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Fracture toughness of sintered Mo–La2O3 alloy and the toughening mechanism
International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
Fracture toughness of sintered Mo±La2O3 alloy and the toughening mechanism Zhang Jiuxing *, Liu Lu, Zhou Meiling, Hu Yancao, Zuo Tieyong Department of Materials Science and Engineering, Beijing Polytechnic University, Beijing 100022, People's Republic of China Received 10 October 1998; accepted 27 September 1999
Abstract The KIC of sintered Mo±La2 O3 alloy and pure molybdenum was tested and the micro-structure was investigated by SEM, TEM and AES. The results show that the KIC of Mo±La2 O3 alloy reaches 24.76 MPa m1=2 , which is 2.5 times as much as that of pure Mo. The sintered Mo±La2 O3 alloy and pure molybdenum have similar equiaxed grain structure. The AES analysis revealed that the same content of interstitial impurities exist on grain boundaries of Mo±La2 O3 alloy and pure molybdenum. A toughening mechanism was proposed to be that large number of dislocations were pinned around La2 O3 particles, shortening the ecient slip length and decreasing the dislocation pile-up on grain boundaries of Mo. The improvement of toughness of Mo±La2 O3 alloy was attributed to the relief of stress concentration at the grain boundaries of Mo±La2 O3 alloy and the weaker tendency to intergranular fracture on grain boundaries. Ó 2000 Published by Elsevier Science Ltd. All rights reserved. Keywords: Molybdenum; La2 O3 ; Fracture toughness; Toughening mechanism
1. Introduction Sintered molybdenum is widely used in the industry as an electrode for smelting furnace of glass and refractory, the crucible of rare earth metallurgy, and the thimbles for producing seamless steel tube and so on. Some research work on Mo±La2 O3 alloy done previously indicated that with the addition of La2 O3 , the molybdenum has not only higher strength and better creep-resistant property at high temperature [1±4], but also the low-temperature brittleness of Mo is improved greatly and the DBTT of Mo±La2 O3 sheet annealed at high temperature is 80±103°C lower than that of pure Mo sheet [5,6] There are several opinions on the toughening mechanism of molybdenum. Hiraoka et al. attributed the improvement of low-temperature brittleness of Si±Al±K±Mo to the elongated coarse grain structure [7], Deng shiqiang et al. thought that rare earth could decrease the density of C, N, O on the grain boundaries [8], whereas Zhang et al. regarded the second phase of La2 O3 particles as the main contribution to Mo toughness improvement [9]. *
Corresponding author. Tel.: +86-10-6739-2169; fax: +86-10-67395281.
Up to now, little work has been done on the measurement of the fracture toughness of Mo, and the KIC of Mo±La2 O3 alloy has so far not been reported. It is very important to measure the fracture toughness of Mo±La2 O3 alloy precisely not only for evaluating the eect of toughening but also extending the application of Mo±La2 O3 alloy.
2. Experimental procedure 2.1. Specimen preparation The specimen of Mo and Mo±La2 O3 alloy was prepared by powder metallurgical processing. The rare earth oxide La2 O3 was added to Mo oxide as aqueous solutions of La(NO3 )3 . The doped oxide powders were reduced into metallic molybdenum powder in dry hydrogen. The average particle size of the powder is 2.0± 2.5 lm. The pure molybdenum powder and the Mo± La2 O3 powder were pressed into sheets with the same size, then sintered at 1850°C for 4 h in hydrogen. The densities of Mo±La2 O3 alloy and pure Mo were 9.80 and 9.60 g/cm3 , respectively. The main chemical composition is shown in Table 1.
0263-4368/99/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 9 ) 0 0 0 3 0 - X
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Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
Table 1 Main chemical composition Specimen
Composition
Mo±La2 O3 Pure Mo
Mo
La2 O3 (wt.%)
C (lm)
Fe (lm)
Ni (lm)
W (lm)
Balance Balance
1.02 0.0
50 50
23 23
16 16
50 50
2.2. Specimen dimension According to GB4161-84, the specimen thickness B must obey relation (1). 2
B P 2:5 KIC =ry ;
1
where, B is the thickness of the specimen, KIC the fracture toughness and ry is the yield stress. Referring to paper [10], we select the specimen thickness B as 8 mm ®rst. The geometry and dimension of the compact tension specimen are shown in Fig. 1. The experimental results indicate that the 8-mm thick specimen of Mo±La2 O3 does not obey the planar strain conditions. So according to GB4161-84, we extend the
specimen thickness B from 8 to 12 mm and do the experiment for another time. 2.3. Initiate fatigue cracks At stress ratio R 0:1 and the frequency of f 50 Hz, a fatigue crack of approx. 2±3 mm length could easily be produced in the specimen of Mo±La2 O3 . However, the pure Mo specimen fatigue precracking is dicult to produce, thus in our experiment a crack cut by molybdenum wire was applied instead. 3. Results and discussions The P±D curves of Mo±La2 O3 alloy and Mo are shown in Fig. 2, and Mo±La2 O3 alloy exhibits greater load and position. The toughness results are listed in Table 2. From Table 2 it can be seen that the fracture toughness of sintered Mo±La2 O3 alloy reaches 24.76 MPa m1=2 , which is 2.5 times as much as that of pure Mo, and which is also 5.76 MPa m1=2 higher than that of hot-forged-air-cooled TZM alloy [10], as shown in Table 3. 4. Toughening mechanism
Fig. 1. Geometry and dimension of the compact tension specimen.
As is well-known, the brittleness of molybdenum results mainly from the instinctive brittleness of grain
Fig. 2. P±D curves of sintered Mo±La2 O3 alloy and Mo.
Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
407
Table 2 KIC of sintered Mo±La2 O3 alloy and pure Mo Specimen
B (mm)
W (mm)
a (mm)
PQ (kN)
Pmax (kN)
KQ (MPa m1=2 )
KIC (MPa m1=2 )
Pure Mo Mo±La2 O3 Mo±La2 O3
8 8 12
32 32 40
15.0 15.6 20.0
1.69 2.75 6.10
1.69 4.54 6.69
9.78 15.85 24.76
9.78 ± 24.76
Table 3 The comparison of fracture toughness of Mo [10] Materials
Pure Mo
Mo±La2 O3
Pure Mo
TZM
State KIC (MPa m1=2 )
Sintered 9.78
Sintered 24.76
Hot-forged-air-cooled 8.0
Hot-forged-air-cooled 19.0
boundary and C, N, O interstitial impurity segregation at grain boundaries. From Fig. 3, it is noticed that the sintered Mo±La2 O3 alloy and pure molybdenum have similar equiaxed grain structure. But the spherical particles of La2 O3 were observed in Mo±La2 O3 alloy and the fractograph is greatly dierent. Fig. 4 shows an SEM fractograph of sintered PMo and Mo±La2 O3 alloy. There are some fatigue micro-cracks proceeding from electrospark crack as shown in zone (I) of Fig. 4(a). In some cases fatigue cracks can also start from favorably orientated grain boundaries, as seen in zone (II). After that, some holes were observed in zone (III). In the tensile fracture zone, sintered Mo±La2 O3 alloy mainly fractured by cleavage, as shown in Fig. 4(b), but a ®brous fractograph has also been found in some cases, as seen in Fig. 4(c). Pure molybdenum's fractograph is quite dierent from that of
sintered Mo±La2 O3 alloy, as seen in Fig. 4(d). It wholly fractured in intergranular mechanism. Both Mo±La2 O3 alloy and pure Mo specimens were broken at a temperature of liquid nitrogen. The fracture surface were eroded by Ar for 5 min, and then analyzed by means of AES. It is interesting to ®nd that the two specimens has same content of C, N, O on the grain boundaries, as seen in Fig. 5. These is no obvious evidence to prove that the addition of La2 O3 may decrease the segregation of C, N, O interstitial impurities on the grain boundaries. TEM observation shows that a number of dislocations are pinned around La2 O3 particles, and the number of dislocations which pile at grain boundaries are decreased greatly, as seen in Fig. 6(a) and (b), respectively. Since the large number of dislocations were pinned around La2 O3 particles, shortening the ecient
Fig. 3. SEM observation of micro-graph of sintered Mo (a) and Mo±La2 O3 alloy (b).
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Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
Fig. 4. SEM observation of fractography of Mo±La2 O3 alloy and PMo. (a) the three dierent fractographic zone of Mo±La2 O3 alloy; (b) ®brous fractogarphy of Mo±La2 O3 alloy; (c) cleavage of Mo±La2 O3 alloy; (d) intergranular fractography of PMo.
Fig. 5. AES analysis on fracture surface.
slip length and decreasing the dislocation pile-up on grain boundaries of Mo. It is due to relief of stress concentration on grain boundaries that the micro-crack
nucleation tendency on grain boundaries becomes weaker. Thus the fracture toughness of molybdenum was greatly improved.
Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
409
Fig. 6. Interactions of dislocations with La2 O3 particles (TEM): (a) much dislocations around La2 O3 particles in grains; (b) few dislocations near to grain boundaries.
5. Conclusions
References
1. The KIC of sintered Mo±La2 O3 alloy reaches 24.76 MPa m1=2 , which is 2.5 times as much as that of sintered pure Mo, and which is also 5.76 MPa m1=2 higher than that of hot-forged-air-cooled TZM alloy. The toughening eect is very obvious by addition of La2 O3 . 2. The results of AES prove that the addition of La2 O3 did not obviously decrease the density of interstitial impurities on the grain boundaries. 3. The main toughening mechanism is that a number of dislocations were pinned around La2 O3 particles, so that the dislocations piled at grain boundaries decreased greatly, resulting in the relief of stress concentration at the grain boundaries and the weaker tendency to intergranular fracture, thus the fracture toughness of molybdenum was greatly improved.
[1] Endo M. Proceedings of the 12th International Plansee. SeminarÕ89, Reutte, Tirol, Austria, 12±16 May 1989. Plansee Metall AG., Reutte, Tirol, Austria 1989;1:37. [2] Eck R. Proceedings of the 12th International Plansee. SeminarÕ89, Reutte, Tirol, Austria, 12±16 May 1989. Plansee Metall AG., Reutte, Tirol, Austria, 1989;1:829. [3] Eck R. Proceedings of the 12th International Plansee. SeminarÕ89, Reutte, Tirol, Austria, 12±16 May 1989. Plansee Metall AG., Reutte, Tirol, Austria, 1989;1:1047. [4] Zhou ML, Li J, Zuo TY. Transactions of Nfsoc 1994;4(3):57. [5] Zhang JX, Zhou ML, Zuo TY. Proceedings of the 13th International Plansee SeminarÕ93, Reutte, Tirol, Austria, 24±28 May 1993. Plansee Metall AG., Reutte, Tirol, Austria 1993;1:599. [6] Liu XY, Wang DZ. Transactions of Nfsoc 1995;5(4):100. [7] Hiraska Z. Metallkd 1988;79(8):519. [8] Deng Shiqiang et al., Proceedings of the Sixth Refractory Metal 1990:103. [9] Zhang JX, Zhou ML, Zuo TY. Proceedings of the 13th International Plansee SeminarÕ93, Reutte, Tirol, Austria, 24±28 May 1993. Plansee Metall AG., Reutte, Tirol, Austria 1993;1:93. [10] Chen DL, Weiss B, Sticker R, Witwer M, Leicht®ed G, Hodl H. Proceedings of the 13th International Plansee SeminarÕ93, Reutte, Tirol, Austria, 24±28 May 1993. Plansee Metall AG., Reutte, Tirol, Austria 1993;1:621.
Acknowledgements This program is ®nancially supported by Beijing Science Star Foundation and 863 High-Tech Plan.
Fracture toughness of sintered Mo±La2O3 alloy and the toughening mechanism Zhang Jiuxing *, Liu Lu, Zhou Meiling, Hu Yancao, Zuo Tieyong Department of Materials Science and Engineering, Beijing Polytechnic University, Beijing 100022, People's Republic of China Received 10 October 1998; accepted 27 September 1999
Abstract The KIC of sintered Mo±La2 O3 alloy and pure molybdenum was tested and the micro-structure was investigated by SEM, TEM and AES. The results show that the KIC of Mo±La2 O3 alloy reaches 24.76 MPa m1=2 , which is 2.5 times as much as that of pure Mo. The sintered Mo±La2 O3 alloy and pure molybdenum have similar equiaxed grain structure. The AES analysis revealed that the same content of interstitial impurities exist on grain boundaries of Mo±La2 O3 alloy and pure molybdenum. A toughening mechanism was proposed to be that large number of dislocations were pinned around La2 O3 particles, shortening the ecient slip length and decreasing the dislocation pile-up on grain boundaries of Mo. The improvement of toughness of Mo±La2 O3 alloy was attributed to the relief of stress concentration at the grain boundaries of Mo±La2 O3 alloy and the weaker tendency to intergranular fracture on grain boundaries. Ó 2000 Published by Elsevier Science Ltd. All rights reserved. Keywords: Molybdenum; La2 O3 ; Fracture toughness; Toughening mechanism
1. Introduction Sintered molybdenum is widely used in the industry as an electrode for smelting furnace of glass and refractory, the crucible of rare earth metallurgy, and the thimbles for producing seamless steel tube and so on. Some research work on Mo±La2 O3 alloy done previously indicated that with the addition of La2 O3 , the molybdenum has not only higher strength and better creep-resistant property at high temperature [1±4], but also the low-temperature brittleness of Mo is improved greatly and the DBTT of Mo±La2 O3 sheet annealed at high temperature is 80±103°C lower than that of pure Mo sheet [5,6] There are several opinions on the toughening mechanism of molybdenum. Hiraoka et al. attributed the improvement of low-temperature brittleness of Si±Al±K±Mo to the elongated coarse grain structure [7], Deng shiqiang et al. thought that rare earth could decrease the density of C, N, O on the grain boundaries [8], whereas Zhang et al. regarded the second phase of La2 O3 particles as the main contribution to Mo toughness improvement [9]. *
Corresponding author. Tel.: +86-10-6739-2169; fax: +86-10-67395281.
Up to now, little work has been done on the measurement of the fracture toughness of Mo, and the KIC of Mo±La2 O3 alloy has so far not been reported. It is very important to measure the fracture toughness of Mo±La2 O3 alloy precisely not only for evaluating the eect of toughening but also extending the application of Mo±La2 O3 alloy.
2. Experimental procedure 2.1. Specimen preparation The specimen of Mo and Mo±La2 O3 alloy was prepared by powder metallurgical processing. The rare earth oxide La2 O3 was added to Mo oxide as aqueous solutions of La(NO3 )3 . The doped oxide powders were reduced into metallic molybdenum powder in dry hydrogen. The average particle size of the powder is 2.0± 2.5 lm. The pure molybdenum powder and the Mo± La2 O3 powder were pressed into sheets with the same size, then sintered at 1850°C for 4 h in hydrogen. The densities of Mo±La2 O3 alloy and pure Mo were 9.80 and 9.60 g/cm3 , respectively. The main chemical composition is shown in Table 1.
0263-4368/99/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 9 ) 0 0 0 3 0 - X
406
Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
Table 1 Main chemical composition Specimen
Composition
Mo±La2 O3 Pure Mo
Mo
La2 O3 (wt.%)
C (lm)
Fe (lm)
Ni (lm)
W (lm)
Balance Balance
1.02 0.0
50 50
23 23
16 16
50 50
2.2. Specimen dimension According to GB4161-84, the specimen thickness B must obey relation (1). 2
B P 2:5 KIC =ry ;
1
where, B is the thickness of the specimen, KIC the fracture toughness and ry is the yield stress. Referring to paper [10], we select the specimen thickness B as 8 mm ®rst. The geometry and dimension of the compact tension specimen are shown in Fig. 1. The experimental results indicate that the 8-mm thick specimen of Mo±La2 O3 does not obey the planar strain conditions. So according to GB4161-84, we extend the
specimen thickness B from 8 to 12 mm and do the experiment for another time. 2.3. Initiate fatigue cracks At stress ratio R 0:1 and the frequency of f 50 Hz, a fatigue crack of approx. 2±3 mm length could easily be produced in the specimen of Mo±La2 O3 . However, the pure Mo specimen fatigue precracking is dicult to produce, thus in our experiment a crack cut by molybdenum wire was applied instead. 3. Results and discussions The P±D curves of Mo±La2 O3 alloy and Mo are shown in Fig. 2, and Mo±La2 O3 alloy exhibits greater load and position. The toughness results are listed in Table 2. From Table 2 it can be seen that the fracture toughness of sintered Mo±La2 O3 alloy reaches 24.76 MPa m1=2 , which is 2.5 times as much as that of pure Mo, and which is also 5.76 MPa m1=2 higher than that of hot-forged-air-cooled TZM alloy [10], as shown in Table 3. 4. Toughening mechanism
Fig. 1. Geometry and dimension of the compact tension specimen.
As is well-known, the brittleness of molybdenum results mainly from the instinctive brittleness of grain
Fig. 2. P±D curves of sintered Mo±La2 O3 alloy and Mo.
Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
407
Table 2 KIC of sintered Mo±La2 O3 alloy and pure Mo Specimen
B (mm)
W (mm)
a (mm)
PQ (kN)
Pmax (kN)
KQ (MPa m1=2 )
KIC (MPa m1=2 )
Pure Mo Mo±La2 O3 Mo±La2 O3
8 8 12
32 32 40
15.0 15.6 20.0
1.69 2.75 6.10
1.69 4.54 6.69
9.78 15.85 24.76
9.78 ± 24.76
Table 3 The comparison of fracture toughness of Mo [10] Materials
Pure Mo
Mo±La2 O3
Pure Mo
TZM
State KIC (MPa m1=2 )
Sintered 9.78
Sintered 24.76
Hot-forged-air-cooled 8.0
Hot-forged-air-cooled 19.0
boundary and C, N, O interstitial impurity segregation at grain boundaries. From Fig. 3, it is noticed that the sintered Mo±La2 O3 alloy and pure molybdenum have similar equiaxed grain structure. But the spherical particles of La2 O3 were observed in Mo±La2 O3 alloy and the fractograph is greatly dierent. Fig. 4 shows an SEM fractograph of sintered PMo and Mo±La2 O3 alloy. There are some fatigue micro-cracks proceeding from electrospark crack as shown in zone (I) of Fig. 4(a). In some cases fatigue cracks can also start from favorably orientated grain boundaries, as seen in zone (II). After that, some holes were observed in zone (III). In the tensile fracture zone, sintered Mo±La2 O3 alloy mainly fractured by cleavage, as shown in Fig. 4(b), but a ®brous fractograph has also been found in some cases, as seen in Fig. 4(c). Pure molybdenum's fractograph is quite dierent from that of
sintered Mo±La2 O3 alloy, as seen in Fig. 4(d). It wholly fractured in intergranular mechanism. Both Mo±La2 O3 alloy and pure Mo specimens were broken at a temperature of liquid nitrogen. The fracture surface were eroded by Ar for 5 min, and then analyzed by means of AES. It is interesting to ®nd that the two specimens has same content of C, N, O on the grain boundaries, as seen in Fig. 5. These is no obvious evidence to prove that the addition of La2 O3 may decrease the segregation of C, N, O interstitial impurities on the grain boundaries. TEM observation shows that a number of dislocations are pinned around La2 O3 particles, and the number of dislocations which pile at grain boundaries are decreased greatly, as seen in Fig. 6(a) and (b), respectively. Since the large number of dislocations were pinned around La2 O3 particles, shortening the ecient
Fig. 3. SEM observation of micro-graph of sintered Mo (a) and Mo±La2 O3 alloy (b).
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Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
Fig. 4. SEM observation of fractography of Mo±La2 O3 alloy and PMo. (a) the three dierent fractographic zone of Mo±La2 O3 alloy; (b) ®brous fractogarphy of Mo±La2 O3 alloy; (c) cleavage of Mo±La2 O3 alloy; (d) intergranular fractography of PMo.
Fig. 5. AES analysis on fracture surface.
slip length and decreasing the dislocation pile-up on grain boundaries of Mo. It is due to relief of stress concentration on grain boundaries that the micro-crack
nucleation tendency on grain boundaries becomes weaker. Thus the fracture toughness of molybdenum was greatly improved.
Zhang Jiuxing et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 405±409
409
Fig. 6. Interactions of dislocations with La2 O3 particles (TEM): (a) much dislocations around La2 O3 particles in grains; (b) few dislocations near to grain boundaries.
5. Conclusions
References
1. The KIC of sintered Mo±La2 O3 alloy reaches 24.76 MPa m1=2 , which is 2.5 times as much as that of sintered pure Mo, and which is also 5.76 MPa m1=2 higher than that of hot-forged-air-cooled TZM alloy. The toughening eect is very obvious by addition of La2 O3 . 2. The results of AES prove that the addition of La2 O3 did not obviously decrease the density of interstitial impurities on the grain boundaries. 3. The main toughening mechanism is that a number of dislocations were pinned around La2 O3 particles, so that the dislocations piled at grain boundaries decreased greatly, resulting in the relief of stress concentration at the grain boundaries and the weaker tendency to intergranular fracture, thus the fracture toughness of molybdenum was greatly improved.
[1] Endo M. Proceedings of the 12th International Plansee. SeminarÕ89, Reutte, Tirol, Austria, 12±16 May 1989. Plansee Metall AG., Reutte, Tirol, Austria 1989;1:37. [2] Eck R. Proceedings of the 12th International Plansee. SeminarÕ89, Reutte, Tirol, Austria, 12±16 May 1989. Plansee Metall AG., Reutte, Tirol, Austria, 1989;1:829. [3] Eck R. Proceedings of the 12th International Plansee. SeminarÕ89, Reutte, Tirol, Austria, 12±16 May 1989. Plansee Metall AG., Reutte, Tirol, Austria, 1989;1:1047. [4] Zhou ML, Li J, Zuo TY. Transactions of Nfsoc 1994;4(3):57. [5] Zhang JX, Zhou ML, Zuo TY. Proceedings of the 13th International Plansee SeminarÕ93, Reutte, Tirol, Austria, 24±28 May 1993. Plansee Metall AG., Reutte, Tirol, Austria 1993;1:599. [6] Liu XY, Wang DZ. Transactions of Nfsoc 1995;5(4):100. [7] Hiraska Z. Metallkd 1988;79(8):519. [8] Deng Shiqiang et al., Proceedings of the Sixth Refractory Metal 1990:103. [9] Zhang JX, Zhou ML, Zuo TY. Proceedings of the 13th International Plansee SeminarÕ93, Reutte, Tirol, Austria, 24±28 May 1993. Plansee Metall AG., Reutte, Tirol, Austria 1993;1:93. [10] Chen DL, Weiss B, Sticker R, Witwer M, Leicht®ed G, Hodl H. Proceedings of the 13th International Plansee SeminarÕ93, Reutte, Tirol, Austria, 24±28 May 1993. Plansee Metall AG., Reutte, Tirol, Austria 1993;1:621.
Acknowledgements This program is ®nancially supported by Beijing Science Star Foundation and 863 High-Tech Plan.