Effect of thermal exposure on the microstructure and mechanical properties of Ti60 alloy

The effect of thermal exposure on the microstructure and mechanical properties of Ti60 alloy was investigated in the present study. Meanwhile, the fatigue fracture microscopic appearance characteristics at elevated temperature were analyzed compared, providing the basis to further improve the performance of the series of high temperature titanium alloy. The results show that the yield strength and tensile strength were basically stable under long-term thermal exposure below 600°C, while the elongation decreased with the increase of exposure temperature. Thermal exposure below 800°C did not change the microstructure type of Ti60 alloy, but at higher temperature local β coursing occurred at the grain boundary. When the alloy was exposed above 600°C, Si was obviously concentrated in the grain boundary region, and the maximum concentration was up to 2.5%. With the increase of thermal exposure temperature, the characteristics of high-temperature fatigue fracture of Ti60 alloy change from trans-granular toughness to intergranular brittleness.


Introduction
High-temperature titanium alloy is capable for long-time service above 400℃ due to the advantages of high-temperature strength and oxidation resistance compared with the ordinary structural titanium alloy in service at room temperature [1] .With the accelerated development of aerospace manufacturing industry, the operating temperature and strength requirements of key components are gradually increased, as a result researchers worldwide are actively committed to the development of high-temperature titanium alloy [2] .Since 1966, the United States developed Ti6246, Ti6242, Ti6242S, Ti1100, the operating temperature was raised from 450℃ to 600℃.IMI834 of Britain and BT36 of Russia were aimed at application above 600℃ and reported successfully in key components of high thrust ratio aircraft engines [3] .The development of high-temperature titanium alloy in China started comparatively late, and imitation of the mature designation featured the early stage.The nominal composition of early developed TC11 and TA19 are basically the same as that ofBT9 and Ti6242Srespectively [4] .In order to realize independent innovation, Chinese researchers have developed high-temperature titanium alloy designated Ti60 by adding an appropriate amount of rare earth element Nd and Ta with the nominal composition of Ti-5.8Al-4.0Sn-4.0Zr-0.7Nb-1.5Ta-0.4Si-0.06C [5].The alloy is strengthened by multiple solution strategy.Nb and Ta have relatively large solubility in α phase, which can enhance the solution strengthening effect of α phase and improve the overall oxidation resistance and thermostability [6] .By adding small amount of C, the thermal processing window in α+β phase is enlarged endowing the alloy good processing adaptability.
A good match of creep resistance, fatigue strength and thermal stability can be obtained by appropriate thermo-mechanical treatment process [7,8] .In order to evaluate the mechanical properties, change of Ti60 under different thermal serving conditions, the present research focus on the effect of thermal exposure on the microstructure and mechanical properties.Meanwhile, the fatigue fracture microscopic appearance characteristics at both room and elevated temperature were analyzed and compared, providing the basis to further improve the performance of the series of high temperature titanium alloy.

Materials and experimental
The chemical composition of the main alloying elements in Ti60 alloy used in the present research are shown in Table 1.In addition, the content of impurity elements is as follows: Fe≤0.025 wt.%, C≤0.1 wt.%, O≤0.15 wt.%, N≤0.05 wt.%, H≤0.01 wt.%.After forged in the α+β region and subsequently annealed, the optical microscopy and XRD patterns of the original microstructure before thermal exposure are shown in Figure 1.The alloy is of typical bi-modal microstructure consisting of equiaxed primary α phase and transformed β of lamellar structure.
(a) Optical microscopy of Ti60 (b) XRD pattern of Ti60 Figure 1.Microstructure and phase components of Ti60 before thermal exposure The thermal exposure experiment of Ti55 alloy was carried out at 400℃, 500℃, 600℃, 700℃, 800℃, and the exposure time was controlled at 2h, 4h, 8h, and 12h, with a total of 20 groups.The tensile test at room temperature was carried out by AG-XPLUS100KN testing machine at a speed of 0.5mm/min.Epsilon3442 axial extensor was used to measure the instantaneous elongation during tensile test to determine the yield strength.The tensile strength was calculated from the ratio of the maximum force to the original cross-sectional area, and the elongation was obtained in terms of (  −  0 )/ 0 , with  0 the initial gauge length and   the elongated gauge length at fracture.The microstructure of Ti55 was firstly observed by ICX41M optical microscope.The metallographic samples were ground to 5000# sandpaper, mechanically polished with 50nm amorphous silica polishing solution, and then etched in HF: HNO3: H2O solution with volume ratio of 1:3:7 for 10~30 seconds.Phase analysis was performed with SmartLab X-ray diffractometer at a scanning speed of 5°/min and a scanning range of 20°~100°.Electron backscatter diffraction (EBSD) analysis was performed on ApreO2C field emission scanning electron microscope, and the scanned surface was pretreated by Ilion II 697 argon ion polishing system.EPMA line scan of chemical elements was carried out to measure the distribution of elements in the microstructure by JXA-8530F field emission electron probe.Before the element scanning the samples were mechanically polished and slightly etched to reveal the microstructure.In order to characterize the fracture feature of Ti55 alloy of different states, the sheet samples with one side of the surface polished and etched were pulled to fracture in the above tensile test mode, and then the microstructure morphology of the crack near the fracture was observed by SEM.Finally, the high temperature fatigue fracture of Ti55 at 400℃ and 700℃ was prepared on MTS fatigue testing machine equipped with high temperature furnace, and the morphology of the fracture was analyzed by SEM.

Results and Discussion
The effect of thermal exposure on tensile mechanical properties of Ti60 alloy at room temperature after thermal exposure for various time at different temperatures was investigated, and the results are shown in Figure 2. The yield strength in Figure 2(a) manifests maximum value of 875MPa under the aging at 650℃ for 8 h, and with the aging thermal exposure period decreased the overall yield strength decreased from 860MPa to 820MPa.In Figure 2(b), the tensile strength was higher than 930MPa when the temperature was below 600℃, and the value maintained relatively stable for long-time exposure.Under the temperatures above 600℃, aging effect was followed by an obvious decrease in tensile strength especially exposed at 800℃ for 12h. Figure 2(c) shows that elongation depended mainly on the thermal exposure temperature, below 600℃ elongation was higher than 8%, and the effect of time-length is negligible.As the exposure was above 600℃, the elongation decreased obviously with the increase of temperature and time, which is consistent with the variation of tensile strength.Figure 2(d) comprehensively represents the relationship among yield strength, tensile strength, elongation and exposure temperature.As exposed from 400℃ to 800℃, the elongation decreased from 16% to 5%, meanwhile the yield strength increased due to the aging effect and the tensile strength decreased due to the deteriorative work hardening effect combined with a sudden drop of strength near 6% elongation.Figure 4 shows the optical microstructure under different thermal exposure conditions.Compared with the original state in Figure 1(a), the microstructure morphology is basically the same as that of the original state, which is typical bi-modal microstructure consisting of equiaxed primary α phase and transformed β of lamellar structure.As a result, thermal exposure experiment below the β transition temperature of 1050℃ does not change the microstructure type of the alloy.Since chemical etching of polished surface is required before optical metallographic structure observation, contour fluctuation is formed at the phase interface, which is easy to cover the phase distribution difference at the interface.Moreover, because the magnification of optical microscope is usually less than 1000 times, it is not suitable for comparing the subtle phase distribution difference at the interface.To solve this problem, EBSD test was further applied in this study to analyze the distribution of α phase and β phase under two different thermal exposure conditions with high magnification, as shown in Figure 5. Figure 5(a) is the microstructure of the sample exposed at 400℃ for 12h, while Figure 5(b) is the microstructure of the sample exposed at 800℃ for 12h, with the red area representative of α phase, and the blue area β phase.By comparison, it can be found that the content of blue β phase in Figure 5 (b) was significantly higher than that in Figure 5 (a).Fine β phases were found evenly distributed in the intersections of the red transformed β, but no significant β phases were found within the inner-grain area.In contrast, in Figure 5(b), there were not only uniformly distributed fine β phases, but also more β bulks with a width of phases about 1μm distributed intermittently in the in the boundary.It can be concluded that local β coursing occurred at the grain boundary in Ti60 alloy when exposed at higher temperature such as 800℃.6, in which the distribution of Mo and Si is obviously affected by temperature.It can be found that β -type stable element, Mo aggregated between the α -phase bundles.With the increase of thermal exposure temperature from 550℃ to 800℃, the maximum concentration of Mo increased from 2% to 4%.For the element Si, it uniformly distributed in the inner-grain and grain boundary region at 600℃, and the average content was about 0.4%, which is consistent with the results in Table 1.When the temperature was increased to 800℃, Si was obviously concentrated in the grain boundary region, including the interface around the primary α phase and the interface between the lamellar in the β transition microstructure, and the maximum concentration was up to 2.5%.As an interstitial eutectoid element, the main function of Si in high-temperature titanium alloys is to form Si compounds and improve the creep resistance.There are two kinds of Si compounds: S1 type Ti5Si3 and S2 type Ti6Si3.When other alloying elements were added, part of Ti or Si would be replaced, forming new Si compounds with the same crystal structure and different lattice constants, such as Ti3(Al,Si) and Ti5(Al,Si)3 phases in Ti-Al-Si system alloys.Due to the large difference in atomic size between Si and Ti, Si tends to converge at the dislocation in the solid solution, which seriously hinders dislocation movement and affects the overall plastic deformation coordination ability.Therefore, the amount of silicon added in high temperature titanium alloy should not exceed the maximum solid solubility of α phase, generally about 0.25%.In this study, it was found that thermal exposure at 800℃ resulted in significant aggregation of Si at the microstructural boundary, which was related to the decrease of elongation in Figure 3 7(a) showed that after exposed at 600℃ for 12h the parallel slip lines appeared, suggesting local plastic deformation occurred and the crack originated from the boundary between the transformed β grains.These morphological characteristics indicate that Ti60 can still maintain coordination ability after long-term thermal exposure at 600℃, which is consistent with the elongation feature in Figure 2(c) as well as related to the uniform distribution of Si in Figure 6(a).In contrast, the morphologies in Figure 7(b) after exposed at 800℃ for 12h showed poor plastic deformation and coordination ability, since part of the surface was smooth and slip line was significantly less.Moreover, a crack initiated from the β phase in the grain boundary with high Si content, as shown in Figure 6(b).As a result, the Si accumulation at the interface after exposure at 800℃ significantly reduced the local plastic deformation ability, and thus caused the crack initiation in the region between the transformed β grains, and seriously worsened the plasticity of the alloy.
(a) exposed at 600℃ for 12h (b) exposed at 800℃ for 12h Figure 7. Scanning electron micrograph of the area near the tensile fracture For Ti60 alloy, the segregation of Si caused by thermal exposure above 550℃ will significantly reduce its plasticity, and this phenomenon was also reflected in the morphology of high temperature fatigue fracture.Figure 8

Conclusions
(1) The effect of thermal exposure on mechanical properties of Ti60 alloy is mainly reflected in that the yield strength and tensile strength of Ti60 alloy are basically stable under long-term exposure below 600℃, while the elongation decreases from 15% to 6% with the increase of exposure temperature from 400℃ to 800℃.
(2) Thermal exposure below 800℃ does not change the microstructure type of Ti60 alloy, but at higher temperature local β coursing occurs at the grain boundary in Ti60 alloy when the alloy is exposed at higher temperature such as 800℃.
(3) When exposed at 800℃, Si is obviously concentrated in the grain boundary region, including the interface around the primary α phase and the interface between the lamellar in the β transition microstructure, and the maximum concentration is up to 2.5%.( 4) With the increase of thermal exposure temperature, the characteristics of high-temperature fatigue fracture of Ti60 alloy change from trans-granular toughness to intergranular brittleness.

2 . 3 .
(a) Yield strength (b) Tensile strength (c) Elongation (d) Relationship of strength and ductility Figure Tensile mechanical properties of Ti60 alloy at room temperature In order to further reflect the influence of thermal exposure temperature and time on the elongation and yield strength of Ti60 alloy, diagrams are drawn, as shown in Figure 3.In the range of 400℃~600℃ the elongation decreased almost linearly with the increase of temperature at a rate of -4%//100℃ and maintained in the range of 6%~8% from 600℃ to 800℃.As shown in Figure 3(b) the exposure temperature above 600℃ undermined the yield strength obviously.As a result, Ti60 is able to serve for long time below 600℃, mainly reflected in the stable tensile strength in this temperature range shown in Figure 2(b), and the simultaneous deterioration of the yield strength and elongation above 600℃, as shown in Figure 3. (a) Influence on elongation (b) Influence on yield strength Figure Effects of thermal exposure temperature and time on elongation and yield strength According to molybdenum equivalent calculation formula of titanium alloy, [Mo]eq = [wt.%Mo] + 0.2 [wt.%Ta] +0.28 [wt.%Nb] + 0.4 [wt.%W] + 0.67 [wt.%V] + 1.25 [wt.%Cr] + 1.25 [wt.%Ni] + 1.7 [wt.%Mn] + 1.7 [wt.%Co] +2.5 [wt.%Fe], it can be calculated that the [Mo]eq of Ti60 is 0.75.According to aluminum equivalent calculation formula, [Al]eq = [ wt.% Al] + 0.17 [wt.%Zr] + 0.33 [wt.%Sn] + 10 [wt.%O], and the [Al]eq of Ti60 is 8.2.Therefore, Ti60 alloy belongs to near α titanium alloy, and the α+β→β transition temperature is about 1050℃.

Figure 4 .
(a) exposed at 400℃ for 12h (b)exposed at 800℃ for 12h Optical microstructure of Ti60 after different thermal exposure

Figure 5 .
(a) exposed at 400℃ for 12h (b) exposed at 800℃ for 12h Phase distribution under different thermal exposure conditions obtained by EBSD test In order to further analyze the influence of thermal exposure on the distribution of elements in Ti60, results of EPMA line scan test are shown in Figure (a).

Figure 6 .
Figure 6.Elements distribution under different thermal exposure conditions obtained by EPMA After thermal exposure, Ti60 was processed into tensile test samples with the surface polished and metallographically etched.After the tensile test the morphological characteristics of the crack initiation area near the fracture were observed by SEM, and the microstructure is shown in Figure 7.Figure 7(a) showed that after exposed at 600℃ for 12h the parallel slip lines appeared, suggesting local plastic deformation occurred and the crack originated from the boundary between the transformed β grains.These morphological characteristics indicate that Ti60 can still maintain coordination ability after long-term thermal exposure at 600℃, which is consistent with the elongation feature in Figure 2(c) as well as related to the uniform distribution of Si in Figure 6(a).In contrast, the morphologies in Figure 7(b) after exposed at

Figure
Figure 6.Elements distribution under different thermal exposure conditions obtained by EPMA After thermal exposure, Ti60 was processed into tensile test samples with the surface polished and metallographically etched.After the tensile test the morphological characteristics of the crack initiation area near the fracture were observed by SEM, and the microstructure is shown in Figure 7.Figure 7(a) showed that after exposed at 600℃ for 12h the parallel slip lines appeared, suggesting local plastic deformation occurred and the crack originated from the boundary between the transformed β grains.These morphological characteristics indicate that Ti60 can still maintain coordination ability after long-term thermal exposure at 600℃, which is consistent with the elongation feature in Figure 2(c) as well as related to the uniform distribution of Si in Figure 6(a).In contrast, the morphologies in Figure 7(b) after exposed at (a) and (b) show the fatigue fracture at room temperature, and Figure 8(c) and (d) show the fatigue fracture at 750℃.The river pattern in Figure 8(a) indicates that the room temperature fatigue crack propagates in trans-granular mode, while the obvious intergranular mode is shown in Figure 8(c) at 750℃, which is also caused by the segregation of Si element at grain boundaries.The morphology of Ti60 fatigue band is shown in Figure 8(b) and (d) which is fine and arc-shaped, and the center of the circle points were to the direction of the fatigue source.
Figure 8 reflects the typical characteristics of high temperature fatigue fracture of Ti60 alloy.

Figure 8 .
(a) River patterns at room temperature (b) Fatigue bands at room temperature (c) Secondary cracks at 750℃ (d) Fatigue bands at 750℃ Scanning electron micrograph of high temperature fatigue fracture