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Microcrystalline or nanocrystalline grain size in two-phase alloys after mechanical alloying
Materials Science and Engineering, AIM ( 1991 ) 1418-1421
1418
Microcrystalline or nanocrystalline grain size in two-phase alloys after mechanical alloying D. G. Morris and M. A. Morris Institute of Structural Metallurgy, University of Neuchdtel, 2000, Neuch~tel (Switzerland)
Abstract Mechanical alloying may be used to prepare materials in the form of powders which have an extremely small grain size. The thermal stability of such materials is of critical importance since powder consolidation methods invariably require high temperatures. The possibility of using fine, dispersed particles to pin grain boundaries and thereby maintain the small grain size is examined for grain sizes down towards the nanocrystalline scale. It appears that such pinning mechanisms are effective for grain sizes below 40 nm and allow the relatively easy preparation of bulk materials with such small grain sizes.
1. Introduction
the fine nature of these structures is still not clear
Materials with a nanometre-scale grain size are of fundamental scientific interest as well as, potentially, offering new properties or property combinations for engineering applications [1, 2]. These materials are characterized as having a high proportion of atoms at grain boundaries, where the atomic positions may be somewhat relaxed from those of the grain lattices. Image forces acting on dislocations may lead to dislocation annihilation leading to an interesting question of the deformation mechanisms of such materials. For example, grain boundary sliding or grain boundary diffusion may be important. At the same time, ductility may be expected to be good on the grounds of the absence of weak sites or stress concentrations. Initially, nanocrystalline powders have been made by a vapour condensation method [1, 2]. Attempts have also been made to consolidate such powders at low temperatures to give bulk material. A n inherent problem of such materials, associated with the high surface area and energy, as well as with the possibility of rapid grain boundary diffusion, is the instability of the fine structural scale which makes it difficult to use high temperatures and ensure proper powder consolidation. As a result of relaxation or diffusional processes at the grain boundaries, the structures present may readily be modified and
Recent experiments have shown that mechanical alloying can lead to nanocrystalline grain sizes, at least in some cases [5]. In other cases it has been shown that mechanical alloying can produce a fine dispersion of stable particles within a fine grain sized material [6]. One interesting question, addressed here, is whether particle pinning of grain boundaries can act to restrain grain growth even down at the level of nanocrystalline grain sizes. The basic mechanism of grain boundary pinning proposed by Zener [7] considers that a particle interacts with the grain boundary to reduce the energy of the boundary-particle system and thereby restrains the boundary. The restraining effect can be related to the number of particles at the boundaries and the average grain boundary curvature, thus providing a relationship between size and number of particles and the equilibrium grain size. A modification to such a pinning principle has been published recently [8], with the proposal that very small particles may be ineffective pinning obstacles because of thermally-assisted breakaway of the boundary: such particle sizes represent a lower limit to particle size and then to the grain size which can be stabilized. The present study considers particle-dispersed materials of fine grain size prepared by mechanical alloying and high temperature powder corn-
0921-5093/91/$3.50
[3, 4].
© Elsevier Sequoia/Printed in The Netherlands
1419
paction. It is clear that thermally stable partmles are needed such that the pinning particles are themselves not significantly coarsened during annealing. For this reason we consider ceramic particles, produced in situ during milling and annealing, dispersed very finely within the metallic matrix. The relationships between particle size and grain size will be examined, specifically to see whether the Zener-type pinning equations still apply at very small grain sizes, and to see whether nanocrystalline materials can be produced. 2. Experimental
The experimental techniques of ball milling and powder consolidation by cold pressing and hot extrusion have been described previously [6, 9] and are only briefly summarized here. Starting powders of pre-alloyed Cu-Sat.%Zr are ball milled in a planetary ball mill under an argon atmosphere. An organic fluid of molecular weight 90 was added to the powders to avoid premature welding to the balls and container. The amount of additive was adjusted to ensure complete reaction with the zirconium to produce zirconium carbide and oxide during milling or subsequent heat treatments. About 10.8 vol.% of particles should form in this way. Milled powders were cold pressed to billets, heated to 700 °C or 800 °C and extruded to produce bar samples. Microstructures were examined by SEM and TEM techniques, and particle and grain sizes and size distributions analysed using a quantitative image analyser. 3. Results and discussion
Figure I shows the evolution of the microstructure of the powders during milling. The structure changes from a coarse dendritic morphology in the starting powders to a fine, lamellar structure after 10 h milling, reaching a structureless state at the SEM resolution after 15-30 h milling. After milling and compaction the microstructure showed a very fine grain size with fine particles (Fig. 2). The particles were analysed by electron diffraction as being consistent with the zirconium carbide phase. The grain size and particle size varied according to the milling time, 15 h or 30 h, the consolidation temperature, 700°C or 800°C, and subsequent annealing treatments, varying in the range 4-7 nm for particle size and 38-60 nm for grain size for consoli-
Fig. 1. Evolution of the microstructure of powder milled for (a) 10 h and (b) 30 h. Scanning electron micmgraphs taken in back-scattered electron mode.
dated material, and reaching as much as 23 nm (particle size) and 135 nm (grain size) after heat treatments. Figures 3 and 4 show the experimental grain sizes measured on consolidated and sometimes subsequently heat treated materials. These sizes are related to the grain size predicted by Zener based on the number and size of the particles present, namely
3f where ¢ is the particle size, f the volume fraction of the pinning phase and D the grain size. If the grain sizes observed in this study are the steady state sizes as predicted by Zener, there should be a direct equality of the Zener grain size and the experimental grain size, as indicated by the corresponding line in Figs. 3 and 4. An alternative relationship has been proposed by Giadman [10],
1420 60C
i
• Cu-V
oN
/ / ( . , .::deo
= 40~
. / O~o
z .~_ 20C
/-/"
26o
460
66o
86o
Z e n e r Grain Size (nm)
Fig. 3. Comparison of experimental grain size with values predicted by the Zener and Gladman models based on size and distribution of particles. Someof the data is taken from earlier work [11].
~" 150 oN "~ 100 O
50
Zefier Grain Size (nm)
Fig. 4. Comparison of experimental grain size with values predicted by Zener and Gladman models. Some of the data is taken from earlier work [11].
Fig. 2. Transmission electron micrographs showing the microstructures of powders milled for 15 h extruded at 700 °C: (a) bright field image showing general grain size; (b) dark field image showingparticles.
taking into account the distribution of grain sizes within a given material. For reasonable values of grain size distributions the relationship proposed is D =0.41 -~
f
and the corresponding line relating the Gladman grain size and Zener grain size is also shown in Figs. 3 and 4. At small grain sizes, namely over
the range of experimental grain size of 38-150 nm, the Zener relationship is seen to provide an excellent description of the grain size in the consolidated materials. For larger grain sizes, namely above 150 nm, the Gladman equation may provide a better description of the material analysed. In comparing the experimental data with the various models, it should be emphasized that the excellent agreement between experiment and theory may be somewhat fortuitous, and related to the particular choice of parameters inserted in the models to produce the two equations shown. It is, however, interesting to note that the classical ideas relating stable grain size to particle distributions can be extended to grain sizes below 40 nm. The reason for the change in the experimental data, from agreeing with the Zener formula for very small grain sizes to agreeing with the Gladman formula for larger grain sizes, is not understood. The change seen is equivalent to more effective pinning by the particles at very large grain sizes and less effective pinning at smaller
1421
grain sizes. The explanation is perhaps to be found in the distribution of grain size or particle size, or perhaps the location of the particles (on triple points, for example), but remains unclear. As a final point, it is interesting to note that for particle sizes down to 4 nm and for heat treatments above 700 °C, namely above 0.7 Tm, the pinning models apply well, with no indication of reduced effectiveness of the pinning produced by the particles. The analysis of thermally activated unpinning of boundaries [8] shows that for a 4 nm particle, thermal activation begins to play a role above 0.7 Tm (for ferrous-based materials). As such the conditions examined here may represent about the lower limit of applicability of the grainsize pinning approach. 4. Conclusions
(1) Mechanical alloying can be used to produce copper powders of very fine grain size dispersed with very small ceramic particles. (2) The microstructures obtained are fairly stable for temperatures of 700-800 °C allowing high temperature powder consolidation to be carried out while maintaining a fine grain size (below 40 nm, for example).
(3) The grain size obtained can be related to the size and distribution of the second phase particles according to the classical Zener-Gladman equations, at least for particles of size larger than 4 nm.
References 1 H. Gleiter and P. Marquardt, Z. Metall., 75 (1984) 263. 2 R. Birringer, Mater. Sci. Eng., A l l 7 ( 1 9 8 9 ) 33. 3 G. J. Thomas, R. W. Siegel and J. A. Eastman, Scr. Metall., 24 (1990) 201. 4 W. Wunderlich, Y. lshida and R. Maurer, Scr. Metall., 24 (1990) 403. 5 W. Schlump and H. Grewe, in E. Arzt and k. Schultz (eds.), New Materials" by Mechanical Alloying Techniques', DGM, Oberursel, 1989, p. 307. 6 M. A. Morris and D. G. Morris, Mater. Sci. Eng., A l l l (1989) 115. 7 C. Zener, private communication to C. S. Smith, Trans. AIME, 175 (1949) 15. 8 M.J. Gore, M. Grujicic, G. B. Olson and M. Cohen, Acta Metall., 37 (1989) 2849. 9 D.G. Morris and M. A. Morris, submitted to Acta Metall. 10 T. Gladman, t'roc. Roy. Sot., A294 (1966) 298. 11 D. G. Morris and M. A. Morris, in F. H. Froes and J. J. de Barbadillo (eds.), Proc. ASM Conf. on Structural Applications of Mechanical Alloying, Myrtle Beach, SC (1990), ASM, Materials Park, Ohio, in press.
1418
Microcrystalline or nanocrystalline grain size in two-phase alloys after mechanical alloying D. G. Morris and M. A. Morris Institute of Structural Metallurgy, University of Neuchdtel, 2000, Neuch~tel (Switzerland)
Abstract Mechanical alloying may be used to prepare materials in the form of powders which have an extremely small grain size. The thermal stability of such materials is of critical importance since powder consolidation methods invariably require high temperatures. The possibility of using fine, dispersed particles to pin grain boundaries and thereby maintain the small grain size is examined for grain sizes down towards the nanocrystalline scale. It appears that such pinning mechanisms are effective for grain sizes below 40 nm and allow the relatively easy preparation of bulk materials with such small grain sizes.
1. Introduction
the fine nature of these structures is still not clear
Materials with a nanometre-scale grain size are of fundamental scientific interest as well as, potentially, offering new properties or property combinations for engineering applications [1, 2]. These materials are characterized as having a high proportion of atoms at grain boundaries, where the atomic positions may be somewhat relaxed from those of the grain lattices. Image forces acting on dislocations may lead to dislocation annihilation leading to an interesting question of the deformation mechanisms of such materials. For example, grain boundary sliding or grain boundary diffusion may be important. At the same time, ductility may be expected to be good on the grounds of the absence of weak sites or stress concentrations. Initially, nanocrystalline powders have been made by a vapour condensation method [1, 2]. Attempts have also been made to consolidate such powders at low temperatures to give bulk material. A n inherent problem of such materials, associated with the high surface area and energy, as well as with the possibility of rapid grain boundary diffusion, is the instability of the fine structural scale which makes it difficult to use high temperatures and ensure proper powder consolidation. As a result of relaxation or diffusional processes at the grain boundaries, the structures present may readily be modified and
Recent experiments have shown that mechanical alloying can lead to nanocrystalline grain sizes, at least in some cases [5]. In other cases it has been shown that mechanical alloying can produce a fine dispersion of stable particles within a fine grain sized material [6]. One interesting question, addressed here, is whether particle pinning of grain boundaries can act to restrain grain growth even down at the level of nanocrystalline grain sizes. The basic mechanism of grain boundary pinning proposed by Zener [7] considers that a particle interacts with the grain boundary to reduce the energy of the boundary-particle system and thereby restrains the boundary. The restraining effect can be related to the number of particles at the boundaries and the average grain boundary curvature, thus providing a relationship between size and number of particles and the equilibrium grain size. A modification to such a pinning principle has been published recently [8], with the proposal that very small particles may be ineffective pinning obstacles because of thermally-assisted breakaway of the boundary: such particle sizes represent a lower limit to particle size and then to the grain size which can be stabilized. The present study considers particle-dispersed materials of fine grain size prepared by mechanical alloying and high temperature powder corn-
0921-5093/91/$3.50
[3, 4].
© Elsevier Sequoia/Printed in The Netherlands
1419
paction. It is clear that thermally stable partmles are needed such that the pinning particles are themselves not significantly coarsened during annealing. For this reason we consider ceramic particles, produced in situ during milling and annealing, dispersed very finely within the metallic matrix. The relationships between particle size and grain size will be examined, specifically to see whether the Zener-type pinning equations still apply at very small grain sizes, and to see whether nanocrystalline materials can be produced. 2. Experimental
The experimental techniques of ball milling and powder consolidation by cold pressing and hot extrusion have been described previously [6, 9] and are only briefly summarized here. Starting powders of pre-alloyed Cu-Sat.%Zr are ball milled in a planetary ball mill under an argon atmosphere. An organic fluid of molecular weight 90 was added to the powders to avoid premature welding to the balls and container. The amount of additive was adjusted to ensure complete reaction with the zirconium to produce zirconium carbide and oxide during milling or subsequent heat treatments. About 10.8 vol.% of particles should form in this way. Milled powders were cold pressed to billets, heated to 700 °C or 800 °C and extruded to produce bar samples. Microstructures were examined by SEM and TEM techniques, and particle and grain sizes and size distributions analysed using a quantitative image analyser. 3. Results and discussion
Figure I shows the evolution of the microstructure of the powders during milling. The structure changes from a coarse dendritic morphology in the starting powders to a fine, lamellar structure after 10 h milling, reaching a structureless state at the SEM resolution after 15-30 h milling. After milling and compaction the microstructure showed a very fine grain size with fine particles (Fig. 2). The particles were analysed by electron diffraction as being consistent with the zirconium carbide phase. The grain size and particle size varied according to the milling time, 15 h or 30 h, the consolidation temperature, 700°C or 800°C, and subsequent annealing treatments, varying in the range 4-7 nm for particle size and 38-60 nm for grain size for consoli-
Fig. 1. Evolution of the microstructure of powder milled for (a) 10 h and (b) 30 h. Scanning electron micmgraphs taken in back-scattered electron mode.
dated material, and reaching as much as 23 nm (particle size) and 135 nm (grain size) after heat treatments. Figures 3 and 4 show the experimental grain sizes measured on consolidated and sometimes subsequently heat treated materials. These sizes are related to the grain size predicted by Zener based on the number and size of the particles present, namely
3f where ¢ is the particle size, f the volume fraction of the pinning phase and D the grain size. If the grain sizes observed in this study are the steady state sizes as predicted by Zener, there should be a direct equality of the Zener grain size and the experimental grain size, as indicated by the corresponding line in Figs. 3 and 4. An alternative relationship has been proposed by Giadman [10],
1420 60C
i
• Cu-V
oN
/ / ( . , .::deo
= 40~
. / O~o
z .~_ 20C
/-/"
26o
460
66o
86o
Z e n e r Grain Size (nm)
Fig. 3. Comparison of experimental grain size with values predicted by the Zener and Gladman models based on size and distribution of particles. Someof the data is taken from earlier work [11].
~" 150 oN "~ 100 O
50
Zefier Grain Size (nm)
Fig. 4. Comparison of experimental grain size with values predicted by Zener and Gladman models. Some of the data is taken from earlier work [11].
Fig. 2. Transmission electron micrographs showing the microstructures of powders milled for 15 h extruded at 700 °C: (a) bright field image showing general grain size; (b) dark field image showingparticles.
taking into account the distribution of grain sizes within a given material. For reasonable values of grain size distributions the relationship proposed is D =0.41 -~
f
and the corresponding line relating the Gladman grain size and Zener grain size is also shown in Figs. 3 and 4. At small grain sizes, namely over
the range of experimental grain size of 38-150 nm, the Zener relationship is seen to provide an excellent description of the grain size in the consolidated materials. For larger grain sizes, namely above 150 nm, the Gladman equation may provide a better description of the material analysed. In comparing the experimental data with the various models, it should be emphasized that the excellent agreement between experiment and theory may be somewhat fortuitous, and related to the particular choice of parameters inserted in the models to produce the two equations shown. It is, however, interesting to note that the classical ideas relating stable grain size to particle distributions can be extended to grain sizes below 40 nm. The reason for the change in the experimental data, from agreeing with the Zener formula for very small grain sizes to agreeing with the Gladman formula for larger grain sizes, is not understood. The change seen is equivalent to more effective pinning by the particles at very large grain sizes and less effective pinning at smaller
1421
grain sizes. The explanation is perhaps to be found in the distribution of grain size or particle size, or perhaps the location of the particles (on triple points, for example), but remains unclear. As a final point, it is interesting to note that for particle sizes down to 4 nm and for heat treatments above 700 °C, namely above 0.7 Tm, the pinning models apply well, with no indication of reduced effectiveness of the pinning produced by the particles. The analysis of thermally activated unpinning of boundaries [8] shows that for a 4 nm particle, thermal activation begins to play a role above 0.7 Tm (for ferrous-based materials). As such the conditions examined here may represent about the lower limit of applicability of the grainsize pinning approach. 4. Conclusions
(1) Mechanical alloying can be used to produce copper powders of very fine grain size dispersed with very small ceramic particles. (2) The microstructures obtained are fairly stable for temperatures of 700-800 °C allowing high temperature powder consolidation to be carried out while maintaining a fine grain size (below 40 nm, for example).
(3) The grain size obtained can be related to the size and distribution of the second phase particles according to the classical Zener-Gladman equations, at least for particles of size larger than 4 nm.
References 1 H. Gleiter and P. Marquardt, Z. Metall., 75 (1984) 263. 2 R. Birringer, Mater. Sci. Eng., A l l 7 ( 1 9 8 9 ) 33. 3 G. J. Thomas, R. W. Siegel and J. A. Eastman, Scr. Metall., 24 (1990) 201. 4 W. Wunderlich, Y. lshida and R. Maurer, Scr. Metall., 24 (1990) 403. 5 W. Schlump and H. Grewe, in E. Arzt and k. Schultz (eds.), New Materials" by Mechanical Alloying Techniques', DGM, Oberursel, 1989, p. 307. 6 M. A. Morris and D. G. Morris, Mater. Sci. Eng., A l l l (1989) 115. 7 C. Zener, private communication to C. S. Smith, Trans. AIME, 175 (1949) 15. 8 M.J. Gore, M. Grujicic, G. B. Olson and M. Cohen, Acta Metall., 37 (1989) 2849. 9 D.G. Morris and M. A. Morris, submitted to Acta Metall. 10 T. Gladman, t'roc. Roy. Sot., A294 (1966) 298. 11 D. G. Morris and M. A. Morris, in F. H. Froes and J. J. de Barbadillo (eds.), Proc. ASM Conf. on Structural Applications of Mechanical Alloying, Myrtle Beach, SC (1990), ASM, Materials Park, Ohio, in press.