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Grain size stability of nanocrystalline cryomilled Fe-3wt.%Al alloy
NanoStructured Materials, VoL 9. pp. 71-74, 1997 Elsevier Scie~tce Lid O 1997 Aeta Metallurgica Inc. Prinml in the USA. All rights reserved 0965-9773/97 $17.00 + .00
Pergamon pll S0965-9773(97)00021-4
GRAIN SIZE STABILITY OF NANOCRYSTALLINE CRYOMILLED Fe-3wt.%Al ALLOY R.J. Perez, H.G. Jiang and E.J. Lavernia Department of Chemical Engineering and Materials Science University of California-Irvine Irvine, CA 92697
Abstract -- Nanocrystalline Fe-3wt. %Al alloy powders are synthesized using cryogenic mechanical alloying in liquid N. Uptake of 0 by the powders is significantly reduced by controlling the atmosphere and pressure during milling. The initial 18 nm grain size of the cryomilled powder is found to experience grain growth at temperatures above 500 °C (0.42 Tm), as determined by X-ray diffraction. Cryomilled pure Fe, in contrast, is found to be significantly less stable. Transmission electron microscopy indicates that the grain growth in the Fe3wt. %Al is abnormal, while lattice parameter measurements indicate limited dissolution of Al in the Fe matrix. ©1997 Acta Metallurgica Inc.
INTRODUCTION In order to produce high density structural materials from ball milled nanocrystalline powders, hot consolidation techniques are typically employed. The retention of nanocrystalline microstructure during consolidation is directly related to the thermal stability against grain growth of the milled powders. As a result, the identification of mechanisms by which nanocrystalline grains may be stabilized has emerged as a topic of foremost importance. Recently, significant thermal stability has been observed in Fe-10wt.%Al powders produced by cryogenic milling (cryomilling) in liquid N (1) as well as liquid Ar (2). In these studies, it was hypothesized that the stable nanocrystalline microstructure was due to the in-situ formation of oxide and nitride particles during the cryomilling process; a mechanism analogous to that proposed by Luton et al. in earlier cryomilling work performed on AI alloys (3). The present investigation seeks to expand upon these findings by providing new experimental evidence which addresses two critical issues. First, the uptake of O in the Fe-AI powders during the milling process, which is believed to facilitate the in-situ formation of oxide particles, will be controlled by minimizing the concentration of O in the milling atmosphere. Second, the concentration of A1 in the starting powder blend will be reduced, relative to previous work, so as to more accurately provide the minimum amount of A1 atoms necessary to promote the formation of A1 oxides.
71
RJ PEREZ,HG JiJu~GAND EJ LAVERNIA
72
EXPERIMENTAL
The cryogenic milling experiments were performed in a modified Union Process 01HD attritor. The ball to powder mass ratio was 10:1 and the milling time was 25 hours. The powders were continually submerged in liquid N during the process and a positive pressure of 500 Pa was maintained by venting the volatilized N2 to a methanol-filled bubbler. Cryomilled powder samples were placed on A1203 boats, sealed in stainless steel envelopes and heat treated for one hour at various temperatures. Grain size and lattice strain were determined by X-ray diffraction (CuKct radiation) using the procedure described by Klug and Alexander (4). Chemical analysis was performed by Luvak Inc. (Boylston, Massachusetts). Additional powder samples were hot pressed at 550°C for 30 minutes in argon, mechanically ground and jet polished (10% Nital solution) for transmission electron microscopy (TEM), which was performed using a Philips CM20 operating at 200kV.
RESULTS After 25 hours of cryomilling, the average grain size of the Fe-3wt.%A1 powder was found to be 18 nm. The rate of grain growth increased rapidly during heat treatment at temperatures above 500°C, as shown in Figure 1. For comparison, data are presented for pure Fe cryomilled for 25 hours under a non-controlled atmosphere. Transmission electron microscopy performed on Fe-3wt.%Al samples compacted at 550°C for 30 minutes revealed an inhomogeneous distribution of grain sizes ranging from less than 9 nm to larger than 110 nm. Chemical analysis results, shown in Table 1, indicate that measurable quantities of O and N were incorporated into the powders during cryomilling. In addition, the results reflect the loss of AI powder (from 5.0 wt.% to 2.6 wt.%) to the N 2 exhaust gas stream during the initial stages of cryomilling. The lattice parameter of the cryomilled powders was found to be 0.2867 nm, which corresponds to the equilibrium lattice spacing of a/~e. A lattice expansion of 0.08% was observed following heat treatment for one hour at 950°C. TABLE 1 Chemical Analysis of Powders Cryomilled in Liquid Nitrogen Cryomilling Time (hrs)
Fe (wt.%)
AI (wt.%)
O (wt.%)
N (wt.%)
0
94.8
5.00
0.13
0.019
25
96.2
2.62
0.28
0.880
25
98.6
0
1.18
0.155
GRAINSIZESTABILITYOFNANOCRYSTALLINECRYOMILLEDFe-3wr.% AI ALLOY
150
f
'
[
'
'
'
I
I
'
~
- -A1 - Fe_3wt.%A ¢
I
73
'
,' ' II
100 l/
,2 t..
.
50
0
200
400 600 Temperature (°C)
800
1000
Figure 1. Grain size of cryomilled powder specimens after heat treatment for one hour.
DISCUSSION The results indicate that the maintenance of positive N2 pressure during cryomilling was effective in reducing the O uptake of the powders. For example, data from Table 1 yield an average oxygen uptake rate of 0.006 wt.%/hr when the atmosphere is controlled, versus 0.042 wt.%/hr when it is not. Chemical analysis performed on samples compacted at 550°C for 30 minutes indicated that O and N concentrations remained constant, within experimental error. Experimental control over the oxygen content in the cryomilled material provides control over the in-situ formation of oxide particles. According to conventional grain growth theory, if the volume fraction of a particle dispersion is reduced, the critical particle radius required for effective pinning will be reduced. This is illustrated by the following relationship, developed by Gladman (5), which relates average grain size, R o, particle volume fraction, f, and grain size inhomogeneity, Z, to the critical particle size, r, 6Roe ( 3 2~ -1 r= n \'2-ZJ [1] As an approximation of the present case, one may assume that all O goes toward the formation of A1203, resulting in a volume fraction of 0.011. When combined with a Z of 5 and R o of 18 nm, this yields a critical particle radius of 0.34 nm. A homogeneous distribution of A1203 particles of this size seems unlikely, given that the lattice parameter of y-AI203 is 0.79 nm. From Figure 1, the cryomilled Fe-AI powders appear to offer an improvement in stability relative to pure Fe. The pure Fe grain growth results are comparable to those reported for nanocrystalline Fe produced using SPEX milling by Malow et al. (6), in which the grain size increased to 57 nm after one hour at 552°C.
74
RJ PEREZ,HG J=ANGANDEJ LAVERNIA
The lattice parameter results indicate that a negligible amount of AI was dissolved in the Fe matrix following cryomilling. This implies that 2.3 wt.% A1 (the amount which may not be accounted for in the formation of oxides) was inhomogeneously distributed in the microstructure. Only during subsequent heat treatment at 950°C for 1 hour was a 0.08% lattice expansion observed, indicative of the dissolution of 0.9 wt.% AI in Fe (7). Recent work by the authors has indicated that the presence of A1 may impede grain growth of nanocrystalline Fe produced by room temperature SPEX milling (8). This suggests that the presence of AI atoms may produce a solute drag effect in the present system.
CONCLUSIONS Nanocrystalline Fe-3wt.%Al was synthesized using cryomilling. The grain size was found to increase in an inhomogeneous manner above 500°C, yet maintain an average size smaller than that of pure Fe. Based upon the low O content, the probability of oxide pinning was unlikely. The presence of undissolved A1 in the Fe microstructure may have contributed to the observed stability.
ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by the Office of Naval Research (Grants No. N00014-94-1-0017 and No. N00014-93-1-1072). Thanks to Drs. James Rawers and Cindy Dogan for their valuable assistance and insight on TEM and chemical analysis. Thanks also to M. Lau and V.L. Tellkamp for their help with the powder synthesis.
REFERENCES 1. 2. 3.
4. 5. 6.
7. 8.
R.J. Perez, B. Huang and E.J. Lavernia, NanoStructured Matls. in press (1996). B. Huang, R.J. Perez and E.J. Lavernia, in MRS Fall Meeting, in press, Boston, MA (1995). M.J. Luton, C.S. Jayanth, M.M. Disko, S. Matras and J. Vallone, in Multicomponent Ultrafine Microstructures, p. 79, L.E. McCandlish, et al., editors, Materials Research Society Symposium Proceedings, Pittsburgh, P.A. (1989). H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures., p. 661, John Wiley & Sons, New York (1974). T. Gladman, Proc. R. Soc. London, Ser. A 294, 298 (1966). T.R. Malow and C.C. Koch, in Synthesis and Processing of Nanocrystalline Powder, p. 33, D.L. Bourell, editor The Minerals Metals and Materials Society, Warrendale, Pennsylvania (1996). F. Lihl and H. Ebel, Arch. Eisenhuttenw. 32, 483 (1961). H.G. Jiang, University of California, Irvine, Current Research, (1996).
Pergamon pll S0965-9773(97)00021-4
GRAIN SIZE STABILITY OF NANOCRYSTALLINE CRYOMILLED Fe-3wt.%Al ALLOY R.J. Perez, H.G. Jiang and E.J. Lavernia Department of Chemical Engineering and Materials Science University of California-Irvine Irvine, CA 92697
Abstract -- Nanocrystalline Fe-3wt. %Al alloy powders are synthesized using cryogenic mechanical alloying in liquid N. Uptake of 0 by the powders is significantly reduced by controlling the atmosphere and pressure during milling. The initial 18 nm grain size of the cryomilled powder is found to experience grain growth at temperatures above 500 °C (0.42 Tm), as determined by X-ray diffraction. Cryomilled pure Fe, in contrast, is found to be significantly less stable. Transmission electron microscopy indicates that the grain growth in the Fe3wt. %Al is abnormal, while lattice parameter measurements indicate limited dissolution of Al in the Fe matrix. ©1997 Acta Metallurgica Inc.
INTRODUCTION In order to produce high density structural materials from ball milled nanocrystalline powders, hot consolidation techniques are typically employed. The retention of nanocrystalline microstructure during consolidation is directly related to the thermal stability against grain growth of the milled powders. As a result, the identification of mechanisms by which nanocrystalline grains may be stabilized has emerged as a topic of foremost importance. Recently, significant thermal stability has been observed in Fe-10wt.%Al powders produced by cryogenic milling (cryomilling) in liquid N (1) as well as liquid Ar (2). In these studies, it was hypothesized that the stable nanocrystalline microstructure was due to the in-situ formation of oxide and nitride particles during the cryomilling process; a mechanism analogous to that proposed by Luton et al. in earlier cryomilling work performed on AI alloys (3). The present investigation seeks to expand upon these findings by providing new experimental evidence which addresses two critical issues. First, the uptake of O in the Fe-AI powders during the milling process, which is believed to facilitate the in-situ formation of oxide particles, will be controlled by minimizing the concentration of O in the milling atmosphere. Second, the concentration of A1 in the starting powder blend will be reduced, relative to previous work, so as to more accurately provide the minimum amount of A1 atoms necessary to promote the formation of A1 oxides.
71
RJ PEREZ,HG JiJu~GAND EJ LAVERNIA
72
EXPERIMENTAL
The cryogenic milling experiments were performed in a modified Union Process 01HD attritor. The ball to powder mass ratio was 10:1 and the milling time was 25 hours. The powders were continually submerged in liquid N during the process and a positive pressure of 500 Pa was maintained by venting the volatilized N2 to a methanol-filled bubbler. Cryomilled powder samples were placed on A1203 boats, sealed in stainless steel envelopes and heat treated for one hour at various temperatures. Grain size and lattice strain were determined by X-ray diffraction (CuKct radiation) using the procedure described by Klug and Alexander (4). Chemical analysis was performed by Luvak Inc. (Boylston, Massachusetts). Additional powder samples were hot pressed at 550°C for 30 minutes in argon, mechanically ground and jet polished (10% Nital solution) for transmission electron microscopy (TEM), which was performed using a Philips CM20 operating at 200kV.
RESULTS After 25 hours of cryomilling, the average grain size of the Fe-3wt.%A1 powder was found to be 18 nm. The rate of grain growth increased rapidly during heat treatment at temperatures above 500°C, as shown in Figure 1. For comparison, data are presented for pure Fe cryomilled for 25 hours under a non-controlled atmosphere. Transmission electron microscopy performed on Fe-3wt.%Al samples compacted at 550°C for 30 minutes revealed an inhomogeneous distribution of grain sizes ranging from less than 9 nm to larger than 110 nm. Chemical analysis results, shown in Table 1, indicate that measurable quantities of O and N were incorporated into the powders during cryomilling. In addition, the results reflect the loss of AI powder (from 5.0 wt.% to 2.6 wt.%) to the N 2 exhaust gas stream during the initial stages of cryomilling. The lattice parameter of the cryomilled powders was found to be 0.2867 nm, which corresponds to the equilibrium lattice spacing of a/~e. A lattice expansion of 0.08% was observed following heat treatment for one hour at 950°C. TABLE 1 Chemical Analysis of Powders Cryomilled in Liquid Nitrogen Cryomilling Time (hrs)
Fe (wt.%)
AI (wt.%)
O (wt.%)
N (wt.%)
0
94.8
5.00
0.13
0.019
25
96.2
2.62
0.28
0.880
25
98.6
0
1.18
0.155
GRAINSIZESTABILITYOFNANOCRYSTALLINECRYOMILLEDFe-3wr.% AI ALLOY
150
f
'
[
'
'
'
I
I
'
~
- -A1 - Fe_3wt.%A ¢
I
73
'
,' ' II
100 l/
,2 t..
.
50
0
200
400 600 Temperature (°C)
800
1000
Figure 1. Grain size of cryomilled powder specimens after heat treatment for one hour.
DISCUSSION The results indicate that the maintenance of positive N2 pressure during cryomilling was effective in reducing the O uptake of the powders. For example, data from Table 1 yield an average oxygen uptake rate of 0.006 wt.%/hr when the atmosphere is controlled, versus 0.042 wt.%/hr when it is not. Chemical analysis performed on samples compacted at 550°C for 30 minutes indicated that O and N concentrations remained constant, within experimental error. Experimental control over the oxygen content in the cryomilled material provides control over the in-situ formation of oxide particles. According to conventional grain growth theory, if the volume fraction of a particle dispersion is reduced, the critical particle radius required for effective pinning will be reduced. This is illustrated by the following relationship, developed by Gladman (5), which relates average grain size, R o, particle volume fraction, f, and grain size inhomogeneity, Z, to the critical particle size, r, 6Roe ( 3 2~ -1 r= n \'2-ZJ [1] As an approximation of the present case, one may assume that all O goes toward the formation of A1203, resulting in a volume fraction of 0.011. When combined with a Z of 5 and R o of 18 nm, this yields a critical particle radius of 0.34 nm. A homogeneous distribution of A1203 particles of this size seems unlikely, given that the lattice parameter of y-AI203 is 0.79 nm. From Figure 1, the cryomilled Fe-AI powders appear to offer an improvement in stability relative to pure Fe. The pure Fe grain growth results are comparable to those reported for nanocrystalline Fe produced using SPEX milling by Malow et al. (6), in which the grain size increased to 57 nm after one hour at 552°C.
74
RJ PEREZ,HG J=ANGANDEJ LAVERNIA
The lattice parameter results indicate that a negligible amount of AI was dissolved in the Fe matrix following cryomilling. This implies that 2.3 wt.% A1 (the amount which may not be accounted for in the formation of oxides) was inhomogeneously distributed in the microstructure. Only during subsequent heat treatment at 950°C for 1 hour was a 0.08% lattice expansion observed, indicative of the dissolution of 0.9 wt.% AI in Fe (7). Recent work by the authors has indicated that the presence of A1 may impede grain growth of nanocrystalline Fe produced by room temperature SPEX milling (8). This suggests that the presence of AI atoms may produce a solute drag effect in the present system.
CONCLUSIONS Nanocrystalline Fe-3wt.%Al was synthesized using cryomilling. The grain size was found to increase in an inhomogeneous manner above 500°C, yet maintain an average size smaller than that of pure Fe. Based upon the low O content, the probability of oxide pinning was unlikely. The presence of undissolved A1 in the Fe microstructure may have contributed to the observed stability.
ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by the Office of Naval Research (Grants No. N00014-94-1-0017 and No. N00014-93-1-1072). Thanks to Drs. James Rawers and Cindy Dogan for their valuable assistance and insight on TEM and chemical analysis. Thanks also to M. Lau and V.L. Tellkamp for their help with the powder synthesis.
REFERENCES 1. 2. 3.
4. 5. 6.
7. 8.
R.J. Perez, B. Huang and E.J. Lavernia, NanoStructured Matls. in press (1996). B. Huang, R.J. Perez and E.J. Lavernia, in MRS Fall Meeting, in press, Boston, MA (1995). M.J. Luton, C.S. Jayanth, M.M. Disko, S. Matras and J. Vallone, in Multicomponent Ultrafine Microstructures, p. 79, L.E. McCandlish, et al., editors, Materials Research Society Symposium Proceedings, Pittsburgh, P.A. (1989). H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures., p. 661, John Wiley & Sons, New York (1974). T. Gladman, Proc. R. Soc. London, Ser. A 294, 298 (1966). T.R. Malow and C.C. Koch, in Synthesis and Processing of Nanocrystalline Powder, p. 33, D.L. Bourell, editor The Minerals Metals and Materials Society, Warrendale, Pennsylvania (1996). F. Lihl and H. Ebel, Arch. Eisenhuttenw. 32, 483 (1961). H.G. Jiang, University of California, Irvine, Current Research, (1996).