Materials Letters 164 (2016) 267–269
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Formation of cavitation-induced nanosize precipitates on the eroded surface for Inconel 718 alloy Zhen Li a,b, Jiansong Zhou a, Jiesheng Han a, Jianmin Chen a,n a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 September 2015 Accepted 25 October 2015 Available online 30 November 2015
The cavitation erosion of Inconel 718 nickel-base alloy was investigated using an ultrasonic vibratory apparatus. The morphologies and microstructure evolution of the eroded surface were observed by scanning electron microscopy (SEM) and cold ﬁeld emission scanning electron microscope (FESEM) and the cavitation-induced nanosize precipitates are found in the local zones of eroded surface. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nickel-base alloy Cavitation erosion Microstructure Nanosize
1. Introduction Inconel 718 is a Nb-modiﬁed nickel-base superalloy widely used as an important structure material in gas turbines, pumps, rocket engines and containers due to its excellent corrosion resistance, wear resistance and high temperature strength [1,2]. Cavitation erosion is a serious damage by the implosion of bubbles on surfaces of hydraulic components, e.g. hydraulic turbines, vessels, pumps, valves, etc. Some critical components of nuclear devices, steam turbine, and liquid fuelled rockets are exposed to complex working conditions in the presence of cavitation erosion. Serious cavitation damage reduces the machine lifetime and threatens the security operation of the whole device [3–5]. In recent years, the cavitation erosion behaviors of various materials have been extensively studied [6–8]. The results indicate that erosion resistance is greatly affected by the properties and microstructure of materials. In fact cavitaion erosion of the material is a dynamic damage process and the microstructure evolution of eroded surface can occur with cavitation erosion. The formation of new microstructures on the eroded surface can cause a signiﬁcant change in cavitation erosion behavior and cavitation erosion mechanism of materials [9,10]. The cavitation-induced martensitic transformation exerts signiﬁcant effects on the erosion resistance of the iron-based alloys by steadily absorbing the cavity collapse energy and improving surface mechanical behavior [11,12]. As n
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http://dx.doi.org/10.1016/j.matlet.2015.10.122 0167-577X/& 2015 Elsevier B.V. All rights reserved.
mentioned above, Inconel 718 is an important structure material and the mechanical properties are very sensitive to microstructure. In this paper, cavitation erosion of Inconel 718 was investigated and microstructure evolution was observed.
2. Experimental procedure Commercial Inconel 718 alloy was selected as test material for the cavitation erosion. The nominally major chemical compositions are listed in Table 1. Before the cavitation tests, the specimens were machined into blocky shape by cutting machine. The specimens were ﬁnally polished with alumina suspension down to 0.05 μm, then etched by a mixture solution of HCl, C3H8O3 and HNO3 with a volume ratio of 3:2:1. Cavitation erosion tests were carried out by ultrasonic vibratory apparatus according to ASTM G32-06 standard . The vibratory frequency and peak-to-peak amplitude were 2070.5 kHz and 50 mm75%, respectively. Fig. 1 shows the schematic diagram of the ultrasonic vibratory apparatus. The upper specimen made of 1Cr18Ni9Ti stainless was ﬁxed on the tip of the horn (Fig. 1) with a diameter of 15.970.05 mm. The test specimen was placed in the specimen holder as the lower specimen with a distance of 0.5 mm to the upper specimen face. The immersion depth of the test specimen surface was 1274 mm. Pure water was used as the test liquid. The water temperature was kept at approximately 25 °C by temperature control device. The morphologies and surface distribution of eroded surface were analyzed by scanning electron microscopy (JSM5600LV, JEOL) with energy dispersive spectroscopy (EDS) and cold ﬁeld emission scanning electron microscope (JSM-6701F).
Z. Li et al. / Materials Letters 164 (2016) 267–269
Table 1 Nominally chemical compositions of Inconel 718 (wt%). Ni 50.0–55.0
Co 1.0 max
Mn 0.35 max
Si 0.35 max
S 0.015 max
Cu 0.30 max
C 0.08 max
Fig. 3. Typical FESEM micrograph of the eroded surface of Inconel 718 after cavitation erosion for 4 h.
Fig. 1. Schematic diagram of the ultrasonic vibratory apparatus.
3. Results and discussion The SEM micrographs of the eroded surface at different time intervals are shown in Fig. 2. As shown in Fig. 2a, some small cavities were clearly observed at the twin boundaries and the junctions of twin lamella and grain boundary after cavitation erosion for 2 h. A local magniﬁcation micrograph (Fig. 2a) shows that the cavity extends towards subsurface and two sides of twin boundary. The main cavitation damage occurs initially from twin boundaries and grain boundaries after 2 h. With the erosion time increasing to 4 h, the cavity at the junction of twin lamella and grain boundary becomes more apparent than that of 2 h (Fig. 2b). Large area of materials was removed from the junctions of twin lamella and grain boundary, twin boundary and grain boundary (Fig. 2b and c). Many slip lines appear in the grains as noted by the ellipse (Fig. 2b) after 4 h. Furthermore, the obvious sub-surface cracks were observed clearly from the local magniﬁcation image (labeled A region in Fig. 2c) after 4 h. The massive material is torn off because of the propagation of micro-cracks and the join of small cavities after 4 h. An important ﬁnding in our study is the formation of nanosize precipitates as shown in Fig. 3. The nanosize precipitates agglomerate and locate at the serious eroded regions as pointed by arrows (Fig. 3). Long et al.  investigated the change in surface microstructures of Ti3Al–Nb alloy during cavitation erosion. The nanocrystallized and amorphous structures were found in some zones of the eroded surface. They proposed that the mechanical and thermal effects of micro-jets resulted in the formation of
nanocrystallized and amorphous structures in the eroded surface. Thermal aging treatment or thermomechanical effect can induce the precipitation of second phase particles in Inconel 718 superalloy [2,15]. In the present study, we found that the cavitationinduced nanosize precipitates were formed in Inconel 718 alloy due to repeated impacting and thermal effect generated by imploding bubbles. It is well known that the shock waves and/or micro-jets generated by bubble collapse produce impacting pressure near a material surface ranging from several hundreds to 1000 MPa . Furthermore bubble collapse can also produce distinct thermal effect. Both the high impacting pressure and thermal effect, similar to the thermomechanical action in nature, may result in the change of microstructure of cavitation erosion surface for nickel-base alloy. Furthermore, the micro-defects (e.g. cavities and micro-cracks) generated by cavitation erosion in the eroded regions may be sites where precipitation preferentially nucleates. Thus, it is possible that the cavitation-induced nanoprecipitation occurs preferentially at the serious eroded region because of strongly mechanical action and thermal effect produced by bubble collapsing. Fig. 4 shows surface distribution of major elements of Inconel 718 after cavitation erosion for 4 h. It can be found that the elements of Ni and Cr distribute uniformly except the serious cavitation regions where the content of both Ni and Cr is slightly low (Fig. 4b and c). The distribution of Nb is not uniform, the content of Nb at the seriously eroded region is relatively low. While high content and agglomerates of Nb were found in the local zones of the seriously eroded surface as noted by the ellipse (Fig. 4d). This is in good agreement with the agglomerates of nanosize precipitates as shown in Fig. 3. So we proposed the composition of the nanosize precipitates is Ni3Nb. The metastable body centered tetragonal precipitate γ″ Ni3Nb phase is a major strengthening phase of Inconel 718 alloy , which is easily formed during the thermal aging treatment or thermomechanical effect. In speculation the nanosize precipitates of intermetallic phase Ni3Nb is easy to form during the cavitation erosion. The cavitation-induced
Fig. 2. Typical SEM micrograph of the eroded surface of Inconel 718 at different time intervals for different regions: (a) for 2 h; (b) and (c) for 4 h.
Z. Li et al. / Materials Letters 164 (2016) 267–269
Fig. 4. Image of Inconel 718 surface (a) and corresponding surface distribution of major elements (b) Ni, (c) Cr and (d) Nb after cavitation erosion for 4 h.
martensitic transformation exerted signiﬁcant effects on the cavitation erosion resistance and the erosion damage mechanism for the iron-based alloys by steadily absorbing collapse energy of cavities and obstructing the propagation of cracks from grain boundaries into the grain [10,12]. We proposed that the precipitation of cavitation-induced nanosize precipitates can delay the erosion damage by steadily absorbing the cavity collapse energy. Because of the restriction of experimental conditions it is difﬁcult to completely prove above hypothesis. Further work is still needed to be done to clarify the formation of cavitation-induced nanoprecipitates and their effect on the cavitation behavior of nickel-base alloy.
4. Conclusions The main cavitation damage occurs initially from the twin boundaries, grain boundaries, the connection of twin lamella and grain boundary. The cavitation-induced nanosize precipitates were found at the seriously eroded regions due to strongly mechanical action and thermal effect produced by bubble collapsing during cavitation erosion.
Acknowledgment The authors would like to acknowledge the ﬁnancial support from National Natural Science Foundation of China (No: 51475444).
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