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Boron-induced softening in polycrystalline stoichiometric ni3al alloys
Volume 6, number 5,6
BORON-INDUCED
MATERIALS LETTERS
SOFTENING
IN POLYCRYSTALLINE
March I988
STOICHIOMETRIC
N&Al ALLOYS
X.R. QIAN and Y.T. CHOU Depurtment ofMaterials Science and Engineering, Lehigh University. Bethlehem, F4 18015. USA Recewed
23 December
1987
.Alloy softening was observed occurred both in grain interiors
in microhardness measurements of Ni,Al alloy with 0.2 at% boron. The boron-induced softening and along grain boundaries. Possible mechanisms for the softening effects are discussed.
1. Introduction Nickel aluminide, Ni,Al, has recently received considerable attention because of its potential for engineering applications at elevated temperatures. The attractive feature of this alloy is that its yield stress increases with increasing temperature. However, the polycrystalline stoichiometric binary compound exhibits poor ductility and fails preferentially along the grain boundary. Aoki and Izumi [ 1] first observed the beneficial effects of boron in Ni3A1 and obtained a tensile elongation of about 35% at room temperature. Liu and Koch [ 21 subsequently reported a higher tensile elongation, exceeding 50% for the boron-doped Ni3Al containing 24 at% Al. These results stimulated a new wave of research on the structure and the mechanical properties of this ordered alloy. It is believed that the improved ductility of NixAl is attributable to boron segregation at grain boundaries [ 3 ] which increases its cohesive strength [ 4 1. It has also been postulated that boron exhibits a significant strengthening effect in Ni,Al due to the large lattice strain produced by the interstitial B atoms [ 5 1. However, it has been difficult to investigate experimentally the distribution of boron in Ni3Al and hence the reasons for its beneficial effect. Recently the distribution of boron in a rapidly solidified Ni-24 at% Al alloy containing 0.24 at% B was investigated by Horton and Miller [ 6 ] using the atom probe field-ion microscopy. Boron was found te segregate at both antiphase boundaries and grain
The effect of boron on mechanical properties has been studied by macrostrain and macrohardness measurements which reflect the average properties of a bulk sample, including both grain interiors and grain boundaries. On the other hand, the microhardness of rapidly solidified Ni&l containing 0,0.25 and 0.5 at% B was studied by Koch et al. [ 71. Their results showed no obvious variation in hardness as a function of boron concentration, contrary to the boron strengthening effect deduced from the macrostrain and macrohardness tests [ 5,8]. This paper reports an investigation of the boron effect in polycrystalline NixA alloys based on microhardness measurements. The results demonstrate an interesting role of boron which has not been discussed previously.
2. Experimental The binary polycrystalline Ni3A1 alloys used in this study were supplied by the Oak Ridge National Laboratory. The compositions are Ni-25 at% Al and Ni-25 at% Al-O.2 at% B. The cast rod-shaped alloys with a diameter of z 10 mm were vacuum ( = lop2 Torr) encapsulated in fused silica tubes, annealed at 1200°C ( + 1O’C) for a period of 72 h and fumacecooled to room temperature. The homogenized alloys have an average grain size of 0.9 mm and were also used in the diffusion study I9 1. The annealed samples were cut from each rod at different angles with the longitudinal axis, i.e. at 450,
boundaries.
0167-571x/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division \
B.V.
157 __
Volume 6, number
MATERIALS
5,6
90’ and 180’. To minimize the errors due to surface preparation, a set of six samples from the two alloys (three boron free and three boron doped) was mounted in the same bakelite mold. The mount was mechanically ground to 600 grit emery papers, then polished with 6 micron diamond paste and 0.3 micron alumina powder. In order to obtain reliable microhardness data, the mount was polished further with 0.05 micron silicon oxide powder. Finally, it was chemically etched in a solution containing equal parts of HNO,, H2S04 and H,PO, by volume for 45 s. Vickers hardness (DPH) was measured at room temperature on a TUKON microhardness tester equipped with a microprocessor. Two levels of loading were used. For grain interiors, a load of 500 g was chosen, and each data point refers to an individual grain and represents the average of five indentation readings within the grain. More indentations in a grain would lead to interference in deformation from the adjacent indentation sites. The spacing of two adjacent indentations was thus chosen to be about six units of edge length of the indentation (fig. la). The experimental errors involved in the measurements were within +3%. For hardness measurements on grain boundaries, a load of 5 g was selected, giving an indentation diagonal of 5.0 to 7.0 urn. In this case, ten measurements were taken for each data point with an error of ? 7% (fig. lb).
LETTERS
March
1988
3. Results and discussion Figs. 2 and 3 illustrate the frequency distributions of 30 averaged Vickers hardness numbers (DPH) in grain interiors (a total of 150 indentations) and on grain boundaries (a total of 300 indentations) for the Ni3.41 alloys with and without boron. The mean values and standard deviations are shown in table 1. The statistical data show that the average microhardness of the boron-doped alloy is lower than that of the boron-free alloy. The result is true for measurements both in the grain interiors and on the grain boundaries. Thus an important observation in the present study is that with the addition of 0.2 at% (500 ppm by weight) of boron to the stoichiometric Ni3Al alloy, the grain boundary hardness was decreased. This indicates that the addition of boron softens the grain boundaries of Ni3Al. Schulson and co-workers [ 10,111 have shown that the boron addition to Ni&l reduced the effectiveness of grain boundary strengthening and suggested that the addition of boron would soften the grain boundaries. Softer boundaries were more accommodating to slip and therefore did not fracture intergranularly, resulting in a lower strength but much higher ductility. Our results of grain boundary microhardness support this argument.
b
Fig. 1. Vickers hardness boundary (5 g load).
158
indentations
on stoichiometric
Ni,Al alloy with 0.2 at% boron:
(a) in grain interior
(500 g load);
(b) on grain
Volume 6, number
MATERIALS
5.6
LETTERS
March
1988
IO7-
-
T’
IO)
I
6;
5t-
i , z I:
4-
rJ I
(01
rl
I
8-
’ 1
(b)
1
7-
L-l
rA I
3-
9-
(b)
I Ll
rd
6-
I
2
Ll I
I0
’ 260
I 270
I 280
I 290
300
310
320
Mlcrohardness.
I 330
, 340
’ ;; III 350
II 360
DPH
Fig. 2. Frequency distributions of 30 averaged Vickers hardness numbers (DPH) on grain boundaries of stoichiometric Ni,Al alloy at a 5 g load: (a) alloy doped with 0.2 at% boron (mean= 283.7); (b) boron-free alloy (mean=329.2).
A more interesting finding of the present study is that the addition of boron also lowers the microhardness in the grain interior. It is commonly believed that in the matrix of a Ni3A1 alloy, boron resides interstitially and generates lattice strain which imparts a marked increase in lattice resistance to slip. Obviously, our result shows the opposite effect. It has been recognized [ 121 that the introduction of point defects (substitutional, interstitial and selfinterstitial) in bee metals may produce a softening effect at low temperatures, known as “alloy softening” (AS). There are a number of mechanisms proposed for the process and two of these seem to be pertinent to the present case of boron-induced softening. One mechanism suggests that the softening effect is due to the ( 111) to ( 11 i ) cross-slip of screw dislocations promoted by their interaction with the consecutive, properly oriented interstitial boron dipoles. This mechanism has been used to explain the
Table 1 Statistical
data of Vickers hardness
r
01 ” I80
190
I 200
II
II
210
220
Microhardness.
I
II
I
230
240
250
DPH
Fig. 3. Frequency distributions of 30 averaged Vickers hardness numbers (DPH) in grain interiors of stoichiometric Ni,.41 alloy at a 500 g load: (a) alloy doped with 0.2 at% boron (mean= 201.7); (b) boron-freealloy (mean=225.2).
alloy softening in the Fe-N system near 175 K [ 13,141. It may likely be responsible for the boron softening in Ni3Al. A second softening mechanism suggests that boron would interact with impurities and remove them from solution, resulting in a reduction in flow stress [ 15- 18 1. This mechanism also seems to be an acceptable candidate for the softening effect. The applicability of these mechanisms, however, remains to be verified by experiment. Nakada and Keh [ 19 1, in their study of the Fe-N system, remarked that alloy softening and alloy strengthening are two concurrent but competitive processes in the course of deformation. Under certain (apparently more frequent) circumstances, al-
in N,Al alloys
Alloy
boron free boron doped
Hardness in grain interior
Hardness on grain boundary
mean
standard deviation
mean
225.2 201.7
6.9 6.3
329.2 283.7
standard deviation 10.3
10.0
159
Volume 6, number
5,6
MATERIALS
loy strengthening is the dominant mechanism, while in other situations alloy softening plays a superior role. The balance of the two processes depends on the interplay of the process parameters, i.e. temperature, alloy concentration, crystallographic orientation, grain size and strain rate, etc. On macrostraining of boron-doped Ni3A1 (i.e. tensile deformation and hardness) it was reported that alloy softening occurs at small grain size and alloy strengthening at large grain size [ 10,201. The salient point of the present investigation is to show that alloy softening occurs also on microstraining (microhardness ). More detailed study is needed to extend the preliminary results and to clarify the factors that promote the boroninduced softening effect in Ni3Al alloys.
Acknowledgement
The authors wish to thank Dr. C.T. Liu and Dr. D.M. Kroeger of the Oak Ridge National Laboratory for helpful discussions. The work was sponsored by the Division of Materials Sciences, U.S. Department of Energy under Grant DE-FG02-86ER45256.
References [ 1] K. Aoki and 0. (1979) 1190.
160
Izumi,
Nippon
Kinzoku
Gakkaishi
43
LETTERS
March
1988
[2] C.T. Liu and C.C. Koch, Technical aspects of critical materials used by the steel industry, Vol. 2B, NBSIR83-26799 (National Bureau of Standards, Washington, 1983). [3] C.T. Liu, C.L. White and J.A. Horton, Acta Metall. 33 (1985) 213. [4] T. Takasugi, 0. Izumi and N. Masahashi, Acta Metall. 33 (1985) 1259. [5] S.C. Huang, AI. Taub and K.M. Chang, Acta Metall. 32 (1984) 1703. [6] J.A. Horton and M.K. Miller, Acta Metall. 35 (1987) 133. [7]C.C. Koch, J.A. Horton, C.T. Liu, O.B. Cavin and J.O. Scarbough, in: Rapid solidification processing, principles and technologies, Vol. 3, ed. R. Mehrabian (National Bureau of Standards, Washington, 1983) p. 264. [ 8 ] T.P. Weihs, V. Zinoviev, D.V. Viens and E.M. Schulson, Acta Metall. 35 (1987) 1109. [9] T.C. Chou and Y.T. Chou, in: Proceedings of the MRS Symposium on High Temperature Ordered Intermetallic Alloy, eds. C.C. Koch, C.T. Liu and N.S. Stoloff (Materials Research Society, Pittsburgh, 1985) p. 46 1. [IO] E.M. Schulson, T.P. Weihs, I. Baker, H.J. Frost and J.A. Horton, Acta Metall. 34 ( 1986) 1395. [ 111 I. Baker, E.M. Schulson and J.A. Horton, Acta Metall. 35 (1987) 1533. [ 121 E. Pink and R.J. Arsenault, Prog. Mater. Sci. 24 ( 1979) 1. [ 131 B.W. Christ, Acta Metall. 17 (1969) 17. [ 141 CL. Formby, Phil. Mag. 14 (1966) 745. [ 15 ] C.A. Edwards, D.L. Phillips and H.N. Jones, J. Iron Steel Inst. 142 (1940) 199. [ 161 W.P. Rees, B.E. Hopkins and H.R. Tipler, J. Iron Steel Inst. 177 (1954) 93. [ 17 ] B.C. Allen and R.I. Jaffee, Trans. ASM 56 (1963) 387. [ 18 ] R.W. Guard and J.H. Westbrook, Trans. AIME 2 15 ( 1959) 807. [ 191 Y. Nakada and AS. Keh, Acta Metall. 16 ( 1968 ) 903. [20] P.S. Khadkikar, K. Vedula and B.S. Shabel, Metall. Trans. ISA (1987) 425.
BORON-INDUCED
MATERIALS LETTERS
SOFTENING
IN POLYCRYSTALLINE
March I988
STOICHIOMETRIC
N&Al ALLOYS
X.R. QIAN and Y.T. CHOU Depurtment ofMaterials Science and Engineering, Lehigh University. Bethlehem, F4 18015. USA Recewed
23 December
1987
.Alloy softening was observed occurred both in grain interiors
in microhardness measurements of Ni,Al alloy with 0.2 at% boron. The boron-induced softening and along grain boundaries. Possible mechanisms for the softening effects are discussed.
1. Introduction Nickel aluminide, Ni,Al, has recently received considerable attention because of its potential for engineering applications at elevated temperatures. The attractive feature of this alloy is that its yield stress increases with increasing temperature. However, the polycrystalline stoichiometric binary compound exhibits poor ductility and fails preferentially along the grain boundary. Aoki and Izumi [ 1] first observed the beneficial effects of boron in Ni3A1 and obtained a tensile elongation of about 35% at room temperature. Liu and Koch [ 21 subsequently reported a higher tensile elongation, exceeding 50% for the boron-doped Ni3Al containing 24 at% Al. These results stimulated a new wave of research on the structure and the mechanical properties of this ordered alloy. It is believed that the improved ductility of NixAl is attributable to boron segregation at grain boundaries [ 3 ] which increases its cohesive strength [ 4 1. It has also been postulated that boron exhibits a significant strengthening effect in Ni,Al due to the large lattice strain produced by the interstitial B atoms [ 5 1. However, it has been difficult to investigate experimentally the distribution of boron in Ni3Al and hence the reasons for its beneficial effect. Recently the distribution of boron in a rapidly solidified Ni-24 at% Al alloy containing 0.24 at% B was investigated by Horton and Miller [ 6 ] using the atom probe field-ion microscopy. Boron was found te segregate at both antiphase boundaries and grain
The effect of boron on mechanical properties has been studied by macrostrain and macrohardness measurements which reflect the average properties of a bulk sample, including both grain interiors and grain boundaries. On the other hand, the microhardness of rapidly solidified Ni&l containing 0,0.25 and 0.5 at% B was studied by Koch et al. [ 71. Their results showed no obvious variation in hardness as a function of boron concentration, contrary to the boron strengthening effect deduced from the macrostrain and macrohardness tests [ 5,8]. This paper reports an investigation of the boron effect in polycrystalline NixA alloys based on microhardness measurements. The results demonstrate an interesting role of boron which has not been discussed previously.
2. Experimental The binary polycrystalline Ni3A1 alloys used in this study were supplied by the Oak Ridge National Laboratory. The compositions are Ni-25 at% Al and Ni-25 at% Al-O.2 at% B. The cast rod-shaped alloys with a diameter of z 10 mm were vacuum ( = lop2 Torr) encapsulated in fused silica tubes, annealed at 1200°C ( + 1O’C) for a period of 72 h and fumacecooled to room temperature. The homogenized alloys have an average grain size of 0.9 mm and were also used in the diffusion study I9 1. The annealed samples were cut from each rod at different angles with the longitudinal axis, i.e. at 450,
boundaries.
0167-571x/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division \
B.V.
157 __
Volume 6, number
MATERIALS
5,6
90’ and 180’. To minimize the errors due to surface preparation, a set of six samples from the two alloys (three boron free and three boron doped) was mounted in the same bakelite mold. The mount was mechanically ground to 600 grit emery papers, then polished with 6 micron diamond paste and 0.3 micron alumina powder. In order to obtain reliable microhardness data, the mount was polished further with 0.05 micron silicon oxide powder. Finally, it was chemically etched in a solution containing equal parts of HNO,, H2S04 and H,PO, by volume for 45 s. Vickers hardness (DPH) was measured at room temperature on a TUKON microhardness tester equipped with a microprocessor. Two levels of loading were used. For grain interiors, a load of 500 g was chosen, and each data point refers to an individual grain and represents the average of five indentation readings within the grain. More indentations in a grain would lead to interference in deformation from the adjacent indentation sites. The spacing of two adjacent indentations was thus chosen to be about six units of edge length of the indentation (fig. la). The experimental errors involved in the measurements were within +3%. For hardness measurements on grain boundaries, a load of 5 g was selected, giving an indentation diagonal of 5.0 to 7.0 urn. In this case, ten measurements were taken for each data point with an error of ? 7% (fig. lb).
LETTERS
March
1988
3. Results and discussion Figs. 2 and 3 illustrate the frequency distributions of 30 averaged Vickers hardness numbers (DPH) in grain interiors (a total of 150 indentations) and on grain boundaries (a total of 300 indentations) for the Ni3.41 alloys with and without boron. The mean values and standard deviations are shown in table 1. The statistical data show that the average microhardness of the boron-doped alloy is lower than that of the boron-free alloy. The result is true for measurements both in the grain interiors and on the grain boundaries. Thus an important observation in the present study is that with the addition of 0.2 at% (500 ppm by weight) of boron to the stoichiometric Ni3Al alloy, the grain boundary hardness was decreased. This indicates that the addition of boron softens the grain boundaries of Ni3Al. Schulson and co-workers [ 10,111 have shown that the boron addition to Ni&l reduced the effectiveness of grain boundary strengthening and suggested that the addition of boron would soften the grain boundaries. Softer boundaries were more accommodating to slip and therefore did not fracture intergranularly, resulting in a lower strength but much higher ductility. Our results of grain boundary microhardness support this argument.
b
Fig. 1. Vickers hardness boundary (5 g load).
158
indentations
on stoichiometric
Ni,Al alloy with 0.2 at% boron:
(a) in grain interior
(500 g load);
(b) on grain
Volume 6, number
MATERIALS
5.6
LETTERS
March
1988
IO7-
-
T’
IO)
I
6;
5t-
i , z I:
4-
rJ I
(01
rl
I
8-
’ 1
(b)
1
7-
L-l
rA I
3-
9-
(b)
I Ll
rd
6-
I
2
Ll I
I0
’ 260
I 270
I 280
I 290
300
310
320
Mlcrohardness.
I 330
, 340
’ ;; III 350
II 360
DPH
Fig. 2. Frequency distributions of 30 averaged Vickers hardness numbers (DPH) on grain boundaries of stoichiometric Ni,Al alloy at a 5 g load: (a) alloy doped with 0.2 at% boron (mean= 283.7); (b) boron-free alloy (mean=329.2).
A more interesting finding of the present study is that the addition of boron also lowers the microhardness in the grain interior. It is commonly believed that in the matrix of a Ni3A1 alloy, boron resides interstitially and generates lattice strain which imparts a marked increase in lattice resistance to slip. Obviously, our result shows the opposite effect. It has been recognized [ 121 that the introduction of point defects (substitutional, interstitial and selfinterstitial) in bee metals may produce a softening effect at low temperatures, known as “alloy softening” (AS). There are a number of mechanisms proposed for the process and two of these seem to be pertinent to the present case of boron-induced softening. One mechanism suggests that the softening effect is due to the ( 111) to ( 11 i ) cross-slip of screw dislocations promoted by their interaction with the consecutive, properly oriented interstitial boron dipoles. This mechanism has been used to explain the
Table 1 Statistical
data of Vickers hardness
r
01 ” I80
190
I 200
II
II
210
220
Microhardness.
I
II
I
230
240
250
DPH
Fig. 3. Frequency distributions of 30 averaged Vickers hardness numbers (DPH) in grain interiors of stoichiometric Ni,.41 alloy at a 500 g load: (a) alloy doped with 0.2 at% boron (mean= 201.7); (b) boron-freealloy (mean=225.2).
alloy softening in the Fe-N system near 175 K [ 13,141. It may likely be responsible for the boron softening in Ni3Al. A second softening mechanism suggests that boron would interact with impurities and remove them from solution, resulting in a reduction in flow stress [ 15- 18 1. This mechanism also seems to be an acceptable candidate for the softening effect. The applicability of these mechanisms, however, remains to be verified by experiment. Nakada and Keh [ 19 1, in their study of the Fe-N system, remarked that alloy softening and alloy strengthening are two concurrent but competitive processes in the course of deformation. Under certain (apparently more frequent) circumstances, al-
in N,Al alloys
Alloy
boron free boron doped
Hardness in grain interior
Hardness on grain boundary
mean
standard deviation
mean
225.2 201.7
6.9 6.3
329.2 283.7
standard deviation 10.3
10.0
159
Volume 6, number
5,6
MATERIALS
loy strengthening is the dominant mechanism, while in other situations alloy softening plays a superior role. The balance of the two processes depends on the interplay of the process parameters, i.e. temperature, alloy concentration, crystallographic orientation, grain size and strain rate, etc. On macrostraining of boron-doped Ni3A1 (i.e. tensile deformation and hardness) it was reported that alloy softening occurs at small grain size and alloy strengthening at large grain size [ 10,201. The salient point of the present investigation is to show that alloy softening occurs also on microstraining (microhardness ). More detailed study is needed to extend the preliminary results and to clarify the factors that promote the boroninduced softening effect in Ni3Al alloys.
Acknowledgement
The authors wish to thank Dr. C.T. Liu and Dr. D.M. Kroeger of the Oak Ridge National Laboratory for helpful discussions. The work was sponsored by the Division of Materials Sciences, U.S. Department of Energy under Grant DE-FG02-86ER45256.
References [ 1] K. Aoki and 0. (1979) 1190.
160
Izumi,
Nippon
Kinzoku
Gakkaishi
43
LETTERS
March
1988
[2] C.T. Liu and C.C. Koch, Technical aspects of critical materials used by the steel industry, Vol. 2B, NBSIR83-26799 (National Bureau of Standards, Washington, 1983). [3] C.T. Liu, C.L. White and J.A. Horton, Acta Metall. 33 (1985) 213. [4] T. Takasugi, 0. Izumi and N. Masahashi, Acta Metall. 33 (1985) 1259. [5] S.C. Huang, AI. Taub and K.M. Chang, Acta Metall. 32 (1984) 1703. [6] J.A. Horton and M.K. Miller, Acta Metall. 35 (1987) 133. [7]C.C. Koch, J.A. Horton, C.T. Liu, O.B. Cavin and J.O. Scarbough, in: Rapid solidification processing, principles and technologies, Vol. 3, ed. R. Mehrabian (National Bureau of Standards, Washington, 1983) p. 264. [ 8 ] T.P. Weihs, V. Zinoviev, D.V. Viens and E.M. Schulson, Acta Metall. 35 (1987) 1109. [9] T.C. Chou and Y.T. Chou, in: Proceedings of the MRS Symposium on High Temperature Ordered Intermetallic Alloy, eds. C.C. Koch, C.T. Liu and N.S. Stoloff (Materials Research Society, Pittsburgh, 1985) p. 46 1. [IO] E.M. Schulson, T.P. Weihs, I. Baker, H.J. Frost and J.A. Horton, Acta Metall. 34 ( 1986) 1395. [ 111 I. Baker, E.M. Schulson and J.A. Horton, Acta Metall. 35 (1987) 1533. [ 121 E. Pink and R.J. Arsenault, Prog. Mater. Sci. 24 ( 1979) 1. [ 131 B.W. Christ, Acta Metall. 17 (1969) 17. [ 141 CL. Formby, Phil. Mag. 14 (1966) 745. [ 15 ] C.A. Edwards, D.L. Phillips and H.N. Jones, J. Iron Steel Inst. 142 (1940) 199. [ 161 W.P. Rees, B.E. Hopkins and H.R. Tipler, J. Iron Steel Inst. 177 (1954) 93. [ 17 ] B.C. Allen and R.I. Jaffee, Trans. ASM 56 (1963) 387. [ 18 ] R.W. Guard and J.H. Westbrook, Trans. AIME 2 15 ( 1959) 807. [ 191 Y. Nakada and AS. Keh, Acta Metall. 16 ( 1968 ) 903. [20] P.S. Khadkikar, K. Vedula and B.S. Shabel, Metall. Trans. ISA (1987) 425.