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Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy
Journal of Alloys and Compounds xxx (xxxx) xxx
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Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy Yiwen Lei a, b, *, Xiaolong Li a, Ronglu Sun a, Ying Tang a, Wei Niu a a b
School of Mechanical Engineering,Tianjin Polytechnic University, Tianjin, 300387, China Tianjin Key Laboratory of Advanced Mechatronics Equipment Technology, Tianjin, 300387, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2019 Received in revised form 18 October 2019 Accepted 30 October 2019 Available online xxx
A nickel-based superalloy was prepared through powder metallurgy (P/M). In order to balance their microstructure and properties, heat treatments were carried on the alloys. The microstructures of the alloys were characterized by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The solidification process in the alloys during the sintering process was calculated using Thermo-Calc software. A good quality nickel-based superalloy with a homogeneous microstructure and fine grain size can be obtained at the sintering temperature of 1140 C. The alloys mainly consist of irregular polygon CrB particles, irregular morphology Ni3B particles, hexagonal M7C3 particles, (g-Ni þ Ni3B) eutectics and bright g-Ni solid solution matrix. The calculated results show that the solidification process in the alloys during the sintering process is liquid / liquidþg-Ni / liquidþg-Ni þ M7C3 / liquidþ g-Ni þ M7C3þCrB / g-Ni þ M7C3þCrBþ(g-Ni þ Ni3B). The calculation results are helpful to understand the microstructure evolution in the nickel-based superalloy and agree well with the experimental data. The microstructure of the alloys after solution treatment is mainly composed of supersaturated g-Ni solid solution and M7C3. During the aging treatment, the supersaturated g-Ni solid solution decomposed and many small particles were uniformly precipitated in the g-Ni solid solution. As the aging temperature increased, the decomposition process of supersaturated g-Ni solid solution was accelerated and the precipitated particles became coarsen according to Ostwald ripening theory. The hardness of the alloys increased when aging treated at 300 C and 500 C due to the second phase strengthening effect, and then the hardness decreased when aging treated at 700 C due to the coarsening of precipitated particles. © 2019 Elsevier B.V. All rights reserved.
Keywords: Nickel-based superalloy Sintering Microstructure Heat treatment Thermo-Calc
1. Introduction Nickel-based superalloy has been widely used in nuclear reactors, sparking electrodes, furnace components and high temperature rotating parts of the advanced aero-engines owing to its excellent resistance to various types of corrosion, high temperature rupture life, fatigue crack growth resistance and creep strength [1e5]. Nickel-based superalloys are mainly produced by various melting and casting techniques or by powder metallurgy (P/M). Although several advanced melting and casting techniques have been developed for the nickel-based superalloys, the related
* Corresponding author. School of Mechanical Engineering, Tianjin Polytechnic University, Tianjin 300387, China. E-mail address: [email protected] (Y. Lei).
problems such as macro-segregation and formation of freckles could not completely be avoided in big ingots. Elimination of such segregation is a time consuming and expensive process [6]. Compared with melting and casting techniques, powder metallurgy can virtually eliminate the macro-segregation and freckles problems. It is a near-net-shape manufacturing method in which metal powder are compressed with or without other materials and then heated for solidification [7]. The powder metallurgical nickelbased superalloys with more uniform composition and microstructure, smaller microcrystalline structures and more consistent grain possess excellent mechanical properties, fatigue and oxidation resistance at elevated temperatures. They are widely used in aircraft parts or turbine blades [8]. In this study, a nickel-based superalloy was fabricated by powder metallurgy technique. The demand for improved jet engine industry has resulted in the continued development of nickel-based superalloy with higher properties. It has spurred the development of the alloys with the
https://doi.org/10.1016/j.jallcom.2019.152882 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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desired microstructures, properties, and cost effectiveness [9]. The microstructure of the alloys, which is mainly determined by the sintering processing, plays a very important role in determining the mechanical and physical properties of superalloys [10]. In order to extend the alloys’ future application, it is pivotal to study the microstructure and properties of the P/M nickel-based superalloy after different heat treatment [11]. Nickel possesses an extensive solid solubility for many alloying elements and the microstructure of nickel-based superalloy usually consists of the face-centered cubic structure solid-solution matrix with various uniformly dispersed precipitate particles in it. It is mainly strengthened by solid-solution hardening and precipitation hardening [12]. The content of alloying elements dissolved in the matrix and the shape, size and distribution of the precipitations are significantly affected by heat treatment, which is an important step for nickel-based superalloy to achieve the desired distribution of the precipitate in the matrix and the optimum mechanical properties [13]. Previous studies illustrated that the cooling rate after solution treatment affects the size and shape of precipitates [14] and the mean size of precipitates increases with thermal exposure time during aging treatment [15]. In this study, it aims to provide experimental basis for the development of various heat treatment methods for the purpose of optimizing the microstructure and the properties of nickel-based superalloys. The heat treatment is divided into two steps, termed as solution and aging treatment [13]. The first step solution treatment is processed at 1200 C for 2 h. The second step aging treatment is at 850 C for 8 h. Alloy development is still an expensive, time consuming and generally empirical processing. This is because the alloy usually contains many elements, and the actions of alloying elements within the alloy are very complicated [16]. To reduce processing time and gain a deep and theoretical understanding of the phase transformation and microstructure evolution in the alloy during the sintering process, many alloy designers employ thermodynamic modeling tools to explore the phase transformation and the complex relationships between the phases in the alloys [17]. These thermodynamic tools provide information on a variety of aspects, such as the occurred reactions, the formed phases, as well as the mechanical properties. Different authors have used modeling tools to predict the thermodynamic of powder metallurgical processing. Wu et al. simulated the effects of alloying elements on the microstructure and thermodynamics during the liquid phase sintering process in FeeNieMoeCeB steel [18]. Sophia A. Tsipas presented the results of MoeAl co-deposition treatments on titanium alloy. Thermodynamic analysis was carried out to better understand the thermodynamics of the deposition process [19]. According to the above literatures, the thermodynamics software could be used to predict the solidification behavior and phase transformation in the nickelbased superalloy during sintering process using a Java based material processing software, Thermo-Calc software [20]. The software, based on CALPHAD (Calculation of Phase Diagrams), requires a mathematical description of the thermodynamics of the system [21]. The thermodynamic package was used to predict the various equilibria phases and microstructure evolutions that occurs during the cooling process from the sintering temperature to room temperature by a Gibbs energy minimization process [22]. The prediction data were subsequently compared to experimental results to evaluate the validity and applicability of the thermodynamic software. In the present paper, a nickel-based superalloy was prepared by powder metallurgy. The effects of sintering temperature and heat treatment on microstructure and mechanical properties are investigated. The phase constituents and microstructure of the samples were analyzed in detail. The temperature dependent phase
fractions and compositions in the nickel-based superalloy were calculated using the most developed Thermo-Calc software [20]. Combining with the calculated phase fractions and compositions, the solidification process, precipitation behavior and reaction scheme in the nickel-based superalloy during the sintering process were analyzed.
2. Experimental procedure Commercially available powders of (wt.%) Ni (10 mm, 99.5% purity), Cr (5 mm, 99% purity), Fe (10 mm, 99% purity), Si (5 mm, 99% purity) and C (20 mm, 99% purity) were used as the raw materials. A mixture of (wt.%) 15% Fe, 16% Cr, 4% Si, 0.8% C, 3% C and balance Ni was chosen in this paper. The mixture was blended on a universal planetary ball-milling at a rotation velocity of 200 rpm for 1 h with a ball-to-powder mass ratio of about 5:1. Hardened stainless steel milling vessel and 100Cr6 hardened steel balls with a diameter of 5 mm were used as milling media. After mixing, 1.5 wt% of PVB was added to the mixture to improve its compaction ability. To form disc specimens with of size 12 mm in diameter and 6 mm in height, appropriate quantities of mixed powders were weighed and poured into a stainless steel floating die and then bidirectionally pressed at 140 MPa for 3 min using zinc stearate as die lubricant. The sintering process of the green samples was carried out in a tube furnace by keeping the samples on an aluminium oxide plate under 99.999% argon atmosphere with a flow rate of 5 L/min during the sintering run. The sintering process step is shown as in Fig. 1. The thermal debinding of PVB was performed by heating the green samples to 700 C with a heating rate of 15 C/min and a dwelling time of 40 min. After thermal debinding, a pre-sintering process was performed by reaching to 1000 C with a heating rate of 10 C/ min and a dwelling time of 30 min. The final sintering process was performed at different temperature with a heating rate of 6 C/min and a dwelling time of 30 min, and followed by cooling down to the room temperature with a rate of 15 C/min. To examine the influence of the sintering temperature on sintered parts, the sintering processes were carried out at temperature of 1050 C, 1080 C, 1110 C, 1140 C and 1170 C, respectively. As the mechanical properties of the nickel-based superalloy were significantly affected by the precipitates and microstructure in the alloys [18], it is an important to investigate the effects of postsintering heat treatment on the alloys to achieve the desired microstructure [14]. The heat treatment consisted of solid solution followed by aging procedure [13]. The heat treatment was carried
Fig. 1. Sintering process step for the green samples.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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out in a furnace filled with argon to prevent the oxidation of specimens. The solution treatment was at 900 C for 1.5 h followed by quenching in water, and the aging treatment was at 300 C, 500 C and 700 C for 8 h followed by slow cooling in argon atmosphere in the furnace. For metallographic observation, the samples were cut using a high speed wire electrical-discharge machining. Mounted specimen were polished using successively finer silicon carbide abrasive papers of 350e1200 grit size followed by a sequence of 3 mm and 1 mm diamond abrasive slurry. The polished specimens were etched using an etchant (HF: HNO3: H2O ¼ 1: 2: 4) with an etching time of 10e30 s. The specimens were examined using an Olympus GX-51 optical microscopy and QUANTA200 scanning electron microscopy with an Oxford Link energy dispersive spectroscopy. The phase constituent in the alloys were identified using a Rikagu XRD with Cu Ka radiation, wavelength l ¼ 1.54056 nm, operating at an applied voltage of 40 kV and a current of 40 mA. Rockwell-C hardness test (HRC) as a function of sintering temperature and aging time under various aging temperature was carried out on polished samples. It was measured using a HRS-150 Digital Rockwell Hardness Tester under load of 150 kgf and time of 15s. The samples were polished using silicon carbide abrasive papers of 350e1200 grit size and cleaned by alcohol to avoid the effects of surface oxide films before testing. Every sample was measured five times, and then the average value is adopted in the paper. 3. Results and discussion 3.1. Microstructure observation and XRD spectrum of the sintered alloys Fig. 2 shows the morphology of the nickel-based superalloys sintered at different temperature. When the alloys were sintered at relatively low temperature (1050 C), small amount of liquid phase formed during the sintering process and the grains possessed low integrity characteristics. The morphology of grains in the alloys is relatively irregular and clutter, as shown in Fig. 2a. When the sintering temperature was 1080 C and 1110 C, more liquid phase formed during the sintering process and the grains possessed higher integrity characteristics. Some regular polygon M7C3 particles and irregular polygon CrB particles were formed in the alloys as the sintering temperature increased. However, the microstructure of the alloys is inhomogeneous and the normal grain growth occurs, as shown in Fig. 2b and c. When the sintering temperature was 1140 C, an uniform microstructure with many regular polygon M7C3 particles and irregular polygon CrB particles was formed in the alloys, as shown in Fig. 2d. As the sintering temperature further increased to 1170 C, most of the alloying elements dissolve into the liquid phase and many second phase precipitates in the coarse primary g-Ni dendrites matrix. The amount of CrB and M7C3 particles in the alloys reduced. The alloys were mainly composed of the seriously coarse dendrites with obviously over-heated characteristic, as shown in Fig. 2e. On the other hand, the hardness of the sintered samples increased from 38 HRC at 1040 C to the maximum value 51 HRC at 1140 C and then decreased to 42 HRC at1170 C, as shown in Fig. 8. According to the above analysis, a good quality nickel-based superalloys with uniform microstructure and the highest hardness can be obtained at the sintering temperature of 1140 C. Fig. 3 shows the X-ray diffraction spectra of the nickel superalloys sintering at 1140 C. The alloys mainly consist of g-Ni solid solution, CrB, Ni3B and M7C3 (where M is the metallic carbideforming elements). Fig. 4 and Fig. 5 show the SEM-EDS analysis results of different morphological phases in the alloys sintered at 1140 C. The
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composition of the irregular polygon particles (Point A) determined by SEM EDS is enriched in Cr and B. There is also a certain amount of C, as shown in Figs. 4 and 5a. The quantitative composition analysis result of Point A is (wt.%, the same as follows) 23.17B, 74.10Cr and 2.73C. The composition of the irregular morphology particles (Point B) is mainly composed of Ni and B, as shown in Figs. 4 and 5b. The quantitative analysis result of Point B is 13.13B, 73.46Ni, 8.77Fe and 4.64C. The composition of the bright matrix phase (Point C) is enriched in Ni and Fe and a certain amount of C and Si, as shown in Figs. 4 and 5c. The quantitative analysis result of Point C is 77.79Ni, 14.37Fe, 3.28Si and 4.56C. The composition of the regular hexagonal particles (Point D) is mainly composed of Cr and C and a certain amount of Ni and Fe, as shown in Figs. 4 and 5d. The quantitative analysis result of Point D is 8.91C, 69.50Cr, 10.11Ni and 11.48Fe. By a combination of XRD and EDS analyses, the irregular polygon particles (Point A) were identified as CrB, the irregular morphology particles (Point B) were Ni3B, the bright matrix phase (Point C) were g-Ni solid solution with Fe, Cr and C in it and the regular hexagonal particles (Point D) were M7C3 (where M is the Cr, Fe and Ni alloy elements). 3.2. Phase fraction and reaction scheme of the alloys The microstructure evolution in the alloys during the sintering process is complicated and unpredictable due to the different cooling rate and chemical composition [23]. To gain a theoretical understanding of the microstructure evolution in the alloys and to fabricate high quality alloys with desirable microstructure and properties, it is necessary to analyze the phase transformation, such as the temperature dependent phase fractions and compositions in the alloys during the sintering process [24]. In Fig. 10, the horizontal axis denotes the temperature and the vertical one denotes the mole fractions of the phases for the alloys, so the relationship between phase fractions and temperature can be expressed from this figure. The lines express the phase fractions and compositions calculated by Thermo-Calc [20]. According to Fig. 6 and the above analysis, the crystallization process and reaction scheme in the nickel-based superalloys during the sintering process is as follows: i) Primary g-Ni dendrites nucleated and grew up from the molten pool at 1304 C. The mole fraction of g-Ni phase continuously increased and the amount of liquid phase reduced until the temperature decreased to 950 C. The maximal mole fraction of g-Ni phase in the alloys is (mol.%, the same as follows) 57.8% which formed the bright matrix of the alloys. ii) At approximately 1137 C, M7C3 started to crystallize from the remaining liquid phase. Many M7C3 particles with regular hexagonal shape and different particle size were formed in the alloys. According to the solidification process, most of M7C3 particles distributed among the primary g-Ni dendrites and there are also some smaller M7C3 particles were in the primary g-Ni dendrites, as shown in Fig. 2d. The calculated mole fraction of M7C3 phase is about 7.2%. iii) At the temperature range of 1072 Ce950 C, CrB phase with irregular shape crystallized from the liquid. The calculated mole fraction of CrB increased from 0 to 17.7%. At 950 C. A small quantity of remaining liquid phase filled in the gaps among the g-Ni dendrites, M7C3 and CrB phase. The liquid phase reached the eutectic composition. ⅳ) At temperature range from 950 C to 929 C, g-Ni and Ni3B phase co-precipitated from the interdendritic remaining liquid phase. A binary eutectic reaction Liquid / g-Ni þ Ni3B occurred and all the remaining liquid phase transformed into
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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Fig. 2. Morphology of the alloys sintered at different temperature (a) 1050 C, (b) 1080 C, (c) 1110 C, (d) 1140 C, (e) 1170 C.
Fig. 3. X-ray diffraction spectra of the alloys sintering at 1140 C. Fig. 4. SEM morphology of the alloys sintered at 1140 C.
a eutectic (g-Ni þ Ni3B). The calculated mole fraction of the g-Ni þ Ni3B eutectic is about 22%, as shown in Fig. 6. The primary g-Ni dendrite uniformly distributed in the alloys,
while the eutectic distributed among g-Ni dendrites, M7C3 and CrB phase, as shown in Fig. 2d.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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Fig. 5. SEM-EDS results of different phases in the alloys sintered at 1140 C. (a) Point A, (b) Point B, (c) Point C, (d) Point D.
Fig. 6. The phase fractions versus temperature in the nickel-based super alloys.
ⅴ) When the temperature was lower than about 929 C, all the liquid phase transformed into solid phase and a solid-state transformation took place in the alloy. During this process, the fraction of g-Ni phase decreased from 55.5% to 53% and the fraction of M7C3 phase increased from 4 to 7% at the temperature range of 929 Ce600 C. The fraction of CrB and Ni3B almost kept unchanged, as shown in Figs. 6 and 2d. It illustrated that part of g-Ni transformed into M7C3 during solid-state transformation process. The area fraction of different phase in the morphology of the alloy sintered at 1140 C (Fig. 2d) was assessed using a quantitative metallography analyzing system. The measured area fractions of the g-Ni, M7C3, CrB particle and g-Ni þ Ni3B eutectic in this field were 48.0%, 6.4%, 19.6% and 25.9%, respectively. The calculation results obtained from Thermo-Calc software agree well with the
ones from the experiment. According to the metallographic examination and the calculation results obtained from Thermo-Calc software, the complete solidification process and reaction scheme in the alloys during the sintering process is liquid / liquidþ g-Ni / liquidþ g-Ni þ M7C3 / liquidþ g-Ni þ M7C3þCrB / gNi þ M7C3þCrBþ(g-Ni þ Ni3B), as shown in Fig. 6a. As mentioned above, a solid-phase transformation occurred in the alloys during the subsequent cooling process. To further investigate the transformation process the phase composition is calculated as a function of temperature [25]. Fig. 12 is the relationship between the phase composition and temperature in the alloys at the temperature range from 929 C to 600 C. The calculation results indicated that g-Ni phase is a solid solution with a certain amount of Cr, Fe, Si and C in it. When the temperature decreased from 929 C to 600 C, the content of Fe and Ni in g-Ni phase slightly increased, the content of Si almost kept unchanged, while the content of Cr and C slightly decreased, as shown in Fig. 7a. It indicated that an alloyed carbide M7C3 phase precipitated from g-Ni solid solution. M7C3 is an alloyed carbide with a certain amount of Ni, Fe and a small amount of B in it. In this temperature range, the content of Ni in M7C3 significantly decreased, Fe and B almost kept unchanged and Cr increased, as shown in Fig. 7b. The calculation results indicated amount of Ni diffused from M7C3 to gNi phase and it resulted in the decreasing of Ni and increasing of Cr in M7C3. CrB is an alloyed boride with a certain amount of Ni and Fe in it and its composition almost remain unchanged during the subsequent cooling process, as shown in Fig. 7c. Ni3B is also an alloyed compound with a small amount of Cr, Fe and C in it. The content of these alloying elements decreased during the subsequent cooling process, as shown in the calculation results Fig. 7d. It indicated that a slight amount of alloyed carbide M7C3 phase precipitated from Ni3B phase. Above analysis illustrated that amount of M7C3 precipitated in g-Ni and Ni3B phase during solidstate transformation process. It lead to the decreasing fraction of gNi phase and the increasing fraction of M7C3 phase at the temperature range of 929 Ce600 C, as shown in Fig. 6.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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Fig. 7. The phase compositions versus temperature in the alloys. (a) g-Ni, (b) M7C3, (c) CrB, (d) Ni3B.
The calculated results are well consistent with the experimental data of the nickel-based superalloys. It is beneficial to helping understand the solidification process and reaction scheme of the nickel-based superalloy during sintering process and the alloy design for desirable microstructural and mechanical properties [26]. 3.3. Hardness of the sintered alloys The influence of sintering temperature on the hardness of the alloys is shown in Fig. 8. The alloys sintered at 1050 C has a hardness of 38 HRC, the lowest hardness value in this research work. When the sintering temperature is relatively low, small amount of liquid phase was forms in the compacts and it is difficult to ensure high bonding strength among the grains. It results in low hardness of the alloy, as shown in Fig. 2a. This explains the low hardness of the alloys sintered at 1050 C. At the sintering temperature of 1140 C, the hardness of alloys increased to the maximum value 51 HRC. At this sintering temperature, it is capable of generating enough amount of liquid phase and it results in high bonding strength among the grains. The alloys are perfectly crystallized with fine and uniform microstructure and many regular shaped carbides and irregular shaped borides were formed in the alloys, as shown in Fig. 2d. According to fine-grained strengthening and second-phase strengthening theories, the alloys sintered at 1140 C possessed the maximum value 51 HRC. At the sintering
Fig. 8. Hardness of the alloys sintered at different temperature.
temperature of 1170 C, the hardness decreased to 42 HRC, a drop of 17% compared to the alloys sintered at 1140 C. At this sintering temperature, the average size of the grains in the alloys significantly increased, and the amount of carbides and borides in the alloys substantially reduced. The microstructure of alloys mainly
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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composed of coarse dentritics and the alloys were over-heated, as shown in Fig. 2e. It resulted in the dropped hardness for the alloys sintered at 1170 C.
3.4. Microstructure and XRD spectrum of the heat treated alloys To achieve the better precipitation-hardening effect, the solution treatment is key factor to obtain the maximum amounts of the soluble hardening alloying elements dissolving in the alloy matrix and it should be carefully designed to meet the requirements. During this process, it is necessary to produce a solid solution by heating the alloy to a temperature range above the solvus temperature, holding at this temperature range for a sufficient time to obtain a quasi single-phase structure, and then retaining this structure at ambient temperatures by cooling rapidly. The appropriate solution temperature and soak time is required to obtain a satisfactory degree of solution and to achieve good homogeneity of the solid solution. In general, the solubility of the alloying element in the solid solution increases with the increasing of the temperature. During the solution treatment, part of the small second phase redissolved into g-Ni solid solution, almost all of the eutectic transformed into g-Ni solid solution and a nearly homogeneous solid solution matrix was achieved as the alloys were soaked in a furnace at 900 C for a long enough time. And then the alloys were quenched in water at room temperature. Due to the rapid cooling process, the alloying element can not precipitate from the solid solution and a supersaturated solid solution was obtained. This process can preserve the solid solution formed at the solution heat-treating temperature to near room temperature. The microstructure of the alloys after solution treatment is mainly composed of g-Ni solid solution, CrB and small quantity of M7C3, as shown in Fig. 9. After solution treatment, the solute atoms and a certain number of vacant lattice sites were retained in solution. The structure is supersaturated with respect to the solute and is unstable. It is the optimum condition for precipitation hardening. When the alloys were reheated at a suitable temperature for a suitable time, the desolation transformation and precipitation process occurred. The conditions, such as the time and temperature of aging treatment and initial state of the alloys, affect the type, fraction, size, and distribution of the precipitated particles which govern properties of the alloys. An understanding of the compound type and its morphology formed at aging treatment is beneficial to the alloy designer. The aging treatment should be carefully considered to
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produce optimum precipitate size and distribution pattern. During this process, carbon and boron reacts with the alloy elements to form carbides and borides which uniformly distribute in g-Ni solid solution matrix of the alloys. Fig. 10 is the microstructure of the alloys aging treatment at 300 C, 500 C and 700 C, respectively. During the aging treatment process, these precipitates nucleate and have a higher solute content than the matrix and the region in the matrix surrounding them is reduced in solute content. A concentration gradient is formed in the solid solution, so the solute atoms diffuse from the matrix toward the particles and the precipitates continue to grow. As the aging temperature increased, the diffusion and migration ability of the atoms improve and the desolation transformation in the supersaturated g-Ni solid solution was preceded more completely. In Fig. 10a, the desolation transformation slightly occurred in the supersaturated g-Ni solid solution due to the low aging temperature (300 C). Many small precipitates were uniformly distributed in gNi solid solution matrix. There are also a small amount of fine precipitates in M7C3 and CrB particles. When the alloys aging treatment temperature increased to 500 C, the diffusion and migration ability of the atoms further improved and the desolation transformation in the supersaturated g-Ni solid solution processed more quickly. Many relatively larger particles were uniformly precipitated in the g-Ni solid solution matrix, as shown in Fig. 10b. As the aging temperature further increased (700 C), the desolation transformation in the supersaturated g-Ni solid solution was further accelerated. According to Ostwald ripening theory of second phases in dilute solute, the smaller particles dwindle and the larger particles grow. So the precipitated particles in the alloys became coarsened. Many particles with larger size were observed in the g-Ni solid solution, as shown in Fig. 10c. Fig. 11 shows the X-ray diffraction spectra of the nickel-based superalloys in different state. According to the XRD index analysis results, the sintered and heat treated alloys almost possess the same phase constitution, but show various characteristic. They are mainly composed of g-Ni, Ni3B, CrB, and M7C3 (where M is the metallic carbide-forming elements). Comparing with the XRD spectra of the sintered alloys, the peaks of the heat treated alloy become broadened and shift a certain degree and some weak peaks disappeared, as shown in Fig. 11. During the solution treatment, some of small second particles redissolve into g-Ni solid solution and a supersaturated g-Ni solid solution was obtained. It results in the variation of lattice constants and decreases the crystalline perfection of the g-Ni solid solution. When the alloys were continuously engaged in aging treatment after solution treatment, the supersaturated g-Ni solid solution is decomposed and many small particles were uniformly precipitated in the g-Ni solid solution matrix. Some sharp and weak diffraction peaks appeared in X-ray diffraction spectra of the aging treated alloys. Stronger and sharper diffraction peaks were observed in the X-ray diffraction spectra as the temperature of aging treatment increased. As the aging temperature further increased, the decomposition process of supersaturated g-Ni solid solution was more complete. It increases the crystalline perfection of the g-Ni solid solution. On the other hand, the precipitated particles in the alloys became coarsen according to Ostwald ripening theory. 3.5. Hardness of the heat treated alloys
Fig. 9. Microstructure of the alloys after solution treatment.
The influence of heat treatment on the hardness of the nickelbased superalloys sintered at 1140 C is shown in Fig. 12. The hardness of the sintered alloys is relatively high, it is about 51 HRC. After solution treatment, the hardness of the alloys significantly decreased and reached the minimum values (39 HRC). In nickelbased superalloys, the hardness could be enhanced mainly by
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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Fig. 10. Microstructure of the alloys after solution and aging treatment.
Fig. 12. Hardness of the alloys in different state. Fig. 11. X-ray diffraction spectra of the alloys in different state.
two mechanisms: solid solution hardening and precipitation hardening [24]. During the solution treatment process, some of small second particles redissolve into g-Ni solid solution and a supersaturated g-Ni solid solution was obtained. In general, the strengthening effect of solid solution for materials is weaker than that of the second phase. So the hardness of the solution-treated alloys is significantly lower than that of the sintering state alloy, as shown in Fig. 12. In the subsequent aging process, the hardness of the nickel-based superalloys increased from 47 HRC to 60 HRC when aging treated at 300 C and 500 C, and then, decreased to 55 HRC when aging treated at 700 C, as shown in Fig. 12. During the aging treatment, the supersaturated g-Ni solid solution is decomposed and many particles were uniformly precipitated in the g-Ni solid solution matrix. The precipitated particles enhance the precipitation hardening and decrease the solid solution hardening because the precipitation would consume part of the solute
elements Cr, Fe and C in the matrix [27]. The high hardness of the alloys after aging at 300 C and 500 C indicated that precipitation hardening was the dominant factor for the changes in the hardness. The hardness change during aging treatment in the experimental alloy was primarily affected by the microstructural evolution of the precipitates. The maximum hardness with the value of 60 HRC appeared at the aging temperature of 500 C because a large number of tiny particles were precipitated at this aging temperature. Then, the hardness decreased slowly with the increase of the aging temperature. An obvious coarsening of precipitated particles can be seen from 500 C to 700 C due to Ostwald ripening theory and the coarsening of the precipitated particles would decrease the hardening effect of the particles [28], as shown in Fig. 10. The fact that the hardness gradually decreased during the aging from 500 C to 700 C (Fig. 12) indicated that coarsening of the precipitated particles reduced the precipitation hardening and had an adverse effect on the hardness of the alloys.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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4. Conclusions In this study, a nickel-based superalloy was fabricated by powder metallurgy, and then the sintered alloys were given solution and aging treatment. The main conclusions are summarized as follows: (1) A good quality nickel-based superalloy with a homogeneous microstructure and fine grain size can be obtained at the sintering temperature of 1140 C. (2) The sintered nickel-based superalloys mainly consist of CrB particles, Ni3B particles, M7C3 particles, (g-Ni þ Ni3B) eutectics and g-Ni solid solution matrix. (3) The calculated solidification process in the alloys is liquid / liquidþg-Ni / liquidþg-Ni þ M7C3 / liquidþgNi þ M7C3þCrB / g-Ni þ M7C3þCrBþ (g-Ni þ Ni3B). (4) Many small particles were uniformly precipitated in the matrix during the aging treatment and the particles became coarsen as the aging temperature increased. (5) The hardness of the alloys increased when aging treated at 300 C and 500 C, and then the hardness decreased when aging treated at 700 C. Acknowledgement The authors would like to thank the National Natural Science Foundation of China (No. 51371125) for their support. References [1] D.X. Wen, Y.C. Lin, H.B. Li, X.M. Chen, J. Deng, L.T. Li, Hot deformation behavior and processing map of a typical Ni-based superalloy, J. Mater. Sci. Eng. A 591 (2014) 183e192. [2] K. Wu, G.Q. Liu, B.F. Hu, F. Li, Y.W. Zhang, Y. Tao, J.T. Liu, Hot compressive deformation behavior of a new hot isostatically pressed Ni-Cr-Co based powder metallurgy superalloy, J. Mater. Des. 32 (2011) 1872e1879. [3] L.T. Chang, W.R. Sun, Y.Y. Cui, F.Q. Zhang, R. Yang, Effect of heat treatment on microstructure and mechanical properties of the hot-isostatic-pressed Inconel 718 powder compact, J. Alloy. Comp. 590 (2014) 227e232. [4] H.B. Zhang, K.F. Zhang, Z. Lu, C.H. Zhao, X.L. Yang, Hot deformation behavior and processing map of a g-hardened nickel-based superalloy, J. Mater. Sci. Eng. A 604 (2014) 1e8. [5] Y.H. Liu, Z.K. Yao, Y.Q. Ning, Y. Nan, Effect of deformation temperature and strain rate on dynamic recrystallized grain size of a powder metallurgical nickel-based superalloy, J. Alloy. Comp. 691 (2017) 554e563. [6] G.A. Rao, M. Kumar, M. Srinivas, D.S. Sarma, Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy inconel 718, Mater. Sci. Eng. A 355 (2003) 114e125. [7] A. Arifin, A.B. Sulong, N. Muhamad, J. Syarif, M.I. Ramli, Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials
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Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy Yiwen Lei a, b, *, Xiaolong Li a, Ronglu Sun a, Ying Tang a, Wei Niu a a b
School of Mechanical Engineering,Tianjin Polytechnic University, Tianjin, 300387, China Tianjin Key Laboratory of Advanced Mechatronics Equipment Technology, Tianjin, 300387, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2019 Received in revised form 18 October 2019 Accepted 30 October 2019 Available online xxx
A nickel-based superalloy was prepared through powder metallurgy (P/M). In order to balance their microstructure and properties, heat treatments were carried on the alloys. The microstructures of the alloys were characterized by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The solidification process in the alloys during the sintering process was calculated using Thermo-Calc software. A good quality nickel-based superalloy with a homogeneous microstructure and fine grain size can be obtained at the sintering temperature of 1140 C. The alloys mainly consist of irregular polygon CrB particles, irregular morphology Ni3B particles, hexagonal M7C3 particles, (g-Ni þ Ni3B) eutectics and bright g-Ni solid solution matrix. The calculated results show that the solidification process in the alloys during the sintering process is liquid / liquidþg-Ni / liquidþg-Ni þ M7C3 / liquidþ g-Ni þ M7C3þCrB / g-Ni þ M7C3þCrBþ(g-Ni þ Ni3B). The calculation results are helpful to understand the microstructure evolution in the nickel-based superalloy and agree well with the experimental data. The microstructure of the alloys after solution treatment is mainly composed of supersaturated g-Ni solid solution and M7C3. During the aging treatment, the supersaturated g-Ni solid solution decomposed and many small particles were uniformly precipitated in the g-Ni solid solution. As the aging temperature increased, the decomposition process of supersaturated g-Ni solid solution was accelerated and the precipitated particles became coarsen according to Ostwald ripening theory. The hardness of the alloys increased when aging treated at 300 C and 500 C due to the second phase strengthening effect, and then the hardness decreased when aging treated at 700 C due to the coarsening of precipitated particles. © 2019 Elsevier B.V. All rights reserved.
Keywords: Nickel-based superalloy Sintering Microstructure Heat treatment Thermo-Calc
1. Introduction Nickel-based superalloy has been widely used in nuclear reactors, sparking electrodes, furnace components and high temperature rotating parts of the advanced aero-engines owing to its excellent resistance to various types of corrosion, high temperature rupture life, fatigue crack growth resistance and creep strength [1e5]. Nickel-based superalloys are mainly produced by various melting and casting techniques or by powder metallurgy (P/M). Although several advanced melting and casting techniques have been developed for the nickel-based superalloys, the related
* Corresponding author. School of Mechanical Engineering, Tianjin Polytechnic University, Tianjin 300387, China. E-mail address: [email protected] (Y. Lei).
problems such as macro-segregation and formation of freckles could not completely be avoided in big ingots. Elimination of such segregation is a time consuming and expensive process [6]. Compared with melting and casting techniques, powder metallurgy can virtually eliminate the macro-segregation and freckles problems. It is a near-net-shape manufacturing method in which metal powder are compressed with or without other materials and then heated for solidification [7]. The powder metallurgical nickelbased superalloys with more uniform composition and microstructure, smaller microcrystalline structures and more consistent grain possess excellent mechanical properties, fatigue and oxidation resistance at elevated temperatures. They are widely used in aircraft parts or turbine blades [8]. In this study, a nickel-based superalloy was fabricated by powder metallurgy technique. The demand for improved jet engine industry has resulted in the continued development of nickel-based superalloy with higher properties. It has spurred the development of the alloys with the
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Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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desired microstructures, properties, and cost effectiveness [9]. The microstructure of the alloys, which is mainly determined by the sintering processing, plays a very important role in determining the mechanical and physical properties of superalloys [10]. In order to extend the alloys’ future application, it is pivotal to study the microstructure and properties of the P/M nickel-based superalloy after different heat treatment [11]. Nickel possesses an extensive solid solubility for many alloying elements and the microstructure of nickel-based superalloy usually consists of the face-centered cubic structure solid-solution matrix with various uniformly dispersed precipitate particles in it. It is mainly strengthened by solid-solution hardening and precipitation hardening [12]. The content of alloying elements dissolved in the matrix and the shape, size and distribution of the precipitations are significantly affected by heat treatment, which is an important step for nickel-based superalloy to achieve the desired distribution of the precipitate in the matrix and the optimum mechanical properties [13]. Previous studies illustrated that the cooling rate after solution treatment affects the size and shape of precipitates [14] and the mean size of precipitates increases with thermal exposure time during aging treatment [15]. In this study, it aims to provide experimental basis for the development of various heat treatment methods for the purpose of optimizing the microstructure and the properties of nickel-based superalloys. The heat treatment is divided into two steps, termed as solution and aging treatment [13]. The first step solution treatment is processed at 1200 C for 2 h. The second step aging treatment is at 850 C for 8 h. Alloy development is still an expensive, time consuming and generally empirical processing. This is because the alloy usually contains many elements, and the actions of alloying elements within the alloy are very complicated [16]. To reduce processing time and gain a deep and theoretical understanding of the phase transformation and microstructure evolution in the alloy during the sintering process, many alloy designers employ thermodynamic modeling tools to explore the phase transformation and the complex relationships between the phases in the alloys [17]. These thermodynamic tools provide information on a variety of aspects, such as the occurred reactions, the formed phases, as well as the mechanical properties. Different authors have used modeling tools to predict the thermodynamic of powder metallurgical processing. Wu et al. simulated the effects of alloying elements on the microstructure and thermodynamics during the liquid phase sintering process in FeeNieMoeCeB steel [18]. Sophia A. Tsipas presented the results of MoeAl co-deposition treatments on titanium alloy. Thermodynamic analysis was carried out to better understand the thermodynamics of the deposition process [19]. According to the above literatures, the thermodynamics software could be used to predict the solidification behavior and phase transformation in the nickelbased superalloy during sintering process using a Java based material processing software, Thermo-Calc software [20]. The software, based on CALPHAD (Calculation of Phase Diagrams), requires a mathematical description of the thermodynamics of the system [21]. The thermodynamic package was used to predict the various equilibria phases and microstructure evolutions that occurs during the cooling process from the sintering temperature to room temperature by a Gibbs energy minimization process [22]. The prediction data were subsequently compared to experimental results to evaluate the validity and applicability of the thermodynamic software. In the present paper, a nickel-based superalloy was prepared by powder metallurgy. The effects of sintering temperature and heat treatment on microstructure and mechanical properties are investigated. The phase constituents and microstructure of the samples were analyzed in detail. The temperature dependent phase
fractions and compositions in the nickel-based superalloy were calculated using the most developed Thermo-Calc software [20]. Combining with the calculated phase fractions and compositions, the solidification process, precipitation behavior and reaction scheme in the nickel-based superalloy during the sintering process were analyzed.
2. Experimental procedure Commercially available powders of (wt.%) Ni (10 mm, 99.5% purity), Cr (5 mm, 99% purity), Fe (10 mm, 99% purity), Si (5 mm, 99% purity) and C (20 mm, 99% purity) were used as the raw materials. A mixture of (wt.%) 15% Fe, 16% Cr, 4% Si, 0.8% C, 3% C and balance Ni was chosen in this paper. The mixture was blended on a universal planetary ball-milling at a rotation velocity of 200 rpm for 1 h with a ball-to-powder mass ratio of about 5:1. Hardened stainless steel milling vessel and 100Cr6 hardened steel balls with a diameter of 5 mm were used as milling media. After mixing, 1.5 wt% of PVB was added to the mixture to improve its compaction ability. To form disc specimens with of size 12 mm in diameter and 6 mm in height, appropriate quantities of mixed powders were weighed and poured into a stainless steel floating die and then bidirectionally pressed at 140 MPa for 3 min using zinc stearate as die lubricant. The sintering process of the green samples was carried out in a tube furnace by keeping the samples on an aluminium oxide plate under 99.999% argon atmosphere with a flow rate of 5 L/min during the sintering run. The sintering process step is shown as in Fig. 1. The thermal debinding of PVB was performed by heating the green samples to 700 C with a heating rate of 15 C/min and a dwelling time of 40 min. After thermal debinding, a pre-sintering process was performed by reaching to 1000 C with a heating rate of 10 C/ min and a dwelling time of 30 min. The final sintering process was performed at different temperature with a heating rate of 6 C/min and a dwelling time of 30 min, and followed by cooling down to the room temperature with a rate of 15 C/min. To examine the influence of the sintering temperature on sintered parts, the sintering processes were carried out at temperature of 1050 C, 1080 C, 1110 C, 1140 C and 1170 C, respectively. As the mechanical properties of the nickel-based superalloy were significantly affected by the precipitates and microstructure in the alloys [18], it is an important to investigate the effects of postsintering heat treatment on the alloys to achieve the desired microstructure [14]. The heat treatment consisted of solid solution followed by aging procedure [13]. The heat treatment was carried
Fig. 1. Sintering process step for the green samples.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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out in a furnace filled with argon to prevent the oxidation of specimens. The solution treatment was at 900 C for 1.5 h followed by quenching in water, and the aging treatment was at 300 C, 500 C and 700 C for 8 h followed by slow cooling in argon atmosphere in the furnace. For metallographic observation, the samples were cut using a high speed wire electrical-discharge machining. Mounted specimen were polished using successively finer silicon carbide abrasive papers of 350e1200 grit size followed by a sequence of 3 mm and 1 mm diamond abrasive slurry. The polished specimens were etched using an etchant (HF: HNO3: H2O ¼ 1: 2: 4) with an etching time of 10e30 s. The specimens were examined using an Olympus GX-51 optical microscopy and QUANTA200 scanning electron microscopy with an Oxford Link energy dispersive spectroscopy. The phase constituent in the alloys were identified using a Rikagu XRD with Cu Ka radiation, wavelength l ¼ 1.54056 nm, operating at an applied voltage of 40 kV and a current of 40 mA. Rockwell-C hardness test (HRC) as a function of sintering temperature and aging time under various aging temperature was carried out on polished samples. It was measured using a HRS-150 Digital Rockwell Hardness Tester under load of 150 kgf and time of 15s. The samples were polished using silicon carbide abrasive papers of 350e1200 grit size and cleaned by alcohol to avoid the effects of surface oxide films before testing. Every sample was measured five times, and then the average value is adopted in the paper. 3. Results and discussion 3.1. Microstructure observation and XRD spectrum of the sintered alloys Fig. 2 shows the morphology of the nickel-based superalloys sintered at different temperature. When the alloys were sintered at relatively low temperature (1050 C), small amount of liquid phase formed during the sintering process and the grains possessed low integrity characteristics. The morphology of grains in the alloys is relatively irregular and clutter, as shown in Fig. 2a. When the sintering temperature was 1080 C and 1110 C, more liquid phase formed during the sintering process and the grains possessed higher integrity characteristics. Some regular polygon M7C3 particles and irregular polygon CrB particles were formed in the alloys as the sintering temperature increased. However, the microstructure of the alloys is inhomogeneous and the normal grain growth occurs, as shown in Fig. 2b and c. When the sintering temperature was 1140 C, an uniform microstructure with many regular polygon M7C3 particles and irregular polygon CrB particles was formed in the alloys, as shown in Fig. 2d. As the sintering temperature further increased to 1170 C, most of the alloying elements dissolve into the liquid phase and many second phase precipitates in the coarse primary g-Ni dendrites matrix. The amount of CrB and M7C3 particles in the alloys reduced. The alloys were mainly composed of the seriously coarse dendrites with obviously over-heated characteristic, as shown in Fig. 2e. On the other hand, the hardness of the sintered samples increased from 38 HRC at 1040 C to the maximum value 51 HRC at 1140 C and then decreased to 42 HRC at1170 C, as shown in Fig. 8. According to the above analysis, a good quality nickel-based superalloys with uniform microstructure and the highest hardness can be obtained at the sintering temperature of 1140 C. Fig. 3 shows the X-ray diffraction spectra of the nickel superalloys sintering at 1140 C. The alloys mainly consist of g-Ni solid solution, CrB, Ni3B and M7C3 (where M is the metallic carbideforming elements). Fig. 4 and Fig. 5 show the SEM-EDS analysis results of different morphological phases in the alloys sintered at 1140 C. The
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composition of the irregular polygon particles (Point A) determined by SEM EDS is enriched in Cr and B. There is also a certain amount of C, as shown in Figs. 4 and 5a. The quantitative composition analysis result of Point A is (wt.%, the same as follows) 23.17B, 74.10Cr and 2.73C. The composition of the irregular morphology particles (Point B) is mainly composed of Ni and B, as shown in Figs. 4 and 5b. The quantitative analysis result of Point B is 13.13B, 73.46Ni, 8.77Fe and 4.64C. The composition of the bright matrix phase (Point C) is enriched in Ni and Fe and a certain amount of C and Si, as shown in Figs. 4 and 5c. The quantitative analysis result of Point C is 77.79Ni, 14.37Fe, 3.28Si and 4.56C. The composition of the regular hexagonal particles (Point D) is mainly composed of Cr and C and a certain amount of Ni and Fe, as shown in Figs. 4 and 5d. The quantitative analysis result of Point D is 8.91C, 69.50Cr, 10.11Ni and 11.48Fe. By a combination of XRD and EDS analyses, the irregular polygon particles (Point A) were identified as CrB, the irregular morphology particles (Point B) were Ni3B, the bright matrix phase (Point C) were g-Ni solid solution with Fe, Cr and C in it and the regular hexagonal particles (Point D) were M7C3 (where M is the Cr, Fe and Ni alloy elements). 3.2. Phase fraction and reaction scheme of the alloys The microstructure evolution in the alloys during the sintering process is complicated and unpredictable due to the different cooling rate and chemical composition [23]. To gain a theoretical understanding of the microstructure evolution in the alloys and to fabricate high quality alloys with desirable microstructure and properties, it is necessary to analyze the phase transformation, such as the temperature dependent phase fractions and compositions in the alloys during the sintering process [24]. In Fig. 10, the horizontal axis denotes the temperature and the vertical one denotes the mole fractions of the phases for the alloys, so the relationship between phase fractions and temperature can be expressed from this figure. The lines express the phase fractions and compositions calculated by Thermo-Calc [20]. According to Fig. 6 and the above analysis, the crystallization process and reaction scheme in the nickel-based superalloys during the sintering process is as follows: i) Primary g-Ni dendrites nucleated and grew up from the molten pool at 1304 C. The mole fraction of g-Ni phase continuously increased and the amount of liquid phase reduced until the temperature decreased to 950 C. The maximal mole fraction of g-Ni phase in the alloys is (mol.%, the same as follows) 57.8% which formed the bright matrix of the alloys. ii) At approximately 1137 C, M7C3 started to crystallize from the remaining liquid phase. Many M7C3 particles with regular hexagonal shape and different particle size were formed in the alloys. According to the solidification process, most of M7C3 particles distributed among the primary g-Ni dendrites and there are also some smaller M7C3 particles were in the primary g-Ni dendrites, as shown in Fig. 2d. The calculated mole fraction of M7C3 phase is about 7.2%. iii) At the temperature range of 1072 Ce950 C, CrB phase with irregular shape crystallized from the liquid. The calculated mole fraction of CrB increased from 0 to 17.7%. At 950 C. A small quantity of remaining liquid phase filled in the gaps among the g-Ni dendrites, M7C3 and CrB phase. The liquid phase reached the eutectic composition. ⅳ) At temperature range from 950 C to 929 C, g-Ni and Ni3B phase co-precipitated from the interdendritic remaining liquid phase. A binary eutectic reaction Liquid / g-Ni þ Ni3B occurred and all the remaining liquid phase transformed into
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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Fig. 2. Morphology of the alloys sintered at different temperature (a) 1050 C, (b) 1080 C, (c) 1110 C, (d) 1140 C, (e) 1170 C.
Fig. 3. X-ray diffraction spectra of the alloys sintering at 1140 C. Fig. 4. SEM morphology of the alloys sintered at 1140 C.
a eutectic (g-Ni þ Ni3B). The calculated mole fraction of the g-Ni þ Ni3B eutectic is about 22%, as shown in Fig. 6. The primary g-Ni dendrite uniformly distributed in the alloys,
while the eutectic distributed among g-Ni dendrites, M7C3 and CrB phase, as shown in Fig. 2d.
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Fig. 5. SEM-EDS results of different phases in the alloys sintered at 1140 C. (a) Point A, (b) Point B, (c) Point C, (d) Point D.
Fig. 6. The phase fractions versus temperature in the nickel-based super alloys.
ⅴ) When the temperature was lower than about 929 C, all the liquid phase transformed into solid phase and a solid-state transformation took place in the alloy. During this process, the fraction of g-Ni phase decreased from 55.5% to 53% and the fraction of M7C3 phase increased from 4 to 7% at the temperature range of 929 Ce600 C. The fraction of CrB and Ni3B almost kept unchanged, as shown in Figs. 6 and 2d. It illustrated that part of g-Ni transformed into M7C3 during solid-state transformation process. The area fraction of different phase in the morphology of the alloy sintered at 1140 C (Fig. 2d) was assessed using a quantitative metallography analyzing system. The measured area fractions of the g-Ni, M7C3, CrB particle and g-Ni þ Ni3B eutectic in this field were 48.0%, 6.4%, 19.6% and 25.9%, respectively. The calculation results obtained from Thermo-Calc software agree well with the
ones from the experiment. According to the metallographic examination and the calculation results obtained from Thermo-Calc software, the complete solidification process and reaction scheme in the alloys during the sintering process is liquid / liquidþ g-Ni / liquidþ g-Ni þ M7C3 / liquidþ g-Ni þ M7C3þCrB / gNi þ M7C3þCrBþ(g-Ni þ Ni3B), as shown in Fig. 6a. As mentioned above, a solid-phase transformation occurred in the alloys during the subsequent cooling process. To further investigate the transformation process the phase composition is calculated as a function of temperature [25]. Fig. 12 is the relationship between the phase composition and temperature in the alloys at the temperature range from 929 C to 600 C. The calculation results indicated that g-Ni phase is a solid solution with a certain amount of Cr, Fe, Si and C in it. When the temperature decreased from 929 C to 600 C, the content of Fe and Ni in g-Ni phase slightly increased, the content of Si almost kept unchanged, while the content of Cr and C slightly decreased, as shown in Fig. 7a. It indicated that an alloyed carbide M7C3 phase precipitated from g-Ni solid solution. M7C3 is an alloyed carbide with a certain amount of Ni, Fe and a small amount of B in it. In this temperature range, the content of Ni in M7C3 significantly decreased, Fe and B almost kept unchanged and Cr increased, as shown in Fig. 7b. The calculation results indicated amount of Ni diffused from M7C3 to gNi phase and it resulted in the decreasing of Ni and increasing of Cr in M7C3. CrB is an alloyed boride with a certain amount of Ni and Fe in it and its composition almost remain unchanged during the subsequent cooling process, as shown in Fig. 7c. Ni3B is also an alloyed compound with a small amount of Cr, Fe and C in it. The content of these alloying elements decreased during the subsequent cooling process, as shown in the calculation results Fig. 7d. It indicated that a slight amount of alloyed carbide M7C3 phase precipitated from Ni3B phase. Above analysis illustrated that amount of M7C3 precipitated in g-Ni and Ni3B phase during solidstate transformation process. It lead to the decreasing fraction of gNi phase and the increasing fraction of M7C3 phase at the temperature range of 929 Ce600 C, as shown in Fig. 6.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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Fig. 7. The phase compositions versus temperature in the alloys. (a) g-Ni, (b) M7C3, (c) CrB, (d) Ni3B.
The calculated results are well consistent with the experimental data of the nickel-based superalloys. It is beneficial to helping understand the solidification process and reaction scheme of the nickel-based superalloy during sintering process and the alloy design for desirable microstructural and mechanical properties [26]. 3.3. Hardness of the sintered alloys The influence of sintering temperature on the hardness of the alloys is shown in Fig. 8. The alloys sintered at 1050 C has a hardness of 38 HRC, the lowest hardness value in this research work. When the sintering temperature is relatively low, small amount of liquid phase was forms in the compacts and it is difficult to ensure high bonding strength among the grains. It results in low hardness of the alloy, as shown in Fig. 2a. This explains the low hardness of the alloys sintered at 1050 C. At the sintering temperature of 1140 C, the hardness of alloys increased to the maximum value 51 HRC. At this sintering temperature, it is capable of generating enough amount of liquid phase and it results in high bonding strength among the grains. The alloys are perfectly crystallized with fine and uniform microstructure and many regular shaped carbides and irregular shaped borides were formed in the alloys, as shown in Fig. 2d. According to fine-grained strengthening and second-phase strengthening theories, the alloys sintered at 1140 C possessed the maximum value 51 HRC. At the sintering
Fig. 8. Hardness of the alloys sintered at different temperature.
temperature of 1170 C, the hardness decreased to 42 HRC, a drop of 17% compared to the alloys sintered at 1140 C. At this sintering temperature, the average size of the grains in the alloys significantly increased, and the amount of carbides and borides in the alloys substantially reduced. The microstructure of alloys mainly
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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composed of coarse dentritics and the alloys were over-heated, as shown in Fig. 2e. It resulted in the dropped hardness for the alloys sintered at 1170 C.
3.4. Microstructure and XRD spectrum of the heat treated alloys To achieve the better precipitation-hardening effect, the solution treatment is key factor to obtain the maximum amounts of the soluble hardening alloying elements dissolving in the alloy matrix and it should be carefully designed to meet the requirements. During this process, it is necessary to produce a solid solution by heating the alloy to a temperature range above the solvus temperature, holding at this temperature range for a sufficient time to obtain a quasi single-phase structure, and then retaining this structure at ambient temperatures by cooling rapidly. The appropriate solution temperature and soak time is required to obtain a satisfactory degree of solution and to achieve good homogeneity of the solid solution. In general, the solubility of the alloying element in the solid solution increases with the increasing of the temperature. During the solution treatment, part of the small second phase redissolved into g-Ni solid solution, almost all of the eutectic transformed into g-Ni solid solution and a nearly homogeneous solid solution matrix was achieved as the alloys were soaked in a furnace at 900 C for a long enough time. And then the alloys were quenched in water at room temperature. Due to the rapid cooling process, the alloying element can not precipitate from the solid solution and a supersaturated solid solution was obtained. This process can preserve the solid solution formed at the solution heat-treating temperature to near room temperature. The microstructure of the alloys after solution treatment is mainly composed of g-Ni solid solution, CrB and small quantity of M7C3, as shown in Fig. 9. After solution treatment, the solute atoms and a certain number of vacant lattice sites were retained in solution. The structure is supersaturated with respect to the solute and is unstable. It is the optimum condition for precipitation hardening. When the alloys were reheated at a suitable temperature for a suitable time, the desolation transformation and precipitation process occurred. The conditions, such as the time and temperature of aging treatment and initial state of the alloys, affect the type, fraction, size, and distribution of the precipitated particles which govern properties of the alloys. An understanding of the compound type and its morphology formed at aging treatment is beneficial to the alloy designer. The aging treatment should be carefully considered to
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produce optimum precipitate size and distribution pattern. During this process, carbon and boron reacts with the alloy elements to form carbides and borides which uniformly distribute in g-Ni solid solution matrix of the alloys. Fig. 10 is the microstructure of the alloys aging treatment at 300 C, 500 C and 700 C, respectively. During the aging treatment process, these precipitates nucleate and have a higher solute content than the matrix and the region in the matrix surrounding them is reduced in solute content. A concentration gradient is formed in the solid solution, so the solute atoms diffuse from the matrix toward the particles and the precipitates continue to grow. As the aging temperature increased, the diffusion and migration ability of the atoms improve and the desolation transformation in the supersaturated g-Ni solid solution was preceded more completely. In Fig. 10a, the desolation transformation slightly occurred in the supersaturated g-Ni solid solution due to the low aging temperature (300 C). Many small precipitates were uniformly distributed in gNi solid solution matrix. There are also a small amount of fine precipitates in M7C3 and CrB particles. When the alloys aging treatment temperature increased to 500 C, the diffusion and migration ability of the atoms further improved and the desolation transformation in the supersaturated g-Ni solid solution processed more quickly. Many relatively larger particles were uniformly precipitated in the g-Ni solid solution matrix, as shown in Fig. 10b. As the aging temperature further increased (700 C), the desolation transformation in the supersaturated g-Ni solid solution was further accelerated. According to Ostwald ripening theory of second phases in dilute solute, the smaller particles dwindle and the larger particles grow. So the precipitated particles in the alloys became coarsened. Many particles with larger size were observed in the g-Ni solid solution, as shown in Fig. 10c. Fig. 11 shows the X-ray diffraction spectra of the nickel-based superalloys in different state. According to the XRD index analysis results, the sintered and heat treated alloys almost possess the same phase constitution, but show various characteristic. They are mainly composed of g-Ni, Ni3B, CrB, and M7C3 (where M is the metallic carbide-forming elements). Comparing with the XRD spectra of the sintered alloys, the peaks of the heat treated alloy become broadened and shift a certain degree and some weak peaks disappeared, as shown in Fig. 11. During the solution treatment, some of small second particles redissolve into g-Ni solid solution and a supersaturated g-Ni solid solution was obtained. It results in the variation of lattice constants and decreases the crystalline perfection of the g-Ni solid solution. When the alloys were continuously engaged in aging treatment after solution treatment, the supersaturated g-Ni solid solution is decomposed and many small particles were uniformly precipitated in the g-Ni solid solution matrix. Some sharp and weak diffraction peaks appeared in X-ray diffraction spectra of the aging treated alloys. Stronger and sharper diffraction peaks were observed in the X-ray diffraction spectra as the temperature of aging treatment increased. As the aging temperature further increased, the decomposition process of supersaturated g-Ni solid solution was more complete. It increases the crystalline perfection of the g-Ni solid solution. On the other hand, the precipitated particles in the alloys became coarsen according to Ostwald ripening theory. 3.5. Hardness of the heat treated alloys
Fig. 9. Microstructure of the alloys after solution treatment.
The influence of heat treatment on the hardness of the nickelbased superalloys sintered at 1140 C is shown in Fig. 12. The hardness of the sintered alloys is relatively high, it is about 51 HRC. After solution treatment, the hardness of the alloys significantly decreased and reached the minimum values (39 HRC). In nickelbased superalloys, the hardness could be enhanced mainly by
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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Fig. 10. Microstructure of the alloys after solution and aging treatment.
Fig. 12. Hardness of the alloys in different state. Fig. 11. X-ray diffraction spectra of the alloys in different state.
two mechanisms: solid solution hardening and precipitation hardening [24]. During the solution treatment process, some of small second particles redissolve into g-Ni solid solution and a supersaturated g-Ni solid solution was obtained. In general, the strengthening effect of solid solution for materials is weaker than that of the second phase. So the hardness of the solution-treated alloys is significantly lower than that of the sintering state alloy, as shown in Fig. 12. In the subsequent aging process, the hardness of the nickel-based superalloys increased from 47 HRC to 60 HRC when aging treated at 300 C and 500 C, and then, decreased to 55 HRC when aging treated at 700 C, as shown in Fig. 12. During the aging treatment, the supersaturated g-Ni solid solution is decomposed and many particles were uniformly precipitated in the g-Ni solid solution matrix. The precipitated particles enhance the precipitation hardening and decrease the solid solution hardening because the precipitation would consume part of the solute
elements Cr, Fe and C in the matrix [27]. The high hardness of the alloys after aging at 300 C and 500 C indicated that precipitation hardening was the dominant factor for the changes in the hardness. The hardness change during aging treatment in the experimental alloy was primarily affected by the microstructural evolution of the precipitates. The maximum hardness with the value of 60 HRC appeared at the aging temperature of 500 C because a large number of tiny particles were precipitated at this aging temperature. Then, the hardness decreased slowly with the increase of the aging temperature. An obvious coarsening of precipitated particles can be seen from 500 C to 700 C due to Ostwald ripening theory and the coarsening of the precipitated particles would decrease the hardening effect of the particles [28], as shown in Fig. 10. The fact that the hardness gradually decreased during the aging from 500 C to 700 C (Fig. 12) indicated that coarsening of the precipitated particles reduced the precipitation hardening and had an adverse effect on the hardness of the alloys.
Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882
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4. Conclusions In this study, a nickel-based superalloy was fabricated by powder metallurgy, and then the sintered alloys were given solution and aging treatment. The main conclusions are summarized as follows: (1) A good quality nickel-based superalloy with a homogeneous microstructure and fine grain size can be obtained at the sintering temperature of 1140 C. (2) The sintered nickel-based superalloys mainly consist of CrB particles, Ni3B particles, M7C3 particles, (g-Ni þ Ni3B) eutectics and g-Ni solid solution matrix. (3) The calculated solidification process in the alloys is liquid / liquidþg-Ni / liquidþg-Ni þ M7C3 / liquidþgNi þ M7C3þCrB / g-Ni þ M7C3þCrBþ (g-Ni þ Ni3B). (4) Many small particles were uniformly precipitated in the matrix during the aging treatment and the particles became coarsen as the aging temperature increased. (5) The hardness of the alloys increased when aging treated at 300 C and 500 C, and then the hardness decreased when aging treated at 700 C. Acknowledgement The authors would like to thank the National Natural Science Foundation of China (No. 51371125) for their support. References [1] D.X. Wen, Y.C. Lin, H.B. Li, X.M. Chen, J. Deng, L.T. Li, Hot deformation behavior and processing map of a typical Ni-based superalloy, J. Mater. Sci. Eng. A 591 (2014) 183e192. [2] K. Wu, G.Q. Liu, B.F. Hu, F. Li, Y.W. Zhang, Y. Tao, J.T. Liu, Hot compressive deformation behavior of a new hot isostatically pressed Ni-Cr-Co based powder metallurgy superalloy, J. Mater. Des. 32 (2011) 1872e1879. [3] L.T. Chang, W.R. Sun, Y.Y. Cui, F.Q. Zhang, R. Yang, Effect of heat treatment on microstructure and mechanical properties of the hot-isostatic-pressed Inconel 718 powder compact, J. Alloy. Comp. 590 (2014) 227e232. [4] H.B. Zhang, K.F. Zhang, Z. Lu, C.H. Zhao, X.L. Yang, Hot deformation behavior and processing map of a g-hardened nickel-based superalloy, J. Mater. Sci. Eng. A 604 (2014) 1e8. [5] Y.H. Liu, Z.K. Yao, Y.Q. Ning, Y. Nan, Effect of deformation temperature and strain rate on dynamic recrystallized grain size of a powder metallurgical nickel-based superalloy, J. Alloy. Comp. 691 (2017) 554e563. [6] G.A. Rao, M. Kumar, M. Srinivas, D.S. Sarma, Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy inconel 718, Mater. Sci. Eng. A 355 (2003) 114e125. [7] A. Arifin, A.B. Sulong, N. Muhamad, J. Syarif, M.I. Ramli, Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials
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Please cite this article as: Y. Lei et al., Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152882