Stability of the microstructure and properties of Ti2AlNb alloys under thermal exposure conditions

In this paper, the microstructure and property stability of Ti2AlNb alloy specimens treated by thermal exposure at 800°C for 0~12h were investigated using universal tensile tester, X-ray diffractometer, scanning electron microscope, electron backscattering diffraction, and in-situ tensile scanning electron microscope. The results show that with the extension of thermal exposure time, the tensile strength and yield strength of the alloy slightly increased, the B2-phase of the alloy decreased while the O-phase increased, and the content of the α2-phase also slightly decreased. It has been found that the properties changes are related to the changes of microstructure. The tensile cracks expanded along the grain boundaries between the O-phase and B2-phase as well as along the O-phase grain boundaries. The slight changes in microstructure and properties indicate that the Ti2AlNb alloy is stable and reliable for short-term service at 800 °C.


1.Introduction
Ti2AlNb-based alloys are a new class of metal-question compound materials developed in the early 1990s since they have many advantages, such as, high yield strength at high temperatures, high fracture toughness, high creep resistance, low coefficient of thermal expansion, nonmagnetic and good flame-retardant properties, compared with Ni-based high temperature alloys, as well as high specific strength in the temperature range from room temperature to 750 °C.Compared with high-temperature Ti alloys, the service temperature of Ti2AlNb-based alloys is 200 °C higher.For example, the newly expanded TiAl intermetallic compound alloys containing Nb can be used for a long time in the temperature range of 600 °C to 800 °C, having a broad application prospect in the field of aerospace [1-4] .The development of Ti2AlNb alloys can be divided into three stages according to the content of Nb in the alloy: in the first stage, Nb <15%; in the second stage, Nb is 15-20%; and in the third stage, Nb >20%.Most of the properties of the materials in this category are improved with the increase of Nb content.The third class of alloys with more than 20% Nb has better high-temperature yield strength, creep

Experimental
Thermal environment simulation was carried out in the tube high temperature furnace, the temperature control accuracy of this tube furnace was ± 3 ℃.In this study, the temperature was set at 800°C and the time range was 0 to 12 hours, and the room temperature tensile properties of the specimens were measured using a Shimadzu-made AG-XPLUS 100KN universal testing machine.The specimens were mechanically smoothed and polished and corroded with titanium alloy corrosive solution, which was hydrofluoric acid, nitric acid, and aqueous solution with the volume ratio of 1:3:7, and the corrosion time was about 10~20 seconds.Metallographic microstructure observation was carried out under a Zeiss metallographic microscope.The microstructure and elemental composition of the quenched specimens were analyzed using Zeiss scanning electron microscope (SEM).The weave structure and grain size of the samples were observed by electron backscatter diffraction (EBSD) at an accelerating voltage of 15 kV and a step size of 2.0 µm.X-ray diffraction was used to test the composition and type of alloy phases.The micromorphological features at the crack initiation were in-situ observed using high resolution scanning electron microscopy.1a with the heat exposure time increasing to 8 hours, the yield strengths of the alloys gradually increased from the initial 780 MPa to 816 MPa, and then decreased to 810 MPa when the heat exposure time increased from 8 h to 12 h.And Figure 1b show that the tensile strength increased from the initial 880 MPa to 916 MPa when the heat exposure time increased to 12 h. .The microstructure observation for the alloy after thermal exposure at 800 °C for different times of 0h, 4h, 8h and 12h was carried out by OM, and the results are shown in Figure 2. The metallographic photographs in Figure 2 did not reflect the microstructure differences after different time at high temperature environment.For example, the simulated metallographic microstructure of Ti2AlNb after 12h of thermal environment at 800 ℃ does not show any obvious difference in microstructure features compared with the original state of Figure 2 a.It was still a band-like microstructure distributed in the direction of zapping and distributed in the matrix in a non-uniform state, and the metallographic photographs of all the states were mainly similar.Therefore, it may be concluded that the microstructure of the alloy is stable during the thermal exposure at 800 °C.  3 shows SEM images of the Ti2AlNb alloy before and after thermal exposure at 800 °C for 12 h.Three regions of different height can be clearly seen in Figure 3: firstly, the highest region is interconnected to form the matrix microstructure of the material; secondly, the region of intermediate height is elliptical and needle-like; thirdly, the lowest region is distributed in the central part of the elliptical region.The three different regions belong to different phases and the difference in corrosion rates results in a height difference in the morphology.This primitive microstructure is transformed in a high-temperature environment, thus changing the material properties [14][15][16].Comparing the microstructures of Ti2AlNb in Figures 3a and 3b at two temperatures, 400 °C and 800 °C, with a magnification of 25,000 times, it can be seen that the total area of the equiaxed region is increased at 800 °C, the number of needle-like microstructures in the matrix is reduced to zero, and the distribution of short bar-like microstructures, which have the worst corrosion resistance in the equiaxed region, is also reduced.These changes are microstructural factors that contribute to the change in properties.The phase composition of the sample can be determined from the characteristic position of the XRD peaks: B2 phase is an ordered body-centered cubic structure, O phase is an ordered orthorhombic structure and α2 phase is a densely arranged hexagonal structure [17][18][19].It is precisely because of the different crystal structures of the phases, which result in the formation of different diffraction peaks, that it is possible to calibrate the phases by the position of the diffraction peaks and to qualitatively predict the changes in the contents of the phases by the changes in the relative strengths of the diffraction peaks.It is also possible to speculate qualitatively about the change in content from the change in relative strength of the diffraction peaks.Comparing the changes in the relative strengths of the O-phase 62° and α2-phase 65° diffraction peaks in Figures 4a and 4b, it can be judged that the α2-phase is almost absent in the diffraction spectrum of the corresponding Figure 4b when the temperature is increased to 800℃.Combined with the characteristic difference between Figures 3a and 3b of the scanning electron micrographs, the reduction in the content of the short rod-like tissue distributed in the matrix in Figure 3b, it can be tentatively judged that the region of the short rods with the most severe corrosion in the scanning electron micrographs is the α2 phase, and that the other two phases, the B2 phase and the subordinate region of the O phase, need further analysis.[13].Combined with the electron micrographs in Figure 3a and Figure 3b, it can be judged that the matrix microstructure with the lightest degree of corrosion is the B2 phase, the equiaxial region is the O phase, the short rod-like microstructure distributed in the equiaxial region and the region with the deepest degree of corrosion is the α2 phase.This result is in agreement with the XRD analysis results, reflecting the superiority and accuracy of the multi-detection means of analysis.As the ambient temperature increases to 800°C, comparing Figures 5a and 5b, the B2 phase decreases significantly while the O phase increases, and at the same time the content of the α2 phase decreases slightly.Therefore, the increase in temperature leads to the transformation of the B2 phase into the O phase.The different colors in Figure 6 represent different crystallographic orientations, and it is worth noting that the phases in the tissue have essentially the same crystallographic orientation [20,21].Taking Figure 6a as an example, the green areas are the same crystal orientation features of the O phase, and the red areas are the same crystal orientation features of the B2 phase.The consistency of the crystal orientation is related to the material preparation process, in the Ti2AlNb sheet hot rolling process, the internal B2 phase is bound to dominate, with the rolling of the material within the crystals showing a preferential orientation characteristic, "weaving characteristics".During the subsequent aging process, the O phase is formed in the B2 phase matrix, and since phase transition nucleation follows specific crystal phase relationships, the O phase formed in the uniformly oriented B2 phase matrix will also have a uniform crystal orientation.In order to observe in detail, the crack initiation and propagation during tensile deformation up to the final fracture of the material, the microstructure and morphological features of the crack initiation in situ were observed using a high-resolution scanning electron microscope, as shown in Figure 7a to Figure 7b.For the Ti2AlNb alloy, the crack initiates at the grain boundary position between the O and B2 phases and extends along the grain contour of the O phase, reflecting certain along-grain features [22].As the B2 phase decreases and the O phase increases after thermal exposure of the alloy, this will result in larger O phase grains, which in turn will make the alloy more difficult to fracture along the O phase grain boundaries [23], thus improving the strength of the alloy.

Conclusions
In this paper, Ti2AlNb temperature of 800 ℃ time range of 0-12 hours of high-temperature environment simulation test; and then through various types of organizational properties characterization experiments and get the following conclusions.
(1) After thermal exposure of Ti2AlNb at a temperature of 800 ℃ for 0-12 hours, the tensile and yield strengths gradually increased with the increase of thermal exposure time, and the strengths peaked at 12 hours.
(2) The microstructure of Ti2AlNb consists of B2, O and α2 phases, where the B2 phase is matrix and distributed between the equiaxed O phases, the α2 phase is a short rod and distributed within the O phase, and the B2 and O phases have the characteristics of selective orientation.Increasing the temperature causes the B2 phase to transform into the O phase and the α2 phase to decrease in Ti2AlNb, while the acicular O phase distributed inside the B2 phase also gradually disappears.The strength of the alloy increases as the B2 phase transforms into the O phase.
(3) The cracks start at the grain boundary between the O phase and the B2 phase and extend along the grain contour of the O phase, thus reflecting certain along grain characteristics.As the B2 phase decreases and the O phase increases after thermal exposure of the alloy, while the O phase grain becomes larger, it will result in the alloy becoming more difficult to fracture along the O phase grain boundary, thus improving the strength of the alloy.

Figure 1 .
Figure 1.Mechanical properties of Ti2AlNb alloy after thermal exposure at 800 °C for different times (a) Yield strength; (b) Tensile strength As can be seen from Figure1awith the heat exposure time increasing to 8 hours, the yield strengths of the alloys gradually increased from the initial 780 MPa to 816 MPa, and then decreased to 810 MPa when the heat exposure time increased from 8 h to 12 h.And Figure1bshow that the tensile strength increased from the initial 880 MPa to 916 MPa when the heat exposure time increased to 12 h.

Figure 2 .
Figure 2. Metallographic microstructure of Ti2AlNb alloy after thermal exposure at 800°C for different times (a) 0h; (b) 4h (c) 8h; (d) 12h.The microstructure observation for the alloy after thermal exposure at 800 °C for different times of 0h, 4h, 8h and 12h was carried out by OM, and the results are shown in Figure2.The metallographic photographs in Figure2did not reflect the microstructure differences after different time at high temperature environment.For example, the simulated metallographic microstructure of Ti2AlNb after 12h of thermal environment at 800 ℃ does not show any obvious difference in microstructure features compared with the original state of Figure2 a.It was still a band-like microstructure distributed in the direction of zapping and distributed in the matrix in a non-uniform state, and the metallographic photographs of all the states were mainly similar.Therefore, it may be concluded that the microstructure of the alloy is stable during the thermal exposure at 800 °C.

Figure 3 .
Figure 3. SEM images of Ti2AlNb alloy before and after thermal exposure at 800 °C for 12 h, (a) 0h; (b) 12h Figure3shows SEM images of the Ti2AlNb alloy before and after thermal exposure at 800 °C for 12 h.Three regions of different height can be clearly seen in Figure3: firstly, the highest region is interconnected to form the matrix microstructure of the material; secondly, the region of intermediate height is elliptical and needle-like; thirdly, the lowest region is distributed in the central part of the elliptical region.The three different regions belong to different phases and the difference in corrosion rates results in a height difference in the morphology.This primitive microstructure is transformed in a high-temperature environment, thus changing the

Figure 4 .
Figure 4. XRD pictures of Ti2AlNb alloy after thermal exposure at 800°C for different times (a) 0h; (b) 12hThe phase composition of the sample can be determined from the characteristic position of the XRD peaks: B2 phase is an ordered body-centered cubic structure, O phase is an ordered orthorhombic structure and α2 phase is a densely arranged hexagonal structure[17][18][19].It is precisely because of the different crystal structures of the phases, which result in the formation of different diffraction peaks, that it is possible to calibrate the phases by the position of the diffraction peaks and to qualitatively predict the changes in the contents of the phases by the changes in the relative strengths of the diffraction peaks.It is also possible to speculate qualitatively about the change in content from the change in relative strength of the diffraction peaks.Comparing the changes in the relative strengths of the O-phase 62° and α2-phase 65° diffraction peaks in Figures4a and 4b, it can be judged that the α2-phase is almost absent in the diffraction spectrum of the corresponding Figure4bwhen the temperature is increased to 800℃.Combined with the characteristic difference between Figures3a and 3bof the scanning electron micrographs, the reduction in the content of the short rod-like tissue distributed in the matrix in Figure3b, it can be tentatively judged that the region of the short rods with the most severe corrosion in the scanning electron micrographs is the α2 phase, and that the other two phases, the B2 phase and the subordinate region of the O phase, need further analysis.

Figure 5 .
Figure 5. Physical phase distribution of Ti2AlNb alloy after thermal exposure at 800 °C for different times (a) 0h; (b) 12h

Figure 6 .
Figure 6.Crystal orientation distribution of Ti2AlNb alloy after thermal exposure at 800 °C for different times (a) 0h; (b) 12hThe three colors in the figure represent the three phases in Ti2AlNb, of which blue represents the B2 phase, purple the O phase and yellow the α2 phase[13].Combined with the electron micrographs in Figure3aand Figure3b, it can be judged that the matrix microstructure with the lightest degree of corrosion is the B2 phase, the equiaxial region is the O phase, the short rod-like microstructure distributed in the equiaxial region and the region with the deepest degree of corrosion is the α2 phase.This result is in agreement with the XRD analysis results, reflecting the superiority and accuracy of the multi-detection means of analysis.As the ambient temperature increases to 800°C, comparing Figures5a and 5b, the B2 phase decreases significantly while the O phase increases, and at the same time the content of the α2 phase decreases slightly.Therefore, the increase in temperature leads to the transformation of the B2 phase into the O phase.The different colors in Figure6represent different crystallographic orientations, and it is worth noting that the phases in the tissue have essentially the same crystallographic orientation[20,21].Taking Figure6aas an example, the green areas are the same crystal orientation features of the O phase, and the red areas are the same crystal orientation features of the B2 phase.The consistency of the crystal orientation is related to the material preparation process, in the Ti2AlNb sheet hot rolling process, the internal B2 phase is bound to dominate, with the rolling of the material within the crystals showing a preferential orientation characteristic, "weaving characteristics".During the subsequent aging process, the O phase is formed in the B2 phase matrix, and since phase transition nucleation follows specific crystal phase relationships, the O phase formed in the uniformly oriented B2 phase matrix will also have a uniform crystal orientation.

Figure 7 .
Figure 7. In-situ tensile SEM images of Ti2AlNb alloy after thermal exposure at 800°C for different times (a) 0h; (b) 12h