Microstructural evolu on and Mechanical Proper es of a Newly Developed Ti2AlNb-based alloy

Abstract: The effects of the microstructure on the tensile and creep proper es of the alloy at room temperature and high temperature were inves gated by controlling the microstructures of the alloy by different hot working processes. It is found that the lath microstructure obtained by forging in B2 single phase zone has high tensile strength. The tensile strength is 1188 MPa at room temperature and 950 MPa at high temperature. The equiaxed structure obtained by forging in O+B2 phase region has the characteris cs of high plas city, creep resistance and low tensile strength. The elonga on at room temperature is 9.0%, and the elonga on at high temperature is 36%. The ambient temperature, high temperature tensile proper es of the dual microstructure obtained by forging in the three-phase zone of α2+O+B2 are between the lath and the equiaxed microstructure.


Introduc on
Due to higher strength-to-density ra o, be�er room-temperature duc lity and more reasonable elevated temperature mechanical proper es than conven onal tanium aluminides [1][2][3][4], Ti 2 AlNb based alloy has been considered as poten al structural materials for aircra� engine applica on at elevated temperature. However, their poor oxida on resistance at high temperatures of above 750 ℃ [5] and insufficient elevated-temperature strength [6][7] s ll limit their wide applica on. In the resent years, Efforts have been made to improve the high temperature oxida on resistance by adding alloying elements to Ti 2 AlNb based alloys. The previous research [8][9][10][11] have been found that mul ple addi ons of Mo, V, Zr and Si are effec ve to improve oxida on resistance and creep strength. It was reported a Ti 2 AlNb-based alloy with Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si showing excellent high temperature oxida on resistance. the weight gain of Ti 2 AlNb-based alloy oxidized at 850℃ for 100 h was only 0.41 mg/cm -2 [12]. And the creep strain at 650℃ /150 MPa is only 0.12% under 100 h loading [13]. However, it exhibits poor room temperature tensile elonga on of less than 5%.
The mechanical proper es of the Ti 2 AlNb alloys are affected extensively by their microstructures [14]. However, phase equilibria and microstructural evolu on in orthorhombic alloys are complicated. The volume frac on, size, and morphology of the cons tuent phases are dependent on the thermal processing and the heat treatment [15,16]. Thus, it is necessary to further explore the microstructure and mechanical property rela onship and to op mize the microstructure to improve the duc lity and strength of the of Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si orthorhombic alloy.

Experimental procedure
An ingot with the nominal composi on of Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si was prepared by vacuum non-consumable arc mel ng under vacuum atmosphere in a water-cooled copper crucible, and it was re-melted for three mes in order to ensure composi on homogeneity.
The ingot with 280 mm in diameter and 1200 mm in length was forged several mes above and below the phase transi on point, and the final forging billet size was 250 mm *1000 mm. The billet was divided into three parts and forged in B2, α 2 +B2+O and B2+O phases respec vely. Samples from the as-forged specimens were solu on-treated at 975 °C for 1.5 h followed by water quenching, and then they were aged at 750°C followed by air cooling. The tensile tests at room-temperature were carried out in air by using Instron 1185 mechanical tes ng machine. The tensile tests at 650 °C were performed by using a MTS mechanical tes ng machine.
The microstructures were characterized by scanning electron microscopy (SEM, Tescan MIRA 3). The polished samples were etched by Kroll's reagent for more accurate dis nguish of different phase. Image Pro Plus (IPP) so�ware was used to count the content and size of each phase in different samples on the basis of their microstructural images. Ten images were selected for each sample at least. Fine microstructural characteriza on of samples was conducted using transmission electron microscopy (TEM, JEM-200CX TEM, JEOL, Japan). TEM samples were prepared by cu ng slices from as forged specimens. The samples were mechanically grounded to 30-50 μm in thickness by using SiC paper, and then punched into 3 mm discs. Finally, electrochemical polishing was used to thin the sample using a solu on of 6% perchloric acid, 34% n-butanol, and 60% carbinol. Figure.1 shows the backsca�er morphology of mul -component Ti 2 AlNb-based alloy forged in B2 single phase region under air cooling. Based on the imaging principle of backsca�ering, the microstructure of the alloy is composed of three colors contrast, hence it can be preliminarily determined that the α 2 phase based on Ti 3 Al composi on is black, the contrast of B2 matrix containing more beta elements is bright, and the O phase between α 2 phase and B2 is gray. A�er forging in B2 single-phase zone, O phase laths of different sizes and direc ons are distributed on the matrix, showing typical characteris cs of lath structure. the volume frac on of the α 2 phase, B2 and O was analyzed by Image-Pro Plus 6.0 image analysis so�ware, the volume frac ons of O, α 2 phase and B2 were 40.8%, 0.7% and 58.5% respec vely. Figure.2 shows the backsca�er morphology of mul -component Ti 2 AlNb-based alloy forged in α 2 +B2+O phase region under air cooling. It was found that the microstructures are mainly composed of equiaxed α 2 phase par cles, acicular O phase and B2 matrix, and the content of equiaxed α 2 phase is less than 30%. According to the classifica on criteria for typical structures of Ti 2 AlNb-based alloys, primary α 2 phase or O-phase equiaxed par cles and secondary lath are con nuously distributed in the matrix, and the structure with primary α 2 phase content less than 30% is dual-microstructure. Combining with the characteris cs of mul -component Ti 2 AlNb-based alloy in the three-phase zone of α 2 +B2+O, it can be concluded that the microstructure forged at three-phase zone of α 2 +B2+O is dual-microstructure. Compared with the lath microstructure forging in B2 phase zone, the α 2 phase is formed in the forging at α 2 +B2+O zone, which plays the role of pinning and restraining the growth of B2 grains. The acicular O phase is formed in the cooling process a�er forging, so it is uniformly distributed in the crystal. The volume frac ons of O, α 2 phase and B2 phase were analyzed, and the corresponding contents were 58.8%, 1.1% and 40.1% respec vely.  Fig.3 shows the microstructure of the alloy forging at the B2+O two-phase region. Compared with the forging structure in three-phase zone, the equiaxed par cles of α 2 /O phase are more uniform and the needle-like O phase is smaller. The volume frac ons of the alloys corresponding to the three phases of O, α 2 and B2 are 68.7%, 1.1% and 30.2%, respec vely. and the volume frac on of O phase increases while that of B2 phase decreases.

Mechanical Proper es of the alloy
Table1 shows the room and high temperature tensile proper es of mul -component Ti 2 AlNb-based alloy specimens forged in three phases and then aged at 750 ℃ a�er heat treatment. From the test results, it can be seen that the lath structure obtained by forging and heat treatment in B2 single-phase zone shows high strength and low plas city. The average tensile strength at room temperature is 1188 MPa, yield strength is 1134 MPa and elonga on is 2.1%. The high temperature tensile strength, yield strength and elonga on at 650℃ are 950 MPa, 895 MPa and 2.9% respec vely. The average tensile strength at room temperature is 1100 MPa and the yield strength is 985 MPa, which is slightly lower than the tensile and yield strength at B2 single-phase forging. However, the elonga on and cross-sec on shrinkage, which characterize the excellent plas city index, are greatly increased. The elonga on and cross-sec on shrinkage at room temperature reach 9.0% and 12%. The average tensile strength and yield strength at high temperature are 910 MPa and 822 MPa, which are also slightly lower than those forged in B2 single-phase zone, but the high temperature plas c elonga on increases to 36%. The strength and plas city of dualmicrostructure forged in the α 2 +B2+O three-phase zone are between the equiaxed structure and the lath structure.  Fig.4 (a) and (b) show the fracture morphology of the lath structure obtained by forging in B2 single phase zone at room temperature. It can be seen from the figure that there is almost no necking phenomenon in the tensile fracture at room temperature. The macro surface of the fracture is ver cal to the tensile stress, and the fracture surface is flat, with dark gray color and no metallic luster. Riverlike pa�erns can be observed in the radia on area of the fracture surface, which shows obvious bri�le fracture characteris cs. The results are consistent with the plas city index of the alloy itself. show the fracture morphology of the dual structure obtained by forging in α 2 +B2+O phase zone at room temperature. Compared with the lath structure obtained by forging in B2 single-phase zone, the effect of α 2 on grain boundary pinning during forging in α 2 +B2+O three-phase zone makes the forged grains fine and the room temperature plas city of the dual-phase structure slightly increased. Fig.4 (e) and (f) show the fracture morphology of equiaxed Ti 2 AlNb-based alloy forged in B2+O phase region a�er tension at room temperature, which is showed quasi-cleavage fracture characteris cs due to Fiber region, radia on region and shear lip region of the fracture.

Fig.5 High temperature tensile fractographs of multi-component Ti 2 AlNb based alloy: (a), (c) and (e) macro-fractograph; (b) (b), (d) and
(f)micro-fractograph Fig.5 shows high temperature tensile fracture morphologies of mul -component Ti 2 AlNb-based alloy bars forged in three phases. Fig. 5(a) and (b) are the fracture morphology of high-temperature tensile lath structure obtained by forging in B2 singlephase region. It can be seen from the figure that there is almost no necking phenomenon in the high-temperature tensile fracture, and the fracture morphology is crystalline ice sugar block, and there are micro-cracks along the grain boundary. Fig.5(e) is the macro-fracture morphology of equiaxed structure obtained by forging in B2+O phase region a�er high temperature tension. From the graph, it can be seen that the fracture surface is cup-cone shape, the fracture height is different, there is obvious plas c deforma on necking characteris cs. Fig.5(f) is the micro-morphology of the fibrous zone of the fracture. The fibrous zone, radia on zone and shear lip zone of the fracture are also obvious. The fibrous zone of the fracture is large, and the fracture is filled with dimples of different depths. Compared with the fracture morphology of the dual microstructure ( Fig.5(c), 5(d)), there are no micro-cracks and deep dimples in the fracture surface of the forged structure in the zone of B2+O, and the fracture mode is duc le fracture. The results show that the equiaxed structure obtained by forging in B2+O phase region has good plas city at high temperature, which is consistent with the experimental results of high elonga on index and high sec on shrinkage of alloy at high temperature (

Discussions
The mechanical proper es of Ti 2 AlNb-based alloys under different microstructures show that the proper es of Ti 2 AlNbbased alloys are basically the same as those of other intermetallic, showing the characteris cs of high strength and low plas city. However, strong plas city varies with different microstructures. The tensile strength of lath structure is high, the plas city of equiaxed structure is good, and the strength-plas city of duplex structure is between the two. For the three kinds of structures, the main influence is the difference of forging temperature. lath microstructure is obtained by forging at higher temperature. The grain size of B2 obtained by forging at higher temperature is the largest. While that the equiaxed microstructure is the smallest. According to the principle of fine grain strengthening, equiaxed structure should have high yield strength theore cally, but the result is contrary to the conclusion. The volume frac on of B2 phase in lath microstructure is the highest, while that of B2 phase as plas c phase can move and slip more. Therefore, the lath microstructure should have high plas city index, but the result is just the opposite. It is shown that besides the grain size, phase type and phase content of B2, the micro-variables such as morphology and thickness of various phases in mul -component Ti 2 AlNb-based alloys need to be taken into account. Therefore, it is necessary to combine fracture mode analysis.  Fig.6(a) and (b) show the transmission morphology of lath and dual microstructures. It can be seen from the figure that the long cellular phase is covered in the thick lath. Diffrac on analysis shows that the long cellular phase is α 2 phase, while the outer thick lath is O phase. The width of the α 2 /O lath is about 1.2 um, and the disloca on density in the lath is very low. Fig.6(c) is TEM with equiaxed structure. Compared with lath and dual microstructure, equiaxed structure is mainly composed of lath O phase and B2 phase. There is no α 2 /O phase in TEM, and the width of O phase is about 0.5 um, the disloca on density is higher. From the analysis of the tensile fracture in Fig7, it is found that the tensile fracture is ini ated with the coarsening of α 2 /O, and the fine O-phase lath can hinder the crack propaga on, thus improving the strength of the alloy. Con nuous crack propaga on will intersect with many spiral disloca ons to form a step with sufficient height, which will become a river pa�ern visible under the electron microscope, and result to the plas city of the alloy is poor. Due to the fine and uniform size of O phase and high disloca on density, the equiaxed structure exhibits excellent plas city in tension.

Conclusion
The lath structure was obtained by forging in B2 single-phase zone, which was composed of lath-like α 2 /O phase, needle-like O phase and B2 matrix. Dual microstructure was obtained by forging in α 2 +O+B2 three-phase zone, which consisted of primary equiaxed α 2 /O phase, lath-like O phase, needle-like O phase and B2 matrix. The equiaxed micro-structure was obtained by forging in O+B2 phase zone, which is mainly composed of primary equiaxed O phase and B2 matrix. Under the same heat treatment regime, the tensile proper es of lath microstructure at room temperature and high temperature show the characteris cs of high strength and low plas city. The tensile strength at room temperature is 1188 MPa, and the tensile strength at high temperature is 950 MPa. The equiaxed structure shows the characteris cs of low plas city and high strength, with