Interface characteristic and mechanical performance of TiAl/Ti2AlNb diffusion bonding joint with pure Ti interlayer
State Key Laboratory of Solidification Processing,Northwestern Polytechnical University
Shaanxi Key Laboratory of High-Performance Precision Forming Technology and Equipment,Northwestern Polytechnical University
作者简介：*Bin Tang,e-mail:toby@nwpu.edu.cn;
收稿日期：25 March 2020
基金：financially supported by the National Natural Science Foundation of China (No.51771150);the National Key Research and Development Program of China (No. 2016YFB0701303);the Aeronautical Science Foundation of China (No.201936053001);the Research Fund of the State Key Laboratory of Solidification (NWPU),China (No.2019-TS-07);
Interface characteristic and mechanical performance of TiAl/Ti2AlNb diffusion bonding joint with pure Ti interlayer
Lei Zhu Bin Tang Ming-Xuan Ding Yan Liu Xiao-Fei Chen Shao-Peng Yan Jin-Shan Li
State Key Laboratory of Solidification Processing,Northwestern Polytechnical University
Shaanxi Key Laboratory of High-Performance Precision Forming Technology and Equipment,Northwestern Polytechnical University
Abstract：
Solid-state diffusion bonding(DB) of TiAl alloy and Ti2 AlNb alloy was carried out using pure Ti as an interlayer at 1000℃ under 20 MPa for 60-120 min.The effects of bonding times on the interfacial microstructure and mechanical performance of the TiAl/Ti/Ti2 AlNb bonded joints at room temperature(RT) were investigated detailly.The results demonstrated that the diffusion layers(DLs) mainly consisted of four characteristic layers,(Ⅰ) single coarse α2 phase adjacent TiAl alloy,(Ⅱ) single refined α2 phase at the bonding interface,(Ⅲ) equiaxed/acicular α2 phase embedded in β phase adjacent Ti2 AtNb alloy and(IV) both equiaxed α2 phase and acicular O phase embedded in β phase adj acent Ti2 AlNb alloy,respectively.The thickness of the four layers increased with the increasing of the bonding time.The growth of DLs is controlled by diffusion and the reaction rate constant k for region Ⅰ,Ⅱ,Ⅲ and Ⅳ are 1.22×10-6,1.27×10-6,2.6×10-7 and 7.7×10-7 m·s-1/2,respectively.Meanwhile,the interface α2 grain grows up without texture.The maximum tensile strength of 281 MPa was maintained at1000℃ for 90 min under the pressure of 20 MPa.Consequently,the phase transformation and dynamic recrystallization behavior of the DLs were discussed.
Keyword：
Sputtering; Bonding interface; Microstructure; Growth kinetics; Mechanical performance;
Received： 25 March 2020
1 Introduction
As one of pivotal high-temperature structure engineering material,TiAl-based alloy has been used in aerospace and auto industry for several years due to its attractive"strong and light"properties,such as excellent high-temperature strength,low density,good creep and outstanding oxidation resistance
On the other hand,there are still many difficulties for direct diffusion bonding dissimilar intermetallic compounds on account of the different thermal expansion coefficients and chemical compositions.Accordingly,it is difficult to bond the dissimilar intermetallic compounds without defects and internal stresses.Therefore,to inhibit the formation of harmful IMCs and reduce the residual stress at the bonding interface,the addition of a proper interlayer was conducted to obtain a resultant joint
The objective of this present work is to investigate the influences of bonding time on the interfacial microstructures and the mechanical properties of the TiAl/Ti/Ti2AlNb bonding joints.Besides,the feasibility of diffusion bonding of TiAl alloy and Ti2AlNb alloy using pure Ti as an interlayer,which is essential for enlarging their engineering applications,was also evaluated.The relationship between the microstructural evolution and the mechanical performance of the bonding joints at room temperature (RT) was investigated.Moreover,the dynamic recrystallization and phase transformation behavior were discussed.Finally,diffusion bonding mechanism was analyzed based on the microstructural observation.
2 Experimental
The ingot of TiAl alloy was manufactured by vacuum arc remelting (VAR) three times following with hot isostatic pressing (HIP) at 1280℃for 4 h under the pressure of140 MPa.After HIP,the ingot was forged at 1170℃to obtain a forging pancake with a size ofΦ420 mm×100mm.The initial microstructure with equiaxed structure is displayed in Fig.1a,including theβ/B2 phase (bright),α2phase (gray) andγphase (dark),as given in Fig.1c.The actual chemical composition is shown in Table 1.
The Ti2AlNb alloy with a nominal composition of Ti-22Al-25Nb (at%) used in this work was fabricated by triple VAR,and then,the ingot was free forged by a multi-pass forging process.The size of the forging bar wasΦ220 mm×500 mm finally.The forging microstructure of the Ti2AlNb alloy is shown in Fig.1b.The microstructure consists of gray rim O phase around the darkest ovalα2 phase and the gray acicular O phase enmeshed in the brightestβ/B2 phase.Figure 1d also demonstrates X-ray diffraction (XRD) pattern of Ti2AlNb,including O phase,α2 phase andβ/B2 phase.The detailed chemical composition is also shown in Table 1.
The forging TiAl alloy and Ti2AlNb alloy were machined by electric-discharge machining into cuboids with the dimensions of 50 mm×30 mm×20 mm.The faying surface was grounded subtly followed by mechanical polished using 1.5 mesh abrasive paste,and the asreceived specimens were ultrasonic cleaned in alcohol for～5 min to clean the faying surface.Finally,the samples were vacuum encapsulated for diffusion bonding.The interlayer of pure Ti was deposited by DC magnetron sputtering.The total thickness of the interlayer was about10μm,which was composed of two 5μm layers sputtering on the faying surface of two substrates.
Diffusion bonding was taken place in a vacuum diffusion bonding machine (ZC-ZK YL40) at 1000℃and20 MPa for different bonding time of 60-120 min.The selection of temperature and time were based on the previous research results
The microstructure of the interface was characterized by scanning electron microscope (SEM,TESCAN VEGA3)with electron back-scattered diffraction (EBSD),and the dispersion of elements in the diffusion layers (DLs) and metallic compounds was measured by electron probe micro-analyzer (EPMA,JEOL JXA-8230).Notably,the schematic diagram of the coordinate system in EBSDanalysis is shown in Fig.3a.The specimens for EBSDwere produced by the once mechanical grinding and electropolishing on a Struers Lectropol-5 (Denmark,Struers Corporation) in a solution of 15 vol%n-butyl alcohol,5vol%perchloric acid and 80 vol%methanol with a voltage of 35 V holding for 30 s.EBSD analytical test was executed with a scan step size of 0.2μm to obtain the detailed orientation information of the interface,the geometrically necessary dislocations (GND) maps from EBSD data were calculated by using the software named HKL Channel 5.Transmission electron microscopy (TEM) measurements were conducted at Themis Z Double Cs Corrector microscope operated at 300 kV.TEM specimens were preparedby focused ion/electron double beam electron microscopy(FIB).Moreover,to assess the joints quality,a nonstandard tensile test was designed to appraise the tensile properties of the joints in an electronic universal testing machine controlled by microcomputer (SUNS) with an initial strain rate 1×10-4 s-1 at RT.Figure 3b shows the schematic diagram of nonstandard tensile specimens whose bonding interface is situated in the middle of the tensile samples.
Fig.1 SEM images of based materials in BSE mode:a TiAl alloy and b Ti2AlNb alloy;XRD patterns of c TiAl alloy and d Ti2AlNb alloy
Table 1 Chemical compositions of TiAl and Ti2AlNb alloys (at%)
Fig.2 Curve of diffusion bonding process
3 Results and discussion
diffusion behavior of bonding interface
3.1 Microstructure characteristic and elemental
Figure 4a,b shows the typical micros true ture of TiAl/Ti2AlNb bonding interface without pure Ti interlayer andwith pure Ti interlayer at 1000°C-20 MPa-90 min,respectively.It is self-evident that there exists a discon?tinuous equiaxed white grain produced at the bonded interface without an interlayer,which is similar to our previous research
Fig.3 Schematic diagram of EBSD and mechanical performance test:a EBSD test coordinate system and b tensile specimen
Fig.4 Typical SEM images of TiAl/Ti2AlNb bonded interface at 1000℃-20 MPa-90 min:a without an interlayer and b with pure Ti interlayer
Moreover,two layers are produced at the bonding interface from SEM image.One is a single gray strip layeradjacent TiAl alloy,and the other is a layer containing white band inlaid with gray needles and black equiaxed phase near the Ti2AlNb alloy.Figure 5 shows the line and plane scan of the major element across the bonding interface at 1000℃-20 MPa-90 min.There is an interdiffusion of major elements obviously across the bonding interface,as shown in Fig.5a.Figure 5b shows the plane distribution of the major elements within the white rectangle.There is an obvious enrichment of Nb element at the bonding interface,which is the main reason for the formation of"beach"micros true ture at the interface between the two DLs.In addition,the thickness of the DLs is approximately twice as thick as the joint without an interlayer approximately,as shown in Fig.4.This will be discussed in Sect.3.3.
Table 2 Elemental quantitative analysis results of points in Fig.4b determined by EMPA
To further confirm the products within DLs,EBSDtechnology was implemented to research the crystallographic characterization of the interfacial microstructure.The phase distribution of the bonded joint is shown in Fig.6a,where theγphase is presented as blue,βphase as yellow,α2 phase as red and O phase as green,respectively.Also,the low angle grain boundaries (LAGBs) were measured with a misorientation (θ) of grain boundaries between 2°and 15°(light blue line),and theθvalue of high angle grain boundaries (HAGBs) is higher than 15°(black line).The boundaries with 57°<θ<63°are considered as twin boundaries (TB)(purple line).One can see that both gray acicular phase and gray equiaxed phase (Points 3and 4) areα2 phases.There is almost no LAGB at the bonding interface.It proves that dynamic recrystallization(DRX) occurs on the interfacialα2 phase ultimately.It is conducive to dislocation movement and the evolution of the contact interface,and the density of dislocation at the DRX region is reduced as the corresponding GND map shows in Fig.6b.The similar phenomenon has also occurred in the bonding of Ti alloy
Fig.5 EPMA analysis of major elements across bonding interface at 1000℃-20 MPa-90 min:a line scan of black line and b plane distribution within white rectangle
Furthermore,the grain size of the interfacialα2 phase is perse.Scilicet,the grain size of theα2 phase in the middle (I-α2) of the bonding interface is the smallest,on account of being induced by DC magnetron sputtering.The grain size of theα2 phase adjacent to Ti2AlNb alloy(Ti2AlNb-α2) is larger than that of the intermediateα2phase.The grain size of theα2 phase adjacent to TiAl alloy(TiAl-α2) is the largest.The I-α2 is equiaxed grain,while the Ti2AlNb-α2 and TiAl-α2 are elongated grains (Fig.6a).It can be revealed that the crystal nucleates at the I-α2 and grows toward the two based alloys with the process of the bonding.This behavior is the criterion to measure the thickness of the DLs in EBSD maps,following which the thickness of every reaction layer was measured.The diffusion direction of the interlayer Ti atom is reciprocal to the two based alloy at the initial stage and tend to be gentle gradually,as shown in Fig.7c.In particular,the phase transformation is found at the side of Ti2AlNb based alloy,and the newα2 phase is always close to the O phase,asshown by black rectangle region (A and B) in Fig.6a.This phenomenon indicates that lots of O phases form thisα2phase.The orientation relationship between equiaxed/acicularα2 phase and O phase can be described by
Figure 7 shows TEM bright-filed image of the hightemperature TiAl/Ti2AlNb bonding interface,in which the black line shows the recrystallized grain of I-α2 interlayer.To further study the I-α2 interlayer,the magnification of region 1 in Fig.7a was conducted.Figure 7b shows the corresponding HRTEM image,there is no other nanophase in I-α2 interlayer based on fast Fourier transformation(FFT) patterns of Regions A,B and C.
Fig.6 EBSD maps of bonding interface at 1000℃-20 MPa-90 min:a phases map,b GND map,and c,d pole figures
Fig.7 TEM analysis of bonding interface:a TEM bright-field micrograph and b HRTEM image of Region 1 (insets being FFT images);c line scan of green area in a
Besides,to determine the direction of interface growth during the diffusion bonding,the grain orientation distribution of TiAl/Ti2AlNb joint is shown in Fig.8.Figure 8a-c shows the grain orientation distribution of normal direction (ND),bonding direction (BD) and transverse direction (TD) at the bonding interface,respectively.According to Fig.8,it can be found out that there exists hardly preferred orientation of the grains across the binding interface along with ND,BD or TD directions.Moreover,the full pole figures (Fig.9a) and inverse pole figures (Fig.9b) ofα2 grains in the bonding interface were investigated,which shows that the texture of interfacialα2grains is free.The grain grows freely during the bonding due to the effect of nanointerlayer,i.e.,there are a number of grain boundaries at the bonding interface to furnish the vacancy and promote the diffusion path for atoms.In addition,when the grain boundary is parallel to the stress,the stress direction is identical to the BD during the diffusion bonding,and the formation energy of the vacancy is high.Nevertheless,the formation energy of the vacancy is lower as the grain boundary is perpendicular to stress.Hence,the vacancy will assemble along the BD.Consequently,both bar-like grains and sawtooth phase interface of Ti2AlNb-α2 and TiAl-α2 are formed at the bonding interface,respectively.It can be inferred that the growth of the bonding interface is controlled by vacancy diffusion.
3.2 Microstructural evolution of bonding interface
Figure 10 reveals the impact of bonding time on the interfacial microstructure of TiAl/Ti2AlNb joints bonded using a pure Ti interlayer at 1000℃under a pressure of20 MPa.It is observed that the same interfacial microstructure is formed in the joints and the thickness ofthe DLs gradually increases with the increase of bonding time,as shown in Fig.10a-c.Besides,both the thicknesses of the reaction layer adjacent Ti2AlNb alloy and the reaction layer adj acent TiAl alloy have the same trend with the total thickness.With a lower bonding time of 60 min,as shown in Fig.10a,some coarse white clusters together with many black refined particles are stacked on the clad interface,which shows an insufficient diffusion.Further,with the increase of the bonding time to 90 min,Fig.10b shows that the cohesive interface is smooth,the black particles are dispersed gradually,and the interfacial microstructure is uniform.As shown in Fig.10c,the bonding interface is almost straight when the bonding time is 120 min.However,it is found that the grain of DLs adjacent TiAl alloy is coarse,which is detrimental to the mechanical properties.
3.3 Grow kinetics of diffusion reaction layers
For the sake of clarifying the growth behavior of the DLs detailly,the color-coded phase maps of the bonding joints at1000℃-20 MPa with different bonding time obtained from EBSD analysis were conducted,as shown in Fig.11.As mentioned above,the DLs can be pided into two layers from SEM,but the DLs should be broken into four layers from EBSD analysis in further research.Here,the thickness of DLs was measured by the minimum of diffusion distance according to the phase boundary,which is indicated by the black line in Fig.11.According to Fig.11,the DLs contains four layers,including (Ⅰ) single coarseα2phase adjacent TiAl alloy,(Ⅱ) single refined phase at the bonding interface,(Ⅲ) equiaxed/acicularα2 phase embedded inβphase adjacent Ti2AlNb alloy and (Ⅳ) both equiaxedα2 phase and acicular O phase embedded inβphase adjacent Ti2AlNb alloy,respectively.Of course,there has an error of artificial microscale measurement.The diffusion thicknesses of four layers are marked in Fig.11,which are averaged by three measures.
Fig.8 Grain orientation distributions of TiAl/Ti2AlNb joint:a ND,b BD,and c TD
Fig.9 a Full and b inverse pole figures ofα2 grains in DLs
Fig.10 SEM images of bonded joint at 1000℃-20 MPa with different bonding time:a 60 min,b 90 min,and c 120 min
Fig.11 EBSD color-coded phase maps of bonding joints at 1000℃-20 MPa with different bonding time obtained:a 60 min,b 90 min,and c 120 min (color-coded agreeing with former)
Figure 11a indicates the thickness of DLs at different bonding time.Note that the thickness of each layer of thejoints increases as the bonding time increases,as also shown in Fig.12a.In order to illuminate the layer growth behavior,it is crucial to analyze the growth kinetics of the four reaction layers.The thickness of a reaction layer by elemental diffusion at a given temperature can be expressed by the following relationship
Fig.12 a Evolution of diffusion thicknesses of interfacial reaction layers at different bonding time;b linear regressions according to Eq.(2);c linear regressions according to Eq.(3)
where X is the thickness of reaction layer,k is the reaction rate constant,t is diffusion time,and n is reaction kinetic exponent.Distinct value of n represents the different growth mechanism of reaction layers,standard diffusion limit growth is expressed when n is 0.5,and n>0.5suggests an interface-controlled growth
The experimental data shown in Fig.11 are processed into Eq.(2) for linear regression analysis,and the result is shown in Fig.12b.As shown in Fig.12b,the reaction kinetic exponent (n) for different layers are 0.46,0.56,0.13and 0.31,respectively.As for LayersⅠandⅡ,the value of n is close to 0.5,showing that the layers are controlled by a standard diffusion.However,for LayersⅢandⅣ,the value of n is lower than 0.5,which may be caused by the phase transformation at the based Ti2AlNb alloy.Also,it is necessary to take the experimental error into account.As a result,it is inferred that the layer growth belongs to a standard diffusion limited growth.
Based on Eq.(3),the experimental data could be expressed as a linear analysis,as shown in Fig.12c.Hence,it can be given that the values of k for RegionsⅠ,Ⅱ,ⅢandⅣare 1.22×10-6,1.27×10-6,2.60×10-7 and7.70×10-7 m·s-1/2,respectively.
3.4 Mechanical properties of bonding joints
The RT tensile strength of the TiAl/Ti/Ti2AlNb DB joint obtained at different bonding time is shown in Fig.13.It is clear that the tensile strength increases firstly and then decreases with the increase of bonding time.Figure 13reveals that the RT tensile strength is relatively lowbecause of an insufficient diffusion with a short bonding time of 60 min.There is a visible improvement of tensile strength with the bonding time increasing to 90 min.And the maximum tensile strength of 281 MPa is achieved,which is about twice as strong as that of the joint without any interlayer marked by the red line in Fig.13.Table 3shows the comparison of the strength of the dissimilar diffusion bonding joint of TiAl or Ti2AlNb alloy with/without the interlayer.Note that the addition of the interlayer can enhance the strength of the joint as the results in Refs.
Fig.13 Tensile strength of TiAl/Ti2AlNb DB joint using pure Ti interlayer
Table 3 Strength of dissimilar diffusion bonding joint of TiAl or Ti2AlNb alloy
Tensile strength was marked by T,and S represented shear strength
Fig.14 Schematic diagrams of microstructural evolution of TiAl/Ti2AlNb DB joint:a before DB,b under DB,and c after DB
3.5 Diffusion bonding mechanism
To study the diffusion bonding of the TiA alloy and Ti2AlNb alloy,the pure Ti layer fabricated by DC magnetron sputtering is used.The pure Ti interlayer has a significant effect on micros true tural evolution and mechanical performance.The pure Ti interlayer could buffer the mechanical stress at the faying interface before diffusion bonding,thus decreasing the internal stress in the bonding joint.During the bonding process,many grain boundaries provided by the nano pure Ti interlayer can promote the diffusion of atoms and voids sealed,which is importance for the microstructural evolution of the DLs.Therefore,the resultant joint will be produced after furnace cooling.The diffusion bonding mechanism is schematically illustrated in Fig.14.Figure 14a shows the state before diffusion bonding.And the interlayer has a great number of grain boundaries to adjust the deformation and facilitate diffusion of the element,as shown in Fig.14b,during the DB processing.Besides,a metallurgical bonded joint with excellent properties is formed in Fig.14c.
4 Conclusion
TiAl alloy and Ti2AlNb alloy were successfully bonded using a pure Ti interlayer by diffusion bonding.The results show that the DLs mainly consisted of four characteristic layers:(Ⅰ) single coarseα2 phase adjacent TiAl alloy,(Ⅱ)single refinedα2 phase at the bonding interface,(Ⅲ)equiaxed/acicularα2 phase embedded inβphase adjacent Ti2AlNb alloy and (Ⅳ) both equiaxedα2 phase and acicular O phase embedded inβphase adjacent Ti2AlNb alloy.
The thickness of the four layers increased with the increase of the bonding time.The growth of DLs is limited by diffusion growth and the reaction rate constant (k) for RegionsⅠ,Ⅱ,ⅢandⅣare 1.22×10-6,1.27×10-6,2.60×10-7 and 7.70×10-7 m.s-1/2,respectively.And the orientation of the interfacialα2 phase grows up without texture.The RT tensile strength of the joints increased initially,and after that,it decreased with the bonding time increasing.The RT tensile strength of 156 MPa is obtained without the interlayer,and RT tensile strength increases to281 MPa with a nano interlayer at 1000℃-20 MPa-90 min.
参考文献
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