On the interpretation of the crystal structure of the 18R martensite in duplex FeMnAlC alloys

Scripta METALLURGICA et MATERIALIA

Vol. 26, pp. 1863-1868, 1992 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

ON THE INTERPRETATION OF THE CRYSTAL STRUCTURE OF THE 18R MARTENSITE IN DUPLEX Fe-Mia-A1.-C ALLOYS Cheng-Hu Chao and New-Jin Ho Institute of Materials Science and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan, R. O. C. (Received March 4, 1992) (Revised April 14, 1992) Introduction In the 1960's, Lysak and Nikolin [1, 2] discovered a long period stacking order (LPSO) structure in disordered manganese-containing steels (0.4 - 0.7wt%C, 10 - 12wt%Mn) subjected to cyclic cooling and heating. The unit cell of this martensitic phase consisted of 18 layers in the close-packed plane of the FCC T-austenite matrix. The intensity of the x-ray diffraction spots showed that its stacking sequence could be expressed as 18R and (5T) 3 in terms of the Ramsdell notation and the Zhdanov symbol, respectively. The other two close-packed structures, i.e., 18R(4 2-)3 and 18R(1 ~ 31-)3, were ruled out. Late in the 1970's, the same disordered LPSO structure was confirmed with conventional transmission electron microscopy (CTEM) by Oka et al. [3, 4] in a similar experiment. The way they differentiate this martensite from the other ones is mainly by the microphotometer tracing of the intensities of electron diffraction spots. Recently, a series of TEM works had concentrated on the phase transformation of duplex (austenite plus ferrite) Fe-Mn-AI-C alloys during 1300°C quenching [5 - 9]. Many needle-like precipitates were found within ferrite matrix, and much of them were identified as the disordered 18R(5~)3 martensite. The ratio of lattice parameters in the orthorhombic unit cell was suggested to be a : b : c = ~ : 2 : 18-2,/'~. Tomida et al. [10, 11] had shown two kinds of plate-like products within ferrite columnar grains of the meltquenched 8/Tduplex stainless steel ribbons by CTEM and high resolution electron microscopy (HREM). They are FCC austenite and 18R(4 2-)3 martensite, respectively. With CTEM, both the relative positions and relative intensities of diffraction spots were utilized to discern the (4~) and the (51-) structures. As part of a study of the feasibility of duplex Fe-Mn-AI-C alloy by laser welding, the authors have observed a variety of interesting microstructures in the weld regions based on optical microscopy (OM) and scanning electron microscopy (SEM), and the martensitic transformation from BCC ~-ferrite was seemly included [12]. Further detailed CTEM and HREM studies have confirmed a disordered 18R(4~) 3 structure existed instead of the 18R(5 1-')3 type. In this preliminary paper, a criterion to identify and differentiate these two confusing structures by selected area diffraction pattern (SADP) of the CTEM is described. It is used to examine the martensite-like product in the fusion boundary. Discussion is also inferred to the re-identification of the previous articles. Identification and Differentiation of 18R(4 ~ 3 and 18R(5 ~ 3 Structures The stacking sequences and/or atomic arrangements of the disordered 18R(4 2-)3 and 18R(51-)3 martensites can be described schematically in Figure 1. Both consist of 18-layer stacki_ngs of close-packed planes parallel to the basal plane with the orthorhombic unit cell ( lattice constants a : b : c = q3:1 : 182 ~ ) . They are divided into three periods, and each period contains six layers with the same stacking order. The stacking order of (42-) means four forward (or positive) plus two backward (or negative) stacking planes. The (5 1-) type order is composed of five forward and one backward stacking planes. Three types of close-packed stacking planes, namely A, B, and C,

1863 0956-716X/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.

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containing two atoms per unit layer in each, have the relationships of B = A + ( 1 / 3 , 0 ) and C = A + ( 2 / 3 , 0 ) . The two dimensional atom sites in type A plane are ( 0 , 0 ) and ( 1 / 2 , 1 / 2 ) . According to these crystal lattices, we can construct their reciprocal lattices based on the indices of a*, b*, and c* axes as (6 0 0), (0 2 0), and (0 0 18), respectively. Obviously, the best way to identify these structures is viewing into the a and c axes plane, i.e., from a direction parallel to the b axis, [0 1 0]. Differentiation can be easier by examining the atom arrangements along the c axis. Therefore, the structure factors whose square values are propotional to the diffracted intensities can be calculated and rearranged into the following equations, as modified from Ref. [10]: F A = f { 1 + exp [ 27ti (A/2 + ,~/2)] } F42 = F A (1 + x+ x 2 + x 3 + x 4 + x4y) • ~,exp [2him (2A/3 + ~/3)] ~t

FS1 = F A (1 + x + x 2 + x 3 + x 4 + x 5 ) • ~,exp [2him ( A/3 +.t/3)] where f is atomic structure factor, x = exp [2rti (A/3 + t/18)] is a parameter in B-type stacking, y = exp [2~i ( A/3 + t/18)] indicates a parameter in C-type stacking, and (A ,~ £) are Miller indices of a plane and its reflection; FA, F42, and Fs1 are structure factors of A-type stacking, 18R(4~)3, and 18R(5"T)3, respectively. The geometrical values of the orthorhombic lattice [13]: interplanar spacing, d, and interplanar angle, ~, can be calculated with respect to the + (0 0 18) plane: d00ls/d = 0.816Z cos 00 = 0.068(~/Z) where Z = (A2/3 + ~2 + £2/216)1/2. According to the above equations, data of relative intensity (I/Imax), relative d-spacing, and angle in two periods of reflections (2 0 £), ~ = 18n + 1, 4, 7, 10, 13, and 16 along the c* axis are listed in Table 1. They show some facts: firstly, the angle between (0 0 18) and (6 0 0) is 90 deg; secondly, the relative intensities among (2 0 ~) reflections are very different between 18R(4 g)3 and 18R(5 ]')3; thirdly, the most intense reflections in the former structure are (2 0 8) and (2 0 10), but (2 0 7) and (2 0 11) for the latter one, which have different values of relative d-spacing and angle; lastly, there are two less strong reflections beside the most intense one, and their spacings and angles are also discernible. TABLE 1. Calculated Values of Relative Intensity, Relative d-spacing, and Relative Angle of Reflections.

0018 202 205 208 2011 2014 2017

(4~)3 IBmax

(5i)3 I/Imax.

2 77 100 33 18 9

3 3 7 100 26 5

d0m 8/d 1 0.950 0.983 1.042 1.124 1.223 1.335

O0 0 83.2 73.6 64.8 57.1 50.5 45.0

A ,~ ~ 6 0 0 2 0 16 2 0 13 2 0 10 2 0 7 2 0 4 2 0 1

(4~)3 IBmax

(5i)3 I/Imax

2 77 100 33 18 9

3 3 7 100 26 5

d0ms/d 2.828 1.296 1.188 1.094 1.020 0.969 0.945

¢~ 90.0 46.7 52.6 59.5 67.6 76.7 86.6

To clarify these facts, SADPs in 10 1 0] zone of these two structures by a computer simulation are given in Figure 2. Both of them indicate six spots in each period along c* axis, and spots are not symmetrical with respect to the a* axis. Detailed analysis in the SADPs of [0 1 0l and [0T0] zones of 18R(4-2)3 and 18R(5 l'i')3 are shown schematically in Figure 3. The intensity is represented by the closed area of a circle. Angles between + (0 0 18) and three strong reflections are marked. There are two periods of (2 0 ~) reflections on both sides of a* axis. In

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the comparison between [0 1 0] and [0T0] zones of the same structure, i.e., Figure 3(a) to 3(b) and Figure 3(c) to 3(d) pairs, they have the same relative intensity sequences. However, these two periods of reflections are reversed, and they can be differentiated by angles. Comparing Figure 3(a) to 3(c) of [010] zones, both the intensity distribution and the position of each reflection are different, so does in the comparison of [0 T 0] zones in 3(b) and 3(d). For example, the relative 0-angle difference between (5 0 8-) and ~ 0 7--3is 2.8°, and the others are between 2.1 ° and 3.1 °. Although the relative positions of two reflection periods in Figure 3(a) and 3(d) are the same, positions of the three stronger spots are different. They still can be differentiated by angles. Figure 3(b) and 3(c) are also discernible. Therefore, the direct and easy criterion to identify the exact ferrous 18R martensite is by the measurement and comparison of angles of two or three intense (2 0 ~) reflections in each reflection period. It can be re-confirmed by the relative d-spacings. According to this criterion, the necessary information about relative intensity is qualitative rather than quantitative. Exoerimental Procedures A 100 x 30 x 6.4 mm cold-rolled plate, with the composition Fe-28.69Mn-8.90A1-0.56C-0.34Si (in wt%) was used for the present work. It was welded by a continuous wave CO2 laser machine with parameters of 2500W (power), 10mm/s (speed), and on focus. After welding, pertinent regions of the weldment were prepared into thin foils by the following procedures: cutting, grinding and etching to reveal weld area, spark machining into 3 mm disc, grinding down to 80 t.tm thickness, and thinning electrolytically with a solution of 10% perchloric + 20% ethal + 70% acetic acid. The analysis was performed in JEM 200CX electron microscope operating at 200 kV and equipped with a double-tilt specimen holder. Results

The TEM work on the plate-like products within the ferrite grains in the fusion boundary of the weld is shown in Figure 4. Figure 4(a) is a bright field, image taken by a two-beam condition near [1 1 1] zone of the matrix, which shows four variants of the plates and some dislocations. After tilting along + 12" 1 Kikuchi line pairs to the [3 4 5] zone, the exact zone of the blacker plate (marked as A) was attained. Figure 4(b) shows the diffraction pattern of matrix and indices of spots. Obviously, DO3 reflections co-exists with ferrite reflections (underline mark). The diffraction pattern of this plate is given in Figure 4(c), where spots belonging to the matrix were included. Detailed analysis of reflections by the plate alone is schematically shown in Figure 4(d). Both the angles and spacings of strong (~0 ~) reflections match with those shown in Table 1 and Figure 3(b), especially in the most intense reflections. In addition, the relative intensities are qualitatively different from those shown in Figure 3(c) and (d). Therefore, it is identified as the [ 0 i 0 ] zone of 18R(4 2--'3) martensite. It is noted that the DO3 superlattice shown in the BCC matrix is not concerned with the ordering of the 18R martensite during high temperature quenching, and this will be present elsewhere. Discussion The results suggest that the structure formed by rapid quenching from temperatures up to the melting point of ferrite in duplex Fe-Mn-A1-C alloy is 18R(4 2-)3 martensite instead of the 18R(5 1)3 structure reported in the previous investigations [5 - 9]. Both the alloy composition and thermal history of the present work and those are very similar, and these two kinds of LPSO structures are not so easy to differentiate. In order to verify the real rapid-quenched structure in duplex Fe-Mn-A1-C steels and to test the reliability of the criterion we present, all of the [0 1 0] zone SADPs of 18R martensite shown in steels are re-identified as follows. Figure 5 is the angles measured from Figure 5(b) of Ref. [3], and are almost the same as those in Figure 3(b), not to mention the relative intensities. That is to say that it is indeed a [0 1 0] zone pattern of 18R(5 1-)3. Figure 6 lists data of six intense ~ 0 ~) reflections in Figure 3(b) of Ref. [5]. Both the quantitative angles and qualitative intensities strongly indicate a [0 1 0] zone of 18R(4 2-')3 structure. As in the same reason, Figure 4(b) of Ref. [6], Figure 2(b) of Ref. [9], and Figure 3 of Ref. [11] were taken from the [0 1 0] zone of 18R(42) 3 martensite. On the other hand, Figure 6(a) of Ref. [7], Figure 7 of Ref. [8], and Figure 8(b) of Ref. [10] all belong to the [0T0] zone of 18R(42) 3. Therefore, it is evident that the 18R martensites in steels can be identified according to the qualitative intensity and quantitative angles of the (2 0 ~) reflections. It is also inferred that the (42-) type stacking order prefers to transform from BCC parent phase than the (5 1-) type stacking order.

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Conclusions The large amount of plate-like products fomaed during high temperature rapid quenching within ferrite grains of duplex Fe-Mn-A1-C alloys have interpreted to be the 18R(4~) 3 martensite rather than the previously misidentified 18R(5 T)3 structure. Acknowledgement~ The authors would acknowledge the financial support by the National Science Council of R. O. C. under the grant No. NSC81-0405-E 110-02. References 1. L. I. Lysak and B. I. Nikolin, Dokl. Akad. Nauk S.S.S.R. 153, 812(1963). 2. L. I. Lysak and B. I. Nikolin, Fiz. metal, metalloved. 20, 547(1965). 3. M. Oka, Y. Tanaka and K. Shimizu, Jpn. J. Appl. Phys. 11, 1073(1972). 4. M. Oka, Y. Tanaka and K. Shimizu, Trans. Jpn. Inst. Met. 14, 148(1973). 5. K. H. Hwang, S. K. Chen, W. S. Yang, T. B. Wu, C. M. Wan and J. G. Byrne, Scripta Metall. Mat. 24, 495(1990). 6. W. S. Yang, T. B. Wu and C. M. Wan, Scripta Metall. Mat. 24, 895(1990). 7. K. H. Hwang, C. M. Wan and J. G. Byrne, Scripta Metall. Mat. 24, 979(1990). 8. K. H. Hwang, C. M. Wan and J. G. Byrne, Mater. Sci. Eng. A132, 161(1991). 9. K. H. Hwang, W. S. Yang, T. B. Wu, C. M. Wan and J. G. Byrne, Acta Metall. Mater. 39, 825(1991). 10. T. Tomida, Y. Maehara and Y. Ohmori, Mater. Trans. Jpn. Inst. Met 30, 326(1989). 11. T Tomida, J. Jpn. Inst. Met. 53, 1208(1989).(in Japanese) 12. C. H. Chao and N. J. Ho, J. Mater. Sci. 27, (1992), in press. 13. B. D. Cullity, Elements of X-ray Diffraction, p.501, Addison-Wesley Publishing Co., Massachusetts (1978).

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