Mech 473 Lectures

Professor Rodney Herring b

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 -phase – Al and the interstitials C B O and N

Partially stabilize the -phase – Mn Fe Cr Cu Ni H

Fully stabilize the -phase – Mo V Ta Nb

In addition, combinations of solutes are added to give intermediate effects.

Compositions and Properties of Ti-Alloys

Compositions and Properties of Ti-Alloys

All of the and 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 -alloy and the Ti-6Al-4V alloy can also be rolled into plate and sheet and strip

An – 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 alloys to increase high temperature creep resistance – due to insoluble silicide phases

The -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 C dissolve 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 y (kpsi) y (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 TitaniumAs 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 -Ti - and raises the transformation temperature from 883 to 1172 oC

An alloy with 16 wt% Al will precipitate the brittle -phase on cooling – so -phase solid solution alloys are usually limited to <7 wt% Al

-Ti-Al Alloys

In the hcp -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 (-Ti Alloy Systems

The transition metal solutes Mn Fe Cr Ni and Cu – only weakly stabilize the -phase –so it decomposes by a eutectoid reaction – while the -phase solid solution is reduced to a few percent

798 oC

Eutectoid (Ti-Cu Phase Diagram

+ Ti2Cu

Microstructures of and () Ti Alloys

Grade A70 Ti – -phase alloy – Cold worked and annealedBlack particles are -phase – due to Fe impurity

Microstructures of and () Ti Alloys

Ti-6Al-4V alloy – water quenched and tempered 2 h 740 oCDark phase is -phase precipitated from matrix

Plate like structure within matrix is hcp martensite formed on quenching

() Ti Alloys

These alloys contain both - and -phase stabilizers – Al + V or Mo

In Ti-6Al-4V Al promotes -phase – while V promotes -phaseIn Ti-6Al-2Sn-4Zr-6Mo Al promotes – while Mo promotes - while Sn and Zr are added for increased strength

As the -phase is only partially stable – the () alloys can be further strengthened by heat treatment

() Ti Alloys

These ()Ti alloys are solution treated in the phase region – when additional is taken into solution

On quenching – decomposes to hcp martensite

On tempering – small particles of 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.

-phase Ti Alloys

V Mo Nb and Ta form continuous -phase solid solutions with Ti at high temperatures – and the -phase solid solutions that form at lower temperatures – with the maximum compositions given below – can be suppressed by rapid coolingV = 3.5 wt% Mo = 0.8 wt% Nb = 4.0 wt% Ta = 12.0 wt%

The Ti-V Phase Diagram

phase

Microstructures of -Ti Alloys

Ti-13V-11Cr-3Al 100% -phase alloy – cold rolled into sheet –then solution treated and quenched

Microstructures of -Ti Alloys

Ti-13V-11Cr-3Al 100% -phase alloy – same as before but tempered 24 hat 485 oC – dark network is precipitated from the matrix

-phase Alloys

100% -phase alloys have excellent formability – but are more susceptible to contamination from oxygen and nitrogen

Ti-13V-11Cr-3Al – does contains some -phase on slow cooling – but 100% can be retained on quenching – so this alloy can be strengthened by heat treatment

After solution treatment and quenching – the relatively soft -alloy can be readily formed into complex shapes

-phase Alloys

During tempering - -phase particles are precipitated from the -matrix

The -matrix is deformed as it precipitates from the -matrix – because it has a greater specific volume that – and the -precipitates also simultaneously deform the -matrix

The alloy is thus strengthened – because the dislocation density in both phases is increased – so that further deformation is more difficult.

The End

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