Diffusion Bonding of Ti6al4v to Al2o3 Using Ni/Ti Reactive Multilayers

metals

Article Diffusion Bonding of Ti6Al4V to Al2O3 Using Ni/Ti Reactive Multilayers

Marcionilo Silva Jr. 1,2,3, Ana S. Ramos 4 , M. Teresa Vieira 4 and Sónia Simões 2,3,*

1 Department of Mechanical Engineering, Federal University of Amazonas, General Rodrigo Octavio Jordão Ramos ST., Manaus 69067-005, Brazil; [email protected] 2 Department of Metallurgical and Materials Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal 3 LAETA/INEGI—Institute of Science and Innovation in Mechanical and Industrial Engineering, Rua. Dr. Roberto Frias, 4200-465 Porto, Portugal 4 Department of Mechanical Engineering, University of Coimbra, CEMMPRE, R. Luís Reis Santos, 3030-788 Coimbra, Portugal; soﬁ[email protected] (A.S.R.); [email protected] (M.T.V.) * Correspondence: [email protected]; Tel.: +35-12-2041-3113

Abstract: This paper aims to investigate the diffusion bonding of Ti6Al4V to Al2O3. The potential of the use of reactive nanolayered thin ﬁlms will also be investigated. For this purpose, Ni/Ti multilayer thin ﬁlms with a 50 nm modulation period were deposited by magnetron sputtering onto the base materials. Diffusion bonding experiments were performed at 800 ◦C, under 50 MPa and a dwell time of 60 min, with and without interlayers. Microstructural characterization of the interface was conducted through scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). The joints experiments without interlayer were unsuccessful. The interface is

characterized by the presence of a crack close to the Al2O3 base material. The results revealed that the Ni/Ti reactive multilayers improved the diffusion bonding process, allowing for sound joints to be obtained at 800 ◦C for 60 min. The interface produced is characterized by a thin thickness and Citation: Silva, M., Jr.; Ramos, A.S.; is mainly composed of NiTi and NiTi2 reaction layers. Mechanical characterization of the joint was Vieira, M.T.; Simões, S. Diffusion assessed by hardness and reduced Young’s modulus distribution maps that enhance the different Bonding of Ti6Al4V to Al2O3 Using phases composing the interface. The hardness maps showed that the interface exhibits a hardness Ni/Ti Reactive Multilayers. Metals 2021, 11, 655. https://doi.org/ distribution similar to the Al2O3, which can be advantageous to the mechanical behavior of the joints. 10.3390/met11040655 Keywords: multilayers; diffusion bonding; Ti6Al4V alloys; Al2O3; microstructure; interface; sputtering Academic Editor: Mohammad Jahazi

Received: 22 March 2021 Accepted: 14 April 2021 1. Introduction Published: 17 April 2021 In recent years, research into the development of similar and dissimilar joining technologies of titanium alloys has attracted much attention in the scientific community [1–5]. Publisher’s Note: MDPI stays neutral This is due to the attractive properties of these alloys, such as low density, high temperature with regard to jurisdictional claims in properties and excellent creep and corrosion resistance [6–8]. Ti6Al4V, TiAl and TiNi are the published maps and institutional affil- most attractive titanium alloys for the aerospace industry. However, the major challenge iations. for their application in the aerospace industry is related to the service temperature. The joining of these alloys to other materials will enhance the successful implementation of these alloys since a combination of unique properties will occur. The successful joining of metal to advanced ceramics can contribute to overcome the Copyright: © 2021 by the authors. challenges that titanium alloys present when the application requires operating tempera- Licensee MDPI, Basel, Switzerland. tures above 550 ◦C[7]. Besides, due to the properties of advanced ceramics, such as high This article is an open access article wear resistance and high thermal stability [9,10], this dissimilar joining will promote the distributed under the terms and formation of components with a combination of desirable properties. conditions of the Creative Commons However, joining materials with such different properties is quite demanding, making Attribution (CC BY) license (https:// it a challenging task to obtain sound joints with good mechanical properties. One of the creativecommons.org/licenses/by/ main difficulties is the development of residual stresses at the interface. The coefficients 4.0/).

Metals 2021, 11, 655. https://doi.org/10.3390/met11040655 https://www.mdpi.com/journal/metals Metals 2021, 11, 655 2 of 12

of thermal expansion (CTE) of ceramics are generally considerably lower than the CTE of metals. This CTE mismatch and the different mechanical behavior induce the formation of residual stresses at the interface of the joint during cooling. Hence, in recent years, researchers have been working to understand the mechanisms that affect the metal to ceramic joining process and propose alternatives [11–21]. Brazing [11–14] and diffusion bonding [15–21] are the most reported technologies for metal to ceramic joining. Brazing is a very attractive process since it is a cost-effective process, and a correct selection of the filler metal promotes the successful joining of titanium alloys to ceramics such as Al2O3 and ZrO2. The most-reported fillers used are Ti-based and Ag-based alloys. Ag-based alloys promote the formation of a significant extension of (Ag), which degrades the service temperature of the joints, while the Ti-based alloys require high processing temperatures that promote microstructural changes of base materials such as Ti6Al4V alloy. Diffusion bonding [15–21] has the potential of obtaining sound joints without requiring the use of Ag-based fillers, and also the possibility of using lower bonding temperatures than those required with Ti-based fillers. However, in the conventional diffusion bonding process, high temperatures are required for joint production success during long durations. Several approaches have been developed in order to reduce the bonding conditions for the diffusion bonding of titanium alloys. The use of varying pressure, interlayers with ceramic nanoparticles or reactive interlayers has been referred to as a potential approach for overcoming the problems in dissimilar diffusion bonding [22–28]. The use of interlayers in diffusion bonding can reduce the processing conditions and promote the formation of a joint with good mechanical properties. In recent years, the joining of dissimilar metals using interlayers has evolved into reactive multilayer foils/films with nanometric bilayer thickness (modulation period) and chemical elements that react exothermically with each other, favoring diffusivity and reactivity, as well as acting as a localized source of heat/energy, e.g., [5,25–28]. The use of reactive multilayers for ceramic/metal joining is scarce, and only a few works have been reported so far [20,21]. Yi et al. [20] investigated the diffusion bonding of Al2O3 to copper assisted by Al/Ti reactive nano multilayers with different modulation periods. The diffusion bonding experiments were performed at 900 ◦C for 10 min. The results demonstrated that a sound joint can be obtained. Several intermetallic compounds were formed at the interface. The joint strength is strongly dependent on the modulation period of the multilayers. The use of reactive multilayers proves to be an effective approach in the joining of Cu to Al2O3. Cao et al. [21] investigated the diffusion bonding of TiAl to TiC using Ni/Al multilayers with a total thickness of ~30 µm. Microstructural characterization revealed that the interface consists of a layered structure; a Ni3(AlTi) layer, a Ni2AlTi layer, a (Ni, Al, Ti) layer and a Ni diffusion layer were observed from the interlayer towards the TiAl base material. Ni/Al multilayers’ application improved the joints’ quality based on the shear strength values obtained. The applicability of different reactive multilayer systems produced by magnetron sputtering has been studied for similar and dissimilar joints of NiTi, Ti6Al4V and TiAl alloys, as well as for Ni-based superalloys and steels [5,25–28]. The similar and dissimilar joints were produced with success for these base materials and under less demanding bonding conditions (time, temperature and/or pressure) than the ones required without multilayers. Nevertheless, the conditions are dependent on the reactive multilayer system used and the microstructure and mechanical properties at the interface are strongly influenced by the chemical composition of the base materials. Reliable joints produced for these base materials led us to consider an excellent approach to use these multilayers to join ceramic and titanium alloys. In this context, the objective of the present work consists in the study of the feasibility of joining Ti6Al4V to Al2O3 by diffusion bonding using Ni/Ti reactive multilayers prepared by magnetron sputtering. The microstructure analysis of the interface was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS)) in Metals 2021, 11, 655 3 of 12

order to evaluate the integrity of the joint. Nanoindentation tests across the joint interface were carried out to understand the role of the reaction phases at the interface. Diffusion bonding experiments without multilayers were also conducted in order to evaluate the potential of this interlayer in dissimilar Ti6Al4V/Al2O3 joints.

2. Materials and Methods 2.1. Base Materials

Ti6Al4V alloy and Al2O3, supplied by Goodfellow in rods with diameters of 7 and 4 mm, respectively, were cut 5 mm in length and ground and polished down to 1 µm diamond suspension. The base materials were cleaned with acetone, ethanol and deionized water in an ultrasonic bath and dried with heat blow air before the multilayer’s deposition. The quality of the polished surfaces was assessed by optical microscopy (OM) (DM4000, Leica Microsystems (Leica Microsystems, Wetzlar, Germany)) and by the determination of the average roughness (Ra) of the polished base materials obtained by proﬁlometry (Perthometer SP4, with laser probe (Mahr Perthometer SP4, Göttingen, Germany)). This evaluation was crucial for Al2O3 due to the presence of porosity. The polished process was adjusted in order to obtain different Ra values for Al2O3 (from 0.10 to 1.27 µm).

2.2. Deposition of Ni/Ti Multilayer Thin Films Nickel and Titanium nanolayers were deposited alternately onto the polished surfaces of the base materials (substrates) by direct current magnetron sputtering using titanium (99.99% pure) and nickel (Ni-7V wt.%) targets (150 mm × 150 mm × 3–7 mm thick). After achieving a base pressure below 5 × 10−4 Pa in the sputtering chamber, Ar was introduced (P ≈ 1.5 × 10−1 Pa), and the substrate materials were cleaned by heating followed by Ar+ (current of 20A) etching using an ion gun. The depositions, carried out at a 4 × 10−1 Pa Ar pressure, started immediately after the substrates’ cleaning. To avoid substrate heating and thus prevent reactions during the deposition process, a thick copper block was used as substrate holder, which acted as a heat sink. The power density applied to Ti and Ni targets was ~7.20 × 10−2 Wmm−2 and ~2.8 × 10−2 Wmm−2, respectively, to obtain a near equiatomic average chemical composition (50 at. % Ni). According to the literature, in B2-NiTi vanadium can be located in Ni-substitutional site, as well as in Ti-substitutional site [29]. Therefore, a Ni:Ti atomic ratio close to 1 should be obtained, in order to guarantee that upon heating B2-NiTi is formed. The substrate holder’s rotation speed defines the time that the rotating substrates are in front of each target, determining the thickness of the individual layers, and, consequently, the period or bilayer thickness. In this work, a substrates’ rotation speed of ~2.0 rpm was used to achieve ≈ 50 nm of modulation period, and a deposition time of ~30 min was selected for a total thickness close to 3.0 µm. The initial layer was Ti to assure a good adhesion between the base materials and the multilayer thin films. This is particularly relevant for the Al2O3 base material because the lack of adhesion can compromise the diffusion bonding process. The top layer was Ni to prevent oxidation. The adhesive strength between multilayer film and Al2O3 was evaluated by a procedure similar to that used by Lim et al. [30]. The adhesion strength between the film and substrate was measured using a mechanical tester with a load cell of 500 N. The mechanical pull test was carried out with a loading speed of 10.0 µm/min. The bonding strength is calculated as the average tensile stress at failure.

2.3. Diffusion Bonding Process

Ti6Al4V/Al2O3 diffusion bonding was performed in an apparatus consisting of a mechanical testing machine (LLOYD Instruments LR 30K (AMETEK Test & Calibration Instruments Lloyd Materials Testing, West Sussex, UK)), vertical infrared radiation furnace, molybdenum punctures (to apply pressure), quartz tube and vacuum system, as described in previous works [26,27]. Metals 2021, 11, 655 4 of 12

Diffusion bonding experiments were performed at a vacuum level better than 10−2 Pa. The dissimilar base materials were joined with and without the reactive multilayers to evaluate the interlayer’s potential in the joining process. The diffusion bonding experiments were carried out at 800 ◦C, by applying a pressure of 50 MPa during 60 min. Different rates of heating and cooling were investigated. In the ﬁrst step, the rate of heating and cooling was 10 ◦C/min and in a second step, the heating rate was 10 ◦C/min up to 800 ◦C, and the cooling rate was 5 ◦C/min up to 500 ◦C, followed by 3 ◦C/min down to room temperature.

2.4. Microstructural Characterization Microstructural and chemical characterization of the multilayer thin films and diffusion bonding joints were carried out by scanning electron microscopy (SEM) (FEI Quanta 400FEG ESEM/EDAX Genesis X4M (FEI Company, Hillsboro, OR, USA)) operating at an accelerating voltage of 15 keV, coupled with energy-dispersive X-ray spectroscopy (EDS) (Oxford Instrument, Oxfordshire, UK). The EDS measurements were made at an accelerating voltage of 15 keV by the standardless quantification method. The results obtained by this method provide a fast quantification with automatic background subtraction, matrix correction, and normalization to 100% for all the elements in the peak identification list. The cross-sections of the films and of the joints’ interfaces were prepared using standard metallographic procedures.

2.5. Mechanical Characterization Nanoindentation tests were carried out across the joints’ interface by using a Micro Materials-NanoTest equipment with a Berkovich diamond indenter. The depth-sensing indentation tests were performed in load control mode, at a maximum load of 5 mN. Several indentation matrixes formed by 8 rows and 12 columns (96 measurements) were defined along the joint interface in order that each matrix starts on the Ti6Al4V side, cross the interface, and finish on the alumina side. The distance between columns was 3 µm to guarantee that some indentations fall in the region corresponding to the interlayer thin films, and the distance between rows was 5 µm. Fused quartz was used as reference material to determine the Berkovich tip area function. Hardness and reduced Young’s modulus were determined by the Oliver and Pharr method [31]. Before the indentation tests, the joints were polished using standard metallographic procedures.

3. Results and Discussion 3.1. Characterization of Ni/Ti Multilayer Thin Films Deposited onto Al2O3 Figure1 shows SEM images of an as-deposited Ni/Ti multilayer thin film deposited onto the Al2O3 base material. During the sputtering process, multilayer reaction was not detected. SEM images revealed that the multilayer’s structure was preserved, being possible to observe the Ni- (light grey) and Ti-rich (dark grey) alternated nanolayers clearly present in the high magnification SEM image of Figure1b. The Ni/Ti multilayer film’s total thickness is approximately 3.0 µm and the modulation period (Λ) is close to 50 nm. The success of obtaining sound joints depends strongly on the multilayer thin films’ good adhesion to the base materials. Due to the high challenges for a good preparation of the Al2O3 surface, an evaluation of the adhesion and morphology of the films deposited onto this base material was crucial. Metals 2021, 11, 655x FOR PEER REVIEW 5 5of of 12

Ni/Ti 266.3 nm Λ≈53 nm

Al2O3

(a) (b)

Figure 1. ((a)a) Scanning electron microscopy (SEM) imagesimages ofof aa Ni/TiNi/Ti multilayer thin film ﬁlm with ~50∼50 nm modulation period (Λ) deposited onto Al2O3 and (b) higher magnification of the region marked in (a). (Λ) deposited onto Al2O3 and (b) higher magniﬁcation of the region marked in (a).

The multilayer thin films’ films’ adhesion to the Al22O3 basebase material material is strongly influenced influenced by the the surface surface topography. topography. In In contrast contrast to tothe the observed observed average average roughness roughness for Ti6Al4V for Ti6Al4V (Ra =(Ra 0.06 = 0.06± 0.01± 0.01 µm)µ m)prepared prepared using using the the standard standard metallographic metallographic procedure, procedure, alumina prepared by the same process exhibitsexhibits aa RaRa ofof 1.271.27 ±± 0.010.01 µm.µm. Although Although due to this higher value the Al2OO33 surfacesurface peaks peaks could could act act as as mechanical anchoring zones to obtain good ceramic/multilayer film film adhesion, adhesion, the the value value is is higher higher than than that considered adequate for thethe diffusion bonding process [[30,32,33].30,32,33]. To To obtain an AlAl2O3 surfacesurface with with less Ra, an alternative approach to the metallographic metallographic pr procedureocedure was conducted using solutions with high diamond concentration (75,(75, 6 and 1 µµm),m), which resultedresulted inin RaRa closeclose toto 0.100.10 ±± 0.010.01 µm.µm. The adhesive adhesive strength strength between between multilayer multilayer film film and and ceramic ceramic substrate substrate was was measured. measured. Pull-off teststests werewere done done to to evaluate evaluate the the adhesive adhesive strength strength between between Al2O 3Aland2O Ni/Ti3 and multi-Ni/Ti multilayerlayer thin filmsthin films with lowwith (0.1 lowµ (0.1m) andµm) highand high (1.27 (1.27µm) Ra.µm) As Ra. referred As referred in the in literature the literature [33], [33],a good a good value value of adhesive of adhesive strength strength should should be around be 10around MPa. 10 As MPa. expected, As expected, the adhesion the adhesionis better for is better the base for materialthe base withmaterial higher with Ra higher (16 MPa). Ra (16 For MPa). lower For Ra lower values, Ra an values, adhesive an adhesivestrength betweenstrength between 0 to 1 MPa 0 to was 1 MPa observed. was observed. It means It thatmeans reducing that reducing the roughness the roughness of the ofceramic the ceramic surface surface impairs impairs its adhesion its adhesion to the to Ni/Ti the Ni/Ti reactive reactive multilayers. multilayers. Therefore, Therefore, in this in thiswork, work, ceramic ceramic base base materials materials with with high high roughness roughness were were used. used. 3.2. Characterization of Diffusion Bonding Joints 3.2. Characterization of Diffusion Bonding Joints The effectiveness of using Ni/Ti multilayer thin films to reaction-assist the diffusion The effectiveness of using Ni/Ti multilayer thin films to reaction-assist the diffusion bonding of Ti6Al4V alloy to Al2O3 was assessed through microstructural and mechanical bonding of Ti6Al4V alloy to Al2O3 was assessed through microstructural◦ and mechanical characterization. Joining of Ti6Al4V to Al2O3 was processed at 800 C, applying a pressure characterization. Joining of Ti6Al4V to Al2O3 was processed at 800 °C, applying a pressure of 50 MPa for 60 min, without and with Ni/Ti multilayer thin films as interlayer material. of 50 MPa for 60 min, without and with Ni/Ti multilayer thin films as interlayer material. Experiments conducted using base materials without interlayers allowed the multilayer Experimentsthin films’ role conducted in these dissimilar using base joints materials to be understood.without interlayers allowed the multilayer thin films’ role in these dissimilar joints to be understood. 3.2.1. Diffusion Bonding without Interlayer 3.2.1. Diffusion Bonding without Interlayer Initially, the experiments were carried out with heating and cooling rates of 10 ◦C/min. ObservationsInitially, inthe optical experiments microscopy were (OM) carried revealed out awith high heating deformation and cooling in the Ti6Al4V rates of alloy 10 °C/min.after joining Observations without a multilayer.in optical microscopy Figure2 shows (OM) OM revealed images where a high plastic deformation deformation in the is Ti6Al4Vclearly observed alloy after for joining the joint without produced. a multilayer. In order Figure to reduce 2 shows the residual OM images stresses, where a heating plastic deformationrate of 10 ◦C/min is clearly up toobserved 800 ◦C, for and the cooling joint produced. rate of 5 ◦InC/min order up to toreduce 500 ◦ theC, followedresidual stresses,by 3 ◦C/min a heating down rate to roomof 10 °C/min temperature, up to were800 °C, adopted and cooling for all rate the of subsequent 5 °C/min up diffusion to 500 °C,bonding followed experiments. by 3 °C/min down to room temperature, were adopted for all the subsequent diffusion bonding experiments.

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AlAl2O2O33

Ti6Al4VTi6Al4V

FigureFigure 2. 2. Optical Optical microscopy microscopy (OM) (OM) image image of of the the Ti6Al4V/Al Ti6Al4V/Al2O2O3 3joint joint processed processed at at 800 800 °C °C ◦under under 50 50 Figure 2. Optical microscopy (OM) image of the Ti6Al4V/Al2O3 joint processed at 800 C under MPa for 60 min with heating and cooling rates of 10 °C/min. 50MPa MPa for for 60 60 min min with with heating heating and and cooling cooling rates rates of of10 10 °C/min.◦C/min.

TheThe microstructure microstructure of of the the interfaceinterface ofof theththee jointsjoints producedproduced produced withoutwi withoutthout interlayer interlayer was was characterizedcharacterized by byby SEM. SEM. Backscattered Backscattered Backscattered electron electron electron (BSE) (BSE) (BSE) SEM SEM SEM images images images of of the ofthe thejoint joint joint produced produced produced can can becanbe observed observed be observed in in Figure Figure in Figure 3. 3. Although Although3. Although the the thesamp samp samplelele was was was apparently apparently apparently bonded bonded bonded at at the atthe theend end end of of the ofthe diffusionthediffusion diffusion bondingbonding bonding process,process, process, anan an unbondedunbonded unbonded lalayer layeryer cancan can bebe observed observedobserved after after metallographicmetallographicmetallographic preparation.preparation. This ThisThis may maymay have havehave been beenbeen due due to to re residualresidualsidual stressesstresses that that occurred occurred during during cooling cooling leadingleading to to the the detachment detachment of of the the base base material materials materialss or or even even to to the the formation formation of of a a brittle brittle phase phase thatthat resulted resulted in in crack crack nucleation nucleation and and propagation propagation during during sample sample preparation. preparation. Anyway, Anyway, a a reactionreaction layer layer is is observed observed close close to to the the Ti6Al4VTi6A Ti6Al4Vl4V basebase base material.material. material. SEMSEM SEM observationobservation observation revealedrevealed revealed thatthat this this reaction reaction zone zone exhibits exhibits a a thickness thickness of of 3.1 3.1 µµm. µm.m.

Ti6Al4VTi6Al4V I I AlAl2O2O3 3

(a(a) ) (b(b) ) ◦ FigureFigureFigure 3. 3. 3.( a(a) )(a) Backscattered Backscattered Backscattered electron electron electron (BSE) (BSE (BSE SEM ) SEM)SEM images images images of of diffusionof diffusion diffusion bonded bonded bonded Ti6Al4V/Al Ti6Al4V/Al Ti6Al4V/Al2O23O2Ojoint3 3joint joint processed processed processed at at 800 at 800 800C °C and°C and and (b ) higher magniﬁcation of the region( markedb(b) )higher higher in magnification (magnificationa). of of the the region region marked marked in in ( a(a).).

TheThe chemicalchemical chemical compositioncomposition composition of of reactionof reaction reaction zone zone zone Z1 wasZ1Z1 was obtainedwas obtained obtained by EDS, by by EDS, resultingEDS, resulting resulting in 48.5% in in 48.5%Ti,48.5% 39.3% Ti,Ti, O, 39.3%39.3% 11.3% O,O, Al 11.3%11.3% and 0.9% AlAl and Vand (atomic%). 0.9%0.9% VV (atomic%).(atomic%). This zone isThisThis due zonezone to the isis reaction duedue toto betweenthethe reactionreaction the betweenelementsbetween the thatthe elements elements diffused that fromthat diffused diffused the base from materialsfrom the the ba ba tosese the materials materials center ofto to thethe the interface. center center of of Althoughthe the interface. interface. the AlthoughEDSAlthough does the not the EDS allowEDS does does a correct not not allo allo analysisww a a correct correct of the analysis analysis oxygen, of basedof the the ox onoxygen,ygen, these based based results on on it these isthese clear results results that itit is is clear clear that that in in this this layer layer reaction reaction occurred occurred between between the the elements elements of of the the Ti Ti base base material material

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within this some layer of reaction the oxygen, occurred and between eventually the elementsAl, coming of thefrom Ti the base ceramic material base with material. some of Accordingthe oxygen, to and these eventually results, at Al, 800 coming °C it is from not thepossible ceramic to diffusion base material. bond According Ti6Al4V to to Al these2O3, ◦ evenresults, by atapplying 800 C itpressures is not possible up to 50 to MPa diffusion during bond 60 min. Ti6Al4V to Al2O3, even by applying pressures up to 50 MPa during 60 min. 3.2.2. Diffusion Bonding Using Ni/Ti Reactive Multilayers 3.2.2. Diffusion Bonding Using Ni/Ti Reactive Multilayers Diffusion bonding of Ti6Al4V to Al2O3 was processed under the same conditions Diffusion bonding of Ti6Al4V to Al O was processed under the same conditions using Ni/Ti reactive multilayers. Nanometric2 3 interlayers on the contact surfaces of the using Ni/Ti reactive multilayers. Nanometric interlayers on the contact surfaces of the base materials aim to improve the diffusivity through the interface e.g., [5,26,27], but can base materials aim to improve the diffusivity through the interface e.g., [5,26,27], but can also contribute to reduce or eliminate the formation of residual stresses. Microstructural also contribute to reduce or eliminate the formation of residual stresses. Microstructural characterization reveals that interfaces with apparent soundness were produced during characterization reveals that interfaces with apparent soundness were produced during solid-state diffusion bonding of Ti6Al4V to Al2O3 using Ni/Ti multilayer thin films. The solid-state diffusion bonding of Ti6Al4V to Al O using Ni/Ti multilayer thin films. The cross-section of the joint was observed and analysed2 3 by SEM. The BSE images in Figure 4 cross-section of the joint was observed and analysed by SEM. The BSE images in Figure4 show an interface with a thickness of 6.4 ± 0.2 µm without voids or cracks along the show an interface with a thickness of 6.4 ± 0.2 µm without voids or cracks along the interface. Although pores characterized the Al2O3 base material, they did not affect the interface. Although pores characterized the Al2O3 base material, they did not affect the effectivenesseffectiveness of of the the joining joining process. process. Two Two regions can be distinguished at the interface, one darker zone adjacent to the Ti6Al4V alloy and a brighter zone adjacent to the Al2O3. The darker zone adjacent to the Ti6Al4V alloy and a brighter zone adjacent to the Al2O3. The bondlinebondline is is perceptible perceptible at the center of the in interface.terface. Previous Previous work [25] [25] demostrated demostrated that thethe interface interface of of similar similar TiAl TiAl joints using Ni/Ti reactive reactive multilayers multilayers exibith exibith a a line line rich rich in in Ti Ti thatthat divided divided the the interface. interface. The The identification identification of of the the possible possible phases phases at at the interface was performedperformed taking taking into into account account the the chemical composition obtained obtained by by EDS in the regions highlightedhighlighted in in red red in Figure 44b,b, combinedcombined withwith thethephase phasediagrams diagrams[ [34,35].34,35]. The EDS resultsresults are are presented presented in in Table Table 1.1 .During During the the di diffusionffusion bonding bonding process, process, Ni Niand and Ti from Ti from the multilayerthe multilayer thin thin film film reacted reacted to toform form inte intermetallicrmetallic compounds. compounds. According According to to previous works [34,35] [34,35] and considering considering the the average average chem chemicalical composition composition (Ni:Ti (Ni:Ti atomic ratio close toto 1), 1), austenitic austenitic NiTi NiTi should should form. form. However, However, du duee to to diffusion, diffusion, a Ti Ti enrichment is observed atat the the interface interface (zone (zone Z2 Z2 in in Figure Figure 44b).b). Cavaleiro etet al.al. [[36]36] studied studied the the interaction interaction between between Ni/TiNi/Ti multilayer multilayer thin thin films films and and Ti6Al4V Ti6Al4V substrates substrates during during heat heat treatment treatment and confirmed confirmed thethe diffusion diffusion of of Ni Ni from from the thin films films towards the β-phase of the substrate. In fact, when comparedcompared to to the the Ti6Al4V Ti6Al4V in in Figure Figure 33,, inin FigureFigure4 4 the the Ti6Al4V Ti6Al4V base base material material (zone (zone Z1) Z1) has has moremore bright bright phase phase along along the the αα-Ti-Ti grain grain boundaries, boundaries, which which should should correspond correspond to β–Ti–Ti with aa higher higher concentration concentration of of β-stabilizer-stabilizer elements, elements, such such as as Ni Ni and and V V [26,35–37]. [26,35–37].

Ti6Al4VI Al2O3

(a) (b)

Figure 4. (a) BSE SEM images of diffusion bonded Ti6Al4V/Al2O3 joint processed at 800 °C◦ using a Ni/Ti multilayer and Figure 4. (a) BSE SEM images of diffusion bonded Ti6Al4V/Al2O3 joint processed at 800 C using a Ni/Ti multilayer and (b) higher magniﬁcation of the region(b) higher marked magnification in (a). of the region marked in (a).

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Ni and V diffused from the interlayer towards the Ti6Al4V, as can be observed in the elemental distribution profiles shown in Figure 5, as well as in the elemental distribution mapsTable of 1. ChemicalFigure 6. compositionThe vanadium obtained content by EDS in the of the interlayer regions highlighted is due to its in Figurepresence4. in the Ni target used for producing the Ni/Ti multilayer thin films, as referred in Section 2.2. The Element (at.%) chemicalZone composition of zone Z1, combined with the Ti-Al-V and Ti-Al-NiPossible ternary Phases phase diagrams [38,39],Ti indicates Alas possible Vphases α-Ti Ni and β-Ti (Table O 1). The presence of Ni and V1 along the 86.0 α–Ti grain 9.9boundaries 2.8 is confirmed 1.3 by their *distribution αon-Ti the + β Ti6Al4V-Ti side (Figure2 6). 64.3The decrease 1.9 of the 1.7Ni content 32.1 in zone Z2 * (compared to NiTi zone2 Z3) is explained3 due to 46.5 the high diffusion - coefficient 3.1 of 50.4Ni in Ti6Al4V * [40], i.e., Ni diffused NiTi from 4 - 43.8 - - 56.2 Al O the interlayer thin film towards the base metal, as explained above. The content2 of3 Ti and Ni* Not available quantified. in Z2, combined with the Ni-Ti phase diagram, points to the presence of the NiTi2 phase, and is also reported in the literature [26,36]. Ni and V diffused from the interlayer towards the Ti6Al4V, as can be observed in the Tableelemental 1. Chemical distribution composition profiles obtained shown by in EDS Figure of the5, regions as well highlighted as in the elemental in Figure 4. distribution maps of Figure6. The vanadium content in the interlayer is due to its presence in the Element (at.%) NiZone target used for producing the Ni/Ti multilayer thin films, as referredPossible in Phases Section 2.2. The chemical compositionTi Al of zone Z1,V combinedNi with theO Ti-Al-V and Ti-Al-Ni ternary phase1 diagrams 86.0 [38 ,39], indicates9.9 as possible2.8 phases1.3 α-Ti and * β-Ti (Tableα1-Ti). The+ β-Ti presence of Ni2 and V along64.3 the α–Ti1.9 grain boundaries1.7 32.1 is confirmed * by their distribution NiTi2 on the Ti6Al4V3 side (Figure46.5 6). The decrease- of3.1 the Ni content50.4 in zone* Z2 (compared NiTi to zone Z3) is explained4 due to- the high diffusion43.8 coefficient- of- Ni in Ti6Al4V56.2 [40], i.e., Ni Al diffused2O3 from *the Not interlayer quantified. thin film towards the base metal, as explained above. The content of Ti and Ni available in Z2, combined with the Ni-Ti phase diagram, points to the presence of the NiTiThe2 phase, chemical and iscomposition also reported of inzone the literatureZ3 is similar [26, 36to ].the as-deposited multilayer thin film (The∼49 chemicalat.% Ni, composition47 at.% Ti and of zone4 at.% Z3 V). is similarApparently to the no as-deposited significant multilayerinterdiffusion thin film (~49 at.% Ni, 47 at.% Ti and 4 at.% V). Apparently no significant interdiffusion occurred between multilayer and Al2O3 base material, which can be explained by the joint temperatureoccurred between (800 °C) multilayer being not and enough Al2O3 [40-42].base material, At 800 which°C, the can diffusion be explained coefficient by the of joint Ni temperature (800 ◦C) being not enough [40–42]. At 800 ◦C, the diffusion coefficient of Ni in NiTi2 (Z2) is lower than in Ti6Al4V; as a result, NiTi2 works like a barrier, retarding the diffusionin NiTi2 of(Z2) Ni is[40]. lower Chemical than in composition Ti6Al4V; as in a zone result, Z3, NiTi combined2 works with like the a barrier, Ni-Ti [43] retarding phase diagram,the diffusion confirms of Ni the [40 presence]. Chemical of B2-NiTi composition phase. in Zone zone Z4 Z3, in combined Figure 4 is with located the Ni-Ti far from [43] thephase interface, diagram, so confirmsits chemical the presence composition of B2-NiTi (Table phase. 1) corresponds Zone Z4 in to Figure the 4alumina is located base far from the interface, so its chemical composition (Table1) corresponds to the alumina base material. As expected, on the alumina side the EDS maps confirm the presence of Al and material. As expected, on the alumina side the EDS maps confirm the presence of Al and O. O.

Figure 5. SEM image and EDS proﬁles across the joint processed at 800 ◦C using a Ni/Ti multilayer. Figure 5. SEM image and EDS profiles across the joint processed at 800 °C using a Ni/Ti multilayer.

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FigureFigure 6. SEM 6. image SEM andimage EDS and elemental EDS elemental distribution distribution maps across maps theacro jointss the processed joint processed at 800 ◦ Cat using800 °C an using Ni/Ti an multilayer. Ni/Ti multilayer. Nanoindentation tests were performed across the interface and adjacent base materials. FigureNanoindentation7 shows the SEM imagetests ofwere a nanoindentation performed ac matrixross the and interface the corresponding and adjacent hardness base andmaterials. reduced Figure Young’s 7 modulusshows the (Er ) maps.SEM image The residual of a imprintsnanoindentation of the indenter matrix cannot and bethe clearlycorresponding seen in Al hardness2O3 and zoneand reduced Z2 (interface Young’s adjacent modulus to the (E Ti6Al4Vr) maps. base The material) residual becauseimprints theof indentationsthe indenter arecannot too smallbe clearly due toseen the in high Al2 hardnessO3 and zone values. Z2 Nevertheless,(interface adjacent part ofto the the matrixTi6Al4V is perceptible base material) in Figure because7 with the the indentatio 8 rows distinguishablens are too small on due the Ti6Al4Vto the high side, hardness while severalvalues. smaller Nevertheless, indentations part canof the be observedmatrix is inperceptible zone Z3 (interface in Figure adjacent 7 with to the the Al8 rows2O3 basedistinguishable material). on the Ti6Al4V side, while several smaller indentations can be observed in zoneThe Z3 hardness (interface and adjacent reduced to Young’s the Al2 modulusO3 base material). (Er) maps clearly exhibit the differences betweenThe the hardness different and zones reduced composing Young’s the modulus interface (E andr) maps the baseclearly materials. exhibit the As differences expected, thebetween metallic the base different material zones has composing significantly the lower interface hardness and andthe base Er values materials. than theAs expected, alumina basethe metallic material. base At thematerial interface, has significantly zone Z3, adjacent lower tohardness the alumina, and E hasr values rather than low the hardness alumina andbase E material.r values, At while the interface, zone Z2 iszone characterized Z3, adjacent by to highthe alumina, hardness has and rather Er values.low hardness The mechanicaland Er values, properties while atzone the jointZ2 is interface characterized corroborate by high the phasehardness identification and Er values. based The on themechanical EDS results, properties i.e., NiTi at 2theand joint B2-NiTi interface identified corroborate at zones the phase Z2 and identification Z3, respectively. based Inon athe recent EDS work, results, a higheri.e., NiTi hardness2 and B2-NiTi was also identified measured at zones by nanoindentation Z2 and Z3, respectively. in the region In a correspondingrecent work, a to higher the NiTi hardness2 phase was at thealso interface measured of NiTiby nanoindentation to Ti6Al4V diffusion in the bondsregion assistedcorresponding by Ni/Ti to multilayer the NiTi2 thinphase films at the [44 ].interface The high of hardness NiTi to valueTi6Al4V of thediffusion NiTi2 phase bonds confirmsassisted itsby brittleNi/Ti multilayer character andthin canfilms compromise [44]. The high the mechanicalhardness value strength of the of NiTi the2 joints. phase Brittleconfirms behavior its brittle of the character NiTi2 was and also can confirmed compromise by Huthe etmechanical al. [45]. It shouldstrength be of noted the joints. that the interface is characterized by mechanical properties close to the Al O base material. In Brittle behavior of the NiTi2 was also confirmed by Hu et al. [45]. It2 should3 be noted that the essential, the nanoindentation results obtained for the different matrixes are similar, the interface is characterized by mechanical properties close to the Al2O3 base material. In meaningthe essential, that thetheresults nanoindentation presented areresults representative obtained for of the the different mechanical matrixes properties are similar, of the ceramic/metalmeaning that the joint. results presented are representative of the mechanical properties of the To summarize, the diffusion bonding of Ti6Al4V to Al O can be improved by using ceramic/metal joint. 2 3 Ni/Ti multilayer thin films. At 800 ◦C, the joining between these dissimilar materials was To summarize, the diffusion bonding of Ti6Al4V to Al2O3 can be improved by using unsuccessful without multilayers. The interface obtained is characterized by a reaction layer Ni/Ti multilayer thin films. At 800 °C, the joining between these dissimilar materials was close to Ti6Al4V and a crack along the interface with Al O . This may have happened due unsuccessful without multilayers. The interface obtained2 3 is characterized by a reaction to crack nucleation and propagation during sample preparation resulting in the formation layer close to Ti6Al4V and a crack along the interface with Al2O3. This may have happened of a brittle phase or residual stresses. The use of Ni/Ti multilayer thin films shows to due to crack nucleation and propagation during sample preparation resulting in the be effective, and it was possible to obtain a sound joint between Ti6Al4V and Al O , at formation of a brittle phase or residual stresses. The use of Ni/Ti multilayer thin2 3films a rather low temperature. The multilayer reaction and the interdiffusion between the shows to be effective, and it was possible to obtain a sound joint between Ti6Al4V and elements of the base and interlayer materials promoted the formation of two reaction layers Al2O3, at a rather low temperature. The multilayer reaction and the interdiffusion between

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the elements of the base and interlayer materials promoted the formation of two reaction atlayers the interface. at the interface. The hardness The hardness values at va thelues interface at the areinterface close toare the close Al2 Oto3 basethe Al material,2O3 base conﬁrmingmaterial, confirming that the use that of thesethe use multilayers of these multilayers have potential have to potential be applied to be in theapplied diffusion in the bondingdiffusion between bonding metals between and meta ceramicls and materials. ceramic materials.

(a) (b) Max. Max. Reduced Young modulus (GPa) Hardness (GPa)

Min. Min.

FigureFigure 7. 7.(a (,ba) SEMand ( imagesb) SEM of images the nanoindentation of the nanoindentation matrix, (c )matrix, hardness (c) andhardness (d) reduced and (d Young’s) reduced modulus Young’s map modulus across themap jointacross processed the joint at processed 800 ◦C using at 800 a Ni/Ti °C using multilayer a Ni/Ti thinmultilayer ﬁlm. thin film.

4.4. Conclusions Conclusions Diffusion bonding of Ti6Al4V to Al2O3 was processed at 800◦ °C for 60 min without Diffusion bonding of Ti6Al4V to Al2O3 was processed at 800 C for 60 min without andand with with Ni/Ti Ni/Ti multilayer multilayer thin thin film asfilm interlayer as interlayer material. material. The joining The experimentsjoining experiments without interlayerwithout interlayer were unsuccessful. were unsuccessful. The interfaces The inte arerfaces characterized are characterized by the presence by the presence of a crack of a crack close to the Al2O3 base material. The heating and cooling rates during the diffusion close to the Al2O3 base material. The heating and cooling rates during the diffusion bondingbonding have have a a significant significant effect effect on on the the plastic plastic deformation deformation of of the the Ti6Al4V Ti6Al4V base base material. material. InIn order order to to decrease decrease the the deformation deformation of of the the base base material, material, slow slow cooling cooling rates rates need need to to be be applied. The use of a Ni/Ti multilayer to assist the diffusion bonding of Ti6Al4V to Al2O3 applied. The use of a Ni/Ti multilayer to assist the diffusion bonding of Ti6Al4V to Al2O3 provesproves to to be be a a good good approach. approach. Sound Sound interfaces interfaces were were obtained obtained using using this this multilayer multilayer thin thin filmfilm with with aa modulationmodulation period close close to to 50 50 nm. nm. The The interfaces interfaces are are mainly mainly composed composed of two of tworeaction reaction layers. layers. Due to Due the to diffusion the diffusion of Ni towa of Nirds towards Ti6Al4V, Ti6Al4V, the zone the adjacent zone adjacent to this base to thismaterial base materialis enriched is enriched in Ti, corresponding in Ti, corresponding to NiTi2 to. The NiTi interdiffusion2. The interdiffusion between between alumina ◦ aluminaand interlayer and interlayer material material is insignificant is insignificant at 800 at 800°C; thus,C; thus, the thezone zone adjacent adjacent to to Al Al2O2O3 3is isconstituted constituted by by NiTi, NiTi, due due to to the the reaction reaction of of Ni Ni and and Ti Ti from from the the multilayer multilayer and and according according to toits its average average chemical chemical composition. composition. The The hardness hardness and reducedreduced YoungYoung modulusmodulus maps maps obtainedobtained by by nanoindentation nanoindentation corroborate corroborate the the microstructural microstructural characterization characterization and and clearly clearly distinguisheddistinguished the the different different reaction reaction layers layers composing composing the the interface. interface. The The hardness hardness maps maps showedshowed that that the the interface interface exhibited exhibited hardness hardness values values close toclose the Alto 2theO3 baseAl2O material,3 base material, which canwhich be favorable can be favorable to the mechanical to the mechanical behavior behavior of the joints. of the joints.

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Author Contributions: Conceptualization, M.S.J.; methodology, S.S., A.S.R. and M.T.V.; validation, S.S., A.S.R. and M.T.V.; investigation, M.S.J., S.S., A.S.R. and M.T.V.; writing—original draft preparation M.S.J.; writing—review and editing, S.S. and A.S.R.; funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Project PTDC/CTM-CTM/31579/2017—POCI-01- 0145- FEDER- 031579- funded by FEDER funds through COMPETE2020—Programa Operacional Com- petitividade e Internacionalização (POCI) and by national funds (PIDDAC) through FCT/MCTES. This research was also supported by FEDER funds through the program COMPETE —Programa Operacional Factores de Competitividade, and by national funds through FCT—Fundação para a Ciência e a Tecnologia, under the project UIDB/EMS/00285/2020. Data Availability Statement: Not applicable. Acknowledgments: The authors are grateful to CEMUP-Centre of Materials of University of Porto for expert assistance with SEM. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Cao, J.; Qi, J.; Song, X.; Feng, J. Welding and joining of titanium aluminides. Materials 2014, 7, 4930–4962. [CrossRef][PubMed] 2. Cao, R.; Sun, J.H.; Chen, J.H. Mechanisms of joining aluminium A6061- T6 and titanium Ti–6Al–4V alloys by cold metal transfer technology. Sci. Technol. Weld. Join. 2013, 18, 425–433. [CrossRef] 3. Tomashchuk, I.; Sallamand, P. Metallurgical strategies for the joining of titanium alloys with steels. Adv. Eng. Mater. 2018, 20, 1700764. [CrossRef] 4. Xue, Z.; Yang, Q.; Gu, L.; Hao, X.; Ren, Y.; Geng, Y. Diffusion bonding of TiAl based alloy to Ti-Al-4V alloy using amorphous interlayer. Mater. Wiss. Werkstofftech 2015, 46, 40–46. [CrossRef] 5. Simões, S.; Ramos, A.S.; Viana, F.; Vieira, M.T.; Vieira, M.F. Joining of TiAl to steel by diffusion bonding with Ni/Ti reactive multilayers. Metals 2016, 6, 96. [CrossRef] 6. Leyens, C.; Peters, M. Titanium and Titanium Alloys: Fundamentals and Applications; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2003; pp. 89–145. ISBN 9783527305346. 7. Dai, J.; Zhu, J.; Chem, C.; Weng, F. High temperature oxidation behavior and research status of modifications on improving high temperature oxidation resistance of titanium alloys and titanium aluminides: A review. J. Alloys Compd. 2016, 685, 784–798. [CrossRef] 8. Dimiduk, D.M. Gamma titanium aluminide alloys—An assessment within the competition of aerospace structural materials. Mater. Sci. Eng. A 1999, 263, 281–288. [CrossRef] 9. Aldinger, F.; Weberruss, V.A. Advanced Ceramics and Future Materials: An Introduction to Structures, Properties, Technologies, Methods; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2010; pp. 170–260. ISBN 978-3-527-32157-5. 10. Richerson, D.W.; Lee, W.E. Modern Ceramic Engineering: Properties, processing, and Use in Design, 4th ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, USA, 2018; ISBN 9781498716918. 11. Yang, Z.; Lin, J.; Wang, Y.; Wang, D. Characterization of microstructure and mechanical properties of Al2O3/TiAl joints vacuum- brazed with Ag-Cu-Ti+W composite filler. Vacuum 2017, 143, 294–302. [CrossRef] 12. Niu, G.B.; Wang, D.P.; Yang, Z.W.; Wang, Y. Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag-Cu-Ti filler metal. Ceram. Int. 2016, 42, 6924–6934. [CrossRef] 13. Dai, X.; Cao, J.; Wang, Z.; Wang, X.; Chen, L.; Huang, Y.; Feng, J. Brazing ZrO2 ceramic and TC4 alloy by novel WB reinforced Ag-Cu composite filler: Microstructure and properties. Ceram. Int. 2017, 43, 15296–15305. [CrossRef] 14. Liu, Y.; Hu, J.; Shen, P.; Guo, Z.; Liu, H. Effect of fabrication parameters on interface of zirconia and Ti-6Al-4V joints using Zr55Cu30Al10Ni5 amorphous filler. J. Mater. Eng. Perform. 2013, 22, 2602–2609. [CrossRef] 15. Travessa, D.; Ferrante, M. The Al2O3-titanium adhesion in the view of the diffusion bonding process. J. Mater. Sci. 2002, 37, 4385–4390. [CrossRef] 16. Correia, R.N.; Emiliano, J.V.; Moretto, P. Microstructure of diffusional zirconia-titanium and zirconia-(Ti-6wt% Al-4 wt% V) alloy joints. J. Mater. Sci. 1998, 33, 215–221. [CrossRef] 17. Barrena, M.I.; Matesanz, L.; Gómez de Salazar, J.M. Al2O3/Ti6Al4V diffusion bonding joints using Ag-Cu interlayer. Mater. Charact. 2009, 60, 1263–1267. [CrossRef] 18. Cao, J.; Liu, J.; Song, X.; Lin, X.; Feng, J. Diffusion bonding of TiAl intermetallic and Ti3AlC2 ceramic: Interfacial microstructure and joining properties. Mater. Des. 2014, 56, 115–121. [CrossRef] 19. Liu, J.; Cao, J.; Song, X.; Wang, Y.; Feng, J. Evaluation on diffusion bonded joints of TiAl alloy to Ti3SiC2 ceramic with and without Ni interlayer: Interfacial microstructure and mechanical properties. Mater. Des. 2014, 57, 592–597. [CrossRef] 20. Yi, J.L.; Zhang, Y.P.; Wang, X.X.; Dong, C.L.; Hu, H.C. Characterization of Al/Ti nano multilayer as a jointing material at the interface between Cu and Al2O3. Mater. Trans. 2016, 57, 1494–1497. [CrossRef] Metals 2021, 11, 655 12 of 12

21. Cao, J.; Song, X.G.; Wu, L.Z.; Qi, J.L.; Feng, J.C. Characterization of Al/Ni multilayers and their application in diffusion bonding of TiAl to TiC cermet. Thin Solid Films 2012, 520, 3528–3531. [CrossRef] 22. AlHazaa, A.; Haneklaus, N.; Almutairi, Z. Impulse pressure-assisted diffusion bonding (IPADB): Review and outlook. Metals 2021, 11, 323. [CrossRef] 23. Cooke, K.O.; Atieh, A.M. Current trends in dissimilar diffusion bonding of titanium alloys to stainless steels, aluminium and magnesium. J. Manuf. Mater. Process. 2020, 4, 39. [CrossRef] 24. Hu, A.; Janczak-Rusch, J.; Sano, T. Joining technology innovations at the macro, micro and nano levels. Appl. Sci. 2019, 9, 3568. [CrossRef] 25. Simões, S.; Viana, F.; Ramos, A.S.; Vieira, M.T.; Vieira, M.F. TiAl diffusion bonding using Ni/Ti multilayers. Weld. World 2017, 61, 1267–1273. [CrossRef] 26. Simões, S.; Viana, F.; Ramos, A.S.; Vieira, M.T.; Vieira, M.F. Reaction zone formed during diffusion bonding of TiNi to Ti6Al4V using Ni/Ti nanolayers. J. Mater. Sci. 2013, 58, 7718–7727. [CrossRef] 27. Simões, S.; Viana, F.; Ramos, A.S.; Vieira, M.T.; Vieira, M.F. Diffusion bonding of TiAl using reactive Ni/Al nanolayers and Ti and Ni foils. Mater. Chem. Phys. 2011, 128, 202–207. [CrossRef] 28. Ramos, A.S.; Vieira, M.T.; Simões, S.; Viana, F.; Vieira, M.F. Reaction-assisted diffusion bonding of advanced materials. Defect Diffus. Forum 2010, 297–301, 972–977. [CrossRef] 29. Yamamoto, S.; Yokomine, T.; Sato, K.; Terai, T.; Fukuda, T.; Kakeshita, T. Ab initio Prediction of Atomic Location of Third Elements in B2-Type TiNi. Mater. Trans. 2018, 59, 353–358. [CrossRef] 30. Lim, J.D.; Lee, P.M.; Rhee, D.M.W.; Leong, K.C.; Chen, Z. Effect of surface treatment on adhesion strength between magnetron sputtered copper thin films and alumina substrate. Appl. Surf. Sci. 2015, 355, 509–515. [CrossRef] 31. Oliver, W.C.; Pharr, G.M. Improved technique for determining hardness and elastic modulus using load and displacements sensing indentation experiments. J. Mater. Res. 1992, 7, 1564–1583. [CrossRef] 32. Uday, M.B.; Ahmad-Fauzi, M.N.; Noor, A.M.; Rajoo, S. Current Issues and Problems in the Joining of Ceramic to Metal. In Joining Technologies; Ishak, M., Ed.; IntechOpen: London, UK, 2016. [CrossRef] 33. Lim, J.D.; Lee, P.M.; Chen, Z. Understanding the bonding mechanisms of directly sputtered copper thin film on an alumina substrate. Thin Solid Films 2017, 634, 6–14. [CrossRef] 34. Cavaleiro, A.J.; Santos, R.J.; Ramos, A.S.; Vieira, M.T. In-situ thermal evolution of Ni/Ti multilayer thin films. Intermetallics 2014, 51, 11–17. [CrossRef] 35. Cavaleiro, A.J.; Ramos, A.S.; Martins, R.M.S.; Fernandes, F.M.B.; Morgiel, J.; Baehtz, C.; Vieira, M.T. Phase transformations in Ni/Ti multilayers investigated by synchrotron radiation-based X-ray diffraction. J. Alloys Compd. 2015, 646, 1165–1171. [CrossRef] 36. Cavaleiro, A.; Ramos, A.S.; Fernandes, F.; Baehtz, C.; Vieira, M.T. Interaction between Ni/Ti nanomultilayers and bulk Ti-6Al-4V during heat treatment. Metals 2018, 8, 878. [CrossRef] 37. Lu, Y.; Zhu, M.; Zhang, Q.; Hu, T.; Wang, J.; Zheng, K. Microstructure evolution and bonding strength of the Al2O3/Al2O3 interface brazed via Ni-Ti intermetallic phases. J. Eur. Ceram. Soc. 2020, 40, 1496–1504. [CrossRef] 38. Hayes, F.H. The Al-Ti-V (aluminum-titanium-vanadium) system. J. Phase Equilibria 1995, 16, 163–176. [CrossRef] 39. Humeau, B.; Rogl, P.; Zeng, K.; Schmid-Fetzer, R.; Bohn, M.; Bauer, J. The ternary system Al-Ni-Ti Part I: Isothermal section at 900 ◦C.; Experimental investigation and thermodynamic calculation. Intermetallics 1999, 7, 1337–1345. [CrossRef] 40. He, P.; Liu, D. Mechanism of forming interfacial intermetallic compounds at interface for solid state diffusion bonding of dissimilar materials. Mater. Sci. Eng. A 2006, 437, 430–435. [CrossRef] 41. Hatakeyama, F.; Suganuma, K.; Okamoto, T. Solid-state bonding of alumina to austenitic stainless steel. J. Mater. Sci. 1986, 21, 2455–2461. [CrossRef] 42. Misra, A.K. Reaction of Ti and Ti-Al alloys with alumina. Metall. Trans. A 1991, 22, 715–721. [CrossRef] 43. Hornbuckle, B.C.; Xiao, X.Y.; Noebe, R.D.; Martens, R.; Weaver, M.L.; Thompson, G.B. Hardening behavior and phase decomposi- tion in very Ni-rich Nitinol alloys. Mater. Sci. Eng. A 2015, 639, 336–344. [CrossRef] 44. Cavaleiro, A.J.; Ramos, A.S.; Braz Fernandes, F.M.; Schell, N.; Vieira, M.T. Follow-up structural evolution of Ni/Ti reactive nano and microlayers during diffusion bonding of NiTi to Ti6Al4V in a synchrotron beamline. J. Mater. Process. Technol. 2020, 275, 116354. [CrossRef] 45. Hu, L.; Xue, Y.; Shi, F. Intermetallic formation and mechanical properties of Ni-Ti diffusion couples. Mater. Des. 2017, 130, 175–182. [CrossRef]