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Strength and ductility of microscale brass-steel multilayer composites
Scripta Materialia, Vol. 35, No. 10, pp. 1135-I 140, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights resewed 1359-6462/96 $12.00 + .OO
Pergrunon
PII S1359-6462(96)00289-S
STRENGTH AND DUCTILITY OF MICROSCALE BRASS-STEEL MULTILAYER COMPOSITES KS. Ravichandran,
S.S. Sahay and J.G. Byrne
Department of Metallurgical Engineering, 412 Wm. C. Browning Building The University of Utah, Salt Lake City, UT 84112 (Received April 30, 1996)
Introduction
Multilayer composites or laminates show potential for structural applications due to a good combination of strength and toughness. However, much developmental work, especially in processing, remains to be done before their properties can be fully exploited in high performance applications such as automotive and aerospace. Such composites can be made by a number of techniques including rolling [1,2], co-extrusion and wire drawing [3,4], vapor deposition [5,6] and electro-deposition [7]. Composites including Cu-Nb wires [3], Cu-Fe sheets [4] as well as laminates of Al-Cu [6], Ni-Cu [7] and AlAlzOX [5, 81 have been fabricated by these techniques. With the exception of rolling, laminates with layer thicknessles in the nanometer range have been fabricated by these techniques. In the past [9, lo] as well as more recently [ 111, research was performed to make metallic laminates by roll bonding of sheets of different metals. However, in these studies, the layer thicknesses in the laminates were limited to sizes of the order of millimeters or larger. The composite strength levels followed [9,11] the predictions from the rule of mixtures (ROM). However, making of laminates with layer thicknesses in the micrometer range, yet maintaining the layer discreteness is very challenging, due to layer fragmentation, clustering and interfacial reaction [ 1, 121. For the first time, we have successfully fabri’cated [13] metallic multilayer sheets with layer thicknesses in the nanometer range, while maintaining good layer discreteness, using brass and steel as components. This paves the way for the economic manufacture of laminates with very high specific strength levels. Depending on layer thickness and components, high strength in the laminates can arise from composite strengthening, Hall-Petch type boundary-induced strengthening as well as strengthening due to dislocation density and texture. However, there has been no study of strengthening in microscale laminates made by rolling. In the prefsent work, strength and ductility of rolled microlaminates having alternating layers of brass and steel were investigated. Strength levels are correlated to layer thicknesses and the mechanisms of strengthening are discussed. Experimental
Procedure
The starting materials to make the laminates were the commercially available 0.025 mm thick sheets of AISI 1010 steel and CDA 260 cartridge brass (70%Cu-30%Zn). The sheets, cut into slices of 40 mm 1135
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Figure 1. Microstructures of (a) as-bonded laminate, (b) laminate rolled to 90% reduction (S41), (c) laminate rolled to 96% reduction (S21), (d) laminate rolled to 97% reduction (S32), (e) monolithic brass rolled to 75% reduction and (f) monolithic steel rolled to 54% reduction. All specimens were annealed at 450°C for 1 hr.
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TABLE 1 Thicknessesof Layers in the Rolled Multilayers
x40 mm in size, were stacked alternatively and diffusion bonded in the temperature range of 450 to 550°C for 4 to 8 hrs. under a pressure of about 16 MPa. There was a reduction in thickness of about 7% after bonding. Bonding of monolithic brass and steel stacks were also performed under the same conditions. Colld rolling was performed in a two-high rolling mill having 150 mm diameter rolls. After a reduction of about 30-60% and depending on the instantaneous thickness of the stack, intermediate annealing was given at temperatures ranging from 400 to 500°C for 15 to 30 mins. The monolithic brass and steel stacks were also rolled under the same conditions, with total reductions of 75% and 54%, respectively. Tensile specimens of 5 mm in width and 10 mm in gage length were cut from the rolled sheets. The specimens were annealed at 45O’C for 1 hr. A final polishing of the faces of tensile specimens was done to remove the surface layers affected by the annealing treatment. Transverse sections of the specimens were metallographically polished and etched with 2% nital solution to reveal the steel layers. Microstructures were examined in a Cambridge Stereoscan scanning electron microscope (SEM). Thicknesses of layers were estimated from optical and SEM micrographs. Tensile testing was performed in a 4505 Instron screw-driven testing machine at a strain rate of 2 X 10~s~‘.For thin tensile specimens, aluminum tabs were bonded on both sides of the shoulder region to facilitate gripping. Load and displacement data were obtained and engineering values of 0.2% yield strength (YS), ultimate tensile strength (UTS) were determined. The % total elongation was determined from the displacement at specimen rupture. Results and Discussion
Microstructural observations on rolled multilayers are presented elsewhere [ 131. The microstructure of the as-bonded laminate is shown in Fig. l(a). Figs. l(b-d) show the microstructures of rolled multilayers with different layer thicknesses. These figures indicate that layer discreteness was maintained even TABLE 2 Tensile Properties of Multilayers
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MULTILAYERCOMPOSITES
0
10
20
3”
40
Vol. 35, No. 10
50
Bilayer Thickness (pm)
Figure 2. Experimentally measured YS and the composite YS calculated on the basis of rule of mixtures, as a function of bilayer thickness.
Figure 3. Experimentally measured UTS and the composite UTS calculated on the basis of rule of mixtures, as a function of bilayer thickness.
in the laminate with the smallest layer thickness. Figures l(e & f) illustrate the microstructures of the monolithic brass and steel, respectively, after the annealing treatment. Interestingly, whereas the brass completely recrystallized, the microstructure of steel revealed elongated grains that appear to be not fully recrystallized. The annealing treatment was expected to produce full recrystallization in brass and iron even with a degree of cold work as low as 20% [14]. The thickness of brass and steel layers as a function of % reduction during rolling is given in Table 1. As can be seen from the table, although the brass and steel had equal thicknesses before rolling, the steel layers reduced in thickness preferentially, when compared to the brass layers. The presence of particles rich in Fe were seen in the microstructures [13]. It appears that the loss of Fe due to the formation of these particles contributed to the preferential reduction in the thickness of Fe layers. In Table 2 the tensile data for the multilayer specimens are presented. The tensile tests exhibited good reproducibility and the value for each specimen is an average of two tests. Data for pure brass and steel, bonded and rolled under the same conditions as the multilayers are also included. There is an increase in YS and UTS, but a decrease in % total elongation with a decrease in the layer thicknesses. The YS, UTS and % total elongation were plotted as a function of the combined bilayer thickness of one brass and one steel layer in Figures 2, 3 and 4, respectively. It is not clear at this time whether the tensile properties are controlled by the steel layer or brass layer or a combination of both. Because of this difficulty and due to the fact that either cannot truly classified as harder or softer than the other, it is considered appropriate to use the combined layer thicknesses to plot the data. Also included in Figs. 2 and 3 are the estimated strengths of laminates, on the basis of the ROM, using the strength levels of monolithic brass and steel.
Bilayer Thickness (pm) Figure
4. Experimentally measured % total elongation as a function of bilayer thickness.
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The increase in strength with a decrease in the layer thickness is similar to the observations in laminates processed; by other techniques as well as polycrystalline alloys with lamellar microstructures [36, 8, 151. Microlaminates derive strength by a number of mechanisms including: (i) composite strengthening by virtue of difference in elastic moduli and flow characteristics of components [9], (ii) Hall-Petch type strengthening due to the presence of boundaries between dissimilar layers [8], (iii) Koehler strengthening [16] due to the difference in shear moduli, arising from the preference of a dislocation to remain in a lower shear modulus material to minimize its line tension, (iv) dislocation strengthening 1,171,especially in deformation processed composites involving large strains during deformation and finally (v) texture strengthening [18] arising from the anisotropic structure of layers formed during, for example, vapor deposition or a deformation texture in cold rolling or wire drawing. In the present laminates, ROM strength values actually decreased with layer thicknesses, due to preferential thinning of steel layers with rolling. The increases in strength are more than that offset by the decreases due to the thin steel layer. A calculation [S] of minimum layer thickness of brass (hb) required for Koehler strengthening ( h, = 16xub[(p, - pb) /(p, + p,)] in which CY.= 1, b is the Burgers vector, taken as 0.26 nm, and b and pb are the shear moduli of steel and brass, taken as 80 GPa and 40 GPa, respectively), yields a value of 39 nm which is much smaller than the layer thicknesses encountered in this study. Therefore, both ROM and Koehler strengthening can be considered not relevant. Since brass and steel layers are expected to be recrystallized [ 141,the dislocation strengthening may be considered negligible. Hence, it is clear that the increase in strength must come from other two mechanisms. The dominant mechanism seems to be the Hall-Petch type strengthening, due to layer boundaries as well as t.he grain boundaries within a given layer. The grain sizes in monolithic brass (Fig. l(e)) and steel (grain width, Fig. l(f)) are of the order of 10 pm and 2 pm, respectively. Although the annealing treatment was the same for both monolithics and laminates, because of the presence of layer boundaries, the recrystallized grain sizes could be smaller than these values, in the respective layers of the laminates. However, due to the limited scope of the present work, this could not be ascertained. Additionally, the values of coefficients (k in Hall-Petch equation, <~=c~,+lcX~.~~), when fitted in terms of bilayer thicknesses (h), were 0.46 and 0.29, for yield and ultimate tensile strengths, respectively. The coefficient for yield strength lies within the values for polycrystalline forms of brass (0.31) and steel (0.74). It should be noted that the only other relevant mechanism is texture, since the multilayers showed [ 131 strong (1 lOf brass texture and (lOO} iron texture in the as-rolled condition. However, it is not clear as to the effect of annealing on texture and its impact on strength. Further work is clearly necessary to understand the strengthening in these laminates. The decrease in ductility of microlaminates is consistent with the increase in strength at small layer thicknesses. Similar observations have been made by others [19]. Conclusions
High strength levels were achieved in cold rolled brass/steel microlaminates. The yield and ultimate tensile strength levels increased strongly with decreases in the layer thicknesses of the brass and steel layers. The strength levels approached to the level of steel in the monolithic condition. The % total elongation decreased with a decrease in layer thicknesses. Strengthening in these microlaminates appear to be derived Tom a combination of mechanisms, including primarily the Hall-Petch type boundary strengthening and texture-induced strengthening.
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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
F. Bordeaux and R. Yavari, 7. Metallkde Bd.8, 130, (1990). 0. D. Sherby, T. Oyama, D. W. Kum, B. Walser and .I. Wadsworth, m, 37,50, (1985). W. A. Spitzig. Acta Metall. Mater, 39, 1085, (1991). P. D. Funkenbusch and T. H. Courtney, Scriuta Metall,, 15, 1349 (1981). H. N. G. Wadley, L. M. Hsiung and R. L. Lankey, Como. Enp 5,935, (1995). 49, 5479 (1978). S. L. Lehoczky, J., D. Trench and J. White, Metall. Trans. 15A, 2039 (1984). A. T. Alpas, J. D. Embury, D. A. Hardwick and R. W. Springer, J. Mat. Sci, 25, 1603, (1990). E. S. Wright and A. P. Levitt, in &tall Matrix Comoosites. edited by K. G. Krieder, Academic Press, New York, 4,37 (1974). lOA, 1107, (1979). S. L. Semiatin and H. R. Piehler, Metall. m C. K. Syn, D. R. Lesuer, K. L. Cadwell, 0. D. Sherby and K. R. Brown, in Proc. Conf. onMetal-h, edited by K. Upadhya, TMS, Warrendale, PA, 311, (1991). J. R. Blackford, R. A. Buckley, H. Jones and C. M. Sellers, $&&_&&, 34,721 (1996). S. Sahay, K. S. Ravichandran and J. G. Byrne, Metall. Mater. Trans. A, in press, (1996). C. H. &mans, Metallic The Macmillan Company, NY, 99, (1963). G. Longford, Metall. Tra 8A, 861, (1977). J. S. Koehler, Phv. Rev. B, 2,547, (1970). P. D. Funkenbush, J. K. Lee and T. H. Courtney, Metall. Trans. 18A 1249, (1987). U. Hangen and D. Raabe, Acta Metall. Ma& 43,4075, (1995). R. F. Bunshah, R. Nimmagadda, H. J. Doerr, B. A. Movchan, N. I. Grechanuk and E. V. Dabizha, Thin Solid Films_,72, 261, (1980).
Pergrunon
PII S1359-6462(96)00289-S
STRENGTH AND DUCTILITY OF MICROSCALE BRASS-STEEL MULTILAYER COMPOSITES KS. Ravichandran,
S.S. Sahay and J.G. Byrne
Department of Metallurgical Engineering, 412 Wm. C. Browning Building The University of Utah, Salt Lake City, UT 84112 (Received April 30, 1996)
Introduction
Multilayer composites or laminates show potential for structural applications due to a good combination of strength and toughness. However, much developmental work, especially in processing, remains to be done before their properties can be fully exploited in high performance applications such as automotive and aerospace. Such composites can be made by a number of techniques including rolling [1,2], co-extrusion and wire drawing [3,4], vapor deposition [5,6] and electro-deposition [7]. Composites including Cu-Nb wires [3], Cu-Fe sheets [4] as well as laminates of Al-Cu [6], Ni-Cu [7] and AlAlzOX [5, 81 have been fabricated by these techniques. With the exception of rolling, laminates with layer thicknessles in the nanometer range have been fabricated by these techniques. In the past [9, lo] as well as more recently [ 111, research was performed to make metallic laminates by roll bonding of sheets of different metals. However, in these studies, the layer thicknesses in the laminates were limited to sizes of the order of millimeters or larger. The composite strength levels followed [9,11] the predictions from the rule of mixtures (ROM). However, making of laminates with layer thicknesses in the micrometer range, yet maintaining the layer discreteness is very challenging, due to layer fragmentation, clustering and interfacial reaction [ 1, 121. For the first time, we have successfully fabri’cated [13] metallic multilayer sheets with layer thicknesses in the nanometer range, while maintaining good layer discreteness, using brass and steel as components. This paves the way for the economic manufacture of laminates with very high specific strength levels. Depending on layer thickness and components, high strength in the laminates can arise from composite strengthening, Hall-Petch type boundary-induced strengthening as well as strengthening due to dislocation density and texture. However, there has been no study of strengthening in microscale laminates made by rolling. In the prefsent work, strength and ductility of rolled microlaminates having alternating layers of brass and steel were investigated. Strength levels are correlated to layer thicknesses and the mechanisms of strengthening are discussed. Experimental
Procedure
The starting materials to make the laminates were the commercially available 0.025 mm thick sheets of AISI 1010 steel and CDA 260 cartridge brass (70%Cu-30%Zn). The sheets, cut into slices of 40 mm 1135
1136
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COMPOSITES
Vol. 35, No. 10
Figure 1. Microstructures of (a) as-bonded laminate, (b) laminate rolled to 90% reduction (S41), (c) laminate rolled to 96% reduction (S21), (d) laminate rolled to 97% reduction (S32), (e) monolithic brass rolled to 75% reduction and (f) monolithic steel rolled to 54% reduction. All specimens were annealed at 450°C for 1 hr.
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TABLE 1 Thicknessesof Layers in the Rolled Multilayers
x40 mm in size, were stacked alternatively and diffusion bonded in the temperature range of 450 to 550°C for 4 to 8 hrs. under a pressure of about 16 MPa. There was a reduction in thickness of about 7% after bonding. Bonding of monolithic brass and steel stacks were also performed under the same conditions. Colld rolling was performed in a two-high rolling mill having 150 mm diameter rolls. After a reduction of about 30-60% and depending on the instantaneous thickness of the stack, intermediate annealing was given at temperatures ranging from 400 to 500°C for 15 to 30 mins. The monolithic brass and steel stacks were also rolled under the same conditions, with total reductions of 75% and 54%, respectively. Tensile specimens of 5 mm in width and 10 mm in gage length were cut from the rolled sheets. The specimens were annealed at 45O’C for 1 hr. A final polishing of the faces of tensile specimens was done to remove the surface layers affected by the annealing treatment. Transverse sections of the specimens were metallographically polished and etched with 2% nital solution to reveal the steel layers. Microstructures were examined in a Cambridge Stereoscan scanning electron microscope (SEM). Thicknesses of layers were estimated from optical and SEM micrographs. Tensile testing was performed in a 4505 Instron screw-driven testing machine at a strain rate of 2 X 10~s~‘.For thin tensile specimens, aluminum tabs were bonded on both sides of the shoulder region to facilitate gripping. Load and displacement data were obtained and engineering values of 0.2% yield strength (YS), ultimate tensile strength (UTS) were determined. The % total elongation was determined from the displacement at specimen rupture. Results and Discussion
Microstructural observations on rolled multilayers are presented elsewhere [ 131. The microstructure of the as-bonded laminate is shown in Fig. l(a). Figs. l(b-d) show the microstructures of rolled multilayers with different layer thicknesses. These figures indicate that layer discreteness was maintained even TABLE 2 Tensile Properties of Multilayers
1138
MULTILAYERCOMPOSITES
0
10
20
3”
40
Vol. 35, No. 10
50
Bilayer Thickness (pm)
Figure 2. Experimentally measured YS and the composite YS calculated on the basis of rule of mixtures, as a function of bilayer thickness.
Figure 3. Experimentally measured UTS and the composite UTS calculated on the basis of rule of mixtures, as a function of bilayer thickness.
in the laminate with the smallest layer thickness. Figures l(e & f) illustrate the microstructures of the monolithic brass and steel, respectively, after the annealing treatment. Interestingly, whereas the brass completely recrystallized, the microstructure of steel revealed elongated grains that appear to be not fully recrystallized. The annealing treatment was expected to produce full recrystallization in brass and iron even with a degree of cold work as low as 20% [14]. The thickness of brass and steel layers as a function of % reduction during rolling is given in Table 1. As can be seen from the table, although the brass and steel had equal thicknesses before rolling, the steel layers reduced in thickness preferentially, when compared to the brass layers. The presence of particles rich in Fe were seen in the microstructures [13]. It appears that the loss of Fe due to the formation of these particles contributed to the preferential reduction in the thickness of Fe layers. In Table 2 the tensile data for the multilayer specimens are presented. The tensile tests exhibited good reproducibility and the value for each specimen is an average of two tests. Data for pure brass and steel, bonded and rolled under the same conditions as the multilayers are also included. There is an increase in YS and UTS, but a decrease in % total elongation with a decrease in the layer thicknesses. The YS, UTS and % total elongation were plotted as a function of the combined bilayer thickness of one brass and one steel layer in Figures 2, 3 and 4, respectively. It is not clear at this time whether the tensile properties are controlled by the steel layer or brass layer or a combination of both. Because of this difficulty and due to the fact that either cannot truly classified as harder or softer than the other, it is considered appropriate to use the combined layer thicknesses to plot the data. Also included in Figs. 2 and 3 are the estimated strengths of laminates, on the basis of the ROM, using the strength levels of monolithic brass and steel.
Bilayer Thickness (pm) Figure
4. Experimentally measured % total elongation as a function of bilayer thickness.
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MULTILAYER COMPOSITES
1139
The increase in strength with a decrease in the layer thickness is similar to the observations in laminates processed; by other techniques as well as polycrystalline alloys with lamellar microstructures [36, 8, 151. Microlaminates derive strength by a number of mechanisms including: (i) composite strengthening by virtue of difference in elastic moduli and flow characteristics of components [9], (ii) Hall-Petch type strengthening due to the presence of boundaries between dissimilar layers [8], (iii) Koehler strengthening [16] due to the difference in shear moduli, arising from the preference of a dislocation to remain in a lower shear modulus material to minimize its line tension, (iv) dislocation strengthening 1,171,especially in deformation processed composites involving large strains during deformation and finally (v) texture strengthening [18] arising from the anisotropic structure of layers formed during, for example, vapor deposition or a deformation texture in cold rolling or wire drawing. In the present laminates, ROM strength values actually decreased with layer thicknesses, due to preferential thinning of steel layers with rolling. The increases in strength are more than that offset by the decreases due to the thin steel layer. A calculation [S] of minimum layer thickness of brass (hb) required for Koehler strengthening ( h, = 16xub[(p, - pb) /(p, + p,)] in which CY.= 1, b is the Burgers vector, taken as 0.26 nm, and b and pb are the shear moduli of steel and brass, taken as 80 GPa and 40 GPa, respectively), yields a value of 39 nm which is much smaller than the layer thicknesses encountered in this study. Therefore, both ROM and Koehler strengthening can be considered not relevant. Since brass and steel layers are expected to be recrystallized [ 141,the dislocation strengthening may be considered negligible. Hence, it is clear that the increase in strength must come from other two mechanisms. The dominant mechanism seems to be the Hall-Petch type strengthening, due to layer boundaries as well as t.he grain boundaries within a given layer. The grain sizes in monolithic brass (Fig. l(e)) and steel (grain width, Fig. l(f)) are of the order of 10 pm and 2 pm, respectively. Although the annealing treatment was the same for both monolithics and laminates, because of the presence of layer boundaries, the recrystallized grain sizes could be smaller than these values, in the respective layers of the laminates. However, due to the limited scope of the present work, this could not be ascertained. Additionally, the values of coefficients (k in Hall-Petch equation, <~=c~,+lcX~.~~), when fitted in terms of bilayer thicknesses (h), were 0.46 and 0.29, for yield and ultimate tensile strengths, respectively. The coefficient for yield strength lies within the values for polycrystalline forms of brass (0.31) and steel (0.74). It should be noted that the only other relevant mechanism is texture, since the multilayers showed [ 131 strong (1 lOf
High strength levels were achieved in cold rolled brass/steel microlaminates. The yield and ultimate tensile strength levels increased strongly with decreases in the layer thicknesses of the brass and steel layers. The strength levels approached to the level of steel in the monolithic condition. The % total elongation decreased with a decrease in layer thicknesses. Strengthening in these microlaminates appear to be derived Tom a combination of mechanisms, including primarily the Hall-Petch type boundary strengthening and texture-induced strengthening.
1140
MULTILAYER COMPOSITES
Vol. 35, No. 10
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
F. Bordeaux and R. Yavari, 7. Metallkde Bd.8, 130, (1990). 0. D. Sherby, T. Oyama, D. W. Kum, B. Walser and .I. Wadsworth, m, 37,50, (1985). W. A. Spitzig. Acta Metall. Mater, 39, 1085, (1991). P. D. Funkenbusch and T. H. Courtney, Scriuta Metall,, 15, 1349 (1981). H. N. G. Wadley, L. M. Hsiung and R. L. Lankey, Como. Enp 5,935, (1995). 49, 5479 (1978). S. L. Lehoczky, J., D. Trench and J. White, Metall. Trans. 15A, 2039 (1984). A. T. Alpas, J. D. Embury, D. A. Hardwick and R. W. Springer, J. Mat. Sci, 25, 1603, (1990). E. S. Wright and A. P. Levitt, in &tall Matrix Comoosites. edited by K. G. Krieder, Academic Press, New York, 4,37 (1974). lOA, 1107, (1979). S. L. Semiatin and H. R. Piehler, Metall. m C. K. Syn, D. R. Lesuer, K. L. Cadwell, 0. D. Sherby and K. R. Brown, in Proc. Conf. onMetal-h, edited by K. Upadhya, TMS, Warrendale, PA, 311, (1991). J. R. Blackford, R. A. Buckley, H. Jones and C. M. Sellers, $&&_&&, 34,721 (1996). S. Sahay, K. S. Ravichandran and J. G. Byrne, Metall. Mater. Trans. A, in press, (1996). C. H. &mans, Metallic The Macmillan Company, NY, 99, (1963). G. Longford, Metall. Tra 8A, 861, (1977). J. S. Koehler, Phv. Rev. B, 2,547, (1970). P. D. Funkenbush, J. K. Lee and T. H. Courtney, Metall. Trans. 18A 1249, (1987). U. Hangen and D. Raabe, Acta Metall. Ma& 43,4075, (1995). R. F. Bunshah, R. Nimmagadda, H. J. Doerr, B. A. Movchan, N. I. Grechanuk and E. V. Dabizha, Thin Solid Films_,72, 261, (1980).