Brazing studies of Ti-joint using Ti 20 Zr 20 Cu 60-xNix ( x = 30 , 40 , and 50 ) metallic glass ribbon as filler metal

The present study investigation, our results on characterization of commercially pure-Ti alloy brazed with metallic glass ribbons of Ti20Zr20Cu60-xNix (x = 30, 40, and 50) metallic glass ribbons were produced using a vacuum melt spinner. These ribbons were then used as filler materials for vacuum brazing of two Ti alloy plates at 1268, 1277 and 1279 K for a period of 10 min. Field-Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and the energy dispersive X-ray spectroscopy (EDX). The as-spun ribbons showed fully amorphous structure when examined on both surfaces by XRD and also verified by TEM investigation. The brazing joint of two Ti-plates using the metallic glass ribbon when brazed with Ni50 was found to be of very high strength. FESEM characterization of the cross-section of the brazed joints shows sub-micron size grains uniformly distributed in the matrix with brighter appearance. FESEM and EDX analysis revealed that the sub-micron grains are rich in Ti & Ni while the matrix phase mainly consisted of Ti. BSE image along with EDS Analysis indicated that the brazed joint has a presence of NiTi2 and Cu2 (Ni Zr) phases which could be responsible for increase in the strength of the brazed joint.


Introduction
Brazing is a metal-joining process where a filler metal is heated above its melting point and distribution between two or more closefitting parts [1][2][3][4].This process is considered to be one of the simplest and by far the most widely methods used to join ceramics to metals based on melting, wetting and solidification of a liquid film between the base materials [5,[6][7][8][9][10], as reported by Martens et al. (1996).More recently, Sugar et al. (2006) reported that to improve the wetting and adherence of the metal-foil interlayer where the surfaces are in contact during the joining process [11,12], it is essential that the surfaces are prepared (cleanliness) and/or the materials to be joined are conditioned, i.e., an effective metallization (through wetting or flow) is required.Titanium alloys, due to their high specific strength and good corrosion resistance, are particularly suitable for special applications.CP-Ti (Commercially Pure Titanium), which is unalloyed, ranges in purity from 99.5 to 99.0 wt pct Ti.Titanium exists in two allotropic crystal structures.There area, which has the hexagonal close-packed (HCP) structure, and b, which has the body-centered cubic (BCC) crystal structure.Above the β transus temperature, the hexagonal α-phase is transformed on heating to the BCC β-phase [13][14][15][16].The β transus temperature is strongly affected by the alloying elements in CP-Ti, e.g., Fe and interstitial elements, carbon, oxygen, nitrogen, as well as hydrogen.
The scanning, transmission electron microscope (FESEM/TEM), and X-ray diffractometer (XRD) are not satisfactory in analysis of a brazed joint involved with phase transformation.
Although FESEM observations present quantitative chemical compositions of the specific phases, it lacks structural data to identify them.For example, the transformation between α and β phases in the titanium alloy could not be accurately identified from FESEM/TEM analyses.On the other hand, the width of a brazed joint is usually below 100 µm, so slicing different brazed zones in order to make TEM examination is quite difficult.The electron backscatter diffraction (EBSD) technique has made a great achievement in recent years [17,18].Therefore, the combination of morphology, element mapping, and crystallographic data makes it possible to analyse such a brazed joint undergoing phase transformation.In this work, the objective of the research is concentrated on the brazing CP-Ti using the Ti 20 Zr 20 Cu 60-x Ni x (x = 30, 40, and 50) filler foil.The vacuum brazing has been carried out during the experiment for the sake of comparison.Phase identification, microstructural evolution, and interfacial reaction of the brazed joint are extensively studied in this investigation.

EXPERIMENTAL METHODS
The alloy with nominal composition Ti 20 Zr 20 Cu 60-x Ni x (x = 30, 40, and 50) is prepared from pure elements (purity > 99.9 wt.%) by arc melting in a titanium-gettered argon atmosphere.
For achieving homogeneity in the alloy composition, it is remelted many times.The amorphous ribbon of this compound is prepared using the standard rapid quenching technique.The ribbon is about 25 mm wide and 100 m thick.The X-ray Diffractometers, used to characterize the sample, is a Bruker machine, Model No. D8.The Field Emission Gun (FEG) is usually a wire of Tungsten (W) Zigma, Carl Zeiss, Germany (FE-SEM, Carl ZEISS, FEG, Ultra 55), 30kV, images were obtained at an operating voltage of 15 kV and the working distance was about 8.5 mm.
Vacuum brazing was performed to braze commercially pure CP-Ti plates using the as spun Ti 20 Zr 20 Cu 60-x Ni x (x = 30, 40, and 50) metallic glass ribbon as filler material.The CP-Ti plates measuring 10 mm×7 mm× 3mm were prepared.the lap-butt joint, the CP-Ti plates measuring 5 mm×3 mm×1 mm were first prepared and then steps were cut in EDM using a 0.5 mm wire.Both the brazing ribbons and Ti plates were initially cleaned using acetone and then the ribbons were kept in between the two Ti plates before tightening them using nicrome wire.The samples were then placed in a vacuum furnace (10 -3 mbar) and annealed for 10 minutes.The selected temperature for each sample was 20±5 o C higher than the solidus temperature of respective ribbon.The samples were then furnace cooled.

RESULTS AND DISCUSSION
3.1 Microstructure analysis of Ti/Ti20Zr20Cu30Ni30 /Ti composite joint brazed at 1268 K for a period of 10 min The FESEM analysis of Ti/Ti 20 Zr 20 Cu 30 Ni 30 /Ti composite joint brazed at 1268 K for a period of 10 min is shown in fig. 1. Figure 1(a) shows the cross sectional BSE image of the Ti/Ti 20 Zr 20 Cu 30 Ni 30 /Ti composite joint.It can be seen from Figure 1(a) that the joint was soundly bonded and devoid of imperfections such as cracks and voids.It appears that there was possibly a vigorous interfacial interaction which had occurred during the brazing process and must have had involved dissolution, diffusion and chemical reaction between the Ti substrate and the molten metallic glass filler.This observation indicates a good wetting and intimate bonding between the substrates and the filler alloy.As shown in Figure 1(a), continuous and sound interfacial reaction layers are formed at the interface between the brazing alloy and the substrates [21,22].The reaction layer appeared to have an average thickness of about 4 μm.It had formed at the Ti-alloy/brazed interface.Further, it was thicker than the reaction layer between filler and brazed alloy.The reaction layer appeared to have a thickness of only around 1 μm.As shown in Figure .1a,both reaction layers were primarily composed of Ti.
The corresponding EDX based line scan analysis across 70m length comprising of the CP-Ti pieces on both sides of brazed joint is shown in Figure 1(b).It shows that the amount of Ti was maximum at the left side interface of the joint and was minimum at about 50 to 65 mm from the same interface.Beyond this point the amount of Ti slightly increased as the right side interface of the joint was approached.Compared to that of Ti, the amounts of Ni, Cu and Zr were a little on the lower side.To elucidate the atomic behavior at the solid/liquid interface, the distribution of the primary elements across the brazing seam was measured by EDX.Thus, the corresponding concentration profiles of Ti, Ni, Cu and Zr are shown in Figures 1(c), (d), (e 3(a-d) and Figures 4 (a-d).
Maximum solubilities of Cu and Ni in the β-Ti are 28.5 and 28 at.%, respectively.In                respectively.Table 5 presents the corresponding data on compositional analysis of the same annealed ribbon.These results e.g., figure.17(c) and Table 5 corroborated with the corresponding XRD data (Figure .14).
The homogeneous distribution of the various nanocrystalline phases in amorphous matrix has also been observed (Figures.17

Figure 1 .
Figure 1.FESEM analysis of Ti/ Ti 20 Zr 20 Cu 30 Ni 30 /Ti composite joint brazed at 1268 K for a period of 10 min (a) Cross sectional BSE Image (b) corresponding EDS based line scan analysis across 30μm length comprising the CP-Ti pieces on both sides of brazed joint, corresponding concentration profiles of elements :(c)Ti, (d)Ni (e) Cu and (f) Zr.
) and (f), respectively.Similarly, the corresponding EDX based X-ray maps of the elemental distribution of Ti, Zr, Cu and Ni are shown in Figures.1 (a), (b), (c) and (d), respectively.While the presence of Ti was throughout across the brazed joint, there was a relative increase near the left interface region where the reaction had initially started, Figure 1(c).Similarly, the amounts of Ni Figure 1(d) and Cu Figure 1(e) were the minimum at the left interface and grew to their respective maximum positions at about 65 and 50 m from the left interface [23, 24].Similarly, the amount of Zr (Figure .1f)was the minimum at the left interface and grew to its maximum positions at about 55 to 60 m from the left interface.The inhomogeneous and nonuniform distribution of Ni, Cu and Zr across the interface hinted that the diffusion of Zr, Ni and Cu atoms from the filler into the Ti substrate took place at the solid interface, reducing their concentration in the filler.The quaternary Ti-Zr-Ni-Cu was transformed into a liquid fourmembered Ti-Zr-Ni-Cu system with an uncertain composition as a consequence of the intensive interfacial interaction.It can be anticipated that the formation of joint microstructure during the isothermal solidification and cooling of the Ti-Zr-Ni-Cu molten pool were complex.It may be noted that both the element pairs of (Ti and Zr), and (Cu and Ni), have similar atomic radii, crystal structure, and are completely miscible with each other according to the Ti-Zr and Cu-Ni binary phase diagrams [25, 26].These observations are well corroborated by the X-ray Map data of Ti, Zr, Cu and Ni as shown in Figures .2(a),(b), (c) and (d), respectively.

Figure 2 .Figure 3 .
Figure 2. FESEM analyses of Ti/ Ti 20 Zr 20 Cu 30 Ni 30 /Ti composite joint brazed at 1268 K for a period of 10 min.EDX based X-ray Maps of the distribution of various elements across the brazed joint (a) Ti, (b) Ni, (c) Zr and (d) Cu.

Figure 4 .
Figure 4. EDX spectra of the interface microstructures of Ti/ Ti 20 Zr 20 Cu 30 Ni 30 /Ti composite joint brazed at 1268 K for a period of 10 min: (a) Spectrum 1 collected from Diffusion zone 1 shown in Figure 3 (a), (b) Spectrum 2 collected from Diffusion zone 2 shown in Figure 3 (b), (c) Spectrum 3 collected from discontinuous reaction zone shown in Figure 3(c) and (d) Spectrum 4 collected from central zone shown in Figure 3 (d).

Figure 5 .
Figure 5. FESEM analysis of Ti/ Ti 20 Zr 20 Cu 20 Ni 40 /Ti composite joint brazed at 1004°C for a period of 10 min (a) Cross sectional BSE Image (b) corresponding EDX based line scan analysis across 90 µm length comprising the CP-Ti pieces on both sides of brazed joint, corresponding concentration profiles of elements :(c)Ti, (d) Ni (e) Cu and (f) Zr.

Figure 6 .
Figure 6.FESEM analysis of Ti/ Ti 20 Zr 20 Cu 20 Ni 40 /Ti composite joint brazed at 1277 K for a period of 10 min.EDX based X-ray Maps of the distriution of various elements across the brazed joint (a) Ti, (b)Ni, (c) Zr and (d) Cu.

Figure 7 .
Figure 7.Further FESM study of the interface microstructures of Ti/ Ti 20 Zr 20Cu 20 Ni 40 /Ti composite joint brazed at 1277 K for a period of 10 min:(a)

Figure 8 .
Figure 8. EDX spectra of the interface microstructures of Ti/ Ti 20 Zr 20 Cu 20 Ni 40 /Ti composite joint brazed at 1277 K for a period of 10 min: (a) Spectrum 1 collected from Diffusion zone 1 shown in Figure 6 (a), (b) Spectrum 2 collected from Diffusion zone 2 shown in Figure 6 (b), (c) Spectrum 3 collected from discontinuous reaction zone shown in Figure 6 (c) and (d) Spectrum 4 collected from central zone shown in Figure 6(d).

3. 3 Figure 9 .
Figure 9. FESEM analysis of Ti/ Ti 20 Zr 20 Cu 10 Ni 50 /Ti composite joint brazed at 1279 K for a period of 10 min (a) Cross sectional BSE Image (b) corresponding EDS based line scan analysis across 90 m length comprising the CP-Ti pieces on both sides of brazed joint, corresponding concentration profiles of elements :(c)Ti, (d)Ni (e) Cu and (f) Zr.

Figure 10 .
Figure 10.FESEM analysis of Ti/ Ti 20 Zr 20 Cu 10 Ni 50 /Ti composite joint brazed at 1279 K for a period of 10 min.EDX based X-ray Maps of the distribution of various elements across the brazed joint (a) Ti, (b)Ni, (c) Zr and (d) Cu.

Figure 11 .
Figure 11.Further FESM study of the interface microstructures of Ti/ Ti 20 Zr 20 Cu 10 Ni 50 /Ti composite joint brazed at 1279 K for a period of 10 min: (a) Diffusion zone 1 (b) diffusion zone 2, (c) diffusion zone 3, (d) discontinuous reaction zone and (e) central zone.

Figure 12 .
Figure 12.EDX spectra of the interface microstructures of Ti/Ti 20 Zr 20 Cu 10 Ni 50 /Ti composite joint brazed at 1279 K for a period of 10 min: (a) Spectrum 1 collected from Diffusion zone 1 shown in Figure 11 (a), (b) Spectrum 2 collected from Diffusion zone 2 shown in Figure 11 (b), (c) Spectrum 3 collected from discontinuous reaction zone shown in Figure 11(c) and (d) Spectrum 4, 5 collected from central zone shown in Figure 11 (e).

4 .
Fig.14confirmed that annealing at 778 K for a period of 30 min, produced the crystalline

Figure 14 .
Figure 14.XRD spectra of the Ti 20 Zr 20 Cu 30 Ni 30 Metallic glass Ribbons annealed at 758 K for

Figure 14 .
Figure 14.XRD spectra of the Ti 20 Zr 20 Cu 20 Ni 40 Metallic glass Ribbons annealed at 778 for a

Figure 15 .
Figure 15.The Ti 20 Zr 20 Cu 10 Ni 50 metallic glass ribbon annealed at 783 K for a period of 30 (a-c) represent the bright field (BF), dark field (DF) and corresponding SAED, respectively for theTi 20 Zr 20 Cu 50 Ni 10 metallic glass ribbon annealed at 758 K for a period of 30 min.Table 4 presents the corresponding data on composition analysis of the annealed Ti 20 Zr 20 Cu 30 Ni 30 metallic glass ribbon.These results e.g., figures.16 (c) and Table 4 corroborated with the corresponding XRD data figure.13.The homogeneous distribution of the various nanocrystalline phases in amorphous matrix has been shown in figsure.16(a) and (b).Existence of these nanocrystalline phases were confirmed from both the XRD data figure.13 and the compositional analysis data obtained from the TEM studies Table 4.The range of the size of spherical features in the dark field image figure 16 (b) was about 10 to 100 nm while the average feature size was about 50 nm figure 16 (a).The BF, DF and corresponding SAED pattern of the annealed Ti 20 Zr 20 Cu 20 Ni 40 metallic glass at 778 K for a period of 30 min are shown in Figures 17(a), (b) and (c), ( a) and (b)).Existence of the nanocrystalline phases were confirmed from both the XRD data figure.14 and the compositional analysis data obtained from the TEM studies Table5.The range of the size of spherical features in the dark field image figure17 (b) was about 10 to 100 nm[18,19].The same phenomena followed for BF, DF and the corresponding SAED patterns of the annealed metallic glass ribbons viz., Ti 20 Zr 20 Cu 10 Ni 50 (annealed at 783 K).The corresponding data on compositional analysis of the above three glasses are also shown in TEM studies Tables4-6.Existence of these nanocrystallie phases were confirmed from both the XRD data (Figures.[16][17][18].

Figure 16 .
Figure 16.The annealed ribbon of Ti 20 Zr 20 Cu 30 Ni 30 metallic glass at 758 K for a period of 30 min and the corresponding TEM images: (a)-(c): BF image, DF image and selected area of electron diffraction pattern (SAED), respectively.

Figure 17 .
Figure 17.The annealed ribbon of Ti 20 Zr 20 Cu 20 Ni 40 metallic glass at 778 K for a period of 30 min and the corresponding TEM images: (a)-(c): BF image, DF image and selected area of electron diffraction pattern (SAED), respectively.

Figure 18 .
Figure 18.The annealed ribbon of Ti 20 Zr 20 Cu 10 Ni 50 metallic glass at 778 K for a period of 30 min and the corresponding TEM images: (a)-(c): BF image, DF image and selected area of electron diffraction pattern (SAED), respectively.

ConclusionsA.
The microstructural analysis for brazed samples of Ti 20 Zr 20 Cu 60-x-Ni x (x=30, 40 and 50) metallic glasses with CP-Ti alloy pieces were respectively braze joined using metallic glass ribbon fillers of compositions Ti 20 Zr 20 Cu 60-x-Ni x (x=30, 40 and 50) at 1268 , 1277 K and 1279 K for a period of 10 min.Based on extensive studies of FESEM, EDX line scans, EDX spectrum analysis and Elemental X-Ray Mapping by EDX it was found that depending on the combination of filler composition, brazing condition and cooling cycle of the brazing processes the corresponding joints were characterized by microstructure formed by fine lamellar eutectoids of (a) NiTi 2 , Ti 2 Cu , α-Ti and Ti-rich phases, (b) NiTi 2 , (Ti, Zr) 2 Ni, β(Ti, Zr) and Ti-rich phases and (c) NiTi 2 , Cu 2 (Ni, Zr), NiTi ,α-Ti and Ti-rich phases; in correspondence.B. The XRD spectra shown in produced the crystalline phases NiTi, NiTi 2 , Ni 4 Ti 3 , CuNi 2 Ti, and Ni 2 Ti, NiTi 0.8 Zr 0.2 in the Ti 20 Zr 20 Cu 30 Ni 30 metallic glass, NiTi, NiTi 2 , Ni 2 Ti, Cu 8 Zr 3 , Ni 4 Ti 3 , CuNi 2 Ti, Ni 2 Ti, and NiTi 0.8 Zr 0.2 in the Ti 20 Zr 20 Cu 20 Ni 40 metallic glass and NiTi 2 , Ni 2 Ti, NiTi, Ni 4 Ti 3 , and CuNi 2 Ti in the Ti 20 Zr 20 Cu 10 Ni 50 metallic glass ribbons.C. The Ti 20 Zr 20 Cu 30 Ni 30, Ti 20 Zr 20 Cu 20 Ni 40 and Ti 20 Zr 20 Cu 10 Ni 50 metallic glass ribbons annealed at 758, 778 and 783 K, respectively for a period of 30 min produced the crystalline phases of NiTi, NiTi 2 and NiTi 2, respectively.The homogeneous distribution of the various nanocrystalline phases in the amorphous matrix and the existence of these nanocrystallie phases were confirmed from the XRD, TEM data and compositional analysis data obtained from the EDAX studies.

Table 1 .
EDX analyses Based Elemental Chemical Composition Analysis of interface microstructures of Ti/ Ti 20 Zr 20 Cu 30 Ni 30 /Ti composite joint brazed at 1268 K for a period of 10 min.Thus, based on the experimental data displayed in Figures1-4and Table1, it appears plausible to suggest the following processes to have occurred during the brazed joint formation.Dissolution of CP-Ti substrate into the brazed melt had possibly resulted in isothermal solidification of the molten brazed and had eventually formed primary β-Ti during brazing.The residual melt was solidified via eutectic reaction upon the cooling cycle of brazing.Based on the data of Table1, the eutectic consisted most probably of Ti 2 Cu, (Ti,Zr) 2 Ni, α-Ti and Ti-rich phases as displayed in Figures

Table 2 .
EDX analyses based elemental chemical composition analysis of interface microstructures of Ti/ Ti 20 Zr 20 Cu 20 Ni 40 /Ti composite brazing joint at 1277 K for a period of 10 min

Table 3 .
EDX analyses based elemental chemical composition analysis of interface microstructures of Ti/Ti 20 Zr 20 Cu 10 Ni 50 /Ti composite brazing joint at 1279 K for a period of 10 min

Table 4 .
The analytical composition of the Ti 20 Zr 20 Cu 30 Ni 30 metallic glass ribbon (annealed at 758K for a period of 30 min)

Table 5 .
The analytical composition of the Ti 20 Zr 20 Cu 20 Ni 40 metallic glass ribbon (annealed

Table 6 .
The analytical composition of the Ti 20 Zr 20 Cu 10 Ni 50 metallic glass ribbon (annealed