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Application of surface active substances in mechanical alloying
1346
Materials Science and Engineering, A 134 ( 1991 ) 1346 - 1349
Application of surface active substances in mechanical alloying A. P. Radlinski* and A. Calka Department of Electronic Materials Engineering, The Australian National University, P.O. Box 4, Canberra 2601 (Australia)
B. W. Ninham Department of Applied Mathematics, The Australian National University, P.O. Box 4, Canberra 2601 (Australia)
W. A. Kaczmarek Research School of Chemistry, The Australian National University, P.O. Box 4, Canberra 2601 (Australia)
Abstract Solid state reactions during mechanical alloying of metal-metal and metal-metalloid mixtures can be dramatically affected by small amounts of organic surfactants. We discuss the extended solid solubility of magnesium in aluminium, the formation of highly reactive nanostructures of Ti-C and A1-Ti, and the particle shape and size modifications in (Co-Fe)75Si15B10 soft magnetics. A decreased level of contamination with the milling cell materials is also observed.
1. Introduction
Since Benjamin's synthesis of the Inconel alloy by mechanical alloying [1], there has been an increasing activity in applying this method for fabrication of various alloys. This activity has been further intensified by the recent discovery that mechanical alloying may lead to the formation of amorphous phases [2]. In recent years a wide range of alloys made using mechanical alloying have been studied by various workers. In this paper we describe the modifications caused by small amounts of the organic surface active substances (surfactants) added to the milled mixtures. In particular, we concentrate on the synthesis of lightweight A1-Mg alloys (although other systems are also briefly mentioned) and show how the presence of surfactants can affect the outcome of solid state reactions. Alloys containing aluminium and magnesium are difficult to make using ball milling because of the excessive cold welding. This process results in lumping of the alloyed material, which in turn *Present address: Department of Applied Mathematics, The Australian National University, RO. Box 4, Canberra 2601 (Australia). 0921-5093/91/$3.50
suppresses the solid state reaction. Organic lubricants were used previously [3] to achieve the critical balance between cold welding and fracturing. In particular, it was observed that the organic lubricants helped to keep the particle size small and the size distribution narrow. On the basis of our results we claim that surfactants can be used not only to decrease the particle size, but also to cause both structural and morphological modifications in the mechanically alloyed powders. Although the detailed microscopic mechanism for this process is not understood yet, we qualitatively explain the observed effects by the presence of very intense electric fields at the particle surfaces, caused by the electrically charged surfactant head groups. These fields affect the surfactant properties of powder particles and, consequently, influence the solid state reactions occurring during mechanical alloying. 2. Experimental
Mixtures of aluminium and magnesium powders of two compositions (70at.%A1 and 50at.%A1) were prepared using 99.9% purity elemental powders of mean particle size about 50 /zm. In cases when the surfactant (usually in © Elsevier Sequoia/Printed in The Netherlands
1347
hexane solution) was added, its amount was calculated using the requirement that the whole surface area of the final mixture (assuming a mean particle size of - 1/xm) should be covered with a molecular monolayer of surfactant of a known head-group area. Powders of crystalline (Co-Fe)vsSi~sBI~ were prepared from crystallized amorphous ribbons, as described elsewhere [4]. Powders of carbon, aluminium and titanium used in this work were of 99.9% purity. Structural studies were routinely performed using a computerized X-ray diffractometer (Co K a radiation). Sample purity was studied using the electron microprobe and Rutherford backscattering. The ball milling was performed in a ball mill with externally controlled ball movement, as described elsewhere [5]. Unless stated otherwise, a high-energy milling mode was used.
3. Results and discussion
3.1. Al-Mg alloys In Fig. l(a) and (b) X-ray diffractograms taken from A170-Mg30 mixtures milled without surfactant for 20 h and 80 h, respectively, are shown. A
particular sequence of milling modes was used [5] to avoid excessive cold welding. As the milling proceeds, the diffraction peaks due to magnesium as well as aluminium broaden markedly, indicating the submicron grain size and/or build-up of high internal strain. However, the shift of diffraction peaks originating from scattering on the elemental f.c.c, aluminium lattice was not observed, thus indicating a negligible level of the solid solubility of magnesium in aluminium [6]. In Fig. l(c) we present an X-ray diffractogram taken from a powder of the same initial composition, but milled for 80 h with the addition of sodium1,2 bis (dodecyl carbonyl) ethane-l-sulfonate. From the observed shift of X-ray diffraction peaks due to f.c.c, aluminium we determine 13 at.% solubility of magnesium in aluminium, which is much more than the equilibrium solubility (about 1 at.%). The balance of elemental magnesium present in the final powder does not produce a distinct diffraction pattern, probably because of the small grain size and/or strong deformation.
/ \
(b )
~i il i::!
j,J (b)
'
(c)
v
2
c~
g 0
\~,
J'
20
degrees
g 0
(d)
(c)
.7.
3 o v i
S =
'I
~
~ ~ ¢0
04 ¢q 30
30
50
70 2®
9'0
10
degrees
Fig. 1. X-ray diffractograms of mechanically alloyed elemental mixtures of AI-Mg. The ball milling process was performed in a helium atmosphere. (a) Alloyed for 20 h; (b) alloyed for 80 h; (c) alloyed with the addition of sodium-l,2 bis (dodecyl carbonyl) ethane- 1-sulfonate for 80 h.
50
70
90
110
Fig. 2. X - r a y diffractograms of mechanically alloyed elemental mixtures of Als0-Mgs0. The ball milling process was
performed in a helium atmosphere. (a) Alloyed for 160 h, (b) alloyed with addition of sodium-l,2 bis (dodecyl carbonyl) ethane-l-sulfonate for 160 h; (c) alloyed with the addition of dodecyloxycarbonyl sulfesuccinate for 160 h; (d) alloyed with the addition of diodecyldimethyl ammonium bromide for 160 h.
1348
An X-ray diffractogram taken from Als0-Mgs0 mixture milled without surfactant for 160 h is shown in Fig. 2(a). The diffraction peaks are very broad, indicating an amorphous-like structure. After annealing for 5 rain at 350 °C this structure transforms into a mixture of two equilibrium phases: fl(A13Ug2) and 7(A112Mg17). When various surfactants are added to the mixture, the final products of ball milling change dramatically. Powders milled for 160 h in the presence of sodium-l,2 bis (dodecyl carbonyl) ethane-l-sulfonate (Fig. 2(b)) transform into a mixture of f.c.c, aluminium plus 24 at.% solid solution of magnesium in aluminium plus a small amount of the equilibrium phase Al12Mg17 (the latter is more clearly seen using electron diffraction). In contrast to this, powders milled for 160 h with the addition of lithium-l,2 bis dodecyloxycarbonyl sulfasuccinate transform into a very stable phase (or mixture of phases), which yields only two strong and broad X-ray diffraction peaks (Fig. l(c)). The position of these peaks is consistent with a b.c.c, structure characterized by the lattice parameter a = 4.15 A. Additionally, a washed-out diffraction pattern caused by small amount of f.c.c, aluminium can be identified in the diffractogram. Finally, powders milled for 160 h with the addition of didodecyldimethyl ammonium bromide transform into a mixture of the f.c.c, aluminium phase and the b.c.c, phase described above (Fig. 2(d)). Upon prolonged milling the b.c.c, phase continues growing at the expense of the f.c.c, aluminium phase. It is important to note that the supersaturated solid solution of magnesium and aluminium and the b.c.c, phase obtained with the addition of surfactant are stable up to a temperature of 360 °C. These phases do not form if the ball milling process is performed in dry conditions. 3. 2. Highly reactive materials Ball milling of some mixtures may result in the formation of nanostructures [7], i.e. apparently non-reacted mixtures of very fine particle powders (particle size ~<100 nm). Nanostructures are characterized by a large percentage of the total number of atoms residing on the particle surface rather than in the bulk, and their overall properties are largely affected by the surface phenomena. In particular, when a nanostructure contains easily oxidizing elements, its exposure to air may lead to a violent reaction. Mixtures of Tis0-Cso and Als0-Tis0 powders
form nanostructures upon ball milling (mean particle size ~ 100 nm) and explosively oxidize when exposed to the atmosphere. If enough surfactant is added to cover the particle surface with one molecular layer, however, the resulting nanostructures containing both metal atoms and organic molecules can be safely handled in air and subjected to further thermal and/or mechanical treatment. 3. 3. Soft magnetic materials It was shown recently that application of both cationic and anionic surfactants to ball milling of (Co-Fe)75Si15B10 soft magnetics provides some control over the particle size and shape [8]. In particular, circular shapes (diameter - 1 0 0 nm) were observed for powders milled in the presence of sodium-l,2 bis (dodecyl carbonyl) ethane-1sulfonate and needle-like shapes (length - 1 0 0 nm) were obtained as the result of milling in the presence of ammonium dihexadecyl dimethylacetate. These various particle shapes and sizes affected the values of maximum magnetic saturation as well as the anisotropy field, thus improving the soft magnetic properties of the materials milled with surfactants compared with those milled in dry conditions. 3.4. Contamination with the mill materials A common problem encountered when the ball milling technology is applied for synthesis of alloys is contamination with the elements contained in the milling cell and/or balls. The level of contamination depends on the type of ball mill used and on the milling conditions. The milling device used by us [5] is particularly "clean" in this respect; depending on the materials milled, contamination with elemental iron varies from below the detection levels for Mg-Zn alloys [9] to about 1 at.% for Si-Ti alloys [10], the latter value being the maximum contamination level recorded by us. We have observed that in general the level of contamination with iron is dramatically reduced (by at least a factor of 10) when suffactant is present in the milled mixture. This finding may be of great practical importance when contamination with the surfactant itself does not pose a problem. 4. Conclusion
The addition of small amounts of the organic surface active substances to ball milled powders
1349
can affect the solid state reaction occurring during the milling, in some cases quite dramatically. The composition, purity, particle size and shape, reactivity, and all the consequent structural, thermal, chemical and magnetic properties of the ball milled powders may be affected, thus resulting in materials of novel properties.
References 1 J.S. Benjamin, Metall. Trans., 1 (1970) 2943. 2 C. C. Koch, O. B. Cavin, C. G. McKamey and Y. O. Scarborough, Appl. Phys. Lett., 43 ( 1983) 1017.
3 P. S. Gilman and W. D. Nix, Metall. Trans., 12A (1981) 813. 4 A. Calka, A. E Pogany, R. A. Shanks and H. Engelman, accepted by Mater. Sci. Eng. 5 A. Calka and A. P. Radlinski. Mater. Sei. Eng., A134 (1991) 1350. 6 Bulletin of Alloy Phase Diagrams, 3 (1982) 70. 7 H. E. Schaefer and R. Wurschum, J. Le.s~-Common Metals, 140(1988) 161. 8 W. A. Kaczmarek, R. Bramley, A. Calka and B. W. Ninham, Conj. Proe. lnterrnag 90, Brighton, U.K., April 1990. 9 A. Calka and A. P. Radlinski, Mater. Sei. Eng., A l l 8 (1989) 131. 10 A. Calka and A. P. Radlinski, to be published in Mater. Sei. Eng.
Materials Science and Engineering, A 134 ( 1991 ) 1346 - 1349
Application of surface active substances in mechanical alloying A. P. Radlinski* and A. Calka Department of Electronic Materials Engineering, The Australian National University, P.O. Box 4, Canberra 2601 (Australia)
B. W. Ninham Department of Applied Mathematics, The Australian National University, P.O. Box 4, Canberra 2601 (Australia)
W. A. Kaczmarek Research School of Chemistry, The Australian National University, P.O. Box 4, Canberra 2601 (Australia)
Abstract Solid state reactions during mechanical alloying of metal-metal and metal-metalloid mixtures can be dramatically affected by small amounts of organic surfactants. We discuss the extended solid solubility of magnesium in aluminium, the formation of highly reactive nanostructures of Ti-C and A1-Ti, and the particle shape and size modifications in (Co-Fe)75Si15B10 soft magnetics. A decreased level of contamination with the milling cell materials is also observed.
1. Introduction
Since Benjamin's synthesis of the Inconel alloy by mechanical alloying [1], there has been an increasing activity in applying this method for fabrication of various alloys. This activity has been further intensified by the recent discovery that mechanical alloying may lead to the formation of amorphous phases [2]. In recent years a wide range of alloys made using mechanical alloying have been studied by various workers. In this paper we describe the modifications caused by small amounts of the organic surface active substances (surfactants) added to the milled mixtures. In particular, we concentrate on the synthesis of lightweight A1-Mg alloys (although other systems are also briefly mentioned) and show how the presence of surfactants can affect the outcome of solid state reactions. Alloys containing aluminium and magnesium are difficult to make using ball milling because of the excessive cold welding. This process results in lumping of the alloyed material, which in turn *Present address: Department of Applied Mathematics, The Australian National University, RO. Box 4, Canberra 2601 (Australia). 0921-5093/91/$3.50
suppresses the solid state reaction. Organic lubricants were used previously [3] to achieve the critical balance between cold welding and fracturing. In particular, it was observed that the organic lubricants helped to keep the particle size small and the size distribution narrow. On the basis of our results we claim that surfactants can be used not only to decrease the particle size, but also to cause both structural and morphological modifications in the mechanically alloyed powders. Although the detailed microscopic mechanism for this process is not understood yet, we qualitatively explain the observed effects by the presence of very intense electric fields at the particle surfaces, caused by the electrically charged surfactant head groups. These fields affect the surfactant properties of powder particles and, consequently, influence the solid state reactions occurring during mechanical alloying. 2. Experimental
Mixtures of aluminium and magnesium powders of two compositions (70at.%A1 and 50at.%A1) were prepared using 99.9% purity elemental powders of mean particle size about 50 /zm. In cases when the surfactant (usually in © Elsevier Sequoia/Printed in The Netherlands
1347
hexane solution) was added, its amount was calculated using the requirement that the whole surface area of the final mixture (assuming a mean particle size of - 1/xm) should be covered with a molecular monolayer of surfactant of a known head-group area. Powders of crystalline (Co-Fe)vsSi~sBI~ were prepared from crystallized amorphous ribbons, as described elsewhere [4]. Powders of carbon, aluminium and titanium used in this work were of 99.9% purity. Structural studies were routinely performed using a computerized X-ray diffractometer (Co K a radiation). Sample purity was studied using the electron microprobe and Rutherford backscattering. The ball milling was performed in a ball mill with externally controlled ball movement, as described elsewhere [5]. Unless stated otherwise, a high-energy milling mode was used.
3. Results and discussion
3.1. Al-Mg alloys In Fig. l(a) and (b) X-ray diffractograms taken from A170-Mg30 mixtures milled without surfactant for 20 h and 80 h, respectively, are shown. A
particular sequence of milling modes was used [5] to avoid excessive cold welding. As the milling proceeds, the diffraction peaks due to magnesium as well as aluminium broaden markedly, indicating the submicron grain size and/or build-up of high internal strain. However, the shift of diffraction peaks originating from scattering on the elemental f.c.c, aluminium lattice was not observed, thus indicating a negligible level of the solid solubility of magnesium in aluminium [6]. In Fig. l(c) we present an X-ray diffractogram taken from a powder of the same initial composition, but milled for 80 h with the addition of sodium1,2 bis (dodecyl carbonyl) ethane-l-sulfonate. From the observed shift of X-ray diffraction peaks due to f.c.c, aluminium we determine 13 at.% solubility of magnesium in aluminium, which is much more than the equilibrium solubility (about 1 at.%). The balance of elemental magnesium present in the final powder does not produce a distinct diffraction pattern, probably because of the small grain size and/or strong deformation.
/ \
(b )
~i il i::!
j,J (b)
'
(c)
v
2
c~
g 0
\~,
J'
20
degrees
g 0
(d)
(c)
.7.
3 o v i
S =
'I
~
~ ~ ¢0
04 ¢q 30
30
50
70 2®
9'0
10
degrees
Fig. 1. X-ray diffractograms of mechanically alloyed elemental mixtures of AI-Mg. The ball milling process was performed in a helium atmosphere. (a) Alloyed for 20 h; (b) alloyed for 80 h; (c) alloyed with the addition of sodium-l,2 bis (dodecyl carbonyl) ethane- 1-sulfonate for 80 h.
50
70
90
110
Fig. 2. X - r a y diffractograms of mechanically alloyed elemental mixtures of Als0-Mgs0. The ball milling process was
performed in a helium atmosphere. (a) Alloyed for 160 h, (b) alloyed with addition of sodium-l,2 bis (dodecyl carbonyl) ethane-l-sulfonate for 160 h; (c) alloyed with the addition of dodecyloxycarbonyl sulfesuccinate for 160 h; (d) alloyed with the addition of diodecyldimethyl ammonium bromide for 160 h.
1348
An X-ray diffractogram taken from Als0-Mgs0 mixture milled without surfactant for 160 h is shown in Fig. 2(a). The diffraction peaks are very broad, indicating an amorphous-like structure. After annealing for 5 rain at 350 °C this structure transforms into a mixture of two equilibrium phases: fl(A13Ug2) and 7(A112Mg17). When various surfactants are added to the mixture, the final products of ball milling change dramatically. Powders milled for 160 h in the presence of sodium-l,2 bis (dodecyl carbonyl) ethane-l-sulfonate (Fig. 2(b)) transform into a mixture of f.c.c, aluminium plus 24 at.% solid solution of magnesium in aluminium plus a small amount of the equilibrium phase Al12Mg17 (the latter is more clearly seen using electron diffraction). In contrast to this, powders milled for 160 h with the addition of lithium-l,2 bis dodecyloxycarbonyl sulfasuccinate transform into a very stable phase (or mixture of phases), which yields only two strong and broad X-ray diffraction peaks (Fig. l(c)). The position of these peaks is consistent with a b.c.c, structure characterized by the lattice parameter a = 4.15 A. Additionally, a washed-out diffraction pattern caused by small amount of f.c.c, aluminium can be identified in the diffractogram. Finally, powders milled for 160 h with the addition of didodecyldimethyl ammonium bromide transform into a mixture of the f.c.c, aluminium phase and the b.c.c, phase described above (Fig. 2(d)). Upon prolonged milling the b.c.c, phase continues growing at the expense of the f.c.c, aluminium phase. It is important to note that the supersaturated solid solution of magnesium and aluminium and the b.c.c, phase obtained with the addition of surfactant are stable up to a temperature of 360 °C. These phases do not form if the ball milling process is performed in dry conditions. 3. 2. Highly reactive materials Ball milling of some mixtures may result in the formation of nanostructures [7], i.e. apparently non-reacted mixtures of very fine particle powders (particle size ~<100 nm). Nanostructures are characterized by a large percentage of the total number of atoms residing on the particle surface rather than in the bulk, and their overall properties are largely affected by the surface phenomena. In particular, when a nanostructure contains easily oxidizing elements, its exposure to air may lead to a violent reaction. Mixtures of Tis0-Cso and Als0-Tis0 powders
form nanostructures upon ball milling (mean particle size ~ 100 nm) and explosively oxidize when exposed to the atmosphere. If enough surfactant is added to cover the particle surface with one molecular layer, however, the resulting nanostructures containing both metal atoms and organic molecules can be safely handled in air and subjected to further thermal and/or mechanical treatment. 3. 3. Soft magnetic materials It was shown recently that application of both cationic and anionic surfactants to ball milling of (Co-Fe)75Si15B10 soft magnetics provides some control over the particle size and shape [8]. In particular, circular shapes (diameter - 1 0 0 nm) were observed for powders milled in the presence of sodium-l,2 bis (dodecyl carbonyl) ethane-1sulfonate and needle-like shapes (length - 1 0 0 nm) were obtained as the result of milling in the presence of ammonium dihexadecyl dimethylacetate. These various particle shapes and sizes affected the values of maximum magnetic saturation as well as the anisotropy field, thus improving the soft magnetic properties of the materials milled with surfactants compared with those milled in dry conditions. 3.4. Contamination with the mill materials A common problem encountered when the ball milling technology is applied for synthesis of alloys is contamination with the elements contained in the milling cell and/or balls. The level of contamination depends on the type of ball mill used and on the milling conditions. The milling device used by us [5] is particularly "clean" in this respect; depending on the materials milled, contamination with elemental iron varies from below the detection levels for Mg-Zn alloys [9] to about 1 at.% for Si-Ti alloys [10], the latter value being the maximum contamination level recorded by us. We have observed that in general the level of contamination with iron is dramatically reduced (by at least a factor of 10) when suffactant is present in the milled mixture. This finding may be of great practical importance when contamination with the surfactant itself does not pose a problem. 4. Conclusion
The addition of small amounts of the organic surface active substances to ball milled powders
1349
can affect the solid state reaction occurring during the milling, in some cases quite dramatically. The composition, purity, particle size and shape, reactivity, and all the consequent structural, thermal, chemical and magnetic properties of the ball milled powders may be affected, thus resulting in materials of novel properties.
References 1 J.S. Benjamin, Metall. Trans., 1 (1970) 2943. 2 C. C. Koch, O. B. Cavin, C. G. McKamey and Y. O. Scarborough, Appl. Phys. Lett., 43 ( 1983) 1017.
3 P. S. Gilman and W. D. Nix, Metall. Trans., 12A (1981) 813. 4 A. Calka, A. E Pogany, R. A. Shanks and H. Engelman, accepted by Mater. Sci. Eng. 5 A. Calka and A. P. Radlinski. Mater. Sei. Eng., A134 (1991) 1350. 6 Bulletin of Alloy Phase Diagrams, 3 (1982) 70. 7 H. E. Schaefer and R. Wurschum, J. Le.s~-Common Metals, 140(1988) 161. 8 W. A. Kaczmarek, R. Bramley, A. Calka and B. W. Ninham, Conj. Proe. lnterrnag 90, Brighton, U.K., April 1990. 9 A. Calka and A. P. Radlinski, Mater. Sei. Eng., A l l 8 (1989) 131. 10 A. Calka and A. P. Radlinski, to be published in Mater. Sei. Eng.