Microstructure and hot flow stress at 970℃ of various heat-treated Ti2AlNb sheets
来源期刊:Rare Metals2020年第6期
论文作者:Yong Wu Rong-Lei Fan Xian-Jun Zhou Ming-He Chen
文章页码:695 - 706
摘 要: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 KO=1.14KB2/β.
稀有金属(英文版) 2020,39(06),695-706
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);
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 KO=1.14KB2/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
For Ti2AlNb alloy,the grain shapes and phase transition among theα2,B2/βand O-phase are very complex during the hot working processes
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
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.Td 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
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
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.
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
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°)
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)
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
whereεp is the steady-state strain rate,D0 is the appropriate diffusion coefficient,G is the shear modulus,b is the Burgers vector,kb 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℃
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
In some two-phase Ti-alloy deformation studies
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 fB2/βand fo are the contents of B2/βand O phase,
where dB2/β,0 and dO,0 are the mean grain sizes of B2/βand O phases at un-deformation position for TEST-0,dB2/βand do are the mean grain sizes at the deformation position,KB2/βand KO 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
Fig.12 Relationship between flow stress and sub-grain/grain size for heat-treated specimens:a B2/βphase and b O phase (dB2 and dO being mean grain size of B2 and O phase of undeformed region for tested specimen,respectively;dTEST-0,B2 and dTEST-0,O being mean grain size of B2and O phase of undeformed region for TEST-0,which is reference substance;σandσTEST-0 being flow stress of tested specimen and TEST-0,respectively;n being the stress exponent;fvolume,B2 and fvolume,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 (σ2%) 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
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
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 KO=1.14KB2/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.
参考文献