**Author details**

or striations were identified which is an evidence of frequent change of the crack growth direction. This phenomenon along with secondary crack branching are important reasons for

**Figure 9.** Fatigue fracture surfaces of Ti-6Al-5Mo-5V-1Cr-1Fe alloy cooled from 1020°C at a rate of 0.8°C s-1.

absorb sufficient amounts of energy and retard the crack propagation.

in turn lowers yield stress and tensile strength of the alloys.

therefore increasing fatigue life.

**5. Summary**

The progress of the crack tip through regions of interfacial β-phase is accompanied by the absorption of large amount of energy due to intensive plastic deformation, contributing to lowering the rate of crack propagation. When thickness of β-phase regions decreases, it cannot

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

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

advantageous effect of lamellar microstructure on fatigue behaviour.

78 Titanium Alloys - Advances in Properties Control

J. Sieniawski, W. Ziaja, K. Kubiak and M. Motyka

Rzeszów University of Technology, Dept. of Materials Science, Poland
