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b. Titanium-based Alloys. Titanium is hcp at room temperature – and transform to the bcc structure on heating to 883 o C. Alloying elements are added to Ti to: Stabilize the hcp a -phase – Al and the interstitials C B O and N Partially stabilize the b -phase – Mn Fe Cr Cu Ni H
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Titanium-based Alloys Titanium is hcp at room temperature – and transform to the bcc structure on heating to 883 oC. Alloying elements are added to Ti to: Stabilize the hcp a-phase – Al and the interstitials C B O and N Partially stabilize the b-phase – Mn Fe Cr Cu Ni H Fully stabilize the b-phase – Mo V Ta Nb In addition, combinations of solutes are added to give intermediate effects.
Compositions and Properties of Ti-Alloys All of the a and a+b alloys can be produced as forgings into bars and rings for support members in jet engines Ti alloys can be hot forged with the same equipment used for steels. The oxidized surface layers are machined off after forging – but the high scrap losses are justified by the high strength-weight properties The Ti-5Al-2.5Sn a-alloy and the Ti-6Al-4V a+b alloy can also be rolled into plate and sheet and strip An a+b – phase alloy – Ti-5Al-5V-1Fe-0.5Cu – has similar properties to Ti-5Al-2.5Sn – in both the annealed and heat treated condition
Compositions and Properties of Ti-Alloys Small amounts of Si are added to the a+b alloys to increase high temperature creep resistance – due to insoluble silicide phases The b-phase alloy Ti-13V-11Cr-3Al is used for applications requiring extensive forming – such as light-weight pressure vessels, honeycomb panels for aircraft and missile casings
Interstitial Solutes in Titanium O N and Cdissolve in both hcp and bcc Ti – as interstitials Since chemical analysis of these elements is difficult as their concentrations are low – the purity of Ti is conveniently expressed in terms of its yield strength at 0.2% offset Grade sy (kpsi) sy (MPa) A40 40-60 276-414 A55 55-80 380-552 A70 70-95 483-655 All of these solutes increase hardness and strength – but with lower ductility
Interstitial Solutes in Titanium As O is usually present in the largest concentration – the total effect of interstitials is expressed in terms of the “percent oxygen equivalent” % O equivalent = (%O) + 2(%N) + 0.67(%C)
Ti-Al Alloy System Al is soluble up to ~16 wt% in a-Ti - and raises the a/b transformation temperature from 883 to 1172 oC An alloy with 16 wt% Al will precipitate the brittle d-phase on cooling – so a-phase solid solution alloys are usually limited to <7 wt% Al
a-Ti-Al Alloys In the hcp a-phase – Ti deforms by both slip and twinning At -196 oC twinning is the dominant mechanism At room temperature both methods are active At 700 oC – hot working – slip is the dominant mechanism Since the stresses developed during rolling do not induce twinning – the strength of Ti can be improved – without severely reducing ductility – by the addition of 5 wt% Al – which suppresses twinning in favour of slip
Eutectoid (a+b)-Ti Alloy Systems The transition metal solutes Mn Fe Cr Ni and Cu – only weakly stabilize the b-phase –so it decomposes by a eutectoid reaction – while the a-phase solid solution is reduced to a few percent 798 oC
Eutectoid (a+b), Ti-Cu Phase Diagram a + Ti2Cu
Microstructures of a and (a+b) Ti Alloys Grade A70 Ti – a-phase alloy – Cold worked and annealed Black particles are b-phase – due to Fe impurity
Microstructures of a and (a+b) Ti Alloys Ti-6Al-4V a+b alloy – water quenched and tempered 2 h 740 oC Dark phase is a-phase precipitated from b matrix Plate like structure within b matrix is hcp martensite formed on quenching
(a+b) Ti Alloys • These alloys contain both a- and b-phase stabilizers – Al + V • or Mo • In Ti-6Al-4V Al promotes a-phase – while V promotes b-phase • In Ti-6Al-2Sn-4Zr-6Mo Al promotes a – while Mo promotes b • while Sn and Zr are added for increased strength • As the b-phase is only partially stable – the (a+b) alloys can be further strengthened by heat treatment
(a+b) Ti Alloys These (a+b)Ti alloys are solution treated in the b phase region – when additional a is taken into solution On quenching – b decomposes to hcp martensite On tempering – small particles of a are precipitated with the hcp martensite – and thus strengthen the alloy – but the hardening is not quite so great as the effect in tempered steels The increased strength of these heat treatable alloys gives them a high strength to weight ratio – even though the transition metal solutes themselves are relatively heavy metals.
b-phase Ti Alloys V Mo Nb and Ta form continuous b-phase solid solutions with Ti at high temperatures – and the a-phase solid solutions that form at lower temperatures – with the maximum compositions given below – can be suppressed by rapid cooling V = 3.5 wt% Mo = 0.8 wt% Nb = 4.0 wt% Ta = 12.0 wt%
The Ti-V Phase Diagram b-phase
Microstructures of b-Ti Alloys Ti-13V-11Cr-3Al 100% b-phase alloy – cold rolled into sheet – then solution treated and quenched
Microstructures of b-Ti Alloys Ti-13V-11Cr-3Al 100% b-phase alloy – same as before but tempered 24 h at 485 oC – dark network is a precipitated from the b matrix
b-phase Alloys 100% b-phase alloys have excellent formability – but are more susceptible to contamination from oxygen and nitrogen Ti-13V-11Cr-3Al – does contains some a-phase on slow cooling – but 100% b can be retained on quenching – so this alloy can be strengthened by heat treatment After solution treatment and quenching – the relatively soft b-alloy can be readily formed into complex shapes
b-phase Alloys During tempering - a-phase particles are precipitated from the b-matrix The a-matrix is deformed as it precipitates from the b-matrix – because it has a greater specific volume that b – and the a-precipitates also simultaneously deform the b-matrix The alloy is thus strengthened – because the dislocation density in both phases is increased – so that further deformation is more difficult.
