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Microstructures and Mechanical Properties of Mg-9Al/Ti Metallurgical Bonding Prepared by Liquid-Solid Diffusion Couples Jiahong Dai 1,2 , Bin Jiang 2,3, *, Qiong Yan 1 , Hongmei Xie 1 , Zhongtao Jiang 4 , Qingshan Yang 5 , Qiaowang Chen 4 , Cheng Peng 1, * and Fusheng Pan 2,3 1 2 3 4 5

*

College of Materials Science and Engineering, Yangtze Normal University, Chongqing 408100, China; [email protected] (J.D.); [email protected] (Q.Y.); [email protected] (H.X.) State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China; [email protected] Chongqing Academy of Science and Technology, Chongqing 401123, China Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China; [email protected] (Z.J.); [email protected] (Q.C.) School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing, 401331, China; [email protected] Correspondence: [email protected] (B.J.); [email protected] (C.P.); Tel.: +86-135-9419-0166 (B.J.); +86-136-5826-7036 (C.P.); Fax: +86-0236-5111-140 (B.J.); +86-0237-2790-029 (C.P.)

Received: 23 August 2018; Accepted: 28 September 2018; Published: 29 September 2018







Abstract: Microstructures and mechanical properties of Mg-9Al/Ti metallurgical bonding prepared by liquid-solid diffusion couples were investigated. The results indicate that a metallurgical bonding was formed at the interface Mg-9Al/Ti, and the Mg17 Al12 phase growth coarsening at the interfaces with the increase in heat treatment time. Push-out testing was used to investigate the shear strength of the Mg-9Al/Ti metallurgical bonding. It is shown that the shear strength presents an increasing tendency with the increased heat treatment time. The sequence is characterized, and the results show that the fracture takes place along the Mg-9Al matrix at the interface. The diffusion of Al and Ti elements play a dominant role in the interface reaction of Mg-9Al/Ti metallurgical bonding. By energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and thermodynamic analysis, it was found that Al3 Ti is the only intermetallic compound at the interface of Mg-9Al/Ti metallurgical bonding. These results clearly show that chemical interaction at the interface formation of Al3 Ti improves the mechanical properties of Mg-9Al/Ti metallurgical bonding. Keywords: magnesium; titanium; metallurgical bonding; interface; mechanical properties

1. Introduction Magnesium alloys, as the lightest metal structural materials, have high specific strength, excellent castability, and easy recyclability. However, the application of commercially available Mg alloys is limited to applications because of their lower strength [1–3]. Titanium alloys have been widely used due to their high specific strength, notable impact toughness, excellent corrosion resistance and significant thermal stability [4]. However, the high production cost restricts their widespread applications [5]. Bimetal materials have been widely used in many industrial fields because they combine several promising properties that cannot be provided by monolithic materials. Mg and Ti bimetal materials have drawn great attention due to its unique properties, which can combine low density of magnesium and extraordinarily high specific strength and significant thermal stability of Ti [6].

Metals 2018, 8, 778; doi:10.3390/met8100778

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In the past several years, it has been reported that Al/Mg, Mg/Mg and Al/Ti bimetal materials have already been fabricated by accumulative roll-bonding [7], extrusion forming [8], laser welding– Metals 2018, 8, 778 2 of 13 brazing [9], and so forth. However, these methods still have much room for improvement as their processing procedures are very complex and the products usually have lower interface bonding In the past several years, it has been reported that Al/Mg, Mg/Mg and Al/Ti bimetal strengthmaterials and little opportunity forfabricated mass production. In recent years, the casting have already been by accumulative roll-bonding [7],liquid-solid extrusion forming [8],process laser methodwelding–brazing has been also utilized to produce Al/Mg [10], Mg/Mg [11] and Al/Ti [12] bimetal materials, [9], and so forth. However, these methods still have much room for improvement as which exhibits excellent industrial for the usually preparation of bimetallic materials their processing procedures areapplication very complexprospects and the products have lower interface bonding due to the low and production costs, simple production procedure high interface bonding strength strength little opportunity for mass production. In recent and years, the liquid-solid casting process method has been alsosoutilized to produce Al/Mg [10], Mg/Mg [11] [12] bimetal materials, of the products. However, far, Mg/Ti bimetal materials prepared byand the Al/Ti liquid-solid casting process which exhibits excellent industrial application prospects for the preparation of bimetallic materials method have not been reported yet. due to thea low production costs, simple production procedurebetween and high Mg interface bonding of Obtaining perfect metallurgical bonding in the interface and Ti is verystrength important the products. However, so far, Mg/Ti bimetal materials prepared by the liquid-solid casting process to guarantee the excellent mechanical properties for the Mg/Ti bimetallic materials. However, Mg method have not been reported yet. and Ti are not phase formation. Furthermore, the solid solution between Mg and Ti is very low [13]. Obtaining a perfect metallurgical bonding in the interface between Mg and Ti is very important These indicate that no interfacial reaction or atomic diffusion occurs between Mg and Ti. Hence, an to guarantee the excellent mechanical properties for the Mg/Ti bimetallic materials. However, Mg intermediate element must be added to react with Mg and Ti or solid solubility in Mg and Ti. Al is and Ti are not phase formation. Furthermore, the solid solution between Mg and Ti is very low [13]. the dominant alloying element to improve theormechanical properties Mg alloys andTi.TiHence, alloys, These indicate that no interfacial reaction atomic diffusion occurs of between Mg and which can form intermetallic during solidification the solubility reaction of Al with Mg an intermediate elementcompounds must be added to react with Mg anddue Ti ortosolid in Mg and Ti. Alor Ti. Theishybrid structure of (Mg-Al)-Ti dissimilar metal by welding has been investigated by the the dominant alloying element to improve the mechanical properties of Mg alloys and Ti alloys, which can method form intermetallic compounds during solidification due to the reactioncompound of Al with Ti Mg welding-brazing [14–16]. Brazing interfaces were composed of intermetallic 3Al, or Ti. The hybrid structure of (Mg-Al)-Ti dissimilar metal by welding has been investigated by the which was resistance to the crack propagation and improvement of mechanical properties. Therefore, welding-brazing method Brazing composed intermetallic a perfect metallurgical bond can[14–16]. be formed at theinterfaces interfacewere between Mg-Alofalloy and Ti bycompound the liquidTi Al, which was resistance to the crack propagation and improvement of mechanical properties. 3 solid casting process, but the formation of Mg-Al alloy and Ti metallurgical bonding has not been Therefore, a perfect metallurgical bond can be formed at the interface between Mg-Al alloy and Ti by declared yet. the liquid-solid casting process, but the formation of Mg-Al alloy and Ti metallurgical bonding has not Based on this condition, Mg-9Al alloy and Ti were studied in this paper to produce excellent been declared yet. metallurgical bonding by the liquid-solid diffusion couples. The microstructure and mechanical Based on this condition, Mg-9Al alloy and Ti were studied in this paper to produce excellent behaviors at the interface of Mg-9Al/Ti metallurgical bonding areThe further discussed.and mechanical metallurgical bonding by the liquid-solid diffusion couples. microstructure behaviors at the interface of Mg-9Al/Ti metallurgical bonding are further discussed.

2. Experiments

2. Experiments

Mg-9Al (Mg-9 wt.%Al) casting ingots and Ti rods with dimensions of Φ 8 mm × 100 mm were Mg-9Al (Mg-9 casting ingots and Ti rods with dimensions of Φ 8 mm application × 100 mm were used. The surface of Tiwt.%Al) rods was polished with 1000-grit SiC papers before and used. The surface of Ti rods was polished with 1000-grit SiC papers before application and subsequently subsequently treated with acetone cleaning. Then, Mg-9Al ingots were put in a stainless-steel crucible treated with acetone cleaning. Then, Mg-9Al ingots were put in a stainless-steel crucible with Φ 34 mm with Φ 34 mm × 90 mm (showed in Figure 1), which was kept in a resistance furnace under a × 90 mm (showed in Figure 1), which was kept in a resistance furnace under a protective atmosphere protective atmosphere of CO2 + 0.5 vol.% SF6. Mg-9Al alloy was held at 700 °C for melting. After of CO2 + 0.5 vol.% SF6 . Mg-9Al alloy was held at 700 ◦ C for melting. After melting, Ti rod was melting,rapidly Ti rodsubmerged was rapidly submerged the alloy. molten Mg-9Al alloy. cover was set onwhen top of into the molten into Mg-9Al A steel cover was A setsteel on top of the crucible the crucible when the insertwas experiment was outrod sowould that the rod wouldand be centered kept vertical and the insert experiment carried out so carried that the Ti be Ti kept vertical (shown centered (shown in Figure 1). Next, the liquid-solid diffusion couples of Mg-9Al/Ti were formed. in Figure 1). Next, the liquid-solid diffusion couples of Mg-9Al/Ti were formed. Diffusion couples were kept atwere 700 ◦ C for 0atmin, 60 min. After diffusion couples were furnace cooledwere to Diffusion couples kept 700 30 °Cmin for 0and min, 30 min andthat, 60 min. After that, diffusion couples room temperature. furnace cooled to room temperature.

Figure The schematic sketch of preparingMg-9Al/Ti Mg-9Al/Ti metallurgical metallurgical bonding prepared by liquid-solid Figure 1. The 1.schematic sketch of preparing bonding prepared by liquiddiffusion couples. solid diffusion couples.

The samples were cut from the middle part of the diffusion couples perpendicular to the rod axis with a thickness of 9 mm. Each sample was ground with ground papers from 400 to 1000 grit to

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The samples were cut from the middle part of the diffusion couples perpendicular to the rod cross-sectional scanningofelectron microscopy TESCAN VEGA 3 LMH, Brno, axis with a thickness 9 mm. Each sample(SEM, was ground with ground papersTESCAN from 400Co., to 1000 grit Czech) examinations.scanning SEM equipped energy-dispersive spectroscopy (Oxford Instrument to cross-sectional electronwith microscopy (SEM, TESCAN VEGA (EDS) 3 LMH, TESCAN Co., Brno, Technology Co., Ltd., Oxford, UK) was employed to determine the concentration profiles of the Czech) examinations. SEM equipped with energy-dispersive spectroscopy (EDS) (Oxford Instrument interfaces and phase composition thewas interface. Additionally, the phase structure for the fractured Technology Co., Ltd., Oxford, of UK) employed to determine the concentration profiles of the specimen of and the phase interface was determined by X-ray diffraction the (XRD, D/Max 2500PC, Dandong interfaces composition of the interface. Additionally, phase structure for the fractured Fangyuan Instrument Co., Ltd., Dandong, China). specimen of the interface was determined by X-ray diffraction (XRD, D/Max 2500PC, Dandong The shear strength of theLtd., metallurgical Fangyuan Instrument Co., Dandong, bonding China). interface was investigated by push-out testing (NEW SANSI CMT-5105, (Shanghai) bonding Enterprise Development Co., Ltd., Shanghai, China). The shear strengthXinSanSi of the metallurgical interface was investigated by push-out testing The schematic diagram is illustrated Figure 2. The supporting platform isCo., a diameter of a centered (NEW SANSI CMT-5105, XinSanSiin(Shanghai) Enterprise Development Ltd., Shanghai, China). circular hole of 10diagram mm. The steel 2. cylinder was 6 mm. The loading rate of cross-head The schematic is diameter illustratedofinthe Figure The supporting platform is a diameter of a centered was 1 mm/min. of theofmetallurgical bonding was calculated by the circular hole of Shear 10 mm.strength The diameter the steel cylinder was 6interfaces mm. The loading rate of cross-head following equation: Shear strength of the metallurgical bonding interfaces was calculated by the was 1 mm/min. following equation: F Fmax τ  max (1) τ= (1) 2 π2πrt rt where whereFmax Fmaxisisthe themaximum maximumload, load,r risisthe theradius radiusofofTiTiinsert, insert,and andt tisisthe thespecimen specimenthickness. thickness.

Figure 2. schematic A schematic diagram of the push-out tests. Figure 2. A diagram of the push-out tests.

3. Results and Discussion 3. Results and Discussion 3.1. The Microstructure of the Interface 3.1. The Microstructure of the Interface Cross-sectional backscattered electron (BSE, TESCAN Co., Brno, Czech) micrographs at the Cross-sectional backscattered electron (BSE, TESCAN Co., Brno, Czech) micrographs the interface of Mg-9Al/Ti metallurgical bonding prepared by the liquid-solid diffusion couples at at different interface of Mg-9Al/Ti metallurgical by thethat liquid-solid couples at heat treatment times are presented inbonding Figure 3.prepared It can be noted the Mg17 Aldiffusion 12 phase in the Mg-9Al different heat treatment times are presented in Figure 3. It can be noted that the Mg 17Al12 phase in the matrices are attached to the surface of Ti rods. The Mg17 Al12 phases growth coarsened with the Mg-9Al matrices attached to the surface of Ti rods. The Al Mg17Al12 phases growth coarsened with increase of heatare treatment time at the interfaces. The Mg 17 12 phases are distributed along the grain theboundaries increase ofofheat treatment time at the interfaces. The Mg 17Al12 phases are distributed along the Mg-based matrix. They increased with increasing temperature, and the grain boundaries grain boundaries of Mg-based matrix. They increased with increasing the Because grain wetting transitioned from incompletely wetted to completely wetted bytemperature, the Mg17 Al12and phase. boundaries transitioned wetted to completely by the[17], Mg17 Al12the the contactwetting angle between Ti andfrom Mg17incompletely Al12 phase decreases with increasingwetted temperature and phase. Because the contact angle between Ti and Mg 17Al12 phase decreases with increasing reversible grain boundary(GB) transitioned from incomplete to complete, wetting by a liquid phase temperature [17], and theincreasing reversible temperature. grain boundary(GB) from incomplete to complete, always proceeds with This is transitioned due to the temperature dependence 2σSL (T) wetting a liquid always proceeds with temperature. This is higher due toentropy the alwaysby being more phase steep than the dependence σGBincreasing (T), as the liquid phase possess temperature dependence 2σ SL(T) always being more steep than the dependence σGB(T), as the liquid in comparison to the solid one [18]. Figure 3a shows a cross-sectional BSE micrograph image at the phase possess higher entropy the solidaone 3a shows cross-sectional interface of Mg-9Al/Ti at 700in◦ comparison C for 0 min. to Obviously, gap[18]. wasFigure established at thea interface between BSE micrograph image at the interface of Mg-9Al/Ti at 700 °C for 0 min. Obviously, a interface gap was of Mg-9Al alloy and Ti rod. In addition, the cross-sectional BSE micrograph images at the established at at the700 interface between Mg-9Al alloy Ti rod. addition, BSE a ◦ C for 30 Mg-9Al/Ti min and 60 min are and indicated in In Figure 3b,c. the Thecross-sectional specimens formed micrograph images at thebonding interfaceatofthe Mg-9Al/Ti at interface. 700 °C for 30 min and 60 min are indicated in complete metallurgical Mg-9Al/Ti Figure 3b,c. The specimens formed a complete metallurgical bonding at the Mg-9Al/Ti interface.

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Figure 3.3.Cross-sectional Cross-sectional micrographs ofofinterfaces interfaces between Mg-9Al Figure backscattered electron (BSE) Figure3. Cross-sectionalbackscattered backscatteredelectron electron(BSE) (BSE)micrographs micrographsof interfacesbetween betweenMg-9Al Mg-9Al ◦ matrix and Ti rod obtained at 700 °C for (a) 0 min, (b) 30 min, (c) 60 min. matrix rod obtained atat 700 forfor (a)(a) 0 min, (b)(b) 3030 min, (c)(c) 6060 min. matrixand andTiTi rod obtained 700C°C 0 min, min, min.

Figure shows aamagnified image ofofof the interface between Mg-9Al matrix and Ti Ti after 60 60 min at Figure 4a4a shows image the interface between Mg-9Al matrix and Figure4a shows amagnified magnified image the interface between Mg-9Al matrix and Tiafter after 60min min at 700 °C. The intermetallic compounds have developed around the Ti rod at the interface that can be observed. composition ofofpoint AA was analyzed by by EDS in Figure 4b. The EDS result is taken from observed. The composition analyzed ininFigure 4b. isistaken observed.The The composition ofpoint point Awas was analyzed byEDS EDS Figure 4b.The TheEDS EDSresult result taken the testing position A denoted in Figure 4a. The4a. composition of Al is of 57.12 at.% andat.% Ti isand 39.45 for from the position AAdenoted ininFigure isis39.45 from thetesting testing position denoted Figure 4a.The Thecomposition composition ofAlAlis is57.12 57.12 at.% andTiTiat.% 39.45 point A. The ratio of Al/Ti is 3.06. It can be inferred that the intermetallic compound is Al Ti. This is at.% for point A. The ratio of Al/Ti is 3.06. It can be inferred that the intermetallic compound is Al 3Ti. 3 at.% for point A. The ratio of Al/Ti is 3.06. It can be inferred that the intermetallic compound is Alin 3Ti. agreement with the reports of reports Jie et al.of[19] Li[19] et al. [20] formation of Al3 Ti between Al This with etetal.al. and LiLithat etetal.al. [20] ofofAlAl 3Tiliquid between Thisisisininagreement agreement withthe the reports ofJie Jieand [19] and [20]that thatformation formation 3Ti between and solid Ti, but theTi, results are inconsistent with Cao et al. [14] and etand al. [15,16] studies. Therefore, liquid AlAland solid with Cao etetal.Tan Tan al.al.[15,16] studies. liquid and solid Ti,but butthe theresults resultsare areinconsistent inconsistent with Cao al.[14] [14] and Tanetet [15,16] studies. further proof is needed. As can be seen in Figure 4a, there are a large number of nanostructures Therefore, further proof is needed. As can be seen in Figure 4a, there are a large number ofof Therefore, further proof is needed. As can be seen in Figure 4a, there are a large numberat the interface. It is reported in the literature that these nanostructures will accelerate the diffusion of nanostructures at the interface. It is reported in the literature that these nanostructures will accelerate nanostructures at the interface. It is reported in the literature that these nanostructures will accelerate adatoms and of improve theand wettability ofthe thewettability interface [21–23]. the ofofthe thediffusion diffusion ofadatoms adatoms andimprove improve the wettability theinterface interface[21–23]. [21–23]. ◦ C.°C. 700 The intermetallic compounds at 700 The intermetallic compoundshave havedeveloped developedaround aroundthe theTiTirod rodatatthe theinterface interface that that can can be

Figure (a) The high magnification image ofofthe between Mg-9Al matrix and TiTirob Figure4.4.4. magnification theinterface interface Mg-9Aland matrix rob Figure (a)(a) TheThe highhigh magnification imageimage of the interface betweenbetween Mg-9Al matrix Ti roband obtained obtained after 60 min at 700 °C, (b) Energy-dispersive spectroscopy (EDS) result of the point A. ◦ obtained 60 min at 700 °C, (b) Energy-dispersive spectroscopy after 60 minafter at 700 C, (b) Energy-dispersive spectroscopy (EDS) result(EDS) of theresult pointofA.the point A.

The interface ofofMg-9Al Mg-9Al matrix and TiTirob rob obtained after 6060min min atat700 700 was analyzed using The C°Cwas Theinterface interfaceof Mg-9Almatrix matrixand andTi robobtained obtainedafter after60 minat 700◦°C wasanalyzed analyzedusing using EDS through line scan ininFigure Figure 5.5.At At the interface between Mg-9Al matrix and rod, the element EDS TiTirod, EDSthrough throughline linescan scanin Figure5. Atthe theinterface interfacebetween betweenMg-9Al Mg-9Almatrix matrixand andTi rod,the theelement element TiTitends tends totodecrease decrease atatthe the interface when approaching Mg-9Al matrix. The concentration TiTiin inin Ti ofofTi tendsto decreaseat theinterface interfacewhen whenapproaching approachingMg-9Al Mg-9Almatrix. matrix.The Theconcentration concentrationof Mg 17Al12 phase is obviously higher than that of Mg matrix. Moreover, at the interface between MgMg Al than that of Mg matrix. Moreover, at the between Mg-9Al Mg Al1212phase phaseisisobviously obviouslyhigher higher than that of Mg matrix. Moreover, at interface the interface between Mg1717 9Al matrix and Ti rod, the elements of Mg and Al vary clearly, indicating the decreasing tendency matrix and Tiand rod,Tithe elements of Mg of and Aland varyAlclearly, indicating the decreasing tendency when 9Al matrix rod, the elements Mg vary clearly, indicating the decreasing tendency when ititinferred can ofofTi AlAl and may approaching Ti side.TiTherefore, it can be that thethat diffusion of Al and elements may play an whenapproaching approaching Tiside. side.Therefore, Therefore, canbebeinferred inferred thatthe thediffusion diffusion andTiTielements elements may play an important role during the interface of Mg-9Al/Ti metallurgical bonding. Moreover, because play an important role during the interface of Mg-9Al/Ti metallurgical bonding. Moreover, because ofofthe 3Ti compound is formed at the interface. theinterdiffusion interdiffusionbetween betweenAlAland andTi, Ti,the theAlAl 3Ti compound is formed at the interface.

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important role during the interface of Mg-9Al/Ti metallurgical bonding. Moreover, because of the interdiffusion between Al and Ti, the Al3 Ti compound is formed at the interface. Metals 2018, 8, x FOR PEER REVIEW 5 of 13

Figure 5. Line scan curves of the interface between Mg-9Al matrix and Ti rod obtained after 60 min at 700 °C. Figure 5. Line scan curves of the interface between Mg-9Al matrix matrix and and Ti rod obtained after 60 min ◦ C. at 700 °C. concentration profiles of the interface between Mg-9Al matrix and Ti rod annealed

The after 60 min at 700The °Cconcentration showed in Figureof6thewhich arebetween line scan results inand Figure 3c. The after trend of element Mg-9Al matrix Ti rod annealed 60 min concentrationprofiles profiles of theinterface interface between Mg-9Al matrix and Ti rod annealed after 60 distribution isshowed consistent with results ofscan Figure 5. in According to Hall’s method the impurity at 700at◦ C in Figure 6 the which line results Figure 3c. The trend element distribution min 700 °C showed in Figure 6are which are line scan results in Figure 3c. of The trend[24], of element −15 diffusion coefficients of in Ti and Al5.in Ti, and5. Ti Mg-9Al. The results arethe 1.70(±0.02) is consistent theMg results of Figure to in Hall’s method [24], the impurity diffusion distribution iswith consistent with the results ofAccording Figure According to Hall’s method [24], impurity × 10 − 15 2 −15in 2/s, 2/s,Tirespectively. −15 coefficients of Mg Ti and AlTiinand Ti, and inm Mg-9Al. The results 1.70( ± 0.02) × 10the×m m2/s, 3.77(±0.12) × 10 mof 1.60(±0.23) × 10 Itare can be are seen that diffusion of diffusion coefficients Mg in Al inTi−14 Ti, and in Mg-9Al. The results 1.70(±0.02) 10/s, − 15 2 − 14 2 2 −15 2 −14 2 3.77( 0.12) × 10 × the /s, 1.60( ±0.23) ××10 /s,respectively. respectively. canbe be seen that theimpurity diffusion diffusion m /s, 3.77(±0.12) 10m Mg m /s, 10 inmMg-9Al /s, ItItthan can seen that the diffusion of Al in Ti is ±faster than in1.60(±0.23) Ti. Ti diffuse is faster Al and Mg Al in Ti is ininTi. ininMg-9Al isisfaster than and Mg impurity in Timay is faster faster thanthe theMg Mgthe Ti.TiTidiffuse diffuse Mg-9Al faster than Al and Mg impurity diffusion in Ti, which be than related to state of Ti and Mg-9Al alloy. So Al the diffusion of Aldiffusion and Ti elements Ti, which may bebe related toto the in Ti, which may related thestate stateofofTiTiand andMg-9Al Mg-9Alalloy. alloy.So Sothe thediffusion diffusionof of Al Al and Ti elements play an important role during the interface of Mg-9Al/Ti metallurgical bonding. play an important role during the interface of Mg-9Al/Ti Mg-9Al/Ti metallurgical bonding.

Figure 6. Concentration profiles of the interface between Mg-9Al matrix and Ti rod annealed after 60 min at 700 °C. Figure 6. Concentration profiles of the interface between Mg-9Al matrix and Ti rod annealed after

Figure606.min Concentration profiles of the interface between Mg-9Al matrix and Ti rod annealed after 60 at 700 ◦ C. 3.2. at Mechanical min 700 °C. Behaviors Figure 7 shows the load-displacement curves. It is noticeable that the shear strength of Mg-

3.2. Mechanical Behaviorsbonding presents an increasing tendency for the extension of the heat treatment 9Al/Ti metallurgical time. The average shear stress at the interface can be evaluated by Equation (1). Results of shear

Figure 7 shows the load-displacement curves. It is noticeable that the shear strength of Mgstrength for Mg-9Al/Ti metallurgical bonding under different heat treatment time are listed in Table 9Al/Ti1.metallurgical bonding presents an increasing tendency for the extension of the heat treatment It can be seen that the shear strength of the Mg-9Al/Ti metallurgical bonding with the increase of time. heat The treatment average time shear at the can value be evaluated in stress this study, and interface the maximum is 56 MPa. by Equation (1). Results of shear

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3.2. Mechanical Behaviors Figure 7 shows the load-displacement curves. It is noticeable that the shear strength of Mg-9Al/Ti metallurgical bonding presents an increasing tendency for the extension of the heat treatment time. The average shear stress at the interface can be evaluated by Equation (1). Results of shear strength for Mg-9Al/Ti metallurgical bonding under different heat treatment time are listed in Table 1. It can be seen 8, that the shear Metals 2018, x FOR PEER strength REVIEW of the Mg-9Al/Ti metallurgical bonding with the increase of heat treatment 6 of 13 time in this study, and the maximum value is 56 MPa.

Figure 7. Load-displacement curves obtained of the Mg-9Al/Ti metallurgical bonding by push-out

Figure 7. Load-displacement curves obtained of the Mg-9Al/Ti metallurgical bonding by push-out testing for 0 min, 30 min and 60 min at 700 ◦ C. testing for 0 min, 30 min and 60 min at 700 °C. Table 1. Shear strength of Mg-9Al/Ti metallurgical bonding with different heat treatment time.

Table 1. ShearSample strength of Mg-9Al/Ti metallurgical bonding with different heat treatment time. Maximum Stress (N) Maximum Shear Strength (MPa) 0 min-1# Maximum 3478 16 Strength (MPa) Sample Stress (N) Maximum Shear 0 min-2# 3482 16 0 min-1# 3478 16 30 min-1# 7086 32 30 min-2# 7742 35 0 min-2# 3482 16 60 min-1# 12,174 54 30 min-1# 7086 32 60 min-2# 12,531 56 30 min-2# 7742 35 60 min-1# 12,174 54 Figure 8a–c shows SEM fractographs on the Ti insert of the Mg-9Al/Ti metallurgical bonding 60 min-2# 12,531heat treatment time. It can be seen 56 that the surface roughness push-out samples obtained by different

of the Ti rod face increase with the increase of heat treatment time. Figure 8d shows the magnified ◦ C for image of the shows fractureSEM surface of Ti side at 700 min. of The fracture is characterized by long Figure 8a–c fractographs on the Ti 60 insert the Mg-9Al/Ti metallurgical bonding grooves, all with the same orientation parallel to the push-out direction, characteristic of a ductile push-out samples obtained by different heat treatment time. It can be seen that the surface roughness Figure 8e–f shows EDS of the point treatment B and C in Figure So the B is the Mg-9Al of the fracture. Ti rod face increase with theanalyses increase of heat time. 7d. Figure 8dpoint shows magnified matrix, the point C is the interface of Mg-9Al/Ti metallurgical bonding. Figure 9 shows the distribution image of the fracture surface of Ti side at 700 °C for 60 min. ◦The fracture is characterized by long of Mg, Al and Ti elements for the fracture surface of Ti side at 700 C for 60 min. It is observed that the grooves, all with the same orientation parallel to the push-out direction, characteristic of a ductile Al element in Mg-9Al matrix diffuses into the Ti rod. Those are indicating that the shear stress has fracture. Figure showswhen EDSAlanalyses of the point and C in Figure 7d. So between the point B is Mg-9Al been greatly8e–f improved and Ti interdiffuse andBchemical interaction occurs Mg-9Al matrix, point C boundaries is the interface Mg-9Al/Ti metallurgical Figure wetted 9 shows the andthe Ti, and grain betweenof Ti and Mg-9Al wetting transitionbonding. from incompletely to completely by Ti Mgelements metallurgical firm60and distribution of Mg,wetted Al and for So theMg-9Al/Ti fracture surface of Tibonding side at becomes 700 °C for min. It is 17 Al12 phase. impervious with heat treatment time. observed that the Al element in Mg-9Al matrix diffuses into the Ti rod. Those are indicating that the

shear stress has been greatly improved when Al and Ti interdiffuse and chemical interaction occurs between Mg-9Al and Ti, and grain boundaries between Ti and Mg-9Al wetting transition from incompletely wetted to completely wetted by Mg17Al12 phase. So Mg-9Al/Ti metallurgical bonding becomes firm and impervious with heat treatment time.

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Figure (c)(c) 6060 min; (d)(d) magnified image of the Ti Figure 8. 8. BSE BSE fractographs fractographsof ofTi Tirod rodfor for(a) (a)0 0min, min,(b) (b)3030min, min, min; magnified image of the rod for for 60 min; (e) (e) EDS of the point B; (f) of the point C. C. Ti rod 60 min; EDS of the point B; EDS (f) EDS of the point

To understand the mechanism of crack formation and propagation during shear strength tests, it is necessary to identify where the crack initiation occurs. Figure 10 illustrates the cross-sectional BSE image of the Mg-9Al/Ti metallurgical bonding interface for 60 min. It can be found that the fracture takes place along the Mg-9Al matrix of the interface.

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Figure 9. BSE-EDS mapping of Mg, Al and Ti elements for the fracture surface of Ti rod at 700 °C for 60 min.

To understand the mechanism of crack formation and propagation during shear strength tests, it is necessary to identify where the crack initiation occurs. Figure 10 illustrates the cross-sectional Figure 9. 9. BSE-EDS mappingmetallurgical of Mg, elementsinterface for the the fracture fracture surface ofcan Ti rod rod atfound 700 ◦°C for the BSE image ofBSE-EDS the Mg-9Al/Ti bonding for 60surface min. Itof beat Figure mapping of Mg, Al Al and and Ti Ti elements for Ti 700 Cthat for 60 min. min. fracture takes place along the Mg-9Al matrix of the interface. 60 To understand the mechanism of crack formation and propagation during shear strength tests, it is necessary to identify where the crack initiation occurs. Figure 10 illustrates the cross-sectional BSE image of the Mg-9Al/Ti metallurgical bonding interface for 60 min. It can be found that the fracture takes place along the Mg-9Al matrix of the interface.

◦ C for Mg-9Al/Ti metallurgical Figure 10. BSE cross-sectional view of the Mg-9Al/Ti metallurgical bonding at 700 °C for 60 60 min.

To characterize the intermetallic compounds formed at the interface of the Mg-9Al/Ti metallurgical bonding, X-ray diffraction was carried out for the Mg-9Al/Ti metallurgical bonding fractured surfaces for heat treatment at 700 ◦ C for 60 min. The XRD patterns are shown in Figure 11. Figure 10. BSE cross-sectional view of the Mg-9Al/Ti metallurgical bonding at 700 °C for 60 min.

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To characterize the intermetallic compounds formed at the interface of the Mg-9Al/Ti metallurgical bonding, X-ray diffraction was carried out for the Mg-9Al/Ti fracturedmetallurgical surfaces for To characterize the intermetallic compounds formed atmetallurgical the interfacebonding of the Mg-9Al/Ti Metals 2018, 8, 778 9 of 13 heat treatment at 700 °C for 60 min. The XRD patterns are shown in Figure 11. According the XRD bonding, X-ray diffraction was carried out for the Mg-9Al/Ti metallurgical bonding fracturedtosurfaces for patterns, Mg, Ti, Mg 17Al12 and Al3Ti on both fractured surfaces of the Mg-9Al and Ti fractured surface. heat treatment at 700 °C for 60 min. The XRD patterns are shown in Figure 11. According to the XRD However, Mgtoand Mg 17Al 12 patterns of Ti, theboth Ti 17 fracture surfaces were than surfaces the TiTi patterns at the MgAccording theMg XRD patterns, Mg, Mg Al12 and Al fractured of the Mg-9Al patterns, Mg, Ti, 17Al 12 and Al 3Ti on fractured surfaces ofhigher the Mg-9Al and fractured surface. 3 Ti on both 9Al fracture surfaces. The result showed that fracture path iswere propagated through Mg-9Al matrix. and Ti fractured However, Mgthe and Mg patterns of the Ti fracture surfaces were at higher However, Mg andsurface. Mg 17Al12 patterns of Tithe fracture higher than the Tithe patterns the Mg17 Al12surfaces Al 3 Ti were formed via the reaction of Al and Ti at the interface. This is in agreement with the EDS results than the Tisurfaces. patternsThe at the Mg-9Al fracture surfaces. showed that the fracture path is 9Al fracture result showed that the fractureThe pathresult is propagated through the Mg-9Al matrix. ofAlpropagated the interface between Mg-9Al and Ti matrices obtained after 60 min at 700 °C. through the Mg-9Al matrix. Al Ti were formed via the reaction of Al and Ti at the interface. 3 Ti at the interface. This is in agreement with the EDS results 3Ti were formed via the reaction of Al and is in agreement with the EDS of theobtained interfaceafter between Mg-9Al ofThis the interface between Mg-9Al andresults Ti matrices 60 min at 700and °C. Ti matrices obtained ◦ after 60 min at 700 C.

Figure 11. X-ray diffraction patterns form the fractured specimen at 700 °C for 60 min. Figure 11. X-ray diffraction patterns form the fractured specimen at 700 ◦ C for 60 min.

Figure 11. X-ray diffraction patterns form the fractured specimen at 700 °C for 60 min.

3.3. Thermodynamic Analysis of Intermetallic Compounds Formation at the Interface 3.3. Thermodynamic Analysis of Intermetallic Compounds Formation at the Interface 3.3. From Thermodynamic Analysis of Intermetallic Formation Interface Mg-Ti binary phase diagram, MgCompounds and Ti hardly formsata the chemical reaction. However, the From Mg-Ti binary phase diagram, Mg and Ti hardly forms a chemical reaction. However, the Al-Ti Al-Ti binary phase diagram is shown in Figure 12. It can be seen that several However, intermetallic From Mg-Ti binary phaseindiagram, andbeTiseen hardly forms aintermetallic chemical reaction. binary phase diagram is shown Figure 12.Mg It can that several compounds, namely,the compounds, namely, AlTi, Al 2Ti, Al5Ti2, Al3Ti and AlTi3. Therefore, Mg-Al and Al-Ti intermetallic Al-Ti diagram is 3shown in Figure canintermetallic be seen that several intermetallic AlTi, binary Al2 Ti, Alphase . Therefore, Mg-Al 12. and It Al-Ti compounds are likely 5 Ti2 , Al 3 Ti and AlTi compounds are likelyAlTi, toMg-9Al/Ti form in metallurgical the interfaceAlTi of 3.Mg-9Al/Ti metallurgical bonding. The to form in the interface of The thermodynamics is an important compounds, namely, Al 2Ti, Al5Ti 2, Al3Ti and bonding. Therefore, Mg-Al and Al-Ti intermetallic thermodynamics an whether important whether an intermetallic compound be factor in determining anfactor intermetallic compound can be formed. The Miedema’s modelcan forThe compounds are islikely to form in in thedetermining interface of Mg-9Al/Ti metallurgical bonding. formed. The Miedema’s model for the enthalpy of formation is used to calculate the standard molar the enthalpy of formation is used factor to calculate the standard molar enthalpies of formation of Mg-Ti, thermodynamics is an important in determining whether an intermetallic compound can be enthalpies of formation of Mg-Ti, Mg-Al and Al-Ti binary systems [9,16]. These calculations indicate Mg-Al and Al-Ti binary systems [9,16]. These calculations indicate that the standard molar enthalpy of formed. The Miedema’s model for the enthalpy of formation is used to calculate the standard molar that the standard molar enthalpy Mg-Ti binary system was positive, but Al-Mg and binary Mg-Ti binary system was but Al-Mg and Al-Ti binary systems were negative. TheAl-Ti standard enthalpies of formation ofpositive, Mg-Ti,of Mg-Al and Al-Ti binary systems [9,16]. These calculations indicate molar enthalpy of Al-Ti binary system was more negative than that of Al-Mg binary system, which systems were negative. The standard molar enthalpy of Al-Ti binary system was more negative than that the standard molar enthalpy of Mg-Ti binary system was positive, but Al-Mg and Al-Ti binary indicates that Al-Ti intermetallic compound prefers to be formed at the interface of the Mg-9Al/Ti in that of Al-Mg system, which indicates that Al-Tiofintermetallic prefers to be formed systems werebinary negative. The standard molar enthalpy Al-Ti binarycompound system was more negative than the same condition. atthat theof interface of the Mg-9Al/Ti in the same condition. Al-Mg binary system, which indicates that Al-Ti intermetallic compound prefers to be formed at the interface of the Mg-9Al/Ti in the same condition.

Figure binary phase phasediagram. diagram. Figure12. 12. Al-Ti Al-Ti binary Figure 12. Al-Ti binary phase diagram.

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Previous studies involving the synthesis of titanium aluminides through powder metallurgical routes showed that Al 3Ti forms prior to the formation of any other titanium aluminides belonging to Previous studies involving the synthesis of titanium aluminides through powder metallurgical the binary Ti–Al system [25].prior Formation of Al3Ti has alsoother beentitanium reported during the interaction routes showed that Al3 Ti forms to the formation of any aluminides belonging to between solid Ti and liquid Al [26]. However, the reason was not evaluated clearly. The possible the binary Ti–Al system [25]. Formation of Al3 Ti has also been reported during the interaction between reason for the preference can be explained by considering the thermodynamic drive force at the Mgsolid Ti and liquid Al [26]. However, the reason was not evaluated clearly. The possible reason for the Al/Ti interface. is based the temperature dependence drive of Gibbs energy of formation of preference can beThis explained by on considering the thermodynamic forcefree at the Mg-Al/Ti interface. various Al-Ti compounds from the literature [27,28]. In previous studies, the sublattice model This is based on the temperature dependence of Gibbs free energy of formation of various Al-Ti (Wagner–Schottky was [27,28]. used to In calculate Gibbs free energies of formation AlTi3, AlTi, Al3Ti, compounds from themodel) literature previous studies, the sublattice model of (Wagner–Schottky Al2Ti and Al5Tito 2. The final expressions obtained for Gibbs free energies of formation for the model) was used calculate Gibbs free energies of formation of AlTi3 , AlTi, Al3 Ti, Al2 Ti and Al5 Ti2 . compounds are presented Theenergies values of offormation Gibbs free are calculated in the The final expressions obtainedinforTable Gibbs2.free forenergies the compounds are presented temperature range of 373–1073 K and the results obtained are shown in Figure 13. It can be found in Table 2. The values of Gibbs free energies are calculated in the temperature range of 373–1073 Kin thisthe figure thatobtained in this temperature free energy of the Al3Ti is in lower the AlTi and results are shown inrange, Figurethe 13. Gibbs It can be found in this figure that this than temperature and AlTi 3, and is higher than the Al 2Ti and Al5Ti2, but the Al 2Ti and Al5Ti2 must react with AlTi range, the Gibbs free energy of the Al3 Ti is lower than the AlTi and AlTi3 , and is higher than the Al2 Ti through ofAl reactions. So Al3Ti is expected to be the first phase to form in the Al-Ti system. and Al5 Ti2a, series but the 2 Ti and Al5 Ti2 must react with AlTi through a series of reactions. So Al3 Ti is This is in agreement with the EDS XRD results of the interface between Mg-9Al obtained expected to be the first phase to formand in the Al-Ti system. This is in agreement with theand EDSTiand XRD at 700 °C after 60 min. However, there was only a small number of Al diffusion to the Ti rod this ◦ results of the interface between Mg-9Al and Ti obtained at 700 C after 60 min. However, thereinwas study, so thenumber formation ofdiffusion Al3Ti compound only a small of Al to the Tiwas rod less. in this study, so the formation of Al Ti compound 3

was less.

Table 2. Temperature dependence of Gibbs free energy of formation of various Ti-A1 compounds, Table Temperature data 2. from ref. [21]. dependence of Gibbs free energy of formation of various Ti-A1 compounds, data from ref. [21].

Compound Compound AlTi3 AlTi AlTi3 AlAlTi 3Ti 3 Ti AlAl2Ti Al2 Ti Al5Ti2 Al5 Ti2

Free Energy of Formation, ΔG Free Energy of Formation, ∆G −9,633.6 + 6.70801T −37,445.1 + 16.79376T −9,633.6 + 6.70801T −37,445.1 + 16.79376T −40,349.6 + 10.36525T −40,349.6 + 10.36525T −43,858.4 + 11.02077T −43,858.4 + 11.02077T −40,495.4 + 9.52964T −40,495.4 + 9.52964T

Figure 13. Gibbs energy of formation, ∆G, of different compounds as a function of temperature. Figure 13. Gibbs energy of formation, ΔG, of different compounds as a function of temperature.

3.4. The Metallurgical Reaction Mechanism of the Solid/Liquid Interface 3.4. The Metallurgical Reaction Mechanism of the Solid/Liquid Interface Metallurgical reaction mechanism of Mg-9Al/Ti interface by liquid-solid diffusion couples was Metallurgical reactiondiagram mechanism of Mg-9Al/Ti interface diffusion couples clarified with the schematic shown in Figure 14. Under by theliquid-solid experimental temperature, thewas Ti clarified with the schematic diagram shown in Figure 14. Under the experimental temperature, the rods were rapidly submerged into the molten Mg-9Al alloy. Mg and Ti did not react with each other. Ti rods were rapidly submerged into theofmolten alloy. Mg and Ti did not react with each However, Al and Ti can be the formation severalMg-9Al A1-Ti intermetallic compounds. The Al atoms other. However, Al and Ti can be the formation of several A1-Ti intermetallic compounds. The Al diffuse from the molten Mg-9Al alloy filler to the liquid/solid interface and Ti substrate. Al atoms were atomswith diffuse from for the the molten Mg-9Aland alloy filler tofrom the liquid/solid and Ti substrate. Al mixed Ti atoms Ti substrate diffusing Ti substrateinterface at the liquid/solid interface, atoms were mixed14a,b. with Ti atoms the Ti substrate diffusing from Tithe substrate at potential the liquid/solid indicated in Figure Tan et al.for [16] found that for and the same Ti content, chemical of Al interface, as indicated in Figure 14a,b. Tan et al.For [16] found for the same Ti content, the chemical decreased the Al molar fraction increased. the samethat Al content, the chemical potential of Al potential of Al decreased as the Al molar fraction increased. For the same Al content, the chemical

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decreased with reduction of the Ti molar fraction. low AlTherefore, and high Ti content promoted potential of Al the decreased with reduction of TiTherefore, molar fraction. low Al and high Ti the diffusion of Al the atom from molten to thefrom liquid/solid interface and Ti substrate Al atom content promoted diffusion of Al atom molten to the liquid/solid interface[29]. andThe Ti substrate in the The Ti substrate and interface wassolid/liquid saturated inducing precipitation Al3 Ti phase [29]. Al atom insolid/liquid the Ti substrate and interfacethe was saturated ofinducing the ◦ C, the eutectic indicated in Figure 14c. When the temperature decreased to 325 reaction occurred precipitation of Al3Ti phase indicated in Figure 14c. When the temperature decreased to 325 °C, the with α-Mg+Mg Al12 [30],with as shown in Figure 14d. as Asshown a result, couple form athe complete eutectic reaction17occurred α-Mg+Mg 17Al12 [30], in the Figure 14d.could As a result, couple metallurgical bonding formed at the Mg-9Al/Ti interface. could form a complete metallurgical bonding formed at the Mg-9Al/Ti interface.

Figure interface. Figure 14. 14. Schematic Schematic diagram diagram of of the the metallurgical metallurgical reaction reaction mechanism mechanism of of the the solid/liquid solid/liquid interface. (a,b) diffusion of Ti atoms and Al atoms at the interface, (c) precipitation of Al Ti phase 3 (a,b) diffusion of Ti atoms and Al atoms at the interface, (c) precipitation of Al3Ti phase in in the the Ti Ti substrate and solid/liquid interface, (d) solidification of interfacial zone. substrate and solid/liquid interface, (d) solidification of interfacial zone.

4. Conclusions 4. Conclusions In this paper, the effects of different heat treatment time on the microstructures, mechanical In this paper, the effects of different heat treatment time on the microstructures, mechanical properties, and fractographies of the Mg-9Al/Ti metallurgical bonding by liquid-solid diffusion properties, and fractographies of the Mg-9Al/Ti metallurgical bonding by liquid-solid diffusion couples were investigated. The obtained results can be summarized as follows. couples were investigated. The obtained results can be summarized as follows. (1) TheMg Mg1717 the Mg-9Al matrices was attached surface rods. Grain boundaries (1) The AlAl 12 12 inin the Mg-9Al matrices was attached to to thethe surface of of Ti Ti rods. Grain boundaries Ti Ti and Mg-9Al wetting transition from incompletely wetted to completely wetted by and Mg-9Al wetting transition from incompletely wetted to completely wetted byMg Mg1717Al Al1212 phase.AAmetallurgical metallurgicalbond bondwas wasestablished establishedatat the the interface interface between between Mg-9Al Mg-9Al alloy alloy and and Ti phase. Ti matrices. The impurity diffusion coefficients of Mg in Ti and Al in Ti, and Ti in Mg-9Al matrices. The impurity diffusion coefficients of Mg in Ti and Al in Ti, and Ti in Mg-9Al are are −15 m2 /s, 3.77(±0.12) × 10−15 m2 /s, 1.60(±0.23) × 10−14 m2 /s, respectively. 1.70(±0.02) × −15 10m 2/s, 3.77(±0.12) × 10 −15 m2/s, 1.60(±0.23) × 10−14 m2/s, respectively. The diffusion 1.70(±0.02) × 10 The diffusion of Al and Ti elements play role an important role during of theMg-9Al/Ti interface of Mg-9Al/Ti of Al and Ti elements play an important during the interface metallurgical metallurgical bonding. bonding. (2) Shearstrength strengthof of Mg-9Al/Ti Mg-9Al/Timetallurgical metallurgicalbonding bondingwas was presented presented an an increasing increasing tendency (2) Shear tendency in in perfect accordance with heat treatment time. Shear strength could reach the maximum perfect accordance with heat treatment time. Shear strength could reach the maximum value value of of 56MPa. MPa.The Thefracture fracturewas wastook took place place along along Mg-9Al Mg-9Al matrix matrix at at the the interface. interface. Shear Shear stress 56 stress greatly greatly improveswhen whenAl Aland andTiTiinterdiffused interdiffusedand andchemical chemicalinteraction interactionoccurs occursbetween betweenMg-9Al Mg-9Al and and Ti. Ti. improves (3) ByEDS EDSand andXRD XRDanalysis, analysis,these theseresults resultswere were found found that that Al Al33Ti Ti is is the the only only intermetallic intermetallic compound (3) By theinterface interfaceof ofthe theMg-9Al/Ti Mg-9Al/Ticompound compoundcastings. castings.Gibbs Gibbsfree freeenergies energies of of formation formation of AlTi33, atatthe AlTi, Al Ti, Al Ti and Al Ti were calculated. The result was shown that Al Ti is expected to be AlTi, Al33Ti, Al2Ti 2 and Al55Ti2 2were calculated. The result was shown that Al33Ti is thefirst firstphase phasetotoform formininthe theAl-Ti Al-Tisystem. system. the Author Contributions: B.J., C.P. and F.P. designed the project and guided the research. J.D, Z.J., Q.Y. Author Contributions: B.J., C.P. and F.P. designed the project and guided the research. J.D, Z.J., Q.Y. (Qingshan (Qingshan Yang) and Q.C. theYan). data.and Q.Y. (Qiong Yan). and Yang) and Q.C. performed theperformed experiment the andexperiment analyzed theand data.analyzed Q.Y. (Qiong H.X. investigated the project. J.D. wrote and H.X. investigated thereviewed project. the J.D.manuscript. wrote and reviewed the manuscript. Funding: This research was funded by the Chongqing Science and Technology Commission (Grant No. Funding: This research was funded by the Chongqing Science and Technology Commission (Grant cstc2017jcyjAX0394), the Scientific and Technological Research Program of Chongqing Municipal Education No. cstc2017jcyjAX0394), the Scientific Technological Program of Chongqing Municipal Commission (Grant No. KJ1712301), theand Fuling Science andResearch Technology Commission (Grant No. FLKW, 2017ABA1014), the Fundamental Research Funds for the Yangtze Normal University (Grant No. 2016KYQD15), Education Commission (Grant No. KJ1712301), the Fuling Science and Technology Commission the National Natural Science Foundation of China (Grant No. 51701033), the Talent project of Chongqing (Grant No. FLKW, 2017ABA1014), the Fundamental Research Funds for the Yangtze Normal University of Arts and Sciences (Grant No. 2017RXC24), the undergraduate research project for the Yangtze University (Grant No. 2016KYQD15), the National Natural Science Foundation of China (Grant No. Normal University.

51701033), the Talent Chongqing University ofUniversity) Arts and Sciences (Grant improvement. No. 2017RXC24), Acknowledgments: Theproject authorsof thank Jianyue Zhang (Purdue for the language the undergraduate research project for the Yangtze Normal University.

Conflicts of Interest: The authors declare no conflict of interest.

Acknowledgments: The authors thank Jianyue Zhang (Purdue University) for the language improvement.

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