorphism • Alpha phase • Beta phase • Pure titanium • Titanium alloys • a alloys • a + b alloys • b alloys • Phasetransformationβ α • w-phase • Hardeningmechanisms in Ti • Hardeningofα+βalloys • Hardeningofmetastableβalloys

3. 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

4. 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.

5. HCP – hexagonalclosedpacked a - phase Plane - (hkl) Set ofplanes- {hkl} Direction - [UVW] Set ofdirections – <UVW> c/a(Ti) = 1.588 < 1.633 (optimal) c/a(Cd) = 1.886 c/a(Zn) = 1.856 c/a(Mg) = 1.624 c/a(Co) = 1.623 c/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>

6. HCP – hexagonalclosedpacked 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>

7. 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 • - stabilizing • (eutectoid) • - stabilizing • (izomorphous) a - stabilizing Neutral

8. Theeffectofalloyingelements on phasecomposition • Neutralelements– Zr, Sn • a- stabilizingelements – Al, O, N, C • b - stabilizingelements– isomorphous – Mo, V, Ta, Nb • b - stabilizingelements – eutectoid– Fe, Mn, Cr, Co, Ni, Cu, H • - stabilizing • (eutectoid) • - stabilizing • (isomophous) a - stabilizing Neutral

9. Titaniumalloys • Pure Ti atroomtemperature onlya – phase • Low-contentofb-stabilizingelements (and/oroutweighted by a -stabilizingelements)  onlya – phaseisstableat RT  so-calleda – alloys • Increasedcontentofb-stabilizingelements • b - phasebecomesstableatroom-temperature • a + b alloys

10. Titaniumalloys • Furtherincreasedcontentofb-stabilizingelements • Thetemperature „martenzite start“ ofphasetransitionb  aisdecreasedbelowroomtemperature • Phasea is not formedduringquenching • Metastableb-alloys  Duringannealingbelowb-transustemperature, theaphaseiscreateduntilthe balance compositionisachieved

11. Titaniumalloys • Even more increasedcontentofb-stabilizingelements • Beta transustemperaturecanbedecreasedbelow RT • Afer cooling to roomtemperature, b phaseremainstable • Stableb-alloys

12. Molybdenumequivalence • Mo: oneofthe most importantb – stabilizingelements • Comparisonofb– stabilizingeffectofdifferentelements  so-calledmolybdenumequivalence • [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 contentmustbe 1.5 timeshigherthenMolybdenum to achievethesameeffect on stability of beta phase • i.e. Iron isthreetimesstronger beta stabilizerthanMo a 4x than V • Molybdenumequivalenceisonlyempirical rule based on analysisofbinaryalloys • Molybdenumequivalencecannotbeusedquantitatively to compute beta transustemperatureorequilibriumphasecomposition • Especially in the case ofternary and more complicatedalloys

13. Aluminium equivalence • Lessused analogy ofmolybdenumequivalencefora - stabilizers • Al: oneofthe most important a – stabilizingelements[Al]eq = [Al] + 0,33 [Sn] + 0,17 [Zr] + 10 [O + C +2N] • In the case that Al equivalenceishigherthanapprox 9% (somesourcessay 5%), then Ti3X intermetalicparticles are formed • TheeffectofZrremainsunknown and dependsstrongly on thecontentofotheralloyingelements

14. Phasetransformationb 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

15. Phasetransformations in metastableballoys • Phaseseparation: b blean + brichtakéb b´ + b • Occurs in stronglystabilizedb-alloys • b-stabilizationelementsformclusters via spinodaldecomposition  brichregions • Formationofw-phase • Lessstabilizedb-alloys • Formationofparticlesw • Smallcoherentparticles, size: 1-20 nm • Precipitationofa-phase • fromw-phase • Smallwphaseparticles serve as precursorsofa phaseprecipitation •  precipitatesofaphase are tiny and homogeneouslydistrbiuted •  significantstrengthening

16. w-phase • Hexagonal (but not hcp) phase • Nanometersizedparticles • Createdafterquenching (w-athermal) • Growduringageing Transmissionelectronmicroscopy Devaraj et al., Acta Mat 2012 Ti-9Mo a,b) – quenchedfromb; c)-e) – 475°C/30 mins; f)-h) - 475°C/48 h

17. bwaphasetransformations • Volume fraction of phases in Ti-LCB alloy studied by X-ray diffraction • b+wtransforms to a with increasing ageing temperature and ageing time (isothermal annealing) Ageingat 450°C Ageingat400°C Smilauerova et al., unpublishedresearch

18. bwaphasetransformations • Identified by in-situmethods • Differentialscanningcalorimetry • Measurementsofelectricalresistivity • bwatransformationsidentifiedduringheating 5°C/min Dissolutionofathermalwphase, reversiblediffusionlessprocess Stabilization and growthofisothermalwphase – diffuseprodcess Dissolutionofwphase Precipitationofa phase Dissolutionofa phase Aboveb-transustemperature – b phase

19. w observed by 3D atom probe • Observationisbased on chemicaldifferences Deveraj et al., Scripta Mat 61 (2009)

20. HardeningofTitanium • Inerstitial oxygen atoms • If oxygen content in pure Ti isincreasedfrom 0.18 to 0.4 wt. %  •  thestrengthisincreasedfrom 180 MPa to 480 MPa (!) • Typical oxygen content in commercial Ti alloysis 0.08 – 0.20 hm. % • Higher oxygen contentoftencausesembrittelment • Positionsof oxygen atoms are correlated to vacancies • positron annihilationspectroscopy study • Solid solutionstrengthening • a-stabilizingsubstitutionalelements (Al, Sn) strengthena-phase (Ti-5Al-2.5Sn  800 MPa) • Somefullysolubleb-stabilizingelementsstrengthenb-phase (Mo, Fe, Ta), othershavenegligibleeffect (Nb) • Sizeoftheatoms and electronstructureisdecisiveforthiseffect • Someatoms serve as obstaclesfordislocationmotion

21. Hardening of Titanium 3. 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

22. Hardening of a+balloys • 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 doesnot allow such precipitation • Phase boundaries serve as dislocation motion obstacles • The smaller are morphological features of respective phases, the bigger is strengthening effect

23. Hardeningofmetastableb-alloys • Interstitial, solid solution and dislocationdensitystrengthening • Precipitationhardening/phaseboundaryhardeningcaused by phasetransformationofb-matrix • Phaseseparation – blean+ brich • Formationofw-phaseparticles • Precipitationofa-phaseparticles

24. Lecture 2: Conclusion • Titaniumispolymorphousmaterial – twostablephases • a-phase– hexagonalclose-packed (hcp) • b-phase– body-centeredcubic (bcc) • Belowb-transustemperature- aphaseisformed • Pure Ti – atroomtemperatureonlya-phase • a + b alloys • At roomtemperature mix ofphasesa + b • a phaseformationcannotbesuppressed • Metastableb-alloys • Afterquenching - onlyb-phase • Uponannealinga-phaseisformed • b atransformation • Martensiticfollowed by growth (a + b alloys) • Precipitationfollowed by growth (balloys) • wphase • Nano-sizedparticles, precursorfora-phaseparticlesprecipitation • Hardeningof Ti and Ti alloys • Pure ti ishardenendmainly by oxygen content • Alloys are hardened by solid solutionstrengthening, intermetalicparticles and particlesofotherphases

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