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Influence of alloy elements (Mo, Nb, Ti) on the strength and damping capacity of Fe-Cr based alloy
Materials Science & Engineering A 667 (2016) 326–331
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of alloy elements (Mo, Nb, Ti) on the strength and damping capacity of Fe-Cr based alloy Hui Wang a,b,n, Fu Wang c,nn, Haitao Liu d, Dong Pan b, Qianfu Pan b, Yunming Liu b, Jun Xiao b, Pengcheng Zhang a a
Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box No. 9-35, Huafengxincun, Jiangyou City, Sichuan Province 621908, PR China National Key Laboratory for Nuclear Fuel and Materials, Nuclear Power Institute of China, Chengdu 610041, PR China c School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China d State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2016 Received in revised form 3 May 2016 Accepted 4 May 2016 Available online 6 May 2016
Effects of Mo-, Ti- and Nb-substitution for Al on the strength and damping capacity of the Fe-13Cr-4.5Al alloy were investigated by analyzing the mutual relationships among microstructures, strength, internal stress and damping capacity of the alloys. The obtained results show that the substitution of 0.5Mo for 0.5Al improves both the strength and damping capacity of the alloy. While the substitution of 0.5Ti or 0.5Nb for 0.5Al simply improves the strength but reduces the damping capacity of the alloy. The effect of the alloy elements on the strength and damping capacity lies in the fact that the substitution of the alloy elements generates both local internal stress and pin dislocations. Lower average internal stress leads to higher damping capacity of the alloy. The substitution of 0.5Mo for 0.5Al decreases the interactions between the dislocations and the solute atoms while increases the elastic distortions of the crystalline lattice, resulting in the enhancement of both strength and damping capacity. & 2016 Elsevier B.V. All rights reserved.
Keywords: Fe-Cr-Al based alloy Damping capacity Mechanical strength Internal stress Saturation magnetostriction
1. Introduction In recent years, the problems of vibrations and noise are increasingly serious, which not only affect the safety and service life of mechanical and military equipment, but also are severely harmful to people's physical and mental health. Therefore, the reduction of vibrations and noise is an urgent task at present. The adoption of high damping alloys is one of the effective ways to reduce noise and vibrations. It is well known that Fe–Cr based ferromagnetic damping alloys have received extensive research due to high damping capacity, good mechanical properties, corrosion resistance, wide service temperature range (with high damping capacity even at 500 °C) and the fact that the damping capacity does not vary with vibration frequency [1–4]. The damping mechanism of the Fe-Cr based ferromagnetic damping alloys is, due to the irreversible movement of magnetic domain walls, the vibration energy is transferred to heat energy which is dissipated by heat conduction [2,3]. Internal stress is a key factor to influence the irreversible movement of magnetic n Corresponding author at: Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box No. 9-35, Huafengxincun, Jiangyou City, Sichuan Province 621908, PR China. nn Corresponding author. E-mail address: [email protected] (H. Wang).
http://dx.doi.org/10.1016/j.msea.2016.05.013 0921-5093/& 2016 Elsevier B.V. All rights reserved.
domain wall, and energy fluctuation caused by local internal stress is an indispensable condition for the irreversible movement of magnetic domain walls. However, the excessive internal stresses resulting from defects such as dislocations and second phases have a strong pinning effect on magnetic domain walls, which reduces the damping capacity. Many studies showed that, in order to achieve good damping capacity, the alloys should meet certain requirements [4–7]. Firstly, the content of impurities such as C and N in the alloy must be strictly controlled. The micro internal stresses in an alloy can be significantly reduced when the impurity content is less than 0.01 wt%. Secondly, high temperature recrystallization annealing process is necessary to eliminate the influence of the internal stress resulting from defects such as vacancies and dislocations on the irreversible movement of magnetic domain walls. But the excessively high temperature annealing process is disadvantageous to the strength of an alloy. The mechanical strength of the Fe-Cr based alloys can be greatly enhanced by commonly used strengthening methods such as dispersion strengthening, fine grain strengthening, deformation strengthening and solid-solution strengthening, etc. However, according to the analysis from internal stress model theory, these methods could induce excessive stress concentration in the microstructure of an alloy leading to a difficult movement of the magnetic domain walls and eventually resulting in a significant decrease in its damping capacity. Currently, the enhancement of
H. Wang et al. / Materials Science & Engineering A 667 (2016) 326–331
damping capacity of Fe-Cr-Al based alloys is often at the expense of its strength. Therefore, when the alloys are used in a stringent circumstance, their performances are difficult to meet the requirements of application. Recently, the studies on Fe-Cr-Al based alloys have been focused on the effects of alloy elements and heat treatment on their damping capacity [2–6], but there have been few references to the mechanical properties of the Fe-Cr-Al alloys and even fewer studies on both the damping and mechanical properties. According to the studies [7,8], the Fe-(12–16%) Cr alloy shows good damping capacity when the alloy is annealed at 1100 °C for 1 h. 2 wt% 8 wt% Al doping can further improve the damping capacity of the Fe-Cr based alloys, and the Al doping also effectively improves the mechanical properties of the alloys. Therefore, in this paper, nearly carbon-free (C o0.015 wt%) Fe-13Cr-4.5Al alloy was chosen as the base alloy. Without changing the content of Fe, the Fe-13Cr-4.5Al, Fe-13Cr-4Al-0.5Mo, Fe-13Cr-4Al-0.5Ti and Fe-13Cr-4Al-0.5 Nb alloys were synthesized by vacuum induction melting respectively. The effects of 0.5 wt% Mo-, 0.5 wt% Ti- or 0.5 wt% Nb-substitution on the damping capacity and strength of the Fe-13Cr-4.5Al alloy were investigated, aiming to find out a method to improve both the damping capacity and the strength of the Fe-Cr-Al based alloys.
2. Experimental method 2.1. Alloy melting To reduce the negative influence of C, N, O and interstitial impurity atoms on the damping capacity of the Fe-Cr-Al based alloys, these alloys were prepared using industrial pure Fe (including less than 0.01 wt% C, N and O element), high purity Cr, A1 Ti, Mo and Nb (99.9%). The four samples were melted in a ZG-50 type vacuum induction melting (VIM) furnace with capacity of 25 kg. It can be seen from Table 1 that the chemical composition of the samples is in line with the designed values and the content of impurity elements is perfectly controlled. 2.2. Sample preparation The melted alloy ingots were homogenized at 1200 °C for 2 h before being forged into 25 mm thick plates. The forged plates were further treated with hot rolling (above 800 °C) after annealing at 1150 °C for 40 min and cooled to room temperature at the rate of 3 °C min 1. Finally, 5 mm thick plates were obtained after going through six rolling passes. Then, the plates were processed into different sizes according to the requirements of experiment. The samples for the damping capacity test were wire samples with diameter of 1.0–1.2 mm and length of 150 mm. The samples for Young modulus test were rectangular samples with the size 45 mm 8 mm 0.8 mm. The plates with size of 10 mm 10 mm 10 mm were used for microstructure observation and the plates with size of 30 mm 10 mm 2 mm were used for mechanical tensile test. The samples were annealed in a vacuum quartz tube at 1100 °C for 1 h, and then cooled in the furnace.
327
Table 1 Chemical composition of the Fe-Cr-Al based alloys (wt%). No. Alloy
Cr
1
12.98 4.42 –
2
3
4
Fe-13Cr4.5Al Fe-13Cr4Al0.5Mo Fe-13Cr4Al0.5Ti Fe-13Cr4Al0.5Nb
Al
Mo
Ti
Cu Nb
C
–
–
–
0.015 0.005 0.001 Bal.
13.02 3.91 0.51 –
–
–
0.012 0.005 0.002 Bal.
13.01 3.77 –
0.48 –
–
0.014 0.006 0.002 Bal.
12.96 3.84 –
–
0.49 0.012 0.004 0.001 Bal.
–
N
Fe
⎛ A ⎞ δ = ln⎜ n ⎟ ⎝ An+ 1 ⎠ wherein A is the amplitude, n is the number of vibration, An, An þ 1 is the amplitude of the vibration at nth and (n þ1)th times. The vibration frequency is 1 Hz, the measurement temperature is 30 °C and the strain amplitude of γ is 5 10 6–5 10 4. Logarithmic reduction rate is obtained in the case of free vibrations. A force is applied to the testing sample to deviate its original equilibrium position, when the force is removed, the vibration amplitude will continue to reduce due to continuous consumption of mechanical energy, which is also called free decay, as shown in Fig. 1. High temperature elastic modulus meter (BUZZ-1015, BuzzMac, USA) was used to measure the elastic modulus of the alloy and ASTM standard was adopted. The samples for metallographic structure observation were corroded in hydrochloric acid alcohol solution (10 mL HCI þ90 mL CH3CH2OH) under the voltage of 3– 4 V for 20 s Metallographic structure was observed by means of Optical Microscope (OM). Device name: optical microscope (OLYMPUS), specification and model: BX51. The average grain size was measured by cutting line method [5]. The characteristics of alloy grain boundary were observed by scanning electron microscope (Hitachi, S4800) and the composition of grain boundary precipitates was analyzed with SEM-EDS (Oxford IE450XMax80). The alloy structure change was detected by employing X’Pert PRO diffractometer (PANalytical Company, Holland). Yield strength was measured with a MTS810–100 kN type tensile testing machine at room temperature. Magnetostriction coefficient (λs) was obtained from a magnetostrictive device (Super-ME-II, Beijing, China) for each damping alloy.
3. Results 3.1. Damping capacity Fig. 2 illustrates the vibration of δ with the change of strain amplitude (γ) of the four samples annealed at 1100 °C for 1 h and of the No.2-rolled Fe-13Cr-4Al-0.5Mo sample (rolled Fe-13Cr-4Al0.5Mo sample without annealing). The figure shows that the δ
2.3. Performance test and microstructure observation of the samples Damping capacity of the alloy was tested by using JN-1 type inverted torsion pendulum and characterized using logarithmic decrement δ. The accuracy error of the δ value is lower than 0.002. The logarithmic decrement δ is obtained by taking logarithm of the ratio of two adjacent vibration amplitudes.
O
Fig. 1. Damping curve of free vibration for a sample.
328
H. Wang et al. / Materials Science & Engineering A 667 (2016) 326–331
dispersion strengthening and solid-solution strengthening effect (No.4). Generally speaking, the alloy elements which increase the yield strength of an alloy usually reduce the damping capacity of the alloy (i. e. Ti and Nb). However, there is an exception. The substitution of Mo for Al element improves both the damping capacity and increases the yield strength of the Fe-13Cr-4.5Al alloy.
4. Discussion 4.1. Effect of the alloy elements on the damping capacity
Fig. 2. Variation tendency of the δ values with the change of strain amplitude (γ) for samples annealed at 1100 °C for 1 h and the No.2-rolled Fe-13Cr-4Al-0.5Mo sample.
values of the annealed four samples increase with the values of the strain amplitude. The maximum δ values are reached at the strain amplitude of 9.0–10.0 10 5. Then, the δ values gradually decrease with the increase of vibration strain values. Moreover, the substitution of 0.5 wt% Mo increases the δ values of the Fe-13Cr4.5Al alloy, indicating that an improved damping capacity is achieved. From the figure, it can also be seen that the substitution of 0.5 wt% Nb (No.4) or 0.5 wt% Ti (No.3) for Al element decreases the δ values of the Fe-13Cr-4.5Al alloy, especially for the 0.5 wt% Nb-substituted sample (No.4). These results demonstrate that the substitution of Nb or Ti for Al element reduces the damping capacity of the Fe-13Cr-4.5Al alloy. If the sample not experiences heat treatment, it possesses much lower damping capacity (the No.2-rolled Fe-Cr-Al-0.5Mo sample in Fig. 2).
According to internal friction mechanism of Fe-Cr-Al based ferromagnetic type damping alloys, internal stress theoretical model (S-B theoretical model) developed by Smith and Birchak agrees well with the experimental results in literatures [9,10]. Based on the (S-B theoretical model), logarithmic decrement for ferromagnetic materials under low strain amplitude condition can be expressed as follows:
δ=
4KE2λγ 3σi2
(1)
where K is a dimensionless constant, E is the Young's modulus, λ is the magnetostriction coefficient, γ is the strain amplitude, and σi is the average internal stress. From Formula (1), we know that the value of δ is proportional to γ and inversely proportional to the average value of σi2. Taking derivative with respect to γ in the formula (1), the damping-strain amplitude slope of curve θR can be obtained as:
θR =
dδ 4KE2λ = dγ 3σi2
(2)
Based on the (S-B theoretical model) [11,12], the reciprocal of the maximum value of logarithmic decrement ( δmax ) under high strain conditions can be expressed as follows:
3.2. Yield strength
δmax =
0.34KEλ s σi
(3)
Fig. 3 shows the yield strength of the samples annealed at 1100 °C for 1 h and of the No.4-rolled Fe-13Cr-4Al-0.5Mo sample without annealing. It can be seen from the figure that the hotrolled alloy without annealing (No.4-rolled Fe-13Cr-4Al-0.5Mo sample) shows high yield strength (875 MPa) at room temperature. However, the yield strength of the annealed sample shows much lower yield strength compared with the alloys without annealing. The substitutions of 0.5 wt% Mo, 0.5 wt% Nb and 0.5 wt% Ti for Al element all increase the yield strength of the Fe-13Cr4.5Al alloy (No.2–4). Especially, the Nb substitution shows the best
where, λs is the saturated magnetostriction coefficient. From the formulas (2) and (3), it can be deduced that, for a given ferromagnetic alloy, θR and δmax depend on the average internal stress σi and saturated magnetostriction coefficient λs. An increase of σi and decrease of λs will induce the decrease of the damping-strain amplitude slope θR and the maximum logarithmic decrement δmax . Fig. 4 shows the magnetostriction coefficient curves of the four alloys annealed at 1100 °C for 1 h. The figure shows that the substitution of 0.5 wt% Mo greatly increases the value of λs, but the
Fig. 3. Yield strength of the samples annealed at 1100 °C for 1 h and the No.4-rolled Fe-13Cr-4Al-0.5Mo sample without annealing.
Fig. 4. Magnetostriction curves of the prepared alloys annealed at 1100 °C for 1 h.
H. Wang et al. / Materials Science & Engineering A 667 (2016) 326–331
4.2. Effect of the alloy elements on the strength of the alloy
Table 2 Average internal stress value si of the Fe-Cr-Al-based alloys. Samples
γm 10 5
SDCm
E (GPa)
λs (ppm)
si (MPa)
Fe-13Cr-4.5Al Fe-13Cr-4Al-0.5Mo Fe-13Cr-4Al-0.5Ti Fe-13Cr-4Al-0.5Nb
9.0 9.6 9.3 9.7
0.372 0.418 0.320 0.306
171 169 177 198
20.4 27.1 18.2 20.8
3.10 2.91 3.35 3.50
substitution of 0.5 wt% Ti decreases the value, and the substitution of 0.5 wt% Nb insignificantly affects the saturated magnetostriction coefficient. For anelastic materials, specific damping capacity (SDC) is defined as the damping capacity measuring at the strain amplitude equaling the tenth of its yield strength (s0.2/10) in engineering application [13]. Usually, the value of specific damping capacity can be evaluated using formula
SDC δ = (δ/π < 0.01) 2π π
(4)
The average internal stress σi can be calculated from internal stress model. Based on formula (4), the measured δ-γm data can be expressed using the relationship of SDC-γm data, as shown in Fig. 2. According to the maximum damping capacity and its corresponding vibration amplitude obtained from Fig. 2, the value of K, λs and γi can be calculated according to formulas (5) and (6). For Fe-based damping alloy, K E0.81. Furthermore, the value of λs and si can be calculated from formula (7) according to the elastic modulus of each alloy. Finally, the average internal stress σi is obtained by substituting the saturation magnetostriction λs in Fig. 4, and the results are listed in Table 2. As can be seen from the data, the higher the damping capacity of an alloy (No.2, Fe-13Cr4Al-0.5Mo) is, the smaller the average internal stress is, and vice versa. This result is consistent with the law of the influence of the internal stress on the damping capacity. 2 γm = 1.280 γi2 + γeff
SDCm =
(5)
0.6310Kλ sγi 2 γi2 + γeff
(6)
From Eqs. (5) and (6), the following Eq. (7) is deduced:
γm2 × SDCm = 0.8135λ s
σi E
329
(7)
Fig. 5 shows the microstructure of the Fe-13Cr-4Al-0.5Mo alloy with and without annealing. As can be seen from Fig. 5(a), the alloy shows the stripped structure along the rolling direction after hot rolling, and shows serious work hardening. The high strength of the hot-rolled alloy is ascribed to the very high internal dislocation density and high machining stress which are also the reasons for difficult movement of the magnetic domain walls. Therefore, the prepared hot-rolled alloy without annealing shows limited damping capacity (Fig. 2). In the annealing process, the deformed microstructure re-crystallizes and produces equiaxed ferrite grains. These formed grains also grow fast with annealing time and temperature. As shown in Fig. 5(b), the grain sizes of the alloy annealed at 1100 °C for 1 h are more than 300 mm. Moreover, with the increase in the annealing temperature and the occurrence of reversion and recrystallization, both the dislocation density and the machining stress of the alloy gradually decrease. The elimination of machining stress can be observed in the alloys annealed at 1100 °C. In addition, the grown crystal grains and the decreased number of grain boundaries caused by the annealing process will result in the reduced strength of the alloy. However, the damping capacity will increasingly enhance because of the easy movement of the magnetic domain walls. It has been proved that a second-phase particles Nb(C,N) precipitate within crystals and between grain boundaries in Fe-Cr based stainless steel doped with trace Nb [14–19]. In this study, the same phenomenon is observed for the Fe-13Cr-4Al-0.5Nb alloy sample (No.4), as shown in Fig. 6. The substitution of 0.5 wt% Nb significantly increases the quantity of second-phase precipitates in the alloy. These second phase precipitates have a pinning effect on the growth of grains in the process of annealing resulting in serious restraining of the grain growth. Therefore, the grain size of the alloy increases slowly in the annealing process. Meanwhile, these second phase precipitates also have a pinning effect on the movement of the dislocations and the magnetic domain walls. These are the reasons for the high yield strength and low damping capacity of the Fe-13Cr-4Al-0.5Nb alloy. For Fe-13Cr-4Al-0.5Ti alloy sample, there exist second-phase precipitates within crystals and between grain boundaries. But the quantity of the second-phase precipitates between grain boundaries is significantly decreased compared with the Fe-13Cr-4Al0.5Nb alloy (Fig. 7(a)). Moreover, grain growth is obvious and the grain size of the precipitates within crystals is small (Fig. 7(b)). EDS analysis shows that the precipitates between grain boundaries and within crystals is Ti(C,N). Ti with hcp structure is not easily replaced by bcc structure elements in the process of carbonitride
Fig. 5. Microstructure of the hot-rolled Fe-13Cr-4Al-0.5Mo alloy (a) without annealing and (b) after annealing at 1100 °C for 1 h in a high temperature vacuum furnace.
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H. Wang et al. / Materials Science & Engineering A 667 (2016) 326–331
(b)
(a)
Nb(C,N)
Fig. 6. Precipitated phases of No.4 the Fe-13Cr-4Al-0.5Nb alloy annealed at 1100 °C for 1 h.
(b)
(a)
Ti(C,N)
Fig. 7. Precipitated phases of No.3 the Fe-13Cr-4Al-0.5Ti alloy annealed at 1100 °C for 1 h.
formation. Thus, the substitution of Ti does not lead to large amounts of precipitates within crystal. These are the reasons for the lower yield strength and higher damping capacity of the Fe13Cr-4Al-0.5Ti alloy than those of the Fe-13Cr-4Al-0.5Nb alloy. For hot-rolled alloy after annealing at high temperature, the strength of the annealed alloy is mainly from the elastic interactions between the alloy atoms and the atoms from the substrate alloy, and the contribution of the interactions between the dislocations and the alloy atoms to the strength is negligible [20]. It can be concluded from the strength data in Fig. 3 that the substitution of alloy elements such as Mo, Ti or Nb all enhances these interaction forces. The magnitude of the interaction forces in the structure of the Fe-Cr-Al based damping alloy is about 365 MPa [21]. This magnitude is much higher than the internal stress generated by the irreversible movement of magnetic domain walls. Therefore, it can be deduced that the effect of the substitution elements on the strength and damping capacity is attributed to the two different forces produced by the substitution. Therefore, the improvement of the damping capacity does not necessarily lead to the decrease of the strength of an alloy. For example, the substitution of 0.5 wt% Mo for Al not only improves
the damping capacity but increases the strength of the Fe-13Cr4.5Al alloy. It can be concluded from the effect of the substitution elements on the strength and damping capacity of the prepared alloys that, the deformation zone, the grain boundary and the second-phase particles which can increase the strength of an alloy basically reduce the damping capacity of the alloy.
5. Conclusions The damping capacity and yield strength of the Fe-13Cr-4.5Al, Fe-13Cr-4Al-0.5Mo, Fe-13Cr-4Al-0.5Ti and Fe-13Cr-4Al-0.5Nb alloys were characterized and the effect of alloy elements was analyzed through microstructure observation and internal stress analysis. The following conclusions were drawn: (1) The substitution of 0.5 wt% Mo for Al in the composition of Fe13Cr-4.5Al not only improves the damping capacity but also increases the yield strength of the alloy. The substitution of 0.5 wt% Nb or 0.5 wt% Ti for Al increases the yield strength of
H. Wang et al. / Materials Science & Engineering A 667 (2016) 326–331
the alloy, but reduces its damping capacity. (2) The effects of the substitutional elements on the strength and damping capacity are ascribed to the two different factors: internal stress and pin dislocations. Smaller average internal stresses lead to higher damping. The 0.5 wt% Mo substitution reduces the interactions between the dislocations and the solute atoms, but increases the magnitude of the elastic interactions between the alloy atoms and the atoms from the substrate alloy. Thus, both improved damping capacity and increased strength are obtained.
Acknowledgments Great thanks for the financial support to our work from the Project Funded by China Postdoctoral Science Foundation (58 Group, Grant No: 2015M580799). Great thanks to China Academy of Engineering Physics and Nuclear Power Institute of China for help in terms of experimental conditions.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of alloy elements (Mo, Nb, Ti) on the strength and damping capacity of Fe-Cr based alloy Hui Wang a,b,n, Fu Wang c,nn, Haitao Liu d, Dong Pan b, Qianfu Pan b, Yunming Liu b, Jun Xiao b, Pengcheng Zhang a a
Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box No. 9-35, Huafengxincun, Jiangyou City, Sichuan Province 621908, PR China National Key Laboratory for Nuclear Fuel and Materials, Nuclear Power Institute of China, Chengdu 610041, PR China c School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China d State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2016 Received in revised form 3 May 2016 Accepted 4 May 2016 Available online 6 May 2016
Effects of Mo-, Ti- and Nb-substitution for Al on the strength and damping capacity of the Fe-13Cr-4.5Al alloy were investigated by analyzing the mutual relationships among microstructures, strength, internal stress and damping capacity of the alloys. The obtained results show that the substitution of 0.5Mo for 0.5Al improves both the strength and damping capacity of the alloy. While the substitution of 0.5Ti or 0.5Nb for 0.5Al simply improves the strength but reduces the damping capacity of the alloy. The effect of the alloy elements on the strength and damping capacity lies in the fact that the substitution of the alloy elements generates both local internal stress and pin dislocations. Lower average internal stress leads to higher damping capacity of the alloy. The substitution of 0.5Mo for 0.5Al decreases the interactions between the dislocations and the solute atoms while increases the elastic distortions of the crystalline lattice, resulting in the enhancement of both strength and damping capacity. & 2016 Elsevier B.V. All rights reserved.
Keywords: Fe-Cr-Al based alloy Damping capacity Mechanical strength Internal stress Saturation magnetostriction
1. Introduction In recent years, the problems of vibrations and noise are increasingly serious, which not only affect the safety and service life of mechanical and military equipment, but also are severely harmful to people's physical and mental health. Therefore, the reduction of vibrations and noise is an urgent task at present. The adoption of high damping alloys is one of the effective ways to reduce noise and vibrations. It is well known that Fe–Cr based ferromagnetic damping alloys have received extensive research due to high damping capacity, good mechanical properties, corrosion resistance, wide service temperature range (with high damping capacity even at 500 °C) and the fact that the damping capacity does not vary with vibration frequency [1–4]. The damping mechanism of the Fe-Cr based ferromagnetic damping alloys is, due to the irreversible movement of magnetic domain walls, the vibration energy is transferred to heat energy which is dissipated by heat conduction [2,3]. Internal stress is a key factor to influence the irreversible movement of magnetic n Corresponding author at: Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box No. 9-35, Huafengxincun, Jiangyou City, Sichuan Province 621908, PR China. nn Corresponding author. E-mail address: [email protected] (H. Wang).
http://dx.doi.org/10.1016/j.msea.2016.05.013 0921-5093/& 2016 Elsevier B.V. All rights reserved.
domain wall, and energy fluctuation caused by local internal stress is an indispensable condition for the irreversible movement of magnetic domain walls. However, the excessive internal stresses resulting from defects such as dislocations and second phases have a strong pinning effect on magnetic domain walls, which reduces the damping capacity. Many studies showed that, in order to achieve good damping capacity, the alloys should meet certain requirements [4–7]. Firstly, the content of impurities such as C and N in the alloy must be strictly controlled. The micro internal stresses in an alloy can be significantly reduced when the impurity content is less than 0.01 wt%. Secondly, high temperature recrystallization annealing process is necessary to eliminate the influence of the internal stress resulting from defects such as vacancies and dislocations on the irreversible movement of magnetic domain walls. But the excessively high temperature annealing process is disadvantageous to the strength of an alloy. The mechanical strength of the Fe-Cr based alloys can be greatly enhanced by commonly used strengthening methods such as dispersion strengthening, fine grain strengthening, deformation strengthening and solid-solution strengthening, etc. However, according to the analysis from internal stress model theory, these methods could induce excessive stress concentration in the microstructure of an alloy leading to a difficult movement of the magnetic domain walls and eventually resulting in a significant decrease in its damping capacity. Currently, the enhancement of
H. Wang et al. / Materials Science & Engineering A 667 (2016) 326–331
damping capacity of Fe-Cr-Al based alloys is often at the expense of its strength. Therefore, when the alloys are used in a stringent circumstance, their performances are difficult to meet the requirements of application. Recently, the studies on Fe-Cr-Al based alloys have been focused on the effects of alloy elements and heat treatment on their damping capacity [2–6], but there have been few references to the mechanical properties of the Fe-Cr-Al alloys and even fewer studies on both the damping and mechanical properties. According to the studies [7,8], the Fe-(12–16%) Cr alloy shows good damping capacity when the alloy is annealed at 1100 °C for 1 h. 2 wt% 8 wt% Al doping can further improve the damping capacity of the Fe-Cr based alloys, and the Al doping also effectively improves the mechanical properties of the alloys. Therefore, in this paper, nearly carbon-free (C o0.015 wt%) Fe-13Cr-4.5Al alloy was chosen as the base alloy. Without changing the content of Fe, the Fe-13Cr-4.5Al, Fe-13Cr-4Al-0.5Mo, Fe-13Cr-4Al-0.5Ti and Fe-13Cr-4Al-0.5 Nb alloys were synthesized by vacuum induction melting respectively. The effects of 0.5 wt% Mo-, 0.5 wt% Ti- or 0.5 wt% Nb-substitution on the damping capacity and strength of the Fe-13Cr-4.5Al alloy were investigated, aiming to find out a method to improve both the damping capacity and the strength of the Fe-Cr-Al based alloys.
2. Experimental method 2.1. Alloy melting To reduce the negative influence of C, N, O and interstitial impurity atoms on the damping capacity of the Fe-Cr-Al based alloys, these alloys were prepared using industrial pure Fe (including less than 0.01 wt% C, N and O element), high purity Cr, A1 Ti, Mo and Nb (99.9%). The four samples were melted in a ZG-50 type vacuum induction melting (VIM) furnace with capacity of 25 kg. It can be seen from Table 1 that the chemical composition of the samples is in line with the designed values and the content of impurity elements is perfectly controlled. 2.2. Sample preparation The melted alloy ingots were homogenized at 1200 °C for 2 h before being forged into 25 mm thick plates. The forged plates were further treated with hot rolling (above 800 °C) after annealing at 1150 °C for 40 min and cooled to room temperature at the rate of 3 °C min 1. Finally, 5 mm thick plates were obtained after going through six rolling passes. Then, the plates were processed into different sizes according to the requirements of experiment. The samples for the damping capacity test were wire samples with diameter of 1.0–1.2 mm and length of 150 mm. The samples for Young modulus test were rectangular samples with the size 45 mm 8 mm 0.8 mm. The plates with size of 10 mm 10 mm 10 mm were used for microstructure observation and the plates with size of 30 mm 10 mm 2 mm were used for mechanical tensile test. The samples were annealed in a vacuum quartz tube at 1100 °C for 1 h, and then cooled in the furnace.
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Table 1 Chemical composition of the Fe-Cr-Al based alloys (wt%). No. Alloy
Cr
1
12.98 4.42 –
2
3
4
Fe-13Cr4.5Al Fe-13Cr4Al0.5Mo Fe-13Cr4Al0.5Ti Fe-13Cr4Al0.5Nb
Al
Mo
Ti
Cu Nb
C
–
–
–
0.015 0.005 0.001 Bal.
13.02 3.91 0.51 –
–
–
0.012 0.005 0.002 Bal.
13.01 3.77 –
0.48 –
–
0.014 0.006 0.002 Bal.
12.96 3.84 –
–
0.49 0.012 0.004 0.001 Bal.
–
N
Fe
⎛ A ⎞ δ = ln⎜ n ⎟ ⎝ An+ 1 ⎠ wherein A is the amplitude, n is the number of vibration, An, An þ 1 is the amplitude of the vibration at nth and (n þ1)th times. The vibration frequency is 1 Hz, the measurement temperature is 30 °C and the strain amplitude of γ is 5 10 6–5 10 4. Logarithmic reduction rate is obtained in the case of free vibrations. A force is applied to the testing sample to deviate its original equilibrium position, when the force is removed, the vibration amplitude will continue to reduce due to continuous consumption of mechanical energy, which is also called free decay, as shown in Fig. 1. High temperature elastic modulus meter (BUZZ-1015, BuzzMac, USA) was used to measure the elastic modulus of the alloy and ASTM standard was adopted. The samples for metallographic structure observation were corroded in hydrochloric acid alcohol solution (10 mL HCI þ90 mL CH3CH2OH) under the voltage of 3– 4 V for 20 s Metallographic structure was observed by means of Optical Microscope (OM). Device name: optical microscope (OLYMPUS), specification and model: BX51. The average grain size was measured by cutting line method [5]. The characteristics of alloy grain boundary were observed by scanning electron microscope (Hitachi, S4800) and the composition of grain boundary precipitates was analyzed with SEM-EDS (Oxford IE450XMax80). The alloy structure change was detected by employing X’Pert PRO diffractometer (PANalytical Company, Holland). Yield strength was measured with a MTS810–100 kN type tensile testing machine at room temperature. Magnetostriction coefficient (λs) was obtained from a magnetostrictive device (Super-ME-II, Beijing, China) for each damping alloy.
3. Results 3.1. Damping capacity Fig. 2 illustrates the vibration of δ with the change of strain amplitude (γ) of the four samples annealed at 1100 °C for 1 h and of the No.2-rolled Fe-13Cr-4Al-0.5Mo sample (rolled Fe-13Cr-4Al0.5Mo sample without annealing). The figure shows that the δ
2.3. Performance test and microstructure observation of the samples Damping capacity of the alloy was tested by using JN-1 type inverted torsion pendulum and characterized using logarithmic decrement δ. The accuracy error of the δ value is lower than 0.002. The logarithmic decrement δ is obtained by taking logarithm of the ratio of two adjacent vibration amplitudes.
O
Fig. 1. Damping curve of free vibration for a sample.
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dispersion strengthening and solid-solution strengthening effect (No.4). Generally speaking, the alloy elements which increase the yield strength of an alloy usually reduce the damping capacity of the alloy (i. e. Ti and Nb). However, there is an exception. The substitution of Mo for Al element improves both the damping capacity and increases the yield strength of the Fe-13Cr-4.5Al alloy.
4. Discussion 4.1. Effect of the alloy elements on the damping capacity
Fig. 2. Variation tendency of the δ values with the change of strain amplitude (γ) for samples annealed at 1100 °C for 1 h and the No.2-rolled Fe-13Cr-4Al-0.5Mo sample.
values of the annealed four samples increase with the values of the strain amplitude. The maximum δ values are reached at the strain amplitude of 9.0–10.0 10 5. Then, the δ values gradually decrease with the increase of vibration strain values. Moreover, the substitution of 0.5 wt% Mo increases the δ values of the Fe-13Cr4.5Al alloy, indicating that an improved damping capacity is achieved. From the figure, it can also be seen that the substitution of 0.5 wt% Nb (No.4) or 0.5 wt% Ti (No.3) for Al element decreases the δ values of the Fe-13Cr-4.5Al alloy, especially for the 0.5 wt% Nb-substituted sample (No.4). These results demonstrate that the substitution of Nb or Ti for Al element reduces the damping capacity of the Fe-13Cr-4.5Al alloy. If the sample not experiences heat treatment, it possesses much lower damping capacity (the No.2-rolled Fe-Cr-Al-0.5Mo sample in Fig. 2).
According to internal friction mechanism of Fe-Cr-Al based ferromagnetic type damping alloys, internal stress theoretical model (S-B theoretical model) developed by Smith and Birchak agrees well with the experimental results in literatures [9,10]. Based on the (S-B theoretical model), logarithmic decrement for ferromagnetic materials under low strain amplitude condition can be expressed as follows:
δ=
4KE2λγ 3σi2
(1)
where K is a dimensionless constant, E is the Young's modulus, λ is the magnetostriction coefficient, γ is the strain amplitude, and σi is the average internal stress. From Formula (1), we know that the value of δ is proportional to γ and inversely proportional to the average value of σi2. Taking derivative with respect to γ in the formula (1), the damping-strain amplitude slope of curve θR can be obtained as:
θR =
dδ 4KE2λ = dγ 3σi2
(2)
Based on the (S-B theoretical model) [11,12], the reciprocal of the maximum value of logarithmic decrement ( δmax ) under high strain conditions can be expressed as follows:
3.2. Yield strength
δmax =
0.34KEλ s σi
(3)
Fig. 3 shows the yield strength of the samples annealed at 1100 °C for 1 h and of the No.4-rolled Fe-13Cr-4Al-0.5Mo sample without annealing. It can be seen from the figure that the hotrolled alloy without annealing (No.4-rolled Fe-13Cr-4Al-0.5Mo sample) shows high yield strength (875 MPa) at room temperature. However, the yield strength of the annealed sample shows much lower yield strength compared with the alloys without annealing. The substitutions of 0.5 wt% Mo, 0.5 wt% Nb and 0.5 wt% Ti for Al element all increase the yield strength of the Fe-13Cr4.5Al alloy (No.2–4). Especially, the Nb substitution shows the best
where, λs is the saturated magnetostriction coefficient. From the formulas (2) and (3), it can be deduced that, for a given ferromagnetic alloy, θR and δmax depend on the average internal stress σi and saturated magnetostriction coefficient λs. An increase of σi and decrease of λs will induce the decrease of the damping-strain amplitude slope θR and the maximum logarithmic decrement δmax . Fig. 4 shows the magnetostriction coefficient curves of the four alloys annealed at 1100 °C for 1 h. The figure shows that the substitution of 0.5 wt% Mo greatly increases the value of λs, but the
Fig. 3. Yield strength of the samples annealed at 1100 °C for 1 h and the No.4-rolled Fe-13Cr-4Al-0.5Mo sample without annealing.
Fig. 4. Magnetostriction curves of the prepared alloys annealed at 1100 °C for 1 h.
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4.2. Effect of the alloy elements on the strength of the alloy
Table 2 Average internal stress value si of the Fe-Cr-Al-based alloys. Samples
γm 10 5
SDCm
E (GPa)
λs (ppm)
si (MPa)
Fe-13Cr-4.5Al Fe-13Cr-4Al-0.5Mo Fe-13Cr-4Al-0.5Ti Fe-13Cr-4Al-0.5Nb
9.0 9.6 9.3 9.7
0.372 0.418 0.320 0.306
171 169 177 198
20.4 27.1 18.2 20.8
3.10 2.91 3.35 3.50
substitution of 0.5 wt% Ti decreases the value, and the substitution of 0.5 wt% Nb insignificantly affects the saturated magnetostriction coefficient. For anelastic materials, specific damping capacity (SDC) is defined as the damping capacity measuring at the strain amplitude equaling the tenth of its yield strength (s0.2/10) in engineering application [13]. Usually, the value of specific damping capacity can be evaluated using formula
SDC δ = (δ/π < 0.01) 2π π
(4)
The average internal stress σi can be calculated from internal stress model. Based on formula (4), the measured δ-γm data can be expressed using the relationship of SDC-γm data, as shown in Fig. 2. According to the maximum damping capacity and its corresponding vibration amplitude obtained from Fig. 2, the value of K, λs and γi can be calculated according to formulas (5) and (6). For Fe-based damping alloy, K E0.81. Furthermore, the value of λs and si can be calculated from formula (7) according to the elastic modulus of each alloy. Finally, the average internal stress σi is obtained by substituting the saturation magnetostriction λs in Fig. 4, and the results are listed in Table 2. As can be seen from the data, the higher the damping capacity of an alloy (No.2, Fe-13Cr4Al-0.5Mo) is, the smaller the average internal stress is, and vice versa. This result is consistent with the law of the influence of the internal stress on the damping capacity. 2 γm = 1.280 γi2 + γeff
SDCm =
(5)
0.6310Kλ sγi 2 γi2 + γeff
(6)
From Eqs. (5) and (6), the following Eq. (7) is deduced:
γm2 × SDCm = 0.8135λ s
σi E
329
(7)
Fig. 5 shows the microstructure of the Fe-13Cr-4Al-0.5Mo alloy with and without annealing. As can be seen from Fig. 5(a), the alloy shows the stripped structure along the rolling direction after hot rolling, and shows serious work hardening. The high strength of the hot-rolled alloy is ascribed to the very high internal dislocation density and high machining stress which are also the reasons for difficult movement of the magnetic domain walls. Therefore, the prepared hot-rolled alloy without annealing shows limited damping capacity (Fig. 2). In the annealing process, the deformed microstructure re-crystallizes and produces equiaxed ferrite grains. These formed grains also grow fast with annealing time and temperature. As shown in Fig. 5(b), the grain sizes of the alloy annealed at 1100 °C for 1 h are more than 300 mm. Moreover, with the increase in the annealing temperature and the occurrence of reversion and recrystallization, both the dislocation density and the machining stress of the alloy gradually decrease. The elimination of machining stress can be observed in the alloys annealed at 1100 °C. In addition, the grown crystal grains and the decreased number of grain boundaries caused by the annealing process will result in the reduced strength of the alloy. However, the damping capacity will increasingly enhance because of the easy movement of the magnetic domain walls. It has been proved that a second-phase particles Nb(C,N) precipitate within crystals and between grain boundaries in Fe-Cr based stainless steel doped with trace Nb [14–19]. In this study, the same phenomenon is observed for the Fe-13Cr-4Al-0.5Nb alloy sample (No.4), as shown in Fig. 6. The substitution of 0.5 wt% Nb significantly increases the quantity of second-phase precipitates in the alloy. These second phase precipitates have a pinning effect on the growth of grains in the process of annealing resulting in serious restraining of the grain growth. Therefore, the grain size of the alloy increases slowly in the annealing process. Meanwhile, these second phase precipitates also have a pinning effect on the movement of the dislocations and the magnetic domain walls. These are the reasons for the high yield strength and low damping capacity of the Fe-13Cr-4Al-0.5Nb alloy. For Fe-13Cr-4Al-0.5Ti alloy sample, there exist second-phase precipitates within crystals and between grain boundaries. But the quantity of the second-phase precipitates between grain boundaries is significantly decreased compared with the Fe-13Cr-4Al0.5Nb alloy (Fig. 7(a)). Moreover, grain growth is obvious and the grain size of the precipitates within crystals is small (Fig. 7(b)). EDS analysis shows that the precipitates between grain boundaries and within crystals is Ti(C,N). Ti with hcp structure is not easily replaced by bcc structure elements in the process of carbonitride
Fig. 5. Microstructure of the hot-rolled Fe-13Cr-4Al-0.5Mo alloy (a) without annealing and (b) after annealing at 1100 °C for 1 h in a high temperature vacuum furnace.
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(b)
(a)
Nb(C,N)
Fig. 6. Precipitated phases of No.4 the Fe-13Cr-4Al-0.5Nb alloy annealed at 1100 °C for 1 h.
(b)
(a)
Ti(C,N)
Fig. 7. Precipitated phases of No.3 the Fe-13Cr-4Al-0.5Ti alloy annealed at 1100 °C for 1 h.
formation. Thus, the substitution of Ti does not lead to large amounts of precipitates within crystal. These are the reasons for the lower yield strength and higher damping capacity of the Fe13Cr-4Al-0.5Ti alloy than those of the Fe-13Cr-4Al-0.5Nb alloy. For hot-rolled alloy after annealing at high temperature, the strength of the annealed alloy is mainly from the elastic interactions between the alloy atoms and the atoms from the substrate alloy, and the contribution of the interactions between the dislocations and the alloy atoms to the strength is negligible [20]. It can be concluded from the strength data in Fig. 3 that the substitution of alloy elements such as Mo, Ti or Nb all enhances these interaction forces. The magnitude of the interaction forces in the structure of the Fe-Cr-Al based damping alloy is about 365 MPa [21]. This magnitude is much higher than the internal stress generated by the irreversible movement of magnetic domain walls. Therefore, it can be deduced that the effect of the substitution elements on the strength and damping capacity is attributed to the two different forces produced by the substitution. Therefore, the improvement of the damping capacity does not necessarily lead to the decrease of the strength of an alloy. For example, the substitution of 0.5 wt% Mo for Al not only improves
the damping capacity but increases the strength of the Fe-13Cr4.5Al alloy. It can be concluded from the effect of the substitution elements on the strength and damping capacity of the prepared alloys that, the deformation zone, the grain boundary and the second-phase particles which can increase the strength of an alloy basically reduce the damping capacity of the alloy.
5. Conclusions The damping capacity and yield strength of the Fe-13Cr-4.5Al, Fe-13Cr-4Al-0.5Mo, Fe-13Cr-4Al-0.5Ti and Fe-13Cr-4Al-0.5Nb alloys were characterized and the effect of alloy elements was analyzed through microstructure observation and internal stress analysis. The following conclusions were drawn: (1) The substitution of 0.5 wt% Mo for Al in the composition of Fe13Cr-4.5Al not only improves the damping capacity but also increases the yield strength of the alloy. The substitution of 0.5 wt% Nb or 0.5 wt% Ti for Al increases the yield strength of
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the alloy, but reduces its damping capacity. (2) The effects of the substitutional elements on the strength and damping capacity are ascribed to the two different factors: internal stress and pin dislocations. Smaller average internal stresses lead to higher damping. The 0.5 wt% Mo substitution reduces the interactions between the dislocations and the solute atoms, but increases the magnitude of the elastic interactions between the alloy atoms and the atoms from the substrate alloy. Thus, both improved damping capacity and increased strength are obtained.
Acknowledgments Great thanks for the financial support to our work from the Project Funded by China Postdoctoral Science Foundation (58 Group, Grant No: 2015M580799). Great thanks to China Academy of Engineering Physics and Nuclear Power Institute of China for help in terms of experimental conditions.
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