Scandinavian Journal of Metallurgy 2004; 33: 1–5 Copyright C© Blackwell Publishing 2004Printed in Denmark. All rights reserved SCANDINAVIAN

JOURNAL OF METALLURGY

Characterization of reaction products and their effectson the strength properties of transition joints formed

between titanium and stainless steelMainak Ghosh and Subrata Chatterjee

Department of Metallurgy, Bengal Engineering College (Deemed University), Howrah-711103, West Bengal, India

Transition joints were produced between commercially puretitanium and 304 stainless steel in the temperature range of800–900◦C for 30 min under 3 MPa uniaxial load in a vacuum.The diffusion bonds were characterized by optical and scan-ning electron microscopy, electron probe micro-analyzer andtensile testing. At 850◦C bonding temperature and above, anumber of brittle intermetallics like FeTi and λ phases appearin the reaction zone and are mainly responsible for deterio-ration of bond strength. The highest bond strength (∼74% oftitanium) was obtained for the diffusion-bonded couple pro-cessed at 850◦C due to the absence of discontinuities along

the bond line and the formation of a finer size of intermetalliccompounds.

Key words: β-stabilizers, diffusion bonding, intermetalliccompounds, tensile strength, uphill diffusion, widmanstattenα-β structure.

C© Blackwell Publishing, 2004

Accepted for publication 29 January 2003

In recent years, there has been an upsurge of interestin producing transient joints of dissimilar materials tofabricate primary and secondary components for theaerospace and nuclear industries [1–3]. A number ofadvanced and novel materials combination like metal–metal, metal–ceramic, and ceramic–ceramic have comeinto limelight to produce accessories of intricate shapesand these dissimilar materials can be successfullyjoined by diffusion bonding [4–6]. Diffusion bondingis a micro-deformation solid-state fabrication techniquethat avoids many problems of conventional fusion weld-ing, which have been encountered during the joining ofdissimilar materials [7]. In this process mating surfacesof the materials are brought into intimate and atomic-scale contact without any macroscopic deformation. Atelevated temperature and pressure an interface can beformed by inter-diffusion, which removes barriers tostructural continuum and enlarges the bond area. Thisis a sintering process that closes the voids and maintainsgood mechanical integrity for the whole assembly [8].

Titanium and stainless steel are the two candidates,which can be joined by diffusion bonding [9]. Thesetwo materials can neither be fusion welded owing to thegeneration of chemical, mechanical and structural het-erogeneities nor can they be joined by explosive welding

due to the presence of discontinuities along the bond line[10–11]. Diffusion-bonded components of titanium andstainless steel find wide applications in the reprocessingunit of nuclear industries [12–13].

Existing literature reports the absence of various re-action products during the solid-state direct bonding oftitanium to stainless steel when processed below 904◦C[12]. The same inference has been supported in subse-quent work by another group of researchers who alsodid not observe any intermetallic compounds formationfor diffusion annealing of Ti and 304 stainless steel inthe temperature range of 703–850◦C for 96 h [3]. Still,it has also been shown that different types of inter-metallics like σ phase, Fe2Ti, FeTi, Fe2Ti4O, TiC and χ

phase may form in the reaction zone for the diffusioncouple consisting of commercially pure titanium andAISI 316L stainless steel when processed at a temper-ature of 800◦C temperature [9]. Bond strength has alsobeen evaluated for the bonded assembly consisting ofTi and 304 stainless steel. Kato et al. achieved a bondstrength of ∼70% that of titanium and ≤2.2% ductilityat 927◦C processing temperature for the same transi-tion joint [14]. Attempts have also been made to reducefurther the diffusion bonding temperature and a bondstrength of ∼150 ± 20 MPa has been obtained when

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processed at 850◦C temperature for 2 h under 10 MPaload [13].

In the present investigation, diffusion bonding ofcommercially pure titanium (CP Ti) and 304 stainlesssteel (304 SS) was carried out to raise the bond strengthwith improved ductility for their efficacious use in ser-vice. Efforts were also given to characterize the reactionproducts formed at different processing temperaturesand to observe their effects on the tensile strength of thetransition joints.

Experimental procedure

Parent materials used in this investigation were receivedin the form of a 25 mm diameter rod in a hot-rolled andannealed condition. Chemical compositions and roomtemperature tensile properties of CP Ti and 304 SS aregiven in Tables 1 and 2 respectively.

A pair of cylindrical specimens, 15 mm diameter ×30 mm length were cut off from the rod, prepared bya conventional grinding and polishing technique withultimately a 1 µm diamond finish, cleaned in acetoneand dried in air. Two cylinders are kept in contact in afixture and the assembly was put in a vacuum chamber.Diffusion bonding was carried out at 800◦C, 850◦C and900◦C in (3–4) × 10−4 mbar vacuum for 30 min undera 3MPa uniaxial load. After the operation the sampleswere cooled in vacuum.

The diffusion couples thus prepared were cut off per-pendicularly from the joint portion and prepared bythe usual techniques for optical microscopy. The 304SS side was etched by a mixture of HCl (5 ml), picricacid (1 gm) and ethanol (100 ml) and the CP Ti side bythe solution containing a mixture of HF (10 ml), HNO3

(5 ml) and distilled water (85 ml). Etched and non-etched polished samples were examined by light mi-croscopy (CORRECT SDME TR5) and scanning electronmicroscopy (LEICA S-440) in the backscattered mode(SEM-BSE), respectively, to observe structural changesdue to diffusion near the interface. Quantitative energydispersive spectroscopy (EDS) was performed to deter-mine the concentration of the chemical species at dif-ferent reaction bands in the diffusion zone (OXFORD5431). The JEOL (JXA 8600 Superprobe) electron probemicroanalyser (EPMA) was employed to get elementalconcentration profiles across the reaction zone. The kα

lines of Ti, Fe, Ni and Cr are generated at an operating

Table 1. Chemical compositions of base metals (elements in wt%)

Alloy C Fe Ti Mn Si P S Cr Ni O

Commercially pure titanium 0.02 0.10 Bal – – – – – – 0.15304 stainless steel 0.01 Bal – 1.15 0.23 0.028 0.012 21.12 11.45 –

Table 2. Tensile properties of base metals at room temperature

0.2% proof Ultimate tensile % fractureAlloy stress (MPa) strength (MPa) elongation

Commercially pure 204.60 318.60 22.80titanium

304 stainless steel 131.50 568.60 46.60

Fig. 1. Optical photographs of the diffusion bonded joints processed for30 mins at (a) 800◦C, (b) 850◦C, (c) 900◦C.

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Transition joints of Ti to SS

voltage of 15 kV and a specimen current of 5 × 10−8 A.The LiF crystal was used to diffract the correspondingcharacteristic x-ray radiation. Tensile testing of sub-sizecylindrical test pieces as per ASTM (E8M-96) was per-formed to evaluate the ultimate tensile strength (UTS)and the breaking strain (% BS) of the transition joints, ata cross-head speed of 0.05 mm/min (INSTRON 4204).

Results and discussion

Optical photographs of the bonded couples are given inFig. 1. Region A indicates the austenite phase of 304 SScontaining annealing twins. The darkly etched area Bcorresponds to a combination of different intermetallics[12]. Zone C is the solid solution of Fe, Cr and Ni in ti-tanium. The presence of strong β-stabilizers like Fe andCr influences to the retention of the high-temperaturebody-centred cubic (bcc) structure of Ti even at ambi-ent temperature and hence, is designated as stabilized-β of Ti [4]. Area D exhibits the widmanstatten α-βstructure consisting of β-Ti with transformed α-Ti nee-dles. A small amount of Fe and Cr promotes eutectoidformation in this region by decreasing α-β transfor-mation temperature [15]. Enhancement of joining tem-perature results in substantial growth of regions Band C indicating profuse inter-diffusion of Fe, Cr, Niand Ti.

Fig. 2. SEM-BSE micrographs of the bonded assemblies processed for 30 mins at (a) 800◦C, (b) 850◦C, (c) 900◦C, (d) 900◦C.

Reaction layers in the diffusion zone for the diffusion-bonded assemblies have been revealed in backscatteredimages of scanning electron microscopy (Fig. 2) and thecomposition of these layers has been estimated by EDSanalysis. For 800◦C joining temperature, no intermetal-lic compound formation has been found and this obser-vation is in good agreement with the interpretation ofearlier workers [3]. Perhaps, at this low bonding tem-perature limited diffusion of the chemical species re-duces the concentration of Ti on the SS side and Fe,Cr and Ni on titanium side; hence, intermetallic com-pound formation cannot take place. When the bondingtemperature is raised to 850◦C, two intermediate layersappear in the diffusion zone. Adjacent to the interface,the narrow band consists of Ti (∼51.7 at%), Cr (∼7.41at%), Fe (∼35.73 at%) and Ni (∼5.16 at%). This inter-metallic phase is presumably cubic FeTi with ∼1.8 µmwidth [15]. Close to FeTi, another thin layer (∼2.20 µmwidth) has been noticed in the SS side, containing Ti(∼43.22 at%), Cr (∼14.13 at%), Fe (∼38.87 at%) and Ni(∼3.78 at%). Substantial Cr concentration in this zoneindicates the presence of hexagonal λ phase, which is asolid solution of Fe2Ti and Cr2Ti [15]. Further rise in pro-cessing temperature, i.e. at 900◦C, results in the appear-ance of another intermetallic compound close to λ phase,having the composition of Ti (∼2.5–6.9 at%), Cr (∼27.8–33.4 at%), Fe (∼55.3–65.9 at%) and Ni (∼3.75–4.38

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Fig. 3. Concentration penetration profile for the transition joint processedfor 30 mins at (a) 800◦C, (b) 850◦C, (c) 900◦C.

at%). Hence, this region is a tetragonal σ phase with∼5 µm width [9]. For 900◦C bonding temperature in theFeTi and λ phase, the change in the concentration of thechemical species is marginal; the λ phase consists of Ti(∼40.78 at%), Cr (∼14.55 at%), Fe (∼41.51 at%) and Ni(∼3.15 at%) and that of FeTi contains Ti (∼52.3 at%), Cr(∼7.85 at%), Fe (∼36.46 at%) and Ni (∼3.37 at%). Ear-lier studies on titanium and stainless steel solid-statebonding describe that the intermetallic compounds for-mation may not take place below 904◦C, but the presentwork reveals the presence of FeTi and λ phase at 850◦Cand the σ phase along with FeTi and λ at 900◦C bondingtemperature [12].

The concentration–penetration profile of Ti, Fe, Cr andNi is shown in Fig. 3 for the bonded couples processed

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Fig. 4. Enlarged view of the concentration penetration plots near interface(corresponding Fig. 3 (a), (b), and (c) respectively) for the couple bonded for30 mins at (a) 800◦C, (b) 850◦C, (c) 900◦C.

at 800◦C, 850◦C and 900◦C and an enlarged concentra-tion plot for the same near the interface of the transi-tion joints have been illustrated in Fig. 4. Step formationhas not been identified in the profiles near the interfaceindicating the absence of intermetallic phases. Yet thepresence of the same has been revealed in the SEM-BSEmicrographs and the composition of these diffusion lay-ers was analyzed by EDS. This apparent discrepancy isdue to the limitation of EPMA.

Cr enrichment has been observed for all the bondedjoints. The diffusion of Ti in the 304 SS matrix decreasesthe activity of Cr and diffusion occurs down the activ-ity gradient rather than the concentration gradient. So,

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Transition joints of Ti to SS

Table 3. Tensile properties of the transition joints at ambient temperature

Diffusion bonded Bonding Ultimate tensile Breakingjoints processed temperature strength strainfor 30 min (◦C) (MPa) (%)

800 170.40 3.00850 234.60 6.10900 207.50 5.30

uphill diffusion of Cr is observed [12]. During coolingthis Cr-enriched region transforms to σ phase [9]. At800 and 850◦C diffusion-bonding temperatures, the σ

phase has not been identified by SEM-BSE due to itsfiner width.

The stabilized-β region of Ti shows a compositionvariation: for the 800◦C bonding temperature the av-erage concentration of alloying elements is Ti (∼89–91at%), Fe (∼6.5–7.8 at%), Cr (∼0.6–1.6 at%) and Ni (∼1.3–1.6 at%) and when the joining temperature is raised to850◦C, the composition changes to Ti (∼81–83 at%), Fe(∼10–12 at%), Cr (∼4.1–5.2 at%) and Ni (∼1.3 at%). Fur-ther increase in processing temperature decreases theTi concentration in the same zone and envisages that athigher bonding temperature diffusion of Fe and Cr onthe titanium side becomes more pronounced resultingin reduction in the Ti content in this reaction layer.

Tensile properties of the transition joints are furnishedin Table 3. The presence of intermetallic phases is not ob-served at 800◦C processing temperature but the incom-plete coalescence of the surface asperities has been re-vealed in the SEM-BSE image (Fig. 2(a)), which has led tolow bond strength. This phenomenon is also supportedin the literature and it has been reported that bondingbelow 823◦C may cause splitting due to the developmentof thermal stress [4]. The discontinuous dark band in thephotograph designates debonding (Fig. 2(a)). Enhance-ment in the joining temperature results in a significantrise in bond strength (∼234 MPa) with adequate ductil-ity (∼6%). Such high strength with a substantial amountof breaking strain has not been reported earlier for thedirect solid-state bonding of these two materials [13–14]. Discontinuities are not noticed for this assembly yetthe presence of few intermetallic compounds like FeTiand λ-phase is identified near the interface. These in-termetallics decrease the bond strength of the transitionjoints with respect to the base materials. With furtherrise in the joining temperature, the strength of the tran-sition joints falls as embrittlement becomes severe at the900◦C processing temperature owing to the enhance-ment of the width of FeTi (∼2.2 µm), λ phase (∼2.6 µm)and σ phase.

Conclusions

In the present investigation direct solid-state bondingbetween CP Ti and 304 SS has been carried out in the

temperature range of 800–900◦C for 30 min under the3 MPa uniaxial load. The characterization of the transi-tion joints reveals the following:

� Cr enrichment has been noticed for all the bondedcouples in the SS side near the interface and during cool-ing this region transforms to σ phase. At 800◦C and850◦C though Cr enrichment has been identified, yetσ -phase formation has not been observed, perhaps dueto its finer size.

� The presence of intermetallic phases like FeTi andλ phase has been noticed in the reaction zone for 850◦Cand 900◦C bonding temperature but it has not been ob-served at 800◦C. These brittle intermetallic compoundslower the bond strength of the assembly with respectto the parent metals. These intermetallic phases growsubstantially with the rise in bonding temperature.

� High bond strength (∼74% of the parent CP Ti) witha substantial amount of ductility has been obtained at850◦C processing temperature due to the absence of dis-continuities in the interface and the finer size of the brit-tle intermetallic phases in the diffusion zone.

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Address:Mainak GhoshDepartment of MetallurgyBengal Engineering College (Deemed University)Howrah-711103West BengalIndiae-mail: ghosh mnk@yahoo.com

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