Study on Interface Microstructure and Bonding Properties of Hot-Rolled Nickel-Based Composite Plate

. In this paper, the interface microstructure, elements’ diﬀusion features at the interface, and bonding properties in nickel-based alloy/carbon steel clad composite prepared by vacuum hot-roll bonding were investigated, comprehensively. The inﬂuence of element distribution on the interface bonding strength was revealed as well. The results showed that there was a 13 μ m thick diﬀusion layer at the interface of nickel-based alloy/carbon steel composite plate, which was beneﬁcial to a strong bond between nickel-based alloy and carbon steel, as well as the stable transition of mechanical properties in the thickness direction. Kirkendall voids and ﬁne-grained structure (the grain size is about 41.5nm) were observable by peeling oﬀ the nickel-based alloy cladding, which greatly promoted element diﬀusion and enhanced the interfacial bonding strength of the nickel-based alloy/carbon steel composite plate. The diﬀusion coeﬃcient of Ni at the interface was about 2 orders of magnitude larger than that of nanocrystalline Fe. The shear strength reached up to 453MPa, which was much higher than the minimum of 140MPa deﬁned in ASTM A-264 speciﬁcations. Furthermore, in the shear test, the fracture occurred on the X52 carbon steel side at the contact rather than at the composite plate interface.


Introduction
With the progress of science and technology in modern society in recent years, the fields, such as automobiles, ships, and industry, are increasingly demanding the comprehensive performance of materials, which makes a single material difficult to meet the requirements. erefore, composites integrating the superior performance of various materials have become one of the important research directions [1][2][3][4][5][6][7]. One of the most widely used composite materials is nickelbased alloy composite plate prepared by hot-roll bonding, which not only holds high corrosion resistance from nickelbased alloy [8,9] but also the structural support can be utilized from carbon steel or low alloy steel. In addition, the amount of nickel-based alloys can be reduced [10], which greatly reduces material costs and has a wide application prospect in the chemical industry, petroleum, seawater desalination, and other fields [11,12].
In the composite material, interfacial delamination is one of the major problems, which takes great responsibility for the failure during its service [13][14][15]. erefore, the interface bonding strength plays a decisive role in the overall mechanical properties and subsequent processing properties of composite materials [16]. e composite interface can affect the stress which transfers between the substrate and cladding metal, the crack growth, and the stability of the medium. us, the interface microstructure and the bonding strength determine the overall performance of the composite plate. Hence, it is of great significance to reveal the relationship between the interface microstructure and the bonding properties of composite plates [17,18]. Recently, much research studies on the bonding mechanism, interface element diffusion, interface microstructure, and performance evaluation of rolled composite plates have been conducted, but most studies only give a qualitative explanation rather than quantitative analysis; such as [19][20][21][22]; research studies on the mechanical properties based on microstructure and element distribution, however, did not further reveal the mutual diffusion coefficient between interface elements and the evolution of the interface microstructure. Especially, this paper focuses on the mutual diffusion coefficient of elements, the scale structure characterization of the interface microstructure, and the shear fracture characteristics of NO8825-X52 nickel-based alloy composite plate. To calculate the mutual diffusion coefficient, mathematical fitting is carried out to study the major element distribution in the thickness direction of the composite plate. Furtherly, the interpretation of the mutual diffusion coefficient is presented from microstructure and grain size. en, the relationship between interface microstructure and bonding strength discussed is based on the above quantitative studies. e purpose is to improve the comprehensive performance of composite materials through the control of interface microstructure, which provides a scientific and theoretical basis for the processing of nickelbased alloy composite plates.

Materials and Methods
e nickel-based alloy composite plate was prepared by vacuum hot-roll bonding. X52 carbon steel was chosen as the substrate with the thickness of 9 mm and NO8825 nickel-based alloy was chosen as the cladding with the thickness of 3 mm. e chemical compositions of the metals are listed in Table 1. e 10 × 10 × 12 mm metallographic specimens were cut in the thickness direction at the interface of the composite plate. Due to the difference of corrosion resistance between nickel-based alloy and carbon steel, the metallographic corrosion was conducted on nickel-based alloy and carbon steel separately. e carbon steel composite plate was etched with 4% nitric acid alcohol solution. e interface microstructure and element distribution of the specimens in the thickness direction were observed by optical microscope (OM), scanning electron microscope (SEM, Quanta 200 FEG, FEI), and energy dispersive spectrometer (EDS), respectively. e specimens were peeled off layer by layer from the nickel-based alloy side to the interface of the composite plate [23]. Ferric chloride hydrochloric acid solution was chosen as an etchant to make sure the peeling surface on the carbon side was freshly revealed during the mechanical peeling process. en, after metallographic grinding and polishing, the microstructure of the peeling surface on the carbon steel side was observed and characterized by SEM and X-ray diffractometer (XRD, D/mas-2000PC, Rigaku). Figure 1 shows the shear specimens prepared according to the GB/T 6396-2008 standard [24]. e tensile shear tests were conducted by the electronic universal testing machine (Zwick/Roell, Z250, Germany) with a speed of 1 mm/min. e shear strength was calculated using equation (1), where F is the applied load, S is the area of the shear surface, and S � 4.5 mm × 25 mm in the text, as shown in Figure 1: e strain was monitored using an extensometer. e shear fracture microstructure and element content of the shear specimens were observed by SEM and EDS.

Morphologies at the Interface of Composite Plate.
To investigate the microstructure of the transition zone, crosssection views are obtained and shown in Figures 2(a) and 2(b). It can be seen that the NO8825 nickel-based alloy is well combined with the carbon steel after hot-rolling processing, and the interface line is clear and straight, as well as the grain size at the interface of the carbon steel side decreases. Furthermore, at the X52 carbon steel side, a transition zone with a thickness of about 13 μm near the interface can be observed. An enlarged picture is shown in Figure 2(c). e transition zone consisted of ferrite precipitation with short rod-like grains, which proved the diffusion of carbon components, similar to [25]. e element contents of Ni, Cr, and Fe in the nickel-based alloy and carbon steel sides far away from the interface are unchanged. In the transition zone of the diffusion layer, the content of Cr and Ni elements decreases from the nickel-based alloy side to the carbon steel side, whereas the content of the Fe element increases. It indicates that a metallurgical bonding has been formed between nickel-based alloy and carbon steel. e elements of mutual diffusion take great responsibility for this. At high temperature, mutual diffusion through the bonding interface occurred, which induces the migration of the grain boundary and extends the bonding surface and thus forms a diffusion layer with a certain thickness at the interface [26][27][28].

Element Diffusion at the Interface of the Composite
Plate.
e mutual diffusion of these major elements will directly affect the bonding properties of the hot-roll bonding plate [29,30]. erefore, a fitting analysis of Ni and Fe distribution at the interface is conducted, which is shown in Figure 3. According to Fick's second law, the content of Ni and Fe distribution can be described by the error function. e solutions of the error function for the content of Ni and Fe distribution are as equation (2), respectively: where t � 1 h. us, the diffusion coefficients of elements Ni and Fe at the interface can be obtained as follows: Since the diffusion coefficient of the Fe element is greater than that of the Ni element, therefore, the Kirkendall voids formed in the carbon steel side of the composite plate.
According to [31], the diffusion coefficient of Ni in nanocrystalline Fe increases from 5.79 × 10 −15 m 2 ·s −1 to 8.48 × 10 −15 m 2 ·s −1 with the temperature rises from 650°C to 850°C, which is about 2 orders of magnitude smaller than that in this work. is is likely due to the existence of the Kirkendall voids (see Figure 4), which greatly promoted the element's diffusion and contributed to the formation of metallurgical bonding with enhanced interfacial bonding strength [32,33]. side with less nickel-based alloy was explored. Figure 4(a) shows that an obvious porous structure in the transition zone between nickel-based alloy (austenite phase) and carbon steel is observable. Figure 4(b) illustrates the enlarged view of the porous structure of the red line marked area in Figure 4(a). According to [34,35], the Kirkendall effect is effective to interpret the results. Due to the existence of the Kirkendall voids, the fast channel for the element diffusion is provided, which promotes the element diffusion. It is under the Ni/Fe element diffusion fitting calculation results obtained above.

XRD Characterization of the Peeling Surface on the Carbon Steel Side.
To reveal the interface microstructure, XRD characterization was performed on the fresh revealed peeling surface with less nickel-based alloy (as shown in Figure 4) and the carbon steel substrate side, respectively. Electrolytic polishing was conducted on the carbon steel substrate side before XRD analysis. e XRD results obtained are shown in Figure 5, which reveals the broadening of the full width at half maximum (FWHM) of the (110) and (200) peaks when the peeling surface on the carbon steel side is fresh revealed. Table 2 shows the statistical results of FWHM of the (110) and (200) peaks.
To evaluate the average crystallite size, the Scherrer formula is applied here, which can be express as below [36]: where λ is the X-ray wavelength, λ � 1.54Å in the text, B 0 is the instrument broadening factor, B 0 � 0.091deg in the text, which needs to be converted into radian for calculation, B is the full width at half maximum (FWHM), θ is the diffraction angle, and D is crystallite size. According to equation (3), the larger the FWHM is, the smaller the average crystallite size is. e calculated data are summarized in Table 2. It is shown that the average crystallite size of the fresh revealed peeling surface and electrolytic polishing X52 substrate is 41.5 nm and 113.0 nm, respectively. Crystallite size in the microstructure zone at the fresh revealed peeling surface is about 3 times finer than that of electrolytic polishing X52 substrate, which indicates the elemental transition zone has a fine-grained structure and strengthens the mechanical properties of the interface.

Analysis of Shear Strength and Fracture of Composite
Plate. To explore the bonding strength of the composite, the stress-strain curve is obtained by the shear test, which is  shown in Figure 6. According to the fracture starting point, the shear strength is 453 MPa, which is much higher than the minimum of 140 MPa defined in ASTM A-264 specifications [37]. e bonding strength in this work is higher than the shear strength reported in [38][39][40], which is listed in Table 3. Furthermore, as can be seen from the sheer    [38][39][40].
Shear strength (MPa) Reference Method 395 [37] Vacuum hot rolling 352/340 [38] Asymmetrically hot rolled 282 ± 5 [39] Hot-roll bonding tensile curve, the stress increases with the strain rise until a sharp decrease at the fracture starting point, and then, the stress remains nearly constant until the shear sample is broken. It indicates that the transition zone of the element diffusion layer can effectively share the load and delay the deformation as an interface bonding zone between nickelbased alloy and carbon steel, avoiding the interface fracture. Figures 7(a) and 7(c) are the shear fracture microstructure of both sides of the shear sample. It has obvious dimple characteristics and a ductile fracture has occurred, indicating that the nickel-based composite plate has good ductility and interfacial bonding strength. Furthermore, the EDS analysis of the shear fracture is shown in Figures 7(b) and 7(d). ere is no Cr element, and there is almost no Ni element, but the content of the Fe element on the fracture surface is very high, which indicates that the fracture has occurred on the X52 carbon steel side near the interface rather than at the interface.

Conclusions
Based on the microstructure of the NO8825/X52 composite plate, there is a transition zone with a thickness of about 13 μm at the interface. Due to the major element (Ni, Cr, and Fe) diffusion between nickel-based alloy and carbon steel, a 13 μm thick diffusion layer is formed between nickel-based alloy and carbon steel. Furthermore, Kirkendall voids and fine-grained  Advances in Materials Science and Engineering structure are revealed by peeling off the nickel-based alloy cladding, greatly promoting element diffusion, and improving the interfacial bonding strength of the NO8825/X52 composite plate. e diffusion coefficient of Ni at the interface is about 2 orders of magnitude larger than that of nanocrystalline Fe, and the shear strength is much higher than the minimum of 140 MPa defined in ASTM A264 specifications, as well as the fracture has occurred on the X52 carbon steel side near the interface rather than at the interface of the composite plate in the shear test. us, through the element transition zone of the diffusion layer, the mechanical properties at the interface of the composite plate achieve a stable transition from the nickelbased alloy side to the carbon steel side, which is beneficial to a strong bond between nickel-based alloy and carbon steel.
Data Availability e data presented in this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.