Available online at ScienceDirect

ScienceDirectJ. Mater. Sci. Technol., 2014, 30(5), 493e498

Microstructure and Mechanical Properties of Electron Beam Welded Alloy J75

Shenghu Chen1), Mingjiu Zhao1)*, Hao Liang2), Lijian Rong1)**

1) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China2) Institute of System Engineering, China Academy of Engineering and Physics, Mianyang 621900, China

[Manuscript received February 5, 2013, in revised form April 2, 2013, Available online 3 December 2013]

* CorrespE-mail a** Corresaddress:1005-03JournalLimited.http://dx

Microstructure and mechanical properties of electron beam welded alloy J75 were studied under as-welded andpost-weld aging treatment (PWAT) conditions. The results showed that high-quality welds were produced byelectron beam welding. Under as-welded condition, a fine dendritic structure consisting of gamma dendritematrix and Laves phase was observed in the welds. Better mechanical properties were obtained in the weldzone than that of base metal because of the fine size of the dendritic structure. After PWAT, a discontinuousdistribution of g0 particles existed in the dendritic structure. The presence of a g0 depletion zone in thedendrite core resulted in a significant degradation of mechanical properties of the weld.

KEY WORDS: Electron beam welding; Alloy J75; Precipitation; Mechanical properties

1. Introduction

With the development of hydrogen economy, there is an ever-increasing demand for structural materials with high strengthsuitable for service in hydrogen environment, which motivatesthe development of new materials. A g0-strengthened alloy J75,has been developed at the Institute of Metal Research, ChineseAcademy of Sciences as a hydrogen resistant alloy[1e3]. Thisalloy exhibits high flow stress, low hydrogen embrittlementsensitivity and excellent corrosion resistance. During the indus-trial application, the materials are usually joined by the processof welding to satisfy the requirement of the component designand manufacture. In view of the harsh environment in service,high-quality welds should be guaranteed in the hydrogen resis-tant alloys. Electron beam welding is recognized as a viabletechnique to solve the above problems because of low contam-ination, narrow heat affected zone (HAZ) and high depth ofpenetration[4].As a precipitation hardened alloy, J75 is strengthened by the

precipitation of ordered coherent g0�[Ni3(Al,Ti)], which isformed during high temperature aging. Different from the basemetal, microstructural evolution in welds is responsible forproperties of weld zone. Investigations showed that micro-segregation and non-equilibrium phase produced during welding

onding author. Assoc. Prof., Ph.D.; Tel.: þ86 24 23971985;ddress: mjzhao@imr.ac.cn (M. Zhao).ponding author. Prof., Ph.D.; Tel.: þ86 24 23971979; E-mailljrong@imr.ac.cn (L. Rong).02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2013.11.011

directly affected the microstructural evolution in the welds.Brooks and Krenzer[5,6] found that low melting point eutecticphase was formed in A286 during welding, which resulted inhigh susceptibility to fusion zone cracking and HAZ micro-fissuring. In Inconel 718[7e9] and Incoloy 903[10,11], the forma-tion of non-equilibrium solidification phases (such as MCcarbides and Laves phase) during welding consumed significantamounts of the hardening element of Nb, leading to the reducedstrengthening effects. Moreover, solidification-induced segrega-tion of hardening elements was observed in the welds of theabove alloys, which would affect the precipitation of strength-ening phase. Therefore, a thorough understanding of themicrostructure of welds is vital to establish the applicablewelding process to resolve these problems.However, rare information is available on the microstructure

and properties of electron beam welded J75. In this paper, anattempt was made to clarify the microstructural evolution andmechanical properties of electron beam welded alloy J75 underas-welded and post-weld aging treatment (PWAT) conditions.

2. Experimental

The alloy J75 was produced by vacuum induction meltingtechnique with the chemical composition as follows: 30Nie15Cre1.3Moe1.88Tie0.36Ale0.24Ve0.2Sie0.0008BeFe bal(in wt%). The ingot was homogenized at 1433 K for 20 h, thenforged and rolled into 4 mm-thick sheets. The sheets were givena pre-weld solution treatment at 1253 K for 1 h and subsequentlywater quenched. After milling to the thickness of 2.6 mm, thesheets were bead-on-plate electron beam welded using 60 kVand20 mA current at a welding speed of 100 cm/min. The operatingparameters were optimized through extensive weld trials in order

Fig. 1 Optical micrograph of welds after electron beam welding.

494 S. Chen et al.: J. Mater. Sci. Technol., 2014, 30(5), 493e498

to obtain a full penetration. Eventually, part of the welded sheetswas post-weld aging treated at 1013 K for 8 h followed by aircooling.Post-weld characterization was performed in terms of micro-

structure, hardness and tensile tests. Specimens for optical mi-croscopy (OM) and scanning electron microscopy (SEM) werecut perpendicularly to the welding direction. The cross sectionswere mechanically polished, followed by electro-etching in asolution of 10% chromic acid. A Shimadzu EPMA (electronprobe microanalyzer)-1610 electronic probe microanalyzer wasused to determine the distribution of the alloying elements. Thinfoils for transmission electron microscopy (TEM) were preparedfrom the welds to determine the secondary solidification phaseon FEI Tecnai G220 TEM equipped with an energy dispersiveX-ray analysis (EDX). Two types of tensile specimens with thegauge dimension of 22 mm � 4 mm � 2.6 mm were machinedfrom the sheets: (1) base metal in solutionized and solutionizedplus aged conditions; (2) transverse specimens containing theweld in the center of the gauge length. Then, some of the

Fig. 2 (a) SEM image showing the microstructure of as-welded fusion zo

transverse tensile specimens with the weld were encapsulated inevacuated quartz tubes for PWAT. Tensile tests were carried outat room temperature with a strain rate of 1.5 � 10�3 s�1. Thehardness was measured across the cross section of the weld,using a Vickers microhardness tester with a load of 500 g.

3. Results and Discussion

3.1. Microstructure analysis

Fig. 1 shows a typical macroscopic view on the cross sectionof welds after electron beam welding. It can be seen that nailheadshaped weld is formed and no evidence of liquation cracking isobserved in the as-welded specimens.

3.1.1. As-welded. The microstructure of the as-welded fusionzone is shown in Fig. 2. A fine dendritic structure is formed inthe fusion zone, as shown in Fig. 2(a). It is well-known thatcomplete melting and resolidification occur in the fusion zoneduring welding. A higher magnification SEM image (Fig. 2(b))reveals that the solidification products consist of gamma dendriteand secondary solidification phase (marked with black arrow).The secondary solidification phase was observed to locate at theinterdendritic region. In order to determine the nature of thesecondary solidification phase, TEM thin foils prepared from thefusion zone were examined. Fig. 3(a) shows a bright field TEMmicrograph of the solidification phase, revealing that solidifica-tion phase is formed in the interdendritic region. The inset inFig. 3(a) shows a selected area diffraction pattern of the solidi-fication phase and the corresponding EDX spectrum is given inFig. 3(b). These indicate that the solidification phase is the Lavesphase in the type of hexagonal MgZn2. Compared with the basemetal, the Laves phase is rich in Ti, Mo and Si, and lean in Feand Cr, which seems to the sequence of the elemental segrega-tion during solidification. Fig. 4 shows the distribution of ele-ments in the fusion zone after welding. It can be seen that Tistrongly segregates to the interdendritic regions while incon-spicuous dendritic segregations are observed for other elements.The micro-segregation of Ti or Nb during solidification was alsoobserved in some FeeNi based alloys, such as A286[5], JBK75[7], Incoloy 903[8,9] and Inconel 718[10]. In addition, it can alsobeen seen that Laves phase is formed in the region with thehighest concentration of Ti, suggesting that Ti segregation

ne and (b) higher magnification SEM image of the dendritic structure.

Fig. 3 (a) TEM bright field image and (b) EDX spectrum of the Laves phase in the as-welded fusion zone. The inset in (a) shows a [122] zonediffraction pattern of the Laves phase.

Fig. 4 Backscattered electron image and elemental mapping images of the as-welded fusion zone by EPMA analysis.

S. Chen et al.: J. Mater. Sci. Technol., 2014, 30(5), 493e498 495

determines the formation of secondary solidification phase. TheLaves phase was also observed in the as-welded fusion zone ofInconel 718[10e12] and Incoloy 903[8,9]. It was found that the Nbsegregation was vital to formation of solidification phase andthere was very little segregation of Ti to the Laves phase. It issuggested that the segregation of Ti also promotes the formationof Laves phase in the present work.

Fig. 5 (a) SEM images showing the microstructure of fusion zone in PWATbetween the dendrite core and interdendritic region.

3.1.2. PWAT. As indicated in Fig. 2(b), g0 particles are notobserved in the as-welded fusion zone because the precipitationof g0 is suppressed at a high cooling rate. Therefore, PWAT isusually applied to introduce the g0 strengthening phase. How-ever, a discontinuous distribution of g0 particles exists in thedendritic structure after PWAT, as shown in Fig. 5(a). At highermagnification, it was clearly seen that fine spherical g0 particles

condition, and (b) higher magnification image of g0 particles distribution

Fig. 6 Microhardness profiles measured at three different depths of the weld under as-welded (a) and PWAT (b) conditions.

Table 1 Tensile strength of base metal and welds at room temperature

Condition Tensile strength(MPa)

Failure location

Base metal, solutionized 586 e

Base metal, solutionized þ aged 1067 e

Weld, as-welded 581 Base metalWeld, PWAT 948 Fusion zone

496 S. Chen et al.: J. Mater. Sci. Technol., 2014, 30(5), 493e498

precipitate in the interdendritic region, while a g0 depletion zoneis developed in the dendrite core (Fig. 5(b)). In addition, thedendrite core is observed to be outlined by a ring of large g0

particles.It is well-known that the precipitation of g0 in superalloy can

be attributed to the supersaturation with respect to Al andTi[13,14]. According to the elemental distribution of as-weldedfusion zone in Fig. 4, Ti segregates to the interdendritic re-gions during solidification. The disparity in distribution of Tibetween dendrite core and interdendritic region will result indifferent degrees of supersaturation. Higher degree of supersat-uration is obtained in the interdendritic region after welding andthus high density of g0 particles are observed. In contrast, lowerdegree of supersaturation in dendrite core makes it impossiblefor the nucleation of g0 at the PWAT temperature.

Fig. 7 Micrographs showing the cross section through the fracture surface ofracture surface, (b) higher magnification image of the stepped appea

3.2. Microhardness

Microhardness profiles measured on three different depths ofthe welds under as-welded and PWAT conditions are shown inFig. 6. As shown in Fig. 6(a), the microhardness of the weldzone is higher than that of base metal under as-welded condition.Because the width of the HAZ for electron beam welds is nar-rower than the area of the microindentation (Fig. 1), the hardnessof the HAZ cannot be separated from that of the base metal.After PWAT, a uniform dispersion of g0 particles with the size of10e20 nm is introduced in the base metal[3] and thus leads to asignificant increase in the value of microhardness (Fig. 6(b)).Meanwhile, a discontinuous distribution of g0 particles isobserved in the weld zone (Fig. 2), so a slight increase in thehardness lever occurs after PWAT (Fig. 6(b)). Therefore, it canbe seen that the microhardness of the weld zone is lower thanthat of the base metal after PWAT, as indicated in Fig. 6(b).

3.3. Tensile test

Table 1 presents the average tensile strength of the base metaland welds under various conditions. Under as-welded condition,all the transverse specimens fractured in the base metal, outsidethe weld, which is in agreement with the hardness result that the

f the welds after post-weld aging treatment: (a) macroscopic view of therance in the circle of (a).

Fig. 8 Cross sectional SEM images (a) and higher magnification image (b) near the fracture surface of the welds. In (a), the areas within the dashed linesare the dendrite cores.

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hardness of the weld zone is higher than that of base metal(Fig. 6(a)). Considering that no g0 particles are introduced in theas-welded fusion zone (Fig. 2(b)), the mechanical properties ofwelds zone are mainly determined by the size of the dendriticstructure. It is found that the average secondary dendrite spacingis about 2e3 mm (Fig. 2), which is considerably smaller than thegrain size of base metal (about 50e60 mm)[15]. Therefore, bettermechanical properties are obtained in the weld zone than that ofbase metal.After aging treatment, the average tensile strengths of the base

metal and weld zone reach 1067 MPa and 948 MPa, respectively(Table 1). Contrary to the as-welded condition, all the transversespecimens fractured at the fusion zone and the tensile strength ofthe weld zone is lower than that of the base metal. The fracturesurface of the transverse specimens after PWAT reveals a numberof step-like features, which is described in the publishedwork[16]. In order to provide further insight into the fractureprocess, cross section through the fracture surface of the weldsafter PWAT was examined. A macroscopic view shows thatstepped appearances are observed on the fracture surface(Fig. 7(a)), which directly corresponds with the step-like fea-tures. A detailed investigation of the stepped appearance in-dicates that fracture took places by the cleavage along theinterdendritic region of the cellular dendrites (Fig. 7(b)).Taken together, the fracture process of the welds is postulated

based on the microstructure and fractography analysis. Firstly,localized plastic deformation develops in the dendrite core. AfterPWAT, a g0 depletion zone in the dendrite core (Fig. 5) makes itsofter than the interdendritic region and thus deformation willpreferentially occur in the dendrite core. This notion is demon-strated by the image in Fig. 8(a), showing that deformation bandsare highly concentrated in the dendrite core. Secondly, micro-cracks initiate at the boundary between the dendrite core andinterdendritic region. As deformation continues, the strain con-centration in the dendrite core is greater than that in the inter-dendritic region, requiring a corresponding strain in theinterdendritic region to ensure deformation compatibility[17]. Butit is impossible for the interdendritic region because the g0 pre-cipitates within impede the dislocation motion [18e20]. Therefore,the stress concentration is resulted and the microcracks will beintroduced at the boundary between the dendrite core andinterdendritic region, as evidenced by Fig. 8(b). Finally, themicrocracks propagate and connect to each other driven by the

shear stress. Under tensile test, the microcracks connect to eachother through shear deformation (Fig. 7(b)), leading to thecleavage planes on the fracture surface (Fig. 7(a)). According tothe above analysis, altering the discontinuous distribution of g0

particles in the dendritic structure is vital to the improvement inthe mechanical properties of welds. Further research work isbeing undertaken.

4. Conclusions

(1) Electron beam welding results in a fine dendritic structure,consisting of gamma dendrite matrix and secondarysolidification phase. The secondary solidification phaseformed in the interdendritic region is Laves phase, whichis the sequence of elemental segregation.

(2) A discontinuous distribution of g0 particles exists in thedendritic structure after PWAT. A g0 depletion zone isdeveloped in the dendrite core, which is attributed to thelower degree of supersaturation.

(3) Under as-welded condition, the microhardness of weld zoneis higher than that of base metal and the fracture occurs inthe base metal during tensile test. After PWAT, themicrohardness and tensile strength of the welds are lowerthan those of the base metal.

(4) Under tensile test, the presence of g0 depletion zone in thedendrite core after PWAT makes the deformationpreferentially occur and microcracks initiate at theboundary between the dendrite core and the interdendriticregion. Finally, the microcracks propagate and connect toeach other driven by the shear stress.

AcknowledgmentThis work was financially supported by the National Natural

Science Foundation of China and China Academy of Engineer-ing Physics (Project No. U1230118).

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