2023 Volume 64 Issue 6 Pages 1250-1256
In this paper, Fe–20Mn–xNb alloys were prepared by ball milling and spark plasma sintering techniques. Phase constituents were characterized by X-ray diffractometer and Microstructure were characterized by scanning electron microscopy, and the tensile properties, hardness, compactness and damping properties were tested as well. The results show that after ball milling, the metal powders of Fe–20Mn–3Nb first exhibited an irregular plate-like or flake-like morphology, then large irregular particles were broken into fine particles. The addition of Nb element significantly increases the content of γ and ε phase, promoting the formation of γ and ε, and Fe2(Nb, Mn) phase precipitates in Fe–20Mn–3Nb alloy. And mechanical properties of the alloy are significantly improved by adding Nb element. The UTS and YS of Fe–20Mn–3Nb alloy are the highest, which are 818 MPa and 628.7 Mpa respectively, while the elongation of Fe–20Mn–0.5Nb alloy is the highest, which is 31.7% (20.2% higher than that of 0Nb alloy). The hardness and density of the alloy can be increased by adding Nb. The fracture mechanism of Fe–20Mn–xNb (x = 0.5, 1.5, 3) alloys is typical ductile fracture, while the fracture mechanism of Fe–20Mn alloy is quasi-xcleavage fracture. The damping property of Fe–20Mn alloys increases with increasing strain. The addition of Nb can significantly improve the damping properties of Fe–Mn alloys.
In recent years, high damping alloys such as Twin Crystal alloys (Mn–Cu),1,2) Ferromagnetic alloys (Fe–Cr, Fe–Al)3) and FeMn-based damping alloys4–6) have attracted much attention due to their excellent damping properties in construction, ships and industrial equipment. Among many damping alloys, the new Fe–Mn damping alloy has the advantages of excellent mechanical properties, low price and good damping property of high strain amplitude, and has a good application prospect.7–10) The damping properties of Fe–Mn alloy are mainly derived from the interface movement between ε-martensite and γ-austenite, relative lamellar movement within austenite, stacking fault of martensite and interlamellar movement of martensite.7,11–14) Nb has an important effect on the properties of Fe–Mn alloy. The addition of Nb element to Fe–Mn base alloy can inhibit the reduction of damping property of Fe–Mn base alloy after long time aging, and the precipitated NbC carbide can promote the transition from γ to ε phase.15–17)
At present, Fe–Mn damping alloy is mainly prepared by vacuum melting technology to obtain ingots, and then plastic deformation to obtain samples.5,18–22) During the smelting process, the Mn element is easy to volatilize, serious component segregation. The process is complex and long material preparation cycle, which limits the practical engineering application of the material to a certain extent. Powder metallurgy is a kind of energy saving, material saving, high efficiency, net forming, less pollution of the advanced manufacturing technology, can complete material preparation and molding at the same time.23–25) Spark plasma sintering (SPS), as a new sintering technology of powder metallurgy, has the advantages of uniform heating, fast heating, low sintering temperature, short sintering time and easy microstructure control, which can be used to prepare high performance nanocrystalline or ultrafine metal materials.26,27) High density Fe–Mn alloys were prepared by ball milling and SPS.28) The nanocrystalline Fe–Mn alloy was prepared by SPS technique with a yield strength of 1428 MPa in compression test.29,30) However, there are few reports on the application of the above methods in the preparation of Fe–Mn damping alloys, especially the systematic study on the milling process, tensile properties and fracture morphology.29,31)
In this work, Fe–20Mn–xNb (x = 0, 0.5, 1.5, 3 wt%) damping alloy was prepared by ball milling and SPS process, and the microstructure, mechanical properties and damping properties of the alloy were systematically studied, which provides a basis for future research and engineering applications of the Fe–Mn-based damping alloy.
The Fe–20Mn–xNb alloys were prepared using electrolytic Fe(99.9%), electrolytic Mn(99.5%), electrolytic Nb(99.9%) elemental powders (Jiangsu Vilory Advanced Materials Technology Co. Ltd., Jiangsu, China) via a combination of ball milling and SPS technology. The mixed powders were milled in a planetary ball mill together with stainless steel pot (0.5 L in volume) and stainless steel ball (10 mm and 6 mm in diameter, weight ratio 3:1) at a speed of 400 rpm for 50 hours, with a ball to powder weight ratio of 10:1 in argon atmosphere. Subsequently, the milled powders were sintered into a cylindrical sample with a dimension of 30 mm in diameter and 8 mm in height using a graphite die at 1000°C for 20 min with a heating rate of 100°C/min under a uniaxial pressure of 30 MPa by spark plasma sintering equipment (SPS, LABOX-350, Japan).
The phases of the milled powders and sintered alloys were investigated by a DX-2700 X-ray diffractometer (XRD, Dandong Haoyuan Instrument Co. Ltd., Dandong, China). Test conditions: Cu target Kα ray, tube voltage of 40 kV, tube current of 40 mA, scanning angle range of 30°∼100°, step angle of 0.06°/s, and sampling time of 0.5 s. Microstructure of the milled powders and sintered alloys were characterized by the Quanta 450 FEG field emission scanning electron microscope (FEI Company, Hillsboro, OR, USA). The model of EDS spectrometer is Smartedx, and the analysis modes is element list. Each element is measured 5 times, and its standard deviation is 0.5%. The room temperature tensile test was carried out by using ETM-105D universal testing machine (Shenzhen Wance Test Equipment Co., Ltd., Shenzhen, China). Test conditions: the original gauge length of the tensile specimen was 5 mm, the cross-sectional area was 1.5 mm × 1 mm, and the tensile rate was 0.2 mm/min. To avoid mistakes, five samples of each alloy were evaluated. The hardness of the Fe–20Mn–xNb alloys were performed at RT using Vickers with a load of 1 kgf and a dwell time of 15 s. The Vickers hardness of each alloy reported are average values of seven indentations. Compactness of alloys was determined as the ratio of density to theoretical density, and density of alloys was measured based on the Archimedes method. The damping capacity of the sample was tested by TA Q800 dynamic mechanics analyzer (DMA, TA Instruments, Inc., USA) at room temperature and evaluated by internal friction IF (Q−1). Test conditions: a single cantilever was selected as the test mode, the test frequency was 1 Hz, the amplitude was 100 µm, and the strain amplitude varying from 0 to 600 × 10−6.
Figure 1 shows the microstructure of mixed metal powders at different milling times. The original Fe powder is a nearly spherical particle, and the Mn powder is an irregular block with a size of about 76–248 um as shown in Fig. 1(a). After ball milling, the metal powders exhibited an irregular plate-like or flake-like morphology (Fig. 1(b) (c) (d) (e)), caused by compressive force, impact force, as well as the shear force from the grinding balls.32) with the increase of ball milling time, the number of plate-like or flake-like particles which have some tiny particles on its surface (Fig. 1(f)) increased significantly. After 30 hours of ball milling, the size of the particles increased to about 172–331 um (Fig. 1(e)). After 40 hours of ball milling, large irregular particles were broken into fine particles (Fig. 1(g)). After 50 hours of ball milling, fine particles increased in number and decreased in size (Fig. 1(h)), which is due to heavy working, particle fracturing soon prevails over welding, resulting in finer and more equiaxed particles after milling for more time.28)
Appearance of Fe–20Mn powder after 40 hours of ball milling: (a) 0 h; (b) 5 h; (c) 10 h; (d) 20 h; (e) 30 h; (f) as a detail of e; (g) 40 h; (h) 50 h.
Figure 2 shows the XRD patterns of Fe–20Mn mixed metal powder after ball milling for different times. Figure 2 XRD patterns of the samples: (a) XRD patterns of Fe–20Mn–3Nb powder at different milling times; (b) XRD patterns of amplified peaks near 45°. It can be seen that with the increase of milling time, the diffraction peak of Mn gradually decreases and disappears, while the diffraction peak of Fe shifts to the right. This indicates that Mn atoms are dissolved into the lattice lattice of Fe, and the displacement solid solution is formed, which causes lattice distortion. At the same time, the diffraction peak of Fe is obviously broadened as shown in Fig. 2(b). This is related to the increase of plastic deformation, dislocation density and lattice distortion of mixed metal powder during ball milling.33) After 50 hours of ball milling, the XRD of mixed metal powder is basically unchanged, and its size is small, which is more conducive to sintering. Therefore, the powder with this ball milling time is selected for subsequent sintering.
XRD patterns of the samples: (a) XRD patterns of Fe–20Mn–3Nb powder at different milling times; (b) XRD patterns of amplified peaks near 45°.
Figure 3 shows the XRD patterns of the sintered Fe–20Mn–xNb alloys. It can be seen that the alloy is composed of γ-austenite, α-martensite and ε-martensite. The diffraction peaks intensity of γ-austenite and ε-martensite increase with the addition of Nb element. In the Fe–Mn system, three types of martensitic transformation can be considered: γ-ε, γ-α′ and γ-ε-α′.34) This indicates that the addition of Nb element significantly increases the content of γ-austenite and ε-martensite phase, and promotes the formation of γ-austenite and ε-martensite. At the same time, the addition of Nb leads to the decrease of the diffraction peak intensity of α-martensite, indicating that Nb element can inhibit the formation of α-martensite.
XRD patterns of Fe–20Mn–xNb alloys.
Figure 4 shows the microstructure of Fe–20Mn–xNb alloys. It can be seen that all samples have a large size of dark gray area. The 0Nb alloy has more nearly circular black area, which is small in size, and some of it can be of nanometer grade (Fig. 4(a)). With the increase of Nb content, the amount and size of black area decreased significantly. Some fine white precipitates appeared in the 1.5Nb alloy (Fig. 4(c)). The white precipitates in the 3Nb alloy had the largest amount, and their size was close to 1 um (Fig. 4(d)).
Microstructure of Fe–20Mn–xNb alloys: (a) 0Nb; (b) 0.5Nb; (c) 1.5Nb; (d) 3Nb.
Combining the point scan results in Table 1 with surface scan results in Fig. 5 and the line scan in Fig. 6, it can be seen that Fe elements are enriched in the dark gray area, and the elements in other areas are evenly distributed. The content of Mn in the black area (Tag 1, 68.24%, wt%) was significantly higher than that in the matrix (Tag 2, 28.96%, wt%), indicating that the black area was Mn-rich.
Surface sweep of Fe–20Mn–1.5Nb alloy.
Line sweep of Fe–20Mn–1.5Nb alloy.
The white precipitate was Nb containing precipitate with Nb content up to 31.93% (Tag 9). The atomic fractions of Fe, Mn, Nb are 54.17%, 25.04% and 20.79%, respectively. Chen Feng et al. confirmed that the Nb rich white precipitated phase in Fe–17Mn–xNb alloy was Fe2(Nb, Mn).35)
3.2 Mechanical propertiesFigure 7 shows the engineering stress–strain curve of the Fe–20Mn–xNb alloys at room temperature. The tensile properties, hardness, and compactness of the alloys are shown in Table 2. As can be seen from Fig. 7, all alloys undergo plastic deformation. Mechanical properties of Fe–20Mn alloy are significantly improved by adding Nb element. With the increase of Nb content, the ultimate tensile strength (UTS) and yield strength (YS) of the alloy first decrease and then increase. Among them, the UTS and YS of 3Nb alloy are the highest, which are 818 MPa and 628.7 MPa, respectively, while the UTS and YS of 0Nb Fe–20Mn alloy are 731.5 MPa and 552.9 MPa. Elongation of Fe–20Mn alloy with Nb addition is higher than that without Nb addition, but the elongation decreases with the increase of Nb content, the elongation of 0.5Nb alloy is the highest, 31.7% (20.2% higher than that of 0Nb alloy). Combined with the analysis of SEM, the precipitation of Fe2(Nb, Mn) in the alloy greatly increases the tensile strength and yield strength of 3Nb alloy, reducing the elongation of the alloy. Therefore, the strength and elongation of Fe–20Mn alloy can be significantly improved by adding appropriate Nb element.
Tensile engineering stress-strain curves of Fe–20Mn–xNb alloys at room temperature.
As can be seen from Table 2, with the increase of Nb content, the hard Fe2(Nb, Mn) phase in the alloy increases, resulting in a gradual increase in the hardness of Fe–20Mn alloy, from 220.7HV of 0Nb alloy to 301.6HV of 3Nb alloy. The addition of Nb has a solid solution strengthening effect on the matrix, and more Fe2(Nb, Mn) hard precipitates will also increase the hardness of the alloy.35) At the same time, the compactness of Fe–20Mn alloy with Nb addition is above 98.6%, while the compactness of Fe–20Mn alloy without Nb addition is only 96.9%, indicating that adding Nb can increase the compactness of Fe–20Mn alloy.
Figure 8 shows the typical fracture surfaces of Fe–20Mn–xNb alloys with different Nb content after tensile testing. The cleavage fracture morphology of river pattern and typical dimple characteristics are observed in the Fe–20Mn alloy (Fig. 8(a)), indicating that the 0Nb alloy failed in a quasi-cleavage fracture mode, with the lowest elongation of 11.5%, as show in Table 2. Typical dimple characteristics are observed in both 0.5Nb, 1.5Nb and 3Nb samples, indicating that alloys have undergone considerable plastic deformation and failed in ductile fracture mode, with an elongation scope from 16.3% to 31.7%, as show in Table 2. In Fig. 8, it is observed that a large number of spherical particles occupy the dimples, and the formation of some empty cavities may be caused by the presence of particles in another part of the fracture samples, or the particles of the fracture samples fall off during ultrasonic cleaning and decontamination. In addition, obvious secondary cracks and micro-cavities can be observed in all samples. The dimples in 0.5 Nb alloy of Fig. 8(b) are smaller and their number density is the largest, which is consistent with the maximum elongation (31.7%). Compared with 0.5Nb alloy, the dimples and their number density of 1.5Nb alloy in Fig. 8(c) are smaller, and intergranular fracture can be found in the local region of Fig. 8(c). In Fig. 8(d), in addition to a large number of dimples, there are also large intergranular fractures, so the 3Nb alloy has a low elongation of 16.3%. The second-phase particles can be seen in all alloys, Fig. 8, and when sufficient stress is applied to break the interfacial bond between the particles and the matrix, a cavity is formed around the second phase particles, leading to the final fracture.36) Obviously, the tensile properties of Fe–20Mn alloy can be significantly improved by adding an appropriate amount of Nb element.
Tensile fractographs of Fe–20Mn (a), Fe–20Mn–0.5Nb (b), Fe–20Mn–1.5Nb (c) and Fe–20Mn–3Nb (d).
Figure 9 shows the variation of the internal friction Q−1 with the strain amplitude γ for Fe–20Mn–xNb alloys. The damping property of the alloy increases with the increase of strain amplitude. The damping mechanism of Fe–Mn alloy belongs to static hysteresis damping. When the strain amplitude increases, the movable damping sources increase, which improves the damping performance.37) With the increase of Nb content, the damping property of the alloy increases, and the damping property of 3Nb alloy is the highest. The Q−1 of the 3Nb alloy is 0.035, and the corresponding strain amplitude is approximately 600 × 10−6, which exceeds traditional damping alloys such as FeCr-based38) and sintered MnCu-based alloys,39) reaching the level of vacuum melting Fe–Mn alloy.31,40,41) Nb element refines the microstructure of the alloy and increases the mobile grain boundary. Fe2(Nb, Mn) precipitates increase the mobile energy consumption of the interface and improve the damping property.35)
The variation of the internal friction Q−1 with the strain amplitude γ for Fe–20Mn–xNb alloys.
In this paper, the effects of Nb content on the microstructure, mechanical properties and damping properties of Fe–20Mn alloy prepared by ball milling and spark plasma sintering were systematically studied. The results are as follows:
