High-Temperature Oxidation Behavior of a Ni-Cr-W-Al Alloy

Transcription

1 J. Mater. Sci. Technol., 2011, 27(9), High-Temperature Oxidation Behavior of a Ni-Cr-W-Al Alloy Y.C. Ma, X.J. Zhao, M. Gao and K. Liu Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China [Manuscript received November 29, 2010, in revised form March 7, 2011] A study was conducted to examine the isothermal oxidation behavior of a new Ni-Cr-W-Al alloy in air at 1250 C with different time. Oxidation kinetics was determined from weight-change measurements. The microstructure and composition of the oxide scale were investigated by means of scanning electron microcopy and X-ray diffraction. The results showed that the oxide scales of the alloy were a compact and continuous outer Cr 2 and NiCr 2 O 4 layer and an inner Al 2 layer that was in dendrite shape. Oxides scales with good adherence were formed on the surface of the alloy, which made the alloy perform excellent high-temperature oxidation resistance. KEY WORDS: Oxidation kinetics; Oxide scale; High-temperature oxidation resistance 1. Introduction The high-temperature corrosion occurs when components are in the high temperature and corrosion environments. This type of corrosion includes high-temperature oxidation, decarburizing, hydrogen corrosion, sulfidation corrosion and ablation. The high-temperature oxidation, which happens in the circumstance containing oxygen or oxidizability substances [1 3], is the commonest. Many researchers have focused on investigating the hightemperature oxidation behavior of Fe-based and Nibased alloys [4 15] and reported the formation of regular pattern of oxide scales and the influence factor of oxide scales formation. Tawancy et al. [4] studied the high temperature oxidation behavior of a Ni-Cr- Al-Fe-Y alloy, which was used at the temperature of 1150 C. Chirst et al. [5] investigated the high temperature oxidation behavior of several nickel-based alloys in atmospheres with widely varying oxygen partial pressure. Lou et al. [6] had also studied the high temperature oxidation resistance of sputtered micro-grain superalloy K38G. Quadakkers [16] studied the growth mechanisms of oxide scales and gave the method to increase the adherence of oxide scales. Brady et al. [17] Corresponding author. Ph.D.; Tel.: ; Fax: ; address: ycma@imr.ac.cn (Y.C. Ma). came up with a new formation method of protective oxide scale in terms of alloy design. This paper focus on investigating the high-temperature oxidation behavior of a Ni-Cr-W-Al alloy which is not treated by any extrinsic method and can be used at 1250 C. This work aims to provide some reliable data in order to make the alloy be applied to the high temperature environment. 2. Experimental The chemical composition of Ni-Cr-W-Al alloy is shown in Table 1. The specimens for the high- Table 1 Chemical composition of the Ni-Cr-W-Al alloy C Cr W Al Ni Bal. temperature corrosion testing were cut from the rolled new alloy bar and their dimensions were 15 mm 8.5 mm 2.3 mm. There was a hole drilled in the center of the specimens for hanging the specimen. The surface was ground to 600 grit abrasive paper and was then ultrasonically cleaned in acetone and ethanol. The thermogravimetric measurements were carried out in an open thermobalance which was made in France. The model number of the thermobalance

2 842 Y.C. Ma et al.: J. Mater. Sci. Technol., 2011, 27(9), o C W/A / (mg/cm 2 ) Time / h Fig. 1 Oxidation kinetic curve of Ni-Cr-W-Al alloy at 1250 C for 100 h is SETARAM Microbalance Mtb The sensitivity of the balance is g. The thermogravimetric experiments were carried out at temperature 1250 C. The oxidation times varied from 10 to 100 h. For guaranteeing the air circulation and making specimens be well-oxidized, the thermobalance was open in the course of the experiments. The 10 and 100 h specimens were observed by the optical microscopy (OM) and scanning electron microscopy (SEM). The oxide scales of the specimens were analyzed by X-ray diffraction (XRD) 3. Results and Discussion 3.1 Oxidation kinetics The oxidation kinetic curve of Ni-Cr-W-Al alloy in the air at 1250 C is shown in Fig. 1. The kinetic behavior of the alloy oxidized at 1250 C followed a parabolic rate law [2] which is shown in Fig. 2. The reaction rate constant (K) was calculated from ( W /A) 2 =Kt [18], where W and A are the weight change and the surface area of the specimens, respectively. Figure 2 illustrates the parabolic reaction rate behavior of Ni-Cr-W-Al alloy, which can be divided into two periods. In the first period, the oxidation rate is linear with the testing time. And in the second period, the oxidation rate shows a slight decrease. The results are related with the composition and pattern of oxide scales formed on the surface of specimens at 1250 C. In the first period, the oxide scale is very thin and not continuous in some position so that the oxygen atoms diffuse easily through it and the oxidation reacts quickly. There are some noncontinuous places in the cross-section micrograph of the scale formed at 1250 C for 10 h shown in Fig. 3(a). With the time longer, the oxide scales become more continuous and compact. Figure 3(b) gives the cross-section micrograph of the scale formed at 1250 C for 100 h. The 100 h scale is continuous and compact, which reduces further the diffusion rate of oxygen atom and limites the oxidation reaction. There is a point of inflexion in the parabolic Fig. 2 Parabolic characteristic of Ni-Cr-W-Al alloy oxidation kinetic oxidation kinetic characteristic of Ni-Cr-W-Al alloy shown in Fig. 2. The point of 50 h is a critical time for the formation of the continuous oxide scales. In other words, the continuous oxide scales begin to form at 1250 C for 50 h. The oxidation kinetic results suggest that the oxide scales growth of Ni-Cr-W-Al alloy is governed by the diffusion rate of oxygen atom through the scale. 3.2 Constitution and cross-section morphology of the oxide scales Figure 3 shows the cross-section morphology of oxide scales formed on the surface of the specimens oxidized at 1250 C for 10 and 100 h. And the elemental redistribution of the alloy is shown in Fig. 4. The oxide scales consist of two layers: an external layer and an internal oxide layer. The external layer is approximately 2.5 µm in thickness while the internal oxide layer is approximately 10 µm in thickness after oxidation at 1250 C for 10 h. And, the external layer is approximately 10 µm in thickness while the internal oxide layer is approximately 25 µm in thickness after oxidation at 1250 C for 100 h. The external layer is mostly chromium oxide and the internal layer is mostly aluminum oxide from the X-ray element maps of the scale shown in Fig. 4. A very small amount of Ni and W also dissolves in the outer oxide layer. The alloy oxide scales after 10 and 100 h at 1250 C was examined by XRD. The results in Figs. 5 and 6 show that the oxide mostly contained NiCr 2 O 4, Al 2, Cr 2 and a little amount of W, NiO and Ni 2. Combining the results of elements distribution with the XRD results, the chromium oxide in the outer layer should be Cr 2, NiCr 2 O 4 and the aluminum oxide in the inner layer should be Al 2. The major oxides formed in the region were identified as NiCr 2 O 4, Cr 2 and Al 2. The Cr-depletion area was found beneath the external oxide layer. The transport of the elements across the metaloxide region occurring during oxidation is evident in the respective concentration profiles. In the initial oxidation stage, some kinds of oxides, NiO, Cr 2 etc. were formed on the surface of specimens and the in-

3 Y.C. Ma et al.: J. Mater. Sci. Technol., 2011, 27(9), Fig. 3 Cross-section micrograph of the scale formed on Ni-Cr-W-Al alloy oxidized at 1250 C for 10 h (a) and 100 h (b) Fig. 4 Cross-section micrograph and X-ray element maps of the scale formed on the Ni-Cr-W-Al alloy oxidized at 1250 C for 100 h

4 844 Y.C. Ma et al.: J. Mater. Sci. Technol., 2011, 27(9), Intensity / a.u. W Al 2 NiCr 2 O 4 NiO Cr 2 Ni / deg. Fig. 5 XRD analysis of the composition of surface scale formed at 1250 C for 10 h Intensity / a.u. W Al 2 NiCr 2 O 4 NiO Cr 2 Ni / deg. Fig. 6 XRD analysis of the composition of surface scale formed at 1250 C for 100 h ternal oxidation reaction of aluminum took place simultaneously. Because of the high concentration of chromium in Ni-Cr-W-Al alloy, rich chromic oxide external scale formed in the alloy. The Cr 2 layer was quickly formed on the surface with further oxidation and the oxidation of nickel being inhibited, which led to a rapid mass gain as shown on kinetic curves in Fig. 1. NiO particles were surrounded with Cr 2 and the solid-state reaction occurred to form NiCr 2 O 4 spinel in the scale gradually and dispersed in external oxide layer. The chemical reaction can be written as follows: NiO + Cr 2 NiCr 2 O 4 The NiCr 2 O 4 spinel can also reduce the diffusion velocity of metal ion [2], which decreases the bonded rate of the metal ion and oxygen atom. The oxidation resistance of the alloy will be enhanced. When the oxidation reaction entrances the second oxidation stage, the continuous and compact Cr 2 scale begin to form. The continuous oxide scale decreases further the activity of oxygen in the interface between the scale and the matrix. Due to the high stability of Al 2, aluminum could be selectively oxidized on the matrix/oxide interface and internally oxidized in the matrix even though it is present in a low concentration. The oxide scales growth depends on the diffusion of metal ion and oxygen atom. The diffusion modes of oxygen atom include grain-boundary diffusion and lattice diffusion. The elongated particles in the region of internal oxidation consist of Al 2. The internal oxidation at grain-boundaries extends deeper into the metallic matrix than that in the inner parts of the grains. In the initial oxidation stage, growth of Al 2 scales is dominated by grain-boundary diffusion and the oxides are very easily nucleated in the grain-boundary shown in the Fig. 3(a). With the time longer, the lattice diffusion become more important, the homogenous internal oxidation is formed shown in the Fig. 3(b). From the results of SEM and XRD, we found that the major oxides of the outer layer were Cr 2 and NiCr 2 O 4 which was continuous and compact on the surface of the alloy. And the inner layer is the Al 2 which is in dendrite shape. The oxidation mechanism of the Ni-Cr-W-Al alloy is that the external oxidation and internal oxidation formed simultaneously at the 1250 C in the air. In the initial oxidation stage, the oxygen diffuses down the grain-boundary as a high speed and the external oxide scales Cr 2 and the internal oxide scales Al 2 growth is very quick. With the time longer at the temperature, the diffusion mode of oxygen becomes the lattice diffusion. The continuous and dense external oxide scales combining with the homogenous internal oxide scales adhered firmly to the matrix of the alloy. The adherence of oxide scales prevents effectively the oxygen diffusion from the environment and the alloy exhibits excellent hightemperature oxidation resistance. 4. Conclusions (1) The oxidation kinetic behavior of a new Ni-Cr- W-Al alloy exposed to 1250 C for 100 h obeyed the parabolic law and no spallation was evident. (2) After exposure at 1250 C for 100 h, the external oxide layer of the Ni-Cr-W-Al alloy consisted of Cr 2 and NiCr 2 O 4 and the internal oxide layer was Al 2. The oxidation was controlled by the transmission of metal ion and oxygen atom through the oxide scale. The multi-oxide structure formed on the metal/scale interface was beneficial for oxidation resistance of this new developed alloy. (3) The oxidation mechanism of the Ni-Cr-W-Al alloy was that the external oxidation and internal oxidation formed simultaneously. In the initial oxidation stage, the oxygen diffused down the grain-boundary at a high speed and the external oxide scales Cr 2 and the internal oxide scales Al 2 growth was very quick. With the time longer, the oxygen atom diffused by the lattice diffusion. The continuous and dense external oxide scales combining with the homogenous internal oxide scales adhered firmly to the matrix of the alloy. REFERENCES [1 ] C.L. Zhang: Materials on Ship Handbook, Chemical

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