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High temperature mechanical properties of TZM alloys under different lanthanum doping treatments
Journal of Alloys and Compounds 711 (2017) 64e70
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
High temperature mechanical properties of TZM alloys under different lanthanum doping treatments Ping Hu a, b, *, Fan Yang a, b, Jie Deng a, b, Tian Chang a, b, Boliang Hu a, b, Jiangfei Tan a, b, Kuaishe Wang a, b, Weicheng Cao c, Pengfa Feng c, Hailiang Yu a a b c
School of Metallurgy Engineering, Xi'an University of Architecture and Technology, Xi'an, 710055, China State Local Joint Engineering Research Center for Functional Materials Processing, Xi'an University of Architecture and Technology, Xi'an, 710055, China Jinduicheng Molybdenum Co., Ltd., Xi'an, 710077, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 February 2017 Received in revised form 27 March 2017 Accepted 30 March 2017 Available online 31 March 2017
La(NO3)3 and La2O3 doped titanium-zirconium-molybdenum (TZM) alloys were fabricated by using powder metallurgy method, and their room and high temperature mechanical properties were studied in this paper. Results show that the mechanical properties of La(NO3)3-TZM alloy sheet is better than that of La2O3-TZM alloy sheet in room temperature and the high temperature mechanical properties of both alloys have similar change trend but different sensitivity of temperature. The tiny grains in La(NO3)3-TZM alloy sheet is more sensitive to temperature and the lanthanum doping significantly influences the temperature sensitivity of the alloy. When the temperature is lower than 1400 C, both TZM alloy sheets present a transgranular fracture mode. It is ductile intergranular fracture mode when the temperature is higher than 1400 C, and the temperature of fracture mode transition is 1400 C. © 2017 Elsevier B.V. All rights reserved.
Keywords: Doping methods La-TZM alloy High temperature mechanical properties Fracture
1. Introduction Molybdenum has wide application prospect due to its high melting-point and its excellent high temperature performance [1]. However, the application of pure molybdenum is greatly limited because of its properties such as high ductile-brittle transition temperature and high brittleness at room temperature [2]. Scientists have developed many molybdenum-based alloys to expand their application, such as molybdenum-copper alloy, molybdenumtitanium alloy, rare earth-molybdenum alloy, molybdenum-siliconboron alloy and titanium-zirconium-molybdenum (TZM) alloy [1]. TZM alloy (0.4% 0.55%Ti, 0.06%-0.12%Zr, 0.01%-0.04%C) is one of the most widely used molybdenum alloys due to the characteristics of high melting-point, high strength, high elastic modulus, strong corrosion resistance, low coefficient of expansion, high thermal conductivity and excellent high temperature mechanical properties [3e5]. Although TZM alloy has been used in aerospace, power generation, nuclear reactor, military, and other fields [6,7], its application has also been limited for poor oxidation resistance and
* Corresponding author. School of Metallurgy Engineering, Xi'an University of Architecture and Technology, Xi'an, 710055, China. E-mail address: [email protected] (P. Hu). http://dx.doi.org/10.1016/j.jallcom.2017.03.346 0925-8388/© 2017 Elsevier B.V. All rights reserved.
poor toughness at high temperature [8e13]. Recently, industrial applications of molybdenum alloys have put forward higher requirements. The progress of TZM alloy with stronger oxidation resistance and better mechanical properties at high temperature is imminent. Tan et al. [14] investigated yield strength, tensile strength and elongation for different processed TZM alloy sheets at different temperatures. They found that the tensile strength and ductility decreases greatly when the temperature is higher than 1400 C. Fan et al. [15] investigated the effect of different forms, incorporation methods (TiC, ZrC), preparation process and unequal temperatures on mechanical properties of TZM alloy. The addition of pure Zr is better than that in the form of ZrH2, and the alloy has the highest performance when the mass fraction of Zr reaches 0.1%. The addition of alloying elements Ti in the form of TiH2 is better than other forms, and the alloy has the highest performance when the mass fraction of Ti is 0.8%. Shi et al. [16] studied the fatigue behavior of TZM alloy. They found that TZM alloy has the same isothermal fatigue characteristics at 350 C and 500 C, which indicate TZM alloy maintains excellent high temperature cycle strength. Majumdar et al. [17] investigated the mechanical properties of TZM alloy between 600 and 900 C during hot compression test with strain rates from 0.001 s1 to 1 s1. They found that TZM alloy possesses low strain rate sensitivity at this temperature range. However, there are no
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reports related to the effect of doping methods on the mechanical properties of TZM alloys. Our previous studies [10e14] have shown that the lanthanum doped TZM alloy have high structure density, oxidation resistance, recrystallization temperature, arc ablation resistance and low ductile-to-brittle transition temperature. In this paper, we further studied the high temperature mechanical properties of TZM alloys under different lanthanum doping treatments. The new liquid-solid mixed method was developed for the purpose of improving the mixture uniformity and the refining of particle size by doping La(NO3)3. The La2O3-TZM and La(NO3)3-TZM alloy sintered billets and rolled sheets were prepared by different doping methods of powder metallurgy, whose high temperature performances were analyzed by mechanical properties, microstructure and fracture mode of TZM alloys. 2. Experimental Fig. 1. Room (a) and high (b) temperature tensile geometries of La-TZM alloy sheets (mm).
2.1. Preparation of La-TZM alloy sheets Analytical reagent of Mo powder, TiH2 powder, ZrH2 powder, La2O3 powder, La(NO3)3 powder and stearic acid crystals were chosen to prepare La-TZM alloys. Powder metallurgy process was used to prepare the two kinds of TZM alloys: La2O3-TZM alloy (1#) and La(NO3)3-TZM alloy (2#). Table 1 lists the chemical compositions of the alloys. First, Solid-solid mixing was used to fabricate the La2O3-TZM alloy mixed powder and liquid-solid mixing was improved used to fabricate the La(NO3)3-TZM alloy mixed powder. Second, Ball-milling, pressing, vacuum drying, and sintering processes were used to prepare sintered billets of both alloys. At last, 0.5 mm thick alloy sheets were prepared by hot-rolling, warmrolling and caustic washing. The details for preparing the samples were in the previous publication [9]. 2.2. Room and high temperature tensile tests The tensile specimens were made of the rolled 0.5 mm-thick alloy sheets, whose geometrical parameters are in Fig. 1. The tensile tests were conducted taken along the rolling direction, and the high tensile test was carried out at temperatures of 1000 C, 1200 C, 1400 C and 1600 C respectively under vacuum atmosphere to measure the high temperature tensile strength and elongation by high temperature universal testing machine, which can heat the specimens to 1600 C.
2.5. Micro fracture analysis The fracture morphology of high temperature tensile specimens were tested by using JEOL JSM6460 scanning electron microscope (SEM). The effects on high temperature performance of La2O3 and La(NO3)3 alloy sheets were analyzed through the fracture surface morphology, the second phase particles morphology and distribution. 3. Results and discussion 3.1. Mechanical properties of La-TZM alloy sheets 3.1.1. Room temperature mechanical properties The room temperature mechanical property of La-TZM alloy sheets was obtained by tensile tests. Fig. 2 shows the engineering strain vs. stress curves. The ultimate strength of the La2O3-TZM alloy is 1263 MPa, while the strength of the La(NO3)3-TZM alloy reaches 1405 MPa at room temperature, as shown in Fig. 2. The La(NO3)3 doping method increases the ultimate strength of the TZM alloy by 11.2%. In addition, the tensile result show that the tensile elongation of the La2O3-TZM alloy is 7.9%, while the La(NO3)3-TZM is 7.5%. It is obvious that the La(NO3)3 doping
2.3. Vickers hardness test The Vickers hardness of high temperature tensile specimens were tested by using Wolpert 401MVD Vickers hardness tester with the loading force of 500 g and the holding time of 10 s. Average value of 10 points was obtained for each specimen test. 2.4. Metallographic analysis The sintered billets and rolled sheets were cut into square specimens of 5 5 10 mm and 5 5 0.5 mm. The Nikon SMZ1000 type body optical microscope was used to analyze the metallographic structure of samples. Table 1 Composition design of TZM alloy (wt%). Sample
TiH2
ZrH2
Stearic acid
La2O3
La(NO3)3
Mo
1# 2#
0.5 0.5
0.1 0.1
0.25 0.25
1 0
0 1.99
Balance Balance
Fig. 2. Engineering strain vs. stress curves of La-TZM alloy sheets.
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method has significantly improved the room temperature mechanical property of La2O3-TZM alloy. 3.1.2. High temperature mechanical properties Fig. 3 shows the strength-elongation curves, ultimate strength and tensile elongation of La-TZM alloy rolled sheets with different doping methods at different temperatures. The high temperature ultimate strengths of La2O3-TZM alloy sheet and La(NO3)3-TZM alloy sheet decrease with the increase of temperature, and the high temperature ultimate strength of La(NO3)3-TZM alloy sheet is higher than that of La2O3-TZM alloy sheet at 1000 C, 1200 C and 1400 C, which is opposite at 1600 C. The difference of high temperature ultimate strengths between the two alloys with temperature changes are 85 MPa in 1000 C, 7 MPa in 1200 C, 4 MPa in 1400 C and 3 MPa in 1600 C, as shown in Fig. 3c. The difference turns a negative value to a positive value, and the absolute value of the difference is successively decreased, which indicates that the ultimate strength change rate of La(NO3)3TZM alloy sheet is higher than that of La2O3-TZM alloy sheet at high temperature. That is, La(NO3)3-TZM alloy has higher sensitivity to temperature. Generally, high temperature ultimate strength of both La2O3TZM alloy and La(NO3)3-TZM alloy decrease with the increase of temperature, and the change rate decrease rapidly with the increase of temperature, until it tends to stable. This is mainly because high temperature environment leads to eliminate work hardening below 1200 C, resulting in rapid decline of strength. When the temperature is higher than 1200 C, recrystallization starts and leads to the decrease in strength. The grains grow and the strength dropping rate decreases while the temperature is higher than 1400 C. Besides, the grain size of La(NO3)3-TZM alloy is smaller than that of La2O3-TZM alloy, resulting in that the grain
boundary occupy large proportion in the internal structure. However, the grain boundary strength decreases rapidly with the increase of temperature, which results in the rapidly decrease of La(NO3)3-TZM alloy high temperature ultimate strength. That is why the La(NO3)3-TZM alloy has higher sensitivity to temperature. Fig. 3d shows the high temperature elongation of La(NO3)3-TZM alloy and La2O3-TZM sheets. The high temperature fracture elongations of La2O3-TZM and La(NO3)3-TZM alloy sheets have the similar changes with the temperature, which reaches the maximum value when the temperature is 1200 C, and the elongation of samples gradually decreases with higher temperature than 1200 C. In addition, at 1200 C, the elongation of La(NO3)3TZM alloy sheet is larger than that of La2O3-TZM sheet, while it is opposite at other three kinds of temperature. During tensile tests, the strength of the grain and grain boundary decreases gradually when the La2O3-TZM and La(NO3)3TZM alloy sheets are deformed under high temperature environment due to the dynamic recovery and recrystallization behavior. At the beginning, the dynamic recovery and recrystallization has great effects on eliminating work hardening and stress concentration with the increase in temperature, transforming the alloy from brittle (the grain damaged and the strength reduced due to large deformation of plastic processing in the rolling state) to ductility (semi-hard and soft state). This can explain that the high temperature fracture elongation of La2O3-TZM and La(NO3)3-TZM alloy sheets begin to rise with the temperature rise to 1200 C. Due to the small degree of crystal lattice distortion in the grain boundaries, the atomic diffusion rate increases and the grain boundary strength decreases when the temperature exceeds to the critical value, which causes that the organizational structure strength becomes larger compared with the grain boundary strength. This results in the high temperature tensile ductility
Fig. 3. Strength-elongation curves (a, b), tensile strength-temperature curve (c) and high temperature elongation (d) of La-TZM alloys at different temperatures.
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in Fig. 3c. It also shows that the temperature sensitivity of La(NO3)3TZM alloy sheet is higher than that of La2O3-TZM alloy. 3.2. Metallographic analysis
Fig. 4. Vickers hardness of La-TZM alloy sheets.
reduction of the materials. So, the high temperature fracture elongation of La2O3-TZM and La(NO3)3-TZM alloy sheets begin to decrease when the temperature exceeds the critical temperature (1200 C).
The microstructure of La2O3-TZM alloy and La(NO3)3-TZM alloy sintered billets and rolled sheets was observed in cross-section, using the metallography analysis. The optical micrographs are shown in Fig. 5. In Fig. 5a and b, the second phase particles in the sintered billets distributed uniformly, while major of them districted in the grain and small amount of them distributed near grain boundary. In addition, the size of second phase particle in La(NO3)3-TZM alloy is obviously smaller than that of La2O3-TZM alloy, which results in smaller grain size of La(NO3)3-TZM alloy. Fig. 5c and d shows microstructure of the rolled La2O3-TZM alloy and La(NO3)3-TZM alloy sheets, which both show significant deformation texture after distinct plastic deformation. The microstructure heredity phenomenon obviously exists in the rolled alloy sheets. Similar to the sintered billets, the second phase particles in rolled sheets are fine and uniform in both morphology and size. The second phase and grain size in La(NO3)3-TZM alloy is smaller and more uniform than these in La2O3-TZM alloy. In addition, the fiber texture in La(NO3)3-TZM is more tenuous and longer than that in La2O3-TZM alloy. 3.3. Tensile fracture morphology
3.1.3. Vickers hardness analysis Fig. 4 shows the Vickers hardness of La2O3-TZM and La(NO3)3TZM alloy sheets after high temperature tensile test. The Vickers hardness of both alloy sheets show the same tendency with the change of temperature. The differences between La2O3-TZM and La(NO3)3-TZM alloy sheets are 28.4 HV, 9.1 HV, 9.1 HV, 7.8 HV at 1000 C, 1200 C, 1400 C, 1600 C respectively. This is consistent with the change of the tensile strength of the alloy sheets as shown
Fig. 6 shows the high temperature tensile fracture morphology of La2O3-TZM and La(NO3)3-TZM alloy sheets. It can be seen that La2O3-TZM and La(NO3)3-TZM alloy sheets are of similar features in morphology at the same temperature after high temperature tensile fracture. In Fig. 6a and b, the fracture morphologies of the two La-TZM alloys are honeycomb type after tensile test at 1000 C, which are the dimple morphology after ductile fracture. In Fig. 6c
Fig. 5. Microstructure of sintered billets and rolled sheets: (a, c) La2O3-TZM alloy, (b, d) La(NO3)3-TZM alloy.
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Fig. 6. High temperature tensile fracture morphology of La-TZM alloys at different temperatures. (La2O3-TZM alloy: a-1000 C, c-1200 C, e-1400 C, g-1600 C; La(NO3)3-TZM alloy: b-1000 C, d-1200 C, f-1400 C, h-1600 C).
and d, the fracture morphologies show the majority of dimple and the tongue or fan cleavage plane after cleavage fracture in local areas. In Fig. 6e and f, the fracture morphologies show the most of the rock or rock sugar-like fracture morphology after intergranular fracture with tongue or fan cleavage surface in local areas when the tensile tests were carried out at 1400 C. In Fig. 6g and h, the stone
or rock sugar fracture features appear in the fracture morphology surface as the tensile tests were carried out at 1600 C. Ductile fracture is a kind of high energy absorption fracture process. The expansion and growth of crack is a relatively slow process, which will consume a lot of plastic deformation energy. For La2O3-TZM and La(NO3)3-TZM alloys, a large amount of secondary
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phase particles distributed inside and outside the grain, and these second phase particles have relative lower deformation during rolling, resulting in the hinder of dislocation movement in the deformation process. According to the dislocation theory [18], when the dislocation moved to the second phase particles, it will bypass the second phase particles to move. While rounding the second phase particles, the partial dislocation will pile up around the particles, which causes the stress concentration. During the high temperature tensile tests, cracks firstly initiate from the stress concentration zones, and then slowly propagate until the fracture happens, forming the honeycomb-like fracture morphology. Because second phase particles are mainly distributed in the grain, the tensile fracture morphology of alloys for 1000 C is mainly dimple-type transgranular fracture, and a large number of second phase particles remain in the dimple, as shown in Fig. 6a and b. The cleavage fracture is a kind of transgranular brittle fracture. Under the action of normal stress, it is easy to be destroyed due to the weak bonding of atoms, which causes the fracture of the alloy sheets along the special crystal plane. The cleavage plane of the tongue or fan shape formed in 1400 C tensile fracture morphology of both La-TZM alloys, present the radial river pattern diffused from the second phase particle in the inner phase to the outer of grain, as shown in Fig. 6e and f. For the tensile test at 1200 C, both of the La2O3-TZM and La(NO3)3-TZM alloys test are presenting a kinds of mixed fracture mode of dimple fracture and cleavage fracture, while the dimple fracture is the main fracture mode, as shown in Fig. 6c and d. Intergranular fracture is generally a kind of brittle fracture because a thin layer of continuous or discontinuous network brittle phase appears at grain boundary positions. These brittle phases
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destroy the continuity of the alloy and reduce the bonding strength between the grains. When the alloy sheet under the action of external force, the network brittle phase cannot withstand the load and ruptures, then the cracks propagate quickly, and the fracture occurs along the brittle phase near the grain boundary, thereby forming a rock or rock sugar-like morphology. The tensile fracture morphologies for 1600 C of both alloys show the toughness intergranular fracture mode, as shown in Fig. 6g and h. These present that the La2O3-TZM and La(NO3)3-TZM alloy sheets tensile fracture are transgranular fracture at the temperature range of 1000 Ce1400 C. When the temperature is higher than 1400 C, the fracture modes of both alloy sheets are all intergranular fracture, which is completely consistent with the phenomenon that grain strength and grain boundary strength decrease with increasing temperature. So, the fracture mode transition temperature of two kinds of La-TZM alloy sheets is 1400 C (Teq ¼ 1400 C). The comparison of the fracture morphology shows that the doping of La2O3 or La(NO3)3 have little effect on the fracture mode and morphology. In addition, the grains of the intergranular fracture morphology of La(NO3)3-TZM alloy are slightly smaller than those in La2O3-TZM alloy, which indicates that the size of second phase particles has little effect on the fracture mode and fracture morphology of the alloys under high temperature environment. EDS spectrum analysis was performed on high temperature tensile fracture surface of La2O3-TZM alloy sheet tested at 1000 C and 1600 C, as shown in Fig. 7. The EDS analysis shows that the second phase particles in the fracture of La2O3-TZM alloy sheet at 1000 C are mainly La, Ti, Zr and O, which indicates that the second phase particles exist in the form of oxide. However, the fracture grain composition is major in molybdenum at 1600 C, and the
Fig. 7. The spectrum of the second phase in the La2O3-TZM alloy sheet fracture at different temperatures: (a) 1000 C; (b) 1600 C.
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second phase is not found. It is probably due to the recrystallization behavior at high temperature. The second phase particles in the alloy are wrapped by molybdenum matrix during the nucleation and growth process.
[2]
[3]
4. Conclusions [4]
In summary, the room and high temperature mechanical properties of La(NO3)3 and La2O3 doped TZM alloys were studied systematically. The La(NO3)3 doping method has better room temperature mechanical property than that of La2O3-TZM alloy. During high temperature tensile tests from 1000 C to 1600 C, the toughness of La-TZM alloys improves with higher temperature and it reaches the maximum value at 1200 C, which decreases greatly with further increasing in temperature. The microstructure of La(NO3)3-TZM alloy is more sensitive to temperature, and the lanthanum doping method has a significant impact on the temperature sensitivity of the alloy. When the temperature is lower than 1400 C, La-TZM alloy sheets fracture mode is major controlled by transgranular fracture. When the temperature is higher than 1400 C, it transits into intergranular fracture model. The temperature of fracture mode transition is 1400 C (Teq).
[5]
[6] [7]
[8]
[9]
[10]
[11]
Conflict of interest [12]
The authors declare that they have no conflict of interest. Acknowledgments
[13]
This work was supported by the Science and Technology Coordinating Innovative Engineering Project of Shaanxi Province (2015KTZDGY09-04), the Young Talent Fund of University Association for Science and Technology in Shaanxi China (20150201), the Service Local Special Program of Education Department of Shaanxi Province China (16JF016), the Science and Technology Foundation of Xi'a University of Architecture and Technology (QN1505) and the China Postdoctoral Science Foundation (2016M600770).
[14]
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
High temperature mechanical properties of TZM alloys under different lanthanum doping treatments Ping Hu a, b, *, Fan Yang a, b, Jie Deng a, b, Tian Chang a, b, Boliang Hu a, b, Jiangfei Tan a, b, Kuaishe Wang a, b, Weicheng Cao c, Pengfa Feng c, Hailiang Yu a a b c
School of Metallurgy Engineering, Xi'an University of Architecture and Technology, Xi'an, 710055, China State Local Joint Engineering Research Center for Functional Materials Processing, Xi'an University of Architecture and Technology, Xi'an, 710055, China Jinduicheng Molybdenum Co., Ltd., Xi'an, 710077, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 February 2017 Received in revised form 27 March 2017 Accepted 30 March 2017 Available online 31 March 2017
La(NO3)3 and La2O3 doped titanium-zirconium-molybdenum (TZM) alloys were fabricated by using powder metallurgy method, and their room and high temperature mechanical properties were studied in this paper. Results show that the mechanical properties of La(NO3)3-TZM alloy sheet is better than that of La2O3-TZM alloy sheet in room temperature and the high temperature mechanical properties of both alloys have similar change trend but different sensitivity of temperature. The tiny grains in La(NO3)3-TZM alloy sheet is more sensitive to temperature and the lanthanum doping significantly influences the temperature sensitivity of the alloy. When the temperature is lower than 1400 C, both TZM alloy sheets present a transgranular fracture mode. It is ductile intergranular fracture mode when the temperature is higher than 1400 C, and the temperature of fracture mode transition is 1400 C. © 2017 Elsevier B.V. All rights reserved.
Keywords: Doping methods La-TZM alloy High temperature mechanical properties Fracture
1. Introduction Molybdenum has wide application prospect due to its high melting-point and its excellent high temperature performance [1]. However, the application of pure molybdenum is greatly limited because of its properties such as high ductile-brittle transition temperature and high brittleness at room temperature [2]. Scientists have developed many molybdenum-based alloys to expand their application, such as molybdenum-copper alloy, molybdenumtitanium alloy, rare earth-molybdenum alloy, molybdenum-siliconboron alloy and titanium-zirconium-molybdenum (TZM) alloy [1]. TZM alloy (0.4% 0.55%Ti, 0.06%-0.12%Zr, 0.01%-0.04%C) is one of the most widely used molybdenum alloys due to the characteristics of high melting-point, high strength, high elastic modulus, strong corrosion resistance, low coefficient of expansion, high thermal conductivity and excellent high temperature mechanical properties [3e5]. Although TZM alloy has been used in aerospace, power generation, nuclear reactor, military, and other fields [6,7], its application has also been limited for poor oxidation resistance and
* Corresponding author. School of Metallurgy Engineering, Xi'an University of Architecture and Technology, Xi'an, 710055, China. E-mail address: [email protected] (P. Hu). http://dx.doi.org/10.1016/j.jallcom.2017.03.346 0925-8388/© 2017 Elsevier B.V. All rights reserved.
poor toughness at high temperature [8e13]. Recently, industrial applications of molybdenum alloys have put forward higher requirements. The progress of TZM alloy with stronger oxidation resistance and better mechanical properties at high temperature is imminent. Tan et al. [14] investigated yield strength, tensile strength and elongation for different processed TZM alloy sheets at different temperatures. They found that the tensile strength and ductility decreases greatly when the temperature is higher than 1400 C. Fan et al. [15] investigated the effect of different forms, incorporation methods (TiC, ZrC), preparation process and unequal temperatures on mechanical properties of TZM alloy. The addition of pure Zr is better than that in the form of ZrH2, and the alloy has the highest performance when the mass fraction of Zr reaches 0.1%. The addition of alloying elements Ti in the form of TiH2 is better than other forms, and the alloy has the highest performance when the mass fraction of Ti is 0.8%. Shi et al. [16] studied the fatigue behavior of TZM alloy. They found that TZM alloy has the same isothermal fatigue characteristics at 350 C and 500 C, which indicate TZM alloy maintains excellent high temperature cycle strength. Majumdar et al. [17] investigated the mechanical properties of TZM alloy between 600 and 900 C during hot compression test with strain rates from 0.001 s1 to 1 s1. They found that TZM alloy possesses low strain rate sensitivity at this temperature range. However, there are no
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reports related to the effect of doping methods on the mechanical properties of TZM alloys. Our previous studies [10e14] have shown that the lanthanum doped TZM alloy have high structure density, oxidation resistance, recrystallization temperature, arc ablation resistance and low ductile-to-brittle transition temperature. In this paper, we further studied the high temperature mechanical properties of TZM alloys under different lanthanum doping treatments. The new liquid-solid mixed method was developed for the purpose of improving the mixture uniformity and the refining of particle size by doping La(NO3)3. The La2O3-TZM and La(NO3)3-TZM alloy sintered billets and rolled sheets were prepared by different doping methods of powder metallurgy, whose high temperature performances were analyzed by mechanical properties, microstructure and fracture mode of TZM alloys. 2. Experimental Fig. 1. Room (a) and high (b) temperature tensile geometries of La-TZM alloy sheets (mm).
2.1. Preparation of La-TZM alloy sheets Analytical reagent of Mo powder, TiH2 powder, ZrH2 powder, La2O3 powder, La(NO3)3 powder and stearic acid crystals were chosen to prepare La-TZM alloys. Powder metallurgy process was used to prepare the two kinds of TZM alloys: La2O3-TZM alloy (1#) and La(NO3)3-TZM alloy (2#). Table 1 lists the chemical compositions of the alloys. First, Solid-solid mixing was used to fabricate the La2O3-TZM alloy mixed powder and liquid-solid mixing was improved used to fabricate the La(NO3)3-TZM alloy mixed powder. Second, Ball-milling, pressing, vacuum drying, and sintering processes were used to prepare sintered billets of both alloys. At last, 0.5 mm thick alloy sheets were prepared by hot-rolling, warmrolling and caustic washing. The details for preparing the samples were in the previous publication [9]. 2.2. Room and high temperature tensile tests The tensile specimens were made of the rolled 0.5 mm-thick alloy sheets, whose geometrical parameters are in Fig. 1. The tensile tests were conducted taken along the rolling direction, and the high tensile test was carried out at temperatures of 1000 C, 1200 C, 1400 C and 1600 C respectively under vacuum atmosphere to measure the high temperature tensile strength and elongation by high temperature universal testing machine, which can heat the specimens to 1600 C.
2.5. Micro fracture analysis The fracture morphology of high temperature tensile specimens were tested by using JEOL JSM6460 scanning electron microscope (SEM). The effects on high temperature performance of La2O3 and La(NO3)3 alloy sheets were analyzed through the fracture surface morphology, the second phase particles morphology and distribution. 3. Results and discussion 3.1. Mechanical properties of La-TZM alloy sheets 3.1.1. Room temperature mechanical properties The room temperature mechanical property of La-TZM alloy sheets was obtained by tensile tests. Fig. 2 shows the engineering strain vs. stress curves. The ultimate strength of the La2O3-TZM alloy is 1263 MPa, while the strength of the La(NO3)3-TZM alloy reaches 1405 MPa at room temperature, as shown in Fig. 2. The La(NO3)3 doping method increases the ultimate strength of the TZM alloy by 11.2%. In addition, the tensile result show that the tensile elongation of the La2O3-TZM alloy is 7.9%, while the La(NO3)3-TZM is 7.5%. It is obvious that the La(NO3)3 doping
2.3. Vickers hardness test The Vickers hardness of high temperature tensile specimens were tested by using Wolpert 401MVD Vickers hardness tester with the loading force of 500 g and the holding time of 10 s. Average value of 10 points was obtained for each specimen test. 2.4. Metallographic analysis The sintered billets and rolled sheets were cut into square specimens of 5 5 10 mm and 5 5 0.5 mm. The Nikon SMZ1000 type body optical microscope was used to analyze the metallographic structure of samples. Table 1 Composition design of TZM alloy (wt%). Sample
TiH2
ZrH2
Stearic acid
La2O3
La(NO3)3
Mo
1# 2#
0.5 0.5
0.1 0.1
0.25 0.25
1 0
0 1.99
Balance Balance
Fig. 2. Engineering strain vs. stress curves of La-TZM alloy sheets.
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method has significantly improved the room temperature mechanical property of La2O3-TZM alloy. 3.1.2. High temperature mechanical properties Fig. 3 shows the strength-elongation curves, ultimate strength and tensile elongation of La-TZM alloy rolled sheets with different doping methods at different temperatures. The high temperature ultimate strengths of La2O3-TZM alloy sheet and La(NO3)3-TZM alloy sheet decrease with the increase of temperature, and the high temperature ultimate strength of La(NO3)3-TZM alloy sheet is higher than that of La2O3-TZM alloy sheet at 1000 C, 1200 C and 1400 C, which is opposite at 1600 C. The difference of high temperature ultimate strengths between the two alloys with temperature changes are 85 MPa in 1000 C, 7 MPa in 1200 C, 4 MPa in 1400 C and 3 MPa in 1600 C, as shown in Fig. 3c. The difference turns a negative value to a positive value, and the absolute value of the difference is successively decreased, which indicates that the ultimate strength change rate of La(NO3)3TZM alloy sheet is higher than that of La2O3-TZM alloy sheet at high temperature. That is, La(NO3)3-TZM alloy has higher sensitivity to temperature. Generally, high temperature ultimate strength of both La2O3TZM alloy and La(NO3)3-TZM alloy decrease with the increase of temperature, and the change rate decrease rapidly with the increase of temperature, until it tends to stable. This is mainly because high temperature environment leads to eliminate work hardening below 1200 C, resulting in rapid decline of strength. When the temperature is higher than 1200 C, recrystallization starts and leads to the decrease in strength. The grains grow and the strength dropping rate decreases while the temperature is higher than 1400 C. Besides, the grain size of La(NO3)3-TZM alloy is smaller than that of La2O3-TZM alloy, resulting in that the grain
boundary occupy large proportion in the internal structure. However, the grain boundary strength decreases rapidly with the increase of temperature, which results in the rapidly decrease of La(NO3)3-TZM alloy high temperature ultimate strength. That is why the La(NO3)3-TZM alloy has higher sensitivity to temperature. Fig. 3d shows the high temperature elongation of La(NO3)3-TZM alloy and La2O3-TZM sheets. The high temperature fracture elongations of La2O3-TZM and La(NO3)3-TZM alloy sheets have the similar changes with the temperature, which reaches the maximum value when the temperature is 1200 C, and the elongation of samples gradually decreases with higher temperature than 1200 C. In addition, at 1200 C, the elongation of La(NO3)3TZM alloy sheet is larger than that of La2O3-TZM sheet, while it is opposite at other three kinds of temperature. During tensile tests, the strength of the grain and grain boundary decreases gradually when the La2O3-TZM and La(NO3)3TZM alloy sheets are deformed under high temperature environment due to the dynamic recovery and recrystallization behavior. At the beginning, the dynamic recovery and recrystallization has great effects on eliminating work hardening and stress concentration with the increase in temperature, transforming the alloy from brittle (the grain damaged and the strength reduced due to large deformation of plastic processing in the rolling state) to ductility (semi-hard and soft state). This can explain that the high temperature fracture elongation of La2O3-TZM and La(NO3)3-TZM alloy sheets begin to rise with the temperature rise to 1200 C. Due to the small degree of crystal lattice distortion in the grain boundaries, the atomic diffusion rate increases and the grain boundary strength decreases when the temperature exceeds to the critical value, which causes that the organizational structure strength becomes larger compared with the grain boundary strength. This results in the high temperature tensile ductility
Fig. 3. Strength-elongation curves (a, b), tensile strength-temperature curve (c) and high temperature elongation (d) of La-TZM alloys at different temperatures.
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in Fig. 3c. It also shows that the temperature sensitivity of La(NO3)3TZM alloy sheet is higher than that of La2O3-TZM alloy. 3.2. Metallographic analysis
Fig. 4. Vickers hardness of La-TZM alloy sheets.
reduction of the materials. So, the high temperature fracture elongation of La2O3-TZM and La(NO3)3-TZM alloy sheets begin to decrease when the temperature exceeds the critical temperature (1200 C).
The microstructure of La2O3-TZM alloy and La(NO3)3-TZM alloy sintered billets and rolled sheets was observed in cross-section, using the metallography analysis. The optical micrographs are shown in Fig. 5. In Fig. 5a and b, the second phase particles in the sintered billets distributed uniformly, while major of them districted in the grain and small amount of them distributed near grain boundary. In addition, the size of second phase particle in La(NO3)3-TZM alloy is obviously smaller than that of La2O3-TZM alloy, which results in smaller grain size of La(NO3)3-TZM alloy. Fig. 5c and d shows microstructure of the rolled La2O3-TZM alloy and La(NO3)3-TZM alloy sheets, which both show significant deformation texture after distinct plastic deformation. The microstructure heredity phenomenon obviously exists in the rolled alloy sheets. Similar to the sintered billets, the second phase particles in rolled sheets are fine and uniform in both morphology and size. The second phase and grain size in La(NO3)3-TZM alloy is smaller and more uniform than these in La2O3-TZM alloy. In addition, the fiber texture in La(NO3)3-TZM is more tenuous and longer than that in La2O3-TZM alloy. 3.3. Tensile fracture morphology
3.1.3. Vickers hardness analysis Fig. 4 shows the Vickers hardness of La2O3-TZM and La(NO3)3TZM alloy sheets after high temperature tensile test. The Vickers hardness of both alloy sheets show the same tendency with the change of temperature. The differences between La2O3-TZM and La(NO3)3-TZM alloy sheets are 28.4 HV, 9.1 HV, 9.1 HV, 7.8 HV at 1000 C, 1200 C, 1400 C, 1600 C respectively. This is consistent with the change of the tensile strength of the alloy sheets as shown
Fig. 6 shows the high temperature tensile fracture morphology of La2O3-TZM and La(NO3)3-TZM alloy sheets. It can be seen that La2O3-TZM and La(NO3)3-TZM alloy sheets are of similar features in morphology at the same temperature after high temperature tensile fracture. In Fig. 6a and b, the fracture morphologies of the two La-TZM alloys are honeycomb type after tensile test at 1000 C, which are the dimple morphology after ductile fracture. In Fig. 6c
Fig. 5. Microstructure of sintered billets and rolled sheets: (a, c) La2O3-TZM alloy, (b, d) La(NO3)3-TZM alloy.
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Fig. 6. High temperature tensile fracture morphology of La-TZM alloys at different temperatures. (La2O3-TZM alloy: a-1000 C, c-1200 C, e-1400 C, g-1600 C; La(NO3)3-TZM alloy: b-1000 C, d-1200 C, f-1400 C, h-1600 C).
and d, the fracture morphologies show the majority of dimple and the tongue or fan cleavage plane after cleavage fracture in local areas. In Fig. 6e and f, the fracture morphologies show the most of the rock or rock sugar-like fracture morphology after intergranular fracture with tongue or fan cleavage surface in local areas when the tensile tests were carried out at 1400 C. In Fig. 6g and h, the stone
or rock sugar fracture features appear in the fracture morphology surface as the tensile tests were carried out at 1600 C. Ductile fracture is a kind of high energy absorption fracture process. The expansion and growth of crack is a relatively slow process, which will consume a lot of plastic deformation energy. For La2O3-TZM and La(NO3)3-TZM alloys, a large amount of secondary
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phase particles distributed inside and outside the grain, and these second phase particles have relative lower deformation during rolling, resulting in the hinder of dislocation movement in the deformation process. According to the dislocation theory [18], when the dislocation moved to the second phase particles, it will bypass the second phase particles to move. While rounding the second phase particles, the partial dislocation will pile up around the particles, which causes the stress concentration. During the high temperature tensile tests, cracks firstly initiate from the stress concentration zones, and then slowly propagate until the fracture happens, forming the honeycomb-like fracture morphology. Because second phase particles are mainly distributed in the grain, the tensile fracture morphology of alloys for 1000 C is mainly dimple-type transgranular fracture, and a large number of second phase particles remain in the dimple, as shown in Fig. 6a and b. The cleavage fracture is a kind of transgranular brittle fracture. Under the action of normal stress, it is easy to be destroyed due to the weak bonding of atoms, which causes the fracture of the alloy sheets along the special crystal plane. The cleavage plane of the tongue or fan shape formed in 1400 C tensile fracture morphology of both La-TZM alloys, present the radial river pattern diffused from the second phase particle in the inner phase to the outer of grain, as shown in Fig. 6e and f. For the tensile test at 1200 C, both of the La2O3-TZM and La(NO3)3-TZM alloys test are presenting a kinds of mixed fracture mode of dimple fracture and cleavage fracture, while the dimple fracture is the main fracture mode, as shown in Fig. 6c and d. Intergranular fracture is generally a kind of brittle fracture because a thin layer of continuous or discontinuous network brittle phase appears at grain boundary positions. These brittle phases
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destroy the continuity of the alloy and reduce the bonding strength between the grains. When the alloy sheet under the action of external force, the network brittle phase cannot withstand the load and ruptures, then the cracks propagate quickly, and the fracture occurs along the brittle phase near the grain boundary, thereby forming a rock or rock sugar-like morphology. The tensile fracture morphologies for 1600 C of both alloys show the toughness intergranular fracture mode, as shown in Fig. 6g and h. These present that the La2O3-TZM and La(NO3)3-TZM alloy sheets tensile fracture are transgranular fracture at the temperature range of 1000 Ce1400 C. When the temperature is higher than 1400 C, the fracture modes of both alloy sheets are all intergranular fracture, which is completely consistent with the phenomenon that grain strength and grain boundary strength decrease with increasing temperature. So, the fracture mode transition temperature of two kinds of La-TZM alloy sheets is 1400 C (Teq ¼ 1400 C). The comparison of the fracture morphology shows that the doping of La2O3 or La(NO3)3 have little effect on the fracture mode and morphology. In addition, the grains of the intergranular fracture morphology of La(NO3)3-TZM alloy are slightly smaller than those in La2O3-TZM alloy, which indicates that the size of second phase particles has little effect on the fracture mode and fracture morphology of the alloys under high temperature environment. EDS spectrum analysis was performed on high temperature tensile fracture surface of La2O3-TZM alloy sheet tested at 1000 C and 1600 C, as shown in Fig. 7. The EDS analysis shows that the second phase particles in the fracture of La2O3-TZM alloy sheet at 1000 C are mainly La, Ti, Zr and O, which indicates that the second phase particles exist in the form of oxide. However, the fracture grain composition is major in molybdenum at 1600 C, and the
Fig. 7. The spectrum of the second phase in the La2O3-TZM alloy sheet fracture at different temperatures: (a) 1000 C; (b) 1600 C.
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second phase is not found. It is probably due to the recrystallization behavior at high temperature. The second phase particles in the alloy are wrapped by molybdenum matrix during the nucleation and growth process.
[2]
[3]
4. Conclusions [4]
In summary, the room and high temperature mechanical properties of La(NO3)3 and La2O3 doped TZM alloys were studied systematically. The La(NO3)3 doping method has better room temperature mechanical property than that of La2O3-TZM alloy. During high temperature tensile tests from 1000 C to 1600 C, the toughness of La-TZM alloys improves with higher temperature and it reaches the maximum value at 1200 C, which decreases greatly with further increasing in temperature. The microstructure of La(NO3)3-TZM alloy is more sensitive to temperature, and the lanthanum doping method has a significant impact on the temperature sensitivity of the alloy. When the temperature is lower than 1400 C, La-TZM alloy sheets fracture mode is major controlled by transgranular fracture. When the temperature is higher than 1400 C, it transits into intergranular fracture model. The temperature of fracture mode transition is 1400 C (Teq).
[5]
[6] [7]
[8]
[9]
[10]
[11]
Conflict of interest [12]
The authors declare that they have no conflict of interest. Acknowledgments
[13]
This work was supported by the Science and Technology Coordinating Innovative Engineering Project of Shaanxi Province (2015KTZDGY09-04), the Young Talent Fund of University Association for Science and Technology in Shaanxi China (20150201), the Service Local Special Program of Education Department of Shaanxi Province China (16JF016), the Science and Technology Foundation of Xi'a University of Architecture and Technology (QN1505) and the China Postdoctoral Science Foundation (2016M600770).
[14]
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