Microstructure in a plasma-nitrided Fe–18 mass% Cr alloy

Microstructure in a plasma-nitrided Fe–18 mass% Cr alloy

Acta Materialia 54 (2006) 4771–4779 www.actamat-journals.com Microstructure in a plasma-nitrided Fe–18 mass% Cr alloy G. Miyamoto a a,* , A. Yonemo...

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Acta Materialia 54 (2006) 4771–4779 www.actamat-journals.com

Microstructure in a plasma-nitrided Fe–18 mass% Cr alloy G. Miyamoto a

a,*

, A. Yonemoto a, Y. Tanaka a, T. Furuhara b, T. Maki

a

Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan b Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received 10 November 2005; received in revised form 8 May 2006; accepted 5 June 2006 Available online 7 September 2006

Abstract Microstructures in a nitrided zone of an Fe–18 mass% Cr binary alloy plasma-nitrided at temperatures between 743 and 943 K were investigated by means of electron microscopy. The morphology of the nitrided zone changed drastically depending on the nitriding temperatures. On nitriding at 943 K, disk- and rod-shaped CrN and rod-shaped Cr2N were observed in the original ferrite grains. Examination of orientation relationships between the ferrite matrix and the rod-shaped Cr nitrides suggested that a transition from Cr2N to CrN occurs during nitriding at 943 K. On nitriding at 843 K, elongated columnar-shaped ferrite grains containing regularly spaced lamellar CrN precipitates developed from the specimen surface and grew inwards. Most of the columnar ferrite grains held R9 coincidence site lattice relationships with respect to the original ferrite grains. The columnar ferrite tends to form at a lower nitriding temperature or a higher Cr content.  2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plasma nitriding; Precipitation; EBSD; TEM; Steel

1. Introduction Nitriding of stainless steels, especially austenitic and martenstic stainless steels, has been studied and put to practical use for the past two decades. Most attention has been focused on properties and macroscopic microstructure changes at various nitriding conditions. However, understanding the microstructure of precipitates (nitrides) formed by the nitriding of high-Cr-alloyed steels is also important for broadening the application of the nitriding of stainless steels since surface hardening is induced by precipitation of fine nitrides or clusters [1–6]. Mortimer et al. [6] studied the effects of Cr content and N potential between 500 and 1000 C on the stability of CrN and Cr2N in gaseous nitriding of Fe–Cr binary alloys. They reported that CrN is stable with less than 20 mass% Cr, and

*

Corresponding author. Tel.: +81 75 753 5469; fax: +81 75 753 4861. E-mail address: [email protected] (G. Miyamoto).

with more than this critical content, Cr2N is always formed at high temperatures and CrN at lower temperatures. Mittemeijer et al. [7–11] have studied gaseous nitriding behaviors systematically in various Fe–Cr alloys. They showed that after precipitation of CrN in ferrite (a) matrix, cellular structures composed of (a + CrN) lamellae are formed at the original a grain boundaries near the specimen surface in an Fe–7% Cr alloy below 853 K while the precipitation of CrN in the original a grain takes place without formation of the cells at 973 K [9]. They reported that the cellular reaction occurs even in a monocrystalline specimen of an Fe–20% Cr alloy nitrided at 853 K [10]. Recently, Sennour et al. [12] showed that a boundaries between the cell and original a grains are predominantly near R9 coincidence site lattice (CSL) boundary in a gas-nitrided Fe–3Cr alloy using the electron backscatter diffraction (EBSD) technique. They suggested that discontinuous precipitation of CrN at preexisting a grain boundaries causes plastic deformation and recrystallization of a due to the volumetric misfit between CrN and a, leading to the formation of the cellular structure. As described above, the microstructure in

1359-6454/$30.00  2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.06.006

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nitrided Cr-containing alloys has been studied fairly well. However, most of those studies focused on alloys of lower Cr contents, typically less than 10 mass% Cr. The microstructure formed by precipitation in the nitriding of Fe high-Cr alloys has not yet been studied in detail. In the study reported in the present paper, we examined the microstructure of a nitrided zone in an Fe–18 mass% Cr alloy by means of transmission electron microscopy (TEM) and EBSD. Plasma nitriding is adopted due to its ability to nitride efficiently high-Cr steels. In particular, we focus on the effect of nitriding temperature on the precipitation mode of Cr nitrides in the nitrided zone.

Thin foil specimens for TEM observation were prepared either by ion milling or by Twin-jet electrolytic polishing using the same electrolyte as the preparation for EBSD measurements. In order to investigate an orientation relationship between nitride and a matrix, Kikuchi patterns obtained by a convergent beam method were analyzed using software for orientation determination developed by Zaefferer [13]. Furthermore, to determine the phase after nitriding, X-ray diffraction (XRD) experiments were performed using Cu Ka radiation.

2. Experimental

Fig. 1 shows cross-sectional SEM images and the corresponding a orientation maps of the Fe–18Cr specimens nitrided at 943 K for 18 ks (Fig. 1a and b) and 843 K for 18 ks (Fig. 1c and d). The area with bright contrast on the left hand side corresponds to the nitrided zone in both Fig. 1a and c. After nitriding at 943 K, the a orientation map of Fig. 1b shows no difference in a orientations between the nitrided and unnitrided zones, and an initial a grain boundary is preserved during the nitriding at 943 K. On the other hand, on nitriding at 843 K for 18 ks, columnar-shaped a grains, which are elongated along the nitriding direction and exhibit orientations different from that of the original a grains, are seen in Fig. 1c and d. In Fig. 2a, Æ0 0 1æa axes of the columnar a in Fig. 1d are plotted as filled circles with gray color on the [0 0 1]a standard stereographic projection of the original a grain in the unnitrided zone. Open squares represent cube axes for crystals holding R9 CSL orientation relationships (ORs) given by a rotation of 38.9 around Æ0 1 1æa axis with respect to the original a grain. It is seen that orientation of the columnar a grains can be well explained by R9 CSL OR. A Æ0 1 1æ pole figure of a in the nitrided and unnitrided zones of Fig. 2b shows distribution of the common Æ0 1 1æ rotation axes of R9 OR for the a in nitrided and unnitrided zones. The R9 OR has 12 variants since there are six Æ0 1 1æa rotation axes and two rotation directions for each axis. However, arrows in Fig. 2b indicate that variants of which rotation axes are nearly perpendicular to the nitriding direction are missing. Such a tendency in the R9 variant selection was observed in most cases investigated in the present study, although the reason for this behavior has not been clarified yet. Fig. 3 shows XRD patterns obtained from the surface of the Fe–18Cr specimens nitrided at 843 or 943 K for 18 ks. These patterns reveal the presence of body-centered cubic (bcc) a, MN of a B1 structure and a small amount of face-centered cubic (fcc) c 0 -M4N for these nitriding conditions. Since the intensities of a diffraction peaks are strong, it is indicated that the compound layer is thinner than the penetration depth of X-rays, which is about 2–5 lm. Fig. 4 shows SEM images of the nitrides precipitated at various depths from the surface in the Fe–18Cr specimens nitrided at 943 K for 18 ks shown in Fig. 1a. Fine disk-shaped nitrides are observed near the surface (Fig. 4a) whereas with an increase in the depth from the surface, coarse

In the present study, Fe–Cr binary alloys containing 0.94, 2.86, 9.69, 18.4, and 30.2 mass% Cr (Fe–1Cr, Fe– 3Cr, Fe–10Cr, Fe–18Cr, and Fe–30Cr, respectively) were used. The Fe–18Cr alloy was prepared by vacuum melting, whereas the others were prepared by arc melting. After a homogenization treatment at 1453 K for 86.4 ks, the Fe– 1Cr, Fe–3Cr and Fe–10Cr alloys were air-cooled to room temperature. Those specimens were subsequently annealed at 1073 K for 18 ks to remove dislocations introduced by transformation from austenite (c) to a during the air-cooling. The Fe–18Cr and Fe–30Cr alloys water-quenched after the same homogenization treatment were used for nitriding since no transformation takes place during the quenching. Cubic specimens of 5 · 5 · 5 (mm3) were cut from the heat-treated specimens, and the surfaces were mechanically polished and cleaned in acetone before nitriding. In a nitriding furnace, the specimen surface was first sputtered for cleaning with a gas mixture of 50% H2–50% Ar at the nitriding temperature for 3.6 ks. Subsequently, plasma nitriding at temperatures in the range 743–943 K for various periods was carried out with a gas mixture of 20% N2–80% H2 at a total pressure of 700 Pa; the specimen was cooled in the furnace after the nitriding. The nitriding temperature was controlled by monitoring the temperature of a dummy sample, of the same size as the nitriding specimens, to which a thermocouple was attached. Microstructure observations were performed using optical microscopy (OM), field-emission scanning electron microscopy (SEM; JSM-6500F operated at 15 kV) equipped with an EBSD orientation imaging microscopy system and a field-emission TEM instrument (CM200FEG operated at 200 kV). For OM and SEM observations and EBSD measurements, the nitrided specimens were mechanically polished after being electroplated with Ni to preserve the edges of the specimens during polishing. For OM and SEM observations, 5% nital was used for etching of the Fe–3Cr alloys, whereas electrolytic etching with a solution of 450 ml CH3COOH + 50 ml HClO4 acid was used for the Fe–18Cr and Fe–30Cr alloys. For EBSD measurements, electrolytic polishing with a solution of 500 ml CH3COOH + 20 ml H2O + 100 g CrO3 was performed at 283 K.

3. Results

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Fig. 1. SEM images and corresponding a orientation maps in the Fe–18Cr alloy nitrided at (a, b) 943 K for 18 ks and (c, d) 843 K for 18 ks. Dotted contrasts observed near the boundary between the specimen and plated Ni in (b) and (d) are caused by failure of measurements.

Fig. 2. Crystallographic orientations of a corresponding to Fig. 1d: (a) [0 0 1]a standard stereographic projection showing the orientation relationships between a in the nitrided and unnitrided zones, and (b) Æ0 1 1æ pole figure of a in the nitrided and unnitrided zones.

rod-shaped nitrides are frequently observed besides the fine disk-shaped nitrides (Fig. 4b). In the region near the growth front, the density of nitrides significantly decreases and most of them are coarse and rod-shaped (Fig. 4c). In order to examine the change in phases precipitated in the nitrided zone with depth, TEM observations were per-

formed. Fig. 5 shows TEM images of the nitrides precipitated in the Fe–18Cr specimen nitrided at 943 K for 18 ks at depths of 40–60 lm from the specimen surface. In this region, two kinds of nitrides, fine disk-shaped and coarse rod-shaped, are observed. The disk-shaped nitrides are CrN of a B1 structure as seen in the selected area diffrac-

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Fig. 3. XRD patterns recorded from the surface of the Fe–18Cr alloy nitrided at (a) 843 K for 18 ks and (b) 943 K for 18 ks.

around the BN OR is less than 20%, and other ORs, such 1 0 1CrN as Kurdjumov–Sachs (KS) ðð1 1 1ÞCrN ==ð0 1 1Þa ; ½ ==½1 11a ; ½1 21CrN ==½2 11a Þ, Nishiyama–Wasserman (NW) ðð111ÞCrN ==ð011Þa ;½1 12CrN ==½0 11a ;½1 10CrN ==½100a Þ and inverse NW ðð010ÞCrN ==ð110Þa ;½101CrN ==½1 11a ;½101CrN ==½112a Þ ORs were also nearly held. Furthermore, onethird of rod-shaped CrN do not have any of the specific ORs described above with a matrix. At a depth of 90–130 lm near the growth front region, selected area diffraction reveals that rod-shaped nitrides are not CrN but Cr2N (Fig. 6). Here Cr2N is indexed as hexagonal while Cr2N is actually trigonal when ordering of N is taken into account [14]. The ORs of those rod-shaped Cr2N with respect to a matrix are distributed between the Burgers OR ðð0 0 0 1ÞCr2 N ==ð0 1 1Þa ; ½12 1 0Cr2 N ==½1 1 1a ; ½1 0  1 0Cr2 N == ½2 1 1a Þ and the Pitsch–Schrader (PS) OR ðð0 0 0 1ÞCr2 N ==ð0 1 1Þa ; ½1 1 2 0Cr2 N ==½1 0 0a ; ½1 1 0 0Cr2 N ==½0 1 1a Þ. Fig. 7 shows TEM images of the Fe–18Cr specimen nitrided at 843 K for 18 ks at depths of 10–40 lm from the surface. a and CrN are arranged alternately and form a lamellar structure as shown in Fig. 7a. The thickness of lamellar a is about 20 nm and that of lamellar CrN is about 10 nm. Those CrN lamellae have the BN OR with respect to the a matrix (Fig. 7b and c) with the habit plane lying on the (0 0 1)a//(0 0 1)CrN. At all nitriding temperatures, the square of the nitrided zone thickness (L2) was almost proportional to the nitriding period (t). Fig. 8 shows an Arrhenius plot of the parabolic rate constant (K), where K is related to L and t by the equation L2 = Kt. Fig. 8 indicates that K varies linearly with the inverse of the nitriding temperature in spite of the remarkable change in the morphology of nitrides depending upon the nitriding temperature. From Fig. 8, an empirical activation energy of the growth of the nitrided zone is estimated at 75.6 kJ/mol, which is close to that of bulk diffusion of N, 79.1 kJ/mol [15]. A decrease in a growth rate with an increase in a temperature, previously reported by Granito et al. [16] was not observed in the present study. 4. Discussion 4.1. Formation of columnar a on nitriding at 873 K

Fig. 4. SEM images of nitrides observed in the Fe–18Cr alloy nitrided at 943 K for 18 ks: (a) 5 lm, (b) 80 lm, and (c) 100 lm (near the growth front) from the specimen surface.

tion analysis of Fig. 5b and c. Those disk-shaped CrN have the Baker–Nutting (BN) OR ðð0 0 1ÞCrN ==ð0 0 1Þa ; ½1 0 0CrN ==½1 1 0a ; ½0 1 0CrN ==½ 1 1 0a Þ with respect to the a matrix and its habit plane lies on {0 0 1}a//{0 0 1}CrN. Micro-beam diffraction taken from the rod-shaped nitride indicated by a white arrow in Fig. 5d reveals that this rod-shaped nitride is also CrN of a B1 structure (Fig. 5e). ORs between those rod-shaped CrN and a matrix are scattered in a wide range (Table 1). The fraction of rod-shaped CrN holding ORs

In the present study, two distinctive types of microstructures were observed in the nitrided zone of the Fe–18Cr alloy. Fig. 9 shows schematically the two types. On nitriding at 943 K (Fig. 9a), disk- and rod-shaped CrN are precipitated in a large part of the nitrided zone, and only rodshaped Cr2N is observed near the growth front. The change in microstructure of nitrides with the nitriding depth for Fe–Cr alloys has not been reported previously. When the nitriding temperature is lowered to 843 K, columnar a grains containing lamellar CrN are newly formed near the specimen surface and grow inward (Fig. 9b). Most of the columnar a grains hold near R9 CSL OR with respect to the original a grains.

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Fig. 5. TEM microstructures in the Fe–18Cr alloy nitrided at 943 K for 18 ks at a depth of 40–60 lm: (a) bright field image, (b) selected area diffraction pattern taken from (a), (c) corresponding key diagram to (b), (d) bright field image taken from another area, and (e) micro-beam diffraction taken from the precipitate indicated by a white arrow in (d).

Table 1 Distributions of ORs between the rod-shaped CrN particles and a matrix in the Fe–18Cr alloy nitrided at 943 K for 18 ks at a depth of 40–60 lm OR

Number of ORs deviating less than 5 from exact one

BN NW KS Inverse NW Without specific OR

5 7 8 11 10

Total number investigated is 28. The summation of numbers in the table exceeds 28 because some of ORs observed are located within a deviation of 5 from two or three ORs.

It has been reported that (a + CrN) lamella is formed from original a grain boundaries in gaseous nitriding of Fe–Cr alloys [6–12] and a in lamellae holds predominantly near R9 CSL ORs with the original a grain [12]. However, the formation behavior of columnar a containing (a + CrN) lamella nucleated near the specimen surface has not been revealed yet. In order to clarify the formation condition of the columnar a, the microstructures of Fe–(1, 3, 10, 30)Cr alloys nitrided in the temperature range between 743 and 943 K were investigated. Fig. 10a and b show OM images of the Fe–3Cr alloy nitrided at 843 K for 18 ks and Fe–

30Cr alloy nitrided at 943 K for 18 ks, respectively. It is revealed that the columnar a is not formed in the Fe– 3Cr alloy nitrided at 843 K; the columnar a is observed in the Fe–18Cr alloy. On increasing the Cr content up to 30%, columnar a is formed at 943 K (Fig. 10b). Fig. 10c summarizes the effect of Cr content and nitriding temperature on the formation of columnar a in plasma-nitrided Fe–Cr alloys including the data from previous studies [16,17]. The columnar a containing lamellar CrN tends to form at a higher Cr content and a lower nitriding temperature in agreement with the tendency in gaseousnitrided Fe–Cr alloys [9]. The formation of the columnar a holding different orientations with the original a might be attributed to one of the following mechanisms. One is the formation of non-equilibrium austenite (c) by enrichment of N and a subsequent eutectoid transformation to form a lamellar (a + CrN) structure. The other is recrystallization of a followed by discontinuous precipitation of CrN at grain boundaries between the recrystallized and original a grains. It is difficult to apply the eutectoid transformation model to the present case because columnar a tends to form at high-Cr and a low temperature where c is more unstable with respect to a in Fig. 10c. On the other hand, the recrystallization model describes well the tendency observed when plastic deformation around CrN accommo-

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Fig. 6. TEM images of rod-shaped Cr2N formed in the Fe–18Cr alloy nitrided at 943 K for 18 ks at a depth of 90–130 lm from the surface (near the growth front): (a) bright-field image, (b) selected area diffraction pattern taken from a precipitate indicated by a white arrow, and (c) the corresponding key diagram to (b).

Fig. 8. Arrhenius plot of the parabolic rate constant (K) of zone growth in plasma nitriding of the Fe–18Cr alloy.

Fig. 7. TEM images of the Fe–18Cr alloy nitrided at 843 K for 18 ks at a depth of 10–40 lm: (a) bright-field image, (b) selected area diffraction pattern taken from (a), and (c) corresponding key diagram to (b).

dating volumetric mismatch between CrN and a matrix induces the recrystallization of a as proposed by Sennour et al. [12]. When the nitriding temperature is lower or Cr content is higher, the density of CrN precipitate is higher and also recovery of a is less. A higher density of dislocations is then accumulated by the CrN precipitation, causing recrystallization of a. On the other hand, at higher

nitriding temperature and lower Cr content, plastic deformation induced by the CrN precipitation is smaller and the annihilation of dislocations by recovery occurs more frequently. Thus, recrystallization of a is difficult to occur and thus the continuous precipitation of CrN occurs without forming new a grains. Sennour et al. [12] supposed that discontinuous precipitation of CrN from an original a grain boundary induces the recrystallization of a holding R9 CSL OR with the original a. On the other hand, since columnar a was also formed in the middle of the original a grain during hightemperature + low-temperature two-step nitriding in the present study [18], we consider that the continuous precipitation of CrN can also induce the recrystallization inside the original a grain. Nakashima et al. [19,20] reported

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Fig. 9. Schematics of microstructures in the Fe–18Cr alloy nitrided at (a) 943 K and (b) 843 K.

Fig. 10. Optical micrographs of (a) the Fe–3Cr alloy nitrided at 843 K for 18 ks and (b) the Fe–30Cr alloy nitrided at 943 K for 18 ks. (c) Summary of the formation conditions of columnar a in plasma-nitrided Fe–Cr alloys. The cross and asterisk represent the data reported in Refs. [16,17], respectively.

that a R9 boundary has a larger mobility than those of low-angle and high-angle random boundaries below 1300 K in an Fe–3 mass% Si alloy. Thus, besides preferential nucleation of R9 grains proposed by Sennour et al. [12], it could be also supposed that the preferential growth due to a high growth rate of a R9 boundary results in the preferential formation of the R9 a grains. However, it is necessary to investigate the initial stage of formation of columnar a and also reveal the cause of R9 CSL ORs in order to clarify the formation mechanism of columnar a. 4.2. Transition of Cr nitrides on nitriding at 943 K Since disk-shaped CrN is known to form in a due to good matching between (0 0 1)CrN and (0 0 1)a planes with the BN OR, the rod-shaped CrN largely deviating from the BN OR in the present study is a peculiar case. It is expected that rod-shaped CrN is formed through the trans-

formation from Cr2N because Cr2N is precipitated with a rod-shaped morphology in a matrix [14]. An isothermal section at 973 K of Fe–Cr–N ternary phase diagrams [21] indicates that enrichment of N in the Fe–18Cr alloy results in the transition of Cr nitrides at 943 K as follows: stage 1

stage 2

stage 3

a ! a þ Cr2 N ! a þ Cr2 N þ CrN ! a þ CrN Thereby, it is realized that rod-shaped Cr2N is formed near the growth front at 943 K due to low N concentration (stage 1). As the nitriding proceeds, local N concentration increases causing reactions of stages 2 and 3, where CrN is formed. The precipitation of CrN might occur either by nucleation of CrN separately in the a matrix or by in situ transition from Cr2N to CrN. In the case of CrN separately nucleated in a, disk-shaped CrN with the BN OR is formed. However, when CrN transformed from Cr2N, the rod-shaped morphology of Cr2N could be imparted to CrN.

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Transformation from a hexagonal close-packed (hcp) lattice to a fcc lattice usually occurs with the retention of parallel relations between close packed planes, (0 0 0 1)hcp//(1 1 1)fcc, and close packed directions, ½1  2 1 0hcp ==½ 1 0 1fcc . By assuming those relations, CrN transformed from Cr2N having the Burgers (0 0 0 1)hcp// (0 1 1)bcc, ½1  2 1 0hcp ==½1  1 1bcc or the PS ORs (0 0 0 1)hcp// (0 1 1)bcc, ½1 1  2 0hcp ==½1 0 0bcc with respect to a matrix would have the KS OR (1 1 1)fcc//(0 1 1)bcc, ½ 1 0 1fcc ==½ 1 1 1bcc , or the NW OR, (1 1 1)fcc//(0 1 1)bcc, ½1  1 0fcc ==½1 0 0bcc with a, respectively, as shown in Fig. 11. Thus the observed ORs around the KS and NW ORs between the rod-shaped CrN and a matrix support the view that the rod-shaped CrN are formed by the in situ transformation from Cr2N. However, rod-shaped CrN having near BN, inverse NW and non-specific, irrational ORs with respect to a matrix are also observed (Table 1). The deviations from the KS and NW ORs might be caused by deformation of surrounding a matrix with precipitation of CrN due to large misfit between CrN and a, resulting in a gradual change in ORs with respect to a matrix by growth of CrN. Also, irrational ORs between precipitate and matrix were observed when VC is precipitated on a MnS inclusion incoherent to c matrix [22]. Hence, in the present case, it is considered that nucleation of CrN on the interface between Cr2N and a matrix might result in the irrational ORs with respect to a matrix. On nitriding at 943 K, the fraction of rod-shaped CrN (or Cr2N) increases with an increase in the depth from the surface as seen in Fig. 4. In the early stage of nitriding, supersaturation of N tends to be larger due to a parabolic growth rate of the nitrided zone. Since Cr2N is stable only at low N concentration, a large supersaturation of N promotes direct nucleation of CrN in a matrix, and then a large part of Cr is precipitated as disk-shaped CrN rather than rod-shaped Cr2N. As a result, the fraction of rodshaped nitrides changes with depth from the surface.

Fig. 11. Schematics of OR transition of Cr2N and CrN with respect to a matrix.

5. Summary Microstructures of the nitrided zone in an Fe–18 mass% Cr binary alloy plasma-nitrided at temperatures between 743 and 943 K were investigated by means of EBSD and TEM. The results obtained are summarized as follows. (1) On nitriding at 843 K, a grains containing lamellar CrN are newly formed near the specimen surface and grow inward, resulting in the formation of columnar a grains in the nitrided zone. Most of the columnar a grains hold R9 CSL relationships with the original a grain. The columnar a tends to form at a lower nitriding temperature or a higher Cr content. (2) On nitriding at 943 K, disk- and rod-shaped CrN and rod-shaped Cr2N are precipitated in a grains. The disk-shaped CrN has the BN relationship with respect to a matrix whereas orientation relationships between the rod-shaped CrN and a are scattered largely from the BN relationship. Based on the orientation relationships between Cr nitrides and a matrix, it is proposed that a transition from Cr2N to CrN causes the formation of rod-shaped CrN. Acknowledgements The financial supports of the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists and the Iron and Steel Institute of Japan are gratefully acknowledged. The authors wish to thank Prof. K. Osamura (Department of Materials Science and Engineering, Kyoto University) for allowing the use of a field-emission SEM instrument and Prof. Murakami (Department of Materials Science and Engineering, Kyoto University) for use of an X-ray diffractometer. References [1] Jack DH. Acta Metall 1976;24:137. [2] Rickerby DS, Henderson S, Hendry A, Jack KH. Acta Metall 1986;34:1687. [3] Rickerby DS, Hendry A, Jack KH. Acta Metall 1986;34:1925. [4] Podgurski HH, Knechtel HE. Trans Metall Soc AIME 1969;245:1595. [5] Miyamoto G, Tomio Y, Furuhara T, Maki T. Mater Sci Forum 2005;492–493:539. [6] Mortimer B, Grieveson P, Jack KH. Scand J Met 1972;1:203. [7] Mittemeijer EJ, Vogels ABP, Van Der Schaaf PJ. J Mater Sci 1980;15:3129. [8] Hekker PM, Rozendaal HCF, Mittemeijer EJ. J Mater Sci 1985;20:718. [9] Schacherl RE, Graat PCJ, Mittemeijer EJ. Z Metallkd 2002;93: 468. [10] Schacherl RE, Graat PCJ, Mittemeijer EJ. Metall Mater Trans A 2004;35A:3387. [11] Hosmani SS, Schacherl RE, Mittemeijer EJ. Mater Sci Technol 2005;21:113. [12] Sennour M, Jouneau PH, Esnouf C. J Mater Sci 2004;39:4521. [13] Zaefferer S. J Appl Crystallogr 2000;33:10. [14] Bywater KA, Dyson DJ. Met Sci 1975;9:155.

G. Miyamoto et al. / Acta Materialia 54 (2006) 4771–4779 [15] Grieveson P, Turkdogan ET. Trans Metall Soc AIME 1964;230: 1604. [16] Granito N, Kuwahara H, Aizawa T. J Mater Sci 2002;37:835. [17] Alves Jr C, Rodrigues JA, Martinelli AE. Mater Sci Eng A 2000;279A:10. [18] Miyamoto G. Doctoral thesis, Kyoto University; 2006.

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[19] Nakashima H, Ueda T, Tsurekawa S, Ichikawa K, Yoshinaga H. Testu-to-Hagane 1996;82:238. [20] Uehara M, Toshida H, Nakashima H. Tetsu-to-Hagane 1998;84:212. [21] Raghavan V. Phase diagrams of ternary iron alloys. Materials Park, OH: ASM International; 1987. p. 171. [22] Furuhara T, Kimori T, Maki T. Metall Mater Trans A 2006;32:951.