Microstructures and tensile properties of a high strength β titanium alloy Ti5Al4Zr8Mo7V by near β forging and β-transus forging

A novel high-strength metastable β-Ti alloy Ti-5Al-4Zr-8Mo-7V has been successfully developed by improved Bo-Md map combined with critical Mo equivalent criteria. Near β forging combined with different heat treatments was introduced to tailor the Bi-modal structure. The alloy exhibits good strength-ductility combination with UTS~1390 MPa and El~10% after solution treated (ST) at 800°C followed by 570°C aging. After β-transus forging combined with subsequent heat treatment, an excellent combination of strength UTS~1460MPa and El~10% is achieved which is attributed to the Tri-modal structure consisting of primary α (αp), sub-micro α rods (αr) and nano-scale α platelets (αs). Plastic strain is effectively partitioned in both αp, αr and β matrix during deformation, which results in a more homogeneously strain distribution and improved ductility. Nano-scale distribution of αs effectively block the dislocation motion in β matrix and strengthen the alloy.

on a metallographic sandpaper, electrochemical polished and etched in a Kroll's reagent. For tensile property evalua on, ASTM-E8/E8M plate samples (gauge length ~ 25 mm) were prepared and then tested on Instron 1302 tes ng system. Table 1 Chemical composi on of the novel Ti-5Al-4Zr-8Mo-7V ingot.

Composi on determina on
An ab-ini o based model known as the 'd-electron alloy' design method originally developed by Morinaga has been used to determine the alloy composi on [16]. The 'd-electron theory' provides a physical background to the phase stability which has been successfully applied developing low modulus tanium alloys and TRIP/TWIP alloys [17][18][19][20]. In this paper, expansion of this approach to high strength tanium alloy is performed. As is shown in Fig.1, an improved Bo-Md diagram based on more than sixty commercial alloys is plo�ed to determine the composi on of high strength β tanium alloy. For determining the composi on of high strength β alloys, three rules are used based on this Bo-Md diagram.
First, the composi on of the alloy should locate slightly upper along the β/α+β alloy boundary to obtain high strength. Second, by considering some typical high strength alloys, such as Ti1023, Ti5553, BT22, Ti-17 and Ti7333 which consis ng a blue do�ed contour line in Fig. 1, the composi on of the alloy should be located inside of these target alloys in order to acquire an enhance mechanical property. Third, design of uncertainty strategy is used which means that the composi on is located in the unexploited region of the Bo-Md diagram. Finally, a composi on of Ti-5%Al-4%Zr-8%Mo-7%V in mass percent with the electronic parameters Md=2.376 and Bo=2.774 was determined.

Microstructure and tensile property a�er near β forging.
Near β forging followed by subsequent heat treatment is tradi onal process to produce Bi-modal structure. As is clearly seen in Fig.2a, primary α phase (a p ) with globular morphology is homogeneously distributed in β matrix, which is a�ributed to the dynamic recrystalliza on of α

Fig. 3 (a) and (b) represents the microstructure of the alloy solu on treated at 780℃ and aged at 510℃ for 6h, (c) and (d) is the alloy solu on treated at 810℃
and aged at 510℃ for 6h. Fig. 3 shows the effect of solu on temperature on microstructure evolu on of the alloy. As is clearly seen in Fig. 3a and 3c, volume frac on of α p decreases drama cally with increasing solu on temperature and the size of α s is finer a�er solu on treated at lower temperature. By using the Image-pro Plus so�ware, sta s cal results of volume frac on of α p is recorded, which decreases from 7.1% to 3.1% as the ST temperature increasing from 780℃ to 810℃. The decrease of α p leads to a decrease of b phase stability, which will increase the driving force for α nuclea on during aging. Thus high frac on nano-scaled a s is precipitated a�er high temperature solu on treatment, as seen in Fig.3c and 3d. The tensile property of the alloy solu on treated at 780℃ and 810℃ is shown in Table 2. The strength of the alloy is increasing with eleva ng temperature.
The alloy solu on treated at high temperature (810 ℃ ) followed by low temperature (510 ℃ ) aging shows an extremely high strength UTS~1632MPa but limited duc lity El~6.2%. This is a�ributed to the high frac on nano-scale α s which significantly hinders disloca ons slip in β matrix. The larger amount of finer α s , the higher would be the strength.   [2]. In general, the size of α s increases and the frac on decreases with incremental aging temperature. As for aging at low temperature, owing to the large driving force for nuclea on and sluggish kine cs for growth, large amouts of nano-scale a s precipated (see Fig. 4a and b). On the contary, coarse a s with lower frac on is precipa ed during aging at high temperature (see in Fig. 4e and 4f). In additon, it's obviously seen that the size distribu on of a s is inhomogeneous at 600℃ aging. The large coarse a s region probably yields at low stress and results in strain localiza on. Owing to the large slip length of β matrix in coarse a s region, disloca on behaviour is relaxed comparing to small slip length of beta matrix from aging at low tempertature aging, which could result in a decrease in strength.   Table 3. The alloy exhibits extremely high tensile strength (UTS~1512-1632 MPa) but a rela vely poor elonga on (El~5.1%-6.2%) aging at 510℃. A good combina on of strength and duc lity is abtained a�er solu on treatment at 800℃ followed by aging at 540℃ and 570℃. High tensile strength (UTS~1390MPa) with prominent elonga on (El~10%) is obtained a�er 570℃ aging.   6 shows the microstructure of the alloy a�er β transus forging followed by solu on treated at 760℃ and 600℃ aging. Owing to the insufficient reduc on in α/β two phase region, grain boundary α-phase is in form of necklace-like shape. The intragranular α-precipitates are in three types of morphologies, (i.e. the Tri-modal structure), which is consisted of elongated primary α-phase (α p ), submicron α-rods (α r ) and nano-scaled α-platelets (α s ), as seen in Fig. 6b and 6c.   Fig. 7a presents uniaxial tensile curves of the Tri-modal sample in comparison with Bi-modal structure of equivalent strength (Bi-modal 1) or elonga on (Bi-modal 2). It can be seen that Tri-modal structure exhibits high yield strength with large uniform elonga on. The ul mate tensile strength was measured to be ~1460 MPa and the elonga on at fracture reaches 10%. As is shown in Fig. 7b, tensile proper es of the Tri-modal sample are compared with that of other as-reported high-strength β-Ti alloys [1,7,10,12,22,23]. It is indicated that via β transus forging and the following heat treatments, the Ti-5Al-4Zr-8Mo-7V alloy exhibits Tri-modal structure which possesses an excellent combina on of strength and duc lity in comparison to the other high-strength β-Ti alloys such as Ti5553, Ti-1023, Ti-17, BT22 and Ti-7333. Table 3 Tensile property of the alloy with Bi-modal structure a�er different heat treatments