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Influence of the addition of Lanthanum on a W–Mo–Ni–Fe heavy alloy
International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
In¯uence of the addition of Lanthanum on a W±Mo±Ni±Fe heavy alloy G.C. Wu *, Q. You, D. Wang Central Iron and Steel Research Institute, No. 76 Xueyuan Nanlu, Beijing 100081, People's Republic of China Received 25 September 1998; accepted 26 January 1999
Abstract In order to improve the mechanical properties of a W±Mo±Ni±Fe heavy alloy, the in¯uence of adding rare earth element La on heavy alloy and its action mechanism have been studied in this paper. The results showed that the mechanical properties of this alloy can remarkably be improved when a proper amount of La is added. Scanning electron microscopy, transmission electron microscopy, X-ray diraction pattern and auger electron spectroscopy experiments revealed that the improvement of mechanical properties was primarily due to the formation of stable LaMnO3 and a small amount of Mn3 O4; which changed the existence form and distribution state of the impurity element oxygen, reduced the interfacial segregation of oxygen and thus increased the bonding strength of W-matrix interfaces. Consequently, the mechanical properties of the alloy were improved. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Mechanical properties; Lanthanum; W±Mo±Ni±Fe heavy alloys
Notation SEM AES EDS TEM XRD
scanning electron microscopy auger electron spectroscopy energy dispersive spectrometry transmission electron microscopy X-ray diraction pattern
1. Introduction Heavy W-alloys have widely been used in medical, military and industrial installations. Many previous studies showed that the tensile and impact properties depended critically on the interfacial segregation of some impurity elements such as H, S, P, O [1±5]. The segregation of these impurities on interfaces not only can lower the bonding force between W grains and matrix phase sharply but also may form the bulky inclusion which deteriorates the continuity of the matrix and is the source of fracture of the alloy. Therefore, it is very important to control the concentration of impurities and to improve their state of existence and distri-
*
Corresponding author. E-mail: [email protected]
bution in the alloy. As it is well known, rare earth elements (RE) have been applied in many ®elds because of their extraordinary chemical and physical properties. In steel, the temper embrittlement due to P and Sn can eectively be reduced by adding La [6±8]. The aim of this study is to investigate the possibility of improving the mechanical properties of the heavy alloy by adding a certain RE and to analyze its action mechanism. 2. Experimental The material used in this study is a W±Mo±Ni±Fe heavy alloy, which is dierent from the traditional heavy alloy such as W±Ni±Fe and W±Ni±Cu in that it contains approximately 17 wt.% Mo. This alloy has been applied in industry for some years. The technical speci®cation requires that sintering density must strictly be controlled in the range of 15.73 0.08 Mg á mÿ3 . This alloy, especially used as large dimensional workpieces, has not only very low elongation to failure but also causes brittle crack easily during mechanical working due to a strictly limited density and a high content of Mo which results in the embrittlement of the heavy alloy [9±11]. In this paper, we investigate the reason for the embrittlement of
0263-4368/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 9 ) 0 0 0 1 5 - 3
300
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
Table 1 Alloy composition (wt%)
Table 2 The sintered density of alloys (Mg á mÿ3 )
Alloy type
W
Mo
Ni
Fe
La
Mn
I II III IV V
78 78 78 78 78
17.0 17.1 16.9 16.7 16.5
2.5 2.5 2.5 2.5 2.5
2.3 2.0 2.0 2.0 2.0
0.0 0.2 0.4 0.6 0.8
0.2 0.2 0.2 0.2 0.2
this W±Mo±Ni±Fe heavy alloy and put forward a simple and feasible way of improving mechanical properties. The dierent W±Mo±Ni±Fe heavy alloys were made by traditional powder metallurgy processing. The pure ®ne metal powders of W, Mo, Ni, Fe, Mn and Ni5 La were mixed up and the powder mixture was cold-isostatically pressed under 200 MPa. All compacts were sintered at 1510°C, for 1.5 h in a hydrogen furnace. The compositions of the samples are listed in Table 1. The density and mechanical properties of the sintered specimens were measured. For the experimental investigation of tensile strength, circular specimens with overall dimensions of U5 ´ 65 mm were machined, and the length of the gauge section is 30 mm. At least four specimens were tested for each experimental condition.
Alloy type
Measured density Theoretical density Relative density (%)
I II III IV V
15.75 15.72 15.74 15.78 15.69
15.83 15.81 15.81 15.80 15.79
99.49 99.41 99.58 99.87 99.37
Table 3 The mechanical properties of W±Mo±Ni±Fe alloys Alloy type
rb (MPa)
d (%)
HRC
I II III IV V
291 650 903 846 766
0 2.2 4.7 4.1 3.2
37 34 30 30 31
The microstructural study included optical metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, auger electron spectroscopy (AES), TEM, SEM, X-ray diraction (XRD) analyses are performed to explain the properties of this alloy and determine the phase composition and phase structure.
Fig. 1. Microstructure of W±Mo±Ni±Fe heavy alloys.
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
Fig. 2. EDS analysis of two new phases in the alloy.
Fig. 3. XRD patterns of the new phase extracted from the W±Mo±Ni±Fe heavy alloy; (a) LaMnO3 , (b) Mn3 O4 .
Fig. 4. Indexing of two new phases by diraction pattern.
301
302
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
Fig. 5. AES analysis of the oxygen distribution at the W-matrix interface and in depth (a) (b) segregation of oxygen and its distribution in depth in alloy I (c) (d) segregation of oxygen and its distribution in depth in alloy III.
3. Results and discussion The densi®cation behaviour of W±Mo±Ni±Fe heavy alloys containing 0.0, 0.2, 0.4, 0.6, 0.8 mass% La are listed in Table 2. All sintered specimens appeared to be fully dense, with densities between the range of 15.69± 15.78 Mg á mÿ3 , which satisfactorily meets the special requirement of this W±Mo±Ni±Fe heavy alloy in use. The results obtained from the mechanical property tests at varying La content are shown in Table 3. Tensile strengths rb increased from 291 to 903 MPa, and elongation to failured d changed from 0% to 4.7%, while hardness HRC decreased from 37 to 30, which greatly improved the mechanical properties of this alloy. The total microstructure observed for all W±Mo± Ni±Fe samples showed a typical heavy alloy structure, which is similar to that reported elsewhere for W±Ni± Fe heavy alloys, as illustrated in Fig. 1. The polished cross sections showed spherical W grains embedded in a more or less continuous Ni matrix phase. The grain size of tungsten does not vary with the addition of Lanthanum.
However, there is a new micro phase in the alloy containing La in addition to the W-phase and the matrix phase which are present in conventional heavy alloys. The particles of the new phase are gray round balls and possess distinctive color dierent from W grains and matrix phase. Moreover, it is very stable and can clearly be seen in unetched specimens (Fig. 1). The chemical composition analysis by SEM-EDS shows that most of this new phase is rich in La, W and Mn while the remainder is rich only in Mn (Fig. 2). Because the amount of this phase in the alloy is very small, the new phase was extracted as an electrolytic residue by a certain method [12] in order to determine its kind and structure. The XRD analysis of the extracted powder identi®ed that the new phase, produced by adding rare earth element La in the W±Mo±Ni±Fe heavy alloy, was LaMnO3 and a small amount of Mn3 O4 compounds. The indexing of the two new phases is shown in Figs. 3 and 4. For the liquid sintering heavy alloy, the properties are very sensitive to such impurity elements as O, S, P, H, especially the oxygen segregation on the interface. It is thermodynamically more preferable for these impurities
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
303
Fig. 6. Scanning electron micrographs of surfaces of the alloys.
to segregate to the phase interface and grain boundary than to dissolve in intergranularey. The segregation of impurities to W-matrix interfaces deteriorates the wettability and bonding strength microscopically, which cause the brittleness of the alloy. When rare earth element La is added into this alloy, the chemically active La combines with oxygen to form LaMnO3 and a small amount of Mn3 O4 compounds at W-matrix interfaces or in the matrix phase, which alter the existence and distribution of the impurity element oxygen, purify most of the phase interfaces and enhance the bonding force of the W-matrix interfaces. Thus the tensile strength and elongation of the alloy are improved. The AES analysis showed that the addition of La decreased the segregation of oxygen to interfaces and the distribution of oxygen in depth eectively, which is shown in Fig. 5. The eects of La addition are also proved by fractographic observation. The fracture surfaces of two specimens ± one without any La and the other with 0.4% La are shown in Fig. 6. Due to the interface segregation of impurity element oxygen, the specimen without any La (Fig. 6(a)) shows a typical brittle fracture mode-the intergranular fracture at the boundary between W grains and the decohesion of the Wmatrix interfaces, which is associated with low tensile strength and poor ductility of the alloy. After La is added, the formation of LaMnO3 and some Mn3 O4
reduces the concentration of oxygen segregation to the phase interfaces which increases the bonding strength between phases. Thus, the specimen with 0.4% La (Fig. 6(b)) shows a ductile fracture surface with the transgranlar cleavage of W grains and some ductile rupture of the matrix phase with a dimple appearance, which is associated with satisfactory mechanical properties, especially the elongation to failure. However, when the content of La was higher than 0.4%, the number of the new La-rich phase was increased and its size was enlarged. These bulky new phases greatly deteriorated the continuity of the matrix and became a kind of the source of fracture of the alloy, and thus tensile strength and elongation decreased rapidly, which is shown in Table 3. 4. Conclusions (1) The results of the study indicate that the mechanical properties of W±Mo±Ni±Fe heavy alloys can be improved by adding the rare earth element La. (2) The addition of La makes a remarkable improvement on the mechanical properties of this alloy: tensile strength changed from 291 to 903 MPa, elongation increased from 0% to 4.7% when about 0.4% La was added to the alloy.
304
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
(3) The addition of La alters the existence and distribution state of impurity element oxygen, reduces the segregation of oxygen at W-matrix interfaces because it forms stable LaMnO3 and a small amount of Mn3 O4 . As a result, the bonding force between W grains and matrix phase is increased. Therefore, the mechanical properties of the alloy are improved. References [1] Lea C, Muddle BC, Edmonds DV. Metall Trans A 1983;14:667± 77.
[2] German RM, Hanafee JE, DiGiallonardo SL. Metall Trans 1984;A15:121±28. [3] Muddle BC. Metall Trans 1984;A15:1089±98. [4] Scrikanth V et al. R&HM 1986;5:49±53. [5] Muddle BC, Edmonds DV. Met Sci 1983;17:209±18. [6] Garrison WG. J. Miner. Met. Mater. Soc. 1990;42:20±24. [7] Seah MP, Spencer PJ, Hondros ED. Met Sci 1979; 13:307±14. [8] Garcia GI et al. J Met 1985;37:22±28. [9] Bose A et al. Inter J Refr Metals and Hard Mater 1988;7: 98±102. [10] Bose A, German RM. Metall Trans 1988;A19:3100±03. [11] Bose A, German RM. Metall Trans 1990;A21:1325±27. [12] Wu GC et al. Internal report of Central Iron and Steel Research Institute 1995.
In¯uence of the addition of Lanthanum on a W±Mo±Ni±Fe heavy alloy G.C. Wu *, Q. You, D. Wang Central Iron and Steel Research Institute, No. 76 Xueyuan Nanlu, Beijing 100081, People's Republic of China Received 25 September 1998; accepted 26 January 1999
Abstract In order to improve the mechanical properties of a W±Mo±Ni±Fe heavy alloy, the in¯uence of adding rare earth element La on heavy alloy and its action mechanism have been studied in this paper. The results showed that the mechanical properties of this alloy can remarkably be improved when a proper amount of La is added. Scanning electron microscopy, transmission electron microscopy, X-ray diraction pattern and auger electron spectroscopy experiments revealed that the improvement of mechanical properties was primarily due to the formation of stable LaMnO3 and a small amount of Mn3 O4; which changed the existence form and distribution state of the impurity element oxygen, reduced the interfacial segregation of oxygen and thus increased the bonding strength of W-matrix interfaces. Consequently, the mechanical properties of the alloy were improved. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Mechanical properties; Lanthanum; W±Mo±Ni±Fe heavy alloys
Notation SEM AES EDS TEM XRD
scanning electron microscopy auger electron spectroscopy energy dispersive spectrometry transmission electron microscopy X-ray diraction pattern
1. Introduction Heavy W-alloys have widely been used in medical, military and industrial installations. Many previous studies showed that the tensile and impact properties depended critically on the interfacial segregation of some impurity elements such as H, S, P, O [1±5]. The segregation of these impurities on interfaces not only can lower the bonding force between W grains and matrix phase sharply but also may form the bulky inclusion which deteriorates the continuity of the matrix and is the source of fracture of the alloy. Therefore, it is very important to control the concentration of impurities and to improve their state of existence and distri-
*
Corresponding author. E-mail: [email protected]
bution in the alloy. As it is well known, rare earth elements (RE) have been applied in many ®elds because of their extraordinary chemical and physical properties. In steel, the temper embrittlement due to P and Sn can eectively be reduced by adding La [6±8]. The aim of this study is to investigate the possibility of improving the mechanical properties of the heavy alloy by adding a certain RE and to analyze its action mechanism. 2. Experimental The material used in this study is a W±Mo±Ni±Fe heavy alloy, which is dierent from the traditional heavy alloy such as W±Ni±Fe and W±Ni±Cu in that it contains approximately 17 wt.% Mo. This alloy has been applied in industry for some years. The technical speci®cation requires that sintering density must strictly be controlled in the range of 15.73 0.08 Mg á mÿ3 . This alloy, especially used as large dimensional workpieces, has not only very low elongation to failure but also causes brittle crack easily during mechanical working due to a strictly limited density and a high content of Mo which results in the embrittlement of the heavy alloy [9±11]. In this paper, we investigate the reason for the embrittlement of
0263-4368/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 9 ) 0 0 0 1 5 - 3
300
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
Table 1 Alloy composition (wt%)
Table 2 The sintered density of alloys (Mg á mÿ3 )
Alloy type
W
Mo
Ni
Fe
La
Mn
I II III IV V
78 78 78 78 78
17.0 17.1 16.9 16.7 16.5
2.5 2.5 2.5 2.5 2.5
2.3 2.0 2.0 2.0 2.0
0.0 0.2 0.4 0.6 0.8
0.2 0.2 0.2 0.2 0.2
this W±Mo±Ni±Fe heavy alloy and put forward a simple and feasible way of improving mechanical properties. The dierent W±Mo±Ni±Fe heavy alloys were made by traditional powder metallurgy processing. The pure ®ne metal powders of W, Mo, Ni, Fe, Mn and Ni5 La were mixed up and the powder mixture was cold-isostatically pressed under 200 MPa. All compacts were sintered at 1510°C, for 1.5 h in a hydrogen furnace. The compositions of the samples are listed in Table 1. The density and mechanical properties of the sintered specimens were measured. For the experimental investigation of tensile strength, circular specimens with overall dimensions of U5 ´ 65 mm were machined, and the length of the gauge section is 30 mm. At least four specimens were tested for each experimental condition.
Alloy type
Measured density Theoretical density Relative density (%)
I II III IV V
15.75 15.72 15.74 15.78 15.69
15.83 15.81 15.81 15.80 15.79
99.49 99.41 99.58 99.87 99.37
Table 3 The mechanical properties of W±Mo±Ni±Fe alloys Alloy type
rb (MPa)
d (%)
HRC
I II III IV V
291 650 903 846 766
0 2.2 4.7 4.1 3.2
37 34 30 30 31
The microstructural study included optical metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, auger electron spectroscopy (AES), TEM, SEM, X-ray diraction (XRD) analyses are performed to explain the properties of this alloy and determine the phase composition and phase structure.
Fig. 1. Microstructure of W±Mo±Ni±Fe heavy alloys.
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
Fig. 2. EDS analysis of two new phases in the alloy.
Fig. 3. XRD patterns of the new phase extracted from the W±Mo±Ni±Fe heavy alloy; (a) LaMnO3 , (b) Mn3 O4 .
Fig. 4. Indexing of two new phases by diraction pattern.
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302
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
Fig. 5. AES analysis of the oxygen distribution at the W-matrix interface and in depth (a) (b) segregation of oxygen and its distribution in depth in alloy I (c) (d) segregation of oxygen and its distribution in depth in alloy III.
3. Results and discussion The densi®cation behaviour of W±Mo±Ni±Fe heavy alloys containing 0.0, 0.2, 0.4, 0.6, 0.8 mass% La are listed in Table 2. All sintered specimens appeared to be fully dense, with densities between the range of 15.69± 15.78 Mg á mÿ3 , which satisfactorily meets the special requirement of this W±Mo±Ni±Fe heavy alloy in use. The results obtained from the mechanical property tests at varying La content are shown in Table 3. Tensile strengths rb increased from 291 to 903 MPa, and elongation to failured d changed from 0% to 4.7%, while hardness HRC decreased from 37 to 30, which greatly improved the mechanical properties of this alloy. The total microstructure observed for all W±Mo± Ni±Fe samples showed a typical heavy alloy structure, which is similar to that reported elsewhere for W±Ni± Fe heavy alloys, as illustrated in Fig. 1. The polished cross sections showed spherical W grains embedded in a more or less continuous Ni matrix phase. The grain size of tungsten does not vary with the addition of Lanthanum.
However, there is a new micro phase in the alloy containing La in addition to the W-phase and the matrix phase which are present in conventional heavy alloys. The particles of the new phase are gray round balls and possess distinctive color dierent from W grains and matrix phase. Moreover, it is very stable and can clearly be seen in unetched specimens (Fig. 1). The chemical composition analysis by SEM-EDS shows that most of this new phase is rich in La, W and Mn while the remainder is rich only in Mn (Fig. 2). Because the amount of this phase in the alloy is very small, the new phase was extracted as an electrolytic residue by a certain method [12] in order to determine its kind and structure. The XRD analysis of the extracted powder identi®ed that the new phase, produced by adding rare earth element La in the W±Mo±Ni±Fe heavy alloy, was LaMnO3 and a small amount of Mn3 O4 compounds. The indexing of the two new phases is shown in Figs. 3 and 4. For the liquid sintering heavy alloy, the properties are very sensitive to such impurity elements as O, S, P, H, especially the oxygen segregation on the interface. It is thermodynamically more preferable for these impurities
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
303
Fig. 6. Scanning electron micrographs of surfaces of the alloys.
to segregate to the phase interface and grain boundary than to dissolve in intergranularey. The segregation of impurities to W-matrix interfaces deteriorates the wettability and bonding strength microscopically, which cause the brittleness of the alloy. When rare earth element La is added into this alloy, the chemically active La combines with oxygen to form LaMnO3 and a small amount of Mn3 O4 compounds at W-matrix interfaces or in the matrix phase, which alter the existence and distribution of the impurity element oxygen, purify most of the phase interfaces and enhance the bonding force of the W-matrix interfaces. Thus the tensile strength and elongation of the alloy are improved. The AES analysis showed that the addition of La decreased the segregation of oxygen to interfaces and the distribution of oxygen in depth eectively, which is shown in Fig. 5. The eects of La addition are also proved by fractographic observation. The fracture surfaces of two specimens ± one without any La and the other with 0.4% La are shown in Fig. 6. Due to the interface segregation of impurity element oxygen, the specimen without any La (Fig. 6(a)) shows a typical brittle fracture mode-the intergranular fracture at the boundary between W grains and the decohesion of the Wmatrix interfaces, which is associated with low tensile strength and poor ductility of the alloy. After La is added, the formation of LaMnO3 and some Mn3 O4
reduces the concentration of oxygen segregation to the phase interfaces which increases the bonding strength between phases. Thus, the specimen with 0.4% La (Fig. 6(b)) shows a ductile fracture surface with the transgranlar cleavage of W grains and some ductile rupture of the matrix phase with a dimple appearance, which is associated with satisfactory mechanical properties, especially the elongation to failure. However, when the content of La was higher than 0.4%, the number of the new La-rich phase was increased and its size was enlarged. These bulky new phases greatly deteriorated the continuity of the matrix and became a kind of the source of fracture of the alloy, and thus tensile strength and elongation decreased rapidly, which is shown in Table 3. 4. Conclusions (1) The results of the study indicate that the mechanical properties of W±Mo±Ni±Fe heavy alloys can be improved by adding the rare earth element La. (2) The addition of La makes a remarkable improvement on the mechanical properties of this alloy: tensile strength changed from 291 to 903 MPa, elongation increased from 0% to 4.7% when about 0.4% La was added to the alloy.
304
G.C. Wu et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 299±304
(3) The addition of La alters the existence and distribution state of impurity element oxygen, reduces the segregation of oxygen at W-matrix interfaces because it forms stable LaMnO3 and a small amount of Mn3 O4 . As a result, the bonding force between W grains and matrix phase is increased. Therefore, the mechanical properties of the alloy are improved. References [1] Lea C, Muddle BC, Edmonds DV. Metall Trans A 1983;14:667± 77.
[2] German RM, Hanafee JE, DiGiallonardo SL. Metall Trans 1984;A15:121±28. [3] Muddle BC. Metall Trans 1984;A15:1089±98. [4] Scrikanth V et al. R&HM 1986;5:49±53. [5] Muddle BC, Edmonds DV. Met Sci 1983;17:209±18. [6] Garrison WG. J. Miner. Met. Mater. Soc. 1990;42:20±24. [7] Seah MP, Spencer PJ, Hondros ED. Met Sci 1979; 13:307±14. [8] Garcia GI et al. J Met 1985;37:22±28. [9] Bose A et al. Inter J Refr Metals and Hard Mater 1988;7: 98±102. [10] Bose A, German RM. Metall Trans 1988;A19:3100±03. [11] Bose A, German RM. Metall Trans 1990;A21:1325±27. [12] Wu GC et al. Internal report of Central Iron and Steel Research Institute 1995.