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Electron microscope observations of phase separation near spinodal boundary in a sodium borosilicate glass
Journal of Non-Crystalline Solids 20 (1976) 141-148 O North-Holland Publishing Company
ELECTRON MICROSCOPE OBSERVATIONS OF PHASE SEPARATION NEAR SPINODAL BOUNDARY IN A SODIUM BOROSILICATE GLASS
G.R. SRINIVASAN IBM Dept. 76,4, System Products Division, East FiskiU Facility, Rt. 52, Hopeweli Junction, N Y 12533, USA
A. SARKAR Senior Research Engineer, General Cable Corporation Union, NJ 07083, USA
P.K. GUPTA and P.B. MACEDO Vitreous State Laboratory, The Catholic University of America, Washington, DC20064, USA Received 25 September 1974
Time evolution of volume fraction of the minor phase is studied in an alumina-doped sodium borosilicate glass inside the immiscible region. It is shown that such a study permits a distinction between the two mechanisms of phase separation; namely spinodal decomposition and nucleation and growth. For spinodal decomposition, the volume fraction decreases initially, whereas for nucleation and growth, it increases with heat-treatment time.
1. Introduction
In recent years several studies (see fc,~"a review ref. [ 1]) have attempted to distinguish between two commonly accepted mechanisms of phase separation in glasses, namely nucleation and growth (N & C) and spinodal decomposition (SD). The studies using light or small angle X-ray scattering can directly verify Cahn's spinodal decomposition theory [2] which predicts the preferential growth of a selected band of Fourier components of the composition fluctuations in the scattering spectra. The prediction for N & G mechanism is quite different, in that no such selective growth is expected. Electron-microscope studies are based on the notion that while the N & G process produces initially isolated minor phase particles distributed randomly in the matrix phase (assuming no foreign surfaces exist in the melt), the SD produces an interconnected morphology in which both phases are connected over large distances in the sample [3]. It is emphasized that these predictions are four early stages of the phase separation, and coarsening in later stages is expected to produce a break in the interconnectivity of the spinodal microstructure [4]. On the other hand, interconnectivity in N & G microstructure can result by an intersecting
G.R. Srinivasan et al. / Phase separation in a sodium borosilicate glass
142
growth mechanism as proposed by Hailer [5]. In view of these difficulties, the morphological method of distinguishing between the N & G and SD mechanism remains controversial. it is the purpose of this paper ~.o propose a new method to distinguish between the N & G and SD mechanisms. ~he method is based on the time dependence of the isothermal volume fraction of the minor phase during phase separation. We present here some of the results obtained for an alumina-doped sodium borosilicate glass which has been used in our previous investigations [6,7]. In these studies, we have characterized this glass in terms of the spinodal and ,coexistence temperatures, and the kinetics of composition fluctuations in the single phase.
2. Relevant theoretical considerations Figure I shows an idealized miscibility gap for a binary system. (The same considerations would apply for a pseudo-binary ~ystem in multicomponent cases.) The miscibility gap can be divided into two thermodynamic regions depending on whether the second derivative, f " , of the solution free energy with respect to composttion is positive or negative. The line representing f " = 0 represents the chemical spinodal boundary. In the region above the spinodal boundary and below the coex.istence curve,f" > O, and this represents a metastable state for the solution, which requires a composition fluctuation large in amplitude (i.e. nucleation) to cause phase separation. The decomposition of the solution is then characterized by the N & G theories. Below the spinodal boundary, on the other hand, f " < 0, and the solution is unstable even to infinitesimal fluctuations in composition. The
single phase metastable (stable) region region
*l
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i
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,,I-w
~- : t.=O
A
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corn
e
NC~.~AT|ON fS~lO0
timel t )
Fig. 1. An idealized coexistence curve (solid line) for a binary system showing the metastable and unstable regions, separated by the spinodal boundary (dashed curve). Fig. 2. Volume fraction, O(t), of the minor phase as a function of time for N & G.
G.R. Srinivasan,et al. /Phase separation in a sodium borosilicate glass
143
coexistence curve
0.5
J
•w U
q.1
i=
Co
composition
time( t )
Fig. 3. Composition fluctuations are symmetric about the average composition in the early stages of spinodal decomposition• Fig. 4. Schematic time dependence of the volume fraction of the minor phase for SD.
kinetics of such a decomposition in the initial stages have been described by Cahn [3,41. When a glqss sample is held at a temperature in the metastable region, nucleation of the minor phase occurs at isolated centers, and phase decomposition is accompanied by growth of the second phase particles. The phenomenological theories of N &. G predict that the volume fraction (¢) of the minor phase will be given by ~(t) = 1 - exp (-ktn), where k and n are parameters characteristic of the geometry and the nature of the process, and t is time after the onset of nucleation. This behavior produces sigmoidal curves as shown schematically ~n fig. 2. Most experimental transformation curves for the isothermal nucleationand growth process are sigmoidal in shape. When a glass is held at a temperature inside the spinodal region, the phase decomposition is expected to occur homogeneously throughout the sample. In the initial stages, one would also expect the fluctuation to be symmetric about the mean composition, providing the mobility of the diffusing species does not depend strongly on the composition. This situation is schematically described in fig. 3. As the spinodal decomposition progresses beyond the initial stages, asymmetry in the composition modulation would develop if the mean composition C0, is not @e symmetric composition with respect to the miscibility gap. In ~le initial stages therefore, the voi,ume of the material with composition C > Co is equal to that for the material for which C < C0. If the volume fractions of the two 'phases' are measured as a fuv ction of reaction time in an isothermal experiment, one would expect a behavior as shown schematically in fig. 4. There is no theory at present which would d~;scribe the exact functional relationship of ¢~t),
144
G.R. Srinivasan et ai. / Phase separation in a sodium borosilicate glass
except to say that initially ~ would be a decreasing function of it. This discussion shows that if the volume fraction of the minor phase is measured as a function of time in an isothermal phase-separation experiment, it should be possible to distinguish between the N & G and SD mechanisms.
3. Experimental The batch composition of the glass under study was (in mol%) 69.3SIO 2, 22.3B203, 6.6Na20 and 1.8Al203. The batch was melted in a platinum crucible at 1500°C and roll-quenched to room temperature. Samples were heat treated for various times in a furnace regulated to within ±0.2°C and were quenched in sand. It was estimated that the initial surface cooling rate was about l0 °C s-1. The glass was fractured and the fractured surface was etched with 0-5% HF for appropriate times (between 2 and l0 s). Carbon replicas were made from these surfaces after shadowing with platinum (using carbonplatinum pellets) at an inclination of about 60 ° to the glass surface in the conventional manner. The replicas were examined in an AEI EM 802 electron microscope at 80 kV. ° In a previous study [7] of this glass, we reported that the coexistence temperature was found to be Tco = 654.5 -. 0.5 °C. In another study of this glass [6] it was shown that the spinodal temperature for this glass is TS = 649 ± 0.5 °C. Therefore at temperatures higher than 649°C in the two-phase region this glass is thermodynamically metastable, and at temperatures below 649°C the glass is thermodynamicaUy unstable relative to separating phases. The volume fraction of the minor phase was measured for isothermal studies at different temperatures within the two-phase region. The estimation was made by measuring the area fraction on two.dimensional micrographs by De!esse's method [9]. The precision in these estimates is of the order of +--10%. The volume fractions of some samples were also measured by lineal analysis. The two methods agreed within the error limits.
4. Results and discussion 4.1. Time dependence o f isothermal volume fraction
The volume fraction results for 648, 650 and 654 °C for various times are shown in fig. 5. It is seen that for the 644 and 648°C heat treatment, the volume fraction is, initially, a decreasing function of time, whereas for temperatures at or above 650°C the volume fraction increases with time. Thus, these experiments indicate that for the present composition the boundary between the N & G and SD regions is between 648 and 650 °C. This is consistent with our previous results of Ts = 648 -. 0.5 °C.
G.R. Srinivasan et aL /Phase separation in a sodium borosilicate glass
145
0.5
0.4
67 hrs. 640Oc
0.3
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145hrs. I00 hrs.
650Oc 6S~C
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96 hrs,
654Oc
i 50
time (hours) -
:
Fig. 5. Isothermal volume fraction as a f u , c t i o n of time for t r e a t m e n t at various temperatures. The error bars indicate a precision of + 10% in the measured values of volume fraction.
It is interesting to note that the equilibrium volume fraction of the minor phase approaches a value of 0.21 for the 648°C heat treatment° According to the regular solution model as applied to phase separation in binary solutions [ 10], the equilibrium volume fraction is ~0.21 at the spinodal temperature. Further, our previous studies [6] have shown that the equilibrium volume fraction in the present glass varies with heat-treatment temperature in close agreement with the regular solution model. This result is also consistent with the observations of Cook and Hilliard [10], who reported that in many phase-separating systems the regular solution model seems to provide good agreement with experiments for phase separation temperatures close to the critical temperature (i.e. within 10% loss of Tc). In view of this agreement between our iesults and the binary regular solution model, it seems adequate to consider the present multicomponent system as a pseudobinary system.
4.2. Morphological differences between the products formed just above and just below the spinodal temperature Figure 6 shows the development of early stage phase morphology at 648 and 650 °C. The inicrostructures at these two temperatures appear quite similar in the 1 h heat treatment. In both cases the microstructure consists of composition fluctuations corresponding to the single phase above the miscibi!iW gap and no phase
146
t~.R. Srinivasan et al. / Phase separation in a sodium borosilicate glass
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G.R. Srinivasan et al. /Phase separation in a sodium borosilicate glass
147
separation is observed. The dynamics of these fluctuations have been reported elsewhere [7]. However, after 4 11the microsctructure shows significant differences for these two temperatures. The 648°C microstructure for this period shows the same fluctuations as observed in 1 h heat treatment, but they are now grown to larger size, whereas the 650°C 4 h microstructure shows the appearance of a number of particles in greater optical contrast ~han the background fluctuations. This observation could be explained if we assume that the particles in high contrast are nucleated particles, and that the higher contrast is due to a larger compositional difference between the matrix and the nucleated particle than between the matrix ark the fluctuations. This interpretation is further substantiated by the fact that particles in greater optical contrast grow in size with heat-treatment time, while the matrix fluctuations remain approximately constant in size. Similar differences between the 648 and 650°C microstructure can be seen for the 8 h heat treatment. On the basis of these observations, we conclude that at 648°C the phase separation occurs by continuous growth of fluctuations, whereas at 650°C the phase separation proceeds by nucleation of minor phase particles which grow in size with time. We further note there have been no other studies reported in the literature which indicate that spinodal decomposition could occur within I°C below the spinodal temperature tbr any glass system. It is emphasized that theoretical understanding of spinodal decomposition very close to the spinodal boundary is lacking, for the present theories require infinitely large diffusion distances for such decomposition at the spinodal temperature. Further work exploring even closer to the spinodal boundary than in this case should throw more light on the nature of the thermodynamic singularity that exists at the spinodal boundary.
5. Conclusions Present studies of phase separation in an alumina-doped sodium borosilicate glass have shown that it is possible to distinguish between spinodal decomposition and nucleation and growth mechanisms of phase separation by studying the time dependence of the isothermal volume fraction of the minor phase. For spinodal decomposition the volume fraction decreases initially, whereas for nuc',eation and growth it increases initially with heat-treatment time. It is possible to distinguish between the spinodal microstructure and the nucleation and growth microstructure by observing phase development in the early stages.
Acknowledgement This work was supported by the Air Force Office of Scientific Research under contract No. AFOSR 72-2203.
148
G.R. Srinivasan et al. / Phase separation in a sodium borostlicate glass
References [I] [2] [3] [4] [5] [6] [7]
R.J. Charles, Bull. Amer. Ceram. Soc. 52 (1974) 673. J.W. Cahn, Acta Metal. 9 (1961) 795. J.W. Cahn, J. Chem. Phys. 42 (1965) 93. J.W. Cahn, Acta Metal. 14 (1966) 1685. W. Hailer, J. Chem. Phys. 42 (1965) 686. A. Sarkar, P.K. Gupta, G.R. Srinivasan and P.,B. Macedo, J. Chem. Phys. 59 (1973) 4246. A. Sarkar, G.R. Srinivasan, V. Volterra and P.B. Macedo, Phys. Chem. Glasses 14 (1973) 114. [8] J.W. Christian, The Theory of Transformations in Metals and Alloys (Pergamon Press, London, 1965). [91 J.E. HilUard, Quantitative Microscopy, ch. 3 (McGraw-Hill, 1968). 11o1 H. Cook and J.E. Hilliard, Trans. Amer. Inst. Metal. Eng. 233 (1965) 142.
ELECTRON MICROSCOPE OBSERVATIONS OF PHASE SEPARATION NEAR SPINODAL BOUNDARY IN A SODIUM BOROSILICATE GLASS
G.R. SRINIVASAN IBM Dept. 76,4, System Products Division, East FiskiU Facility, Rt. 52, Hopeweli Junction, N Y 12533, USA
A. SARKAR Senior Research Engineer, General Cable Corporation Union, NJ 07083, USA
P.K. GUPTA and P.B. MACEDO Vitreous State Laboratory, The Catholic University of America, Washington, DC20064, USA Received 25 September 1974
Time evolution of volume fraction of the minor phase is studied in an alumina-doped sodium borosilicate glass inside the immiscible region. It is shown that such a study permits a distinction between the two mechanisms of phase separation; namely spinodal decomposition and nucleation and growth. For spinodal decomposition, the volume fraction decreases initially, whereas for nucleation and growth, it increases with heat-treatment time.
1. Introduction
In recent years several studies (see fc,~"a review ref. [ 1]) have attempted to distinguish between two commonly accepted mechanisms of phase separation in glasses, namely nucleation and growth (N & C) and spinodal decomposition (SD). The studies using light or small angle X-ray scattering can directly verify Cahn's spinodal decomposition theory [2] which predicts the preferential growth of a selected band of Fourier components of the composition fluctuations in the scattering spectra. The prediction for N & G mechanism is quite different, in that no such selective growth is expected. Electron-microscope studies are based on the notion that while the N & G process produces initially isolated minor phase particles distributed randomly in the matrix phase (assuming no foreign surfaces exist in the melt), the SD produces an interconnected morphology in which both phases are connected over large distances in the sample [3]. It is emphasized that these predictions are four early stages of the phase separation, and coarsening in later stages is expected to produce a break in the interconnectivity of the spinodal microstructure [4]. On the other hand, interconnectivity in N & G microstructure can result by an intersecting
G.R. Srinivasan et al. / Phase separation in a sodium borosilicate glass
142
growth mechanism as proposed by Hailer [5]. In view of these difficulties, the morphological method of distinguishing between the N & G and SD mechanism remains controversial. it is the purpose of this paper ~.o propose a new method to distinguish between the N & G and SD mechanisms. ~he method is based on the time dependence of the isothermal volume fraction of the minor phase during phase separation. We present here some of the results obtained for an alumina-doped sodium borosilicate glass which has been used in our previous investigations [6,7]. In these studies, we have characterized this glass in terms of the spinodal and ,coexistence temperatures, and the kinetics of composition fluctuations in the single phase.
2. Relevant theoretical considerations Figure I shows an idealized miscibility gap for a binary system. (The same considerations would apply for a pseudo-binary ~ystem in multicomponent cases.) The miscibility gap can be divided into two thermodynamic regions depending on whether the second derivative, f " , of the solution free energy with respect to composttion is positive or negative. The line representing f " = 0 represents the chemical spinodal boundary. In the region above the spinodal boundary and below the coex.istence curve,f" > O, and this represents a metastable state for the solution, which requires a composition fluctuation large in amplitude (i.e. nucleation) to cause phase separation. The decomposition of the solution is then characterized by the N & G theories. Below the spinodal boundary, on the other hand, f " < 0, and the solution is unstable even to infinitesimal fluctuations in composition. The
single phase metastable (stable) region region
*l
I~)GQ.
i
IH
,,I-w
~- : t.=O
A
r°
corn
e
NC~.~AT|ON fS~lO0
timel t )
Fig. 1. An idealized coexistence curve (solid line) for a binary system showing the metastable and unstable regions, separated by the spinodal boundary (dashed curve). Fig. 2. Volume fraction, O(t), of the minor phase as a function of time for N & G.
G.R. Srinivasan,et al. /Phase separation in a sodium borosilicate glass
143
coexistence curve
0.5
J
•w U
q.1
i=
Co
composition
time( t )
Fig. 3. Composition fluctuations are symmetric about the average composition in the early stages of spinodal decomposition• Fig. 4. Schematic time dependence of the volume fraction of the minor phase for SD.
kinetics of such a decomposition in the initial stages have been described by Cahn [3,41. When a glqss sample is held at a temperature in the metastable region, nucleation of the minor phase occurs at isolated centers, and phase decomposition is accompanied by growth of the second phase particles. The phenomenological theories of N &. G predict that the volume fraction (¢) of the minor phase will be given by ~(t) = 1 - exp (-ktn), where k and n are parameters characteristic of the geometry and the nature of the process, and t is time after the onset of nucleation. This behavior produces sigmoidal curves as shown schematically ~n fig. 2. Most experimental transformation curves for the isothermal nucleationand growth process are sigmoidal in shape. When a glass is held at a temperature inside the spinodal region, the phase decomposition is expected to occur homogeneously throughout the sample. In the initial stages, one would also expect the fluctuation to be symmetric about the mean composition, providing the mobility of the diffusing species does not depend strongly on the composition. This situation is schematically described in fig. 3. As the spinodal decomposition progresses beyond the initial stages, asymmetry in the composition modulation would develop if the mean composition C0, is not @e symmetric composition with respect to the miscibility gap. In ~le initial stages therefore, the voi,ume of the material with composition C > Co is equal to that for the material for which C < C0. If the volume fractions of the two 'phases' are measured as a fuv ction of reaction time in an isothermal experiment, one would expect a behavior as shown schematically in fig. 4. There is no theory at present which would d~;scribe the exact functional relationship of ¢~t),
144
G.R. Srinivasan et ai. / Phase separation in a sodium borosilicate glass
except to say that initially ~ would be a decreasing function of it. This discussion shows that if the volume fraction of the minor phase is measured as a function of time in an isothermal phase-separation experiment, it should be possible to distinguish between the N & G and SD mechanisms.
3. Experimental The batch composition of the glass under study was (in mol%) 69.3SIO 2, 22.3B203, 6.6Na20 and 1.8Al203. The batch was melted in a platinum crucible at 1500°C and roll-quenched to room temperature. Samples were heat treated for various times in a furnace regulated to within ±0.2°C and were quenched in sand. It was estimated that the initial surface cooling rate was about l0 °C s-1. The glass was fractured and the fractured surface was etched with 0-5% HF for appropriate times (between 2 and l0 s). Carbon replicas were made from these surfaces after shadowing with platinum (using carbonplatinum pellets) at an inclination of about 60 ° to the glass surface in the conventional manner. The replicas were examined in an AEI EM 802 electron microscope at 80 kV. ° In a previous study [7] of this glass, we reported that the coexistence temperature was found to be Tco = 654.5 -. 0.5 °C. In another study of this glass [6] it was shown that the spinodal temperature for this glass is TS = 649 ± 0.5 °C. Therefore at temperatures higher than 649°C in the two-phase region this glass is thermodynamically metastable, and at temperatures below 649°C the glass is thermodynamicaUy unstable relative to separating phases. The volume fraction of the minor phase was measured for isothermal studies at different temperatures within the two-phase region. The estimation was made by measuring the area fraction on two.dimensional micrographs by De!esse's method [9]. The precision in these estimates is of the order of +--10%. The volume fractions of some samples were also measured by lineal analysis. The two methods agreed within the error limits.
4. Results and discussion 4.1. Time dependence o f isothermal volume fraction
The volume fraction results for 648, 650 and 654 °C for various times are shown in fig. 5. It is seen that for the 644 and 648°C heat treatment, the volume fraction is, initially, a decreasing function of time, whereas for temperatures at or above 650°C the volume fraction increases with time. Thus, these experiments indicate that for the present composition the boundary between the N & G and SD regions is between 648 and 650 °C. This is consistent with our previous results of Ts = 648 -. 0.5 °C.
G.R. Srinivasan et aL /Phase separation in a sodium borosilicate glass
145
0.5
0.4
67 hrs. 640Oc
0.3
I
C
.2 0.2
- i
II
J~
E
fl 0
I I0
I 20
30
I 40
145,,hrst
648°C
I
145hrs. I00 hrs.
650Oc 6S~C
I
96 hrs,
654Oc
i 50
time (hours) -
:
Fig. 5. Isothermal volume fraction as a f u , c t i o n of time for t r e a t m e n t at various temperatures. The error bars indicate a precision of + 10% in the measured values of volume fraction.
It is interesting to note that the equilibrium volume fraction of the minor phase approaches a value of 0.21 for the 648°C heat treatment° According to the regular solution model as applied to phase separation in binary solutions [ 10], the equilibrium volume fraction is ~0.21 at the spinodal temperature. Further, our previous studies [6] have shown that the equilibrium volume fraction in the present glass varies with heat-treatment temperature in close agreement with the regular solution model. This result is also consistent with the observations of Cook and Hilliard [10], who reported that in many phase-separating systems the regular solution model seems to provide good agreement with experiments for phase separation temperatures close to the critical temperature (i.e. within 10% loss of Tc). In view of this agreement between our iesults and the binary regular solution model, it seems adequate to consider the present multicomponent system as a pseudobinary system.
4.2. Morphological differences between the products formed just above and just below the spinodal temperature Figure 6 shows the development of early stage phase morphology at 648 and 650 °C. The inicrostructures at these two temperatures appear quite similar in the 1 h heat treatment. In both cases the microstructure consists of composition fluctuations corresponding to the single phase above the miscibi!iW gap and no phase
146
t~.R. Srinivasan et al. / Phase separation in a sodium borosilicate glass
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G.R. Srinivasan et al. /Phase separation in a sodium borosilicate glass
147
separation is observed. The dynamics of these fluctuations have been reported elsewhere [7]. However, after 4 11the microsctructure shows significant differences for these two temperatures. The 648°C microstructure for this period shows the same fluctuations as observed in 1 h heat treatment, but they are now grown to larger size, whereas the 650°C 4 h microstructure shows the appearance of a number of particles in greater optical contrast ~han the background fluctuations. This observation could be explained if we assume that the particles in high contrast are nucleated particles, and that the higher contrast is due to a larger compositional difference between the matrix and the nucleated particle than between the matrix ark the fluctuations. This interpretation is further substantiated by the fact that particles in greater optical contrast grow in size with heat-treatment time, while the matrix fluctuations remain approximately constant in size. Similar differences between the 648 and 650°C microstructure can be seen for the 8 h heat treatment. On the basis of these observations, we conclude that at 648°C the phase separation occurs by continuous growth of fluctuations, whereas at 650°C the phase separation proceeds by nucleation of minor phase particles which grow in size with time. We further note there have been no other studies reported in the literature which indicate that spinodal decomposition could occur within I°C below the spinodal temperature tbr any glass system. It is emphasized that theoretical understanding of spinodal decomposition very close to the spinodal boundary is lacking, for the present theories require infinitely large diffusion distances for such decomposition at the spinodal temperature. Further work exploring even closer to the spinodal boundary than in this case should throw more light on the nature of the thermodynamic singularity that exists at the spinodal boundary.
5. Conclusions Present studies of phase separation in an alumina-doped sodium borosilicate glass have shown that it is possible to distinguish between spinodal decomposition and nucleation and growth mechanisms of phase separation by studying the time dependence of the isothermal volume fraction of the minor phase. For spinodal decomposition the volume fraction decreases initially, whereas for nuc',eation and growth it increases initially with heat-treatment time. It is possible to distinguish between the spinodal microstructure and the nucleation and growth microstructure by observing phase development in the early stages.
Acknowledgement This work was supported by the Air Force Office of Scientific Research under contract No. AFOSR 72-2203.
148
G.R. Srinivasan et al. / Phase separation in a sodium borostlicate glass
References [I] [2] [3] [4] [5] [6] [7]
R.J. Charles, Bull. Amer. Ceram. Soc. 52 (1974) 673. J.W. Cahn, Acta Metal. 9 (1961) 795. J.W. Cahn, J. Chem. Phys. 42 (1965) 93. J.W. Cahn, Acta Metal. 14 (1966) 1685. W. Hailer, J. Chem. Phys. 42 (1965) 686. A. Sarkar, P.K. Gupta, G.R. Srinivasan and P.,B. Macedo, J. Chem. Phys. 59 (1973) 4246. A. Sarkar, G.R. Srinivasan, V. Volterra and P.B. Macedo, Phys. Chem. Glasses 14 (1973) 114. [8] J.W. Christian, The Theory of Transformations in Metals and Alloys (Pergamon Press, London, 1965). [91 J.E. HilUard, Quantitative Microscopy, ch. 3 (McGraw-Hill, 1968). 11o1 H. Cook and J.E. Hilliard, Trans. Amer. Inst. Metal. Eng. 233 (1965) 142.