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Annealing behavior of an ultrafine-grained Fe–Ni–Mn steel during isothermal aging
Materials Science and Engineering A 503 (2009) 156–159
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Annealing behavior of an ultrafine-grained Fe–Ni–Mn steel during isothermal aging S. Hossein Nedjad a,∗ , M. Nili Ahmadabadi b , T. Furuhara c a
Faculty of Materials Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran School of Metallurgy and Materials Engineering, University of Tehran, P.O. Box 14395-731, Tehran, Iran c Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b
a r t i c l e
i n f o
Article history: Received 31 October 2007 Received in revised form 18 December 2007 Accepted 20 December 2007 Keywords: Nanocrystalline Grain boundary Triple junction Precipitation Coarsening
a b s t r a c t An ultrafine-grained maraging steel was fabricated by cold rolling and aging of iron–nickel–manganese lath martensite. Transmission electron microscopy was used to study precipitation behavior at grain boundaries during isothermal aging at 753 K. Relatively coarse precipitates were found at triple junctions of grain boundaries after aging for 0.36 ks. During further aging, precipitates coarsen at triple junctions at the expense of fine precipitates embedded at ultrafine grains. Mass-conservative dissolution of matrix precipitates occurs homogeneously as a result of which the precipitate-free zone around coarse particulates is eliminated. The phenomena of exclusive precipitation at grain boundary triple junctions and homogenous dissolution of matrix precipitates are attributed to the reduced length scale in the ultrafine-grained microstructure. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ultrafine-grained metallic materials fabricated by severe plastic deformation exhibit extraordinary properties, e.g. paradoxical combination of high strength–good ductility, low temperature–high strain rate superplasticity, etc. Often ultrafine grain formation and the extraordinary properties are directly realized by as-deformed structure which is believed to be thermodynamically metastable due to excess amounts of various lattice defects and elastic distortions [1]. Hence, thermally activated restoration processes, e.g. recovery, recrystallization and grain growth, arise during annealing at high temperatures. Occasionally, precisely controlled recovery and recrystallization of severely deformed structures have been used to augment ultrafine grain formation [2,3]. Nevertheless, high-temperature annealing treatments in the grain growth regime degrade ultrafine-grained structures and, therefore, caution is needed to avoid exposure of bulk nanomaterials at high temperatures [4,5]. Previous studies on the annealing behavior of severely deformed steels have frequently demonstrated two-phase microstructures consisting of submicrocrystalline cementite particulates and ultrafine ferrite grains in the steels annealed at relatively high temperatures.
∗ Corresponding author. Tel.: +98 412 345 9449; fax: +98 412 344 4333. E-mail address: [email protected] (S.H. Nedjad). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.12.054
A few researches have been carried out on the thermal stability and kinetics of microstructural evolutions during annealing of severely deformed steels [6–8]. Nevertheless, details of microstructural evolutions during annealing of severely deformed steels still need to be more clarified. Furthermore, the sequence of grain boundary precipitation reactions, differential coarsening of grain boundary precipitates and precipitate-free zone formation in bulk nanostructured steels has not been exclusively studied yet. Recently, Hossein Nedjad et al. [9] reported on the ultrafine grain formation in a martensitic Fe–Ni–Mn steel fabricated by heavy cold rolling and aging treatment. Following the work, this paper is aimed at better understanding of the annealing behavior and grain boundary precipitation reactions of the cold-rolled steel during isothermal aging. 2. Experimental procedure A vacuum induction melted and vacuum arc remelted Fe–10.35Ni–6.88Mn–0.006C–0.007S–0.005P–0.005N–0.003Al (wt.%) maraging steel was encapsulated in a quartz tube purged with argon after evacuation to 10−5 Torr. Homogenizing treatment was carried out at 1473 K for 172.8 ks followed by water quenching and cryogenic treating at 77 K for 3.6 ks. Cold rolling to 85% thickness reduction was carried out at room temperature followed by isothermal aging treatment at 753 K in a neutralized salt bath. Disc-shaped specimens with a diameter of 3 mm and initial thickness of 300 m were cut using an electric discharge
S.H. Nedjad et al. / Materials Science and Engineering A 503 (2009) 156–159
Fig. 1. Bright-field transmission electron micrograph of a cold-rolled specimen aged for 0.36 ks showing an ultrafine-grained microstructure. Inset is a selected-area electron diffraction pattern illustrating high orientation spread among ultrafine grains.
wire cutting machine and mechanically polished to a thickness of ca. 30 m. Further thinning was performed in a solution of CrO3 (200 g), CH3 COOH (500 ml) and H2 O (40 ml) held at 285 K with a twin jet TENUPOL-3 electro-polishing machine. Transmission electron microscopy was carried out in a PHILIPS CM200-FEG microscope operating at 200 kV. All observations were made on the lateral plane normal to the transverse direction of the rolled strips. 3. Results Fig. 1 shows a bright-field transmission electron micrograph of a cold-rolled specimen aged for 0.36 ks which demonstrates an ultrafine-grained microstructure consisting of both equiaxed and elongated grains. A selected-area electron diffraction pattern in the inset shows ring-like configuration, illustrating a relatively random orientation of ultrafine grains. Fig. 2a shows an enlarged brightfield transmission electron micrograph representing an equiaxed ultrafine grain in the specimen aged for 0.36 ks. The dark contrast arisen from elastic strain fields adjacent to the grain boundary extinction contours is denoted by an arrow tip. Further, macroscopic distortion or subdivision of the ultrafine grain is deduced
157
from the heterogeneity of the image contrast. Another brightfield transmission electron micrograph of an ultrafine grain in the specimen aged for 0.36 ks is shown in Fig. 2b illustrating coarse precipitates formed at grain boundary triple junctions as shown by arrow tips. The dark zone adjacent to grain boundaries is again proposed to arise from elastic strain field in the vicinity of grain boundaries. Fig. 3a shows a bright-field transmission electron micrograph of a specimen aged for 86.4 ks demonstrating a multi-phase microstructure consisting of ultrafine grains (white and light gray-colored), coarse particulates (gray-colored) and elongated grains with fine precipitates. A selected-area electron diffraction pattern corresponding to the bright-field micrograph is shown in Fig. 3b. The second-phase particles are identified as face-centered tetragonal (fct) -NiMn intermetallic phase holding a near Kurdjamov–Sachs type orientation relationship with bodycentered cubic (bcc) ␣-iron matrix. Fig. 3c and d shows dark-field micrographs obtained using the spots indicated in Fig. 3b illuminating matrix and second-phase particles, respectively. In Fig. 3c the arrow tip denotes fringes similar to the so-called Ghost fringes as an indicative of solute segregation. Adjacent to the coarse precipitates in Fig. 3d, faint evidence of homogenously dispersed fine precipitates is found around the coarse particles within the ultrafine grain (see arrow tip). 4. Discussion Analogous to the solution-annealed Fe–Ni–Mn maraging steel, cold-rolled steel shows age hardening at intermediate stages which subsequently turns to softening by overaging of precipitates at later stages of aging [10]. The fct -NiMn phase precipitates in the cold-rolled steel both within the ultrafine grains and grain boundary triple junctions in the early stages of aging. By further long-term aging, the fct -NiMn phase coarsens differentially at grain boundary triple junctions at the expense of homophase matrix precipitates within the ultrafine grains. It has been assumed that the ultrafine grains are inherited from deformed structure and, consequently, an extended recovery occurs during isothermal aging which is correlated with coarsening of particles at grain boundary triple junctions [9]. However, peculiarities in the grain boundary precipitation and coarsening behavior are found out in the present steel, e.g. (i) grain boundary precipitates are located only at grain boundary triple junctions while grain boundary planes in a midway between triple junctions are free of coarse precipitates and
Fig. 2. Bright-field transmission electron micrographs of a cold-rolled specimen aged for 0.36 ks showing (a) an ultrafine grain with distortions adjacent to grain boundaries and (b) pronounced precipitation at grain boundary triple junctions.
158
S.H. Nedjad et al. / Materials Science and Engineering A 503 (2009) 156–159
Fig. 3. (a) Bright-field transmission electron micrograph of a specimen aged for 86.4 ks; (b) corresponding selected-area electron diffraction pattern; (c) dark-field transmission ¯ ␣ spot; (d) dark-field transmission micrograph obtained using the (0 2 0) spot. micrograph obtained using the (1¯ 0 1)
(ii) mass-conservative dissolution of matrix precipitates around coarse fct -NiMn particulates occurs homogenously for all of the fine precipitates embedded in an ultrafine grain and, consequently, precipitate-free zone around coarse particulates is not deduced clearly. Both of the aforementioned peculiarities are attributed to the reduced length scale of the ultrafine-grained microstructure. It is assumed that short-grain boundaries of the ultrafine-grained microstructure act as extremely accelerated diffusion pathways for nickel and manganese atoms to build up at triple junctions and, consequently, precipitation reactions take place at grain boundary triple junctions. It is well known that short length of grain boundaries in nanocrystalline materials affects structural periodicity and accommodation of elastic strain fields of grain boundary dislocations at neighboring grains. Those reduced-scale effects along with accumulation of excess amounts of dislocations arrived at grain boundaries during deformation processes give rise to the well-established non-equilibrium nature of short-grain boundaries in the ultrafine-grained materials fabricated by severe plastic deformation [11]. The non-equilibrium short-grain boundaries are proposed to possess higher grain boundary free volume resulting in superdiffusivity at short distances along grain boundaries. Lattice diffusion distance for nickel and manganese atoms through ultrafine grains is also reduced to ultrafine grain diameter during coarsening of the fct NiMn phase. Consequently, the
short diffusion path realizes homogenous dissolution of matrix precipitates, as a result of which conventional precipitate-free zone around coarse particulates does not form subsequently. Any lattice defects inherited in the ultrafine grains from deformation processes may incorporate to enhanced lattice diffusion simultaneously. Further, ultradense distribution of matrix precipitates within the ultrafine grain augments the homogeneity of solute distribution in terms of densely dispersed fine precipitates. Although the principles of precipitate dissolution driven by the concentration gradient through a matrix has been well established already, but the effect of nanometer-sized length scale and excess lattice defects of severely deformed structures on the sequence of precipitate dissolution is not well understood now. The phenomenon needs to be verified in a wide range of nanocrystalline alloys. Nevertheless, to understand the enhanced lattice diffusion and homogenous dissolution of precipitates, it is proposed that local solute exchange between closely spaced precipitates may realize uniform solute drainage from all of the precipitates embedded within the ultrafine grain. A new scheme needs to treat the phenomenon in a quantitative manner by taking the chemical and strain energy gradients at a nanometer scale into account. The aforementioned peculiarities in the precipitation behavior of the present ultrafine-grained steel seem practically overwhelming in terms of microstructure control of nanocrystalline metals. For instance, it is well known that the grain boundary migration
S.H. Nedjad et al. / Materials Science and Engineering A 503 (2009) 156–159
velocity in nanomaterials is manifested by migration velocity of grain boundary triple junctions [11]. Hence, second-phase particles at triple junctions certainly affect triple junction motion. Therefore, precisely controlled precipitation at grain boundary triple junctions of nanocrystalline materials aims higher thermal stability. Further, absence of a precipitate-free zone during second-phase coarsening realizes fabrication of relatively homogenous multi-phase ultrafine-grained alloys in which engineering properties could be tailored by suitable phase compositions and geometries. 5. Conclusions The grain boundary precipitation behavior of cold-rolled nanostructured steel was studied. Main conclusions drawn were as follows: (i) Ultrafine grains with surrounding strained grain boundaries form by cold rolling and aging of a martensitic Fe–Ni–Mn steel. (ii) Pronounced precipitation occurs exclusively at grain boundary triple junctions. The short-grain boundaries between triple junctions are free of coarse precipitates.
159
(iii) During isothermal aging, coarsening of triple junction precipitates proceeds at the expense of homogeneous dissolution of matrix precipitates. (iv) The reduced length scale for precipitation and coarsening alters transformation behavior in the ultrafine-grained microstructure. References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. [2] N. Tsuji, R. Ueji, Y. Minamino, Y. Saito, Scripta Mater. 46 (2002) 305–310. [3] J. Tianfu, G. Yuwei, Q. Guiying, L. Qun, W. Tiansheng, W. Wei, X. Furen, C. Dayong, S. Xinyu, Z. Xin, Mater. Sci. Eng. A 432 (2006) 216–220. [4] J. De Messemaeker, B. Verlinden, J.V. Humbeeck, Mater. Sci. Forum 467–470 (2004) 1295–1300. [5] P.B. Prangnell, J.R. Bowen, M. Berta, P.J. Apps, P.S. Bate, Mater. Sci. Forum 467–470 (2004) 1261–1270. [6] D.H. Shin, Y.S. Kim, E.J. Lavernia, Acta Mater. 49 (2001) 2387–2393. [7] Y. Ivanisenko, R.K. Wunderlich, R.Z. Valiev, H.J. Fecht, Scripta Mater. 49 (2003) 947–952. [8] K.T. Park, D.H. Shin, Mater. Sci. Eng. A334 (2002) 79–86. [9] S. Hossein Nedjad, M. Nili Ahmadabadi, T. Furuhara, Mater. Sci. Eng. A 485 (2008) 544–549. [10] S. Hossein Nedjad, M. Nili Ahmadabadi, T. Furuhara, T. Maki, Solid State Phenom. 114 (2006) 159–164. [11] Y. Gogotsi (Ed.), Nanomaterials Handbook, Taylor & Francis, NW, 2006.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Annealing behavior of an ultrafine-grained Fe–Ni–Mn steel during isothermal aging S. Hossein Nedjad a,∗ , M. Nili Ahmadabadi b , T. Furuhara c a
Faculty of Materials Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran School of Metallurgy and Materials Engineering, University of Tehran, P.O. Box 14395-731, Tehran, Iran c Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b
a r t i c l e
i n f o
Article history: Received 31 October 2007 Received in revised form 18 December 2007 Accepted 20 December 2007 Keywords: Nanocrystalline Grain boundary Triple junction Precipitation Coarsening
a b s t r a c t An ultrafine-grained maraging steel was fabricated by cold rolling and aging of iron–nickel–manganese lath martensite. Transmission electron microscopy was used to study precipitation behavior at grain boundaries during isothermal aging at 753 K. Relatively coarse precipitates were found at triple junctions of grain boundaries after aging for 0.36 ks. During further aging, precipitates coarsen at triple junctions at the expense of fine precipitates embedded at ultrafine grains. Mass-conservative dissolution of matrix precipitates occurs homogeneously as a result of which the precipitate-free zone around coarse particulates is eliminated. The phenomena of exclusive precipitation at grain boundary triple junctions and homogenous dissolution of matrix precipitates are attributed to the reduced length scale in the ultrafine-grained microstructure. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ultrafine-grained metallic materials fabricated by severe plastic deformation exhibit extraordinary properties, e.g. paradoxical combination of high strength–good ductility, low temperature–high strain rate superplasticity, etc. Often ultrafine grain formation and the extraordinary properties are directly realized by as-deformed structure which is believed to be thermodynamically metastable due to excess amounts of various lattice defects and elastic distortions [1]. Hence, thermally activated restoration processes, e.g. recovery, recrystallization and grain growth, arise during annealing at high temperatures. Occasionally, precisely controlled recovery and recrystallization of severely deformed structures have been used to augment ultrafine grain formation [2,3]. Nevertheless, high-temperature annealing treatments in the grain growth regime degrade ultrafine-grained structures and, therefore, caution is needed to avoid exposure of bulk nanomaterials at high temperatures [4,5]. Previous studies on the annealing behavior of severely deformed steels have frequently demonstrated two-phase microstructures consisting of submicrocrystalline cementite particulates and ultrafine ferrite grains in the steels annealed at relatively high temperatures.
∗ Corresponding author. Tel.: +98 412 345 9449; fax: +98 412 344 4333. E-mail address: [email protected] (S.H. Nedjad). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.12.054
A few researches have been carried out on the thermal stability and kinetics of microstructural evolutions during annealing of severely deformed steels [6–8]. Nevertheless, details of microstructural evolutions during annealing of severely deformed steels still need to be more clarified. Furthermore, the sequence of grain boundary precipitation reactions, differential coarsening of grain boundary precipitates and precipitate-free zone formation in bulk nanostructured steels has not been exclusively studied yet. Recently, Hossein Nedjad et al. [9] reported on the ultrafine grain formation in a martensitic Fe–Ni–Mn steel fabricated by heavy cold rolling and aging treatment. Following the work, this paper is aimed at better understanding of the annealing behavior and grain boundary precipitation reactions of the cold-rolled steel during isothermal aging. 2. Experimental procedure A vacuum induction melted and vacuum arc remelted Fe–10.35Ni–6.88Mn–0.006C–0.007S–0.005P–0.005N–0.003Al (wt.%) maraging steel was encapsulated in a quartz tube purged with argon after evacuation to 10−5 Torr. Homogenizing treatment was carried out at 1473 K for 172.8 ks followed by water quenching and cryogenic treating at 77 K for 3.6 ks. Cold rolling to 85% thickness reduction was carried out at room temperature followed by isothermal aging treatment at 753 K in a neutralized salt bath. Disc-shaped specimens with a diameter of 3 mm and initial thickness of 300 m were cut using an electric discharge
S.H. Nedjad et al. / Materials Science and Engineering A 503 (2009) 156–159
Fig. 1. Bright-field transmission electron micrograph of a cold-rolled specimen aged for 0.36 ks showing an ultrafine-grained microstructure. Inset is a selected-area electron diffraction pattern illustrating high orientation spread among ultrafine grains.
wire cutting machine and mechanically polished to a thickness of ca. 30 m. Further thinning was performed in a solution of CrO3 (200 g), CH3 COOH (500 ml) and H2 O (40 ml) held at 285 K with a twin jet TENUPOL-3 electro-polishing machine. Transmission electron microscopy was carried out in a PHILIPS CM200-FEG microscope operating at 200 kV. All observations were made on the lateral plane normal to the transverse direction of the rolled strips. 3. Results Fig. 1 shows a bright-field transmission electron micrograph of a cold-rolled specimen aged for 0.36 ks which demonstrates an ultrafine-grained microstructure consisting of both equiaxed and elongated grains. A selected-area electron diffraction pattern in the inset shows ring-like configuration, illustrating a relatively random orientation of ultrafine grains. Fig. 2a shows an enlarged brightfield transmission electron micrograph representing an equiaxed ultrafine grain in the specimen aged for 0.36 ks. The dark contrast arisen from elastic strain fields adjacent to the grain boundary extinction contours is denoted by an arrow tip. Further, macroscopic distortion or subdivision of the ultrafine grain is deduced
157
from the heterogeneity of the image contrast. Another brightfield transmission electron micrograph of an ultrafine grain in the specimen aged for 0.36 ks is shown in Fig. 2b illustrating coarse precipitates formed at grain boundary triple junctions as shown by arrow tips. The dark zone adjacent to grain boundaries is again proposed to arise from elastic strain field in the vicinity of grain boundaries. Fig. 3a shows a bright-field transmission electron micrograph of a specimen aged for 86.4 ks demonstrating a multi-phase microstructure consisting of ultrafine grains (white and light gray-colored), coarse particulates (gray-colored) and elongated grains with fine precipitates. A selected-area electron diffraction pattern corresponding to the bright-field micrograph is shown in Fig. 3b. The second-phase particles are identified as face-centered tetragonal (fct) -NiMn intermetallic phase holding a near Kurdjamov–Sachs type orientation relationship with bodycentered cubic (bcc) ␣-iron matrix. Fig. 3c and d shows dark-field micrographs obtained using the spots indicated in Fig. 3b illuminating matrix and second-phase particles, respectively. In Fig. 3c the arrow tip denotes fringes similar to the so-called Ghost fringes as an indicative of solute segregation. Adjacent to the coarse precipitates in Fig. 3d, faint evidence of homogenously dispersed fine precipitates is found around the coarse particles within the ultrafine grain (see arrow tip). 4. Discussion Analogous to the solution-annealed Fe–Ni–Mn maraging steel, cold-rolled steel shows age hardening at intermediate stages which subsequently turns to softening by overaging of precipitates at later stages of aging [10]. The fct -NiMn phase precipitates in the cold-rolled steel both within the ultrafine grains and grain boundary triple junctions in the early stages of aging. By further long-term aging, the fct -NiMn phase coarsens differentially at grain boundary triple junctions at the expense of homophase matrix precipitates within the ultrafine grains. It has been assumed that the ultrafine grains are inherited from deformed structure and, consequently, an extended recovery occurs during isothermal aging which is correlated with coarsening of particles at grain boundary triple junctions [9]. However, peculiarities in the grain boundary precipitation and coarsening behavior are found out in the present steel, e.g. (i) grain boundary precipitates are located only at grain boundary triple junctions while grain boundary planes in a midway between triple junctions are free of coarse precipitates and
Fig. 2. Bright-field transmission electron micrographs of a cold-rolled specimen aged for 0.36 ks showing (a) an ultrafine grain with distortions adjacent to grain boundaries and (b) pronounced precipitation at grain boundary triple junctions.
158
S.H. Nedjad et al. / Materials Science and Engineering A 503 (2009) 156–159
Fig. 3. (a) Bright-field transmission electron micrograph of a specimen aged for 86.4 ks; (b) corresponding selected-area electron diffraction pattern; (c) dark-field transmission ¯ ␣ spot; (d) dark-field transmission micrograph obtained using the (0 2 0) spot. micrograph obtained using the (1¯ 0 1)
(ii) mass-conservative dissolution of matrix precipitates around coarse fct -NiMn particulates occurs homogenously for all of the fine precipitates embedded in an ultrafine grain and, consequently, precipitate-free zone around coarse particulates is not deduced clearly. Both of the aforementioned peculiarities are attributed to the reduced length scale of the ultrafine-grained microstructure. It is assumed that short-grain boundaries of the ultrafine-grained microstructure act as extremely accelerated diffusion pathways for nickel and manganese atoms to build up at triple junctions and, consequently, precipitation reactions take place at grain boundary triple junctions. It is well known that short length of grain boundaries in nanocrystalline materials affects structural periodicity and accommodation of elastic strain fields of grain boundary dislocations at neighboring grains. Those reduced-scale effects along with accumulation of excess amounts of dislocations arrived at grain boundaries during deformation processes give rise to the well-established non-equilibrium nature of short-grain boundaries in the ultrafine-grained materials fabricated by severe plastic deformation [11]. The non-equilibrium short-grain boundaries are proposed to possess higher grain boundary free volume resulting in superdiffusivity at short distances along grain boundaries. Lattice diffusion distance for nickel and manganese atoms through ultrafine grains is also reduced to ultrafine grain diameter during coarsening of the fct NiMn phase. Consequently, the
short diffusion path realizes homogenous dissolution of matrix precipitates, as a result of which conventional precipitate-free zone around coarse particulates does not form subsequently. Any lattice defects inherited in the ultrafine grains from deformation processes may incorporate to enhanced lattice diffusion simultaneously. Further, ultradense distribution of matrix precipitates within the ultrafine grain augments the homogeneity of solute distribution in terms of densely dispersed fine precipitates. Although the principles of precipitate dissolution driven by the concentration gradient through a matrix has been well established already, but the effect of nanometer-sized length scale and excess lattice defects of severely deformed structures on the sequence of precipitate dissolution is not well understood now. The phenomenon needs to be verified in a wide range of nanocrystalline alloys. Nevertheless, to understand the enhanced lattice diffusion and homogenous dissolution of precipitates, it is proposed that local solute exchange between closely spaced precipitates may realize uniform solute drainage from all of the precipitates embedded within the ultrafine grain. A new scheme needs to treat the phenomenon in a quantitative manner by taking the chemical and strain energy gradients at a nanometer scale into account. The aforementioned peculiarities in the precipitation behavior of the present ultrafine-grained steel seem practically overwhelming in terms of microstructure control of nanocrystalline metals. For instance, it is well known that the grain boundary migration
S.H. Nedjad et al. / Materials Science and Engineering A 503 (2009) 156–159
velocity in nanomaterials is manifested by migration velocity of grain boundary triple junctions [11]. Hence, second-phase particles at triple junctions certainly affect triple junction motion. Therefore, precisely controlled precipitation at grain boundary triple junctions of nanocrystalline materials aims higher thermal stability. Further, absence of a precipitate-free zone during second-phase coarsening realizes fabrication of relatively homogenous multi-phase ultrafine-grained alloys in which engineering properties could be tailored by suitable phase compositions and geometries. 5. Conclusions The grain boundary precipitation behavior of cold-rolled nanostructured steel was studied. Main conclusions drawn were as follows: (i) Ultrafine grains with surrounding strained grain boundaries form by cold rolling and aging of a martensitic Fe–Ni–Mn steel. (ii) Pronounced precipitation occurs exclusively at grain boundary triple junctions. The short-grain boundaries between triple junctions are free of coarse precipitates.
159
(iii) During isothermal aging, coarsening of triple junction precipitates proceeds at the expense of homogeneous dissolution of matrix precipitates. (iv) The reduced length scale for precipitation and coarsening alters transformation behavior in the ultrafine-grained microstructure. References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. [2] N. Tsuji, R. Ueji, Y. Minamino, Y. Saito, Scripta Mater. 46 (2002) 305–310. [3] J. Tianfu, G. Yuwei, Q. Guiying, L. Qun, W. Tiansheng, W. Wei, X. Furen, C. Dayong, S. Xinyu, Z. Xin, Mater. Sci. Eng. A 432 (2006) 216–220. [4] J. De Messemaeker, B. Verlinden, J.V. Humbeeck, Mater. Sci. Forum 467–470 (2004) 1295–1300. [5] P.B. Prangnell, J.R. Bowen, M. Berta, P.J. Apps, P.S. Bate, Mater. Sci. Forum 467–470 (2004) 1261–1270. [6] D.H. Shin, Y.S. Kim, E.J. Lavernia, Acta Mater. 49 (2001) 2387–2393. [7] Y. Ivanisenko, R.K. Wunderlich, R.Z. Valiev, H.J. Fecht, Scripta Mater. 49 (2003) 947–952. [8] K.T. Park, D.H. Shin, Mater. Sci. Eng. A334 (2002) 79–86. [9] S. Hossein Nedjad, M. Nili Ahmadabadi, T. Furuhara, Mater. Sci. Eng. A 485 (2008) 544–549. [10] S. Hossein Nedjad, M. Nili Ahmadabadi, T. Furuhara, T. Maki, Solid State Phenom. 114 (2006) 159–164. [11] Y. Gogotsi (Ed.), Nanomaterials Handbook, Taylor & Francis, NW, 2006.