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Phase constitution and microstructure of the Fe–Si–Cr ternary ferritic alloys
Scripta Materialia 50 (2004) 977–981 www.actamat-journals.com
Phase constitution and microstructure of the Fe–Si–Cr ternary ferritic alloys Keisuke Yamamoto *, Yoshisato Kimura, Yoshinao Mishima Department of Materials Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received 18 March 2003; received in revised form 12 December 2003; accepted 10 January 2004
Abstract Phase constitution and microstructure of Fe-corner of the ternary Fe–Si–Cr system are investigated particularly on the multiphase fields involving bcc disordered phase, A2 phase, and bcc ordered phases, B2 and D03 phases. The (A2 + D03 ) two-phase region is found to exist in compositions between 12 and 15 at.% Si at 10% Cr at 873 K. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Annealing; Transmission electron microscopy (TEM); Iron; Transition metal silicides; Ordering
1. Introduction In an attempt to develop a new class of heat resistant ferritic steel strengthened solely by an intermetallic compound, a ternary system Fe–Si–Cr system would be one of the candidates because there are two kinds of bcc ordered intermetallic phase, D03 and B2, in the vicinity of A2, ferrite, in the Fe–Si binary phase diagram shown in Fig. 1 [1]. Crystallographic relations between A2 and these ordered phases, if co-exist, is expected to be coherent with a small misfit due to the similarity in crystal structures. Since additions of Cr are inevitable to the binary system to assure the oxidation resistance, the phase constitutions at the Fe-corner of the ternary Fe–Si–Cr system should be investigated particularly on the multiphase fields involving A2 and the ordered bcc phases. Raghavan [2] have determined the isothermal section of the Fe–Si–Cr system at 1173 K, which result is reproduced in Fig. 2. It was shown that B2 and D03 ordered phases are absent at this temperature but there are a wide compositional field for single phase a-Fe. Kozakai et al. [3–5] have energetically investigated experimentally and theoretically on the phase diagram and the route of phase separations in ternary Fe–X–Y (X ¼ Al, Si and Y ¼ Co, V, Ge, Ni) ordering alloy systems. However, little work has so far been available in
*
Corresponding author. Tel./fax: +81-459245495. E-mail address: [email protected] (K. Yamamoto).
published phase diagram in Fe–Si–Cr ternary system including order–disorder transition in a-Fe phase field. In the present work, phase constitution and microstructure of the Fe-corner of Fe–Si–Cr are investigated including an isothermal section at 873 K and a vertical section at a constant 10 at.% Cr. The compositional and annealing conditions investigated are so chosen as to realize the present results to become basis for a design of new heat resistant ferritic steels.
2. Experimental procedure Alloys were prepared by arc melting of Fe (99.9%), Cr (99.99%), Si (99.999%) and Ti (99.5%) under an Ar atmosphere. The nominal compositions of ternary alloys are given in the isothermal section at 1173 K shown in Fig. 2. After homogenization for 24 h at 1473 K under an Ar gas flow followed by water quenching, cube specimens with about 5 mm edge were annealed at temperature between 873 and 1373 K, and then quenched into water. Microstructures of the alloys were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Chemical compositions of the phases were determined by electron probe microanalysis (EPMA) using JEOL JXA-8900. Thin foils for TEM observation were prepared by twin-jet electropolishing using a solution of 10% perchloric acid and 90% methanol at below 223 K. A JEOL JEM-2011 TEM and
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.01.006
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K. Yamamoto et al. / Scripta Materialia 50 (2004) 977–981
emphasis on the identification of the phases coexisting with A2 matrix. It is first necessary to distinguish bcc disordered phase (A2 phase) from bcc ordered phases (B2 and D03 phases), and then to distinguish B2 from D03 phase. Therefore dark-field examinations are of most importance [6]. Fig. 3 shows the selected area diffraction (SAD) pattern, a bright field image, 2 0 0 and 1 1 1 dark-field images of alloy H (Fe–12Si–10Cr) annealed at 873 K for
Fig. 1. Fe–Si binary phase diagram.
Philips CM200 TEM equipped with EDS were used at an accelerating voltage of 200 kV. Critical temperatures corresponding to order–disorder transitions were determined by differential scanning calorimetry (DSC) and thermo-gravimetry (TG) using NETZSCH STA449. Curie temperature can be measured by weight change using the TG equipped with a magnet outside the furnace. Specimens were heated to 1273 K and cooled to room temperature at 20 K min1 under Ar gas flow. 3. Results and discussion 3.1. Two-phase microstructures in Fe–Si–Cr ternary system Examinations of microstructures of the ternary Fe– Si–Cr alloys were performed by TEM with particular
Fig. 3. Microstructure of Fe–12Si–10Cr alloy annealed at 873 K for 100 h: (a) SAD pattern, (b) bright field image and dark-field images from (c) 2 0 0 and (d) 1 1 1 spots, respectively.
Fig. 2. Nominal compositions of the present alloys represented on the isothermal section at 973 K of the Fe–Si–Cr ternary system at 1173 K evaluated previously reported in the literature.
K. Yamamoto et al. / Scripta Materialia 50 (2004) 977–981
100 h. The indices on the SAD pattern shown in Fig. 3(a) are double bcc indices, because the lattice parameter of D03 structure is twice as large as that of A2 structure. The 2 0 0 and 1 1 1 super lattice reflections are seen on a diffraction pattern in Fig. 3(a). The super lattice reflection 2 0 0 appears in both B2 and D03 superstructures, while the 1 1 1 reflection only appears in the D03 superstructure. The image contrast is the same between 2 0 0 dark-field image in Fig. 3(c) and the 1 1 1 dark-field image in Fig. 3(d). Therefore, the phases in contrast are D03 phase and the phase out of contrast is A2 and this alloy is in the (A2 + D03 ) two-phase field at 873 K. It is noted that (A2 + D03 ) two-phase region cannot be found in the Fe–Si binary phase diagram shown in Fig. 1. The microstructure of alloy H contains small volume fraction of D03 phase with rod shape. The width of D03 phase with the {1 0 0} habit planes is tens of nm and the length is a few hundreds of nm. Fig. 4 shows dark-field images of the alloy I (Fe– 13Si–10Cr) and J (Fe–15Si–10Cr) annealed at 873 K for 100 h. In both figures the bright parts are D03 phase and the dark parts A2 phase. It seems that with a small increase in Si content, volume fraction of D03 phase seems to increase rapidly. 3.2. Vertical sections at a constant 10 at.% Cr in Fe–Si– Cr ternary system Fig. 5 shows typical DSC and TG curves during cooling obtained on alloy J. Two exothermic peaks shown by circles which accompany weight changes are observed at about 820 and 880 K. It is considered that the alloy J of (A2 + D03 ) two-phase microstructure has two magnetic transformation temperatures, one at higher temperature for A2 phase, and the other at lower temperature for D03 phase. The ordering temperature was determined by an exothermic peak shown by a solid circle at about 1135 K during cooling.
Fig. 4. Dark-field images of the alloys annealed at 873 K for 100 h: (a) Fe–13Si–10Cr alloy and (b) Fe–15Si–10Cr alloy.
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Fig. 5. Typical DSC and TG curves of Fe–15Si–10Cr alloy during cooling.
In order to confirm that the exothermic peak observed is due to the ordering reaction, the alloy is annealed at various temperatures up to 1373 K, quenched into water, and TEM observations were performed at room temperature. It is generally very difficult to suppress a second order transition by quenching, however it is possible to deduce from TEM observations whether a disordered or an ordered state existed at high temperature. This can be done by looking at the size of antiphase domain boundaries (APBs) which are observed in darkfield images using super lattice reflections. During quenching from a disordered state, a large number of small antiphase domains are formed. On the contrary, if the ordering reactions take place at an annealing temperature, there is a time for the antiphase domains to grow much larger, usually larger than a selected area of TEM observation. Fig. 6 shows 2 0 0 and 1 1 1 dark-field images of alloy J quenched from 1173 K for 30 h (Fig. 6(a) and (b)) and 1273 K for 1h (Fig. 6(c) and (d)). The microstructural feature of alloy J quenched from 1173 K, Fig. 6(a) and (b), exhibiting a rectangular texture to lie on cube planes can be seen even in 2 0 0 dark-field image. The alloy is a mixture of extremely small D03 particles with bright contrast and disordered A2 phase with dark contrast. B2 APBs cannot be observed because they are much larger in size during annealing at 1173 K for the scale for Fig. 6(a). The microstructure of this alloy annealed at 1073 K was a coarse (A2 + D03 ) two-phase mixture and much coarser than that in Fig. 4(b) at 873 K. It can be concluded that this alloy is B2 single phase at 1173 K. It should be noted that B2/D03 transition temperature corresponds to an exothermic peak observed at about 1135 K in the DSC measurement. The 2 0 0 dark-field image of alloy J quenched from 1273 K, Fig. 6(c), shows B2 APBs formed during cooling, indicating that A2 to B2 phase transition has
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K. Yamamoto et al. / Scripta Materialia 50 (2004) 977–981
Fig. 7. The vertical section along the constant 10 at.% Cr in Fe–Si–Cr ternary system.
Fig. 6. Dark-field images of Fe–15Si–10Cr alloy; (a, b) annealed at 1173 K for 30 h and (c, d) annealed at 1273 K for 1 h, where (a, c) using 2 0 0 spot and (b, d) using 1 1 1 spot.
occurred between 1273 and 1173 K during cooling. The alloy is in the single A2 phase at 1273 K, passing through B2 phase region, and then transforms into (A2 + D03 ) during cooling to exhibit the microstructure shown in Fig. 6(c) and (d). With all these information gathered, a vertical section at a constant 10 at.% Cr of the Fe–Si–Cr phase diagram is constructed as shown in Fig. 7. Curie temperature was determined by using a TG equipped with a magnet. It is considered that the temperature of an exothermic peak in DSC measurement during cooling corresponds to the transition from B2 to D03 phase, which increases with increasing Si contents. By comparing Fig. 7 with Fig. 1, it is found that the (A2 + D03 ) phase field in the ternary system at 10 at.% Cr and (B2 + D03 ) in the binary system lie within the similar Si concentration range. 3.3. An isothermal section at 873 K in Fe–Si–Cr ternary system In order to establish an isothermal section diagram at 873 K in the Fe–Si–Cr ternary system, the microstruc-
Fig. 8. Back-scattered electron images of the alloys annealed at 873 K for 258 h: (a) Fe–15Si–15Cr alloy and (b) Fe–20Si–15Cr alloy.
tures and composition analysis of the alloys with 15 and 20 at.% Cr are investigated. Fig. 8(a) and (b) show the back-scattered electron images in alloy N (Fe– 15Si–15Cr) and O (Fe–20Si–15Cr) annealed at 873 K for 258 h. Precipitates with bright contrast are r-FeCr phase, while with a dark contrast Cr3 Si phase of A15 crystal structure. The matrix is D03 and both Cr3 Si
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Table 1 Compositions of the D03 , Cr3 Si and r-phases evaluated by EPMA (Fe: Bal.) Alloy
D03 (at.%)
Cr3 Si
Si
Cr
Si
Cr
Si
Cr
Fe–15Si–15Cr Fe–20Si–15Cr
14.5 19.3
13.3 12.2
20.0 19.8
48.4 55.2
14.3 –
35.4 –
r
4. Conclusions
Fig. 9. The isothermal section of Fe–Si–Cr ternary system at 873 K evaluated in the present work.
and r-phases exist preferentially at grain boundaries as shown in Fig. 8(a). In alloy O, as shown in Fig. 8(b), Cr3 Si phase precipitates at grain boundaries and within grains. Chemical compositions of the D03 , r and Cr3 Si phases determined by EPMA are listed in Table 1. With all the information obtained so far together, an isothermal section diagram in the Fe–Si–Cr ternary system at 873 K is established as shown in Fig. 9. Through the present work, in comparison with that at 1173 K by previous workers in Fig. 2, (A2 + D03 ) twophase region is found to exist in compositions between 12 and 15 at.% Si, and also is found that (D03 + Cr3 Si) two-phase region extends to lower temperatures.
Phase constitution and microstructure of the Fecorner of Fe–Si–Cr are investigated including an isothermal section at 873 K and a vertical section at a constant 10 at.% Cr. (A2 + D03 ) two-phase region is found to exist at compositions between 12 and 15 at.% Si in the Fe–Si–Cr ternary system at 873 K. Phase boundaries among A2, B2, D03 single phase and some two-phase regions are determined on the vertical section at a constant 10 at.% Cr of the ternary system. It is found that B2/D03 transition temperature increases with increasing Si contents. References [1] Okamoto H, Subramanian PR, Kacprzak L. In: Massalski TB et al., editors. Binary alloy phase diagrams, 2nd ed. ASM International; 1990. p. 1772. [2] Raghavan V. Phase diagrams of ternary iron alloys, Part 1. ASM International; 1987. p. 34. [3] Kozakai T, Miyazaki T. ISIJ Int 1994;34:373. [4] Zhao PZ, Kozakai T, Miyazaki T. J Japan Inst Metals 1989;53:266. [5] Miyazaki T, Isobe K, Kozakai T, Doi M. Acta Metal 1987;35:317. [6] Mendiratta MG, Ehlers SK, Lipsitt HA. Metall Trans A 1987;18: 509.
Phase constitution and microstructure of the Fe–Si–Cr ternary ferritic alloys Keisuke Yamamoto *, Yoshisato Kimura, Yoshinao Mishima Department of Materials Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received 18 March 2003; received in revised form 12 December 2003; accepted 10 January 2004
Abstract Phase constitution and microstructure of Fe-corner of the ternary Fe–Si–Cr system are investigated particularly on the multiphase fields involving bcc disordered phase, A2 phase, and bcc ordered phases, B2 and D03 phases. The (A2 + D03 ) two-phase region is found to exist in compositions between 12 and 15 at.% Si at 10% Cr at 873 K. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Annealing; Transmission electron microscopy (TEM); Iron; Transition metal silicides; Ordering
1. Introduction In an attempt to develop a new class of heat resistant ferritic steel strengthened solely by an intermetallic compound, a ternary system Fe–Si–Cr system would be one of the candidates because there are two kinds of bcc ordered intermetallic phase, D03 and B2, in the vicinity of A2, ferrite, in the Fe–Si binary phase diagram shown in Fig. 1 [1]. Crystallographic relations between A2 and these ordered phases, if co-exist, is expected to be coherent with a small misfit due to the similarity in crystal structures. Since additions of Cr are inevitable to the binary system to assure the oxidation resistance, the phase constitutions at the Fe-corner of the ternary Fe–Si–Cr system should be investigated particularly on the multiphase fields involving A2 and the ordered bcc phases. Raghavan [2] have determined the isothermal section of the Fe–Si–Cr system at 1173 K, which result is reproduced in Fig. 2. It was shown that B2 and D03 ordered phases are absent at this temperature but there are a wide compositional field for single phase a-Fe. Kozakai et al. [3–5] have energetically investigated experimentally and theoretically on the phase diagram and the route of phase separations in ternary Fe–X–Y (X ¼ Al, Si and Y ¼ Co, V, Ge, Ni) ordering alloy systems. However, little work has so far been available in
*
Corresponding author. Tel./fax: +81-459245495. E-mail address: [email protected] (K. Yamamoto).
published phase diagram in Fe–Si–Cr ternary system including order–disorder transition in a-Fe phase field. In the present work, phase constitution and microstructure of the Fe-corner of Fe–Si–Cr are investigated including an isothermal section at 873 K and a vertical section at a constant 10 at.% Cr. The compositional and annealing conditions investigated are so chosen as to realize the present results to become basis for a design of new heat resistant ferritic steels.
2. Experimental procedure Alloys were prepared by arc melting of Fe (99.9%), Cr (99.99%), Si (99.999%) and Ti (99.5%) under an Ar atmosphere. The nominal compositions of ternary alloys are given in the isothermal section at 1173 K shown in Fig. 2. After homogenization for 24 h at 1473 K under an Ar gas flow followed by water quenching, cube specimens with about 5 mm edge were annealed at temperature between 873 and 1373 K, and then quenched into water. Microstructures of the alloys were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Chemical compositions of the phases were determined by electron probe microanalysis (EPMA) using JEOL JXA-8900. Thin foils for TEM observation were prepared by twin-jet electropolishing using a solution of 10% perchloric acid and 90% methanol at below 223 K. A JEOL JEM-2011 TEM and
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.01.006
978
K. Yamamoto et al. / Scripta Materialia 50 (2004) 977–981
emphasis on the identification of the phases coexisting with A2 matrix. It is first necessary to distinguish bcc disordered phase (A2 phase) from bcc ordered phases (B2 and D03 phases), and then to distinguish B2 from D03 phase. Therefore dark-field examinations are of most importance [6]. Fig. 3 shows the selected area diffraction (SAD) pattern, a bright field image, 2 0 0 and 1 1 1 dark-field images of alloy H (Fe–12Si–10Cr) annealed at 873 K for
Fig. 1. Fe–Si binary phase diagram.
Philips CM200 TEM equipped with EDS were used at an accelerating voltage of 200 kV. Critical temperatures corresponding to order–disorder transitions were determined by differential scanning calorimetry (DSC) and thermo-gravimetry (TG) using NETZSCH STA449. Curie temperature can be measured by weight change using the TG equipped with a magnet outside the furnace. Specimens were heated to 1273 K and cooled to room temperature at 20 K min1 under Ar gas flow. 3. Results and discussion 3.1. Two-phase microstructures in Fe–Si–Cr ternary system Examinations of microstructures of the ternary Fe– Si–Cr alloys were performed by TEM with particular
Fig. 3. Microstructure of Fe–12Si–10Cr alloy annealed at 873 K for 100 h: (a) SAD pattern, (b) bright field image and dark-field images from (c) 2 0 0 and (d) 1 1 1 spots, respectively.
Fig. 2. Nominal compositions of the present alloys represented on the isothermal section at 973 K of the Fe–Si–Cr ternary system at 1173 K evaluated previously reported in the literature.
K. Yamamoto et al. / Scripta Materialia 50 (2004) 977–981
100 h. The indices on the SAD pattern shown in Fig. 3(a) are double bcc indices, because the lattice parameter of D03 structure is twice as large as that of A2 structure. The 2 0 0 and 1 1 1 super lattice reflections are seen on a diffraction pattern in Fig. 3(a). The super lattice reflection 2 0 0 appears in both B2 and D03 superstructures, while the 1 1 1 reflection only appears in the D03 superstructure. The image contrast is the same between 2 0 0 dark-field image in Fig. 3(c) and the 1 1 1 dark-field image in Fig. 3(d). Therefore, the phases in contrast are D03 phase and the phase out of contrast is A2 and this alloy is in the (A2 + D03 ) two-phase field at 873 K. It is noted that (A2 + D03 ) two-phase region cannot be found in the Fe–Si binary phase diagram shown in Fig. 1. The microstructure of alloy H contains small volume fraction of D03 phase with rod shape. The width of D03 phase with the {1 0 0} habit planes is tens of nm and the length is a few hundreds of nm. Fig. 4 shows dark-field images of the alloy I (Fe– 13Si–10Cr) and J (Fe–15Si–10Cr) annealed at 873 K for 100 h. In both figures the bright parts are D03 phase and the dark parts A2 phase. It seems that with a small increase in Si content, volume fraction of D03 phase seems to increase rapidly. 3.2. Vertical sections at a constant 10 at.% Cr in Fe–Si– Cr ternary system Fig. 5 shows typical DSC and TG curves during cooling obtained on alloy J. Two exothermic peaks shown by circles which accompany weight changes are observed at about 820 and 880 K. It is considered that the alloy J of (A2 + D03 ) two-phase microstructure has two magnetic transformation temperatures, one at higher temperature for A2 phase, and the other at lower temperature for D03 phase. The ordering temperature was determined by an exothermic peak shown by a solid circle at about 1135 K during cooling.
Fig. 4. Dark-field images of the alloys annealed at 873 K for 100 h: (a) Fe–13Si–10Cr alloy and (b) Fe–15Si–10Cr alloy.
979
Fig. 5. Typical DSC and TG curves of Fe–15Si–10Cr alloy during cooling.
In order to confirm that the exothermic peak observed is due to the ordering reaction, the alloy is annealed at various temperatures up to 1373 K, quenched into water, and TEM observations were performed at room temperature. It is generally very difficult to suppress a second order transition by quenching, however it is possible to deduce from TEM observations whether a disordered or an ordered state existed at high temperature. This can be done by looking at the size of antiphase domain boundaries (APBs) which are observed in darkfield images using super lattice reflections. During quenching from a disordered state, a large number of small antiphase domains are formed. On the contrary, if the ordering reactions take place at an annealing temperature, there is a time for the antiphase domains to grow much larger, usually larger than a selected area of TEM observation. Fig. 6 shows 2 0 0 and 1 1 1 dark-field images of alloy J quenched from 1173 K for 30 h (Fig. 6(a) and (b)) and 1273 K for 1h (Fig. 6(c) and (d)). The microstructural feature of alloy J quenched from 1173 K, Fig. 6(a) and (b), exhibiting a rectangular texture to lie on cube planes can be seen even in 2 0 0 dark-field image. The alloy is a mixture of extremely small D03 particles with bright contrast and disordered A2 phase with dark contrast. B2 APBs cannot be observed because they are much larger in size during annealing at 1173 K for the scale for Fig. 6(a). The microstructure of this alloy annealed at 1073 K was a coarse (A2 + D03 ) two-phase mixture and much coarser than that in Fig. 4(b) at 873 K. It can be concluded that this alloy is B2 single phase at 1173 K. It should be noted that B2/D03 transition temperature corresponds to an exothermic peak observed at about 1135 K in the DSC measurement. The 2 0 0 dark-field image of alloy J quenched from 1273 K, Fig. 6(c), shows B2 APBs formed during cooling, indicating that A2 to B2 phase transition has
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K. Yamamoto et al. / Scripta Materialia 50 (2004) 977–981
Fig. 7. The vertical section along the constant 10 at.% Cr in Fe–Si–Cr ternary system.
Fig. 6. Dark-field images of Fe–15Si–10Cr alloy; (a, b) annealed at 1173 K for 30 h and (c, d) annealed at 1273 K for 1 h, where (a, c) using 2 0 0 spot and (b, d) using 1 1 1 spot.
occurred between 1273 and 1173 K during cooling. The alloy is in the single A2 phase at 1273 K, passing through B2 phase region, and then transforms into (A2 + D03 ) during cooling to exhibit the microstructure shown in Fig. 6(c) and (d). With all these information gathered, a vertical section at a constant 10 at.% Cr of the Fe–Si–Cr phase diagram is constructed as shown in Fig. 7. Curie temperature was determined by using a TG equipped with a magnet. It is considered that the temperature of an exothermic peak in DSC measurement during cooling corresponds to the transition from B2 to D03 phase, which increases with increasing Si contents. By comparing Fig. 7 with Fig. 1, it is found that the (A2 + D03 ) phase field in the ternary system at 10 at.% Cr and (B2 + D03 ) in the binary system lie within the similar Si concentration range. 3.3. An isothermal section at 873 K in Fe–Si–Cr ternary system In order to establish an isothermal section diagram at 873 K in the Fe–Si–Cr ternary system, the microstruc-
Fig. 8. Back-scattered electron images of the alloys annealed at 873 K for 258 h: (a) Fe–15Si–15Cr alloy and (b) Fe–20Si–15Cr alloy.
tures and composition analysis of the alloys with 15 and 20 at.% Cr are investigated. Fig. 8(a) and (b) show the back-scattered electron images in alloy N (Fe– 15Si–15Cr) and O (Fe–20Si–15Cr) annealed at 873 K for 258 h. Precipitates with bright contrast are r-FeCr phase, while with a dark contrast Cr3 Si phase of A15 crystal structure. The matrix is D03 and both Cr3 Si
K. Yamamoto et al. / Scripta Materialia 50 (2004) 977–981
981
Table 1 Compositions of the D03 , Cr3 Si and r-phases evaluated by EPMA (Fe: Bal.) Alloy
D03 (at.%)
Cr3 Si
Si
Cr
Si
Cr
Si
Cr
Fe–15Si–15Cr Fe–20Si–15Cr
14.5 19.3
13.3 12.2
20.0 19.8
48.4 55.2
14.3 –
35.4 –
r
4. Conclusions
Fig. 9. The isothermal section of Fe–Si–Cr ternary system at 873 K evaluated in the present work.
and r-phases exist preferentially at grain boundaries as shown in Fig. 8(a). In alloy O, as shown in Fig. 8(b), Cr3 Si phase precipitates at grain boundaries and within grains. Chemical compositions of the D03 , r and Cr3 Si phases determined by EPMA are listed in Table 1. With all the information obtained so far together, an isothermal section diagram in the Fe–Si–Cr ternary system at 873 K is established as shown in Fig. 9. Through the present work, in comparison with that at 1173 K by previous workers in Fig. 2, (A2 + D03 ) twophase region is found to exist in compositions between 12 and 15 at.% Si, and also is found that (D03 + Cr3 Si) two-phase region extends to lower temperatures.
Phase constitution and microstructure of the Fecorner of Fe–Si–Cr are investigated including an isothermal section at 873 K and a vertical section at a constant 10 at.% Cr. (A2 + D03 ) two-phase region is found to exist at compositions between 12 and 15 at.% Si in the Fe–Si–Cr ternary system at 873 K. Phase boundaries among A2, B2, D03 single phase and some two-phase regions are determined on the vertical section at a constant 10 at.% Cr of the ternary system. It is found that B2/D03 transition temperature increases with increasing Si contents. References [1] Okamoto H, Subramanian PR, Kacprzak L. In: Massalski TB et al., editors. Binary alloy phase diagrams, 2nd ed. ASM International; 1990. p. 1772. [2] Raghavan V. Phase diagrams of ternary iron alloys, Part 1. ASM International; 1987. p. 34. [3] Kozakai T, Miyazaki T. ISIJ Int 1994;34:373. [4] Zhao PZ, Kozakai T, Miyazaki T. J Japan Inst Metals 1989;53:266. [5] Miyazaki T, Isobe K, Kozakai T, Doi M. Acta Metal 1987;35:317. [6] Mendiratta MG, Ehlers SK, Lipsitt HA. Metall Trans A 1987;18: 509.