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Phase morphology effect on elevated temperature mechanical behavior of nanostructures
Materials Letters 61 (2007) 1465 – 1468 www.elsevier.com/locate/matlet
Phase morphology effect on elevated temperature mechanical behavior of nanostructures A.V. Sergueeva a,⁎, N.A. Mara a , D.J. Branagan b , A.K. Mukherjee a a
University of California, Chemical Engineering and Material Science Department, Davis, CA 95616, USA b Institute of NanoMaterials Research and Development, The NanoSteel Company, 505 Lindsay Boulevard, Idaho Falls, Idaho, 83402, USA Received 7 October 2005; accepted 20 July 2006 Available online 4 August 2006
Abstract A devitrification procedure by annealing was applied to a multicomponent Fe-based metallic glass in order to obtain nanocrystalline materials. Phase composition and phase morphology were strongly dependent on the annealing conditions. An elevated temperature mechanical behavior of nanostructures was evaluated by tensile testing. A strong effect of phase morphology on the mechanical response of the material was revealed. A most attractive combination of strength and plasticity was observed in the nanostructure with approximately equal grain sizes of crystallized phases. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Mechanical properties
1. Introduction Together with constitution and scale, the phase morphology of a material is known to be critical in determining its properties. For example, mechanical properties such as tensile strength, tensile elongation and impact strength for a particular polymer blend vary considerably with the morphology [1]. Phase morphology in stainless steel has also a strong influence on the material behavior [2]. At the same time, an analysis of the structure/properties relationships in nanomaterials is mostly restricted to a grain size effect alone. Recently, crystallization of amorphous solids has been successfully used as one of the methods of nanocrystalline material production in various alloy systems, e.g. in Fe-, Ni-, and Co-based alloys, as well as in some elements (e.g. Se, Si) [3]. To obtain a nanoscale structure, the crystallization process should proceed with the largest nucleation rate possible while suppressing the crystal growth rate as much as practicable. Such conditions can be obtained for some alloy compositions by applying specific methods of heat ⁎ Corresponding author. E-mail address: [email protected] (A.V. Sergueeva). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.055
treatment. It was shown theoretically by Crespo et al. [4] and experimentally by Köster et al. [5] that among metallic glasses those that undergo primary crystallization with a time-dependent, long-range diffusion controlled growth rate are the most suitable candidates for nanocrystallization. Many Fe-based metallic glasses do not require any special heat treatment to be converted into nanocrystalline materials [6–8]. Isothermal annealing at temperatures close to the crystallization temperature for a typical time of 1 h, i.e., the so-called conventional annealing, is widely used by many researchers [7]. On the other hand, it was found [9] that the application of relatively low temperature and longer annealing times also facilitates the creation of a nanocrystalline structure. The size, morphology of crystallites, mechanism of crystallization and crystallization products themselves depend on the temperature of thermal treatment of a metallic glass. In order to develop the best strategy to attain maximum improvement in ultimate properties of nanostructures the critical factors on which these properties rely must be determined including understanding on the phase morphology and crystallization behavior of the metallic glasses. In this investigation, a devitrification procedure of trough annealing was applied to a multicomponent Fe-based metallic glass. Tensile testing was applied in order to evaluate an elevated
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temperature mechanical behavior of nanostructures as a function of phase morphology in the crystallized metallic glass. 2. Experimental Fe-based melt-spun metallic ribbons of (Fe0.8Cr0.2)81B17W2 alloy were used in the current investigation. Initial ribbons were formed by first induction melting followed by ejecting the liquid melt onto a rapidly moving nitrogen cooled copper chill wheel. Heat treatment with constant heating rate to predetermined temperatures, followed by furnace cooling was applied for forming nanocrystalline structures. The size, morphology and composition of the crystallization products were investigated using TEM and X-ray analysis. Microhardness measurements and tensile tests on a custom-built computer controlled constant strain rate tensile test machine were conducted to determine correlation between microstructural characteristics and mechanical properties of the material. Differential scanning calorimeter (DSC) measurements were carried out between 25 °C and 725 °C with a heating rate of 40 °C/min, which mimics the heating rate found in our tensile apparatus. Fig. 2. X-ray data for the alloy after annealing for 100 h at different temperatures.
3. Results DSC measurements have revealed one exothermic peak corresponding to the crystallization process. The material shows the onset of crystallization (Tx) at 520 °C at a heating rate of 40 °C/min (Fig. 1). Ribbons in the as-cast state were annealed for 100 h at temperatures below (450 °C and 500 °C) and above (600 °C and 700 °C) the onset of the crystallization. X-ray data for the ribbons in the initial and annealed states are shown in Fig. 2. It is seen that the initial ribbons are not fully amorphous and some crystallites of α-Fe are present. Annealing of the material for 100 h, even at 500 °C that is below Tx determined by DSC (Fig. 1) results in crystallization of additional phases one of which is Fe3B (Fig. 2). Significant broadening of the peaks suggests an extremely small grain size. A detailed analysis of the phase composition is part of an ongoing investigation. An increase in annealing temperature leads to more extensive crystallization of α-Fe phase. The process of crystallization at T b Tx results in a considerable increase in the microhardness of the material
(Table 1) with a peak at around 500 °C. With increasing annealing temperature above Tx, a decrease in hardness was observed down to the value of the initial state (∼ 11 GPa). Some residual amorphous phase was suggested to exist after annealing at temperatures below the onset of crystallization (450 °C and 500 °C). In order to complete the crystallization process, an additional annealing at 600 °C for 1 h was applied. Moreover, to preserve a resulting grain size in the nanometer range, the annealing time at temperatures higher than the onset of crystallization was reduced to 1 h at 600 °C and 0.5 h at 700 °C. Fig. 3 demonstrates the fully crystallized structures obtained by annealing the (Fe0.8Cr0.2)81 B17W2 alloy at different conditions. It is seen that pre-crystallization of the material at lower temperatures results in the formation of the multiphase homogeneous structure with approximately equal grain sizes of different phases (Fig. 3a, b). In contrast, a higher temperature annealing at 600 °C alone leads to a segregation of the phase seemingly crystallized last, along the grain boundaries of the prior crystallized phases, thus forming a continuous network (Fig. 3c). Such a structure demonstrated a very high degree of thermal stability and an increase in annealing time up to 1000 h (at 600 °C) has not revealed any significant grain growth. Considerable grain growth, even at shorter times of annealing, was observed after heat treatment at 700 °C (Fig. 3d). It
Table 1 Microhardness of the (Fe0.8Cr0.2)81B17W2 alloy crystallized at different conditions
Fig. 1. DSC curves at heating rate of 40 °C/min.
Annealing temperature, °C
Annealing time, h
Microhardness, GPa
Initial materials, unannealed 450 500 600 700
100 100 100 100
11.0 ± 0.2 17.5 ± 0.6 18.2 ± 0.8 16.2 ± 0.6 11.55 ± 0.3
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Fig. 3. Microstructures of the alloy crystallized by different thermal treatments: (a) 450 °C for 100 h and 600 °C for 1 h; (b) 500 °C for 100 h and 600 °C for 1 h; (c) 600 °C for 100 h; (d) 700 °C for 0.5 h.
should be noted that tensile tests of the crystallized alloy (Fig. 4) revealed very different mechanical behavior of the samples with approximately equal matrix grain sizes (Fig. 3b, c) but different morphology of the crystallized phases.
4. Discussions The possibility of nanocrystallization through annealing has been demonstrated in previous studies on the crystallization of Febased metallic glasses [6,7,9–11]. All of these investigations included information on the formation of nanocrystals inside a residual amorphous matrix. In the current work it is shown that metallic glasses can be successfully used for the production of crystallized nanocrystalline materials with different phase morphology by annealing under specific conditions. Some evidence of the effect of annealing conditions on the size, morphology and composition of the crystallization product has been already reported in the literature. It was found [7] that the decrease of annealing temperature of Fe83B17 results in much smaller crystals and a change in morphology. Instead of dendrites with an arm span of ∼ 100 nm, spherical crystals with an average diameter of ∼ 35 nm were observed. In the current work, by applying two-step annealing (at 500 °C for 100 h and 600 °C for 1 h) the multiphase homogeneous structure with approximately equal grain sizes of different phases (Fig. 3b) was obtained in the (Fe0.8Cr0.2)81B17W2 alloy. In this state the alloy has demonstrated the highest microhardness during
the current investigation. A decrease in the pre-annealing temperature to 450 °C increases the inhomogeneity of the microstructure (Fig. 3a). The volume fraction of primary crystals seems to be much smaller after annealing at 450 °C. In this case, crystallization of the residual amorphous phase by annealing at higher temperature leads to inhomogeneity of the microstructure and a decrease in hardness. A completely different microstructure is formed in the alloy after annealing at temperatures above the onset of crystallization.
Fig. 4. Stress–stain curves of the alloy after two-step (curve A) and one-step (curve B) annealing.
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At 600 °C (∼ 0.5Tm) the crystallization process seems to proceed very rapidly. Nanosized grains of the crystallized phases suggest a high nucleation rate at this temperature. Earlier reports in the literature agree with this findings, where very small crystallites formed after annealing at ∼ 0.5 of melting temperature [3]. In this situation the nucleation rate was shown to be maximized while the grain growth rate was limited. Nucleation of the high number of primary crystals of α-Fe (Fig. 2) might result in increased concentration of Al and C in the amorphous phase surrounding the nucleated crystals which is later transformed to the crystalline state. The presence of the crystalline phase along the grain boundaries of the primary crystals retards intergranular diffusion, and as a result, this microstructure has demonstrated high thermal stability at 600 °C. Annealing at this temperature for up to 1000 h has still not revealed any significant changes in the microstructure, which is similar to that shown in Fig. 3c. At higher temperatures (700 °C) even short annealing results in a considerable grain growth (Fig. 3d). A change in the microstructural morphology of the alloy strongly affects its elevated temperature mechanical behavior (Fig. 4). After annealing at 600 °C for 100 h (microstructure in Fig. 3c) the alloy shows much higher yield stress and considerably lower ultimate strength (Fig. 4, curve B) as compared to the equiaxed grained structure with approximately the same grain sizes (microstructure in Fig. 3b) as in curve A (Fig. 4). In this case, test temperature was chosen to be higher than annealing conditions for both states of the alloy microstructure. These results have clearly demonstrated the influence of annealing conditions on the nature of crystallization products and, as a result, on the mechanical behavior of the alloy. Additional experimental work is planned in order to understand the mechanisms responsible for the plastic flow of the nanocrystalline material with different phase morphology at elevated temperatures. 5. Conclusions Fully crystallized nanocrystalline materials have been produced by controlled annealing of Fe-based metallic glass. Phase composition and phase morphology were strongly
affected by the annealing conditions. The material crystallized by a two-step annealing process, which leads to the formation of a structure with approximately equal grain sizes of the two phases, has demonstrated high strength (more than 900 MPa) at an elevated temperature (0.7Tm). The tensile strength of the alloy after one-step annealing at 600 °C (T N Tx) decreases significantly although it has a structure with approximately the same grain size but with different phase morphology. In this case, one of the phases is located along the grain boundaries and results in high thermal stability of the structure. The results show that phase morphology and phase distribution should be taken into account in addition to grain size and grain size distribution during analysis of the microstructure/ property relationship in nanocrystalline materials. Acknowledgements This material is based upon a work supported in part by the National Science Foundation under Grant NSF-DMR-0240144. The authors would like also to acknowledge the support from the Nanosteel Company, Idaho, 83402. References [1] S. Jose, A.S. Aprem, B. Francis, M.C. Chandy, P. Werner, V. Alstaedt, S. Thomas, Eur. Polym. J. 40 (2004) 2105. [2] S. Bugat, J. Besson, A.-F. Gourgues, F. N'Guyen, A. Pineau, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process1. 317 (2001) 32. [3] K. Lu, Mater. Sci. Eng., R Rep. 16 (1996) 161. [4] D. Crespo, T. Pradell, N. Clavaguera, M.T. Clavaguera-Mora, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 238 (1997) 160. [5] U. Köster, U. Schünemann, M. Blank-Bewersdorff, S. Brauer, M. Sutton, G.B. Stephenson, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 133 (1991) 611. [6] M.S. Leu, T.S. Chin, Mater. Res. Soc. Symp. Proc. 577 (1999) 557. [7] T.J. Kulik, J. Non-Cryst. Solids 287 (2001) 145. [8] Y.Q. Wu, T. Bitoh, K. Hono, A. Makimo, A. Inoue, Acta Mater. 49 (2001) 4069. [9] T. Kulik, J. Ferenc, H. Matyja, Mater. Sci. Forum 235–238 (1997) 421. [10] W.J. Botta, D. Negri, A.R. Yavari, Mater. Sci. Forum 312–314 (1999) 387. [11] K. Hono, D.H. Ping, S. Hirosawa, Mater. Res. Soc. Symp. Proc. 577 (1999) 507.
Phase morphology effect on elevated temperature mechanical behavior of nanostructures A.V. Sergueeva a,⁎, N.A. Mara a , D.J. Branagan b , A.K. Mukherjee a a
University of California, Chemical Engineering and Material Science Department, Davis, CA 95616, USA b Institute of NanoMaterials Research and Development, The NanoSteel Company, 505 Lindsay Boulevard, Idaho Falls, Idaho, 83402, USA Received 7 October 2005; accepted 20 July 2006 Available online 4 August 2006
Abstract A devitrification procedure by annealing was applied to a multicomponent Fe-based metallic glass in order to obtain nanocrystalline materials. Phase composition and phase morphology were strongly dependent on the annealing conditions. An elevated temperature mechanical behavior of nanostructures was evaluated by tensile testing. A strong effect of phase morphology on the mechanical response of the material was revealed. A most attractive combination of strength and plasticity was observed in the nanostructure with approximately equal grain sizes of crystallized phases. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Mechanical properties
1. Introduction Together with constitution and scale, the phase morphology of a material is known to be critical in determining its properties. For example, mechanical properties such as tensile strength, tensile elongation and impact strength for a particular polymer blend vary considerably with the morphology [1]. Phase morphology in stainless steel has also a strong influence on the material behavior [2]. At the same time, an analysis of the structure/properties relationships in nanomaterials is mostly restricted to a grain size effect alone. Recently, crystallization of amorphous solids has been successfully used as one of the methods of nanocrystalline material production in various alloy systems, e.g. in Fe-, Ni-, and Co-based alloys, as well as in some elements (e.g. Se, Si) [3]. To obtain a nanoscale structure, the crystallization process should proceed with the largest nucleation rate possible while suppressing the crystal growth rate as much as practicable. Such conditions can be obtained for some alloy compositions by applying specific methods of heat ⁎ Corresponding author. E-mail address: [email protected] (A.V. Sergueeva). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.055
treatment. It was shown theoretically by Crespo et al. [4] and experimentally by Köster et al. [5] that among metallic glasses those that undergo primary crystallization with a time-dependent, long-range diffusion controlled growth rate are the most suitable candidates for nanocrystallization. Many Fe-based metallic glasses do not require any special heat treatment to be converted into nanocrystalline materials [6–8]. Isothermal annealing at temperatures close to the crystallization temperature for a typical time of 1 h, i.e., the so-called conventional annealing, is widely used by many researchers [7]. On the other hand, it was found [9] that the application of relatively low temperature and longer annealing times also facilitates the creation of a nanocrystalline structure. The size, morphology of crystallites, mechanism of crystallization and crystallization products themselves depend on the temperature of thermal treatment of a metallic glass. In order to develop the best strategy to attain maximum improvement in ultimate properties of nanostructures the critical factors on which these properties rely must be determined including understanding on the phase morphology and crystallization behavior of the metallic glasses. In this investigation, a devitrification procedure of trough annealing was applied to a multicomponent Fe-based metallic glass. Tensile testing was applied in order to evaluate an elevated
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temperature mechanical behavior of nanostructures as a function of phase morphology in the crystallized metallic glass. 2. Experimental Fe-based melt-spun metallic ribbons of (Fe0.8Cr0.2)81B17W2 alloy were used in the current investigation. Initial ribbons were formed by first induction melting followed by ejecting the liquid melt onto a rapidly moving nitrogen cooled copper chill wheel. Heat treatment with constant heating rate to predetermined temperatures, followed by furnace cooling was applied for forming nanocrystalline structures. The size, morphology and composition of the crystallization products were investigated using TEM and X-ray analysis. Microhardness measurements and tensile tests on a custom-built computer controlled constant strain rate tensile test machine were conducted to determine correlation between microstructural characteristics and mechanical properties of the material. Differential scanning calorimeter (DSC) measurements were carried out between 25 °C and 725 °C with a heating rate of 40 °C/min, which mimics the heating rate found in our tensile apparatus. Fig. 2. X-ray data for the alloy after annealing for 100 h at different temperatures.
3. Results DSC measurements have revealed one exothermic peak corresponding to the crystallization process. The material shows the onset of crystallization (Tx) at 520 °C at a heating rate of 40 °C/min (Fig. 1). Ribbons in the as-cast state were annealed for 100 h at temperatures below (450 °C and 500 °C) and above (600 °C and 700 °C) the onset of the crystallization. X-ray data for the ribbons in the initial and annealed states are shown in Fig. 2. It is seen that the initial ribbons are not fully amorphous and some crystallites of α-Fe are present. Annealing of the material for 100 h, even at 500 °C that is below Tx determined by DSC (Fig. 1) results in crystallization of additional phases one of which is Fe3B (Fig. 2). Significant broadening of the peaks suggests an extremely small grain size. A detailed analysis of the phase composition is part of an ongoing investigation. An increase in annealing temperature leads to more extensive crystallization of α-Fe phase. The process of crystallization at T b Tx results in a considerable increase in the microhardness of the material
(Table 1) with a peak at around 500 °C. With increasing annealing temperature above Tx, a decrease in hardness was observed down to the value of the initial state (∼ 11 GPa). Some residual amorphous phase was suggested to exist after annealing at temperatures below the onset of crystallization (450 °C and 500 °C). In order to complete the crystallization process, an additional annealing at 600 °C for 1 h was applied. Moreover, to preserve a resulting grain size in the nanometer range, the annealing time at temperatures higher than the onset of crystallization was reduced to 1 h at 600 °C and 0.5 h at 700 °C. Fig. 3 demonstrates the fully crystallized structures obtained by annealing the (Fe0.8Cr0.2)81 B17W2 alloy at different conditions. It is seen that pre-crystallization of the material at lower temperatures results in the formation of the multiphase homogeneous structure with approximately equal grain sizes of different phases (Fig. 3a, b). In contrast, a higher temperature annealing at 600 °C alone leads to a segregation of the phase seemingly crystallized last, along the grain boundaries of the prior crystallized phases, thus forming a continuous network (Fig. 3c). Such a structure demonstrated a very high degree of thermal stability and an increase in annealing time up to 1000 h (at 600 °C) has not revealed any significant grain growth. Considerable grain growth, even at shorter times of annealing, was observed after heat treatment at 700 °C (Fig. 3d). It
Table 1 Microhardness of the (Fe0.8Cr0.2)81B17W2 alloy crystallized at different conditions
Fig. 1. DSC curves at heating rate of 40 °C/min.
Annealing temperature, °C
Annealing time, h
Microhardness, GPa
Initial materials, unannealed 450 500 600 700
100 100 100 100
11.0 ± 0.2 17.5 ± 0.6 18.2 ± 0.8 16.2 ± 0.6 11.55 ± 0.3
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Fig. 3. Microstructures of the alloy crystallized by different thermal treatments: (a) 450 °C for 100 h and 600 °C for 1 h; (b) 500 °C for 100 h and 600 °C for 1 h; (c) 600 °C for 100 h; (d) 700 °C for 0.5 h.
should be noted that tensile tests of the crystallized alloy (Fig. 4) revealed very different mechanical behavior of the samples with approximately equal matrix grain sizes (Fig. 3b, c) but different morphology of the crystallized phases.
4. Discussions The possibility of nanocrystallization through annealing has been demonstrated in previous studies on the crystallization of Febased metallic glasses [6,7,9–11]. All of these investigations included information on the formation of nanocrystals inside a residual amorphous matrix. In the current work it is shown that metallic glasses can be successfully used for the production of crystallized nanocrystalline materials with different phase morphology by annealing under specific conditions. Some evidence of the effect of annealing conditions on the size, morphology and composition of the crystallization product has been already reported in the literature. It was found [7] that the decrease of annealing temperature of Fe83B17 results in much smaller crystals and a change in morphology. Instead of dendrites with an arm span of ∼ 100 nm, spherical crystals with an average diameter of ∼ 35 nm were observed. In the current work, by applying two-step annealing (at 500 °C for 100 h and 600 °C for 1 h) the multiphase homogeneous structure with approximately equal grain sizes of different phases (Fig. 3b) was obtained in the (Fe0.8Cr0.2)81B17W2 alloy. In this state the alloy has demonstrated the highest microhardness during
the current investigation. A decrease in the pre-annealing temperature to 450 °C increases the inhomogeneity of the microstructure (Fig. 3a). The volume fraction of primary crystals seems to be much smaller after annealing at 450 °C. In this case, crystallization of the residual amorphous phase by annealing at higher temperature leads to inhomogeneity of the microstructure and a decrease in hardness. A completely different microstructure is formed in the alloy after annealing at temperatures above the onset of crystallization.
Fig. 4. Stress–stain curves of the alloy after two-step (curve A) and one-step (curve B) annealing.
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At 600 °C (∼ 0.5Tm) the crystallization process seems to proceed very rapidly. Nanosized grains of the crystallized phases suggest a high nucleation rate at this temperature. Earlier reports in the literature agree with this findings, where very small crystallites formed after annealing at ∼ 0.5 of melting temperature [3]. In this situation the nucleation rate was shown to be maximized while the grain growth rate was limited. Nucleation of the high number of primary crystals of α-Fe (Fig. 2) might result in increased concentration of Al and C in the amorphous phase surrounding the nucleated crystals which is later transformed to the crystalline state. The presence of the crystalline phase along the grain boundaries of the primary crystals retards intergranular diffusion, and as a result, this microstructure has demonstrated high thermal stability at 600 °C. Annealing at this temperature for up to 1000 h has still not revealed any significant changes in the microstructure, which is similar to that shown in Fig. 3c. At higher temperatures (700 °C) even short annealing results in a considerable grain growth (Fig. 3d). A change in the microstructural morphology of the alloy strongly affects its elevated temperature mechanical behavior (Fig. 4). After annealing at 600 °C for 100 h (microstructure in Fig. 3c) the alloy shows much higher yield stress and considerably lower ultimate strength (Fig. 4, curve B) as compared to the equiaxed grained structure with approximately the same grain sizes (microstructure in Fig. 3b) as in curve A (Fig. 4). In this case, test temperature was chosen to be higher than annealing conditions for both states of the alloy microstructure. These results have clearly demonstrated the influence of annealing conditions on the nature of crystallization products and, as a result, on the mechanical behavior of the alloy. Additional experimental work is planned in order to understand the mechanisms responsible for the plastic flow of the nanocrystalline material with different phase morphology at elevated temperatures. 5. Conclusions Fully crystallized nanocrystalline materials have been produced by controlled annealing of Fe-based metallic glass. Phase composition and phase morphology were strongly
affected by the annealing conditions. The material crystallized by a two-step annealing process, which leads to the formation of a structure with approximately equal grain sizes of the two phases, has demonstrated high strength (more than 900 MPa) at an elevated temperature (0.7Tm). The tensile strength of the alloy after one-step annealing at 600 °C (T N Tx) decreases significantly although it has a structure with approximately the same grain size but with different phase morphology. In this case, one of the phases is located along the grain boundaries and results in high thermal stability of the structure. The results show that phase morphology and phase distribution should be taken into account in addition to grain size and grain size distribution during analysis of the microstructure/ property relationship in nanocrystalline materials. Acknowledgements This material is based upon a work supported in part by the National Science Foundation under Grant NSF-DMR-0240144. The authors would like also to acknowledge the support from the Nanosteel Company, Idaho, 83402. References [1] S. Jose, A.S. Aprem, B. Francis, M.C. Chandy, P. Werner, V. Alstaedt, S. Thomas, Eur. Polym. J. 40 (2004) 2105. [2] S. Bugat, J. Besson, A.-F. Gourgues, F. N'Guyen, A. Pineau, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process1. 317 (2001) 32. [3] K. Lu, Mater. Sci. Eng., R Rep. 16 (1996) 161. [4] D. Crespo, T. Pradell, N. Clavaguera, M.T. Clavaguera-Mora, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 238 (1997) 160. [5] U. Köster, U. Schünemann, M. Blank-Bewersdorff, S. Brauer, M. Sutton, G.B. Stephenson, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 133 (1991) 611. [6] M.S. Leu, T.S. Chin, Mater. Res. Soc. Symp. Proc. 577 (1999) 557. [7] T.J. Kulik, J. Non-Cryst. Solids 287 (2001) 145. [8] Y.Q. Wu, T. Bitoh, K. Hono, A. Makimo, A. Inoue, Acta Mater. 49 (2001) 4069. [9] T. Kulik, J. Ferenc, H. Matyja, Mater. Sci. Forum 235–238 (1997) 421. [10] W.J. Botta, D. Negri, A.R. Yavari, Mater. Sci. Forum 312–314 (1999) 387. [11] K. Hono, D.H. Ping, S. Hirosawa, Mater. Res. Soc. Symp. Proc. 577 (1999) 507.