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Effects of rare earth elements doping on ordered structures and ductility improvement of Fe–6.5 wt%Si alloy
Materials Letters 184 (2016) 294–297
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of rare earth elements doping on ordered structures and ductility improvement of Fe–6.5 wt%Si alloy Xuan Yu a, Zhihao Zhang b, Jianxin Xie a,b,n a b
Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, People's Republic of China Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 April 2016 Received in revised form 9 July 2016 Accepted 13 August 2016 Available online 16 August 2016
Fe–6.5 wt%Si alloy's poor ductility is mainly caused by the large amount of brittle ordered structure, and the significant reduction of Fe–6.5 wt%Si alloy's order degree was found by rare earth elements (yttrium, lanthanum, cerium) doping, hence the average room temperature bending deflection increases to 0.77 mm from 0.62 mm and the average tensile elongation to failure at 400 °C increases to 23.0% from 7.3% by 0.021 wt% cerium doping. The order degree reduction phenomenon was manifested by the obvious decrease of B2 and D03 ordered structure reflection intensity in X-ray diffraction and selected area electron diffraction patterns and the ordered domains refinement in transmission electron microscopy. The rearrangement capability of adjacent atoms during order–disorder transformations is lowered due to the iron and silicon atoms are dragged by rare earth atoms, hence the formation of ordered phases is hindered, which provides a novel order degree reduction mechanism of Fe–6.5 wt%Si alloy. & 2016 Elsevier B.V. All rights reserved.
Keywords: Rare earth Intermetallic alloys and compounds Phase transformation Ordered structure Fe–6.5 wt%Si alloy Ductility
1. Introduction Fe–6.5 wt%Si alloy serves as the key soft magnetic material in high frequency electrical equipment, energy-saving transformers and electric vehicles, etc. due to its excellent soft magnetic properties such as near zero magnetostriction, low iron loss and high permeability, etc. [1]. The brittle B2 (FeSi) and D03 (Fe3Si) ordered structures are abound in Fe–6.5 wt%Si alloy [2], which is an important reason for the alloy's brittleness and restricts its fabrication and application seriously [3]. To improve Fe–6.5 wt%Si alloy's ductility, current studies are focusing on hindering order–disorder transformations by quenching treatment [4–6] and destroying ordered structures by deformation treatment [3,7,8]. Some studies suggest that the rare earth (RE) elements doping can affect structures and phase transformations in some compounds due to the larger atomic radius and unique electron structure of RE elements. For instance, the first-order phase transformation of BaTiO3 is diffused by RE (La, Ce, Nd, S etc.) substitution [9]. The oxygen vacancy is ordering in nano-sized domains in 25 at% RE (Sm, Dy, Y, Yb) doped ceria samples [10]. There exists phase transformation in Bi1 xEuxFeO3 from rhombohedral to orthorhombic at 20 mol% RE Eu substitution [11]. And the corresponding properties of above compounds are improved n Corresponding author at: Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, People's Republic of China. E-mail address: [email protected] (J. Xie).
http://dx.doi.org/10.1016/j.matlet.2016.08.074 0167-577X/& 2016 Elsevier B.V. All rights reserved.
accordingly. Hence, we assume that the ordered structures or phase transformations of intermetallic compound B2 (FeSi) and D03 (Fe3Si) could be affected by RE doping. And it's of a significant academic and application value to verify this assumption. Whereas current studies about RE doping in Fe–6.5 wt%Si alloy were focusing on grain refinement effect to explain ductility improvement and misorientation generated by the Ce addition [12,13]. In this work, the effect of RE (Y, La, Ce) elements doping on Fe–6.5 wt%Si alloy's ordered structure is mainly investigated, and comparisons on mechanical properties between RE undoped and 0.021 wt% Ce doped specimens are given.
2. Material and methods Fe-6.5 wt%Si ingots in 5 kg were prepared by a ZG-0.01 model vacuum medium frequency induction melting furnace, and the doped RE elements content (Y, La, Ce) is 0.02–0.04 wt% given by chemical components analysis. The investigation and test specimens were sampled from the same radius and height location of the casting ingots. The microstructure investigations were conducted on an optical microscope after polished and etched for 1 min with a 15 wt% ammonium persulphate-water solution. The X-ray diffraction (XRD) analyses were conducted on a Rigaku SmartLab X-ray diffractometer with Cu target at scanning speed of 20°/min, the diffraction surface size is 15 20 mm2. The selected area electron diffraction (SAED) and morphologies investigations
X. Yu et al. / Materials Letters 184 (2016) 294–297
of ordered structures were conducted on a FEI Tecnai G2 20 transmission electron microscope (TEM) at accelerating voltage of 200 kV, and TEM specimens were polished to 50 mm thick then electro polished on a twin-jet polisher in a solution consisting of 5 vol% perchloric acid and 95 vol% ethanol at –30 °C and voltage of 50 V. The room temperature 3-points bending tests were conducted with loading rate of 1 mm/min and span of 30 mm on a merer3010 testing machine, the specimen size is of 2 5 35 mm3. The tensile property tests were conducted on a DDL electronic universal testing machine, the test specimens were processed into 3 mm in gauge diameter and 10 mm in gauge length and were tested at 400 °C with a strain rate of 0.05 min–1, and the fracture morphology was obtained on a JEOL–7001F scanning electron microscope (SEM). The ordered structure recovery of Fe–6.5 wt%Si alloy will happen above 500 °C during deformation process [7], hence the tensile temperature was decided as 400 °C to avoid the influence of ordered structure recovery.
3. Results and discussion The Fe–6.5 wt%Si alloy specimens of casting ingots are all equiaxed, the optical microstructures of RE doped specimens are more uniformed and refined in a various degree, as shown in Fig. 1 (a–d). The average grain sizes of the specimens ( 400–800 mm) are in the same order of magnitude. According to the standard free energies of formation of RE compounds, the RE compounds should be formed in alloy melt in the following sequence: RE oxides, RE sulfides, RE oxysulfides and other RE compounds oxides, and they all have high melting points ( 2000–3000 °C) [14]. The refractory RE compounds increased heterogeneous nucleation positions and lead to the microstructure refinement.
295
The XRD patterns are shown in Fig. 2(a), according to the PDF #65-1835 (FeSi) and PDF #65-0146 (Fe3Si), the B2 (200) and D03 (311) characteristic reflection peaks are obvious in the XRD pattern of RE undoped specimen, whereas the intensities of above reflection peaks are decreased in RE doped patterns. The A2 disordered phases, B2 and D03 ordered phases in Fe–6.5 wt%Si alloy are all bcc structure, therefore we calculated the intensity ratio of B2 (200) characteristic reflection peak and A2 (200) fundamental reflection peak as IB2 (200)/IA2 (200) to evaluate the order degree, as shown in Fig. 2(b). The IB2 (200)/IA2 (200) of specimens with RE undoped and RE (Y, La, Ce) doped is 4.6, 0.67, 0.99, 0.39 respectively, which indicates that the RE doping can reduce the order degree of Fe–6.5 wt%Si alloy effectively. To investigate the mechanism further, the RE undoped and 0.021 wt% Ce doped specimens were analyzed by SAED and TEM, as shown in Fig. 3. The diffraction patterns of [001] zone axis are shown in Fig. 3(a, d), and the {100} superlattice spots (circled in Fig. 3) represent B2 and D03 phases. The superlattice reflection intensity of Ce doped specimen is much lower compared to RE undoped specimen. The dark field TEM micrographs of {100} superlattice spots are shown in Fig. 3(b, e). The B2 and D03 ordered domains in RE undoped specimen are bright and with coarse sizes ( 1 mm), and the anti-phase boundaries (APBs) of the domains are clear and discernible. Whereas, the domains in Ce doped specimen are of dispersive distribution and tiny sizes ( 5 nm) and with a much weaker brightness. The ordered domains refinement effect by cerium doping is able to reach and even exceed the level of quenching and deformation treatments in recent results [3,4]. The atomic radius of RE elements (1.80 Å of Ce, 1.87 Å of La, 1.81 Å of Y) is 45%, 51%, 46% larger than of iron (1.24 Å) respectively. The atomic radius difference between RE and iron is significant, hence RE atoms exist in the lattice only in the
Fig. 1. Optical microstructures of the Fe–6.5 wt%Si alloy with (a) RE undoped, (b) 0.036 wt% Y doped, (c) 0.041 wt% La doped, (d) 0.021 wt% Ce doped.
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Fig. 2. (a) XRD patterns of the Fe–6.5 wt%Si alloy with RE undoped and RE (Y, La, Ce) doped, (b) the IB2(200)/IA2(200) intensity ratios in the XRD patterns.
Fig. 3. SAED patterns of [001] zone axis of the Fe–6.5 wt%Si alloy with (a) RE undoped and (d) 0.021 wt% Ce doped, the dark filed TEM micrographs of {100} superlattice spots (representing B2 and D03 phases, circled in SAED patterns) of the alloy with (b) RE undoped and (e) 0.021 wt% Ce doped, the schematic illustrations of atomic movements in (110) plane during order–disorder transformations of the Fe–6.5 wt%Si alloy with (c) RE undoped and (f) Ce doped.
substitution form and the solubility is extremely low. The electronegativity of silicon and iron is 1.90 and 1.83 respectively, whereas the electronegativity of RE elements is much lower (1.12 of Ce, 1.10 of La, 1.22 of Y). The electronegativity difference between RE (Ce, La, Y) and matrix elements (iron and silicon) is in the range of 0.68–0.80 and 0.61–0.71 respectively, which is 10 times as long as the difference (0.07) between silicon and iron. The great electronegativity difference will result in the silicon and iron atoms are much easier to attract electrons from RE atoms.
Combined with the low solubility of RE atoms in the alloy matrix, the electronegativity difference between RE and matrix elements will lead to the ordering combination tendency to RE– iron (Fe17Ce2, Fe17Y2) compounds and RE–silicon (CeSi2, YSi2) compounds formation, according to phase diagrams [15]. According to Ref. [16], the order–disorder transformations of A2–B2 and B2–D03 in Fe–6.5 wt%Si alloy are all second-order phase transformations, during which the long range ordered structures are formed by adjacent atoms rearrangement. In this work, the
X. Yu et al. / Materials Letters 184 (2016) 294–297
297
Fig. 4. (a) Engineering stress–strain curves of the tensile specimens tested at 400 °C, (b) average plastic elongation to failure of the fractured tensile specimens, fracture morphology of fractured specimen with (c) RE undoped and (d) 0.021 wt% Ce doped.
extreme low RE content is not enough to form RE–iron or RE–silicon compounds, which is consistent with XRD patterns in Fig. 2 (a). However, the iron and silicon atoms will be dragged by RE atoms during the adjacent atoms rearrangement process due to the stronger ordering combination capability between RE atoms and matrix atoms (Fe, Si), which will weaken the combination capability between iron and silicon atoms during the B2 and D03 ordered phase formation process, as illustrated in Fig. 3(c, f). Hence, the order–disorder transformations are hindered and the order degree is reduced. Tensile tests of the alloy (Fe–6.56 wt%Si, Fe–6.52 wt%Si– 0.021 wt%Ce) were conducted at 400 °C, the engineering stress– strain curves are shown in Fig. 4(a). The curves of RE undoped specimens contain elastic deformation stage and strain-hardening stage, whereas the curves of Ce doped specimens contain elastic deformation stage, strain-hardening stage and necking stage. The average elongation to failure of Ce doped specimens is 23.0%, which is 3.2 times as long as the 7.3% of RE undoped specimens. Fig. 4(c, d) indicates a fracture mode transformation from brittle intergranular fracture to dimple fracture by Ce doping. And the average room temperature bending deflection increases to 0.777 0.01 mm from 0.62 70.04 mm by Ce doping. In this work, the average grain size of RE undoped specimen ( 800 mm) and 0.021 wt% Ce doped specimen ( 600 mm) are in the same order of magnitude. Li et al. [12] attributed the ductile fracture mode occurrence to the elimination of detrimental grain boundary segregation of sulfur by cerium doping. In this work, the obvious order degree reduction was found. Hence, the order degree reduction is also an important reason for ductility improvement besides the grain boundary strengthening effect. The ductility improvement is of a significance to develop hot-warm-cold rolling method to fabricate the alloy's thin sheets [5].
4. Conclusions The significant order degree reduction of Fe–6.5 wt%Si alloy was found by RE (Y, La, Ce) doping, the average room temperature
bending deflection increases to 0.77 mm from 0.62 mm and the average tensile plastic elongation to failure at 400 °C increases to 23.0% from 7.3% by doping 0.021 wt% cerium. The rearrangement capability of adjacent atoms during order–disorder transformations is lowered due to the iron and silicon atoms are dragged by Ce atoms, hence the formation of ordered phases is hindered, which provides a novel order degree reduction mechanism of Fe–6.5 wt%Si alloy and modifies the alloy’s brittleness combined with intergranular cracking inhibition by Ce doping.
Acknowledgments This work was supported by the National Basic Research Program of China (973 Program): Contract number 2011CB606300.
References [1] K.I. Arai, K. Ishiyama, J. Magn. Magn. Mater. 133 (1994) 233–237. [2] J.S. Shin, J.S. Bae, H.J. Kim, H.M. Lee, T.D. Lee, E.J. Lavernia, Z.H. Lee, Mater. Sci. Eng., A 407 (2005) 282–290. [3] H. Li, Y.F. Liang, W. Yang, F. Ye, J.P. Lin, J.X. Xie, Mater. Sci. Eng. A. 628 (2015) 262–268. [4] X. Wang, H. Li, Z. Liu, H. Liu, G. Wang, Z. Luo, F. Zhan, S. Chen, L. Lyu, Mater. Charact. 111 (2016) 67–74. [5] Y. Liang, F. Ye, J. Lin, Y. Wang, L. Zhang, G. Chen, Sci. China Technol. Sci. 53 (2010) 1008–1011. [6] Z. Zhang, W. Wang, H. Fu, J. Xie, Mater. Sci. Eng. A 530 (2011) 519–524. [7] Y. Mo, Z. Zhang, H. Fu, H. Pan, J. Xie, Mater. Sci. Eng. A 594 (2014) 111–117. [8] H. Fu, Z. Zhang, Q. Yang, J. Xie, Mater. Sci. Eng. A 528 (2011) 1391–1395. [9] F. Han, Y. Bai, L.-J. Qiao, D. Guo, J. Mater. Chem. C 4 (2016) 1842–1849. [10] D.R. Ou, T. Mori, F. Ye, T. Kobayashi, J. Zou, G. Auchterlonie, J. Drennan, Appl. Phys. Lett. 89 (2006) 171911. [11] X. Zhang, Y. Sui, X. Wang, Y. Wang, Z. Wang, J. Alloy. Compd. 507 (2010) 157–161. [12] H.-Z. Li, H.-T. Liu, X.-L. Wang, G.-M. Cao, C.-G. Li, Z.-Y. Liu, G.-D. Wang, Mater. Lett. 165 (2016) 5–8. [13] H.-Z. Li, H.-T. Liu, Z.-Y. Liu, G.-D. Wang, Mater. Charact. 103 (2015) 101–106. [14] P.E. Waudby, Int. Met. Rev. 23 (1978) 74–98. [15] H. Okamoto, Desk Handbook: Phase Diagrams For Binary Alloys, ASM International, Materials Park, OH, 2000. [16] K. Raviprasad, K. Chattopadhyay, Acta Metall. Mater. 41 (1993) 609–624.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of rare earth elements doping on ordered structures and ductility improvement of Fe–6.5 wt%Si alloy Xuan Yu a, Zhihao Zhang b, Jianxin Xie a,b,n a b
Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, People's Republic of China Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 April 2016 Received in revised form 9 July 2016 Accepted 13 August 2016 Available online 16 August 2016
Fe–6.5 wt%Si alloy's poor ductility is mainly caused by the large amount of brittle ordered structure, and the significant reduction of Fe–6.5 wt%Si alloy's order degree was found by rare earth elements (yttrium, lanthanum, cerium) doping, hence the average room temperature bending deflection increases to 0.77 mm from 0.62 mm and the average tensile elongation to failure at 400 °C increases to 23.0% from 7.3% by 0.021 wt% cerium doping. The order degree reduction phenomenon was manifested by the obvious decrease of B2 and D03 ordered structure reflection intensity in X-ray diffraction and selected area electron diffraction patterns and the ordered domains refinement in transmission electron microscopy. The rearrangement capability of adjacent atoms during order–disorder transformations is lowered due to the iron and silicon atoms are dragged by rare earth atoms, hence the formation of ordered phases is hindered, which provides a novel order degree reduction mechanism of Fe–6.5 wt%Si alloy. & 2016 Elsevier B.V. All rights reserved.
Keywords: Rare earth Intermetallic alloys and compounds Phase transformation Ordered structure Fe–6.5 wt%Si alloy Ductility
1. Introduction Fe–6.5 wt%Si alloy serves as the key soft magnetic material in high frequency electrical equipment, energy-saving transformers and electric vehicles, etc. due to its excellent soft magnetic properties such as near zero magnetostriction, low iron loss and high permeability, etc. [1]. The brittle B2 (FeSi) and D03 (Fe3Si) ordered structures are abound in Fe–6.5 wt%Si alloy [2], which is an important reason for the alloy's brittleness and restricts its fabrication and application seriously [3]. To improve Fe–6.5 wt%Si alloy's ductility, current studies are focusing on hindering order–disorder transformations by quenching treatment [4–6] and destroying ordered structures by deformation treatment [3,7,8]. Some studies suggest that the rare earth (RE) elements doping can affect structures and phase transformations in some compounds due to the larger atomic radius and unique electron structure of RE elements. For instance, the first-order phase transformation of BaTiO3 is diffused by RE (La, Ce, Nd, S etc.) substitution [9]. The oxygen vacancy is ordering in nano-sized domains in 25 at% RE (Sm, Dy, Y, Yb) doped ceria samples [10]. There exists phase transformation in Bi1 xEuxFeO3 from rhombohedral to orthorhombic at 20 mol% RE Eu substitution [11]. And the corresponding properties of above compounds are improved n Corresponding author at: Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, People's Republic of China. E-mail address: [email protected] (J. Xie).
http://dx.doi.org/10.1016/j.matlet.2016.08.074 0167-577X/& 2016 Elsevier B.V. All rights reserved.
accordingly. Hence, we assume that the ordered structures or phase transformations of intermetallic compound B2 (FeSi) and D03 (Fe3Si) could be affected by RE doping. And it's of a significant academic and application value to verify this assumption. Whereas current studies about RE doping in Fe–6.5 wt%Si alloy were focusing on grain refinement effect to explain ductility improvement and misorientation generated by the Ce addition [12,13]. In this work, the effect of RE (Y, La, Ce) elements doping on Fe–6.5 wt%Si alloy's ordered structure is mainly investigated, and comparisons on mechanical properties between RE undoped and 0.021 wt% Ce doped specimens are given.
2. Material and methods Fe-6.5 wt%Si ingots in 5 kg were prepared by a ZG-0.01 model vacuum medium frequency induction melting furnace, and the doped RE elements content (Y, La, Ce) is 0.02–0.04 wt% given by chemical components analysis. The investigation and test specimens were sampled from the same radius and height location of the casting ingots. The microstructure investigations were conducted on an optical microscope after polished and etched for 1 min with a 15 wt% ammonium persulphate-water solution. The X-ray diffraction (XRD) analyses were conducted on a Rigaku SmartLab X-ray diffractometer with Cu target at scanning speed of 20°/min, the diffraction surface size is 15 20 mm2. The selected area electron diffraction (SAED) and morphologies investigations
X. Yu et al. / Materials Letters 184 (2016) 294–297
of ordered structures were conducted on a FEI Tecnai G2 20 transmission electron microscope (TEM) at accelerating voltage of 200 kV, and TEM specimens were polished to 50 mm thick then electro polished on a twin-jet polisher in a solution consisting of 5 vol% perchloric acid and 95 vol% ethanol at –30 °C and voltage of 50 V. The room temperature 3-points bending tests were conducted with loading rate of 1 mm/min and span of 30 mm on a merer3010 testing machine, the specimen size is of 2 5 35 mm3. The tensile property tests were conducted on a DDL electronic universal testing machine, the test specimens were processed into 3 mm in gauge diameter and 10 mm in gauge length and were tested at 400 °C with a strain rate of 0.05 min–1, and the fracture morphology was obtained on a JEOL–7001F scanning electron microscope (SEM). The ordered structure recovery of Fe–6.5 wt%Si alloy will happen above 500 °C during deformation process [7], hence the tensile temperature was decided as 400 °C to avoid the influence of ordered structure recovery.
3. Results and discussion The Fe–6.5 wt%Si alloy specimens of casting ingots are all equiaxed, the optical microstructures of RE doped specimens are more uniformed and refined in a various degree, as shown in Fig. 1 (a–d). The average grain sizes of the specimens ( 400–800 mm) are in the same order of magnitude. According to the standard free energies of formation of RE compounds, the RE compounds should be formed in alloy melt in the following sequence: RE oxides, RE sulfides, RE oxysulfides and other RE compounds oxides, and they all have high melting points ( 2000–3000 °C) [14]. The refractory RE compounds increased heterogeneous nucleation positions and lead to the microstructure refinement.
295
The XRD patterns are shown in Fig. 2(a), according to the PDF #65-1835 (FeSi) and PDF #65-0146 (Fe3Si), the B2 (200) and D03 (311) characteristic reflection peaks are obvious in the XRD pattern of RE undoped specimen, whereas the intensities of above reflection peaks are decreased in RE doped patterns. The A2 disordered phases, B2 and D03 ordered phases in Fe–6.5 wt%Si alloy are all bcc structure, therefore we calculated the intensity ratio of B2 (200) characteristic reflection peak and A2 (200) fundamental reflection peak as IB2 (200)/IA2 (200) to evaluate the order degree, as shown in Fig. 2(b). The IB2 (200)/IA2 (200) of specimens with RE undoped and RE (Y, La, Ce) doped is 4.6, 0.67, 0.99, 0.39 respectively, which indicates that the RE doping can reduce the order degree of Fe–6.5 wt%Si alloy effectively. To investigate the mechanism further, the RE undoped and 0.021 wt% Ce doped specimens were analyzed by SAED and TEM, as shown in Fig. 3. The diffraction patterns of [001] zone axis are shown in Fig. 3(a, d), and the {100} superlattice spots (circled in Fig. 3) represent B2 and D03 phases. The superlattice reflection intensity of Ce doped specimen is much lower compared to RE undoped specimen. The dark field TEM micrographs of {100} superlattice spots are shown in Fig. 3(b, e). The B2 and D03 ordered domains in RE undoped specimen are bright and with coarse sizes ( 1 mm), and the anti-phase boundaries (APBs) of the domains are clear and discernible. Whereas, the domains in Ce doped specimen are of dispersive distribution and tiny sizes ( 5 nm) and with a much weaker brightness. The ordered domains refinement effect by cerium doping is able to reach and even exceed the level of quenching and deformation treatments in recent results [3,4]. The atomic radius of RE elements (1.80 Å of Ce, 1.87 Å of La, 1.81 Å of Y) is 45%, 51%, 46% larger than of iron (1.24 Å) respectively. The atomic radius difference between RE and iron is significant, hence RE atoms exist in the lattice only in the
Fig. 1. Optical microstructures of the Fe–6.5 wt%Si alloy with (a) RE undoped, (b) 0.036 wt% Y doped, (c) 0.041 wt% La doped, (d) 0.021 wt% Ce doped.
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Fig. 2. (a) XRD patterns of the Fe–6.5 wt%Si alloy with RE undoped and RE (Y, La, Ce) doped, (b) the IB2(200)/IA2(200) intensity ratios in the XRD patterns.
Fig. 3. SAED patterns of [001] zone axis of the Fe–6.5 wt%Si alloy with (a) RE undoped and (d) 0.021 wt% Ce doped, the dark filed TEM micrographs of {100} superlattice spots (representing B2 and D03 phases, circled in SAED patterns) of the alloy with (b) RE undoped and (e) 0.021 wt% Ce doped, the schematic illustrations of atomic movements in (110) plane during order–disorder transformations of the Fe–6.5 wt%Si alloy with (c) RE undoped and (f) Ce doped.
substitution form and the solubility is extremely low. The electronegativity of silicon and iron is 1.90 and 1.83 respectively, whereas the electronegativity of RE elements is much lower (1.12 of Ce, 1.10 of La, 1.22 of Y). The electronegativity difference between RE (Ce, La, Y) and matrix elements (iron and silicon) is in the range of 0.68–0.80 and 0.61–0.71 respectively, which is 10 times as long as the difference (0.07) between silicon and iron. The great electronegativity difference will result in the silicon and iron atoms are much easier to attract electrons from RE atoms.
Combined with the low solubility of RE atoms in the alloy matrix, the electronegativity difference between RE and matrix elements will lead to the ordering combination tendency to RE– iron (Fe17Ce2, Fe17Y2) compounds and RE–silicon (CeSi2, YSi2) compounds formation, according to phase diagrams [15]. According to Ref. [16], the order–disorder transformations of A2–B2 and B2–D03 in Fe–6.5 wt%Si alloy are all second-order phase transformations, during which the long range ordered structures are formed by adjacent atoms rearrangement. In this work, the
X. Yu et al. / Materials Letters 184 (2016) 294–297
297
Fig. 4. (a) Engineering stress–strain curves of the tensile specimens tested at 400 °C, (b) average plastic elongation to failure of the fractured tensile specimens, fracture morphology of fractured specimen with (c) RE undoped and (d) 0.021 wt% Ce doped.
extreme low RE content is not enough to form RE–iron or RE–silicon compounds, which is consistent with XRD patterns in Fig. 2 (a). However, the iron and silicon atoms will be dragged by RE atoms during the adjacent atoms rearrangement process due to the stronger ordering combination capability between RE atoms and matrix atoms (Fe, Si), which will weaken the combination capability between iron and silicon atoms during the B2 and D03 ordered phase formation process, as illustrated in Fig. 3(c, f). Hence, the order–disorder transformations are hindered and the order degree is reduced. Tensile tests of the alloy (Fe–6.56 wt%Si, Fe–6.52 wt%Si– 0.021 wt%Ce) were conducted at 400 °C, the engineering stress– strain curves are shown in Fig. 4(a). The curves of RE undoped specimens contain elastic deformation stage and strain-hardening stage, whereas the curves of Ce doped specimens contain elastic deformation stage, strain-hardening stage and necking stage. The average elongation to failure of Ce doped specimens is 23.0%, which is 3.2 times as long as the 7.3% of RE undoped specimens. Fig. 4(c, d) indicates a fracture mode transformation from brittle intergranular fracture to dimple fracture by Ce doping. And the average room temperature bending deflection increases to 0.777 0.01 mm from 0.62 70.04 mm by Ce doping. In this work, the average grain size of RE undoped specimen ( 800 mm) and 0.021 wt% Ce doped specimen ( 600 mm) are in the same order of magnitude. Li et al. [12] attributed the ductile fracture mode occurrence to the elimination of detrimental grain boundary segregation of sulfur by cerium doping. In this work, the obvious order degree reduction was found. Hence, the order degree reduction is also an important reason for ductility improvement besides the grain boundary strengthening effect. The ductility improvement is of a significance to develop hot-warm-cold rolling method to fabricate the alloy's thin sheets [5].
4. Conclusions The significant order degree reduction of Fe–6.5 wt%Si alloy was found by RE (Y, La, Ce) doping, the average room temperature
bending deflection increases to 0.77 mm from 0.62 mm and the average tensile plastic elongation to failure at 400 °C increases to 23.0% from 7.3% by doping 0.021 wt% cerium. The rearrangement capability of adjacent atoms during order–disorder transformations is lowered due to the iron and silicon atoms are dragged by Ce atoms, hence the formation of ordered phases is hindered, which provides a novel order degree reduction mechanism of Fe–6.5 wt%Si alloy and modifies the alloy’s brittleness combined with intergranular cracking inhibition by Ce doping.
Acknowledgments This work was supported by the National Basic Research Program of China (973 Program): Contract number 2011CB606300.
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