Spark Plasma Diffusion Bonding of Tial/Ti2alnb with Ti As Interlayer

materials

Article Spark Plasma Diﬀusion Bonding of TiAl/Ti2AlNb with Ti as Interlayer

Boxian Zhang 1, Chunhuan Chen 1,*, Jianchao He 2,3,*, Jinbao Hou 3, Lu Chai 3 and Yanlong Lv 3 1 School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China; [email protected] 2 Institute of Special Environments Physical Sciences, Harbin Institute of Technology, Shenzhen 518055, China 3 Aeronautical Key Laboratory for Welding and Joining Technologies, AVIC Manufacturing Technology of Institute, Beijing 100024, China; [email protected] (J.H.); [email protected] (L.C.); [email protected] (Y.L.) * Correspondence: [email protected] (C.C.); [email protected] (J.H.)

Received: 6 July 2020; Accepted: 22 July 2020; Published: 24 July 2020

Abstract: To solve the problem of poor weldability between TiAl-based and Ti2AlNb-based alloys, spark plasma diffusion bonding was employed to join a TiAl alloy and a Ti2AlNb alloy with a pure Ti foil as interlayer at 950 ◦C/10 KN/60 min. After welding, slow cooling was carried out at a rate of 5 ◦C/min, followed by homogenization at 800 ◦C for 24 h. The microstructural evolution and elemental migration of the joint were analyzed via a scanning electron microscope equipped with an energy dispersive spectrometer, while the mechanical properties of the joint were assessed via microhardness and tensile tests. The results show that the spark plasma diffusion bonding formed a joint of TiAl/Ti/Ti2AlNb without microcracks or microvoids, while also effectively protecting the base metal. Before heat treatment, the maximum hardness value (401 HV) appeared at the Ti2AlNb/Ti interface, while the minimum hardness value (281 HV) occurred in the TiAl base metal. The tensile strength of the heat-treated joint at room temperature was measured to be up to 454 MPa, with a brittle fracture occurring in the interlayer. The tensile strength of the joint at 650 ◦C was measured to be up to 538 MPa, with intergranular cracks occurring in the TiAl base metal.

Keywords: spark plasma diﬀusion bonding; TiAl; Ti2AlNb; microstructure; elemental migration; microhardness; tensile properties

1. Introduction With the development of modern aerospace industry, it has become increasingly difficult for traditional materials to meet the requirements of aerospace structural components, that is, to be lightweight and to offer high performance simultaneously. Titanium aluminum alloys and bi-metal structures are getting increased attention as optimal solutions for this problem. Due to low density, high melting point, high specific modulus, and excellent creep and oxidation resistance, TiAl alloy is one of the important candidate materials to replace some Ni-base superalloys in the temperature range of 650–900 ◦C with the goal of weight loss [1,2]. Ti2AlNb alloy is a new type of lightweight high-temperature structural material developed after TiAl and Ti3Al alloys, with a high performance of fracture toughness and creep resistance caused by the addition of the Nb element [3,4]. For its good low-cycle-fatigue property and high creep resistance, Ti2AlNb alloy has been applied extensively in the aerospace field within 600–750 ◦C. Researching the welding of TiAl and Ti2AlNb is of significant importance, as this will allow the thrust-to-weight ratio of aerospace components to be improved and the application possibilities of such alloys to be broadened. To date, fusion welding techniques such as laser welding [5–8] and electron beam welding [9–12] have been employed when joining TiAl-based alloys. It is found that defects such as high stress, hot

Materials 2020, 13, 3300; doi:10.3390/ma13153300 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 9 importance, as this will allow the thrust-to-weight ratio of aerospace components to be improved and the application possibilities of such alloys to be broadened. To date, fusion welding techniques such as laser welding [5–8] and electron beam welding [9–

12]Materials have2020 been, 13 ,employed 3300 when joining TiAl-based alloys. It is found that defects such as high stress,2 of 9 hot cracks, and brittle intermetallic compounds often occur due to excessively fast cooling speed when laser welding and electron beam welding are used. Solid phase bonding is expected to be used forcracks, the achievement and brittle intermetallic of high-quality compounds bonding of often TiAl occur alloy. due Methods to excessively of solid phase fast cooling welding, speed including when linearlaser welding friction andwelding electron [13] beam and conventional welding are used. diffusion Solid bonding phase bonding [14,15], is can expected generate to be rec usedrystallized for the grainsachievement at the of bonding high-quality interface bonding of TiAl/Ti of TiAl2AlNb alloy. Methods and block of thesolid crossing phase welding, of cracks, including thus further linear improvingfriction welding the welding [13] and quality. conventional diffusion bonding [14,15], can generate recrystallized grains at the bondingSpark plasma interface diffusion of TiAl/ Ti bonding2AlNb and (SPDB) block is the a crossing rapid-bonding of cracks, method thus further that introduces improving the weldingcharacteristics quality. of pulse current to solid phase diffusion bonding, promoting atomic diffusion and materialSpark plastic plasma deformation. diffusion The bonding key advantage (SPDB) isto this a rapid-bonding method is that method the heat thatis mainly introduces generated the atcharacteristics the interface, of which pulse is current definitely to solid different phase from diffusion the overall bonding, heating promoting of the atomic traditional diffusion hot-press and diffusionmaterial plasticbonding. deformation. SPDB has been The keyapplied advantage to the todirect this diffusion method is bonding that the of heat Ti- is48Al mainly-2Cr- generated2Nb alloy [at16 the], Ti interface,-45Al-7Nb which-0.3W is (a definitelyt.%) alloy di[17fferent], TZM30 from alloy, the overall WRE alloy heating [18 of], and the traditional HS-6-7-6-10 hot-press-0.1LaB6 powderdiffusion metallurgy bonding. SPDB high hasspeed been steel applied (PMHSS) to the [ direct19] and di fftousion the connection bonding of Ti-48Al-2Cr-2Nbof SiC [20] with Ta alloy-5W [16 as], Ti-45Al-7Nb-0.3Wthe interlayer. (at.%) alloy [17], TZM30 alloy, WRE alloy [18], and HS-6-7-6-10-0.1LaB6 powder metallurgyWhen highmetallurgical speed steel factors (PMHSS) are taken [19] and into to theconsideration, connection ofit SiCis of [ 20great] with importance Ta-5W as theto interlayer.select the appropriateWhen metallurgical interlayer to obtain factors high are taken-quality into welded consideration, joints. In itthe is diffusion of great importance bonding of toTiAl select alloys the, thereappropriate are many interlayer kinds of to interlayers. obtain high-quality Ti-Nb [21 welded], Ti-Ni-Nb joints. [22] In and the TiZrCuNi diffusion bonding[23] have ofall TiAl been alloys, used tothere improve are many the kindsperformance of interlayers. of a TiAl Ti-Nb alloy [21 bonding], Ti-Ni-Nb interface. [22] and It TiZrCuNihas been found [23] have that all Ti beenhas good used compatibilityto improve the with performance Ti2AlNb alloy of a and TiAl can alloy inhibit bonding the form interface.ation of It brittle has been compounds. found that Ti has good compatibilityTherefore, with in order Ti2AlNb to obtain alloy high and can-quality inhibit welded the formation joints, pure of brittleTi was compounds. selected as the interlayer in the SPDBTherefore, of TiAl in orderand Ti to2AlNb. obtain The high-quality microstructure welded and joints, mechanical pure Ti wasproperties selected of as the the welded interlayer joints in werethe SPDB then ofanalyzed. TiAl and Ti2AlNb. The microstructure and mechanical properties of the welded joints were then analyzed. 2. Experimental Procedure 2. Experimental Procedure An as-extruded TiAl intermetallic compound with a nominal composition of Ti-46Al-2Cr (at.%) madeAn by as-extrudedInstitute of M TiAletal intermetallic Research, Chinese compound Academy with aof nominal Sciences composition (Shenyang, ofChina) Ti-46Al-2Cr was selected. (at.%) Themade room by Institute-temperature of Metal microstructure Research, Chinese of this Academymaterial is of shown Sciences in Figure (Shenyang, 1a. The China) material’s was selected. nearly The room-temperature microstructure of this material is shown in Figure1a. The material’s nearly fully lamellar structure consisted of α phases and an alternate lamellar of γ phases. Forged Ti2AlNb alloyfully lamellar with nominal structure composition consisted of ofα Tiphases-22Al-27Nb and an (at.%) alternate mad lamellare by Central of γ phases. Iron & Forged Steel Research Ti2AlNb Institutealloy with (Beijing, nominal China) composition was also of Ti-22Al-27Nbselected. Figure (at.%) 1b shows made bythe Central base metal Iron &of Steel this Researchmaterial after Institute the (Beijing,800 °C annealing. China) was The also resulting selected. microstructure Figure1b shows was the composed base metal of of massivethis material O pha afterses thethat 800 were◦C distributedannealing. Theon the resulting β/B2 matrix. microstructure As one can was see composed from the of image massive, the O O phases phase thatis dark were and distributed looks gray, on β whilethe / Bthe2 matrix. B2 phase As oneis bright can see white. from Pure the image, titanium the OTA1 phase with is dark0.1 mm and thickness looks gray, was while selected the B2 asphase the interlayer.is bright white. The base Pure metal titanium to be TA1 bonded with was 0.1 mmmachined thickness into was a rod selected with a as diameter the interlayer. of 40 mm The and base a lengthmetalto of be 60bonded mm. was machined into a rod with a diameter of 40 mm and a length of 60 mm.

Figure 1. Microstructures of alloys (a) TiAl and (b) Ti2AlNb. Figure 1. Microstructures of alloys (a) TiAl and (b) Ti2AlNb.

The diﬀusion welding test was carried out using HPD-25-HV/SP spark plasma diﬀusion equipment manufactured by FCT Systeme GmbH (Rauenstein, Germany) with a temperature of 950 ◦C, heating rate of 100 C/min, pulse current of 7000 A, frequency of 1/9 ratio, vacuum degree of 2.3 10 3 Pa, ◦ × − Materials 2020, 13, x FOR PEER REVIEW 3 of 9

MaterialsThe2020 diffusion, 13, 3300 welding test was carried out using HPD-25-HV/SP spark plasma diffusion3 of 9 equipment manufactured by FCT Systeme GmbH (Rauenstein, Germany) with a temperature of 950 °C, heating rate of 100 °C/min, pulse current of 7000 A, frequency of 1/9 ratio, vacuum degree of 2.3 pressure× 10−3 Pa, ofpressure 10 kN, andof 10 holding kN, and time holding of 60 min.time Slowof 60 coolingmin. Slow was cooling performed was atperformed a rate of 5 at◦C a/ minrate afterof 5 welding.°C/min after The welding. welding The assembly welding diagram assembly is shown diagram in Figure is shown2. in Figure 2.

Figure 2. SchematicSchematic diagram diagram of of the the spark spark plasma diffusion diﬀusion bonding method.

After welding, homogenizing heat treatment was performed using a vacuum heat treatment furnafurnacece manufactured by Centorr Vacuum Vacuum Industries (Nashua, (Nashua, NH, NH, USA) USA) at at 800 800 °C◦C for for 24 24 h. h. Microstructure specimens were obtained along the direction perpendicular to to the welding surface, ground andand polished,polished, and and corroded corroded by by aqueous aqueous solutions solutions of hydroﬂuoricof hydrofluoric acid acid and and nitric nitric acid. acid. The jointThe jointmicrostructure microstructure wasobserved was observed and analyzed and analyzed on a Zeisson a Zeiss Suppera55 Suppera55 scanning scanning electron electron microscope microscope at the attest the center test center of the of China the China Aviation Aviation Manufacturing Manufacturing Technology Technology Research Research Institute Institute (Beijing, (Beijing, China), China) and the, andchemical the chemical composition composition of each phaseof each produced phase produced by the interfacial by the interfacial reaction reaction was analyzed was analyzed by an Oxford by an XmaxOxford energy Xmax dispersiveenergy dispersive spectrometer spectro manufacturedmeter manufactured by Carl Zeissby Carl (Shanghai, Zeiss (Shanghai, China). The China) hardness. The hardnessdistribution distribution of the joint of was the testedjoint was by antested HXD-1000 by an HXD microhardness-1000 microhardness tester manufactured tester manufactured by Shanghai by ShanghaiOptical Instrument Optical Instrument Factory (Shanghai, Factory China)(Shanghai, with China) a test load with of a 300 test gf load (294.2 of N) 300 and gf a(294.2 holding N) time and ofa 15holding s. For time the microhardness of 15 s. For the test, microhardness at least ﬁve points test, wereat least selected five points for each were of theselected following for each areas: of TiAl the followingbase metal, areas: TiAl /TiAlTi interface, base metal interlayer,, TiAl/Ti Ti interface,/Ti2AlNb interlayer, interface, andTi/Ti Ti2AlNb2AlNb interface, base metal. and AfterTi2AlNb the base heat metal.treatment, After the the specimens heat treatment, were tested the by specimens a Z100 universal were tested testing by machine a Z100 manufactured universal testing by ZwickRoell machine (Shanghai,manufactured China). by ZwickRoell for tensile tests (Shanghai, at room temperatureChina). for tensile and high tests temperature. at room temperature The tensile specimens and high weretemperature. prepared The according tensile specimens to the HB5214-96 were prepared standard. according to the HB5214-96 standard.

3. Results Results and and Discussion 3.1. Microstructural Evolution and Elemental Migration 3.1. Microstructural Evolution and Elemental Migration The microstructures of the as-welded joints are shown in Figure3. No welding defects were The microstructures of the as-welded joints are shown in Figure 3. No welding defects were observed at the as-welded bonding interface of TiAl and Ti2AlNb in terms of microholes, microcracks, observed at the as-welded bonding interface of TiAl and Ti2AlNb in terms of microholes, microcracks, poor bonding, or other anomalies. Furthermore, a trend of microstructural variation was noticed poor bonding, or other anomalies. Furthermore, a trend of microstructural variation was noticed from TiAl to Ti2AlNb. The width of the bonding interface was estimated to be 150 µm. To simplify from TiAl to Ti2AlNb. The width of the bonding interface was estimated to be 150 μm. To simplify the description of the results, the joint was divided into seven zones: the TiAl base metal (zone I), the description of the results, the joint was divided into seven zones: the TiAl base metal (zone I), the the diﬀusion zone of TiAl (zone II), TiAl/Ti interface (zone III), the Ti interlayer (zone IV), the Ti/Ti2AlNb diffusion zone of TiAl (zone II), TiAl/Ti interface (zone III), the Ti interlayer (zone IV), the Ti/Ti2AlNb interface (zone V), the diﬀusion zone of Ti/Ti2AlNb (zone VI), and the Ti2AlNb base metal (zone VII). interface (zone V), the diffusion zone of Ti/Ti2AlNb (zone VI), and the Ti2AlNb base metal (zone VII).

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Figure 3. Microstructure of the TiAl/Ti TiAl/Ti22AlNb joint diffusion diﬀusion bonded with Ti foil at 950 ◦°CC for 60 min under pressure of 10 MPa and the Vickers microhardnessmicrohardness proﬁleprofile acrossacross thethe joint.joint.

When using SPDB, the heat-aheat-affectedffected range is quite narrow; hence, the TiAl andand TiTi22AlNb base metals were protected eeffectively.ffectively. The analyses suggested very small cluster sizes of the α phase an andd the γ phase at the the TiAl TiAl base metal in in zone zone I. I. Besides, Besides, no no significant significant changes changes in in micro microstructurestructure between zone I and zone II were recorded, indicating that welding temperature had little effect effect on the TiAl base metal. Zone VI represented the diffusion diffusion zone of Ti22AlNb, with a microstructure consisting of numerous O phases distributed in the B2 matrix. As As expected, expected, the the growth growth of of the the O O phase phase was was noticed. noticed. The welding thermal cycle showed little influenceinfluence on the Ti22AlNb base metal, leading to insignificant insignificant changes in the microstructure ofof zonezone VII.VII. Using pure Ti foil asas anan interlayerinterlayer eeffectivelyffectively improvedimproved the continuity in the microstructure of the joint. Zone Zone III III represents represents the the transition transition region region at which which the the TiAl TiAl base metal metal and Ti Ti interlayer were bonded. Though the interface interface looked obvious, obvious, the the bonding bonding line line appeared appeared discontinuous discontinuous and and curved, curved, consistentconsistent with intermetallic compound (IMC) (IMC) Ti Ti33AlAl based based on on the the Ti/Al Ti/Al binary phase phase diagram diagram [ [2424].]. Close toto thethe bonding bonding line, line, the the microstructure microstructure of theof the TiAl TiAl base base metal metal remained remained almost almost unchanged, unchanged, while thewhile interlayer the interlayer side contained side contained a coarse feathery a coarse microstructure. feathery microstructure. The formation The of suchformation a microstructure of such a wouldmicrostructure most likely would relate most to interdi likely ffrelateusion to between interdiffusion TiAl and between Ti. TiAl and Ti. Zone IV comprised the middle area of the interlayer and was mainly composed of residual phases formed after didiffusionffusion and metallurgical reaction. Further Furthermore,more, the microstructure contained precipitated α phases, differently differently from that of the TiAlTiAl/Ti/Ti transitiontransition zone.zone. Zone V represented the transition region of TiTi/Ti/Ti22AlNb, where Ti and Ti22AlNb became didiffusionffusion bonded. No evident bonding interface waswas observedobserved under under the the as-welded as-welded state state of thisof this transition transition zone, zone, and and the grainthe grain size lookedsize looked finer thanfiner that than of that the TiAl of the/Ti interface TiAl/Ti (zone interface III). (zoneAccording III). to According previous studies to previous [25], such studies a microstructure [25], such a wouldmicrostructure be more would likely be to more contain likelyα-Ti to and containβ-Ti. α These-Ti and results β-Ti. These were similarresults were to those similar reported to those by Wangreported Ying by [ 26Wang], who Ying determined [26], who thatdetermined the dark that grey the acicular dark grey precipitates acicular inprecipitates the diffusion in the region diffusion were Nb-deficientregion were Nb and-deficient the light and grey the matrix light grey phase matrix was phase Nb-enriched. was Nb-enriched. However, However, the dark the grey dark acicular grey precipitatesacicular precipitates contained contained higher Alhigher content Al c thanontent the than light the grey light matrix grey matrix phase. phase. Thus, Thus, analysis analysis of each of phaseeach phase in Ti2 inAlNb Ti2AlNb and Tiand suggests Ti suggests that largethat large amounts amounts of Nb of can Nb form can form an infinite an infinite solid solid solution solution with βwith-Ti diβ-ffTiusion diffusion from Tifrom2AlNb Ti2 toAlNb Ti. Thisto Ti. led This to theled formation to the formation of a Nb-enriched of a Nb-enriched zone near zone the interface,near the determinedinterface, determined as a B2-enriched as a B2- region.enriched region. Figure 4 shows the microstructure after homogenization by heat treatment. The microstructure of the TiAl base metal had no sharp changes; however, the phase transition occurred on the Ti2AlNb

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Figure4 shows the microstructure after homogenization by heat treatment. The microstructure of Materials 2020, 13, x FOR PEER REVIEW 5 of 9 the TiAl base metal had no sharp changes; however, the phase transition occurred on the Ti2AlNb base metal.base metal. Zones Zones I and IIII and presented III presented no obvious no obvious change change after heat after treatment. heat treatment. In zone II,In Alzone further II, Al di furtherffused todiffused the interlayer, to the interlayer, and more and Ti wasmore simultaneously Ti was simultaneously replenished replenished from the from interlayer. the interlayer. This led This to theled fastto the growth fast growth and formation and formation of the α ofphase the α along phase the along diffusion the diffusion direction. dire Asction. a result, As a the result, fully the lamellar fully microstructurelamellar microstructure obtained developedobtained developed into laths followinginto laths following one direction. one Zonedirection. VI and Zone zone VIVII and displayed zone VII nodisplayed other di ffnoerences other exceptdifferences in terms except of grain in terms coarsening of grain of thecoarsening O phase, of which the O led phase, to sharpened which led phase to boundariessharpened phase of the boundaries O phase when of the compared O phase to when the as-welded compared joints. to the Theas-welded effect of joints. heat treatmentThe effect onof base-metal Ti AlNb looked obvious as well. In particular, the B2 phase mostly transformed into the O heat treatment2 on base-metal Ti2AlNb looked obvious as well. In particular, the B2 phase mostly phasetransformed and into into small the amountsO phase and of α 2into due small to the amounts existence of ofα2 some due unevenlyto the existence distributed of some Nb unevenly element, and transformation of B2 α2 only occurred in the Nb-less regions. distributed Nb element, and→ transformation of B2→α2 only occurred in the Nb-less regions.

Figure 4. Microstructure of the TiAl/Ti2AlNb joint diffusion bonded with Ti foil after homogenizing Figure 4. Microstructure of the TiAl/Ti2AlNb joint diﬀusion bonded with Ti foil after homogenizing heat treatment at 800 °C for 24 h and the Vickers microhardness profile across the joint. heat treatment at 800 ◦C for 24 h and the Vickers microhardness proﬁle across the joint.

After homogenization, the most significantsignificant change appeared in zone V. Importantly, anan interface emerged betweenbetween Ti andand TiTi22AlNb with great didifferencesfferences inin thethe microstructuresmicrostructures ofof bothboth sidessides nearnear the interface.interface. In the bulk of the interlayer, acicularacicular β clusters were grown and arranged alternately onon thethe α matrix, leading to river-likeriver-like patternspatterns that werewere perpendicularperpendicular toto thethe interface.interface. At the Ti22AlNb side, finefine needle-likeneedle-like structuresstructures appearedappeared andand arrangedarranged unorderly,unorderly, similarsimilar toto base-metalbase-metal TiTi22AlNb.AlNb. DuringDuring thethe slowslow coolingcooling stage stage following following heat heat treatment, treatment, both bothα2 α2 and and O phasesO phases partially partially precipitated precipitated again again and coexistedand coexisted with with the β thephase β phase near near the interface. the interface. The composition composition of the didiffusionffusion bonding bonding interface interface was characterized by EDS, the results of of whichwhich areare providedprovided inin FigureFigure5 .5. TheThe extendedextended widthwidth ofof thethe interlayerinterlayer inin thethe as-weldedas-welded jointjoint waswas estimated to to be be about about 250 250µm. μm. This This value value was much was much higher higher than the than thickness the thickness of the original of the interlayer, original confirminginterlayer, confirming the occurrence the ofoccurrence ample interdi of ampleffusion interdiffusion between the interlayerbetween the and interlayer the base metals and the during base themetals welding during process. the welding process.

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Figure 5. The elemental distributions along TiAl/Ti2AlNb alloy joints bonded with the Ti interlayer: Figure 5. The elemental distributions along TiAl/Ti2AlNb alloy joints bonded with the Ti interlayer: (a) as-welded(a) as-welded and and (b ()b heat-treated.) heat-treated.

Al content decreased gradually from the TiAl to the Ti2AlNb side; it dropped sharply in the Al content decreased gradually from the TiAl to the Ti2AlNb side; it dropped sharply in the Ti/Ti2AlNb transition zone, where the Al content reached the lowest level when compared to both Ti/Ti2AlNb transition zone, where the Al content reached the lowest level when compared to both base base metals. By comparison, Ti content first increased and then declined from TiAl side to Ti2AlNb metals. By comparison, Ti content first increased and then declined from TiAl side to Ti AlNb side, side, reaching its maximum value in the TiAl/Ti transition zone. 2 reaching its maximum value in the TiAl/Ti transition zone. Ti and Al in the Ti2AlNb base metal diffused to the Ti interlayer more quickly than Nb in the Ti and Al in the Ti AlNb base metal diffused to the Ti interlayer more quickly than Nb in the base metal due to the 2smaller atomic radius of Ti and Al when compared to that of Nb. This led to base metal due to the smaller atomic radius of Ti and Al when compared to that of Nb. This led to enrichment of the transition zone of Ti2AlNb/Ti by Nb. The microstructural analyses revealed that enrichmentonly small of amounts the transition of finer O zone had ofprecipitated Ti2AlNb/Ti from by the Nb. B2 The matrix microstructural in this region, analysesindicating revealed depletion that onlyof smallAl due amounts to the region’s of finer stable O had α-phase. precipitated These data from were the B2consistent matrix inwith this those region, from indicating EDS. depletion of Al dueThe to contents the region’s of Ti stable in bothα-phase. TiAl (Ti These-46Al-2Cr) data and were Ti consistent2AlNb (Ti-22Al with-27Nb) those fromwere EDS.close to each otherThe before contents welding, of Ti in while both Ti TiAl distribution (Ti-46Al-2Cr) varied and significantly Ti2AlNb (Ti-22Al-27Nb)after welding. The were Ti closecontent to in each TiAl other beforerose welding, to that ofwhile the Ti interlayer, distribution while varied Ti content significantly in Ti2AlNb after welding. remained The almost Ti content unchanged. in TiAl The rose to thatdistribution of the interlayer, of Ti was while related Ti content to the difference in Ti2AlNb in diffusion remained rates almost of Al unchanged. and Nb. The The Al content distribution was of Ti wastwice related as high to in the TiAl diff thanerence in inTi2 diAlNb,ffusion leading rates to of an Al elevated and Nb. concentration The Al content gradient was twice of Al asat highthe in TiAl/Ti interface when compared to the Ti/Ti2AlNb interface. TiAl than in Ti2AlNb, leading to an elevated concentration gradient of Al at the TiAl/Ti interface when Compared to Ti migrating from the interlayer, more Al located at the TiAl side migrated toward compared to the Ti/Ti2AlNb interface. the interlayer. However, the amount of transferable Al at the Ti2AlNb side was nearly half that of Compared to Ti migrating from the interlayer, more Al located at the TiAl side migrated toward TiAl. Coupled with the low diffusion rate of Nb, migration of Ti from interlayer to Ti2AlNb became the interlayer. However, the amount of transferable Al at the Ti2AlNb side was nearly half that of TiAl. more challenging, leading to an elevated concentration gradient of Ti near the Ti/Ti2AlNb interface. Coupled with the low diffusion rate of Nb, migration of Ti from interlayer to Ti AlNb became more After heat treatment, no obvious changes in the shape of the elemental distribution2 curves were challenging,observed, with leading the exception to an elevated of the concentrationbroadening of the gradient interlayer of Ti width. near Thus, the Ti the/Ti2 elementsAlNb interface. underwent furtherAfter diffusion, heat treatment, but distribution no obvious regularities changes of in Ti, the Al, shape and Nb of theremained elemental unchanged. distribution curves were observed, with the exception of the broadening of the interlayer width. Thus, the elements underwent further3.2. Microhardness diffusion, but D distributionistribution regularities of Ti, Al, and Nb remained unchanged.

3.2. MicrohardnessThe microhardness Distribution distributions of the as-welded and heat-treated joints are displayed in Figures 3 and 4. For the as-welded joints, the highest hardness appeared at the interface of Ti/Ti2AlNb, wThehile microhardnessthe lowest was distributionslocated at the ofTiAl the base as-welded metal. andMoreover, heat-treated the heat joints-treated are displayedjoints showed in Figures the 3 andsame4. For hardness the as-welded distribution joints, regularities, the highest although hardness certain appeared changes atoccurred the interface in different of Ti zones/Ti2AlNb, affected while theby lowest heat treatment. was located The at hardness the TiAl remained base metal. almost Moreover, stable in thethe heat-treatedinterlayer, while joints it increased showed in the the same hardnessregions distribution surrounding regularities,TiAl and decreased although in the certain areas changessurrounding occurred Ti2AlNb. in diﬀerent zones aﬀected by heat treatment. The hardness remained almost stable in the interlayer, while it increased in the regions surrounding TiAl and decreased in the areas surrounding Ti2AlNb. Materials 2020, 13, 3300 7 of 9

Al, as an α stabilization element that could diffuse from base metal to the interlayer during welding, γ α promotedMaterials the 2020 transformation, 13, x FOR PEER REVIEW of some into near the TiAl/Ti interface. This, in turn, increased7 of 9 the hardness at the TiAl/Ti interface between the TiAl base metal and the interlayer. The presence of more Nb atAl, the as Ti an2AlNb α stabilization/Ti interface element led to that the could formation diffuse of from substantial base metal B2 to phases the interlayer in this during area, further welding, promoted the transformation of some γ into α near the TiAl/Ti interface. This, in turn, increasing the hardness of Ti2AlNb. On the other hand, the immigrated Al from TiAl provided an increased the hardness at the TiAl/Ti interface between the TiAl base metal and the interlayer. The additional hardening effect by forming a substitutional solid solution [27]. presence of more Nb at the Ti2AlNb/Ti interface led to the formation of substantial B2 phases in this To yield uniform distribution of α grains, the increase in hardness at the TiAl/Ti interface after heat area, further increasing the hardness of Ti2AlNb. On the other hand, the immigrated Al from TiAl treatmentprovided was associated an additional with hardening more di effectffusion by forming of alloying a substitutional Nb element solid after solution heat treatment[27]. in the region surroundingTo the yieldα uniform+ γ phase distribution in TiAl. of This α grains, strengthened the increase the in TiAl hardness solid at solution the TiAl/Ti and interface enhanced after Vickers hardness.heat Thetreatment importance was associated of solid with solution more diffusion strengthening of alloying in Nb Vickers element hardness after heat treatment has been in reported the in previousregion literature surrounding [28]. the At higherα + γ phase temperatures, in TiAl. This strengthened the B2 phase the decomposed TiAl solid solution into αand2 and enhanced O, in which Vickers hardness. The importance of solid solution strengthening in Vickers hardness has been both plasticity and toughness of the α2 and O phases were better than those of the B2 phase. As a reported in previous literature [28]. At higher temperatures, the B2 phase decomposed into α2 and result, theO, in hardness which both of plasticity the Ti/Ti 2andAlNb toughness interface of the decreased. α2 and O phases were better than those of the B2 phase. As a result, the hardness of the Ti/Ti2AlNb interface decreased. 3.3. Tensile Properties Since3.3. Tensile as-welded Properties joints have no direct practical applications, only heat-treated joints were selected for tensile testing.Since as The-welded average joints tensile have strengthsno direct practicalwere estimated applications, to be only454 heat41 MPa-treated at room joints temperature were selected for tensile testing. The average tensile strengths were estimated to± be 454 ± 41 MPa at room and 538 42 MPa at 650 ◦C. The stress–strain curve data are displayed in Figure6a,d. temperature± and 538 ± 42 MPa at 650 °C. The stress–strain curve data are displayed in Figure 6a,d.

FigureFigure 6. Tensile 6. Tensile test test at at room room temperature: temperature: (a) ( stressa) stress–strain–strain curve, curve, (b) location (b) location of fracture, of (c fracture,) microfracture morphologies. Tensile test at 650 °C: (d) stress–strain curve, (e) location of fracture, (f) (c) microfracture morphologies. Tensile test at 650 ◦C: (d) stress–strain curve, (e) location of fracture, microfracture morphologies. (f) microfracture morphologies. During tensile testing at room temperature, taking into account the very brittle nature of the DuringIMC Ti tensile3Al at room testing temperature at room [ temperature,29], the higher takingAl content into in account TiAl led theto more very Ti brittle3Al at the nature interface of the IMC Ti3Al atof room TiAl/Ti temperature when compared [29], to the Ti/Ti higher2AlNb. Al Furthermore, content in TiAlmost ledjoints to fractured more Ti 3fromAl at the the thin interface IMC layer of TiAl/Ti when comparedprecipitated to at Ti the/Ti 2TiAl/TiAlNb. interface, Furthermore, and a few most others joints fractured fractured at the from Ti/Ti the2AlNb thin interface, IMC layer as shown precipitated in Figure 6b. At high temperature (650 °C), however, both interfaces were no longer the weakest parts at the TiAl/Ti interface, and a few others fractured at the Ti/Ti2AlNb interface, as shown in Figure6b. of the whole joint, and the failure position moved to the TiAl base metal, as displayed in Figure 6e. At high temperature (650 ◦C), however, both interfaces were no longer the weakest parts of the whole The reason for this had to do with the increase in Ti3Al toughness and decline in brittleness at high joint, and the failure position moved to the TiAl base metal, as displayed in Figure6e. The reason for temperatures. Moreover, another possible reason could be that the immigrated Nb markedly this hadstrengthened to do with Ti the3Al and increase the surrounding in Ti3Al toughnessmicrostructures and [30 decline]. in brittleness at high temperatures. Moreover, anotherThe micromorphology possible reason of the could fracture be is that presented the immigrated in Figure 6c,f. Nb Both markedly joints demonstrated strengthened brittle Ti3 Al and the surroundingcharacteristics microstructures related to differences [30]. in the failure location of each joint, with transcrystalline fracture Theat room micromorphology temperature and intercrystalline of the fracture fracture is presented at 650 °C. As in described Figure6 c,f.above, Both the IMC joints Ti3Al demonstrated looked brittle characteristics related to diﬀerences in the failure location of each joint, with transcrystalline fracture at room temperature and intercrystalline fracture at 650 ◦C. As described above, the IMC Ti3Al looked brittle at room temperature with a transgranular propagation of cracks. Numerous Ti3Al Materials 2020, 13, 3300 8 of 9

IMCs migrated to grain boundary and aggregated at high temperatures [31], leading to intercrystalline fracture mode.

4. Conclusions

Spark plasma diﬀusion bonding (SPDB) of TiAl alloy to Ti2AlNb alloy was achieved using a pure titanium interlayer. The microstructures and mechanical properties of the joints were investigated, and the major conclusions that can be drawn from this study are the following:

1. After welding, the joint of TiAl/Ti/Ti2AlNb is composed of TiAl; α + γ, α, α + β, and B2-rich duplex microstructures; and Ti2AlNb. The thickness of the interlayer was found to increase after homogeneous heat treatment due to the further diﬀusion of the elements Ti, Nb, and Al. This process resulted in the joint being composed of TiAl; lamellar α, α, α + β, and β + α2 + O phases; and Ti2AlNb. 2. The maximum hardness after welding (401 HV) appeared at the Ti2AlNb/Ti interface, while the minimum hardness (281 HV) occurred in the TiAl base metal. After heat treatment, the microhardness distribution at the joint became more uniform; it increased signiﬁcantly from 309 HV up to 337 HV at the TiAl/Ti interface, while it decreased slightly at the Ti/Ti2AlNb interface. 3. At room temperature, the tensile fracture of the heat-treated joint occurred at the interlayer with an average tensile strength of 454 MPa, and it was found to be a transgranular fracture. On the other hand, at 650 ◦C, the fracture position moved to the TiAl base metal, with a tensile strength of 538 MPa, and was observed to be an intercrystalline fracture.

Author Contributions: B.Z., J.H. (Jianchao He), J.H. (Jinbao Hou), L.C., and Y.L. conceived the initial research issues and designed the experiments; B.Z., J.H. (Jianchao He), and Y.L. conducted the spark plasma diffusion bonding experiment; B.Z., C.C., J.H. (Jianchao He), and Y.L. performed the microstructural observation and the mechanical tests; B.Z., C.C., and J.H. (Jianchao He) analyzed the data of microstructures, EDS results, and mechanical properties. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the “National Natural Science Foundation of China” under Grant Number 91860115. Conflicts of Interest: The authors declare no potential conflict of interest.

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