目录结构

详情信息展示

参考文献

Rare Metals2020年第6期

Microstructure and hot flow stress at 970℃ of various heat-treated Ti₂AlNb sheets

Yong Wu Rong-Lei Fan Xian-Jun Zhou Ming-He Chen

College of Mechanical and Electrical Engineering,Nanjing University of Aeronautics and Astronautics

作者简介：*Yong Wu,e-mail:wuyong@nuaa.edu.cn;

收稿日期：10 November 2019

基金：financially supported the National Natural Science Foundation of China(No.51805256);

Microstructure and hot flow stress at 970℃ of various heat-treated Ti₂AlNb sheets

Yong Wu Rong-Lei Fan Xian-Jun Zhou Ming-He Chen

College of Mechanical and Electrical Engineering,Nanjing University of Aeronautics and Astronautics

Abstract：

The microstructure and hot tensile behaviors of the different heat-treated Ti₂AlNb sheets were investigated by backscattered electron image (BSE),electron backscattering diffraction (EBSD),transmission electron microscope (TEM) and tensile tests.The grain sizes and contents ofα₂,B2/βand O phases were quantitatively studied.As the heating time increases at 970℃,the mean grain size and content ofα₂-phase increased.The grain shapes and distributions of the O-phase lamellar grains were affected by the heat treatments.The plastic deformation promoted the O→B2/βphase transition and the globularization of O-phase lamellar grains at 970℃.Calculated by the creep equation and the iso-stress method,the grain size exponent wasμ=1.1 and the relationship between the material constants of B2/βand O phase was K_O=1.14K_B2/β.

Keyword：

Ti₂AlNb alloy; Heat treatment; Tensile behaviors; Microstructure evolution;

Received： 10 November 2019

1 Introduction

With the development of the application in aerospace and aircraft,there is an urgent need to develop the lightweight and high-temperature alloy in order to improve the fuel efficiency and thrust-weight ratio.The titanium aluminides,i.e.,Ti₃Al and TiAl alloys,are desirable prospective for applications in aircraft turbine engines due to their excellent properties such as high-temperature performance,high specific strength,good creep resistance and good corrosion resistance [ 1, 2] .However,the applications were limited by their brittleness.Niobium has been recognized as a preferredβstabilizer since it can increase the non-basal slip activity in Ti₃Al.Therefore,the critical problem of insufficient number of slip systems can be improved [ 3] .The Ti₂AlNb alloy,a typical Ti₃Al-Nb alloy,firstly discovered by Banerjee et al. [ 4] and Boehlert [ 5] ,presents excellent comprehensive properties,i.e.,fine ductility and high-temperature creep property.In recent years,the thirdly generation of Ti₂AlNb alloy,such as Ti-22Al-23Nb,Ti-22Al-25Nb and Ti-22Al-27Nb,which consists of B2/β,O and a small amount of a₂ grains,presents a great potential in commercial applications of the engine components market [ 6] .

For Ti₂AlNb alloy,the grain shapes and phase transition among theα₂,B2/βand O-phase are very complex during the hot working processes [ 7, 8] .There were differences in the microstructure and mechanical properties of free forged and rolled Ti-22Al-25Nb [ 9] .After the multistep isothermal forged,the mean grain size of the Ti₂AlNb with the homogeneous nanostructure was only 300 nm [ 10] .The elongation and ultimate strength were 25%and 1400 MPa at room temperature,respectively,while the maximum elongation was 930%and the steady-state flow stress (σ₅₀)was 125 MPa at 900℃with a strain rate of4.2×10^-3 s^-1.Actually,the microstructural evolution and the hot flow stress were also affected by the original microstructure [ 11, 12, 13] .The microstructural evolution and hot deformation behavior of the base metal as well as the fusion zone for the laser-welded Ti-22Al-25Nb alloy joints were different at 990℃ [ 14] .In the studies of hot tensile behavior for the rolled Ti-22Al-25Nb sheet at930-990℃,the deformation mechanisms,including dynamic recovery,dynamic recrystallization,strain softening and O-phase instability,were discussed during the hot tensile tests [ 15, 16] .Meanwhile,the microstructure of Ti₂AlNb was affected by deformation degree and strain state in the hot gas forming and superplastic forming [ 17, 18, 19, 20] .To accurately describe the hot deformation behavior and predict the forming process of the complex shape parts by finite element method (FEM) simulation,it is needed to quantitatively express the microstructural evolution and the relationship between microstructure and hot flow stress.Fortunately,a unified viscoplastic constitutive equations [ 21, 22, 23] ,based on internal physical variables,can offer a good idea to quantitatively describe the relationship among the strain rate,flow stress,strain,grain size,phase volume fractions,damage and dislocation.

In this paper,the Ti₂AlNb sheets were heat-treated by different heat treatment schemes.The tensile specimens were tested at 970℃with strain rate of 0.001 s^-1.The grain sizes and contents ofα₂,B2/βand O for the tested specimens were quantitatively analyzed.The effects of heat treatments and hot tensile on the stability and phase transition ofα₂,B2/βand O were discussed.The effects of phase contents and grain sizes on the flow stress were described by the creep constitution equation.This paper aims to provide basic data for the physical variables constitutive equation and offer reference to the FEM simulation for the hot forming of Ti₂AlNb alloy,including the heterogeneous material.

2 Experimental

The nominal composition of as-received rolled sheet is Ti-22Al-24.5Nb-0.5Mo.The thickness is 2.0 mm.According to the phase diagram of Ti-22Al-xNb [ 24, 25] ,five different heat treatments were designed to get different micros true tures,as shown in Fig.1.HT-1 is a simplified experiment for the forming process of hot gas forming or superplastic forming.The purpose of HT-2 is to study the holding time on microstructure change of Ti₂AlNb sheet.The purpose of HT-3 is to reduce the content ofα₂-phase.It is desirable to obtain a microstructure with full coarse B2/βgrains by treatment HT-4,which is similar to the microstructure of the welding fusion zone [ 14] .The HT-5was chosen to study the microstructure evolution and mechanical properties of the lamellar O-phase grains.

Figure 2 demonstrates the schematic diagram and specimen size of the hot tensile tests.The gage length was15 mm,and the width was 4 mm.The tensile direction was parallel to the rolling direction.The uniaxial tensile tests were carried out by an instron-5500R universal testing machine.T_d was the tensile temperature,and T₀ was initial temperature of the tensile machine.Before the tensile test,the specimens were coated with glass slurry to avoid oxidation.The Ti₂AlNb alloy presents excellent plastic deformation performance at 970℃with strain rate of0.001 s^-1 [ 34] .In this paper,all of the hot tensile specimens were tested at 970℃with the constant strain rate of0.001 s^-1.After the hot tensile tests,the tested specimens were taken out and quickly water-quenched to preserve the high-temperature micros true ture.The micros true ture at970℃was reserved by the water quenching,where the cooling rate was greater than 160℃·s^-1 [ 26, 27] .

Fig.1 Scheme of heat treatments for Ti₂AlNb sheet (WC:water cooling)

Fig.2 Schematic diagram and specimen size of hot tensile tests (mm)

The micros true ture was investigated by backs cattered electron image (BSE),electron backscattering diffraction(EBSD) and transmission electron microscope (TEM).The BSE and EBSD were tested by the ZEISS Supra 55 SAP-PHIRE.The specimens of BSE and EBSD were prepared by the electrochemical polishing.The EBSD data were analyzed by the software CHANNAL.5.The results of the B2/βgrain boundaries were observed by the EBSD tests.TEM was performed on a Tecnai G2F30.The content and grain sizes in the BSE images were calculated by the Image Pro Plus 6.0 software.For each analysis result,at least three images were calculated to decrease the accidental error.

3 Results

3.1 Microstructure of as-received and heat-treated specimens

Figures 3,4 show BSE images and TEM results of asreceived and heat-treated specimens,respectively.In Fig.3 a,the as-received material consists of equiaxedα₂/O,matrix B2/βand lamellar O grains,where the dark region representsα₂ phase,the light region represents B2/βphase and the gray region represents O phase.Three-phase grains were mixed together.Through the analysis of the image Pro Plus 6.0 software,the content of theα₂,B2/βand O phases is 10.8 vol%,22.3 vol%and 66.9 vol%,respectively.Some equiaxedα₂-phase and O-phase grains erratically existed in the matrix microstructure.The bright-field image of TEM and electronic diffraction pattern are shown in Fig.4a,d,e and f.B2 phase is an ordered Pm-3m bodycentered cubic structure.βphase is a disordered Im-3m body-centered cubic structure.The two phases transformed into each other during the heating or cooling process and co-existed in the matrix at most of time.In this paper,the distinction between B2 andβphase was not considered.Plenty of lamellar O-phase grains existed in the matrix B2/βphase.

Figure 3b,c represents BSE images of HT-1 and HT-2.Most of the O-phase grains were transformed to B2/βgrains after the heat treating at 970℃.The microstructure of HT-1 and HT-2 was similar.Figures 3d,4b present BSE image TEM result of HT-3 sheet,respectively.After 100-h aging at 900℃,the content ofa₂,B2/βand O is 2.3 vol%,46.2 vol%and 51.5 vol%,respectively.Compared to the as-received sheet,the contents ofα₂ and O phases decreased,while that of B2/βphase increased.Some rim-0grains were formed by the peritectoid reaction of the equiaxedα₂ and matrix B2/βphases.Because of the slow phase transition ofα₂→O and B2/β+α₂→O,someα₂grains were retained after a long time aging at the B2/β+O phase field.

Figure 3e shows BSE image of HT-4 sheet.After the solution treatment at 1100℃,almost all theα₂ and O grains were transformed to B2/βgrains.The sizes of coarse B2/βgrains were about 80-100μm.Because of the nonuniform elemental composition [ 7] ,a few number of smallα₂ grains were reserved.Figure 3f shows BSE image of HT-5 sheet.Figure 4c shows TEM result of HT-5.After aging treatment at 900℃,plenty of lamellar O-phase grains were precipitated in the coarse B2/βgrains.The contents ofα₂,B2/βand O phases were 0.3 vol%,48.5vol%and 50.2 vol%,respectively.The width and length of the O-phase grains were about 0.1-0.3 and 1-5μm,respectively.

Fig.3 BSE images of Ti₂AlNb alloy for a as-received sheet,b HT-1,c HT-2,d HT-3,e HT-4 and f HT-5

Fig.4 TEM results of Ti₂AlNb alloy for a as-received,b HT-3 sheet and c HT-5 sheet;correspond SAED patterns of marked d O phase,e B2phase and fα₂ phase in a

Fig.5 Tensile properties of the various specimens at 970℃with a strain rate of 0.001 s^-1

3.2 Hot tensile behaviors

Figure 5 shows the stress-strain curves of the different specimens at 970℃with a strain rate of 0.001 s^-1.Table 1 shows the tensile properties of the different heattreated specimens,i.e.,yield strength,tensile strength,flow stress (ε=0.45) and elongation.For as-received,HT-1 and HT-2 sheets,the flow stress kept steady.With the increase in the thermal insulation time,the tensile strength was increased alternatively,and the elongation was reduced.The tensile strengths of as-received,HT-1 and HT-2 were75.3,77.4 and 85.1 MPa,while the elongations were197.3%,150.9%and 143.5%,respectively.The peak stress of HT-3 was 90.7 MPa,which was higher than those of asreceived,HT-1 and HT-2 sheets.However,the flow stress linearly decreased after the peak stress point,which presented the tensile softening phenomena.The peak stresses of HT-4 and HT-5 were 115.1 and 106.8 MPa,respectively.However,the flow stress declined quickly after the peak stress.

下载原图

Table 1 Tensile properties of different specimens at 970℃with a strain rate of 0.001 s^-1

To study microstructural evolution,some hot tensile specimens with true strain of 0.45 were prepared,as shown in Fig.6a.The hot tensile tests were immediately stopped when the elongation was 56.8%,i.e.,the true strain was0.45.Then the hot tensile specimens were taken down quickly and water cooled.TEST-0,TEST-1,TEST-2,TEST-3,TEST-4 and TEST-5 were the tested specimens of as-received sheet deformed to the true strain of 0.45 under different deformation conditions.The width and the thickness of the tested specimens were measured by the micrometer.Figure 6b shows the equivalent strain distribution.Most of the equivalent strains were between 0.35and 0.58.Figure 6c shows the distribution of the ratios between the cross-sectional area and the original sectional area for the tested specimens.For TEST-0,TEST-1,TEST-2,TEST-3,TEST-4 and TEST-5 specimens,the ratios of the cross-sectional area and the original sectional area for the tested specimens were between 0.59 and 0.69,i.e.,the deformation was relatively uniform and there was no obvious necking.So,the main reason of the flow stress softening phenomenon of TEST-3,TEST-4 and TEST-5was the microstructural evolution but not localized necking.

Fig.6 Results of tensile specimens with a strain of 0.45:a tested specimens,b equivalent strain distributions of tested specimens,c ratio distributions of cross-sectional area and original sectional area for tested specimens (TEST-0,TEST-1,TEST-2,TEST-3,TEST-4 and TEST-5being tested specimens with true strain 0.45 of as-received sheet,HT-1,HT-2,HT-3,HT-4 and HT-5,respectively)

Fig.7 BSE images of undeformed positions for tested samples with true strain 0.45:a TEST-0,b TEST-1,c TEST-2,d TEST-3,e TEST-4 and f TEST-5

Fig.8 Phase and BC and Euler images of un-deformation places for tested specimens with true strain 0.45:a,b TEST-0,c,d TETS-1,e,f TEST-2,g,h TEST-3 and i,j TEST-5 (heavy lines meaning HAGBs and fine lines meaning LAGBs;in BC and Euler images,only HAGBs were signed;only 1/3 view fields of EBSD results were shown;in i,j,sizes of lamellar O andα₂ grains were much smaller than step length,so small lamellar O andα₂ grains cannot be observed)

3.3 Microstructure of the tested specimens

3.3.1 Micro structure at undeformed position

To quantitatively study the relationship between microstructure and flow stress,the microstructure at the undeformed position was observed by BSE images and EBSD,as shown in Figs.7 and 8.Figure 9 shows B2/β-phase grains/sub-grain size distributions,which was obtained from EBSD results.

Figure 7a shows BSE images of the undeformed position for TEST-0 specimen.After heating at 970℃,the lamellar O-phase grains of the as-received sheet were fully transferred to B2/βmatrix.Most of the equiaxedα₂ grains were saved.Figure 8a,b shows the phase and BC and Euler images of the undeformed position for TEST-0specimen.The heavy lines illustrate the high-angle grain boundaries (HAGBs,>10°),and the fine lines illustrate the low-angle grain boundaries (LAGBs,2°-10°).The BC and Euler images show the grain shapes,deformation distributions and orientation relationships.The contents ofα₂,B2/βand O can be obtained in the phase maps,which are5.2 vol%,93.8 vol%and 1.0 vol%,respectively.In Fig.8a,because of the strong texture of the Ti₂AlNb alloy rolled sheet,it is difficult to find the B2/β-phase HAGBs although the width of the EBSD observation area is larger than300μm.Actually,the hot metal flow stresses were not only affected by the HAGBs,but also affected by the LAGBs [ 28, 29] .In the study by Alabort et al. [ 30] ,to quantitatively analyze the relationship between microstructure and hot deformation behavior of Ti-6Al-4V,the grain size in steady state was measured by EBSD and the minimum grain misorientation was set as 5°.In this paper,according to the testing results,the minimum grain misorientation of B2/βphase was set as 2°,while the ones ofα₂ and O phase were set as 10°.The details of contents and mean sizes ofα₂,B2/βand O phase grains at the undeformed position for the tested specimens are shown in Table 2 where the volume fractions of TEST-4/TEST-5 were calculated by Image Pro Plus 6.0 software based on the BSE images.

Figure 7b,c shows BSE images of undeformed positions for TEST-1 and TEST-2,respectively.With the increase in the heating time,the content ofα₂ phase increased.Figure 8c,e shows the phase maps of TEST-1and TEST-2,respectively.The contents ofα₂ phase at the undeformed positions for TEST-1 and TEST-2 were 6.2vol%and 11.5 vol%,respectively.Meanwhile,there were some HAGBs in Fig.8c,e.From the BC and Euler images in Fig.8d,f,it is easy to find two different grain orientations.With the increase in the heating time,the LAGBs decreased while HAGBs increased.As shown in Fig.9,the mean sizes of B2/β-phase sub-grains for TEST-0,TEST-1and TEST-2 were 3.04,3.73 and 4.17μm,respectively.

Figure 7d shows BSE images of the undeformed position for TEST-3.Figure 8g,h shows the undeformed EBSD results of TEST-3.Most of O phase existed in the coarse B2/βgrains.The mean size of the B2/βphase subgrains was 6.45μm.The contents ofα₂,B2/βand O phase were 6.3 vol%,77.8 vol%and 15.9 vol%,respectively.Compared to HT-3 sheet,the O-phase content obviously decreased and that of TEST-3 was much bigger than those of TEST-0,TEST-1 and TEST-2.It can be concluded that theα₂,B2/βand O phase grains were not stable atα₂+B2/β+O phase zone.

Figure 7e shows BSE image of the undeformed position for TEST-4 specimen.Plenty of lamellarα₂/O grainsexisted in the B2/βgrains.Because the HT-4 sheet was held at 970℃for about 20 min during the hot tensile test,plenty of lamellarα₂ and O grains were precipitated.Calculated by Image Pro Plus 6.0 software,the contents ofα₂,B2/βand O phase were 2.5 vol%,89.7 vol%and 7.8vol%,respectively.It indicated that the phase transitions of B2/βO and B2/βα₂ in the solution-treated Ti₂AlNb alloy were very quick.

Fig.9 Distributions of B2/βphase sub-grain sizes at undeformed potion for a TEST-0,b TEST-1,c TEST-2,d TEST-3 and e TEST-5(minimum grain misorientation being set as 2°)

下载原图

Table 2 Content and mean grain sizes ofα₂,B2/βand O at unde-formed position for tested specimens

^*Content of TEST-4/TEST-5 calculated by Image Pro Plus 6.0 software based on BSE images

Figure 7f shows BSE image of the undeformed position for TEST-5 specimen.The contents ofα₂,B2/βand O phase were 1.7 vol%,93.5 vol%and 4.8 vol%,respectively.As compared to Fig.7e,the contents ofα₂ and O phases were significantly reduced.EBSD results of TEST-5 are presented in Fig.8i,j.The coarse B2/β-phase grains were hundreds of micrometers in size.The mean size of B2/β-phase sub-grains/grains was about 76.7μm.

3.3.2 Microstructure of deformed position

Figures 10,11 show BSE images and EBSD results of deformed positions with true strain of 0.45.Table 3 shows the details of contents and mean grain sizes ofα₂,B2/βand O phases.

Figure 10a-c shows BSE images of deformed positions for TEST-0,TEST-1 and TEST-2,which were similar to the undeformed microstructure.Theα₂/O grains irregularly existed in the B2/βmatrix.After hot deformation,the B2/βphase LAGBs decreased significantly,as shown in Fig.1la-f.Meanwhile,there were some recrystallized B2/βgrains appeared at the coarse grain boundaries.

Figures 10d and 11g,h show the microstructure of the deformed position for TEST-3.Most of the O-phase grains were elongated and broken up.The content of O-phase reduced to 8.7 vol%,which was still much higher than those of TEST-0,TEST-1 and TEST-2.

Figure 10e shows BSE image of deformed position for TEST-4 specimen.Some lamellarα₂-phase and O-phase grains were broken up.The contents ofα₂,B2/βand O phase were 6.0 vol%,90.8 vol%and 3.2 vol%,respectively.Figure 10f shows BSE image of deformed position for TEST-5 specimen.The lamellarα₂-phase and O-phase grains were also broken up.Figure 1 1i,j shows phase map and BC and Euler image of deformation positions for TEST-5.The coarse B2/βgrains were elongated along the tensile direction.There were some LAGBs in B2/β-phase grains.At the coarse B2/βgrains boundary,there were a lot of sub-grains and some B2/βrecrystallized grains,as shown in Fig.11 i,j.The mean size of the B2/βsub-grains was only 22.43μm.

Fig.10 BSE images of deformed positions for tested specimens:a TEST-0,b TEST-1,c TEST-2,d TEST-3 and e TEST-5 (vertical direction of picture being stretching direction)

4 Discussions

4.1 Microstructural evolution during hot deformation

Actually,some researchers have been set up the Ti-22AlxNb phase diagram,where most of the experimental data points were gotten by the long-time heat treatment(36-200 h) [ 31] .The Ti₂AlNb alloy was a typical metastable metal at high temperature,and it was hard to achieve the phase equilibrium even after 600-1000-h heat treatment [ 25] .For the metastable metal,the plastic deformation can induce element diffusion and phase transition [ 32] .The O-phase structure is intermediate phase between theα₂ and B2 phases [ 25] .Theα₂,B2/βand O phase grains were unstable during the hot deformation.At970℃,various micros true tures of Ti₂AlNb alloy can be gotten by different hot working processes,while the hot deformation behaviors were also affected by the micros true ture.

The heat treatment was not beneficial for the micro structure and the hot formability of Ti₂AlNb sheet.After the heating treatment,the grains were coarse and the flow stresses increased.So,reducing the heating time and avoiding the excessive heating temperature was helpful for the forming technology of the as-received Ti₂AlNb sheet.However,most of the hot working processes were uncontrollable and inevitable,for example the laser welding process,induction heating,superplastic forming and diffusion bond.To accurately predict the hot deformation,it is important to describe the flow stress of the Ti₂AlNb sheet with different heating treatments.

4.2 Effect of grain size and content on flow stress

Generally,the high-temperature creep deformation in crystalline materials may occur by processes associated with dislocation activity,grain boundary sliding (GBS) and diffusion creep.The tensile properties are usually characterized in the form of the following equation [ 33] :

whereε_p is the steady-state strain rate,D0 is the appropriate diffusion coefficient,G is the shear modulus,b is the Burgers vector,k_b is Boltzmann constant,d is the mean grain size,n is the stress exponent,n-1/m,m isthe strain rate sensitivity exponent and is about 0.22-0.23 at 970℃ [ 34] ,A is the materials constant,μis the grain size exponent,T is the absolute temperature andσis the applied stress.

Fig.11 Phase and BC and Euler images of deformation positions for tested specimens:a,b TEST-0,c,d TEST-1,e,f TEST-2,g,h TEST-3 and i,j TEST-5 (heavy lines meaning HAGBs and fine lines meaning LAGB s;in BC and Euler images,only HAGB s were signed;only 1/3 view fields of EBSD results were shown;vertical direction of picture being stretching direction)

For Ti₂AlNb-alloy hot deformation,it is important to consider the coordinated roles of theα₂,B2/βand O phase grains.The effect of the content of phases on material properties can be predicted with three assumptions and utilization of the law of mixtures rule.The first one is that the strain is the same in both phases (iso-strain),the second one is that stress is the same in both phases (iso-stress),and the third one is that the contribution of each phase as well as any other contribution from special mechanisms such as interface sliding should be determined [ 35] .

In some two-phase Ti-alloy deformation studies [ 36, 37, 38] ,the iso-stress method was adopted.For Ti₂AlNballoys,most of theα₂ grains were still equiaxed after hot deformation,which showed that theα₂-phase is harder than B2/β-phase and O-phase in Ti₂AlNb alloy,as shown in Fig.11.In this research,the deformation ofα₂-phase was ignored, .Then,the creep total strain rate was expressed by:

下载原图

Table 3 Content and mean grain sizes ofα₂,B2/βand O at deformed position for tested specimens

^*Content of TEST-4/TEST-5 calculated by Image Pro Plus 6.0 software based on BSE images

where f_B2/βand fo are the contents of B2/βand O phase, is the plastic strain rate of B2/βphase and is the plastic strain rate of O phase.The plastic strain rate of B2/βand O phase was expressed by:

where d_B2/β,0 and d_O,0 are the mean grain sizes of B2/βand O phases at un-deformation position for TEST-0,d_B2/βand do are the mean grain sizes at the deformation position,K_B2/βand K_O are the material constants of B2/βand O phases,respectively.

Figure 12 shows the relationship between flow stress and sub-grain/grain size of TEST-0,TEST-1,TEST-2 and TEST-3.The horizontal axis was the grain size ratio between the testing specimen and as-received sheet.The horizontal ordinate reflected the increase in the flow stress.In this research,the strain rate sensitivity exponent was0.23 [ 34] .From the optimal computation in MATLAB software,the grain size exponent wasμ=1.1 and the relationship between the material constants of B2/βand O phases was K_O=1.14K_B2/β.In the Ti-6Al-4V superplastic deformation studies by Ghosh and Hamilton [ 35] and Cheong et al. [ 39] ,the grain size exponents were calculated as 1.0,2.0 or 2.3.In the study by Bae and Ghosh [ 40] ,the effect of the strain rate on the grain size exponent was considered,where the grain size exponent was 0.27-0.48with high strain rate and the grain size exponent was 2.3with low strain rate.

Fig.12 Relationship between flow stress and sub-grain/grain size for heat-treated specimens:a B2/βphase and b O phase (d_B2 and d_O being mean grain size of B2 and O phase of undeformed region for tested specimen,respectively;d_TEST-0,B2 and d_TEST-0,O being mean grain size of B2and O phase of undeformed region for TEST-0,which is reference substance;σandσ_TEST-0 being flow stress of tested specimen and TEST-0,respectively;n being the stress exponent;f_volume,B2 and f_volume,O being contents of B2 and O phase of undeformed region for tested specimen,respectively)

Fig.13 Results of predicted and experimental flow stresses with a strain of 0.45

Figure 13 shows the results of predicted and experimental flow stresses with strain 0.45 of TETS-0,TETS-1,TEST-2 and TEST-3.The flow stress (σ_2%) was the reference.The predicted results of TEST-0,TEST-1 and TEST-2 agreed well with the experimental results.However,there is an error between the predicted stress and experimental stress of TETS-3,which is mainly because of the neglect of O-phase grains globularization softening [ 34] ,as shown in Figs.6d,9d.Meanwhile,the B2/βgrains of TEST-4 and TEST-5 were very coarse,which were very different from the other testing specimens.Langdon [ 41] studied the effect of grain size on the grain size exponent.The small grains promote the grain boundary sliding.So,with the increase in the grain size,the grain size exponent decreased.From the flow stress with strain of 0.02 and strain of 0.45 of TEST-5,the grain size exponent was estimated asμ=0.71.

In Fig.5,it can be found that the flow stresses of asreceived sheet,HT-1 and HT-2 kept stable,while the flow stress-strain curves of HT-3,HT-4 and HT-5 presented strength softening.What factors lead to the strength softening of the Ti₂AlNb alloy at elevated temperature?In the previous study [ 34] ,the microstructure,including grain size,phase content and grain shape,kept steady during the hot deformation for as-received Ti₂AlNb sheet with strain rate of 0.001 s^-1.In Figs.7 and 10,the microstructure of TEST-1 and TETS-2 also kept steady during the hot deformation.So,the flow stress of as-received sheet,HT-1and HT-2 kept stable.For the TEST-3 specimen,the O-phase content decreased and most of the lamellar O-phase grains changed to the equiaxed grains.For TEST-4 and TEST-5,the B2/β-phase mean grain size decreased and plenty of the lamellar O-phase grains changed to the equiaxed grains during the hot deformation.Therefore,the softening mechanism mainly consists of grains refinement,O-phase content reduction and globularization of O-phase lamellar grains.

5 Conclusion

The microstructures of various heat-treated specimens were significantly different.The hot formability and microstructure of the as-received rolled Ti₂AlNb sheet were better than those of the heat-treated specimens.The as-received sheet consisted of equiaxedα₂/O,matrix B2/βand lamellar O grains;most of lamellar O-phase grains were transformed to B2/β-grains after heating at 970℃;after aging at 900℃for a long time,most of theα₂ phase grains were transformed to O-phase grains and the thicknesses of lamellar O-phase grains significant increased;thecoarse single B2-phase grains were gotten by the solution treatment at 1100℃;plenty of fine O-phase lamellar grains was generated in the coarse B2 grains after the solution and aging treatment.

The Ti₂AlNb alloy was a typical metastable metal at high temperature,where the plastic deformation promoted the phase transition of O→B2/βand the globularization of O-phase lamellar grains at 970℃.The flow stress was obviously affected by the grain sizes,grain shapes and content ofα₂,O and B2/βphase.Calculated by the creep equation and the iso-stress method,the grain size exponent wasμ=1.1 and the relationship between the material constants of B2/βand O phase was K_O=1.14K_B2/β.For the specimens with the unusually coarse grains,the grain size exponent was estimated asμ=0.71.Meanwhile,with the increase in globularization of O-phase lamellar grains,the flow stress of Ti₂AlNb alloy decreased.