The Effects of Thermo-Mechanical Treatments on Microstructure and High-Temperature Mechanical Properties of a Nickel-Based Superalloy

Abstract

The effects of thermo-mechanical treatment and different annealing temperatures on the microstructure and mechanical properties of a nickel-based superalloy were investigated by metallographic microscope, scanning electron microscope, and mechanical properties measurements. The results demonstrated that the tensile strength and elongation of the hot-rolled samples were higher than those of the annealed ones. The ultimate engineering stress and engineering strain of the studied samples solid solution treated at 1175 °C for 4 h were 709 ± 19.8 MPa and 87.2 ± 1.4%, and the product of strength times elongation (PSE) was 61.8 GPa·%. These findings indicated that the thermo-mechanical treatment was an effective method to improve both the strength and the ductility of the nickel-based superalloy.

1. Introduction

Superalloys refers to alloys with the ability to serve under high-temperature (above 600 °C) conditions. For example, the temperature in nuclear reactors where high-temperature alloys are used can reach over 600 °C. The severed superalloys should have high strength, good oxidation resistance, and anti-corrosion property. Chemical compositions of typical superalloys usually include Fe, Ni, Co, Cr, Mo, Al, Ti, Nb, and other elements [1,2,3]. Among these, nickel-based superalloy has aroused extensive attention due to its excellent comprehensive properties in high-temperature resistance, corrosion resistance, fatigue resistance, and radiation resistance. Directionally solidified (single-crystal) nickel-based superalloys are used in advanced aero-engines and gas turbine blades at the hot-end parts [4,5,6]. In the nuclear industry, nickel-based superalloys have been considered as key essential structural materials in advanced nuclear power systems [7,8,9]. Nickel-based superalloy has been broadly used in chemical processing, as well as the energy, nuclear fuel, aviation, aerospace, and military realms [10,11]. The long-term safe operation of nuclear reactors depends on the stability and reliability of core materials in extreme environments, in a great extend [12,13,14]. Therefore, further research on nickel-based superalloys is significantly important, and its future development inexorably tends to highlight the development of low production cost, high strength, and high comprehensive performance. Lee reported that the Inconel 690 alloy was dissolved at 1100 °C for 30 min and annealed at 700 °C for 16 h [15], and the true stress of the tensile specimen reached 764 MPa at room temperature. Amanov [16] reported that after a local heat treatment combined with ultrasonic surface modification nanocrystals process, the ultimate tensile strength (UTS, σ_UTS) and elongation (δ) of the 690 alloy at room temperature reached 996 MPa and 15.7% [17,18,19,20]. The nickel-based superalloys were the candidate materials for the nuclear reactor materials, which severed at nearly 900 °C with an almost permanent service-life. Its mechanical properties, including high strength and high ductility, were essential to extend the nickel-based superalloy’s application. This work aimed to improve the strength and performance of the nickel superalloy under room temperature and high temperature by exploring the influence of the heat treatment process on its microstructure and mechanical properties.

2. Experimental Procedure

Pure Mo (3N), pure Fe(3N), pure Mn(3N5), pure Cr(3N5), pure W(3N), and pure Ni (4N) blocks were prepared for the studied alloy. The raw materials were weighed and prepared according to the designed composition, as shown in

Table 1. Chemical composition of the studied nickel-based superalloy (wt.%).

Fe	Mo	Mn	Cr	W	Ni
12.3	7	1.7	20	3	Bal.

, to fabricate the studied nickel-based superalloy with a vacuum induction furnace. Twice-repeated remelting ensured the uniform and consistent composition of the nickel superalloy. Afterwards, the alloy melt was poured into a steel mold, resulting in ingots with dimensions of 200 × 50 × 30 mm. The alloy ingot was homogenized at 1200 °C for 24 h and with air cooling subsequently.

Deformation of the studied alloy under both high and room temperature was conducted on a two-roll mill, which has a roller dimeter of 420 mm, a roll rotational velocity of 30 m/min. The homogenized alloy ingot was firstly processed by hot rolling at 1000 °C by 60%. The rolled samples with cold rolling by 30%, cold rolling by 50%, and annealing under different temperatures were selected to optimize the thermomechanical treatment. The specimens for the mechanical test were sliced from the rolled plates. The tensile direction is parallel to the rolling direction. The non-standard dog bone sample measured 26 mm in length, with a gauge of 8 mm. Then, the tensile mechanical properties were tested at room temperature and high temperature (900 °C). The microstructure of the as-cast and hot-rolled samples, along with the tensile fracture morphologies at room temperature, were captured. After homogenization at 1200 °C for 24 h, hot rolling at 1175 °C, and solution treatment at different temperatures (1100 °C, 1175 °C, and 1200 °C) for 4 h, the tensile fracture morphologies were captured after the tensile was tested at room temperature and 900 °C. The microstructure of the alloy was portrayed by a metallographic microscope (model: DM-4000M). The mechanical properties were tested using a tensile testing machine (model: UTM 5105, China) at a tensile rate of 1 mm/min at room temperature and 0.5 mm/min at high temperature (900 °C). The fracture morphologies of the tensile specimens were characterized by a field emission scanning electron microscope(FEI, Hillsboro, OR, USA) with 20 kV accelerating voltage.

3. Results and Discussion

3.1. Microstructure of As-Cast Specimens

Figure 1 shows the evolution of microstructure in as-cast and hot-rolled specimens. Dendritic structure with dendritic segregation was detected in the as-cast alloy, and the spacing between secondary dendrite arms was approximately 34 ± 6 μm. After hot rolling, the microstructure was refined, with a grain size smaller than 10 μm. As the coarse grains in the alloy were refined, a significant improvement in mechanical properties of the studied alloy could then be witnessed. The crystal arrangement at the grain boundary was very irregular. The crystal face was interlaced with teeth, bit each other in jagged rows, strengthened the bonding force between metals, and consequently improved the strength of the alloy [21,22,23,24].

Figure 2 shows the elemental distribution in the cast state of the test alloy. It can be seen from the figure that the alloying elements are fairly uniformly distributed within the nickel phase with no obvious precipitate phase formed. Therefore, after subsequent homogenization treatment, the material primarily forms a single-phase solid solution internally.

Figure 3 shows the EBSD results of the rolled studied alloy, which indicates refined grains with high solid solutes atoms. After the experimental high-temperature alloy underwent homogenization treatment, the alloying elements were fully diffused. When the samples, after homogenization treatment, underwent hot rolling deformation, they experienced significant plastic deformation at high temperatures, leading to grain tilt and relative sliding, which caused substantial distortion at the local grain boundaries, becoming high-energy regions where dynamic recrystallisation occurred. As the plastic deformation progressed, dynamic recrystallisation continued and gradually expanded throughout the entire sample, ultimately resulting in a high-temperature alloy material with very fine grains.

3.2. Influences of Hot Rolling on the Mechanical Properties

Figure 4 shows the tensile stress–strain curves of the homogenized and hot-rolled samples. The ultimate tensile strength and the elongation at the break of the homogenized sample was 656 MPa and 56.7% (tested at room temperature), while the results for the hot-rolled one were 944 MPa and 34.8%, correspondingly, shown in Figure 4a, which depicts a 288 MPa higher UTS in the hot-rolled sample than the homogenized one, while the elongation rate is 21.9% lower. As for the samples tested at 900 °C, the ultimate tensile strengths of the homogenized and hot-rolled sample were only 165 MPa and 170 MPa, respectively.

3.3. The Effect of Heat Treatment on the Hot-Rolled Samples’ Strength

After homogenizing some as-cast samples at 1200 °C for 24 h and hot-roll them at 1000 °C to a deforming extent of 60%, the samples underwent heat treatment for 4 h under three different temperatures (1100 °C, 1175 °C, and 1200 °C). Figure 5 shows the room-temperature engineering stress–strain curves of the hot-rolled alloy heat-treated for 4 h under different temperatures. After homogenization at 1200 °C for 4 h and hot rolling at 1000 °C with 60% deformation, the as-hot-rolled samples were directly sent to a tensile test. The UTS and elongation at the break of the as-hot-rolled samples were 949 MPa and 33.7% (tested at room temperature). After annealing for 4 h at 1100 °C, 1175 °C, and 1200 °C, the tensile strength of samples tested at room temperature subsided to 752 MPa, 709 MPa, and 701 MPa, and the elongation at the break ascended to 75.7%, 87.2%, and 87.1%, respectively. Apparently, the tensile strength of the sample decreases with an increase in heat treatment temperature, owing to the fact that the particles grow at high temperature: the higher the temperature, the faster the growth. However, the sample heat-treated at 1175 °C for 4 h performed the highest elongation rate.

Figure 6 shows high-temperature engineering stress–strain curves of the hot-rolled alloy heat-treated for 4 h at different temperatures. The UTS and the elongation at the break of the as-hot-rolled samples tested at 900 °C were 156 MPa and 59.6%. After annealing for 4 h at 1100 °C, 1175 °C and 1200 °C, the high-temperature tensile strength reached 172 MPa, 183 MPa, and 199 MPa, and the elongation at break was 77.1%, 92.6%, and 92.0%, respectively. It has been observed that as the annealing temperature increases, the high-temperature tensile strength of the sample also rises.

As Figure 5 and Figure 6 above depict, the engineering stresses of the studied alloy tested at room-temperature and high-temperature tensile strengths varied in a similar trend, although they were supported by different deformation mechanisms. At the beginning of the room-temperature tensile process, the engineering stress increased with the engineering strain by a linear relationship, remaining in an elastic deformation stage. Then, the stress increased slowly and reached the yield stress (σs), transforming into a plastic deformation stage. The plastic deformation at room temperature was due to the motivation of dislocations, and with an increase in the strain, work hardening started to take place. As for the high-temperature tensile, at the beginning of the process, the stress curve also witnessed a linear part, representing the stage of elastic deformation. However, the engineering stress decreased rapidly after reaching a peak value, transforming into the section for plastic deformation. This is due to the fact that the theory for plastic deformation at high temperature was dynamic recovery and recrystallization. Compared to the softening effect from the dynamic recovery and recrystallization, the effects of work hardening were negligible, which made the engineering stress decrease gradually. Compared with the as-hot-rolled alloys, the room-temperature tensile strength of the heat-treated alloys decreased, while the plasticity increased significantly, owing to the recrystallization of the alloy during the annealing process. The hot-rolled alloy was treated by annealing at 1175 °C and 1200 °C, both for 4 h. During the annealing process, grains grew in the alloy, and the grain size at 1200 °C was larger than that at 1175 °C. According to the Hall–Petch equation, a smaller grain size correlates with a higher strength of the alloy. The grain size increased during the annealing process, which made UTS decrease and the elongation increase.

3.4. Fracture Morphology Analysis

Figure 7 shows the fracture morphology of the tensile samples (tested at room temperature). Dimples in various sizes were detected in different areas of the homogenized samples (Figure 5b), and, at the bottom of several dimples, we found some particles (Figure 5c). The hot-rolled samples contain fewer precipitates but more minor dimples than the homogenized samples, which indicates that with dynamic recrystallization, hot working could result in samples with higher ductility.

Figure 8 shows the side view of a fracture, illustrating the development behavior of cracks. The majority of cracks were expanding inside the grains, rather than propagating along grain boundaries. This indicates components reinforcing grain boundaries, which brings grain boundaries higher strengths and could satisfy the requirements of high-temperature strength and creep-resisting property for alloys.

Figure 9 shows the fracture morphology of the tensile samples tested at room temperature. The room-temperature tensile strength of the alloy, annealed at 1175 °C, was higher than that at 1200 °C, as shown in Figure 5. The fracture was plastic, and precipitation particles formed during the high-temperature treatment were found in the dimples of the incision, while more precipitation particles were observed in the samples treated at 1175 °C for 4 h.

3.5. Influences of Cold Rolling on the Mechanical Properties

Figure 10a,b shows that the engineering stress–strain curves of the annealed samples underwent a cold rolling process after 1175 °C/4 h solution treatment, which were tested under both room temperature (a) and at 900 °C (b). After cold rolling, the UTS increased from 709 MPa to 1223 MPa at room temperature, while decreased from 183 MPa to 104 MPa at 900 °C. Cold rolling is an effective measure in improving the strength of an alloy serving under room-temperature, but it is unsuitable for high-temperature conditions. Figure 10c,d shows the tensile curves of alloys tested at room-temperature and 900 °C, whose samples were cold rolled twice (first deformed by 30% and then 50%), along with intermediate annealing at 950 °C for 1 h. After a second cold rolling, the strength of the alloy under room temperature increased a bit, while the elongation witnessed an obvious 45% improvement. However, the high-temperature strength, which usually stands for serving performances of the alloy, dropped significantly, by 17%, compared with that of those samples that were cold rolled only once (by 30%). This indicates that multiple times of cold working with well-designed heat treatment works wonderfully on optimizing alloys’ mechanical properties at room temperature, whereas it is not the case for the 900 °C serving condition. Dual cold rolling would only worsen the performance of the nickel superalloy, as it serves under an extremely high temperature.

3.6. Comparison in Mechanical Properties of Alloys by Different Process

Figure 11 shows the mechanical properties of the studied alloy and Inconel 690 alloys manufactured through other fabrication processes, including UTS and elongation [5,11,16,25,26,27,28,29,30]. Compared with superalloys fabricated by other processes, the samples in this study show an outstanding comprehensive property. The ultimate engineering stress and engineering strain at room temperature of the studied samples, with a solution treatment at 1175 °C for 4 h, were 709 ± 19.8 MPa and 87.2 ± 1.4%, and the product of strength times elongation (PSE) was 61.8 GPa·%. Moreover, the studied nickel-based superalloy shows an excellent high-temperature tensile strength, which reaches 199 MPa after the alloys underwent a 1200 °C/4 h solution treatment, and is almost 20% higher than that in the as-hot-rolled state. These test results proved the feasibility of achieving high strength-elongation combination and the validity of a series of thermo-mechanical treatment precisely designed in this study.

4. Conclusions

(1)

Thermo-mechanical treatment could improve the strength of the alloy, and the grain size of the hot-rolled sample was refined. Hot-rolled alloys achieved high tensile strength under room temperature. Its room-temperature tensile strength was 949 MPa, and the fracture elongation rate was 33.7%. Moreover, its high-temperature (900 °C) tensile strength was 156 MPa, and the fracture elongation rate was 59.6%.

(2)

According to results of the tensile tests, with the elevation of annealing temperature, the room-temperature tensile strength of the alloy decreased, whereas the strength increased at 900 °C. When the annealing temperature was 1100 °C, the UTS was 752 MPa at room temperature. The UTS of alloys under 900 °C was 199 MPa when the annealing temperature was 1200 °C.

(3)

The rupture morphology of the room-temperature and 900 °C tensile test samples were captured, and the precipitated particles formed during high-temperature annealing were detected. There were some local oxidation phenomena on the fracture surface of the high-temperature test samples.

(4)

The ultimate engineering stress and engineering strain of the studied superalloys with solid solution treated at 1175 °C for 4 h were 709 ± 19.8 MPa and 87.2 ± 1.4%, which achieved the highest strength times elongation (PSE) value, up to 61.8 GPa·%.

The studied superalloy exhibits high-temperature mechanical properties with single-phase and super-refined grains; this shows promising prospects, as it may serve under high-temperature conditions, such as nuclear reactors.