1.Introduction

With the rapid development of modern aerospace industry,the demands for high strength and high plasticity structural materials are increasing in all fields.High strength β titanium alloys have high specific strength,corrosion resistance and good resistance to fatigue crack propagation[1,2].Therefore,the high strength β titanium alloy has become a potential and attractive aerospace structural material[3–5].The high strength β titanium alloy has the advantage of rollability and that the grains can be refined by cold deformation and recrystallization heat treatment[6].The refinement of the β phase grain size is beneficial to improve the plasticity of the aged titanium alloy[7].Cai et al.[8]found that the β grain size of Ti-16V-3.5Al-3Sn alloy can be reduced to 1.3–30µm after cold rolling and annealing.During cold rolling and subsequent recrystallization heat treatment,the β titanium alloy will form textures which can affect the anisotropy of the mechanical properties of the alloys[9,10].

Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe is a high strength β titanium alloy with excellent matching of strength and ductility.Previous studies have focused on the effects of heat treatment and hot deformation on microstructure and mechanical properties of this alloy[11–13].The influences of cold deformation strain path and recrystallization heat treatment have not been studied yet.In this study,the effects of unidirectional and two-step cross cold rolling on microstructure and properties of the Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy were studied.The effects of subsequent recrystallization heat treatment on the anisotropy were also investigated.The purpose of this study is to optimize the cold rolling and heat treatment process of high strength β titanium alloy,and provide theoretical guidance for the development and application of high strength β titanium alloy.

2.Experimental details

The investigations were conducted on a high strength β titanium alloy,which nominal composition is Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe.The alloy was melted using double vacuum consumable arc melting process to obtain an original ingot.The chemical composition analysis was performed at top and bottom of the ingot and the average values of compositions were listed in Table 1.The β transition temperature of the alloy measured by metallographic method is 815±10°C.The ingot was forged at 950°C and then hot rolled at 850°C to produce an alloy sheet having a thickness of 2 mm,then annealing at 800°C for 30 min.The annealed hot rolled microstructure is a coarse grain structure and shown in Fig.1(a).There are two types of cold rolling processes:uni-directional rolling(UDR)and two-step cross rolling(TCR).Rolling process schematic was shown in Fig.1(b).The annealing temperature was chosen at 800°C.After several passes of rolling,two cold rolled sheets with thicknesses of 0.8 mm were obtained finally.

从图4中可以看出，Sentinel-2数据所计算出的表层水分含量指数结果具有较好的地物细节特征，特别是农田地块信息完整，较MODIS模型产品结果大幅提高，空间分辨率20 m适合有针对性的农田干旱监测。

Table 1 Chemical composition of experimental alloy mass fraction/%).

Al Mo V Cr Sn Fe Ti 3.39 5.21 6.00 3.11 2.52 0.44 Bal.

Fig.1.(a)Microstructure of annealed hot rolled;(b)schematic diagram of rolling processes.

The samples for microstructure observation were taken in the center area of the cold rolled sheets of the two processes,and three direction tensile samples were taken on the cold rolled sheets.Fig.2(a)is a sampling schematic.Respectively with the final rolling direction(RD)was 0°,45°,90°angle.The samples were solution treated(ST)at 830°C for 2 min and air cooled.The solution treated samples then aged(STA)at 550°C for 4 h and air cooled.Texture and crystallographic structure measurements were carried out using Rigaku Smartlab type X-ray diffraction(XRD).Quanta FEG 650 type field emission scanning electron microscopy(SEM)was used to characterize the microstructure and fracture microstructure of heat treated samples.The electron backscattered diffraction(EBSD)analysis was conducted using Quanta FEG 650 type field emission scanning electron microscope equipped with an Oxford Instruments Nordlys nano EBSD detector.Flat tensile samples with dimensions shown in Fig.2(b)were performed on Instron 5500 R testing machine at room temperature,driven at crosshead speed of 1 mm/min.

3.Results

3.1.Evolution of microstructure and texture

After 2 min short time solution treatment,the samples inherited the cold-rolled deformation texture which will have a certain influence on the final texture and produce anisotropy of properties. The recrystallized texture and the deformed texture are the same type,and only the strength was weakened.Several studies have found the similar results[9,21].Another important aspect that needs to be considered is the control of the energy storage of the various texture components that recrystallization nucleation.During plastic deformation,part of the total energy is stored in the crystal in the form of elastic energy of the dislocation strain field.Depending on the orientation,different grains have different stored energy.For the rolling of BCC metals,the order of energy storage is {111}<211>, {111}<110>, and{100}<110>[24].The first two are γ-fibers,while the latter is αfiber.From this aspect,it can also explain the reason that why the UDR samples recrystallization speed is faster.

Fig.4 shows the EBSD results of the alloy after different rolling processes. The white lines in the maps represent low-angle grain boundaries(misorientation angle greater than 2°and less than 15°)and the black lines represent high angle grain boundaries(misorientation angle greater than 15°).Fig.4(a)and(b)show mostly the same color,reflecting the similar orientation of the two rolling processes.Interestingly,the characteristics of the low-angle grain boundaries of the two rolling processes were significantly different.For UDR sample,the lowangle grain boundaries were nearly parallel to the transverse direction(TD).In contrast,the low-angle grain boundaries of the TCR sample were both parallel to the TD direction and partly parallel to the RD direction.Fig.4(c)and(d)are the recrystallized fraction maps,in which blue represents a completely recrystallized grain,red represents a deformed grain,and yellow represents a substructure.Through observation,it can be found that the samples composes a large number of deformed structures and substructures,and also a small number of fine recrystallized grains.The presence of recrystallized grains may be due to the occurrence of dynamic recrystallization during cold deformation.

The alloy annealed for a long time before cold rolling deformation will result in grain orientation in a random orientation,and there is no macrotexture in the initial state.Fig.11 shows orientation line of ODFs at different rolling process.During the cold rolling process,the orientation of the BCC metal grains continuously accumulates on the stable orientation line[21].All orientations on α orientation line have a<110>direction parallel to the RD,called the α-fiber.And all orientations on the γ orientation line have a<111>direction parallel to the TD,called the γ-fiber.Both of them have a great influence on the anisotropy of the various properties of the alloy,so their regularity of change has received widespread attention[22,23].In this study,the alloy samples have strong α-fiber and γ-fiber formation during cold deformation. The UDR samples were stronger in the{211}<110>direction, whereas the strong texture of the TCR samples tended to turn to{111}<110>.It shows that in the UDR samples the {211}<110>fiber continued to increase with the number of rolling passes.In the TCR samples,the texture produced by the first step rolling was corrected by the second rolling step,resulting in an increase in the strength of{111}<110>fiber.

Fig.2.Tensile samples(a)sampling position and(b)shape size.

Fig.3.Microstructure of the alloy after different rolling processes:(a)UDR process;(b)TCR process.

Fig.4.EBSD results of different rolling processes for the alloy:(a,b)UDR and TCR orientation maps;(c,d)UDR and TCR recrystallized fraction maps.

Fig.8 shows the microstructure of the two types rolling alloy sheets after STA.It can be observed that a large number of small needle-like α phases precipitated in the β matrix.There are many reasons to affect the size and morphology of the α phase,such as the grain size and dislocation density of β phase[14,15].The decrease of α phase size and the increase of volume fraction can significantly increase the strength of the alloy[16].

3.2.Microstructure and mechanical properties

Fig.7 shows the XRD patterns of the alloy in different states.It can be clearly seen from the patterns that the two rolling processes had the same phase composition in the same state.The phase of the as-rolled samples consisted of β phase matrix and a very small amount of α phase,and no strain-induced α"phase and ω phase were observed.After solution treatment at 830°C for 2 min,only the β phase peaks were observed in the(110),(200)and(211)planes.It indicates that after solution treatment at 830°C for 2 min,the samples were completely transformed into β phase.After aging treatment at 550°C for 4 h,a large amount of α phase has precipitated.

Fig.6 shows that the φ2=45°orientation distribution function(ODF)maps of the alloy for different rolling processes.The ODFs were calculated and plotted using pole figure data obtained by XRD.The most relevant ODF sections for BCC metals corresponded to φ2=45°.It can be observed that the samples after two cold rolling processes both had γ-fiber as the main deformation texture.The overall texture intensity in UDR process sample was higher and sharper,including the γfiber: {111}<110>, {111}<123>, {111}<112>textures and α-fibers{001}<100>,{112}<110>.After solution treatment,there were still γ-fiber textures,but it can be seen that the texture passivated and the texture strength reduced,leading to a significant reduction in the anisotropy of the alloy.

Fig.5.EBSD results of different rolling processes for the alloy after solution treatment:(a,b)UDR and TCR orientation maps;(c,d)UDR and TCR recrystallized fraction maps.

Fig.6.φ2=45°orientation distribution function maps of the alloy for different rolling process:(a)UDR as-rolled;(b)TCR as-rolled;(c)UDR at 830°C for 2 min solution treatment;(d)TCR at 830°C for 2 min solution treatment.

研究对象为5台双西门子SINUMERIK 828D系统的数控机床，因此总的监视容量为10套SINUMERIK 828D系统。同时机床监控系统要求系统的实时性强，因此选择了如下方案：整个系统采用C/S架构，采用面向对象的C#软件作为上位界面的开发软件，采用OPC UA通信协议作为系统数据采集以及数控系统与上位机之间的通信协议，选择SQL Server 2008作为服务器上的数据库软件，其他的客户端通过实时访问服务器的数据库实现客户端与服务器之间的数据同步。其主要的功能模块可分为数据采集单元配置、数据采集、数据同步通信、故障报警以及数据分析等，具体如图1所示。

4.Discussion

4.1.Microstructure

Because of the non-closepacked structure of the BCC metal,it is a structure that is easy to slip.However,the BCC structural metal slip plane is not stable,usually{211}at low temperature,{110}at medium temperature and{321}at high temperature,but the slip direction is very stable,always<111>[17].Therefore,it may have 12–48 slip systems.Normally,{211}<111>and{110}<111>are used as main slip systems during cold deformation of high strength β titanium alloys.In this study,a large number of low-angle grain boundaries were observed in EBSD orientation maps(Fig.4).The structure of the lowangle grain boundary may be regarded as a series of dislocations[18].It can reflect the formation process of dislocations after different cold rolling processes.Fig.10 shows the diagram of dislocation distribution during cold rolling.When a high strength β titanium alloy was rolled uni-directionally,a series of parallel dislocations appeared within the grain,which indicates that the individual crystal grain may be dominated by single system slip.When the rolling direction is not changed at second step,dislocations parallel to each other continue to increase.However,dislocations with different directions were formed in the TCR samples,reflecting the different deformation model of different strain paths in alloy.

Fig.7.XRD patterns of the alloy at different states:(a)UDR process;(b)TCR process.

Due to the large amount of cold deformation of the high strength β titanium alloy,the nucleation mechanism by bulging of initial grain boundaries is not obvious,and the recrystallized nucleus is generated by the sub-crystalline nucleus mechanism[19].In this study,with a large deformation reduction,recrystallization directly takes advantage of the subgrains inside the grains as nucleation sites.After cold rolling,alloy samples formed a large number of low-angle grain boundaries,which can also be seen as the formation of many fine subgrains.The dislocation networks on the adjacent subgrain boundaries with smaller misorientations dissociate,dissipate and transfer to other subgrain boundaries.And as they continue to displace dislocations onto the new subgrain boundary,they gradually turn into high angle grain boundaries,then forming new recrystallized grains[20].

In order to prevent grain growth,solution treatment time was shortened as much as possible.By observing the 2 min solution treatment of the EBSD maps(Fig.4),some features can be found.The distribution of low-angle grain boundaries indicates that the strain distribution of TCR samples is more non-uniform than that of UDR samples during cold rolling.According to the sub-crystalline nucleation mechanism[19],TCR is more likely to form high angle grain boundaries,so it is easier to recrystallize nucleation and leading to a faster recrystallization process.

Fig.9 shows the mechanical properties of alloy after two rolling processes and STA.As shown in Fig.9(a),the mechanical properties of the as-rolled alloy had strong anisotropy.For the UDR process,the ultimate tensile strength and elongation were at maximum when the tensile direction was parallel to RD.With increase of the tensile direction from RD to TD,the elongation decreased sharply.On the contrary,after TCR process,the anisotropy of the mechanical properties of the alloy was not obvious.However,compared with the UDR process,the tensile strength of all tensile directions was higher and the elongation was lower expect when tensile direction parallel to RD.After STA,the tensile strength and elongation both increased,and the anisotropy of mechanical properties of alloy was weakened,as shown in Fig.9(b).For the UDR process,it can be found that the strength and elongation was the lowest when the tensile direction was at an angle of 45°with the rolling direction,.On the other hand,the mechanical properties after STA of the TCR samples in different tensile direction did not change significantly than UDR samples.

As shown in Fig.9(a),it can be found that as-rolled UDR samples have significant anisotropy.Due to the fact that grains are elongated along the final rolling direction,the plasticity decreases significantly with the increase of the angle between the tensile direction and the rolling direction.During the solution treatment,the deformed grains undergo recrystallization,resulting in the refinement of the β grains,and hence the mechanical properties of the samples are improved significantly.At the same time,the recrystallization weakened the anisotropy and texture of the alloy,as shown in Fig.6(c)and(d).After STA treatment,the alloy precipitated a large number of fine needle-like α phases,which is the main strengthening method of β titanium alloy,and therefore the strength of the alloy after STA is significantly improved(Fig.9(b)).Moreover,the UDR process still has anisotropy after aging treatment.Compared to UDR sample,the anisotropy of the tensile properties is relatively weaker,and the TCR samples have similar elongation and tensile strength in different tensile directions.This phenomenon may be due to the fact that the β grains of TCR samples are more uniform than the UDR samples after the short time solution treatment,resulted in non-uniform performance.Since the thickness of the alloy sheets are only 0.8 mm,the thickness of the sheet is much smaller than the dimensions of the other two directions.Therefore,the plane stress state exists in the thickness direction no matter how the tensile stress is applied.The decrease of the thickness and the volume of the plastic deformation area of the alloy will lead to weaker structural stability[25].

4.2.Texture

Fig.5 shows the EBSD results of the alloy after solution treated at 830°C for 2 min Fig.5(a)and(b)show orientation maps,and the same color in the maps indicates the same crystal orientation.Compared with cold rolled microstructure and annealed hot rolled microstructure,the grain refinement was obvious,that the grain size reduced from 500µm to about 50µm.By comparing the grain microstructure of the two rolling processes after short time solution treatment,it can be found that the β grains of the TCR sample were more uniform.As can be seen from the UDR recrystallization fraction map(Fig.5(c))that there were a small number of red and yellow regions,which indicating the presence of deformed and incompletely recrystallized microstructures.However,there was no red area appears in the TCR recrystallization fraction map(Fig.5(d)),indicating that the deformed structure undergone complete recrystallization to form equiaxed β grains after 2 min solution treatment.After solution treatment,the UDR sample still had a certain amount of low-angle grain boundaries,while the lowangle grain boundaries disappeared in TCR sample.

近日，据媒体报道，山东济南又有数十名大学生落入“培训贷”陷阱。原本是为了找工作，结果公司直接把他们的信息输入一个第三方小额贷款平台，导致每个人背上了从9800元到19800元不等的贷款。

Fig.3 shows the microstructure of the alloy after cold rolling in two different processes.During the rolling process,the process annealing temperature is 800°C in the α+β field,therefore the microstructure of as-rolled alloy is mainly included deformed β matrix and a small number of α phase.The microstructure observation of uni-directional rolling(UDR)process sample(Fig.3(a))showed that the β grains were significantly elongated along the rolling direction,but this phenomenon was not more obvious than two-step cross rolling(TCR)process sample(Fig.3(b)).It can be found that there were fine new β grains at the grain boundaries between the deformed β grains.This may be due to the process annealing to recrystallize partially deformed grains.

4.3.Mechanical properties

Fig.8.microstructure of the two types rolling alloy sheets after STA:(a)UDR process;(b)TCR process.

Fig.9.Mechanical properties of the alloy(a)as-rolled;(b)after STA.

Fig.10.Diagram of dislocation distribution during different cold rolling process.

孟导是个行动派，当天晚上就联系了老家，虽然花了半小时应付老家那边催他结婚延续香火的唠叨。总算说服让老家人把那盒子钱原封不动地给他邮了过来。

5.Conclusions

Fig.11.Orientation line of ODFs at different rolling process:(a)α-fiber and(b)γ-fiber.

The microstructure and mechanical properties of high strength Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy sheets prepared by uni-directional cold rolling and two-step cross cold have been investigated.The main results are summarized as follows:

1.The deformation textures of the alloy cold rolled sheets after two cold rolling processes are mainly γ-fiber of {111}<110>,{111}<123>, {111}<112>, and weaker α-fiber of{001}<100>, {112}<110>. After solution treatment at 830°C for a short time,the texture is passivated as well as the strength decreased,the anisotropy of the alloy significantly reduced.

2.The β grains of cold-rolled alloy sheets can be significantly refined by short time solution treatment.The β grains of uni-directional rolling process are non-uniform and the degree of recrystallization is incomplete.The two-step cross cold rolling process exhibits higher degree of recrystallization and more uniform β grains.

回顾改革开放40年中国城镇住房供给侧结构性变迁的历史演进及其客观规律是为了更加科学地全面深化新时代中国特色社会主义住房供给侧改革，构建中国特色社会主义住房供给制度。笔者认为，这包括三个方面内容：一是明晰新时代住房供给侧改革的总体思路，二是提出新时代民生改善的基本方略，三是提出防范新时代住房金融风险的根本举措。

3.Compared with as-rolled alloy,the anisotropy of the cold-rolled alloy sheets can be weakened after solution and aging heat treatment,and the anisotropy of uni-directional rolling process more obvious than that of two-step cross cold rolling process.

Acknowledgment

This work was financially supported by National Natural Science Foundation of China(Grant No.51601099 and Grant No.51861029),Natural Science Foundation of Inner Mongolia Autonomous Region of China (Grant No. 2016BS0506), Inner Mongolia University of Technology(Grant No.ZD201607),Scientific Research Foundation of The Higher Education Institutions (SRFHEI) of Inner Mongolia Autonomous Region of China (Grant No. NJZY18096) and Inner Mongolia Autonomous Region Science and Technology Innovation Guide Project.

上述结果表明，向日葵的产量、千粒重和籽仁率随着耕作方式的增加表现增加，千粒重、籽仁率随着密度的增加而减小。在深松45 cm（S45）和密度在60 000株/hm2（D3）时产量最高，而千粒重和籽仁率最低，说明深松可以有效改善由于密度引起的拥挤效应，从而提高产量。

References

[1] S.L.Nyakana,J.C.Fanning,R.R.Boyer,J.Mater.Eng.Perform.14(2005)799–811.

[2] D.Banerjeea,J.C.Williams,Acta Mater.61(2013)844–879.

[3] R.R.Boyer,R.D.Briggs,R.R.Boyer,S.Tamirisakandala,P.Russo,N.Shchetnikov,J.C.Fanning,J.Mater.Eng.Perform.14(2005)681–685.

[4] J.D.Cotton,R.D.Briggs,R.R.Boyer,JOM 67(2015)1281–1303.

[5] R.Santhosh,M.Geetha,M.N.Rao,Trans.Indian Inst.Met.70(2016)1–8.

[6] O.M.Ivasishin,P.E.Markovsky,Yu.V.Matviychuk,S.L.Semiatin,C.H.Ward,S.Fox,J.Alloy.Compd.457(2008)296–309.

[7] P.E.Markovsky,V.I.Bondarchuk,O.M.Herasymchuk,Mater.Sci.Eng.A 645(2015)150–162.

[8] M.H.Cai,C.Y.Lee,Y.K.Lee,Scr.Mater.66(2012)606–609.

[9] N.P.Gurao,A.A.Ashkar,S.Suwas,Mater.Sci.Eng.A 504(2009)24–35.

[10] T.W.Xu,J.S.Li,S.S.Zhang,F.S.Zhang,X.H.Liu,J.Alloy.Compd.682(2016)404–411.

[11] Y.Y.Chen,Z.X.Du,S.L.Xiao,L.J.Xu,J.Tian,J.Alloy.Compd.586(2014)588–592.

[12] Z.X.Du,S.L.Xiao,L.J.Xu,J.Tian,F.T.Kong,Y.Y.Chen,Mater.Des.55(2014)183–190.

[13] Z.X.Du,S.L.Xiao,Y.P.Shen,J.S.Liu,J.Liu,L.J.Xu,F.T.Kong,Y.Y.Chen,Mater.Sci.Eng.A 631(2015)67–74.

[14] Z.Y.Song,Q.Y.Sun,L.Xiao,J.Sun,L.C.Zhang,Mater.Sci.Eng.A 528(2011)4111–4114.

15 T.Li,D.Kent,G.Sha,M.S.Dargusch,J.M.Cairney,Mater.Sci.Eng.A 605(2014)144–150.

[16] A.Dehghan-Manshadi,R.J.Dippenaar,Mater.Sci.Eng.A 528(2011)1833–1839.

[17] H.J.Bunge,Texture Analysis in Materials Science:Mathematical Methods,Elsevier,2013.

[18] P.Moretti,M.C.Miguel,M.Zaiser,S.Zapperi,Phys.Rev.B 68(2003)1681–1685.

[19] D.A.Hughes,N.Hansen,Acta Mater.45(1997)3871–3886.

[20] P.R.Rios,F.S.Jr,H.R.Z.Sandim,R.L.Plaut,A.F.Padilha,Mater.Res.8(2005)225–238.

[21] A.K.Singh,A.Bhattacharjee,A.K.Gogia,Mater.Sci.Eng.A 270(1999)225–230.

[22] K.Cho,M.Niinomi,M.Nakai,H.H.Liu,P.F.Santos,Y.Itoh,M.Ikeda,M.Gepreel,T.Narushima,J.Alloy.Compd.664(2016)272–283.

[23] Z.S.Zhu,J.L.Gu,R.Y.Liu,N.P.Chen,M.G.Yan,Mater.Sci.Eng.A 280(2000)199–203.

[24] A.Sametmeziou,A.L.Etter,T.Baudin,R.Penelle,Mater.Sci.Forum 558–559(2007)323–328.

[25] H.Nasiri-Abarbekoh,A.Ekrami,A.A.Ziaei-Moayyed,Mater.Des.44(2013)528–534.