**1. Introduction**

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*Elements*, Spolom, Lviv

68 Titanium Alloys - Advances in Properties Control

237–240, pp. 1312–1318

Dumka, Kiev

June, 2012

Two-phase titanium alloys constitute very important group of structural materials used in aerospace applications [1-3]. Microstructure of these alloys can be varied significantly in the processes of plastic working and heat treatment allowing for fitting their mechanical properties including fatigue behaviour to the specific requirements [4-6].

The main types of microstructure are (1) lamellar – formed after slow cooling when deforma‐ tion or heat treatment takes place at a temperature in the single-phase β-field above the socalled beta-transus temperature Tβ (at which the α+β→β transformation takes place), consisting of colonies of hexagonal close packed (hcp) α-phase lamellae within large body centered cubic (bcc) β-phase grains of several hundred microns in diameter, and (2) equiaxed – formed after deformation in the two-phase α+β field (i.e., below Tβ), consisting of globular α-phase dispersed in β-phase matrix [7-8].

The first type of microstructure is characterized by relatively low tensile ductility, moderate fatigue properties, and good creep and crack growth resistance.

The second microstructure has a better balance of strength and ductility at room temper‐ ature and fatigue properties which depend noticeably on the crystallographic texture of the hcp α-phase.

An advantageous balance of properties can be obtained by development of bimodal micro‐ structure consisting of primary α-grains and fine lamellar α colonies within relatively small β-grains (10-20 µm in diameter) [9-10].

In the following sections the relations between microstructure morphology and mechanical properties of selected high strength two-phase titanium alloys were analysed.

Dilatometric tests, microstructure observation and X-ray structural analysis were carried out for cooling rates in the range of 48-0.004°C s-1 and time-temperature-transformation diagrams were developed for continuous cooling conditions (CCT).

duplex annealing (880ºC / 1h / air cooling and following heating 550ºC / 2÷5 h / air cooling) or

Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys

http://dx.doi.org/10.5772/56197

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Ti-6Al-2Mo-2Cr alloy retains its mechanical properties up to 300°C. At the temperature higher than 400°C mechanical properties are reduced due to partitioning of alloying elements

Ti-6Al-5Mo-5V-1Cr-1Fe transition α+β titanium alloy (Kβ = 1.2) is produced in Russia and Ukraine, where is known as VT22. It is characterized by very good mechanical properties thus is mainly used for large, heavy loaded, forged parts for long-term operation at elevated temperature up to 350÷400ºC and short-term up to 750÷800ºC. Typical applications include disks and blades of low pressure compressors, landing gear elements, engine mount struts and

Phase composition of titanium alloys after cooling from β phase range is controlled by cooling rate. Kinetics of phase transformations is related to the value of β-phase stability coefficient

One important characteristic of the alloy is a range of α+β→β phase transformation tempera‐ ture that determines conditions of thermomechanical processing intended for development of suitable microstructure. Start and finish temperatures of α+β→β phase transformation, vary

**temperature, °C Ti-6Al-4V Ti-6Al-2Mo-2Cr Ti-6Al-5Mo-5V-1Cr-1Fe** *<sup>T</sup>*α+β→<sup>β</sup> *ns* <sup>890</sup> <sup>840</sup> <sup>790</sup> *<sup>T</sup>*α+β→<sup>β</sup> *ps* <sup>930</sup> <sup>920</sup> <sup>830</sup>

*<sup>f</sup>* 985 980 880

*<sup>s</sup>* 950 940 850

*<sup>f</sup>* 870 850 810

**Table 2.** Start and finish temperature of the α+β→β phase transformation for selected titanium alloys (vh = vc = 0.08°C s-1)

**3. Development of microstructure during continuous cooling**

Kβ resulting from the chemical composition of the alloy [7].

depending on the contents of β stabilizing elements (Table 2).

**Phase transformation Alloy**

hardening heat treatment (water quenching and ageing) [1,7].

proceeding by diffusion.

others [1,11].

*T*α+β→<sup>β</sup>

*T*β→α+<sup>β</sup>

*T*β→α+<sup>β</sup>

ns – nucleation start ps – precipitation start

s – start f – finish

The influence of the quantitative parameters of lamellar microstructure on the tensile proper‐ ties and fatigue behaviour of selected two-phase titanium alloys was analysed. Rotational bending tests were carried out to determine high cycle fatigue (HCF) strength at 107 cycles.
