Materials Letters 63 (2009) 2462–2465
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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Interface microstructure of diffusion bonded Ni3Al intermetallic alloy and austenitic stainless steel Adnan Çalık ⁎ University of SDU, Faculty of Technical Education, Department of Machine Education, Isparta, Turkey
a r t i c l e
i n f o
Article history: Received 23 July 2009 Accepted 18 August 2009 Available online 23 August 2009 Keywords: Diffusion Intermetallic alloys and compounds Microstructure Metals and alloys
a b s t r a c t The diffusion bonding of a Ni3Al intermetallic alloy to an austenitic stainless steel has been carried out at temperatures 950, 1000 and 1050 °C. The influence of bonding temperature on the microstructural development and hardness across the joint region has been determined. The microvoids in the interface have been found to decrease with increasing bonding temperature. The intermetallic phase Al3Ni has been detected at the Ni3Al side of the diffusion couple. Diffusion of Cr and Fe from the stainless steel to the Ni3Al alloy has been observed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Diffusion bonding (DB) is a joining process which relies on the effect of applied pressure at elevated temperatures. The principal mechanism for joint formation is solid-state diffusion. No melting and only limited macroscopic deformation of the parts occurs during bonding [1–3]. The Ni3Al alloy has excellent corrosion and oxidation resistance over a wide range of temperatures owing to the formation of a stable surface aluminum oxide layer. It also has strong chemical and microstructural stability up to temperatures close to its melting point [4]. A limited number of studies exist on the interface behavior of Ni3Al bonded with other metals, some which are based on Ni3Almatrix composites reinforced with various particulates or fibers [5–7], some which are in the form of coatings [8–10] and very few which are based on laser cladding and diffusion bonding [11–13]. In this study, a Ni3Al intermetallic alloy and an austenitic stainless steel have been chosen for diffusion bonding. The main reason for choosing this couple was to combine the ductile and corrosion resistant properties of austenitic stainless steel with the high temperature corrosion and oxidation resistance of the Ni3Al intermetallic alloy. It is known [3,14] that Cr reduces the oxygen embrittlement and improves the ductility of Ni3Al alloys, therefore it is likely that the diffusion of Cr into the Ni3Al alloy during diffusion bonding would improve the properties of the alloy. Another reason for this study was to determine whether diffusion bonding can be used as an alternative to other methods such as cladding and fusion. The effects
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of bonding temperature on the microstructure and microhardness of the joint were investigated. 2. Materials and methods Sheets/plates of Ni3Al intermetallic alloy and AISI 316L stainless steel were used for the DB process. Table 1 shows the typical compositions of the Ni3Al intermetallic and AISI 316L stainless steel alloys. The DB specimens of the Ni3Al intermetallic alloy, having dimensions of 10 × 10 × 2 mm, were obtained from 2 mm-thick sheets. The AISI 316L stainless steel specimens, having dimensions of 10 × 10 × 10 mm, were machined from plates of 10 mm thickness. Prior to diffusion bonding, the surfaces of the specimens were prepared by grinding with 600 and 1200-grit SiC abrasive papers, followed by polishing with a 3 μm diamond paste. Bonding was carried under argon in the chamber of an induction heating unit specially designed for diffusion bonding. The DB couples were heated to bonding temperatures of 950, 1000 and 1050 °C. A constant pressure of 6 MPa was applied to the couples. A holding time of 30 min was used for each bonding temperature. The samples were cooled at a rate of 15 °C min− 1 to room temperature, prior to removal from the chamber. Metallography of the DB area involved preparing transverse sections of the DB couples, followed by the abovementioned grinding and polishing steps, until a mirror finish was obtained. The samples were then etched in a 1:1 solution of HNO3 and pure water. The DB region was investigated by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD). Microhardness readings from the bonded area were obtained under a load of 10 g.
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Table 1 Chemical compositions of the test samples used for diffusion bonding. Alloy
Composition (wt.%) C
Cr
Ni
Mo
Si
Mn
P
S
Al
Fe
0.007 0.046 Bal. 0.002 0.03 0.024 0.002 0.0004 11.12 0.45 Ni3Al AISI 0.02 17.3 11.0 2.2 0.78 1.8 0.02 0.015 – Bal. 316 L
The DB samples were debonded by the shear test apparatus described elsewhere [15]. Fractography of the sheared surfaces of the Ni3Al sides of the DB couples was evaluated by SEM imaging.
Fig. 2. The (a) concentration and (b) microhardness profile of the specimen bonded at 1050 °C.
3. Results and discussion The electron micrographs for the specimens bonded at 950, 1000 and 1050 °C are shown in Fig. 1. Extensive void formation at the interface was observed for the sample bonded at 950 °C, Fig. 1(a), indicating that the DB
Fig. 1. Electron micrographs of the bond interfaces of the specimens bonded at (a) 950 °C, (b) 1000 °C, and (c) 1050 °C.
Fig. 3. X-ray diffraction pattern of the (a) Ni3Al and (b) AISI 316L side of the bond interface.
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were constructed. Fig. 2(a) shows the diffusion profiles of Ni, Fe, Al and Cr elements. The concentrations of Ni and Al show abrupt changes at the interface, while Fe and Cr show a gradual decrease towards the Ni3Al side. This indicates that significant diffusion of Fe and Cr into the Ni3Al alloy occurred during bonding. Microhardness measurements taken across the DB region for the bonded specimens are shown in Fig. 2(b). The figure shows that hardness drops to a minimum at the interface for all bonding temperatures. This decrease can be attributed mainly to micropores remaining at the interface. It may also be caused by the diffusion of Cr to the Ni3Al side. The decreasing Cr content at the AISI 316L side decreased the hardness of the AISI 316L toward the interface. The Ni3Al side shows increasing hardness toward the interface which also proves the diffusion of Fe and Cr. The XRD patterns of the sheared Ni3Al and AISI 316L surfaces are shown in Fig. 3(a) and (b). Fig. 3(a) shows that Al3Ni is formed at the interface. It is likely that the diffusion of Ni towards the AISI 316L side caused the formation of the Al3Ni phase [17,18]. Fig. 3(b) indicates that the Al3Ni phases have remained mostly in the Ni3Al side after shearing. It must be noted here that the XRD patterns for the Ni3Al and austenite phases are very similar, and that superposed peaks are present. The fractographs of the sheared DB couples are shown in Fig. 4. The fractograph of the sample bonded at 950 °C, Fig. 4(a) shows the presence of hollow regions, which are a result of poor bonding. Fig. 4(b) and (c) indicates that these regions decrease with increasing bonding temperature. 4. Conclusions The effect of bonding temperature on the microstructure and hardness of diffusion bonded Ni3Al intermetallic alloy–AISI 316L stainless steel couples has been investigated. The microvoids at the interface have decreased with increasing temperature, primarily due to enhanced diffusion. Formation of Al3Ni has been identified in the Ni3Al side of the diffusion couple. This phase was probably formed as a result of Ni diffusion to the 316L steel. The hardness increase observed in the Ni3Al side of the diffusion couple was mainly caused by the diffusion of Cr from the AISI 316L side. The shear fracture test combined with electron microscopy has provided to be useful for the investigation of interfaces in diffusion couples. References
Fig. 4. Shear fracture surfaces (SEM) for bonded materials at (a) 950 °C, (b) 1000 °C and (c) 1050 °C.
temperature of 950 °C (or its duration of 30 min) was insufficient for diffusion bonding. Some microvoids were also observed at the interface of the diffusion couple bonded at 1000 °C, Fig. 1(b). The relatively clean interface microstructure of the 1050 °C sample in Fig. 1(c) indicates that most of the microvoids at the interface were eliminated at this temperature as a result of enhanced diffusion. Fairly large and irregularly shaped voids can be seen in the Ni3Al sides of all of the samples in Fig. 1; these voids are etch pits caused by galvanic corrosion [16], which in this case was caused by the etchant applied during metallographic preparation. The sample bonded at 1050 °C showed the presence of a very thin layer produced at the interface and was therefore chosen for detailed XRD experiments. By taking EDX spot analyses across the bond interface of the 1050 °C sample at specific distance intervals, concentration profiles
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