**5. Summary**

Microstructure of two-phase titanium alloys after deformation or heat treatment carried out at a temperature in the range of β-phase stability depends on cooling rate. High cooling rates (>18°C s-1) result in martensitic α'(α") microstructure for alloys having β stability factor Kβ<1 and metastable βM microstructure for alloys with higher contents of β-stabilizers. Low and moderate cooling rates lead to development of lamellar microstructure consisting of colonies of α-phase lamellae within large β-phase grains. Decrease of cooling rate cause increase both in thickness of individual α-phase lamellae and size of the colonies of parallel α-lamellae. This in turn lowers yield stress and tensile strength of the alloys.

Lamellar α-phase microstructure of the alloy heat treated in the β-range has beneficial effect on its fatigue behaviour. This is the result of frequent change in crack direction and secondary crack branching. When α-phase lamellae are too large thin layers of β-phase are not capable to absorb large amounts of energy and retard the crack propagation. In this case the colony of the α-phase lamellae behaves as singular element of the microstructure. This phenomenon is more intensive in the alloy with smaller value of Kβ coefficient (Ti-6Al-4V). Sufficient thickness of β-phase surroundings enables absorption of energy in the process of plastic deformation of regions ahead of the crack tip, contributing to slowing the rate of crack propagation and therefore increasing fatigue life.
