Interaction Between Ni/Ti Nanomultilayers and Bulk Ti-6Al-4V During Heat Treatment

Article Interaction between Ni/Ti Nanomultilayers and Bulk Ti-6Al-4V during Heat Treatment

André João Cavaleiro 1,2,*, Ana Soﬁa Ramos 1 , Francisco Braz Fernandes 3, Carsten Baehtz 4 and Maria Teresa Vieira 1 1 CEMMPRE, Department of Mechanical Engineering, University of Coimbra, R. Luís Reis Santos, 3030-788 Coimbra, Portugal; soﬁ[email protected] (A.S.R.); [email protected] (M.T.V.) 2 INEGI, Instituto de Ciência e Inovação em Engenharia Mecânica e Engenharia Industrial–INEGI, Campus da FEUP, 400 4200-465 Porto, Portugal 3 CENIMAT/I3N, Department of Materials Science, Faculty of Sciences and Technology, Universidade Nova de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal; [email protected] 4 Helmholtz Zentrum Dresden Rossendorf HZDR, Institute of Ion Beam Physics and Materials Research, D-01314 Dresden, Germany; [email protected] * Correspondence: [email protected]; Tel.: +351-239790700

Received: 2 October 2018; Accepted: 25 October 2018; Published: 27 October 2018

Abstract: The diffusion bonding of Ti-6Al-4V to NiTi alloys assisted by Ni/Ti reactive multilayer thin films indicates the diffusion of Ni from the filler material towards bulk Ti-6Al-4V. As a consequence, the fragile NiTi2 intermetallic phase is formed at the joint interface. In this context, the aim of this work is to investigate the occurrence of Ni diffusion from Ni/Ti nanomultilayers towards Ti-6Al-4V substrates. For this purpose, multilayer coated Ti alloys were studied in situ at increasing temperatures using synchrotron radiation. After heat treatment, scanning electron microscopy (SEM) analyses were carried out and elemental map distributions were acquired by electron probe microanalysis (EPMA). The EPMA maps confirm the occurrence of Ni diffusion; the presence of Ni in the substrate regions immediately underneath the nanomultilayers is clearly indicated and becomes more pronounced as the temperature increases. The presence of Ni is observed in the same locations where V is found, thus in β-Ti grains of Ti-6Al-4V. At the same time, the synchrotron results together with SEM analyses allow the increase of the amount of β-Ti phase in Ti-6Al-4V to be identified, while the thin films are mainly constituted by NiTi2. Therefore, the presence of the brittle NiTi2 intermetallic phase can be avoided if Ni diffusion is prevented.

Keywords: Ni/Ti; multilayers; Ni; Ti-6Al-4V; diffusion

1. Introduction Due to their high strength to weight ratio and good corrosion resistance, titanium and titanium based alloys have a wide range of applications, such as in aerospace and biomedical industries. The Ti-6Al-4V alloy, constituted by α-Ti grains surrounded by β-Ti grains, accounts for about 60% of the total titanium production because it is easy to work with and it presents good mechanical properties due to the α/β alloy duality [1]. In order to have β-Ti at room temperature it is necessary to add beta stabilizing elements, such as vanadium. There are two different groups of β alloying elements: β-isomorphous and β-eutectoid. Vanadium, which is β-isomorphous, has high solubility in titanium and as the V content increases the β phase will also increase. On the contrary, since nickel is a β-eutectoid element, beside β-Ti it leads to the formation of intermetallic compounds, even for low Ni contents [2]. To enlarge its ﬁeld of application, appropriate techniques for joining Ti-6Al-4V to itself and to other materials, in particular to NiTi shape memory alloys, must be developed. However, joining of Ti

Metals 2018, 8, 878; doi:10.3390/met8110878 www.mdpi.com/journal/metals Metals 2018, 8, 878 2 of 10 alloys is difficult due to their high reactivity. The formation of oxides and brittle intermetallics can reduce joint strength. In particular, dissimilar joining can be challenging and often a filler material is required. Several joining attempts have been made by laser welding [3–5], brazing [6,7] and diffusion bonding [8–10]. Regarding the dissimilar diffusion bonding of Ti-6Al-4V, it is common to introduce metal foils as intermediate material [11]. In this context, to enhance the Ti-6Al-4V and NiTi diffusion bonding process, Ni/Ti multilayer thin films have been used [12,13]. These previous works reveal the formation of NiTi2 at the bonding interface during the joining process. The presence of the brittle NiTi2 phase led to joint failure. In order to increase joint strength, it is necessary to understand the mechanism behind the formation of the NiTi2 phase and the role of the Ti-6Al-4V alloy. The NiTi2 phase was detected by in situ synchrotron X-ray diffraction of Ni/Ti multilayers deposited onto Ti6Al4V [14]. After heat treatment (HT), the NiTi2 phase was observed close to the Ti-6Al-4V substrate by transmission electron microscopy [14]. However, using conventional high-temperature X-ray diffraction, the formation of NiTi2 in Ni/Ti multilayer thin films deposited onto stainless steel was not identified [15]. In a recent paper, Chen et al. [16] studied the diffusion behavior in Ti-6Al-4V alloys modified by the deposition of Cu/Ni bilayers using the electroplate method. The diffusion of Cu/Ni/Ti was significantly influenced by the HT temperature and caused the formation of several intermetallic phases, including NiTi, NiTi2, and Ni4Ti3 at the Ni/Ti interface. In addition, the Ni layer could not prevent diffusion of Ti towards the Cu side [16]. The aim of this study is to understand the interaction between Ni and Ti-6Al-4V during heat treatment, to clarify the mechanism behind the formation of the NiTi2 phase during diffusion bonding using Ni/Ti multilayer thin films. Two different Ni/Ti nanomultilayer configurations were deposited onto bulk Ti-6Al-4V. The bulk and filler materials were analyzed in situ using synchrotron X-ray diffraction during heat treatment at temperatures typical of those used in diffusion bonding. The evolution of the Ni/Ti nanomultilayers and the interaction with the Ti-6Al-4V substrate as a function of temperature were evaluated using scanning electron microscopy and elemental map distributions.

2. Experimental Details

2.1. Deposition Technique Ti-6Al-4V from Goodfellow with a 2 mm thickness was cut into uniform 10 × 10 mm substrates. The substrates were polished up to 1 µm diamond suspension. Ni/Ti reactive multilayer thin films with a total thickness close to 2.5 µm were deposited onto the polished Ti-6Al-4V substrates using a dual cathode magnetron sputtering equipment (Hartec, Stetten am kalten Markt, Germany). The substrates were mounted on a rotating copper block which acts as a heat sink, to avoid diffusion during the deposition process. The time that the substrates under rotation pass in front of each target, and consequently the individual layer thicknesses, depends on the substrates’ rotation speed. The deposition chamber was evacuated to approximately 3 × 10−4 Pa. Ni and Ti nanolayers were deposited by magnetron sputtering at 0.35 Pa from pure Ti (99.99%) and Ni (99.98%) targets. In order to obtain near equiatomic overall chemical compositions, the target power density used for the Ti and for Ni were 5.1 × 10−2 W·mm−2 and 2.5 × 10−2 W·mm−2, respectively. The magnetic Ni target had a reduced thickness of 1 mm. Multilayer thin films with 12 and 25 nm periods (bilayer thicknesses) were deposited by using different substrate rotation speeds. In order to understand the NiTi2 phase formation, two different configurations were studied: Ni/Ti multilayer thin films with periods of 12 and 25 nm were deposited onto Ti-6Al-4V with or without a tungsten interlayer. Ni was always the first layer of the Ni/Ti films. The W interlayer was ~12 nm thick. The diffusivity of Ni in tungsten is extremely reduced, suggesting that it can be used as a Ni diffusion barrier. In addition, W is also almost insoluble in the NiTi phase [17]. It should be noted that NiTi is the first phase to form upon heat treatment of Ni/Ti multilayers with nanometric Metals 2018, 8, 878 3 of 10 periods [14,18]. Due to its atomic characteristics, tungsten is excellent for contrast, particularly in backscattered electron (BSE) scanning electron microscopy (SEM).

2.2. Characterization Technique The phase evolution of the system with temperature was studied by synchrotron X-ray diffraction (XRD), in scanning mode, at the Materials Research station of the Rossendorf beam line in the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The coated Ti-6Al-4V substrates were placed in a furnace mounted on a six-circle goniometer. The incident X-ray beam was monochromatized to 11.5 keV (λ = 0.1078 nm). A micro-strip detector system (Mythen) for time-resolved experiments with high angular accuracy was used. Steps of 50 ◦C were implemented up to the desired maximum temperature (850 or 900 ◦C). At each temperature step, θ-2θ isothermal scans (20◦ < 2θ < 42◦) were acquired. After the heat treatment carried out during the in situ XRD analysis, elemental distribution maps were obtained by electron probe microanalysis (EPMA) using a five-spectrometer Cameca SX 50 (CAMECA, Gennevilliers, France) equipment in order to visualize the relative variation of element concentrations as a result of diffusion. Scanning electron microscopy was used to characterize the heat treated coated Ti-6Al-4V. For comparison purposes, SEM after low temperature HT (600 ◦C) was also carried out. The analyses were performed at a 15 kV acceleration voltage in secondary and backscattered electron modes using a field emission gun FEI QUANTA 400F (FEI Company, Hillsboro, OR, USA) high-resolution SEM. The cross-sections for EPMA and SEM analyses were prepared by cutting the heat treated samples and mounting them transversely in resin. Following this, standard metallographic preparation was conducted to obtain flat and smooth surfaces.

3. Results

3.1. In Situ Characterization during Heat Treatment The Ni/Ti films were analyzed using the synchrotron based high-temperature X-ray diffraction procedure described. Figures1 and2 present the phase evolution with temperature of the Ni/Ti films under study. It is important to notice that at the bottom of the XRD figures the diffraction peaks indicated for the different phases were based on ICDD (International Centre for Diffraction Data) standards collected at room temperature. Therefore, some deviation to low diffraction angles is expected as the temperature increases. The β-Ti peak presents a deviation to high diffraction angles due to residual stresses, since the β-Ti phase growth is trapped between α-Ti grains. For the multilayers with a Ni bottom layer (Figure1), the evolution towards the B2-NiTi phase is similar to the multilayers starting with Ti, previously studied up to 600 ◦C[14]. However, in the present case, as the temperature increases the intensity of the B2-NiTi main diffraction peak diminishes and the NiTi2 peak increases, suggesting a possible compromise between the formation of one phase and the decrease of the other. Moreover, the β-Ti peak shows a significant increase for high HT temperatures. These results can be explained by the diffusion of Ni from the thin film towards the Ti-6Al-4V alloy. For the 12 nm period multilayer (Figure1a), no B2-NiTi peak can be discerned at the highest temperature (real temperature = ◦ 712 C); only substrate and NiTi2 peaks can be identified, indicating that at this temperature the phase evolution is complete. At temperatures above 700 ◦C, besides the diffusion of Ni, the diffusion of Ti from Ti-6Al-4V towards the thin films becomes important, especially for the lowest period multilayer which has an increased diffusivity. Therefore, at 712 ◦C for the 12 nm period multilayer, the thin film is only constituted by the NiTi2 Ti-rich phase. Although the 25 nm period multilayer reached higher temperatures (real temperature = 772 ◦C) than the 12 nm period multilayer, the larger grain size, and consequently lower reactivity, may explain why it was still possible to identify residual B2-NiTi at the maximum temperature. As the temperature reaches ≈750 ◦C, the β-Ti peak becomes significant. Metals 2018, 8, x FOR PEER REVIEW 4 of 10

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(a)

(b)

FigureFigure 1. 1.In Insitu situ synchrotron synchrotron XRD XRD phase phase evolution evolution of multilayer of multilayer thin thinfilms ﬁlms with with (a) 12 (a nm) 12 and nm ( andb) 25 (b ) nm25 period. nm period.

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FigureFigure 2. 2.In situIn situ synchrotron synchrotron XRD XRD phase phase evolution evolution of multilayer of multilayer thin thinfilms ﬁlms (W interlayer) (W interlayer) with with(a) 12 ( a) nm12 and nm ( andb) 25 (b nm) 25 period nm period..

TheWith photon or without absorption W interlayer coefficient the for structural11.5 keV is evolution ≈1.1 × 104with mm2· temperatureg−1 and ≈2.1 × is 10 similar4 mm2·g (Figure−1 for Ti2 ). andHowever, Ni, respectively for ﬁlms [19]. with SinceΛ = for 12 nmcompounds the B2-NiTi and phasemixtures was the still absorption distinguishable is averaged at the using maximum the ◦ weightHT temperature content of each (real component, temperature if =the 716 weightC), meaning ratio increases that the in tungstena lower absorption might have element changed (in thethis Ni case,diffusion Ti), the kinetics. overall absorption decreases. Before the phase transformation, the Ni/Ti multilayer thin 4 2 −1 4 2 −1 films areThe near photon equiatomic absorption, and coefﬁcient at the end for 11.5of the keV thermal is ≈1.1 cycle× 10 themm NiTi·g 2 isand the≈ dominant2.1 × 10 mm phase,·g resultingfor Ti and in Ni, less respectively absorption [ and19]. enablingSince for compoundsphotons to reach and mixtures the tungsten the absorption layer. The is differences averaged using in the weight content of each component, if the weight ratio increases in a lower absorption element (in

Metals 2018, 8, 878 6 of 10

Metalsthis case, 2018, Ti),8, x FOR the overallPEER REVIEW absorption decreases. Before the phase transformation, the Ni/Ti multilayer6 of 10 thin films are near equiatomic, and at the end of the thermal cycle the NiTi2 is the dominant phase, photonresulting absorption in less absorption explain and why enabling at the photons beginning to reach of the the heat tungsten treatment layer. the The presence differences of in W photon is not detectedabsorption and, explain as soon why as atthe the NiTi beginning2 is formed of, the the heat tungsten treatment peak thestarts presence to emerge. of W is not detected and, as soonBased as theon NiTithe 2synchrotronis formed, theresults, tungsten with peak or without starts to W emerge. interlayer, it can be concluded that the formationBased of on NiTi the2 synchrotron is related with results, the with disappearance or without of W the interlayer, B2-NiTi it phase can be as concluded well as with that the formation ofof NiTiβ-Ti.2 Ais possible related with explanation the disappearance can be related of the with B2-NiTi the diffusion phase as wellof Ni as from with the the multilayer formation thinof β -Ti.film A towards possible the explanation bulk Ti-6Al can-4V, be beca relateduse with Ni is the a betageneous diffusion of Nielement from. the Since multilayer this element thin filmis a titaniumtowards theβ-eutectoid, bulk Ti-6Al-4V, it signifies because that besides Ni is a betageneousincreasing the element. β-Ti phase, Since Ni this should element also ispromote a titanium the formationβ-eutectoid, of itNiTi signifies2 on Ti that-6Al besides-4V. increasing the β-Ti phase, Ni should also promote the formation of NiTi2Inon this Ti-6Al-4V. study, the Ni4Ti3 phase was not detected, as in the study of Chen et al. [16], due to the differentIn this coating study, chemical the Ni4Ti 3 composition,phase was not configuration detected, as in (nanoscale the study multilayer of Chen et versusal. [16], microscale due to the bilayer),different and coating processing chemical technology. composition, configuration (nanoscale multilayer versus microscale bilayer), and processing technology. 3.2. Ex Situ Characterization after Heat Treatment 3.2. Ex Situ Characterization after Heat Treatment Figure 3 shows the presence of β-phase in the grain boundaries of α-Ti (dark-phase); neither the periodFigure nor the3 shows presence the presenceof the W ofinterlayerβ-phase seems in the to grain influence boundaries this microstructure of α-Ti (dark-phase); after HT neither at 600 the°C. ◦ periodHowever, nor the the amount presence of of β the-phase W interlayer increases seemsafter HT to influenceat temperatures this microstructure higher than after600 ° HTC (F atigures 600 C.3 ◦ andHowever, 4). β-Ti the is much amount more of βevident-phase close increases to the after interface HT at thin temperatures film/bulk material, higher than which 600 is attributedC (Figures to3 theand diffusion4). β-Ti is of much Ni from more the evident film towards close to Ti the-6Al interface-4V. As thin a consequence, film/bulk material, the films which become is attributed depleted in to Nithe, diffusiongiving rise of to Ni the from formation the film towardsof NiTi2 Ti-6Al-4V.as already As confirmed a consequence, by synchrotron the films becomeXRD. The depleted SEM (BSE) in Ni, imagesgivingrise clearly to the reveal formation the presence of NiTi2 as of already the W interlayerconfirmed (brightby synchrotron lines in XRD.Figure Thes 4b SEM and (BSE) 5b), images which remainsclearly reveal intact, the even presence after of HT, the indicatingW interlayer unequivocally (bright lines in the Figures initial4 b location and5b), of which the remains interface film/subsintact, eventrate. after In HT, the indicating presence unequivocallyof the W interlayer, the initial the location12 nm period of the interfacemultilayer film/substrate. image shows In that the presencewhen the of NiTi the2 Wrich interlayer, film is in the direct 12 nm contact period with multilayer a β-Ti grain image, the shows connection that when will the beNiTi the 2preferentialrich film is routein direct for contactthe Ni diffusion, with a β-Ti while grain, if the the NiTi connection2 only contacts will be the the α preferential-Ti phase, the route NiTi for2 will the grow Ni diffusion, over the whileW interlayer if the NiTi (Figure2 only 4b). contacts As expected, the α-Ti phase, Ni preferentially the NiTi2 will diffuses grow over towards the W theinterlayer -phase. (Figure Ni is4b). a Asbetageneous expected, element Ni preferentially and due diffusesto the different towards crystallographic the β-phase. Ni isstructures a betageneous (β-Ti is element bcc while and α due-Ti tois hcpthe), different Ni diffusion crystallographic is considerably structures higher on (β-Ti the is β bcc-Ti phase while thanα-Ti in is hcp),α-Ti. Regarding Ni diffusion the is film considerably with a 25 nmhigher period, on the theβ cross-Ti phase-section than is insimilarα-Ti. without Regarding and the with film W with interlayer a 25 nm (Figure period, 5). theIt should cross-section be noted is thatsimilar the without25 nm period and with films W were interlayer treated (Figure at higher5). It shouldtemperatu be notedres (900 that °C the), and 25 nmit is period probable films that were the ◦ Wtreated could at higher not prevent temperatures the Ni (900 diffusion.C), and The it is increase probable of that the the HT W couldtemperature not prevent leads the to Ni deeper diffusion. Ni Thediffusion, increase with of film the HTand temperature substrate becoming leads to almost deeper indiscernible. Ni diffusion, Simultaneously, with film and substrate the presence becoming of - Tialmost significantly indiscernible. decreases Simultaneously, (dark regions). the presence of α-Ti significantly decreases (dark regions).

Figure 3. CrossCross-section-section SEM backscattered electron ( (BSE)BSE) image of a Ni/Ti multilayer thin thin film ﬁlm (W interlayer) with 12 nm period after heat treatment (HT)(HT) at 600 ◦°C.C.

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(a) (b) (a) (b) Figure 4. Cross-section SEM BSE images of Ni/Ti multilayer thin films with 12 nm period (a) without Figureor (b) with 4. CrossCross-section W interlayer-section SEM after BSE HT images at 850 ° ofC. Ni/Ti multilayer multilayer thin thin films ﬁlms with with 12 nm period ( a) without ◦ or ( b) with W interlayer after HT at 850 °CC..

(a) (b) (a) (b) Figure 5. CrossCross-section-section SEM BSE images of Ni/Ti multilayer multilayer thin thin films films with 25 nm period ( a) without ◦ Figureor ( b) with 5. Cross W interlayer-section SEM after BSE HT images at 900 ° CofC.. Ni/Ti multilayer thin films with 25 nm period (a) without or (b) with W interlayer after HT at 900 °C. In orderorder toto analyze analyze the the diffusion diffusion phenomena phenomena in detail,in detail, cross-sections cross-sections of the of heatthe heat treated treated films films were 2 preparedwereIn prepared order and to elemental and anal elementalyze (V,the Nidiffusion (V, and Ni W) and phenomena distributions W) distributions in overdetail, areasover cross areas with-sections with 50 × 5050of ×µthe m50 heatµmwere2 treatedwere carried carried films out wereoutby EPMAby prepared EPMA (Figures (Figures and6 elemental and 6 and7). The 7). (V, The cross-sections Ni cross and- sectionsW) distributions were wer analyzede analyzed over starting areas starting with from from 50 the ×the thin50 thin µm film/substrate 2film/substrate were carried outinterface by EPMA to the (Figures Ti Ti-6Al-4V-6Al -64V and bulk 7). The material, cross -to sections avoid werthe esteep analyzed Ni increase starting due from to the the thin film.film. film/substrate In FiguresFigures 66 interfaceand7 , 7, for for eachto the each map Ti - map6Al the- color4V the bulk scale color material, is scale independent isto independentavoid (from the steep low (from concentration Ni increase low concentration due purple to the to high film. purple concentration In F igures to high 6 andconcentrationred). 7, As for previously each red). map mentioned, As the previously color in scale Ti-6Al-4V mentioned, is independent vanadium in Ti-6Al is (from added-4V vanadium low to induce concentration isthe added formation purple to induce of tothe high β the-Ti phase,concentrationformation therefore of the red). β this-Ti As phase, element previously therefore can be mentioned, usedthis element as a trace in can Ti element- be6Al used-4V foras vanadium a the trace location element is added of for this tothe phase. inducelocation From the of formationthisthe elementalphase. of From the map βthe-Ti distribution, elementalphase, therefore map it is distribution, possiblethis element to identifyit can is possible be used the vanadium toas identifya trace inelement the the vanadiumβ -Tifor andthe locationin this the phase β- ofTi thisandis distributed phase.this phase From around is thedistributed elementalα-Ti grains. around map The distribution,α- tungstenTi grains. distribution itThe is possibletungsten maps to distribution identify do not t showhe maps vanadium any do diffusionnot in show the of βany- WTi andditowardsffusion this thephase of W Ti-6Al-4V towardsis distributed substrate. the Ti -around6Al-4V αsubstrate.-Ti grains. The tungsten distribution maps do not show any diffusion of W towards the Ti-6Al-4V substrate.

(a) (a)

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(a) (b)

Figure 4. Cross-section SEM BSE images of Ni/Ti multilayer thin films with 12 nm period (a) without or (b) with W interlayer after HT at 850 °C.

(a) (b)

Figure 5. Cross-section SEM BSE images of Ni/Ti multilayer thin films with 25 nm period (a) without or (b) with W interlayer after HT at 900 °C.

In order to analyze the diffusion phenomena in detail, cross-sections of the heat treated ﬁlms were prepared and elemental (V, Ni and W) distributions over areas with 50 × 50 µm2 were carried out by EPMA (Figures 6 and 7). The cross-sections were analyzed starting from the thin film/substrate interface to the Ti-6Al-4V bulk material, to avoid the steep Ni increase due to the film. In Figures 6 and 7, for each map the color scale is independent (from low concentration purple to high concentration red). As previously mentioned, in Ti-6Al-4V vanadium is added to induce the formation of the β-Ti phase, therefore this element can be used as a trace element for the location of this phase. From the elemental map distribution, it is possible to identify the vanadium in the β-Ti Metalsand this2018 ,phase8, 878 is distributed around α-Ti grains. The tungsten distribution maps do not show8 ofany 10 diffusion of W towards the Ti-6Al-4V substrate.

Metals 2018, 8, x FOR PEER REVIEW 8 of 10 Metals 2018, 8, x FOR PEER REVIEW 8 of 10 (a)

(b) Figure 6. Electron probe microanalysis (EPMA)( belemental) map distributions of the zone directly underFigure the 6. 6. Electron E heatlectron treated probe probe Ni/Ti microanalysis microanalysis multilayer (EPMA) ( thinEPMA films elemental) elemental with map 12 map nm distributions period distributions (a) of without the of zone the or directly zone (b) with directly under W interlayer.theunder heat the treated heat Ni/Ti treated multilayer Ni/Ti multilayer thin ﬁlms thin with films 12 nm with period 12 nm (a) withoutperiod ( ora) ( withoutb) with W or interlayer.(b) with W interlayer.

(a) (a)

(b) Figure 7.7. EPMAEPMA elemental elemental map map distributions distributions of( ofb ) thethe zone directly under the heat treated treated Ni/Ti Ni/Ti multilayerFigure 7. EPMAthin films films elemental with 25 nm map period distributions (a) without of theor ( b zone) with directly W interlayer. under the heat treated Ni/Ti multilayer thin films with 25 nm period (a) without or (b) with W interlayer. The nickel distribution mapsmaps showshow thatthat thisthis elementelement clearly clearly diffuses diffuses to to the the same same locations locations where where V Vis detected is The detected nickel (the (the βdistribution-Ti β grains).-Ti grains). maps Independent Independen show that of thethist of multilayer element the multilayer clearly configuration, di configuration,ffuses asto thethe temperaturesame as the locations temperature increases, where increases,theV is Ni detected diffusion the (the Ni length di β-ffTiusion increases. grains). length Independen For increases. the 12t nm of For period the the multilayer multilayer 12 nm periodconfiguration, the maximum multilayer as temperature the the temperature maximum was temperaturetheincreases, same without the was Ni the and di sameffusion with without interlayer, length and increases. and with even interlayer, For though the and the 12 Ninmeven diffusionperiod though multilayer lengththe Ni isdi almostff usion the maximum thelength same, is almostwithtemperature the the W same, interlayer was with the same thethe NiW without interlayer diffusion and lengththe with Niis diinterlayer, slightlyffusion length shorter and even is (Figure slightly though6). shorter The the small Ni (Figure di differenceffusion 6). The length in small the is didiffusionalmostfference the lengths insame, the with confirmsdiffusion the W thatlengths interlayer the Wconfirms interlayer the Ni thatdiff works usionthe W aslength interlayer a barrier, is slightly butworks since shorter as its a thickness barrier,(Figure 6).but is The relativelysince small its thicknesssmalldifference (≈12 is in nm) relatively the it di isff notusion small effective lengths (≈12 enoughnm) confirms it is to not completely that effective the W prevent enough interlayer Ni to migration.completely works as In aprevent thebarrier, similar Ni but migration. analysis since its of ◦ InNi/Tithickness the similar multilayer is relatively analys thinis films,ofsmall Ni/Ti starting(≈12 multilayer nm) with it is titanium thinnot filmseffective and, starting heatenough treated with to titaniumcompletely at lower and temperatures prevent heat treated Ni migration. (≤ at500 lowerC), temperaturesitIn wasthe similar not possible (analys500 °C) tois of identify, it Ni/Ti was multilayernot any possible Ni diffusion, thin to filmsidentify indicating, starting any Ni with that diffusion, atitanium minimum indicating and temperature heat that treated a minimum mustat lower be ◦ temperaturereachedtemperatures in order must (500 to be promote°C) reached, it was the in not diffusionorder possible to promote of Nito identify towards the di anyff theusion βNi-Ti diffusion,of phase Ni towards (~700 indicating theC for β- 12Ti that phase nm a periodminimum (~700 and °C ◦ ~750fortemperature 12 Cnm for period 25 must nm). and be For reached~750 the ° 25C infor nm order 25 period nm). to promote For multilayer, the 25 the nm thedi ffperiod temperaturesusion multilayerof Ni towards reached, the the temperatures were β-Ti higher, phase (~700 andreached as °C a werefor 12 higher, nm period and andas a ~750consequence °C for 25 Ninm). diffusion For the is25 more nm period pronounced multilayer and, thethe amounttemperatures of β-Ti reached phase alsowere increases higher, and (Figure as a 7).consequence For this period, Ni diffusion the effect is moreof the pronounced W interlayer and cannot the amountbe evaluated of β- Tibecause phase thealso temperature increases (Figure reached 7). was For diffthiserent period, without the effect and withof the interlayer. W interlayer With cannot the W interlayerbe evaluated, the because sample attainedthe temperature 802 °C and reached the region was diff adjacenterent without to the thin and film with is interlayer. only composed With the of βW-Ti interlayer, as indicated, the sample by the Vattained map. Once 802 ° againC and itthe is confirmedregion adjacent that Nito thediffuses thin filmto β grainsis only andcomposed that the of diffusio β-Ti, asn indicateddepth increases by the withV map. temperature. Once again According it is confirmed to Figure that Ni7b ,diffuses it is possible to β grains to find and Ni thatup to the a depthdiffusio ofn closedepth to increases 50 m. Thewith fact temperature. that Ni di Accordingffusion can to be F igureclearly 7b observed, it is possible as the to temperature find Ni up to increases a depth, ofand close that to β 50-Ti andm. NiTiThe 2fact phases that canNi dibeff clearlyusion can identified be clearly by XRDobserved and SEMas the concurrently, temperature confirms increases that, and the that NiTi β-2Ti phase and formationNiTi2 phases is directlycan be clearly related identified to the Ni by di ffXRDusion and process. SEM concurrently, Therefore, if confirmsan effective that barrier the NiTi is 2placed phase betweenformation Ni/Ti is directly nanomultilayers related to andthe TiNi-6Al diff-4Vusion substrates, process. it Therefore, would be possibleif an effective to prevent barrier Ni diis ffusionplaced between Ni/Ti nanomultilayers and Ti-6Al-4V substrates, it would be possible to prevent Ni diffusion

Metals 2018, 8, 878 9 of 10 consequence Ni diffusion is more pronounced and the amount of β-Ti phase also increases (Figure7). For this period, the effect of the W interlayer cannot be evaluated because the temperature reached was different without and with interlayer. With the W interlayer, the sample attained 802 ◦C and the region adjacent to the thin film is only composed of β-Ti, as indicated by the V map. Once again it is confirmed that Ni diffuses to β grains and that the diffusion depth increases with temperature. According to Figure7b, it is possible to find Ni up to a depth of close to 50 m. The fact that Ni diffusion can be clearly observed as the temperature increases, and that β-Ti and NiTi2 phases can be clearly identified by XRD and SEM concurrently, confirms that the NiTi2 phase formation is directly related to the Ni diffusion process. Therefore, if an effective barrier is placed between Ni/Ti nanomultilayers and Ti-6Al-4V substrates, it would be possible to prevent Ni diffusion and avoid the formation of the brittle NiTi2 intermetallic compound. This would be particularly important when using Ni/Ti multilayer thin films to assist the similar and dissimilar diffusion bonding process of α/β Ti alloys.

4. Conclusions During heat treatment of Ti-6Al-4V coated with Ni/Ti nanomultilayers, Ni diffusion from the thin films towards the bulk material is promoted. According to the results, the occurrence of Ni diffusion is determined by the HT temperature. Ni diffuses to the β-Ti grains of the Ti-6Al-4V alloy. Therefore, the presence of β-Ti in the Ti alloy substrates becomes more pronounced, while the films are mainly formed by the NiTi2 intermetallic phase, although after deposition they had a near equiatomic overall chemical composition. Regarding Ni diffusion, no significant differences were detected for the two multilayer periods studied. The W interlayer deposited between bulk Ti-6Al-4V and multilayer thin film was too thin to prevent Ni diffusion. Nevertheless, the results indicate that W can be used as a diffusion barrier, which should be taken into consideration when using Ni/Ti multilayer thin films as fillers for joining Ti alloys, such as Ti-6Al-4V. This point must be confirmed with further scientific evidence.

Author Contributions: A.J.C., A.S.R., and M.T.V. conceived the experiments; A.J.C. produced the Ni/Ti multilayers and performed the EPMA analyses together with M.T.V.; A.J.C., A.S.R. and F.B.F. carried out the synchrotron experiments, with the support of the beamline scientist C.B.; A.J.C. and A.S.R. wrote the manuscript (with contributions from all the other co-authors). Funding: This research was sponsored by FEDER funds through the program COMPETE 2020 and National Funds through FCT-Portuguese Foundation for Science and Technology, which provided financial support for CEMMPRE under the project UID/EMS/00285/2013, and for CENIMAT/I3N under project number UID/CTM/50025 and SciTech Science and Technology for Competitive and Sustainable Industries. The R&D project was co-financed by Programa Operacional Regional do Norte (“NORTE2020”). The authors would like to thank FCT for the grant SFRH/BPD/109788/2015. The authors also acknowledge ESRF funding of the experiment at the ESRF MA-1985 entitled “In-situ phase evolution of sputtered reactive multilayers: Effect of the substrate”. Conflicts of Interest: The authors declare no conflict of interest.

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