Effect of aging treatment on the microstructure and properties of cold rolled Ti-10V-2Fe-3Al alloy

As a typical metastable titanium alloy with high strength and toughness, Ti-10V-2Fe-3Al titanium alloy has found wide applications in aerospace industry to replace forging steel components for light weight, long service life and high reliability. In the present study the microstructure evolution and mechanical properties of cold rolled Ti-10V-2Fe-3Al alloy undergoing aging treatment were characterized by optical microscopy, X-ray diffraction, transmission electron microscopy and mechanical testing, in attempt to understand the relationship between the microstructural evolution and aging treatment temperature and time. It is revealed that the secondary α phase had a very short propagation time to nucleate, and holding 5 min could give rise to the transformation from β phase to secondary α phase when cold rolled alloy suffered from aging at 550°C. This could be attributed to the large amounts of dislocations and grain boundaries by cold deformation, which provide nucleation sites for α phase. Increasing aging time could accelerate the transformation from β phase to secondary α phase. With aging at 550°C for 30 min, a large amount of fine secondary α phase formed, offering a good combination of strength and ductility. With increasing aging time the coarsening secondary α phase led to a decrement of strength of the alloy.


1.Introduction
As an important class of titanium alloys, metastable  titanium alloys not only offer attractive mechanical properties, but also exhibit multiple functional properties [1~3].The underlying mechanism for the versatility of  titanium alloys lies in that the mechanical or functional properties of the alloys could be tailored by phase transformations including precipitation and martensitic transformations during the processes of working or heat treatment [1,2].The structural stability and phase transformations of  titanium alloys have been drawing persistent attentions of researchers to investigate the response of mechanical behavior to microstructural evolution [2,4].Ti-10V-2Fe-3Al alloy is a typical metastable  titanium alloy developed for applications in aerospace industry due to its good combination of high strength and good fracture toughness [3,5].Owing to the metastable characteristics of the alloy, the microstructural evolutions in working processes and heat treatments could remarkably influence the mechanical behavior of the alloy[6~9].Ti-10V-2Fe-3Al alloy has good tensile ductility or as well as impressive mode duo to the assistance of stress induced martensitic transformation [8,10,11].It has been revealed that the cold rolling deformation induces the formation of large amounts of  martensites in solution treated Ti-10V-2Fe-3Al specimen, which leads to substantial refinement of  and  grains.The principal cold rolling deformation mechanisms of Ti-10V-2Fe-3Al involve stress/strain induced  martensitic transformation and dislocation slip [12].Ti-10V-2Fe-3Al as metastable  titanium alloy can achieve the best match of strength, toughness and fatigue strength through heat treatment, which has abundant phase transition in heat treatment.The mechanical properties of this alloy are related to the heat treatment which controls its microstructure [8,10,13,14].Because the hardness of  martensite phase is not high, the strengthening effect of martensite on the alloy is not significant.The martensite phase decomposition during the aging treatment is one of the ways of strengthening methods of  titanium alloy.However, the decomposition process of  martensite phase in the aging process is relatively complicated, which usually needs to go through a series of intermediate phase transition stages of  phase,  phase and metastable  phase.Moreover, the decomposition process of  martensite phase has a great relationship with aging temperature and aging time.Early studies of martensitic decomposition during aging treatment in Ti-10V-2Fe-3Al originate from 1980's, in which the relationship between phase transformations and aging temperature and aging time are discussed [15].Later, martensite decomposition in  titanium alloys also received persistent attention owing to the response of mechanical properties to the transformation [16,17].For example, He Yizhu et al. investigated the phase decomposition of orthorhombic martensite and the age hardening effect in TC21 alloy during aging treatment, and showed the overall decomposition reaction after aging at 300~700℃ for 4h could be represented by the order of "→"+→+ [16].Nevertheless, these investigations mainly focused on quenching martensitic decomposition transformations during aging treatment.However, few studies were reported on the decomposition of stress induced  martensites in Ti-10V-2Fe-3Al alloy during aging process, especially the stress induced  martensite phase produced by cold rolling.In the present study, the characterizations of microstructural evolution during aging treatment and the effects of stress induced martensitic decomposition on the properties of cold rolled Ti-10V-2Fe-3Al alloy were investigated.The results achieved in the present study might be helpful to understand the relationship between mechanical properties and microstructural evolution during aging treatment in Ti-10V-2Fe-3Al alloy, shedding light on improving and optimizing the microstructures and mechanical properties of cold rolled β titanium alloys.

2.Experimental
A Ti-10V-2Fe-3Al (wt%) ingot of 650 kg and 380 mm diameter was fabricated by vacuum arc melting with sponge titanium and V-Al-Fe ternary intermediate alloy.The ingot was multi-forged into a bar of 45 mm in diameter and followed by multi-pass hot rolling into a bar of 10 mm in diameter.After cooling in air, a bar with 2000 mm in length was solution treated at 833 ℃ for 60 min followed by quenching into water.After removing the surficial contaminations and oxide layers by a coreless lathe, a bar with 8mm in diameter was continuously cold rolled into bar specimens with 28% reduction rates.The chemical compositions (wt%) of the bar are analyzed and listed as follows: 3.04 Al, 10.40 V, 1.66 Fe, 0.007 C, 0.012 N, 0.001 H, 0.074 O and balance Ti.The β-transus temperature of the Ti-10V-2Fe-3Al alloy bar was metallographically measured to be 793 ℃.In heat treatments, the specimens of cold rolled bar were aging treated at 400 °C ~ 650 °C for 5min~60 min and followed by air cooling, as listed in Table 1.The specimens for microstructural characterization were cut from the aging treated bar into a length of 10mm.The tensile specimens with a gauge of 3mm  30mm were cut along the rolling direction of the aging treated bars.Tensile tests were performed to record the stress strain curves on a universal testing instrument (Instron 8801) equipped with a clip-on extensometer.

Table 1. Aging treatment parameters of the Ti-10V-2Fe-3Al alloy cold rolled bar
The microstructure and phase constituent of specimens were characterized by a LEICA DM4000 optical microscope (OM) and a Bruker D8 Discover X-ray diffractometer (XRD), respectively.The specimens for optical microscopy and XRD were carefully ground with abrasive papers and then electrochemically polished to avoid the formation of stress induced martensite.Transmitting electron microscopy (TEM)

Effect of aging temperature on microstructural evolution and mechanical behavior
Figure 1 shows the optical micrographs of Ti-10V-2Fe-3Al specimens with 28% cold rolling deformation and different temperature aging treatment.As the alloy was subjected to aging treatment at 400℃, 450℃, 500℃, 550℃, 600℃ or 650℃ for 60min and air cooling, the optical microstructures shown in figure 1 are difficult to be distinguished from cold rolled of Ti-10V-2Fe-3Al alloy.It may be due to the small and dense distribution of the transformed microstructure inside the crystal, which cannot be distinguished under the low magnification microscope.In order to determine whether phase transition occurred and the correspondence between the microstructure and the phase evolution in the specimens with different temperature aging treatment, XRD characterization was carried out and the corresponding results are illustrated in Figure 2. It can be seen that the phase composition of aging treatment at 400℃, 450℃, 500℃, 550℃, 600℃ or 650℃ for 60min was significantly different from that of the cold-rolled Ti-10V-2Fe-3Al alloy.Upon the aging treated of cold rolled specimen at 400℃, 450℃ and 500℃ for 60min, a little of  phase and matrix  phase diffraction peaks are observed, suggesting that " → → phase transition occurred.However, the diffraction peaks for (020) and (111) plane of ″ phase still existed, and the change had not completed yet.This could be attributable to lower aging treatment temperature and shorter aging treatment time.Some diffraction peaks such as (020) and (111) plane of  needle martensite and (100) and (101) plane of secondary  phase could not be identified as the diffraction peaks broadness duo to fine grain size.When the specimens were aging treated at 550℃, 600℃ and 650℃ for 60min, a large number of  phase formed, and the phase composition of the specimen were characterized by a large amount of  phase and  phase.None of ″ phase diffraction peak was observed, indicating that most of ″ phase reversed to  phase and secondary  phase precipitated in  phase.The tensile stress-strain curves of Ti-10V-2Fe-3Al specimens with 28% cold rolling deformations and different aging temperature heat treatment are shown in Figure 3.The aging treated specimens presented typical single yielding feature.However, as the aging temperature increased from 400℃ to 650℃ the tensile strength of the specimens decreased gradually and the plasticity increased gradually.When the aging temperature increased from 400℃ to 500℃, the change trend of the tensile strength and plasticity was not obvious.At this time, the strength exceeded 1250 MPa ~1400 MPa and the elongation was less than 2%.It can be found that the stress-strain curve for the specimen with aging treatment at 400℃, 450℃ and 500℃ could not be recorded by an extensometer because of low plasticity.It is found that increasing aging temperature from 550℃ to 600℃ led to ductility increase and tensile strength decrease.
Upon aging treatment at 600℃ the ultimate strengths of the specimens was lower than 1000 MPa.It can be seen from Figure 3 that the strength and plasticity of the specimen matched well when the aging temperature was 550 ℃. Figure 4 shows the tensile fracture morphologies of Ti-10V-2Fe-3Al specimens with 28% cold rolling deformations and different temperature aging treatment.It can be observed that the specimen of aging treating at 400℃, 450℃ or 500℃ fractured in a brittle manner with noticeable cleavage platform and relatively flat fracture surface, as shown in figure 5(a) ~ figure 5(c).With the increase of aging temperature from 550℃ to 650℃, obvious necking was observed in tensile specimens, and fracture manner changed from brittle fracture to ductile fracture.There were a large number of small dimples, and the dimples became much deeper with the increase of aging temperature.

3.2.Effect of aging time on microstructural evolution and mechanical behavior
Figure 5 shows the optical micrographs of the Ti-10V-2Fe-3Al alloy at 550℃ with different time aging treatment.When the alloy was subjected to aging treatment at 550℃ for 60 min, 30 min, 10 min or 5 min, the optical microstructures shown in figure 5 were also difficult to be distinguished from cold rolled of Ti-10V-2Fe-3Al alloy.In order to determine whether phase transition occurred and the relationship between microstructure and the phase change behavior of the specimens under different time aging treatment, XRD characterization was carried out and the results are shown in Figure 6.  Figure 7 shows the tensile tress-strain curves of Ti-10V-2Fe-3Al specimens aged at 550℃ for 60 min, 30 min, 10 min or 5 min.The cold rolled deformation specimen with aging treatment at 550℃ for 5min or 10min presents the same level strength feature, exhibiting high strength and low ductility( elongation of 3% or 4%).It is found that the ultimate strength and ductility of Ti-10V-2Fe-3Al specimens with aging treatment at 550℃ for 30min or 60min were comparable to that of cold rolling deformation specimens.However, the yield strength of specimens increased significantly from 729 MPa to 1100 MPa and above.Upon aging treating at 550℃ for 30min or 60min, the strength and plasticity of specimens match well.figure 8 shows the tensile fracture morphologies of the alloy aged at 550℃ for 60 min, 30 min, 10 min or 5 min.It was observed that the specimen of aged at 550℃ for different time, it could be observed that the fractures were in a ductile manner with noticeable necking and many dimples.With the increase of aging time from 5min to 60min, the dimples became much deeper.

Discussion
In order to study the dependence of deformation behavior on the microstructure evolution of Ti-10V-2Fe-3Al alloy, TEM observation for the specimens with aging treating at 550℃ for 60 min, 30 min, 10 min or 5 min were carried out, as shown in Figure 9 ~ Figure 12.In the case of aging treating at 550℃ for 5min, there were still long parallel lath " phases locally, but the interfaces between " phase and the  phase become blurred, which were not as flat and clear as that of cold rolled specimen.Although the aging time was very short, the " phase has locally converted to  phase, as shown in figure 9.As a result of secondary a phase precipitation, the strength of Ti-10V-2Fe-3Al specimen increased significantly and the plasticity decreased obviously.As the aging time increased to 10min (figure 10), " martensite twin were observable in  matrix.Local HRTEM high-resolution observation of the twin was carried out and the results are shown in Figure 10(e).It can be observed from the high resolution that there were a large number of faults in the " martensite twin boundary, which indicates that twins were annealing twins formed due to a large number of delamination during aging treatment.With aging treatment time of 30 min (Figure 11), local long parallel lath " phases largely spheroidized and reversed into  phase.In the same time, a large amount of secondary  phase precipitated in the matrix  phase, that is, the transformation of " phase→ phase→ phase occurred.In the specimen with aging treatment time of 60 min the internal secondary  phase began to largen, resulting in the decrease in strength and the increase in plasticity, as shown in Figure 12.When cold-rolled Ti-10V-2Fe-3Al alloy was aging treated at 550℃, the nucleation incubation period of secondary  phase was very short, and the secondary  phase precipitated after aging treated at 550℃ for 5 min.This is due to that  phase of titanium alloy is easy to nucleate in the  phase dislocation and grain boundary defects [18].A large number of grain boundaries and dislocations appeared in the alloy as the alloy was cold rolled deformation.In this case, a lot of grain boundaries and dislocations of " phase and  phase provided nucleation positions for the subsequent formation of  phase, which promote the precipitation of secondary  phase during aging treatment.With the increase of aging treatment time, the number of secondary a phase increase significantly and the secondary  phase began to become larger in size.

5.Conclusions
The microstructure evolution and mechanical properties of the cold rolled Ti-10V-2Fe-3Al alloy undergoing different aging temperature and time have been investigated.
When cold rolled alloy suffers from aging at 550℃, the secondary  phase has a very short propagation time to nucleate, and holding 5 min could give rise to the transformation from  phase to secondary  phase.This is attributed to large amounts of dislocations and grain boundaries by cold deformation, which provide the nucleation sites for  phase.Increasing aging time can accelerate the transformation from  phase to secondary  phase.With aging at 550℃ for 30 min, a large amount of fine secondary  phase form，offering a good combination of strength and ductility.With increasing aging time the coarsening secondary  phase leads to a decrement of strength of the alloy.

Figure 3 .
Figure 3. Tensile stress-strain curves at RT of cold rolled TB6 titanium alloy specimens with different aging temperature heat treatments (cold rolling deformation of 28%)

Figure 5 .
Figure 5. Optical micrographs of cold rolled TB6 titanium alloy specimens with different aging time heat treatments at 550℃(cold rolling deformation of 28%): (a) 550℃/60min /AC; (b) 550℃ /30min/AC; (c) 550℃/10min /AC; (d) 550℃/5min /AC It can be observed that the phase composition of aging treatment at 550℃ for 60 min was significantly different from that of the cold-rolled Ti-10V-2Fe-3Al alloy.When the alloy was treated with short time aging at 550℃, a large number of  phase diffraction peaks appeared, except for the  matrix phase diffraction peaks, as shown in figure6 (a).In order to study the phase evolution of Ti-10V-2Fe-3Al specimens which were aging treated at 550℃ for 60 min, 30 min, 10 min or 5 min, the diffraction profiles within the 2 angle of 34-42 of the XRD patterns in figure6(a) were magnified, as shown in figure6(b).It can be observed that that the diffraction peaks for (110) planes of  phase weakened gradually with aging treatment time increase from 5min to 60min.When the aging treating time increased to 60min, the diffraction peak for (110) planes of  phase became not obvious.

Figure 7 .Figure 8 .
Figure 7. Tensile stress-strain curves at room temperature of cold rolled TB6 titanium alloy specimens with different aging time heat treatments at 550℃ (cold rolling deformation of 28%)