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Subgrain and dislocation structure changes in hot-deformed high-temperature Fe–Ni austenitic alloy
Materials Chemistry and Physics 81 (2003) 493–495
Subgrain and dislocation structure changes in hot-deformed high-temperature Fe–Ni austenitic alloy K.J. Ducki∗ , K. Rodak, M. Hetma´nczyk, D. Kuc Department of Materials Science, Silesian University of Technology, 8 Krasinskiego Street, Katowice 40-019, Poland
Abstract The influence of plastic deformation on the substructure of a high-temperature austenitic Fe–Ni alloy has been presented. Hot-torsion tests were executed at constant strain rates of 0.1 and 1.0 s−1 , at testing temperatures in the range 900–1150 ◦ C. The examination of the microstructure was carried out, using transmission electron microscopy. Direct measurements on the micrographs allowed the calculation of structural parameters: the average subgrain area, and the mean dislocation density. A detailed investigation has shown that the microstructure is inhomogeneous, consisting of dense dislocation walls, subgrains and recrystallized regions. © 2003 Elsevier Science B.V. All rights reserved. Keywords: A-286 alloy; Torsion test; TEM; Deformation; Dislocations; Recrystallization
1. Introduction The behaviour of metals and alloys during hot plastic working has a complex nature and it varies with changing process parameters such as deformation, strain rate and temperature [1]. High-temperature plastic deformation is related to the dynamic processes of recovery and recrystallization, which influence the structure and properties of alloys. These processes have a particular significance for the determination of the mechanisms of hot plastic deformation and the relationships between strain parameters and structure and properties of the material. In this work, the influence of the initial austenite grain size and parameters of hot plastic deformation on the subgrain and dislocation structure changes has been examined in a high-temperature austenitic Fe–Ni alloy of the A-286 type, precipitation hardened by the ␥ phase.
2. Material and procedure The examinations were performed on rolled bars, 16 mm in diameter, of an austenitic Fe–Ni alloy designated as H15N25T2M. The chemical composition is given Table 1. Structural stability of the alloy was achieved by two variants of preheating, i.e. 1100 ◦ C/2 h and 1150 ◦ C/2 h with subsequent cooling in water. The alloy was heat-treated ∗ Corresponding author. Tel.: +48-32-256-3197; fax: +48-32-256-3197. E-mail address: [email protected] (K.J. Ducki).
under the conditions typical for hot plastic deformations of Fe–Ni austenitic alloys [2]. The examinations of alloy formability were performed by a hot-torsion method using a Setaram torsion plastometer. The plastometric tests were executed at constant strain rates of 0.1 and 1.0 s−1 , with a testing temperature in the range 900–1150 ◦ C. The tests were conducted until total fracture of the samples occurred. Structural inspections were performed on longitudinal microsections taken from the plastically deformed samples after so-called “freezing”, i.e. rapid cooling of the samples in water from the deformation temperature. The examination of the microstructure was carried out using transmission electron microscopy (TEM) with a Jeol JEM-100B. Direct measurements on the TEM micrographs allowed the calculation ¯ of the structural parameters: the average subgrain area, A, and the mean dislocation density, ρ. The mean subgrain areas were determined by a planimetric method with the help of a semi-automatic picture analyser of MOP AMO 3 type. The measurements were conducted on the TEM pictures. The analysed microsections of thin foils involved measurements of about 150 subgrains for each sample. The mean dislocation density was calculated by use of a method based on counting the inter-section points of a network superimposed over the micrograph with dislocation lines. The dislocation density (ρ) was defined for the thin foils according to the relation given by Klaar et al. [3]: ρ=
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00051-8
x(n1 / l1 + n2 / l2 ) t
(m−2 )
(1)
494
K.J. Ducki et al. / Materials Chemistry and Physics 81 (2003) 493–495
Table 1 Chemical composition of the investigated alloy (wt.%) C
Si
Mn
P
S
Cr
Ni
Mo
V
Ti
Al
B
0.05
0.61
1.32
0.028
0.005
13.70
24.30
1.30
0.42
1.90
0.22
0.010
where x is the fraction of invisible dislocations with a Burgers vectors a/2<1 1 1> for the A1 lattice, l1(2) the total length of the horizontal (vertical) lattice lines, n1(2) the number of intersections of the horizontal (vertical) lattice lines with dislocations, and t the thickness of foil.
locations with the creation of new subgrain boundaries and partitions of a polygonal type (Fig. 3). The course of changes in subgrain size as functions of temperature and strain rate are presented in Figs. 4 and 5. An
3. Experimental results and discussion The Fe–Ni alloy in its initial state was characterized by an austenitic structure with an insignificant quantity of undissolved precipitates, i.e. TiC, TiN, Ti4 C2 S2 and Fe2 Ti [4]. The mean density of dislocations in the supersaturated material amounted to ρ = 4.88×1012 m−2 (after 1100 ◦ C/2 h) and ρ = 3.99 × 1012 m−2 (after 1150 ◦ C/2 h). After 900 ◦ C deformation of samples at a rate of 0.1 s−1 , the alloy structure appeared in the advanced stages of a dynamic recovery process. In the austenitic regions of high dislocation density a cellular dislocation structure was found together with subgrains with various densities of dislocations (Fig. 1). In samples deformed at 950 ◦ C and at a rate of 1.0 s−1 , nuclei of recrystallized grains were noticed in the subgrain matrix and the recrystallized micro-areas. The initiated process of dynamic recrystallization is characteristic for both variants of pre-soaking adopted in the experiment. The alloy deformed in the range of 1000–1050 ◦ C exhibited the properties characteristic of dynamic recrystallized structures. The structure of austenite was composed mainly of recrystallized grains free of dislocations, while within the subgrains a progressive process of further improvement of substructure was observed (Fig. 2). At the highest deformation temperatures, in the range of 1100–1150 ◦ C, a constant process of reconstruction was observed in the alloy, named repolygonization. It comprised a new regrouping of the dis-
Fig. 1. Substructure of the alloy after deformation at 900 ◦ C at a rate of 0.1 s−1 . Cellular dislocation substructure. Pre-soaking of the alloy: 1100 ◦ C/2 h.
Fig. 2. Substructure of the alloy after deformation at 1000 ◦ C at a rate of 0.1 s−1 . Nucleation of a new recrystallized grain. Pre-soaking of the alloy: 1150 ◦ C/2 h.
Fig. 3. Substructure of the alloy after deformation at 1100 ◦ C at a rate 0.1 s−1 . Process of austenite repolygonization. Pre-soaking of the alloy: 1150 ◦ C/2 h.
Fig. 4. Influence of temperature and strain rate on the mean subgrain size. Pre-soaking of the alloy: 1100 ◦ C/2 h.
K.J. Ducki et al. / Materials Chemistry and Physics 81 (2003) 493–495
495
Fig. 5. Influence of temperature and strain rate on the mean subgrain size. Pre-soaking of the alloy: 1150 ◦ C/2 h.
Fig. 7. Influence of temperature and strain rate on the mean dislocation density. Pre-soaking of the alloy: 1150 ◦ C/2 h.
increase of the deformation temperature was accompanied by an increase of the subgrain size. The mean size of the subgrains varied from 1.0 to 7.8 m2 . The influence of strain rate on the size of the subgrains depended substantially on the pre-soaking temperature of the alloy, i.e. the initial size of the austenite grains. The course of dislocation density changes as functions of temperature and strain rate are presented in Figs. 6 and 7. An increase of the deformation temperature was accompanied by a decrease of the dislocation density. The values of the dislocation density varied in the range 1013 –1014 m−2 . For both variants of pre-soaking of the alloy, the higher dislocation densities were obtained for the smaller strain rate, 0.1 s−1 . This could be explained by a mechanism of cyclic dynamic recrystallization, operating at small strain rates and resulting in a multiple strengthening and recrystallization of the alloy structure.
4. Summary The performed examinations, concerned with the influence of hot-working parameters upon the substructure of austenitic Fe–Ni alloy, revealed the subsequently occurring processes of dynamic recovery, recrystallization and repolygonization. None of the discovered stages of alloy substructure transformation constitutes a self-contained process. The temperature of the process is an essential technological parameter, which has an influence on the size of the subgrains and dislocation density. The increase of the alloy deformation temperature led to a growth of the size of subgrains with a simultaneous decrease of their internal dislocation density. The influence of the strain rate of the alloy on the size of subgrains and dislocation density is complex in character and depends on the initial size of the austenite grains and on the mechanism of the dynamic recrystallization process. Acknowledgements This work was supported by the Committee of Scientific Research of Poland under grant No.7 T08A 038 18. References
Fig. 6. Influence of temperature and strain rate on the mean dislocation density. Pre-soaking of the alloy: 1100 ◦ C/2 h.
[1] L.X. Zhou, T.N. Baker, Mater. Sci. Eng. A 177 (1994) 1. [2] M. Kohno, T. Yamada, A. Susuki, S. Ohta, in: Proceedings of the Internationale Schmiedetagung, Verein Deutscher Eisenhüttenleute, Düsseldorf, 1981, 4.1.1. [3] H.J. Klaar, P. Schwaab, Ö. Werner, Prakt. Metallogr. 29 (1992) 3. [4] K. Ducki, M. Hetmaczyk, Mater. Eng., Sigma NOT, Warsaw 4 (2001) 290.
Subgrain and dislocation structure changes in hot-deformed high-temperature Fe–Ni austenitic alloy K.J. Ducki∗ , K. Rodak, M. Hetma´nczyk, D. Kuc Department of Materials Science, Silesian University of Technology, 8 Krasinskiego Street, Katowice 40-019, Poland
Abstract The influence of plastic deformation on the substructure of a high-temperature austenitic Fe–Ni alloy has been presented. Hot-torsion tests were executed at constant strain rates of 0.1 and 1.0 s−1 , at testing temperatures in the range 900–1150 ◦ C. The examination of the microstructure was carried out, using transmission electron microscopy. Direct measurements on the micrographs allowed the calculation of structural parameters: the average subgrain area, and the mean dislocation density. A detailed investigation has shown that the microstructure is inhomogeneous, consisting of dense dislocation walls, subgrains and recrystallized regions. © 2003 Elsevier Science B.V. All rights reserved. Keywords: A-286 alloy; Torsion test; TEM; Deformation; Dislocations; Recrystallization
1. Introduction The behaviour of metals and alloys during hot plastic working has a complex nature and it varies with changing process parameters such as deformation, strain rate and temperature [1]. High-temperature plastic deformation is related to the dynamic processes of recovery and recrystallization, which influence the structure and properties of alloys. These processes have a particular significance for the determination of the mechanisms of hot plastic deformation and the relationships between strain parameters and structure and properties of the material. In this work, the influence of the initial austenite grain size and parameters of hot plastic deformation on the subgrain and dislocation structure changes has been examined in a high-temperature austenitic Fe–Ni alloy of the A-286 type, precipitation hardened by the ␥ phase.
2. Material and procedure The examinations were performed on rolled bars, 16 mm in diameter, of an austenitic Fe–Ni alloy designated as H15N25T2M. The chemical composition is given Table 1. Structural stability of the alloy was achieved by two variants of preheating, i.e. 1100 ◦ C/2 h and 1150 ◦ C/2 h with subsequent cooling in water. The alloy was heat-treated ∗ Corresponding author. Tel.: +48-32-256-3197; fax: +48-32-256-3197. E-mail address: [email protected] (K.J. Ducki).
under the conditions typical for hot plastic deformations of Fe–Ni austenitic alloys [2]. The examinations of alloy formability were performed by a hot-torsion method using a Setaram torsion plastometer. The plastometric tests were executed at constant strain rates of 0.1 and 1.0 s−1 , with a testing temperature in the range 900–1150 ◦ C. The tests were conducted until total fracture of the samples occurred. Structural inspections were performed on longitudinal microsections taken from the plastically deformed samples after so-called “freezing”, i.e. rapid cooling of the samples in water from the deformation temperature. The examination of the microstructure was carried out using transmission electron microscopy (TEM) with a Jeol JEM-100B. Direct measurements on the TEM micrographs allowed the calculation ¯ of the structural parameters: the average subgrain area, A, and the mean dislocation density, ρ. The mean subgrain areas were determined by a planimetric method with the help of a semi-automatic picture analyser of MOP AMO 3 type. The measurements were conducted on the TEM pictures. The analysed microsections of thin foils involved measurements of about 150 subgrains for each sample. The mean dislocation density was calculated by use of a method based on counting the inter-section points of a network superimposed over the micrograph with dislocation lines. The dislocation density (ρ) was defined for the thin foils according to the relation given by Klaar et al. [3]: ρ=
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00051-8
x(n1 / l1 + n2 / l2 ) t
(m−2 )
(1)
494
K.J. Ducki et al. / Materials Chemistry and Physics 81 (2003) 493–495
Table 1 Chemical composition of the investigated alloy (wt.%) C
Si
Mn
P
S
Cr
Ni
Mo
V
Ti
Al
B
0.05
0.61
1.32
0.028
0.005
13.70
24.30
1.30
0.42
1.90
0.22
0.010
where x is the fraction of invisible dislocations with a Burgers vectors a/2<1 1 1> for the A1 lattice, l1(2) the total length of the horizontal (vertical) lattice lines, n1(2) the number of intersections of the horizontal (vertical) lattice lines with dislocations, and t the thickness of foil.
locations with the creation of new subgrain boundaries and partitions of a polygonal type (Fig. 3). The course of changes in subgrain size as functions of temperature and strain rate are presented in Figs. 4 and 5. An
3. Experimental results and discussion The Fe–Ni alloy in its initial state was characterized by an austenitic structure with an insignificant quantity of undissolved precipitates, i.e. TiC, TiN, Ti4 C2 S2 and Fe2 Ti [4]. The mean density of dislocations in the supersaturated material amounted to ρ = 4.88×1012 m−2 (after 1100 ◦ C/2 h) and ρ = 3.99 × 1012 m−2 (after 1150 ◦ C/2 h). After 900 ◦ C deformation of samples at a rate of 0.1 s−1 , the alloy structure appeared in the advanced stages of a dynamic recovery process. In the austenitic regions of high dislocation density a cellular dislocation structure was found together with subgrains with various densities of dislocations (Fig. 1). In samples deformed at 950 ◦ C and at a rate of 1.0 s−1 , nuclei of recrystallized grains were noticed in the subgrain matrix and the recrystallized micro-areas. The initiated process of dynamic recrystallization is characteristic for both variants of pre-soaking adopted in the experiment. The alloy deformed in the range of 1000–1050 ◦ C exhibited the properties characteristic of dynamic recrystallized structures. The structure of austenite was composed mainly of recrystallized grains free of dislocations, while within the subgrains a progressive process of further improvement of substructure was observed (Fig. 2). At the highest deformation temperatures, in the range of 1100–1150 ◦ C, a constant process of reconstruction was observed in the alloy, named repolygonization. It comprised a new regrouping of the dis-
Fig. 1. Substructure of the alloy after deformation at 900 ◦ C at a rate of 0.1 s−1 . Cellular dislocation substructure. Pre-soaking of the alloy: 1100 ◦ C/2 h.
Fig. 2. Substructure of the alloy after deformation at 1000 ◦ C at a rate of 0.1 s−1 . Nucleation of a new recrystallized grain. Pre-soaking of the alloy: 1150 ◦ C/2 h.
Fig. 3. Substructure of the alloy after deformation at 1100 ◦ C at a rate 0.1 s−1 . Process of austenite repolygonization. Pre-soaking of the alloy: 1150 ◦ C/2 h.
Fig. 4. Influence of temperature and strain rate on the mean subgrain size. Pre-soaking of the alloy: 1100 ◦ C/2 h.
K.J. Ducki et al. / Materials Chemistry and Physics 81 (2003) 493–495
495
Fig. 5. Influence of temperature and strain rate on the mean subgrain size. Pre-soaking of the alloy: 1150 ◦ C/2 h.
Fig. 7. Influence of temperature and strain rate on the mean dislocation density. Pre-soaking of the alloy: 1150 ◦ C/2 h.
increase of the deformation temperature was accompanied by an increase of the subgrain size. The mean size of the subgrains varied from 1.0 to 7.8 m2 . The influence of strain rate on the size of the subgrains depended substantially on the pre-soaking temperature of the alloy, i.e. the initial size of the austenite grains. The course of dislocation density changes as functions of temperature and strain rate are presented in Figs. 6 and 7. An increase of the deformation temperature was accompanied by a decrease of the dislocation density. The values of the dislocation density varied in the range 1013 –1014 m−2 . For both variants of pre-soaking of the alloy, the higher dislocation densities were obtained for the smaller strain rate, 0.1 s−1 . This could be explained by a mechanism of cyclic dynamic recrystallization, operating at small strain rates and resulting in a multiple strengthening and recrystallization of the alloy structure.
4. Summary The performed examinations, concerned with the influence of hot-working parameters upon the substructure of austenitic Fe–Ni alloy, revealed the subsequently occurring processes of dynamic recovery, recrystallization and repolygonization. None of the discovered stages of alloy substructure transformation constitutes a self-contained process. The temperature of the process is an essential technological parameter, which has an influence on the size of the subgrains and dislocation density. The increase of the alloy deformation temperature led to a growth of the size of subgrains with a simultaneous decrease of their internal dislocation density. The influence of the strain rate of the alloy on the size of subgrains and dislocation density is complex in character and depends on the initial size of the austenite grains and on the mechanism of the dynamic recrystallization process. Acknowledgements This work was supported by the Committee of Scientific Research of Poland under grant No.7 T08A 038 18. References
Fig. 6. Influence of temperature and strain rate on the mean dislocation density. Pre-soaking of the alloy: 1100 ◦ C/2 h.
[1] L.X. Zhou, T.N. Baker, Mater. Sci. Eng. A 177 (1994) 1. [2] M. Kohno, T. Yamada, A. Susuki, S. Ohta, in: Proceedings of the Internationale Schmiedetagung, Verein Deutscher Eisenhüttenleute, Düsseldorf, 1981, 4.1.1. [3] H.J. Klaar, P. Schwaab, Ö. Werner, Prakt. Metallogr. 29 (1992) 3. [4] K. Ducki, M. Hetmaczyk, Mater. Eng., Sigma NOT, Warsaw 4 (2001) 290.