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Characterization of strengthening mechanism and hot deformation behavior of powder metallurgy molybdenum
Materials and Design 34 (2012) 112–119
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Characterization of strengthening mechanism and hot deformation behavior of powder metallurgy molybdenum Meili Xiao a,⇑, Fuguo Li a,b, Hangfang Xie a, Yufeng Wang a a b
School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
Article history: Received 18 May 2011 Accepted 29 July 2011 Available online 3 August 2011
a b s t r a c t The high-temperature deformation behavior of powder metallurgy molybdenum has been investigated based on a series of isothermal hot compression tests, which were carried out on a Gleeble-1500 thermal mechanical simulator in a wide range of temperatures (900–1450 °C) and strain rates (0.01–10 s1). Through the research on the experimental stress–strain curves, it reveals that dynamic recrystallization softening effect of powder metallurgy molybdenum occurs in the temperature range from 1200 °C to 1450 °C, in which the flow stress is significantly sensitive to temperature. In comparison with the value of strain hardening index n which decreases along with the temperature rising, the value of strain-rate sensitivity exponent m does not change obviously; however, it increases slowly with the increasing of temperature at first and achieves a peak value at 1350 °C. Furthermore, relying on the comparison of mean value of n and m, it is suggested that deformation strengthening is the main strengthening mechanism at low temperature while the rheological strengthening changes into the primary strengthening mechanism at high temperature. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Molybdenum is a kind of valuable refractory metal and its melting point is about 2610 °C [1]. The interatomic bonding force of molybdenum is so strong that at room temperature and high temperature its strength is always high [2]. Besides, in contrast with other refractory metals like Ta, Nb, Co, W and so on, molybdenum not only possesses high melting point, excellent thermal conductivity, good electrical conductivity and corrosion resistance, but also has a low coefficient of thermal expansion and high hardness [3]. Therefore, molybdenum and its alloys have been widely used in many fields, for instance, metallurgy, machinery, aviation, aerospace, and nuclear power industry etc. [4,5]. Nonetheless, its usefulness as a structural material is limited by its severe brittleness which is known to come from intergranular fracture. Yoshinaqa [6] considered that the grain boundaries with fairly high energy are responsible for intergranular brittleness in molybdenum. Meanwhile, Watanabe and Tsurekawa [7] have investigated the relationship between intergranular brittleness and grain boundary microstructure in two-dimensional polycrystalline molybdenum and indicated that improvement of brittleness of polycrystalline
⇑ Corresponding author: Tel.: +86 029 88474117. E-mail address: [email protected] (M. Xiao). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.07.065
molybdenum due to intergranular fracture can be achieved by controlling the GBCD through grain boundary engineering. Moreover, owing to their high melting point, high hardness [3], low temperature brittleness [8] and poor capability of high temperature oxidation resistance [9], molybdenum and its alloys could hardly be manufactured by casting, forging and machining except the method of powder metallurgy. However, conventional P/M processing of molybdenum to near full density requires long time sintering at high temperature which leads to excessive grain coarsening and subsequent loss of mechanical properties. Garg et al. [10] have researched the sintering mechanism of molybdenum powders and the accurate density prediction and design of optimum sintering cycles of P/M molybdenum were proposed. On the other side, molybdenum forgings should be processed in a narrow high temperature domain because of large deformation resistance [11]. The micro-structural analysis, mechanical behavior and properties testing of molybdenum and its alloys have been studied by many researchers during recent years. For example, Laribi et al. [12] have discussed the metallurgical and mechanical behavior of molybdenum coating formed by flame spraying based on its microstructure, hardness and tribological resistance. Additionally, Creep behavior at temperatures between 1300 °C and 1600 °C of P/M grade molybdenum sheet as well as elevated temperature properties and recrystallization of molybdenum doped with potassium, silicon and aluminum also have been studied by Ciulik and Taleff
M. Xiao et al. / Materials and Design 34 (2012) 112–119
113
[13] and Wang et al. [14], respectively. Yet, few works have been focused on the hot deformation strengthening mechanism of powder metallurgy molybdenum, so further more detailed researches are necessary. In this paper, firstly, a series of isothermal hot compression tests were performed on a Gleeble-1500 thermal mechanical simulator. On the basis of experimental results, hot deformation behavior of powder metallurgy molybdenum has been investigated. The dependence of flow behavior and micro-structural evolution on strain rate and temperature were analyzed by introducing the Zener–Hollomon parameter. Finally, the dominant strengthening mechanism in deformation process of powder metallurgy molybdenum has been indicated in terms of the value of strain hardening index n and strain-rate sensitivity exponent m. 2. Experiments
Fig. 1. The original microstructure of power metallurgy molybdenum.
Chemical composition of as-received powder metallurgy molybdenum has been given in Table 1. The mesh sizes of molybdenum powder particles are in the range of 2.0–3.5 lm and the intermediate frequency induction sintering process was introduced to produce P/M molybdenum. Meanwhile, the original microstructure of sintered molybdenum by powder metallurgy process has been shown in Fig. 1, where the grain boundary is not clear. It is concluded that the prior particle boundaries emerged in the sintered molybdenum lead to it, subsequent forging or extrusion process is necessary to eliminate the particle boundaries and make the grain boundary more apparent. The diameter of as-received P/M molybdenum bar is 20 mm, from which cylindrical compression specimens are machined with a diameter of 8 mm and a height of 12 mm. By usage of large current resistance-heating system, each specimen was heated to the testing temperature at a rate of 10 °C/s, and held for 5 min at the isothermal condition prior to testing so as to obtain a uniform temperature. The compression tests were performed in the temperature range of 900–1450 °C and the strain rate range of 0.01– 10 s1 on a Gleeble-1500 thermal mechanical simulator, while stress–strain data at different temperatures and strain rates were recorded automatically. In order to reduce the interface friction, mica sheet was used to segregate the specimens and tools. All specimens were compressed to true strain of 0.5, and then immediately air cooled down to room temperature. 3. Results and discussion 3.1. Flow stress Flow stress curves of powder metallurgy molybdenum recorded at different temperatures are shown in Fig. 2, respectively. It could be found from Fig. 2 that in the initial stage the flow stress increases quickly with the increasing of strain. The occurrence of rapid increase in stress chiefly attributes to the sharp increasing of dislocation density at the beginning of deformation. Then, the increase rate of flow stress gradually slows down ascribing to the effect of dynamic recovery. It may be noted that the higher the temperature, the faster increase rate drops. Mandal et al. [15] also proclaimed similar changing regularity in high temperature flow stress curves of D9 alloy and pointed out this is due to the fact that
Table 1 Chemical composition of powder metallurgy molybdenum (wt.%). Fe
Ni
Cu
Ca
Mg
Si
Al
O
Mo
0.40
0.70
0.03
0.08
0.01
0.20
0.20
0.40
Bal.
higher temperature offers higher mobility to the grain boundary which leads to the extent of dynamic softening more significant. The values of peak flow stress at different temperatures and strain rates are presented in Table 2. It is interesting to note that the peak stress decreases with the increasing of deformation temperature at certain strain rate, and increases with the increasing of strain rate at certain deformation temperature. However, at some deformation temperatures (dynamic recrystallization temperature or above), after reaching a peak value the flow stress will continuously decrease with the increasing of strain, which may be caused by the dynamic recrystallization softening effect arising from accumulation of dislocation density and achievement of critical strain. That is in common with the initial conditions of the onset of dynamic recrystallization suggested in Ref. [16] that the local stored energy attains a maximum and critical value as well as the rate of dissipation associated with deformation decreases to a minimum value. In the other hand, the value of critical dynamic recrystallization strain can be changed with temperature; and the higher the temperature the smaller the critical strain. Furthermore, with the deformation temperature rising secondary dynamic recrystallization will take place in compression specimen at suitable strain rate. It can be observed from Fig. 2a–c that flow stress curves finally reach a steady state under most deformation conditions, which means that dynamic recovery is the dominating softening effect in powder metallurgy molybdenum below 1100 °C. Luo et al. [17] have given insight into the hot deformation flow behavior of Ti–6Al–4V alloy in the a + b two-phase region and b single-phase region, and also showed clearly that the steady flow behavior occurs as the dynamic recovery is sufficient to counteract the work-hardening effect of the alloy in the isothermal compression. Simultaneously, an obvious dynamic recrystallization softening phenomenon occurs at 1200 °C in the strain rate of 0.01 s1 as illustrated in Fig. 2d, and the value of critical strain calculated according to the equation demonstrated in Ref. [16] is about 0.36. Similarly, dynamic recrystallization softening phenomenon emerges at most of the strain rates in Fig. 2e, and the value of critical strain is approximate 0.27 for lower strain rate while 0.36 for higher strain rate. As the temperature rising to 1300 °C or even higher, secondary dynamic recrystallization comes into being at certain strain rate and the value of firstly dynamic recrystallization critical strain reduces to 0.23. The curves about the variation of flow stress with temperature at constant strain rate have been represented in Fig. 3. It is apparent that work-hardening feature appears in the temperature range of 900–1100 °C, nevertheless, dynamic recrystallization softening effect begins to happen at the temperature domain from 1200 °C to 1450 °C. Moreover, Fig. 3 also manifests that dynamic recrystalliza-
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Fig. 2. Flow stress curves of powder metallurgy molybdenum at different temperatures: (a) T = 900 °C, (b) T = 1000 °C, (c) T = 1100 °C, (d) T = 1200 °C, (e) T = 1250 °C, (f) T = 1300 °C, (g) T = 1350 °C, (h) T = 1400 °C, and (i) T = 1450 °C.
tion initially occurs in the low strain rate, and then gradually expands to high strain rate with the increase in temperature and deformation degree. The temperature range from 1200 °C to 1450 °C is the critical boundary where high-temperature deformation and low-temperature deformation for powder metallurgy molybdenum was distinguished, and flow stress is significantly sensitive to temperature in this region. In addition, it could be found in Fig. 3 that the specimen deformed with a small strain of 0.2, which
without dynamic recrystallization happening, still remains strain hardening characteristic in the temperature sensitive region. 3.2. Microstructure analysis Prasad et al. [18] have demonstrated that it is not appropriate to predict the deformation mechanisms on the basis of the shapes of the stress–strain curves because of that many hot deformation
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Fig. 2 (continued)
Table 2 The value of peak flow stress at different temperatures and strain rates (Mpa).
e_ =s1
T/°C 900
1000
1100
1200
1250
1300
1350
1400
1450
0.01 0.1 0.5 1 10
258.54 297.79 315.58 326.54 345.25
220.57 260.77 279.80 293.70 326.42
182.77 223.43 242.80 260.44 284.80
149.64 178.71 190.22 206.62 229.34
136.17 169.37 185.70 202.43 212.15
105.08 139.20 147.23 180.11 191.61
103.60 137.34 145.13 176.03 187.53
102.09 122.55 143.47 168.48 173.16
99.57 105.78 131.65 149.69 156.85
mechanism may result in a similar shape of the stress–strain curves. However, by making on the metallographic observation, the deformation mechanisms inferred in terms of the shapes of stress–strain curves are able to be confirmed. The optical micrographs of powder metallurgy molybdenum compressed in different strain rates and temperatures are shown in Fig. 4. For example, Fig. 4a and b display the microstructures of specimens deforming in 900 °C with different strain rates of 0.01 s1 and 0.1 s1, respectively. It can be noted that there is no obvious dynamic recrystallization phenomenon appearing in these two pictures. However, a large number of fine equiaxial dynamic recrystallization grains that distribute around the original grains can be observed in Fig. 4c and d. By means of comparing the volume fraction of dynamic recrystallization grains in Fig. 4c and d, it could be found that under identical strain rate the volume fraction will increase with the increasing of
temperature. Fig. 5a and b represent the optical micrographs of powder metallurgy molybdenum samples compressed in the temperature of 1300 °C and strain rate of 0.5 s1 as well as in the temperature of 1400 °C and strain rate of 1 s1, respectively. The microstructure of continuous dynamic recrystallization grains with serrated boundaries could be easily discovered in Fig. 5a and b, as the arrows indicated. 3.3. Contour line of stress difference The contour lines of stress difference at strains of 0.1and 0.5 as well as at temperatures of 900 °C and 1450 °C are represented in Fig. 6. From Fig. 6a and b, it can be seen that the higher values of stress difference completely concentrate in the low temperature and high strain rate region, where the value of temperature
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Fig. 3. Variation of flow stress with temperature at constant strain rates: (a) e_ = 0.01 s1, (b) e_ = 0.1 s1, (c) e_ = 0.5 s1, (d) e_ = 1.0 s1 and (e) e_ = 10 s1.
Q compensated strain rate factor Z which is defined as Z ¼ e_ expðRT Þ [19] is larger. By comparing Fig. 6a and b, it also could be found that the value of stress difference increases obviously with the strain increasing. As it is known to all that increasing deformation will result in the addition of deformation activation energy Q so that the value of Z parameter will increase under constant deformation temperature and strain rate. Li et al. [20] have revealed that the
increment of Q along with deformation increase may be ascribed to the rapid increase of dislocation density and the interaction between dislocations which makes the dislocation motion more difficult. Therefore, in view of the above analysis, it is suggested that with the deformation going on the increment of deformation activation energy Q will lead to the increase of deformation resistance. In addition, the increasing amplitude of flow stress at low
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Fig. 4. Optical micrographs at 900 °C with different strain rates of: (a) 0.01 s1, (b) 0.1 s1; and at strain rate of 10 s1 with different temperatures of (c) 1200 °C, and (d) 1400 °C.
Fig. 5. Optical micrographs at different strain rates and temperatures: (a) T = 1300 °C, e_ = 0.5 s1 and (b) T = 1400 °C, e_ = 1 s1.
temperature is larger than that at high temperature. That is to say, deformation strengthening is more evident in low temperature. Fig. 6c and d illustrate the contour lines of stress difference at temperatures of 900 °C and 1450 °C, respectively. In Fig. 6c, the larger values of stress difference completely gather in high strain and high strain rate area, in which there is a higher value of Z parameter. And in the meantime, according to the slope of contour lines in Fig. 6c, it should be aware that the contour lines of stress difference are close to parallel to the strain rate axis and the smaller the strain the more obvious the tendency, which implies that stress difference is more sensitive to strain and deformation strengthening is the main strengthening mechanism under low temperature. However, the contour lines of stress difference in Fig. 6d are approximately parallel to the strain axis. It is inferred that stress difference is particularly sensitive to strain rate and rheological strengthening is the primary strengthening mechanism under high temperature. In comparison of Fig. 6c and d, it would
be discovered that the value of stress difference in the former is higher than that in the latter, from which is resulted dynamic recrystallization softening effect counterbalanced work hardening in powder metallurgy molybdenum at high temperature. 3.4. The strain hardening index and strain-rate sensitivity exponent The expressions of strain hardening index n and strain-rate sensitivity exponent m are described as the followings:
@ log r D log r @ log e_ D log e_
ð1Þ
@ log r D log r @ log e D log e
ð2Þ
m¼
n¼
where e_ is the strain rate, e is the strain, and r is the stress.
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Fig. 6. Contour line of stress difference at different strains of: (a)
e = 0.1, (b) e = 0.5; and at different temperatures of: (c) T = 900 °C, (d) T = 1450 °C.
Fig. 7. Variation of the value of strain rate sensitivity exponent m with strain.
Fig. 7 shows the curves of variation of the strain-rate sensitivity exponent m values with strain at different temperature. It is clear that the values of strain-rate sensitivity exponent m above 1300 °C are totally lager than that below 1250 °C under the same strain.
And the value of m will ascend accompanying with the strain increased except that at 1300 °C and 1350 °C, where a drop between strain of 0.4 and 0.5 can be noticed. Moreover, the higher the temperature, the faster the increasing rate of strain-rate sensitivity exponent rises. It is concluded that strain-rate sensitivity exponent is not sensitive to the strain at low temperature, but the contrary at high temperature condition. Fig. 8a and b represent the curves of the value of strain-rate sensitivity exponent m at different strain and strain hardening index n at different strain rate varied with temperature, respectively. It can be observed from Fig. 8a that at low temperature domain the value of strain-rate sensitivity exponent m is relatively small, but at high temperature region the value of m increases quickly and then decreases after reaching a peak value at 1350 °C. Sivapragash et al. [21] also found a similar variation of the value of m in ZE41A magnesium alloy and believed that the change in the value of m indicates a corresponding change in the deformation mechanism. However, the value of strain hardening index n decreases gradually over the entire temperature region. From Fig. 9, it can be found that 1250 °C is a key temperature inflection point where the main strengthening mechanism of powder metallurgy molybdenum changes. As the temperature below 1250 °C, the mean value of n is much bigger than that of m which indicates that deformation strengthening is the main strengthening mechanism at low
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4. Conclusions 1. The high-temperature deformation behavior of powder metallurgy molybdenum has been investigated based on a series of isothermal hot compression tests. Dynamic recrystallization softening effect occurs at the temperature region (1200– 1450 °C), meanwhile secondary recrystallization begins to take place as the temperature reaching to 1300 °C. 2. The value of strain hardening index n is relatively large in lowtemperature stage in which deformation strengthening is the main strengthening mechanism. However, in high temperature phase the value of strain-rate sensitivity exponent m increases sharply and rheological strengthening becomes the primary enforcement mechanism. 3. The temperature range from 1200 °C to 1450 °C is the boundary where high-temperature deformation and low-temperature deformation for powder metallurgy molybdenum was distinguished, and flow stress is significantly sensitive to temperature in this area. In addition, 1250 °C is a key temperature inflection point and the value of m reaches peak value at 1350 °C. References
Fig. 8. Variation of: (a) strain rate sensitivity exponent m and (b) strain hardening index n with temperature.
Fig. 9. Variation of mean values of strain rate sensitivity exponent m and strain hardening index n with temperature.
temperature domain. On the contrary, when the temperature comes up to 1250 °C, the mean value of n declines gradually and becomes smaller than the mean value of m, which suggests that rheological strengthening changes into the primary strengthening mechanism at high temperature area.
[1] Garg Pranav, Park Seong-Jin, Randall M. German. Effect of die compaction pressure on densification behavior of molybdenum powders. Int J Refract Metal Hard Mater 2007;25:16–24. [2] Wang Yufeng, Li Fuguo, et al. Study on hot deformation characteristics of molybdenum based on processing map. Xiyou Jinshu Cailiao Yu Gongcheng 2009;38(8):1358–62. [3] Schade P, Bartha L. Deformation and properties of PM molybdenum and tungsten. Int J Refract Metal Hard Mater 2002;20(4):259–60. [4] Walser DH, Shields DJ. Traditional and emerging application of molybdenum metal and its alloys. 18th annual general meeting of IMOA, Austria; September 14, 2006. [5] Shields Jr, Rozak John A, Gary A. Electronic applications for P/M molybdenum. Int J Powder Metall 2005;41(2):21–8. [6] Yoshinaga Hideo. Grain-boundary structure and strength in high temperature materials. Mater Trans JIM 1990;31(4):233–48. [7] Watanabe Tadao, Tsurekawa Sadahiro. Control of brittleness and development of desirable mechanical properties in polycrystalline systems by grain boundary engineering. Acta Mater 1999;47(15):4171–85. [8] Shigeaki Kobayashi, Sadahiro Tsurekawa, Tadao Watanabe. Grain boundary hardening and triple junction hardening in polycrystalline molybdenum. Acta Mater 2005;53:1051–7. [9] Wang Donghui, Yuan Xiaobo, Li Zhongkui, et al. Progress of research and applications for Mo metal and its alloys. Rare Metals Lett 2006;25(12):1–7. [10] Garg Pranav, Park Seong-Jin, German Randall M. Effect of die compaction pressure on densification behavior of molybdenum powders. Int J Refract Metal Hard Mater 2007;25:16–24. [11] Wang Yufeng, Li Fuguo, et al. Numerical simulation of radial precision forging technology for metal molybdenum. Xiyou Jinshu Cailiao Yu Gongcheng 2009;38(12):2136–40. [12] Laribi M, Vannes AB, Treheux D. Study of mechanical behavior of molybdenum coating using sliding wear and impact tests. Wear 2007;262(11–12):1330–6. [13] Ciulik J, Taleff EM. Power-law creep of powder-metallurgy grade molybdenum sheet. Mater Sci Eng A 2007;463(1–2):197–202. [14] Wang Yong, Gao Jiacheng, Gongming Chen, et al. Properties at elevated temperature and recrystallization of molybdenum doped with potassium, silicon and aluminum. Int J Refract Metal Hard Mater 2008;26(1):9–13. [15] Mandal Sumantra, Sivaprasad PV, et al. Constitutive equations to predict high temperature flow stress in a Ti-modified austenitic stainless steel. Mater Sci Eng A 2009;500:114–21. [16] Poliak EI, Jonas JJ. A one-parameter approach to determining the critical conditions for the initiation of dynamic recrystallization. Acta Mater 1996;44(1):127–36. [17] Luo Jiao, Li Miaoquan, et al. Effect of the strain on the deformation behavior of isothermally compressed Ti–6Al–4V alloy. Mater Sci Eng A 2009;505:88–95. [18] Prasad YVRK, Sasidhara S, Sikka VK. Characterization of mechanisms of hot deformation of as-cast nickel aluminide alloy. Intermetallics 2000;8:987–95. [19] Cai Jun, Li Fuguo, Liu Taiying, et al. Constitutive equations for elevated temperature flow stress of Ti–6Al–4V alloy considering the effect of strain. Mater Des 2011;32:1144–51. [20] Li Dejun, Feng Yaorong, et al. Hot deformation behavior of an austenitic Fe– 20Mn–3Si–3Al transformation induced plasticity steel. J Mater Des 2011;34:713–8. [21] Sivapragash M, Lakshminarayanan PR, et al. Hot deformation behavior of ZE41A magnesium alloy. Mater Des 2009;29:860–6.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Characterization of strengthening mechanism and hot deformation behavior of powder metallurgy molybdenum Meili Xiao a,⇑, Fuguo Li a,b, Hangfang Xie a, Yufeng Wang a a b
School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
Article history: Received 18 May 2011 Accepted 29 July 2011 Available online 3 August 2011
a b s t r a c t The high-temperature deformation behavior of powder metallurgy molybdenum has been investigated based on a series of isothermal hot compression tests, which were carried out on a Gleeble-1500 thermal mechanical simulator in a wide range of temperatures (900–1450 °C) and strain rates (0.01–10 s1). Through the research on the experimental stress–strain curves, it reveals that dynamic recrystallization softening effect of powder metallurgy molybdenum occurs in the temperature range from 1200 °C to 1450 °C, in which the flow stress is significantly sensitive to temperature. In comparison with the value of strain hardening index n which decreases along with the temperature rising, the value of strain-rate sensitivity exponent m does not change obviously; however, it increases slowly with the increasing of temperature at first and achieves a peak value at 1350 °C. Furthermore, relying on the comparison of mean value of n and m, it is suggested that deformation strengthening is the main strengthening mechanism at low temperature while the rheological strengthening changes into the primary strengthening mechanism at high temperature. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Molybdenum is a kind of valuable refractory metal and its melting point is about 2610 °C [1]. The interatomic bonding force of molybdenum is so strong that at room temperature and high temperature its strength is always high [2]. Besides, in contrast with other refractory metals like Ta, Nb, Co, W and so on, molybdenum not only possesses high melting point, excellent thermal conductivity, good electrical conductivity and corrosion resistance, but also has a low coefficient of thermal expansion and high hardness [3]. Therefore, molybdenum and its alloys have been widely used in many fields, for instance, metallurgy, machinery, aviation, aerospace, and nuclear power industry etc. [4,5]. Nonetheless, its usefulness as a structural material is limited by its severe brittleness which is known to come from intergranular fracture. Yoshinaqa [6] considered that the grain boundaries with fairly high energy are responsible for intergranular brittleness in molybdenum. Meanwhile, Watanabe and Tsurekawa [7] have investigated the relationship between intergranular brittleness and grain boundary microstructure in two-dimensional polycrystalline molybdenum and indicated that improvement of brittleness of polycrystalline
⇑ Corresponding author: Tel.: +86 029 88474117. E-mail address: [email protected] (M. Xiao). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.07.065
molybdenum due to intergranular fracture can be achieved by controlling the GBCD through grain boundary engineering. Moreover, owing to their high melting point, high hardness [3], low temperature brittleness [8] and poor capability of high temperature oxidation resistance [9], molybdenum and its alloys could hardly be manufactured by casting, forging and machining except the method of powder metallurgy. However, conventional P/M processing of molybdenum to near full density requires long time sintering at high temperature which leads to excessive grain coarsening and subsequent loss of mechanical properties. Garg et al. [10] have researched the sintering mechanism of molybdenum powders and the accurate density prediction and design of optimum sintering cycles of P/M molybdenum were proposed. On the other side, molybdenum forgings should be processed in a narrow high temperature domain because of large deformation resistance [11]. The micro-structural analysis, mechanical behavior and properties testing of molybdenum and its alloys have been studied by many researchers during recent years. For example, Laribi et al. [12] have discussed the metallurgical and mechanical behavior of molybdenum coating formed by flame spraying based on its microstructure, hardness and tribological resistance. Additionally, Creep behavior at temperatures between 1300 °C and 1600 °C of P/M grade molybdenum sheet as well as elevated temperature properties and recrystallization of molybdenum doped with potassium, silicon and aluminum also have been studied by Ciulik and Taleff
M. Xiao et al. / Materials and Design 34 (2012) 112–119
113
[13] and Wang et al. [14], respectively. Yet, few works have been focused on the hot deformation strengthening mechanism of powder metallurgy molybdenum, so further more detailed researches are necessary. In this paper, firstly, a series of isothermal hot compression tests were performed on a Gleeble-1500 thermal mechanical simulator. On the basis of experimental results, hot deformation behavior of powder metallurgy molybdenum has been investigated. The dependence of flow behavior and micro-structural evolution on strain rate and temperature were analyzed by introducing the Zener–Hollomon parameter. Finally, the dominant strengthening mechanism in deformation process of powder metallurgy molybdenum has been indicated in terms of the value of strain hardening index n and strain-rate sensitivity exponent m. 2. Experiments
Fig. 1. The original microstructure of power metallurgy molybdenum.
Chemical composition of as-received powder metallurgy molybdenum has been given in Table 1. The mesh sizes of molybdenum powder particles are in the range of 2.0–3.5 lm and the intermediate frequency induction sintering process was introduced to produce P/M molybdenum. Meanwhile, the original microstructure of sintered molybdenum by powder metallurgy process has been shown in Fig. 1, where the grain boundary is not clear. It is concluded that the prior particle boundaries emerged in the sintered molybdenum lead to it, subsequent forging or extrusion process is necessary to eliminate the particle boundaries and make the grain boundary more apparent. The diameter of as-received P/M molybdenum bar is 20 mm, from which cylindrical compression specimens are machined with a diameter of 8 mm and a height of 12 mm. By usage of large current resistance-heating system, each specimen was heated to the testing temperature at a rate of 10 °C/s, and held for 5 min at the isothermal condition prior to testing so as to obtain a uniform temperature. The compression tests were performed in the temperature range of 900–1450 °C and the strain rate range of 0.01– 10 s1 on a Gleeble-1500 thermal mechanical simulator, while stress–strain data at different temperatures and strain rates were recorded automatically. In order to reduce the interface friction, mica sheet was used to segregate the specimens and tools. All specimens were compressed to true strain of 0.5, and then immediately air cooled down to room temperature. 3. Results and discussion 3.1. Flow stress Flow stress curves of powder metallurgy molybdenum recorded at different temperatures are shown in Fig. 2, respectively. It could be found from Fig. 2 that in the initial stage the flow stress increases quickly with the increasing of strain. The occurrence of rapid increase in stress chiefly attributes to the sharp increasing of dislocation density at the beginning of deformation. Then, the increase rate of flow stress gradually slows down ascribing to the effect of dynamic recovery. It may be noted that the higher the temperature, the faster increase rate drops. Mandal et al. [15] also proclaimed similar changing regularity in high temperature flow stress curves of D9 alloy and pointed out this is due to the fact that
Table 1 Chemical composition of powder metallurgy molybdenum (wt.%). Fe
Ni
Cu
Ca
Mg
Si
Al
O
Mo
0.40
0.70
0.03
0.08
0.01
0.20
0.20
0.40
Bal.
higher temperature offers higher mobility to the grain boundary which leads to the extent of dynamic softening more significant. The values of peak flow stress at different temperatures and strain rates are presented in Table 2. It is interesting to note that the peak stress decreases with the increasing of deformation temperature at certain strain rate, and increases with the increasing of strain rate at certain deformation temperature. However, at some deformation temperatures (dynamic recrystallization temperature or above), after reaching a peak value the flow stress will continuously decrease with the increasing of strain, which may be caused by the dynamic recrystallization softening effect arising from accumulation of dislocation density and achievement of critical strain. That is in common with the initial conditions of the onset of dynamic recrystallization suggested in Ref. [16] that the local stored energy attains a maximum and critical value as well as the rate of dissipation associated with deformation decreases to a minimum value. In the other hand, the value of critical dynamic recrystallization strain can be changed with temperature; and the higher the temperature the smaller the critical strain. Furthermore, with the deformation temperature rising secondary dynamic recrystallization will take place in compression specimen at suitable strain rate. It can be observed from Fig. 2a–c that flow stress curves finally reach a steady state under most deformation conditions, which means that dynamic recovery is the dominating softening effect in powder metallurgy molybdenum below 1100 °C. Luo et al. [17] have given insight into the hot deformation flow behavior of Ti–6Al–4V alloy in the a + b two-phase region and b single-phase region, and also showed clearly that the steady flow behavior occurs as the dynamic recovery is sufficient to counteract the work-hardening effect of the alloy in the isothermal compression. Simultaneously, an obvious dynamic recrystallization softening phenomenon occurs at 1200 °C in the strain rate of 0.01 s1 as illustrated in Fig. 2d, and the value of critical strain calculated according to the equation demonstrated in Ref. [16] is about 0.36. Similarly, dynamic recrystallization softening phenomenon emerges at most of the strain rates in Fig. 2e, and the value of critical strain is approximate 0.27 for lower strain rate while 0.36 for higher strain rate. As the temperature rising to 1300 °C or even higher, secondary dynamic recrystallization comes into being at certain strain rate and the value of firstly dynamic recrystallization critical strain reduces to 0.23. The curves about the variation of flow stress with temperature at constant strain rate have been represented in Fig. 3. It is apparent that work-hardening feature appears in the temperature range of 900–1100 °C, nevertheless, dynamic recrystallization softening effect begins to happen at the temperature domain from 1200 °C to 1450 °C. Moreover, Fig. 3 also manifests that dynamic recrystalliza-
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Fig. 2. Flow stress curves of powder metallurgy molybdenum at different temperatures: (a) T = 900 °C, (b) T = 1000 °C, (c) T = 1100 °C, (d) T = 1200 °C, (e) T = 1250 °C, (f) T = 1300 °C, (g) T = 1350 °C, (h) T = 1400 °C, and (i) T = 1450 °C.
tion initially occurs in the low strain rate, and then gradually expands to high strain rate with the increase in temperature and deformation degree. The temperature range from 1200 °C to 1450 °C is the critical boundary where high-temperature deformation and low-temperature deformation for powder metallurgy molybdenum was distinguished, and flow stress is significantly sensitive to temperature in this region. In addition, it could be found in Fig. 3 that the specimen deformed with a small strain of 0.2, which
without dynamic recrystallization happening, still remains strain hardening characteristic in the temperature sensitive region. 3.2. Microstructure analysis Prasad et al. [18] have demonstrated that it is not appropriate to predict the deformation mechanisms on the basis of the shapes of the stress–strain curves because of that many hot deformation
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Fig. 2 (continued)
Table 2 The value of peak flow stress at different temperatures and strain rates (Mpa).
e_ =s1
T/°C 900
1000
1100
1200
1250
1300
1350
1400
1450
0.01 0.1 0.5 1 10
258.54 297.79 315.58 326.54 345.25
220.57 260.77 279.80 293.70 326.42
182.77 223.43 242.80 260.44 284.80
149.64 178.71 190.22 206.62 229.34
136.17 169.37 185.70 202.43 212.15
105.08 139.20 147.23 180.11 191.61
103.60 137.34 145.13 176.03 187.53
102.09 122.55 143.47 168.48 173.16
99.57 105.78 131.65 149.69 156.85
mechanism may result in a similar shape of the stress–strain curves. However, by making on the metallographic observation, the deformation mechanisms inferred in terms of the shapes of stress–strain curves are able to be confirmed. The optical micrographs of powder metallurgy molybdenum compressed in different strain rates and temperatures are shown in Fig. 4. For example, Fig. 4a and b display the microstructures of specimens deforming in 900 °C with different strain rates of 0.01 s1 and 0.1 s1, respectively. It can be noted that there is no obvious dynamic recrystallization phenomenon appearing in these two pictures. However, a large number of fine equiaxial dynamic recrystallization grains that distribute around the original grains can be observed in Fig. 4c and d. By means of comparing the volume fraction of dynamic recrystallization grains in Fig. 4c and d, it could be found that under identical strain rate the volume fraction will increase with the increasing of
temperature. Fig. 5a and b represent the optical micrographs of powder metallurgy molybdenum samples compressed in the temperature of 1300 °C and strain rate of 0.5 s1 as well as in the temperature of 1400 °C and strain rate of 1 s1, respectively. The microstructure of continuous dynamic recrystallization grains with serrated boundaries could be easily discovered in Fig. 5a and b, as the arrows indicated. 3.3. Contour line of stress difference The contour lines of stress difference at strains of 0.1and 0.5 as well as at temperatures of 900 °C and 1450 °C are represented in Fig. 6. From Fig. 6a and b, it can be seen that the higher values of stress difference completely concentrate in the low temperature and high strain rate region, where the value of temperature
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Fig. 3. Variation of flow stress with temperature at constant strain rates: (a) e_ = 0.01 s1, (b) e_ = 0.1 s1, (c) e_ = 0.5 s1, (d) e_ = 1.0 s1 and (e) e_ = 10 s1.
Q compensated strain rate factor Z which is defined as Z ¼ e_ expðRT Þ [19] is larger. By comparing Fig. 6a and b, it also could be found that the value of stress difference increases obviously with the strain increasing. As it is known to all that increasing deformation will result in the addition of deformation activation energy Q so that the value of Z parameter will increase under constant deformation temperature and strain rate. Li et al. [20] have revealed that the
increment of Q along with deformation increase may be ascribed to the rapid increase of dislocation density and the interaction between dislocations which makes the dislocation motion more difficult. Therefore, in view of the above analysis, it is suggested that with the deformation going on the increment of deformation activation energy Q will lead to the increase of deformation resistance. In addition, the increasing amplitude of flow stress at low
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Fig. 4. Optical micrographs at 900 °C with different strain rates of: (a) 0.01 s1, (b) 0.1 s1; and at strain rate of 10 s1 with different temperatures of (c) 1200 °C, and (d) 1400 °C.
Fig. 5. Optical micrographs at different strain rates and temperatures: (a) T = 1300 °C, e_ = 0.5 s1 and (b) T = 1400 °C, e_ = 1 s1.
temperature is larger than that at high temperature. That is to say, deformation strengthening is more evident in low temperature. Fig. 6c and d illustrate the contour lines of stress difference at temperatures of 900 °C and 1450 °C, respectively. In Fig. 6c, the larger values of stress difference completely gather in high strain and high strain rate area, in which there is a higher value of Z parameter. And in the meantime, according to the slope of contour lines in Fig. 6c, it should be aware that the contour lines of stress difference are close to parallel to the strain rate axis and the smaller the strain the more obvious the tendency, which implies that stress difference is more sensitive to strain and deformation strengthening is the main strengthening mechanism under low temperature. However, the contour lines of stress difference in Fig. 6d are approximately parallel to the strain axis. It is inferred that stress difference is particularly sensitive to strain rate and rheological strengthening is the primary strengthening mechanism under high temperature. In comparison of Fig. 6c and d, it would
be discovered that the value of stress difference in the former is higher than that in the latter, from which is resulted dynamic recrystallization softening effect counterbalanced work hardening in powder metallurgy molybdenum at high temperature. 3.4. The strain hardening index and strain-rate sensitivity exponent The expressions of strain hardening index n and strain-rate sensitivity exponent m are described as the followings:
@ log r D log r @ log e_ D log e_
ð1Þ
@ log r D log r @ log e D log e
ð2Þ
m¼
n¼
where e_ is the strain rate, e is the strain, and r is the stress.
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Fig. 6. Contour line of stress difference at different strains of: (a)
e = 0.1, (b) e = 0.5; and at different temperatures of: (c) T = 900 °C, (d) T = 1450 °C.
Fig. 7. Variation of the value of strain rate sensitivity exponent m with strain.
Fig. 7 shows the curves of variation of the strain-rate sensitivity exponent m values with strain at different temperature. It is clear that the values of strain-rate sensitivity exponent m above 1300 °C are totally lager than that below 1250 °C under the same strain.
And the value of m will ascend accompanying with the strain increased except that at 1300 °C and 1350 °C, where a drop between strain of 0.4 and 0.5 can be noticed. Moreover, the higher the temperature, the faster the increasing rate of strain-rate sensitivity exponent rises. It is concluded that strain-rate sensitivity exponent is not sensitive to the strain at low temperature, but the contrary at high temperature condition. Fig. 8a and b represent the curves of the value of strain-rate sensitivity exponent m at different strain and strain hardening index n at different strain rate varied with temperature, respectively. It can be observed from Fig. 8a that at low temperature domain the value of strain-rate sensitivity exponent m is relatively small, but at high temperature region the value of m increases quickly and then decreases after reaching a peak value at 1350 °C. Sivapragash et al. [21] also found a similar variation of the value of m in ZE41A magnesium alloy and believed that the change in the value of m indicates a corresponding change in the deformation mechanism. However, the value of strain hardening index n decreases gradually over the entire temperature region. From Fig. 9, it can be found that 1250 °C is a key temperature inflection point where the main strengthening mechanism of powder metallurgy molybdenum changes. As the temperature below 1250 °C, the mean value of n is much bigger than that of m which indicates that deformation strengthening is the main strengthening mechanism at low
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4. Conclusions 1. The high-temperature deformation behavior of powder metallurgy molybdenum has been investigated based on a series of isothermal hot compression tests. Dynamic recrystallization softening effect occurs at the temperature region (1200– 1450 °C), meanwhile secondary recrystallization begins to take place as the temperature reaching to 1300 °C. 2. The value of strain hardening index n is relatively large in lowtemperature stage in which deformation strengthening is the main strengthening mechanism. However, in high temperature phase the value of strain-rate sensitivity exponent m increases sharply and rheological strengthening becomes the primary enforcement mechanism. 3. The temperature range from 1200 °C to 1450 °C is the boundary where high-temperature deformation and low-temperature deformation for powder metallurgy molybdenum was distinguished, and flow stress is significantly sensitive to temperature in this area. In addition, 1250 °C is a key temperature inflection point and the value of m reaches peak value at 1350 °C. References
Fig. 8. Variation of: (a) strain rate sensitivity exponent m and (b) strain hardening index n with temperature.
Fig. 9. Variation of mean values of strain rate sensitivity exponent m and strain hardening index n with temperature.
temperature domain. On the contrary, when the temperature comes up to 1250 °C, the mean value of n declines gradually and becomes smaller than the mean value of m, which suggests that rheological strengthening changes into the primary strengthening mechanism at high temperature area.
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