Effects of Heat Treatment on Mechanical Proper es and Microstructure Evolu ons of Ti-5321 Alloy

This work presents a comprehensive study on the microstructure evolu on and mechanical property under different heat treatment procedures of a new near β type tanium Ti-5321(Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe). Two solu on temperatures(830°C and 900°C) and a group of aging temperatures(300-650°C) were carried out to inves gate the influence of heat treatment on this new alloy. The strengthening mechanism of Ti-5321 a�er solu on and aging treatment was discussed by analyzing the microstructure and its mechanical proper es. The best ul mate tensile strength can be achieved to 1564 MPa with 5% on elonga on when solu on treated at 830°C and aging at 450°C for this new alloy. The lamellar and globular α grains can be found in all 830°C solu on treated specimens which contribute to a be�er duc lity. Ultra-fine α phase can be found in all low aging temperature treated specimens but will coarsen significantly when raising the aging temperature and thus increase the tensile strength and lower the duc lity. All these results can provide a comprehensive guidance on heat treatment for this new near β type tanium in the future.


Introduc on
Titanium and its alloy are widely used in medical, marine and aerospace industries due to their excellent proper es such as low density, high strength and good corrosion resistance. Although hundreds of tanium alloys have been developed during the past 60 years, the Ti-6Al-4V with tensile stress no more than 1200 MPa, is s ll the most produced tanium alloy and sharing 80% of the world market nowadays [1]. With the development of higher speed jets and stronger power engines, the strength and toughness tolerance of tanium raw materials are becoming stricter. Old designed tanium alloy such as Ti-6Al-4V cannot sa sfy all needs for today's aerospace industry anymore. Because the complex phase transforma ons and microstructure development of near β type tanium alloys can contribute to a be�er balance between strength and fracture toughness, dozens of near β type tanium such as Ti-10V-2Fe-3Al (Ti-1023), Ti-5.5Al-5V-5Mo-1.5Cr-1Fe (BT22), Ti-5Al-5Mo-5V-3Cr (Ti5553) and Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531) [2][3][4][5] have been designed during the past decades. These alloys can be categorized to two groups which are higher strength (<1300 MPa) with lower toughness tolerance (<55 MPa·m 1/2 ) alloys and lower strength (<1100 MPa) with higher toughness tolerance (<70 MPa·m 1/2 ) alloys. While, it is hard for these tanium alloys' strength to reach 1200 MPa with toughness tolerance over 65 MPa·m 1/2 simultaneously. Therefore, a new near β type tanium Ti-5321 (Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe) was designed and our previous work indicated that this alloy has an excellent balance between strength and toughness in several condi ons [6,7].
Alloy's microstructure has a huge effect on its mechanical proper es. Heat treatment is one of the most important approaches to change and control tanium alloys microstructure, especially for β or near β type tanium alloys. In this study, two solu on temperatures which are below (830°C) and above (900°C) phase transus temperature (860°C) are involved, a group of aging temperatures (300~650°C) were carried out to study the rela onship between microstructure and mechanical property of this new alloy.

Materials Prepara on
The Ti-5321 alloy used in this study was fabricated through vacuum consumable arc mel ng, cas ng, forging and hot rolling and provided by Northwest Ins tute for Nonferrous Metal Research in China. Table 1 shows chemical composi on of the alloy.  Fig. 2 show the XRD pa�ern and microstructure of original rod before heat treatment respec vely. Both α and β phase can be found in the original rod. All specimens were cut to rod with 100 mm in length and 20 mm in diameter for different heat treatment. The detected phase transus temperature of Ti-5321 alloy is 860°C based on previous study [6]. The specimens were grouped by two different solu on temperatures which are below and above the phase transus temperature. The first group specimens were solu on treated at 830°C (α/β solu on) for 1.5h. The second group specimens were solu on treated at 900°C (β solu on) for 0.5h. All specimens were cooled in air and followed with a group of different aging temperatures. The aging process was carried out at 300°C, 400°C, 450°C, 500°C, 550°C, 600°C and 650°C for 8 hours and cooled in air.

Microstructural characteriza on
All specimens were characterized by conven onal X-ray diffrac on on diffractometer(Rigaku SmartLab 9kW) in the angular (2θ) range of 20-90°. Microstructure were analyzed by op cal microscopy (Zeiss AxioObserver A1m OM) and transmission electron microscopy (FEI TECNAI G20). Bright-field image was used to analysis the size of microstructure and electron diffrac on pa�ern was used to iden fy the phases.

Mechanical property tes ng
Columned tensile specimens with gauge length of 25 mm and a cross sec on diameter of 5 mm were prepared by turning machine and polished with grinding wheel to accurate diameter. An universal tes ng system (INSTRON 5985) equipped with extensometer was used to record the stress-strain data at room temperature. All tensile test was performed under GB/T228.1-2010 standard. Three specimens were used for each test to ensure repeatability and the average value was considered for further analysis. Fig. 3 shows the XRD pa�ern of Ti-5321 alloy which is treated at two different solu on temperatures and aged at 300°C. The results indicate that the phase composi on of Ti-5321 is sensi ve to the solu on temperature. For low aging temperature, 300°C in this case, two phase pa�erns can be found for specimens treated at 830°C (a/β phase temperature) while only β phase can be found when treated at 900°C (β phase temperature). Fig. 4 shows the microstructure of solu on treated Ti-5321 under different solu on temperatures. As can be seen in Fig. 4a and Fig. 4b, the primary α phase can only be found in α/β solu on treated specimens. Fig. 4c and Fig. 4d are showing the TEM image of different solu on treated specimens. The lamellar and globular α grains (iden fied as primary α phase) can be found in all α/β solu on treated specimens with average width or diameter around 1 to 2 μm. Finer acicular α phase (iden fied as secondary α phase) can be found on β matrix. It is well known that the primary α phase can contribute as a stumbling block on the growth of β grains which limits the β grains sizes and mobility significantly. By measuring the grains size shown in Fig. 4a and Fig. 4b, the average β grain size of α/β solu on treated specimens is around 5µm while the β grain size can be over 50µm when solu on treated at β solu on temperature. The precipita on kine cs is quite slow for β or near β type tanium and it is fast enough to keep considerable amount of β grains in the matrix when air cooling from high temperature near phase transus temperature [8]. It is worth to point out that the grains boundaries of β solu on treated specimens are straight which indicates the complementary of recrystalliza on of the deformed grains in this study [9].   Fig. 5a shows the 830°C solu on treated specimens and Fig. 5b shows the 900°C. As we can see in Fig. 5a that the intensity of α phase and β phase did not change much when raising the aging temperature of α/β solu on treated specimens since the α/β solu on treated specimens contains a large amount of primary α phase and some secondary α phase in β matrix. Fig. 5b shows the XRD pa�ern of different aging temperature for the alloy been solu on treated at 900°C. Only the β phase can be detected at lower aging temperature(300~400°C) by X-ray diffractometer. With the increasing aging temperature, metastable β phase decomposed to α phase within the matrix and α phase can only be detected when the aging temperature no less than 450°C.  Fig. 6 shows the TEM bright field images of 450°C and 650°C aging treated specimens a�er α/β and β solu on treated. Fine acicular α grains can be found in the β matrix in all images. As shown in Fig. 6a and Fig. 6b, the average width of the secondary α grains found in the β matrix is 20 to 30 nm when treated at 450°C for 8 hours in both solu on treated specimens. The grain size grows significantly to 150 to 200 nm when increase the aging temperature to 650°C. The results also show that the changing rates of tensile strength and elonga on of these specimens can be divided into two stages when grouping by solu on temperature. For specimens treated under the α/β solu on temperature, the tensile strength increased and the elonga on decreased when specimens aged from 300°C to 450°C firstly. This phenomenon are mainly caused by the metastable β phase transforming to dispersion ultra-fine secondary α phase within the matrix [5,10]. In the second stage, with con nually increasing the aging temperature, the secondary α phase coarsened, and thus cause a lower strength and higher elonga on. Similar results can also be found in β solu on treated specimens except that bri�le fracture happened when aging at low temperature (300°C to 500°C in this case). It is caused by the existence of extremely fine α phase with width less than 50 nm along with coarser β grain size. The turning point of tensile strength and elonga on changing rates can be obtained a�er 550°C aging for this solu on temperature. Since the secondary α phase has a higher strength and lower duc lity than primary α phase and β matrix [9], the duc lity for α/β solu on treatment is be�er than β solu on treatments in all aging temperatures.

Conclusions
In order to inves gate the influence of heat treatment on microstructural characteris cs and tensile proper es of this new near β tanium alloy Ti-5321, a comprehensive heat treatment with two solu on temperatures (above and below the β transus temperature) and a group of aging temperatures were carried out in this study. Following conclusions can be made based on the results: 1. The microstructures and mechanical proper es of near β tanium like Ti-5321 are sensi ve to solu on temperature and aging temperature. The highest tensile strength can reach to 1564 MPa with 5% on elonga on when solu on treated at 830°C for 1.5h and aging at 450°C for 8h for Ti-5321. The best combina on of ul mate tensile