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Microstructural stability of rapidly-solidified CuB alloys
Materials Science and Engineering, 98 (1988) 161 164
161
Microstructural Stability of Rapidly-solidified Cu-B Alloys* E. BATAWI, M. A. MORRIS and D. G. MORRIS
Institute 0/ Structural Metallurgy, University ~)f Neuehguel, 2000 Neuchdtel (Switzerland)
Abstract
The ahilitt' to retain refined microstructures at high temperatures is a necessi o, Jot those materials which will have a high serf, ice temperature or will experience a high-temperature stage during processing, ./'or example during extrusion or .forming. It is therefore essential to understand the .factors controlling stability and coarsening o/` the refined and metastable structures O'Pical o/` rapid solidification. The present stud), examines the stability ~/` Cu B alloys prepared by melt spinning. As-east ribbons exhibit the typical two-zone structure, namely a fine, microcrystalline wheel side with a certain boron solubifiO,, and a columnar-grained free side with boron-rich partich, s outlining the cell walls. This microstructure demonstrates a remarkable resistance to change on initial heat treatment. However, alter a critical time and temperature state is reached, the microstructure becomes locall)' unstable and then changes rapidl)'. The grain boundaries remain immobile as long as particle coarsening is negligible. The boron particles are initially amorphous and h(~hly resistant to coarsening: an interface-controlled process appears to he the cause. Following the crystalli=ation o/` the boron particles, these coarsen very rapt'd(v because o/` enhanced solute d(ffusion along the high-density region o/` the grain boundaries. 1. Introduction
Despite the interest in higher strength and conductivity combinations, relatively little work has been reported to date on alloy development of rapidly solidified microcrystalline copper alloys [1, 2]. This work sets out to examine the response of Cu-B alloys to rapid solidification and to characterize the influence of heat treatments on structural change. 2. Experimental procedure
Ribbons of Cu-3at.%B and Cu 7at.%B were melt spun to a thickness of about 35 #m and heat treated *Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3 7, 1987. 0025-5416/88/$3.50
under an inert atmosphere. Structural analysis was carried out by optical and electron microscopy. Further details, including a relationship with mechanical properties, are reported elsewhere [3]. 3. Results and discussion
The ribbons exhibited the typical two-zone structure in the as-cast state with a fine, microcrystalline structure at the wheel side and columnar grains thereafter (Fig. 1). Optically, there is no evidence of any precipitates. Following heat treatments to 700 C for 0.5 h no change in the microstructure is seen. However, above 750 C a rapid change occurs as large boron grains appear, accompanied by extensive grain growth. The large boron particles exhibit a tendency to concentration near the wheel side of the ribbon. Transmission electron microscopy confirms these results (Fig. 2). The columnar region of Fig. 1 consists of grains about one micron in size, subdivided into cells about micron in size by fine particles. In contrast, the near-wheel region contains fine grains, about ¼micron in size, with an indication of fine precipitation and thus suggesting considerable extension of the solid solubility during casting (Fig. 2(b)). The absence of significant change in microstructure on annealing to 700 :C is emphasized by comparison of Figs. 2(a) and (c): quantitative analysis of such micrographs found no change in particle size nor in grain size. The large precipitate particles and large grain size after annealing at high temperature is shown in Fig. 2(d). Isothermal annealing gave analogous results, confirming an initial resistance to change of the microstructure, followed by very rapid particle and grain coarsening. For example, at 600 ' C no microstructural change was observed during the first 24 h, followed by rapid change thereafter; at 800 C , large particles were already observed after heat treatment for 5 rain. A further detail of interest is the concentration of the large boron particles near the wheel side. This effect was very clear at low annealing temperatures (600-700 C ) but completely lost at very high temperatures (above 800 C ) . C Elsevier Sequoia/Printed in The Netherlands
162
Fig. I. Microstructural evolution on annealing the Cu-3at.%B alloy for 30 min: (a) as cast; (b) 700 °C; (c) 750 °C; (d) 800 °C. The coarsening b e h a v i o u r of the large particles observed after long times at high temperatures has been examined a n d the size is shown to vary with the cube root of the time, as shown in Fig. 3. (Note that these results were o b t a i n e d on a C u - 7 a t . % B alloy.) This relationship is precisely that expected for particle
Fig. 2. Transmission electron mlcrographs showing ribbon structures in Cu 3at.%B: (a) as cast middle; (b) as cast wheel side; (c) annealed 30 min at 700 °C and (d) annealed 30 min at 800 °C.
163 O
g
30
E a:10 I
1000
I
2000
I
3000
I
__
&O00
Time (s)
Fig. 3. Coarsening of large boron particles on annealing Cu 7at.%B alloy at high temperature.
size about one order of magnitude larger. The influence of grain size on enhancement of overall diffusion rate and then coarsening rate is thus confirmed. The total absence of coarsening of the boron particles during the initial heat treatments cannot be understood in terms of this analysis. It should be emphasized that the complete absence of measurable coarsening over many hours at 600 C or I h at 700 C cannot be explained by a simple decrease in the controlling diffusivity: the required diffusivity is now too low by a factor of 10~-10 ~°. An explanation of the different coarsening behaviour is found after noting that the large boron particles are crystalline, whilst
coarsening controlled by diffusion of solute in the bulk matrix between the particles [4, 5], ?3
-
~
8
.
C m VmDbt
where ? and ?o are the final and initial mean particle radii on heat treatment for time t at temperature T. Inserting suitable data for the material parameters and using the experimentally determined slopes of Fig. 3, allows the estimation of solute diffusivity Db as about 1,7 x 10 S m2s i and 9.9 x 10 9 m 2 s 1 at 800 C and 750 ' C respectively. These values are far in excess of the literature values for boron diffusivity in copper [6], i.e. 6 x 10 ~3 and 1 x 10 13 m ~, s ~ (five orders of magnitude greater). The much faster diffusivity deduced here is believed to be caused by a significant contribution of rapid grain-boundary diffusion: the observed r 3 t relationship indicates that coarsening is controlled by transport of solute within the bulk alloy between the large boron particles: this bulk alloy contains very many grain boundaries that lead to an enhancement of the effective, total diffusivity. The total diffusivity can be estimated [7] as
/
riDge\
D,.,r = On ~ 1 + dD b ) with the grain boundary thickness ,5 taken as three times the Burgers vector and Dg b taken from estimates of the influence of grain boundaries on the pre-exponential and activation energy terms of diffusion in copper [8, 9]. Using this equation, the grain size, d, necessary to cause the required increase in effective diffusion coefficient is 3(~40 rim. This value is similar to that of the near-wheel zone (within the Cu-7at. % B alloy), namely a grain size of 3(~50 nm. Coarsening rates for the Cu 3at.%B alloy were about one order of magnitude lower, and correspondingly the grain
Fig. 4. Electron diffraction analysis on particles: (a) microdiffraction on amorphous particles (beam diameter 30nm): (b) selected area diffraction on crystalline particles.
164 the small particles are amorphous (Fig. 4). Diffraction analysis on the large particles allows a partial structural identification as either tetragonal or rhombohedral boron: these two phases possess many similar diffraction characteristics and complete identification has not yet been accomplished. Microdiffraction analysis on the small particles shows a weak and diffuse ring corresponding to the amorphous structure of the weakly scattering boron. The absence of measurable coarsening of particles may be related to an interface kinetics problem associated with boron accretion at an amorphous particle surface: the coarsening of crystalline boron particles takes place here by the dissolution of small particles of amorphous boron, by solute diffusion through the matrix and by boron deposition onto the crystalline particles. Analogously coarsening of amorphous boron particles would require dissolution, diffusional transport and deposition onto an amorphous boron particle. It follows that coarsening of the amorphous particles is inhibited by the difficulty of deposition of boron from solution onto an amorphous particle. According to this model the temperature dependence of the time required for rapid coarsening to begin is really the dependence of the crystallization rate of the amorphous particles. Further work is planned to study this phenomenon. 4. Conclusions Significant solid solution extension of boron by rapid solidification is only possible within a nearwheel zone of melt-spun ribbons where high undercoolings are achieved. The majority of such ribbons have a fine columnar-grained structure with amorphous boron particles decorating grain and cell boundaries. On heat treating, the amorphous particles show a remarkable resistance to coarsening. At the
same time, the grain boundaries are pinned and do not move. The low (zero?) coarsening rate is associated with the difficulty of interface transfer of boron from solution onto the amorphous particle. Following their crystallization, the boron particles coarsen at a very rapid rate because of grain-boundary enhancement of the effective matrix diffusivity. As the boron particles coarsen, so the grain boundaries become depleted of pinning points and rapid grain growth occurs. The concept of very high structural stability associated with amorphous particles is an exciting one offering potential for developing structurally-stable materials and requires further study.
Acknowledgments This work was supported by the Swiss Commission for the Encouragement of Scientific Research, in association with the Boillat Brass Company.
References 1 M. Blank, C. Caersar and U. K6ster, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, North-Holland, Amsterdam, 1985, p. 883. 2 J. V. Wood and C. Y. Elvidge, M.R.S. Syrup. Proc., 58(1986) 451. 3 E. Batawi, M. A. Morris and D. G. Morris, Aeta Metall., in the press. 4 I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids, 19 (1961) 35. 5 C. Wagner, Z. Elektrochem., 65(1961) 581. 6 J. Rexer, Z. Metallkde., 63 (1972) 745. 7 D. A. Porter and E. Easterling, Phase Transformations in Metals and Alloys, Van Nostrand Reinhold, New York, 1981, p. 100. 8 T. E. Volin, K. H. Lie and R. W. Balluffi, Acta Metall., 19 (1971) 263. 9 D. B. Butrymowics, Diffusion Rate Data and Mass Transport Phenomena for Copper Systems, Vol. l, INCRA series on the Metallurgy of Copper, Washington, 1977, p. 56.
161
Microstructural Stability of Rapidly-solidified Cu-B Alloys* E. BATAWI, M. A. MORRIS and D. G. MORRIS
Institute 0/ Structural Metallurgy, University ~)f Neuehguel, 2000 Neuchdtel (Switzerland)
Abstract
The ahilitt' to retain refined microstructures at high temperatures is a necessi o, Jot those materials which will have a high serf, ice temperature or will experience a high-temperature stage during processing, ./'or example during extrusion or .forming. It is therefore essential to understand the .factors controlling stability and coarsening o/` the refined and metastable structures O'Pical o/` rapid solidification. The present stud), examines the stability ~/` Cu B alloys prepared by melt spinning. As-east ribbons exhibit the typical two-zone structure, namely a fine, microcrystalline wheel side with a certain boron solubifiO,, and a columnar-grained free side with boron-rich partich, s outlining the cell walls. This microstructure demonstrates a remarkable resistance to change on initial heat treatment. However, alter a critical time and temperature state is reached, the microstructure becomes locall)' unstable and then changes rapidl)'. The grain boundaries remain immobile as long as particle coarsening is negligible. The boron particles are initially amorphous and h(~hly resistant to coarsening: an interface-controlled process appears to he the cause. Following the crystalli=ation o/` the boron particles, these coarsen very rapt'd(v because o/` enhanced solute d(ffusion along the high-density region o/` the grain boundaries. 1. Introduction
Despite the interest in higher strength and conductivity combinations, relatively little work has been reported to date on alloy development of rapidly solidified microcrystalline copper alloys [1, 2]. This work sets out to examine the response of Cu-B alloys to rapid solidification and to characterize the influence of heat treatments on structural change. 2. Experimental procedure
Ribbons of Cu-3at.%B and Cu 7at.%B were melt spun to a thickness of about 35 #m and heat treated *Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3 7, 1987. 0025-5416/88/$3.50
under an inert atmosphere. Structural analysis was carried out by optical and electron microscopy. Further details, including a relationship with mechanical properties, are reported elsewhere [3]. 3. Results and discussion
The ribbons exhibited the typical two-zone structure in the as-cast state with a fine, microcrystalline structure at the wheel side and columnar grains thereafter (Fig. 1). Optically, there is no evidence of any precipitates. Following heat treatments to 700 C for 0.5 h no change in the microstructure is seen. However, above 750 C a rapid change occurs as large boron grains appear, accompanied by extensive grain growth. The large boron particles exhibit a tendency to concentration near the wheel side of the ribbon. Transmission electron microscopy confirms these results (Fig. 2). The columnar region of Fig. 1 consists of grains about one micron in size, subdivided into cells about micron in size by fine particles. In contrast, the near-wheel region contains fine grains, about ¼micron in size, with an indication of fine precipitation and thus suggesting considerable extension of the solid solubility during casting (Fig. 2(b)). The absence of significant change in microstructure on annealing to 700 :C is emphasized by comparison of Figs. 2(a) and (c): quantitative analysis of such micrographs found no change in particle size nor in grain size. The large precipitate particles and large grain size after annealing at high temperature is shown in Fig. 2(d). Isothermal annealing gave analogous results, confirming an initial resistance to change of the microstructure, followed by very rapid particle and grain coarsening. For example, at 600 ' C no microstructural change was observed during the first 24 h, followed by rapid change thereafter; at 800 C , large particles were already observed after heat treatment for 5 rain. A further detail of interest is the concentration of the large boron particles near the wheel side. This effect was very clear at low annealing temperatures (600-700 C ) but completely lost at very high temperatures (above 800 C ) . C Elsevier Sequoia/Printed in The Netherlands
162
Fig. I. Microstructural evolution on annealing the Cu-3at.%B alloy for 30 min: (a) as cast; (b) 700 °C; (c) 750 °C; (d) 800 °C. The coarsening b e h a v i o u r of the large particles observed after long times at high temperatures has been examined a n d the size is shown to vary with the cube root of the time, as shown in Fig. 3. (Note that these results were o b t a i n e d on a C u - 7 a t . % B alloy.) This relationship is precisely that expected for particle
Fig. 2. Transmission electron mlcrographs showing ribbon structures in Cu 3at.%B: (a) as cast middle; (b) as cast wheel side; (c) annealed 30 min at 700 °C and (d) annealed 30 min at 800 °C.
163 O
g
30
E a:10 I
1000
I
2000
I
3000
I
__
&O00
Time (s)
Fig. 3. Coarsening of large boron particles on annealing Cu 7at.%B alloy at high temperature.
size about one order of magnitude larger. The influence of grain size on enhancement of overall diffusion rate and then coarsening rate is thus confirmed. The total absence of coarsening of the boron particles during the initial heat treatments cannot be understood in terms of this analysis. It should be emphasized that the complete absence of measurable coarsening over many hours at 600 C or I h at 700 C cannot be explained by a simple decrease in the controlling diffusivity: the required diffusivity is now too low by a factor of 10~-10 ~°. An explanation of the different coarsening behaviour is found after noting that the large boron particles are crystalline, whilst
coarsening controlled by diffusion of solute in the bulk matrix between the particles [4, 5], ?3
-
~
8
.
C m VmDbt
where ? and ?o are the final and initial mean particle radii on heat treatment for time t at temperature T. Inserting suitable data for the material parameters and using the experimentally determined slopes of Fig. 3, allows the estimation of solute diffusivity Db as about 1,7 x 10 S m2s i and 9.9 x 10 9 m 2 s 1 at 800 C and 750 ' C respectively. These values are far in excess of the literature values for boron diffusivity in copper [6], i.e. 6 x 10 ~3 and 1 x 10 13 m ~, s ~ (five orders of magnitude greater). The much faster diffusivity deduced here is believed to be caused by a significant contribution of rapid grain-boundary diffusion: the observed r 3 t relationship indicates that coarsening is controlled by transport of solute within the bulk alloy between the large boron particles: this bulk alloy contains very many grain boundaries that lead to an enhancement of the effective, total diffusivity. The total diffusivity can be estimated [7] as
/
riDge\
D,.,r = On ~ 1 + dD b ) with the grain boundary thickness ,5 taken as three times the Burgers vector and Dg b taken from estimates of the influence of grain boundaries on the pre-exponential and activation energy terms of diffusion in copper [8, 9]. Using this equation, the grain size, d, necessary to cause the required increase in effective diffusion coefficient is 3(~40 rim. This value is similar to that of the near-wheel zone (within the Cu-7at. % B alloy), namely a grain size of 3(~50 nm. Coarsening rates for the Cu 3at.%B alloy were about one order of magnitude lower, and correspondingly the grain
Fig. 4. Electron diffraction analysis on particles: (a) microdiffraction on amorphous particles (beam diameter 30nm): (b) selected area diffraction on crystalline particles.
164 the small particles are amorphous (Fig. 4). Diffraction analysis on the large particles allows a partial structural identification as either tetragonal or rhombohedral boron: these two phases possess many similar diffraction characteristics and complete identification has not yet been accomplished. Microdiffraction analysis on the small particles shows a weak and diffuse ring corresponding to the amorphous structure of the weakly scattering boron. The absence of measurable coarsening of particles may be related to an interface kinetics problem associated with boron accretion at an amorphous particle surface: the coarsening of crystalline boron particles takes place here by the dissolution of small particles of amorphous boron, by solute diffusion through the matrix and by boron deposition onto the crystalline particles. Analogously coarsening of amorphous boron particles would require dissolution, diffusional transport and deposition onto an amorphous boron particle. It follows that coarsening of the amorphous particles is inhibited by the difficulty of deposition of boron from solution onto an amorphous particle. According to this model the temperature dependence of the time required for rapid coarsening to begin is really the dependence of the crystallization rate of the amorphous particles. Further work is planned to study this phenomenon. 4. Conclusions Significant solid solution extension of boron by rapid solidification is only possible within a nearwheel zone of melt-spun ribbons where high undercoolings are achieved. The majority of such ribbons have a fine columnar-grained structure with amorphous boron particles decorating grain and cell boundaries. On heat treating, the amorphous particles show a remarkable resistance to coarsening. At the
same time, the grain boundaries are pinned and do not move. The low (zero?) coarsening rate is associated with the difficulty of interface transfer of boron from solution onto the amorphous particle. Following their crystallization, the boron particles coarsen at a very rapid rate because of grain-boundary enhancement of the effective matrix diffusivity. As the boron particles coarsen, so the grain boundaries become depleted of pinning points and rapid grain growth occurs. The concept of very high structural stability associated with amorphous particles is an exciting one offering potential for developing structurally-stable materials and requires further study.
Acknowledgments This work was supported by the Swiss Commission for the Encouragement of Scientific Research, in association with the Boillat Brass Company.
References 1 M. Blank, C. Caersar and U. K6ster, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, North-Holland, Amsterdam, 1985, p. 883. 2 J. V. Wood and C. Y. Elvidge, M.R.S. Syrup. Proc., 58(1986) 451. 3 E. Batawi, M. A. Morris and D. G. Morris, Aeta Metall., in the press. 4 I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids, 19 (1961) 35. 5 C. Wagner, Z. Elektrochem., 65(1961) 581. 6 J. Rexer, Z. Metallkde., 63 (1972) 745. 7 D. A. Porter and E. Easterling, Phase Transformations in Metals and Alloys, Van Nostrand Reinhold, New York, 1981, p. 100. 8 T. E. Volin, K. H. Lie and R. W. Balluffi, Acta Metall., 19 (1971) 263. 9 D. B. Butrymowics, Diffusion Rate Data and Mass Transport Phenomena for Copper Systems, Vol. l, INCRA series on the Metallurgy of Copper, Washington, 1977, p. 56.