Influence of Fe-W intermetallic compound on fracture behavior of Steel/Tungsten HIP diffusion bonding joint: Experimental investigation and first-principles calculation

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Highlights

  • Fracture process of steel/tungsten joint can be divided into four stages.

  • Crack generates at (or near) Fe2W/W interface but extends in Fe2W IMC.

  • Terrible Fe-W bond causes Fe2W IMC shows poor property and as crack extension area.

  • Crack initiation process depends on Fe2W/W interface bonding and charge distribution.

Abstract

The influence of Fe-W intermetallic compound on fracture behavior of steel/tungsten HIP diffusion bonding joint was investigated using both experimental investigation and first-principles calculation. The research results indicate that, the fracture of steel/tungsten joint shows brittle fracture characteristic due to the existence of Fe2W intermetallic compound (IMC), and the fracture process can be divided into four stages, which are crack initiation stage, crack extension stage, imminent fracture stage and complete fracture stage, respectively. The crack generates at (or near) Fe2W/W interface at crack initiation stage and extends in Fe2W IMC at crack extension stage decides the joint property. Fe-W atomic bonds in Fe2W IMC not only weak causing the easy-destroy feature, but also exhibit covalent characteristic leading to the large brittleness, is the essential reason that Fe2W IMC shows the poor mechanical performance, and becomes the crack extension area in steel/tungsten joint. Crack initiation processes at Fe2W/W interface are various and depend on interfacial bonding behavior and charge vacancy distribution. For the weak Fe2W/W interface, which represented by Fe2W (001)/W (111) interface with Fe-W terminated structure, the crack occurs exactly at the interface (1−1' layer) due to the large area of charge vacancy region and the weak interfacial binary Fe-W bonding. While for the strong Fe2W/W interface, which represented by Fe2W (001)/W (111) interface with W-W terminated structure, because of the large charge density at interface and the strong interfacial W-Fe-W ternary bonding, the crack will occurs not at the interface exactly but the interior of Fe2W IMC (2′-3′ layer) near the Fe2W/W interface.

Introduction

Because of the obvious advantages on melting point, hardness, density and radiation resistance, W (and W alloy) shows the extensive application prospects in the manufacturing field of advanced weapon and nuclear equipment, such as hypersonic penetrator, demonstration fusion reactor (DEMO), and so on [[1], [2], [3]]. However, due to the poor room temperature toughness, the further application of W component was restricted extremely [4,5]. The jointing of W with steel, which shows the high toughness, is an effective way to promote the further application of W component.

The high-performance jointing between W and steel is a serious challenge. Considering the large difference on physical properties between W and steel, the fusion welding is not applicable. Currently, the main methods for jointing steel and W are brazing [6], plasma spraying [7] and diffusion bonding [8], in which, the diffusion bonding (especially hot isostatic pressure diffusion bonding) is the most recognized method due to the low bonding temperature, high joint service temperature and small post-weld residual stress [9].

However, during the diffusion bonding between tungsten and steel, the formed Fe-W intermetallic compound deteriorates the joint mechanical properties severely. Basulki [8] jointed tungsten with EUROFER 97−2 steel at 1050 °C using diffusion bonding, and found that the Fe-W intermetallic compound generated at the interface, causing the brittle fracture of the joint with the tensile strength of 93 MPa. Oono [10] studied the diffusion route of diffusion bonding joint between tungsten and oxide dispersion strengthened steel, and found that W spread deeply into the steel but Fe did not penetrate the W to an equivalent depth. Moreover, all the samples were broken at the joining interface near the generated Fe-W intermetallic compound. Noh [11] conducted the diffusion bonding between tungsten and high-Cr ODS ferritic steel at 1240 °C, and calculated the stress distribution on the joint by Finite Element Method (FEM). The results indicated that the bonding residual stress concentration on the Fe-W intermetallic compound between steel substrate and tungsten substrate, which is consistent with the results obtained by the shear test.

Actually, although the phenomenon that Fe-W intermetallic compound plays the negative role on influencing the mechanical properties of steel/tungsten diffusion bonding joint has been acknowledged [8,10,11], the specific influence behavior and influence mechanism have not been discussed yet. The most important reason for limiting this critical problem be further investigated is that the Fe-W intermetallic compound and the interface between intermetallic compound and substrate are too microscopic to be investigated by conventional experimental methods.

First-principles calculation based on density functional theory (DFT) is a novel method for micro-investigation at the atomic scale, which has been widely used in modern science, especially in the research on physical properties and surface/interface properties of solid phases [[12], [13], [14], [15]]. Liu [12] calculated the elastic constants and electronic structures of VC and V4C3 compounds, and indicated that the hardness of V4C3 is lower than that of VC. Yang [13] studied the properties of Cu-Ti intermetallic compounds and found that the shear modulus G and Young's modulus E are positively related to formation enthalpy. Meanwhile, all the Cu-Ti intermetallic compounds exhibit a certain degree of toughness, and the sequence is CuTi3 > Cu3Ti2 > Cu2Ti > Cu4Ti2 > CuTi2 > CuTi > Cu4Ti3, which indicates that the brittle fracture tends to occur on Cu4Ti3. Guan [14] studied the interfacial properties of Ag (110)/ Ni(110) interface, Ag (211)/Ni (110) interface and Ag (100)/Ni (211) interface by investigating the interface bonding energy, charge distribution and electronic structure, and found that Ag (100)/Ni (211) interface shows the strongest stability and crack resistance. Yin [15] studied of the tensile and fracture process of the TiN/VN interface, indicating that the adhesion energy and ideal tensile strength of TiN (111)/VN (111) interface are both larger than TiN (001)/VN (001) interface, and demonstrated that the stacking sequence plays an important role on influencing the interfacial property.

In this work, the tight steel/tungsten joint was achieved using hot isostatic pressure diffusion bonding. The microstructure of the joint was observed, the generated interfacial intermetallic compound was characterized and the joint fracture behavior was investigated. Subsequently, in order to discuss the influence of interfacial intermetallic compound on fracture behavior of joint, the physical properties of interfacial intermetallic compound and the interfacial properties (including tensile fracture process) between intermetallic compound and substrate were researched theoretically using the first-principles calculation.

Section snippets

Experiment methods

The tungsten substrate used in this work was commercially available 90W-7Ni-3Fe (wt.%) alloy produced by ATTL Advanced Materials Co., Ltd. As shown in Fig. 1, the microstructure of W alloy consists of two phases, which are white W particle and black Ni-Fe-W ternary low-melting point solid solution (called (Ni,Fe,W)ss) with the composition of Ni63.05Fe28.55W8.4 (at.%). The steel substrate is Q390 low alloy high strength structural steel (Fe bal., Mn 1.6 wt.%, Si 0.35 wt.%, V 0.15 wt.%, Nb 0.05

Experimental results

Microstructure of the cross sections of steel/tungsten joint is shown in Fig. 3(a). From Fig. 3(a), the joint is well-bonded without any crack or discontinuity. Moreover, the gray intermetallic compound was generated between W and steel substrates. Combined with the EDS analysis result shown in Fig. 3(b), it can be found that the generated intermetallic compound is Fe2W, which is consistent with others’ research results [8]. It is worth noting that the Fe2W intermetallic compound at bonding

Structural property

From the experimental results, it can be found that the rapid expansion of crack in Fe2W IMC is one of the essential factors for the fracture of steel/tungsten joint. In order to investigate the reason for Fe2W IMC shows the poor performance, the physical property was researched combined that of the control groups (Fe and W).

The optimized results for the three bulk phases, which include space group, lattice parameters and bulk modulus, are listed in Table 1. From Table 1, the optimized lattice

Interfacial configuration

Actually, compared with the crack extension area (Fe2W IMC), the crack initiation area (Fe2W/W interface or Fe2W IMC near Fe2W/W interface) deserves more attention. As well known, there are many alternative configurations of Fe2W/W interface, and it is impossible to perform computations on all of them. Therefore, the typical Fe2W/W interface was determined firstly according to Bramfitt theory [37], as described in our previous work [38,39]. The calculated lattice mismatches δ of Fe2W/W low

Discussion

In summary, for steel/tungsten HIP diffusion bonding joint, the Fe2W intermetallic compounds generated at steel/tungsten interface bring two negative effects: (i) its own large brittleness and weak strength, and (ii) the low crack-resistant Fe2W/W interface. Moreover, due to the different interfacial bonding property of Fe2W/W interface, there are two kinds of crack initiation and extension behaviors in steel/tungsten joint.

When the steel/tungsten joint is subjected to the imported strain, the

Conclusion

In this work, the influence of Fe-W intermetallic compound on fracture behavior of steel/tungsten HIP diffusion bonding joint was investigated using both experimental investigation and first-principles calculation. The research results are summary as follows:

  • (1)

    The fracture of steel/tungsten joint fracture shows brittle fracture characteristic, and the maximum strain and tensile strength are 1.749 % and 95 MPa, respectively.

  • (2)

    The fracture process of steel/tungsten joint can be divided into four

Acknowledgments

The authors would like to express their gratitude for project funded by the General Armament Department of China (61409230503) and Fundamental Research Funds for the Central Universities (FRF-GF-18-003B).

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