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Effect of niobium addition on the mechanical properties of Fe3Al-based alloys
ScriptaMetallurgicaethhterialia, Vol. 33, No. 12, pp. 2013-2017,1995 ELwvie.rScience Ltd Copyrig& 0 1995 Acta MetallurgicaInc. Feted in the USA All rightsreserved O!X6-716W95 $9.50 + .OO
Pergamon
0956-716X(95)00437-8
EFFECT OF NIOBIUM ADDITION ON THE MECHANICAL PROPERTIES OF Fe&l-BASED ALLOYS Zhang Zhonghua, Sun Yangshan, Guo Jun Department of Materials Science and Engineering, Southeast University, Nanjing 210096, P. R. China (Received May 19,1995) (Revised July 24, 1995)
Introduction
Alloys based on Fe&l are of interest because of their excellent oxidation resistance and low cost. However, low strength above 600°C and a lack of ductility at room temperature limit their use as structural materials [11.Recent development efforts have indicatedthat adequate engineering ambient-temperature ductility of 1O2O?hand tensile yield strength of as high as 5OOMPacan be achieved through control of composition and microshucture [2]. These improved tensile properties make Fe,Al-based alloys more competitive against conventional austenic and fzrritic steels. The elevated temperature strength and creep resistance of Fe& have been improved by alloyingprocesses and niobium has been found to be very effective on strengthening binary Fe&l at high temperatures [6]. The purpose of the present paper is to investigate the effect of variations of niobium concentration in the polynery ahoy of Fe-28Al-5Cr-0.5Mo-0.05B-O.O5Zr- (up to 08Nb) on mechanical properties at room temperature (RT) and high temperature of 600°C. ExDerimental Procedures
Five alloys of which the compositions are listed in Table 1 were prepared by vacuum-induction melting using commercials melt stock which contained impurities such as Mn, Si, Cu, S, and Mg, and the total amount of impurities was about 0.5%. The composition of the base alloy (ahoy 1) was Fe-28Al-5Cr-O.5Mo-O.O5B0.05Zr and ditl’erentamount of niobium was added in the other alloys (the compositions are reported in atomic percentage). The ingots were first held at 1000 oC for 15 hours for homogenizing and then hot forged to 12mm-thick sheet bars. The sheet bars were hot rolled at 950°C to a 8mm thickness, then warm rolled at 650700°C to 1.5mm sheets. Tensile specimens with a gage section of 15x3.5 x 1.5mm were cut by electric spark maching from rolled sheets. Befbre testing, all the specimens were annealed at 700°C for one hour followed by oil quenching. For microstructrual observation, some specimens were heated at 850°C for one hour (for recrystallization) and cooled in air.
2013
Vol. 33, No. 12
Fe,Al-BASED ALLOYS
2014
TABLE 1 Tensile Properties and Creep Resistance of Alloys Investigated Creep Rupture 600°C 200 MPa Life
Elong.
Alloy Composition
(h)
(%)
Fe-28Al-5Cr-O.5Mo-O.0SZr-O.O5B Fc-28Al-5Cr-0.5Mo-O.05Zr-0.05B-O.5Nh Fe-28Al-5Cr-0.5Mo-0.05Zr-O.05B-0.6Nh Fe-28Al-5Cr-O.5Mo-0.05Zr-O.05B-0.7Nh Fe28Al-2Cr-0.5Mo-O.05Zr-0.05B-0.8Nb
20 65 102 130 296
62.4 51.7 32.8 37.5 59.6
Alloy
Code 1
2 3 4 5
RT Tensile Yield
Wa) 520 545 579 599 615
Ult.
Wa) 1000 992 989 900 810
600°C Tensile
W)
Yield Wa)
Ult. Wa)
Elong. W)
16.0 14.8 14.0 11.3 9.0
400 457 524 550 573
450 511 567 590 625
62.6 53.6 31.6 35.2 37.7
Elong.
Microcharacterizations of deformation behavior and fracture mode were conducted on selected tiacture specimens using various techniques, including optical metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results Table 1 lists the tensile and creep-rupture data for all specimens tested. It can be seen that niobium addition to the Fe-28Al-5Cr-O.5Mo-O.0SZr-O.O5B alloy resulted in sign&ant influence on tensile properties and creep resistance. At the ambient temperature, the ductility decreased while the yield strength increased with the increase of niobium addition, as shown in Fig. 1. The ultimate tensile strength was correlated with tensile ductility, and it decreased with decreasing ductility. Fig. 2 shows the change of tensile properties with the increase of niobium concentration at 600 ‘C. The yield strength was remarkably increased, especially when the amount of niobium addition increased from 0.5 to 0.8%. The ultimate tensile strength at 600 “C was correlated with yield strength, and it increased with increasing yield strength. Another important result caused by niobium addition was in creep resistance. The creep rupture life increased rapidly with the increase of niobium content and reached as high as 296h (at 6OO”C, 200MPa). The creep curves of all the alloys studied are shown in Fig. 3, from which it can be seen that the addition of
950 900 850 800 750 700 650 600 550
508 Figure 1. Variation of room temperature properties versus the Nb concentration.
Nb
addition
(-at%)
Figure 2.Variation of tensile properties versus the Nb concentration at 600 “C.
Vol. 33, No. 12
Fe&BASED
Figure 3. Creep-rupture curws of alloys investigated.
ALLOYS
2015
Figure 4. Fracture mode after creep rupture of alloy 3.
niobium reduced the steady-state creep rate greatly. The creep rate o alloy 5 decreased to 18x 1O-5oh/scompared to 1.6x lO”%/s for the base alloy. SEM observations showed that the RT tensile fracture modes of all the alloys investigated were transgranular cleavage. At 6OO”C, the t?acture mode of tensile and creep ruptured specimens of the base alloy was the mainly ductile dimple. however, some areas of the fracture surface still showed transgranular cleavage. Wtth niobium addition, the specimens failed entirely in ductile dimple mode (as shown in Fig. 4). The main change in microstructure caused by niobium addition was the reduction of the grain size. The grain size in the recrysmlhzed specimen of the base alloy was about 50 unr in diameter. However, the niobium addition of 0.5% and 0.8% produced grain Size of 3Opm and 25 pm, respectively. Niobium addition also resulted in the formation of fine precipitates distributed in the Fe,Al matrix and grain boundaries. The morphology of the precipitates was revealed using TEM, Fig. 5 is a TEM micrograph taken from a creep ruptured specimen of alloy 2. Microanalysis carried out by AEM showed that the compositions of precipitates were complex. They were niobium-rich and contained zirconium, chromium, iron and aluminum. TEM observations were performed on tensile and creep specimens of all the alloys. For the base alloy (ahoy 1) test& at RT, there were few network dislocations, and both matrix and grain boundaries were fairly precipitate-free. However, with niobium additions, high concentrations of dislocation tangles distributed as
Figure 5. TEM micrograph of precipitates in alloy 2.
Figure 6. Dislocations of tensile rupture specimen of alloy 3 at RT.
2016
Fe&BASED
ALLOYS
Figure 7. TEM micrograph of alloy 1 after creep rupture.
Vol. 33, No. 12
Figure 8. TEM micrograph of alloy 3 a&r creep rupture.
matrix networks were observed, as shown in Fig. 6. When the specimens were held at 600°C and 200MPa, dislocations of active slip system were able to overcome Peierls-Nabarro force and any additional force holding the dislocations so that the dislocations began to move and caused slip. After creep rupture, the matrix of alloy 1 had only a few network dislocations and no dislocations within the high-angle grain boundaries (Fig. 7). The matrix also contained a coarse structure of planar arrays of dislocations forming low-angle subgrain boundaries. By comparison to alloy 1, creep ruptured specimens of alloys (containing niobium) had a higher concentmtions of creep-produced dislocations distributed around the grain boundaries and precipitates. There was some recovery of dislocations into a subgrain boundary structure (Fig. 8), but far less than found in the base alloy (ahoy 1) tested at the same temperature and stress. Discussion Niobium addition to the binary Fe,Al alloy has been found to increase the yield strength at temperature up to 650°C [4,5]. In the present work, the effect of niobium addition to the polynary Fe&U-based alloy is similar to that to the binary alloy. The solubility of niobium in the Fe,Al matrix is very low, hence the obvious mechanism by which niobium strengthens the Fe,Al-based alloy is precipitation hardening [5]. From Fig. 1 and Fig. 2 it can be seen that the yield strength increases slightly at both ambient temperature and high temperature of 600°C when a small amount (0.5%) of niobium is added to the base alloy. However, a large increase is produced when the niobium addition increases from 0.5% to 0.8%. This can be attributed to the diEerence of the strengthening mechanism. When the content of niobium is lower than O.S%, it mainly goes into the solution and the increase of yield strength is a result of solid-solution strengthening. The further addition of niobium results in the formation of niobium-rich precipitates and precipitation hardening takes the main effect on strengthening the alloy. The precipitates formed in the ternary ahoy of Fe-28ALNb has been found to be Fe$lb [5], and the strengthening &it of the precipitates can be maintained at temperatures up to 600°C. In the present work, although the compositions of the precipitates observed in the alloys with niobium addition are complex, they seem to be more effective to strengthen the alloy at high temperatures and further investigation on characterization of the precipitates is under way The creep resistance of the binary Fe&l alloy is very poor, the creep rupture life of the Fe-28Al alloy is reported to be only 0.6h at 650°C and 138MPa [4]. The short creep-rupture life in the binary alloy is due to weak high-angle grain boundaries combined with the reduced ability of the dislocations to either interact and produce the strain-hardening or resist the recovery process. By adding 1%Nb, the creep-rupture life of the
Vol.
33, No. 12
Fe&BASED
ALLOYS
binary alloy is significantly increased to 304h [4]. Improvement of creep resistance caused by niobium addition has also been found in polynary Fe&l-based alloys. TBM observations reveal the difference of dislocation structures between the base alloy and the alloys containing niobium. In creep ruptured specimens of alloys containing more than 0.5% niobium, the concentration of creep-produced dislocations is higher and the recovety of dislo&.ions into subgrain boundaries is less than that in the base alloy This is also accounted for by the formation of niobium-rich precipitates which are effective climb or recovery obstacles. In the creep ruptured specimens (as shown in Fig. S), these fine precipitates can still be observed, indicating that they play the role of strengthening the matrix and grain boundaries against creep. Conclusion Small amount (0.5-0.8ato/o) of niobium addition to the polynery alloys based on Fe&l (Fe-28Al-5Cr-OSMoO.OSZr-O.OSB) increases the yield strength and creep resistance greatly, but is not beneficial to room temperature ductility. Results of microanalysis indicate that the addition of niobium causes the formation of a lot of precipitates with complex compositions which strengthen both matrix and grain boundaries. Reference 1. 2. 3. 4. 5. 6.
C. G. Mckamey, J. H. Devan, P. F. Tortorelli and V. K Sikka, J. Mater. Res., 6, 1779 (1991). V. K. Sikka, SAMPE, 22,2 (1991). C. G. Mckamey and J. A. Horton, Metall. Trans., 2OA, 751(1989). G. G. Mckamey, P. J. Maziasy and J. W. Jones, J. Mater. Res., 7,2089 (1992). Sun Cbao, Guo Jianting, Acta. Metallurgica Sinica, Vol. 28A, No. 11(1992) 481. J. R. Knibloe and R. N. Wright, Mat. Sci. Eng., A153,382 (1992).
Pergamon
0956-716X(95)00437-8
EFFECT OF NIOBIUM ADDITION ON THE MECHANICAL PROPERTIES OF Fe&l-BASED ALLOYS Zhang Zhonghua, Sun Yangshan, Guo Jun Department of Materials Science and Engineering, Southeast University, Nanjing 210096, P. R. China (Received May 19,1995) (Revised July 24, 1995)
Introduction
Alloys based on Fe&l are of interest because of their excellent oxidation resistance and low cost. However, low strength above 600°C and a lack of ductility at room temperature limit their use as structural materials [11.Recent development efforts have indicatedthat adequate engineering ambient-temperature ductility of 1O2O?hand tensile yield strength of as high as 5OOMPacan be achieved through control of composition and microshucture [2]. These improved tensile properties make Fe,Al-based alloys more competitive against conventional austenic and fzrritic steels. The elevated temperature strength and creep resistance of Fe& have been improved by alloyingprocesses and niobium has been found to be very effective on strengthening binary Fe&l at high temperatures [6]. The purpose of the present paper is to investigate the effect of variations of niobium concentration in the polynery ahoy of Fe-28Al-5Cr-0.5Mo-0.05B-O.O5Zr- (up to 08Nb) on mechanical properties at room temperature (RT) and high temperature of 600°C. ExDerimental Procedures
Five alloys of which the compositions are listed in Table 1 were prepared by vacuum-induction melting using commercials melt stock which contained impurities such as Mn, Si, Cu, S, and Mg, and the total amount of impurities was about 0.5%. The composition of the base alloy (ahoy 1) was Fe-28Al-5Cr-O.5Mo-O.O5B0.05Zr and ditl’erentamount of niobium was added in the other alloys (the compositions are reported in atomic percentage). The ingots were first held at 1000 oC for 15 hours for homogenizing and then hot forged to 12mm-thick sheet bars. The sheet bars were hot rolled at 950°C to a 8mm thickness, then warm rolled at 650700°C to 1.5mm sheets. Tensile specimens with a gage section of 15x3.5 x 1.5mm were cut by electric spark maching from rolled sheets. Befbre testing, all the specimens were annealed at 700°C for one hour followed by oil quenching. For microstructrual observation, some specimens were heated at 850°C for one hour (for recrystallization) and cooled in air.
2013
Vol. 33, No. 12
Fe,Al-BASED ALLOYS
2014
TABLE 1 Tensile Properties and Creep Resistance of Alloys Investigated Creep Rupture 600°C 200 MPa Life
Elong.
Alloy Composition
(h)
(%)
Fe-28Al-5Cr-O.5Mo-O.0SZr-O.O5B Fc-28Al-5Cr-0.5Mo-O.05Zr-0.05B-O.5Nh Fe-28Al-5Cr-0.5Mo-0.05Zr-O.05B-0.6Nh Fe-28Al-5Cr-O.5Mo-0.05Zr-O.05B-0.7Nh Fe28Al-2Cr-0.5Mo-O.05Zr-0.05B-0.8Nb
20 65 102 130 296
62.4 51.7 32.8 37.5 59.6
Alloy
Code 1
2 3 4 5
RT Tensile Yield
Wa) 520 545 579 599 615
Ult.
Wa) 1000 992 989 900 810
600°C Tensile
W)
Yield Wa)
Ult. Wa)
Elong. W)
16.0 14.8 14.0 11.3 9.0
400 457 524 550 573
450 511 567 590 625
62.6 53.6 31.6 35.2 37.7
Elong.
Microcharacterizations of deformation behavior and fracture mode were conducted on selected tiacture specimens using various techniques, including optical metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results Table 1 lists the tensile and creep-rupture data for all specimens tested. It can be seen that niobium addition to the Fe-28Al-5Cr-O.5Mo-O.0SZr-O.O5B alloy resulted in sign&ant influence on tensile properties and creep resistance. At the ambient temperature, the ductility decreased while the yield strength increased with the increase of niobium addition, as shown in Fig. 1. The ultimate tensile strength was correlated with tensile ductility, and it decreased with decreasing ductility. Fig. 2 shows the change of tensile properties with the increase of niobium concentration at 600 ‘C. The yield strength was remarkably increased, especially when the amount of niobium addition increased from 0.5 to 0.8%. The ultimate tensile strength at 600 “C was correlated with yield strength, and it increased with increasing yield strength. Another important result caused by niobium addition was in creep resistance. The creep rupture life increased rapidly with the increase of niobium content and reached as high as 296h (at 6OO”C, 200MPa). The creep curves of all the alloys studied are shown in Fig. 3, from which it can be seen that the addition of
950 900 850 800 750 700 650 600 550
508 Figure 1. Variation of room temperature properties versus the Nb concentration.
Nb
addition
(-at%)
Figure 2.Variation of tensile properties versus the Nb concentration at 600 “C.
Vol. 33, No. 12
Fe&BASED
Figure 3. Creep-rupture curws of alloys investigated.
ALLOYS
2015
Figure 4. Fracture mode after creep rupture of alloy 3.
niobium reduced the steady-state creep rate greatly. The creep rate o alloy 5 decreased to 18x 1O-5oh/scompared to 1.6x lO”%/s for the base alloy. SEM observations showed that the RT tensile fracture modes of all the alloys investigated were transgranular cleavage. At 6OO”C, the t?acture mode of tensile and creep ruptured specimens of the base alloy was the mainly ductile dimple. however, some areas of the fracture surface still showed transgranular cleavage. Wtth niobium addition, the specimens failed entirely in ductile dimple mode (as shown in Fig. 4). The main change in microstructure caused by niobium addition was the reduction of the grain size. The grain size in the recrysmlhzed specimen of the base alloy was about 50 unr in diameter. However, the niobium addition of 0.5% and 0.8% produced grain Size of 3Opm and 25 pm, respectively. Niobium addition also resulted in the formation of fine precipitates distributed in the Fe,Al matrix and grain boundaries. The morphology of the precipitates was revealed using TEM, Fig. 5 is a TEM micrograph taken from a creep ruptured specimen of alloy 2. Microanalysis carried out by AEM showed that the compositions of precipitates were complex. They were niobium-rich and contained zirconium, chromium, iron and aluminum. TEM observations were performed on tensile and creep specimens of all the alloys. For the base alloy (ahoy 1) test& at RT, there were few network dislocations, and both matrix and grain boundaries were fairly precipitate-free. However, with niobium additions, high concentrations of dislocation tangles distributed as
Figure 5. TEM micrograph of precipitates in alloy 2.
Figure 6. Dislocations of tensile rupture specimen of alloy 3 at RT.
2016
Fe&BASED
ALLOYS
Figure 7. TEM micrograph of alloy 1 after creep rupture.
Vol. 33, No. 12
Figure 8. TEM micrograph of alloy 3 a&r creep rupture.
matrix networks were observed, as shown in Fig. 6. When the specimens were held at 600°C and 200MPa, dislocations of active slip system were able to overcome Peierls-Nabarro force and any additional force holding the dislocations so that the dislocations began to move and caused slip. After creep rupture, the matrix of alloy 1 had only a few network dislocations and no dislocations within the high-angle grain boundaries (Fig. 7). The matrix also contained a coarse structure of planar arrays of dislocations forming low-angle subgrain boundaries. By comparison to alloy 1, creep ruptured specimens of alloys (containing niobium) had a higher concentmtions of creep-produced dislocations distributed around the grain boundaries and precipitates. There was some recovery of dislocations into a subgrain boundary structure (Fig. 8), but far less than found in the base alloy (ahoy 1) tested at the same temperature and stress. Discussion Niobium addition to the binary Fe,Al alloy has been found to increase the yield strength at temperature up to 650°C [4,5]. In the present work, the effect of niobium addition to the polynary Fe&U-based alloy is similar to that to the binary alloy. The solubility of niobium in the Fe,Al matrix is very low, hence the obvious mechanism by which niobium strengthens the Fe,Al-based alloy is precipitation hardening [5]. From Fig. 1 and Fig. 2 it can be seen that the yield strength increases slightly at both ambient temperature and high temperature of 600°C when a small amount (0.5%) of niobium is added to the base alloy. However, a large increase is produced when the niobium addition increases from 0.5% to 0.8%. This can be attributed to the diEerence of the strengthening mechanism. When the content of niobium is lower than O.S%, it mainly goes into the solution and the increase of yield strength is a result of solid-solution strengthening. The further addition of niobium results in the formation of niobium-rich precipitates and precipitation hardening takes the main effect on strengthening the alloy. The precipitates formed in the ternary ahoy of Fe-28ALNb has been found to be Fe$lb [5], and the strengthening &it of the precipitates can be maintained at temperatures up to 600°C. In the present work, although the compositions of the precipitates observed in the alloys with niobium addition are complex, they seem to be more effective to strengthen the alloy at high temperatures and further investigation on characterization of the precipitates is under way The creep resistance of the binary Fe&l alloy is very poor, the creep rupture life of the Fe-28Al alloy is reported to be only 0.6h at 650°C and 138MPa [4]. The short creep-rupture life in the binary alloy is due to weak high-angle grain boundaries combined with the reduced ability of the dislocations to either interact and produce the strain-hardening or resist the recovery process. By adding 1%Nb, the creep-rupture life of the
Vol.
33, No. 12
Fe&BASED
ALLOYS
binary alloy is significantly increased to 304h [4]. Improvement of creep resistance caused by niobium addition has also been found in polynary Fe&l-based alloys. TBM observations reveal the difference of dislocation structures between the base alloy and the alloys containing niobium. In creep ruptured specimens of alloys containing more than 0.5% niobium, the concentration of creep-produced dislocations is higher and the recovety of dislo&.ions into subgrain boundaries is less than that in the base alloy This is also accounted for by the formation of niobium-rich precipitates which are effective climb or recovery obstacles. In the creep ruptured specimens (as shown in Fig. S), these fine precipitates can still be observed, indicating that they play the role of strengthening the matrix and grain boundaries against creep. Conclusion Small amount (0.5-0.8ato/o) of niobium addition to the polynery alloys based on Fe&l (Fe-28Al-5Cr-OSMoO.OSZr-O.OSB) increases the yield strength and creep resistance greatly, but is not beneficial to room temperature ductility. Results of microanalysis indicate that the addition of niobium causes the formation of a lot of precipitates with complex compositions which strengthen both matrix and grain boundaries. Reference 1. 2. 3. 4. 5. 6.
C. G. Mckamey, J. H. Devan, P. F. Tortorelli and V. K Sikka, J. Mater. Res., 6, 1779 (1991). V. K. Sikka, SAMPE, 22,2 (1991). C. G. Mckamey and J. A. Horton, Metall. Trans., 2OA, 751(1989). G. G. Mckamey, P. J. Maziasy and J. W. Jones, J. Mater. Res., 7,2089 (1992). Sun Cbao, Guo Jianting, Acta. Metallurgica Sinica, Vol. 28A, No. 11(1992) 481. J. R. Knibloe and R. N. Wright, Mat. Sci. Eng., A153,382 (1992).