Titanium Alloys and Its Heat Treatment | Metallurgy

Titanium alloy is used as prostheses for implanting in the human body. Wrought forms of titanium alloys find major applications. Because of its high melting point (1720°C), its alloys have potentials for high temperature applications, but are a bit expensive to produce them from ores.

As in pure iron, the low temperature HCP (α) titanium on heating undergoes allotropic transformation at 882°C to change to BCC (β) titanium to be stable up to melting point. The addition of the alloying elements in titanium stabilises either the α or the β phases.

Thus, effect of alloying elements may be given as:

(a) Elements which stabilise a-titanium by dissolving preferentially in it, and thus raise α/β transformation temperature. For example- Al, O, C, N.

(b) Neutral effect: For example, Zr, Sn, Si.

(c) Elements which stabilise β-titanium by dissolving preferentially in it, and thus depress α/β temperature.

These elements can be classified:

(i) Which result in isomorphous type solid solution, such as Mo, W, V, Ta.

(ii) Which undergo eutectoid reaction – Such as Cu, Mn, Cr, Fe, Ni, Co.

Titanium alloys are classified as:

(a) α alloys

(b) (α + β) alloys

(c) β alloys

Basics of Heat Treatment:

Fig. 14.2 illustrates the effect of alloying elements on α, and or β in alloys. It also shows that the strength of annealed alloys increases linearly and gradually with solute content, or as amount of β-phase increases. Quenched alloys transform β to martensite.

The resulting effect in increasing strength is not much except at low concentration of solute. The maximum increase in strength occurs by age-hardening by decom­posing retained β-phase. Two phase alloys are stronger than single alpha phase as beta phase is stronger than alpha phase.

The alloys having α and nearly-α (β max 2%) are given stress-relieving and annealing heat treatment. The two-phase alloys containing α + β show maximum response to hardening and strengthening.

Heat Treatment Processes for Titanium Alloys:

(a) Stress relieving to relieve residual stresses induced during fabrication.

(b) Annealing to have maximum ductility, machinability. The stable micro-structure results in dimen­sional stability also.

(c) Solution heat treatment and precipitation hardening to improve the mechanical strength and hardness.

Stress Relieving:

It is the process of heating to a high temperature (table 14.13), soaking at this temperature to relieve stresses without causing an undesirable amount of precipitation, or strain ageing in α+ β and β-type alloys, or undesirable recrystallisation in single phase alloys which depend on cold work for strength and hardness.

Stress relieving does not cause adverse effect on strength, or ductility. A separate stress relieving treatment may be omitted when the sequence in production employs annealing or hardening.

Annealing:

Quite a few components of titanium alloys are used in service in the annealed state.

The aims of annealing are:

(a) To induce maximum ductility at room temperature

(b) To improve machinability

(c) To induce adequate toughness

(d) To induce dimensional and microstructural stability at high temperatures i.e., to induce maximum creep resistance.

Thermal stability is obtained by producing stable beta, which is capable of resisting further transformation, when’ exposed to elevated temperatures in service. Annealing should be able to obtain this beta-phase from, if formed, brittle intermediate omega phase. The annealing process consists of heating to 650° to 790°C (depending on the composition), soaking and then slow cooling (3°C/min), to 540°C, and cooling in air.

‘Duplex’ annealing helps to obtain maximum creep resistance in alloys such as Ti-8 Al- 1 Mo – 1 V. It consists of heating the alloy in range of 790°-900°C for a time of 5 minutes for sheets and to 1 hour for thicker bars, and then cooled in air. The alloy is then given ‘stabilising anneal’ by heating it to 480° to 700°C.

Table 14.14 gives annealing temperatures for some titanium alloys:

Precipitation Hardening:

It is possible to strengthen several titanium alloys with adequate ductility by giving them precipitation hardening:

Solution Treatment:

All the three types of titanium alloys are given precipitation hardening. The solution treatment is given in (α + β) region, which for commercial α + β alloys is 760°C to 1000°C. Changing the solution temperature alters the amount of β-phase. If solutionising is done in 100%, β-phase region, the resulting ductility is impaired. During ageing, it is the β-phase which decomposes to yield high strength levels. Thus, depending on the temperature of solution treatment, the amount of β-phase is varied (increases with temperature), which causes resulting age hardening with required ductility.

Quenching:

Normally β-alloys are air-quenched from solution temperature, (α + β) alloys are water-quenched, or 5% brine-quenched. (Table 14.15 gives the solution and ageing temperatures of some alloys). Actually, cooling rate from solution temperature plays an important role in strengthening the alloys. A slower cooling allows the diffusion to occur during cooling itself the changed β phase may not provide effective strengthening during ageing.

However, rapid rates of quenching is less critical in many alloys, but an alloy Ti-6 Al-4 V should be cooled from 930°C to 540°C in less than 2 seconds to develop maximum strength after ageing. Section size directly affects the effectiveness of quenching in such cases. Alloy Ti-4 Al-3 Mo-1 V may be cooled in oil, and still provides good strength after ageing.

Ageing:

Ageing decomposes the supersaturated beta-phase retained on quenching. Proper ageing produces high strength with adequate ductility. In α + β alloys, various combination of strength, ductility and fabricability can be obtained by modifying the solution and ageing treatments.

In (α + β) alloys, quenching results invariably in beta transforming to a metastable but, brittle phase omega severe quenching and rapid heating to high ageing temperature can avoid it. Ageing above 430°C, can change omega (to β). Table 14.15 gives ageing treatment for some titanium alloys.

The only 100% beta alloy Ti- 13V-11 Cr-3 Al which forged and rolled in beta range, and air cooled after hot working yields solution treated condition. This alloy when aged at 500°C for 8 to 100 hrs. yields a UTS of 1100-1380 MN/m².