Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 93
β
Temperature
α
α + β
β + γ
α + γ
→
M s
3.12 Schematic phase diagram for age-hardening Ti alloys; M S is the
martensite start temperature.
α/β-alloys
α/β-alloys have phase diagrams of the general form indicated in Fig. 3.13,
and are of the greatest commercial importance, with Ti–6Al–4V comprising
over half the world sales of Ti alloys. Their high strength, coupled with
improved formability over the α-alloys, have made them attractive contenders
for forged components such as the fan blades of jet engines. Fine-grained
Ti–6Al–4V sheet material is used in the aero-engine industry for the superplastic
forming of numerous components.
The microstructure and properties of the α/β alloys can be varied by heattreatment.
If the alloy is heated into the β-phase field and then quenched
rapidly it undergoes a diffusionless phase transformation to form a martensitic
phase, α′. This transformation starts at temperature M s and finishes at
temperature M f , as indicated in Fig. 3.13.
The change of strength with heat-treatment is also indicated in Fig. 3.13.
The annealed alloys gradually increase in strength as the alloying element
content is increased: this (by the lever rule) will be accompanied by a
progressive increase in the volume fraction of β-phase. Quenching to form
α′ produces a modest increase in strength (much less than that encountered
in martensitic steels), the highest value corresponding to that alloy whose M f
temperature is at room temperature, so the microstructure will be wholly
transformed to α′, with no metastable β persisting. If the quenched alloys are