Surface relief effects associated with the formation of grain boundary allotriomorph in an Fe–C alloy

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Acta mater. Vol. 46, No. 8, pp. 2929±2936, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 S1359-6454(97)00482-5

SURFACE RELIEF EFFECTS ASSOCIATED WITH THE FORMATION OF GRAIN BOUNDARY ALLOTRIOMORPH IN AN Fe±C ALLOY XIANG-ZHENG BO and HONG-SHENG FANG Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, P.R. China (Received 15 July 1997; accepted 20 October 1997) AbstractÐSurface relief e€ects associated with the formation of grain boundary allotriomorph (GBA) in an Fe±C alloy have been ®rst studied by scanning tunneling microscopy (STM). It was discovered that the GBAs which exhibit the surface relief are only developed into one austenite grain, which means the interface between the austenite and the GBA which exhibits surface relief is partially coherent, and the interface between the austenite and the GBA nucleated at the same grain boundary but developed into the adjacent austenite grain may be incoherent. The maximum shape deformation of surface relief due to GBA precipitation is ca. 0.39, which is consistent with that of WidmanstaÈtten ferrite plates. Accompanying the formation of WidmanstaÈtten sawteeth, surface relief is produced, which is composed of reliefs associated with several sideplates. By STM the boundaries between GBAs are clear, which indicates that they are resulted from edge-to-edge (ETE) or face-to-face (FTF) sympathetic nucleation. Super ledges (ca. 300 nm high) were observed on the broad face of GBA. All these results can be rationally interpreted by the sympathetic nucleation-ledgewise growth mechanism. # 1998 Acta Metallurgica Inc.

1. INTRODUCTION

The Dube morphological classi®cation system on proeutectoid ferrite in steels [1±3] includes six components, that is, grain boundary allotriomorphs (GBAs), primary and secondary WidmanstaÈtten sideplates, primary and secondary sawteeth, intragranular WidmanstaÈtten plates, grain boundary and intragranular idiomorphs, and massive structure. It is well known that accompanying the formation of WidmanstaÈtten sideplates the surface relief that is tent-like is generated [3±7]. However, whether the formation of GBAs in steels produces surface relief or not has not been examined. Christian pointed out that a macroscopic shape change is possible only if the two lattices are coherent or semi-coherent across a planar interface [8] and recently indicated that the lattice correspondence is not destroyed in some di€usional transformations [9]. Because the WidmanstaÈtten sideplate does not separate from the GBA, the orientation relationship of GBA to the austenite must be the same as that of the sideplate which evolves from the GBA [2], i.e. K±S relationship [10]. It was found that the grain boundary precipitates tend to maintain good coherency with respect to both of adjacent matrix grains as much as possible [11±13]. Tent-like surface relief caused by the formation of GBAs in an Al±4.5Cu (wt%) alloy was observed by light optical microscopy (LOM) [14]. The evidence that interface between grain boundary a precipitate and the matrix b is partially coherent containing growth ledges and

mis®t-compensating ledges [15] leads to a tent-like surface relief when grain boundary a allotriomorph precipitates from the matrix b in a Ti±Cr alloy [16]. Therefore, it is not surprising that the surface relief evolves accompanying the formation of GBA in steels. This paper aims to study the surface relief due to GBA precipitation in steels by scanning tunneling microscopy (STM), which has been proved to have many advantages of examining the surface relief e€ects associated with a solid-to-solid phase transformation [17±22]. The vertical resolution of STM can reach 0.01 nm, and lateral resolution is 0.1 nm [23, 24].

2. EXPERIMENTAL

The steel employed has a composition Fe±0.37 C (wt%). After being homogenized at 1473 K for 48 h, the ingots were cut into 10  10  4 mm plates. The prepolished specimens were encapsulated in quartz tubes with 10ÿ4 Pa vacuum degree, and were austenitized at 1473 K for 20 min, and then continuously quenched into room temperature water without breaking tubes in order to protect the surface of the specimen from being oxidized. A large number of WidmanstaÈtten sideplates formed originating from GBA, and the retained austenite transformed into pearlite, which cannot cause a macroscopic shape change. Before STM observation, the surface relief caused by GBA precipitation was observed by polarized

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BO and FANG: Surface relief

optical microscopy. The microhardness impressions were marked on the relieved specimens so that the morphology of GBA at the same area after being etched was observed. STM was operated in ambient air under a constant tunneling current mode. The constant tunneling current is 1.5 nA, and bias voltage is 30 mV. The tip was made by chemical etching of a tungsten wire in 5 M NaOH solution.

3. RESULTS

Figure 1 shows an optical microstructure of GBA and the WidmanstaÈtten sideplates formed with continuous cooling from 1473 K to room water. It is obvious from Fig. 1(a) that the GBA produces a surface relief e€ect indicated by an arrow A from which originate WidmanstaÈtten sideplates marked by an arrow B. This is in agreement with the observations of Aaronson [2, 3]. Figure 1(b) is an optical micrograph of GBA showing the same area with Fig. 1(a) without any mechanical polishing. In Fig. 1(b) GBAs nucleated in the grain boundary and developed into two austenite grains. However, in Fig. 1(a) the GBAs which exhibit surface relief e€ects only developed into one austenite grain; the GBAs which developed into the adjacent austenite grain are not associated with surface relief. The prerequisite for the formation of surface relief is that the interphase boundary between the precipitate and the matrix is coherent or semi-coherent [8]. Therefore, the interface between the GBA which exhibits surface relief and the austenite is semicoherent, but the interface between the adjacent austenite matrix and the GBA nucleated at the same grain boundary but developed into another austenite may be incoherent. In one austenite grain, secondary sawteeth originate from the GBAs by edgeto-edge (ETE) sympathetic nucleation [25, 26], and the interphase boundary between them is clearly seen in Fig. 1(b). Very small surface relief e€ect is

caused by the formation of secondary sawteeth indicated by an arrow C. The GBA precipitates are ca. 2.0±2.5 mm in width, and the width of GBA relief is nearly the same as that of etched GBA. Thus the surface relief of GBA originates only from the GBA, not from both the GBA and its accommodation slip deformation which took place in the matrix proposed by Christian [8]. A STM image of the surface relief due to the formation of GBA is shown in Fig. 2. WidmanstaÈtten ferrite sideplates indicated by WF in Fig. 2(a) develop from GBA. The width of GBA is ca. 0.8 mm, and that of WF is ca. 0.2±1.0 mm. Although the surface relief of GBA seems to be of an invariant plane strain (IPS) type seen from Fig. 2(b), the real nature of its surface relief is still unknown because of the intervention of WidmanstaÈtten sideplates nucleated at the GBA. The aspect ratio of GBA in Fig. 2 is ca. 0.32, which is nearly equal to the average aspect ratio, one third, in the paper [27]. Bradley et al. [27] suggested that because of the maintenance of the non-equilibrium dihedral angle and the presence of a proportion of partially coherent facets at the broad faces of GBA during the GBA growth, the aspect ratio of GBA is independent of reaction time, reaction temperature, and carbon content. The existence of partially coherent area in the interphase boundary between GBA and austenite grain suggests the lattice correspondence [9] and atomic site correspondence [28] which lead to the formation of surface relief that is IPS [29±32] or tent-like [3± 7, 16, 33, 34] even in the di€usional transformations with solute partitioning or drastic change in long rang order. Figure 3 is another STM image showing the surface relief of GBA. In Fig. 3(a) the GBAs marked by A, B, C, and D is ca. 200 nm in width, and the aspect ratio is ca. one third on average. Since the boundaries between the GBAs are obvious, they

Fig. 1. Optical micrograph of grain boundary allotriomorph in an Fe±0.37 C (wt%) steel. (a) surface relief of GBA; (b) the same area after etching in nital.

BO and FANG: Surface relief

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Fig. 2. STM image showing the surface relief of grain boundary allotriomorph. (a) STM image, GBA: grain boundary allotriomorph, WF: WidmanstaÈtten ferrite; (b) pro®le along the line GG' in (a), GB: grain boundary.

formed either by edge-to-edge (ETE) sympathetic nucleation [25, 26] or by the impingement of the GBAs which nucleated separately. If they formed by the impingement of two precipitates, the boundaries caused by the impingement would be irregular, because the impingement is random. However, in Fig. 3(a), the boundaries are straight and almost parallel to each other, which means the boundaries belong to an identical crystallographic plane, therefore, it is sympathetic nucleation other than impingement that operates during the growth of GBAs shown in Fig. 3(a). It is clear that all the GBAs originate from the austenite grain boundary indicated by GB and develop only into one austenite grain, which is consistent with the Fig. 1. Figure 3(b) reveals the topography along the line HH' in Fig. 3(a). It can be seen that the austenite grain

boundary is concave on the surface, which is lower ca. 400 nm than the surface. Due to the intervention of the grain boundary, whether the surface relief associated with GBA is of IPS type or of tent-shaped type is still unknown. Besides GBA, a WidmanstaÈtten sawtooth exhibits surface relief, as shown in Fig. 4(a). The sawteeth morphology is a compromise between the GBAs and WidmanstaÈtten sideplates [2]. The triangleshaped WidmanstaÈtten sawtooth in Fig. 4(a) is actually made up of three parallel ferrite crystals which appear to be nearly plate-shaped, as indicated by the arrow S1, S2 and S3 in Fig. 4(c). Along the line PP' in Fig. 4(a), the pro®le in Fig. 4(c) supplies the height of surface relief accompanying sawteeth formation, ca. 250 nm. The height di€erence amongst the parallel ferrites is 90±

Fig. 3. STM image of surface relief associated with GBA. (a) STM image; (b) pro®le along the line HH' in (a).

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BO and FANG: Surface relief

Fig. 4. STM image of surface relief caused by the formation of GBA and sawteeth ferrite. (a) STM image; (b) pro®le along the line MM' in (a); (c) pro®le along the line PP' in (a).

100 nm, which is lower than the vertical resolution of LOM (0.01 mm). They originate from GBA, which seems to be composed of two GBA ferrites. The pro®le in Fig. 4(b) reveals the topography of a GBA along the line MM' in Fig. 4(a). In order to clearly observe the GBAs in Fig. 4(a), the scan scope of STM was reduced. The interface between the GBAs is apparent, as shown in Fig. 5(a). Because the GBA marked by B nucleated at the side of the GBA A instead of at the grain boundary, it is impossible for the GBA B to nucleate separately within austenite grain and impinge GBA A. Therefore, GBA B was resulted from faceto-face (FTF) sympathetic nucleation [25, 26]. The undulation along the line QQ' in Fig. 5(a) is shown in Fig. 5(b). The di€erence in height of two GBAs is ca. 80 nm. Figure 6(a) is a STM image, showing three GBAs nucleated at the corner of grain boundaries and developed into one austenite grain. The boundaries between them are clear and parallel to each other. Along the line AA' in Fig. 6(a), the height variation

is shown in Fig. 6(b). The height of three GBAs is not equal, but increases from right GBA to left GBA. Moreover, the size of GBA is increasingly large from right GBA to left GBA. All these imply that the GBAs shown in Fig. 6(a) formed sequentially from left to right. So the GBAs nucleated ETE sympathetically. By STM the ledges on the broad face of GBA were observed, as shown in Fig. 7. Figure 7(a) exhibits a three-dimensional morphology of GBA, on the broad face of which exists a ledge indicated by an arrow. Its height is ca. 300 nm, corresponding to the previous results [27]. Along the line SS' in Fig. 6(a), the pro®le in Fig. 6(b) indicates the height of GBA relief is ca. 350 nm. In Ti-base alloy, the growth ledges (approximately 8±9 nm in height) and mis®t-compensating ledges were observed on the broad face of GBA by TEM [35]. All these results indicate that GBA may form by lateral migration of ledges. From the pro®le shown in the above ®gures, Table 1 summarizes the height and shape deformation of the surface relief associated

BO and FANG: Surface relief

2933

is incoherent, so GBA does not exhibit surface relief e€ects. It is true that not all GBAs produce surface relief. The GBA indicated by an arrow D in Fig. 1(b) is not found correspondingly in Fig. 1(a). Because the GBA lattices have the same lattice relationship with respect to the austenite grains in which they nucleated as the WidmanstaÈtten sideplates which developed from them, the matching of atom varies with the orientation of interphase boundary. When the lattice relationship with respect to the poor atom matching across the boundary, the boundary between the GBA and the austenite grains is incoherent [2], and consequently the macroscopic shape change accompanying GBA formation does not occur. On the other hand, when the atom matching on the interface is good, the boundary structure is semi-coherent, i.e. coherent areas and arrays of dislocations, it may lead to surface relief originating from the GBA formation, as revealed by Fig. 1. One of the two driving forces for the GBA growth is the di€erence in chemical free energy between the ferrite and austenite, and another is the minimization of the interfacial energy. Aaronson [2] pointed out that the interfacial energy minimization serves to be operative even after the chemical free energy di€erence is reduced to zero. The interfacial

Fig. 5. Magni®cation image of Fig. 4 showing two GBAs formed by FTF sympathetic nucleation. (a) STM image; (b) pro®le along the line QQ' in (a).

with GBA. The maximum shape deformation is ca. 0.39, which is in agreement with that of the surface relief associated with the WidmanstaÈtten sideplates [6, 7]. The height is ca. 160±350 nm. 4. DISCUSSION

4.1. Surface relief and partially coherent boundary It is evident from Figs 1±7 that surface relief e€ects occur accompanying the formation of GBA. Watson and McDougall [7] reported that the interphase boundary of GBA precipitate and the matrix

Table 1. Height and shape deformation of surface relief due to GBA precipitation No of ®gure

2

3

4

7

Height (nm) Shape deformation

160 0.37

190 0.14

250 0.39

350 ±

Fig. 6. Surface relief due to GBA precipitation. (a) STM image; (b) pro®le along the line AA' in (a).

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BO and FANG: Surface relief

Fig. 7. STM image showing the ledge on the broad face of GBA. (a) three-dimensional image; (b) pro®le along the line SS' in (a).

energy of dislocation facets (partial coherence) is lower than that of disordered facets. Thus the minimization of interfacial energy can be accomplished by extending the dislocation facets at the expense of ferrite with disordered interface. Afterward, Hall et al. [36] introduce the monatomic structural ledges into f.c.c./b.c.c. interface, and increase the coherent area of interface from 8% to 25%. Then, Russell et al. [37] noted that the disordered area between the structural ledges can be made partially coherent by insertion of a mis®t dislocation. Therefore, the dislocation facet anticipated by Aaronson [2] is actually the closely spaced structural ledges interspersed with mis®t dislocations, and hence is partially coherent, while in Ti-base alloy, GBA do not appear to develop mis®t dislocations which, however, are replaced by ledges at partially coherent boundary [35]. Therefore, the precipitates maintain the coherent boundary with respect to the matrix as much as possible. Although it is possible that the GBA grows into two adjacent austenite grains, the GBA which produces surface relief only develops into one austenite grain. The disordered boundary between the allotriomorph and the austenite matrix does not lead to a macroscopic shape change accompanies the formation of the GBA. However, the GBA nucleating at the same grain boundary maintains a partially coherent interface with respect to the adjacent austenite, which lead to a surface relief e€ect. Therefore, all the GBAs exhibiting surface relief observed by STM only grow into one austenite grain. A tent-shaped surface relief e€ect accompanying the formation of GBA was observed by LOM in an Al±Cu [14] and a Ti±Cr alloy [16]. In these previous study [14, 16], the grain boundary of the matrix are not distinguished between the GBAs. The whole GBA exhibited a tent-shaped relief. The in¯uence of grain boundary on the surface relief e€ect is not considered. TEM analysis has proved that the pre-

cipitates nucleating in the grain boundary in an substitute alloy maintain coherence with both matrices as much as possible [12, 15, 16]. However, in the present investigation, the GBAs separated by the grain boundary are shown in Fig. 1(b). The grain boundary of austenite is apparent. The GBA keeps the K±S relationship with respect to one austenite grain, while the orientation relationship with respect to the adjacent grain is irrational. Carbon di€usion may destroy the partially coherent interface with the irrational orientation relationship. Therefore, the GBA which exhibits the surface relief e€ect only develops into one austenite grain. 4.2. Sympathetic nucleation-ledgewise growth mechanism Sympathetic nucleation is de®ned as the nucleation of a precipitate crystal at an interphase boundary of a crystal of the same phase when these crystals di€er in composition from the matrix phase throughout the transformation process. There are three modes of sympathetic nucleation, i.e. ETE, ETF (edge-to-face), FTF [18, 25, 26]. The ETE sympathetic nucleation is frequently observed in proeutectoid a laths of Ti±X alloy [38] and recently in the lower bainitic sub-subunits of steels containing silicon [18]. In the present investigation, because all the results are observed on the specimen which formed into surface relief without any polishing and etching by STM, and the boundaries between the sympathetic crystal and substrate crystal are of the same crystallographic plane respectively in Fig. 2(a) and Fig. 7(a), the ETE sympathetic nucleation other than impingement [26] in GBA can be con®rmed. Although the morphological con®guration of WidmanstaÈtten sideplate which appears to have resulted from sympathetic nucleation at GBA has been observed [2, 5], such examples are rare and observed only in etched specimen. Figure 2(a) shows the surface reliefs caused by the formation of a number of WidmanstaÈtten sideplates sympatheti-

BO and FANG: Surface relief

cally nucleating at the surface relief associated with GBA. The boundary between the GBA and sideplates is clear and its width, i.e. the spacing between sideplates and GBA, is ca. 40±80 nm. They also resulted from ETE sympathetic nucleation [26]. Up to now, it has not been discovered that the con®guration of GBA results from FTF sympathetic nucleation, as shown in Fig. 5(a). In Fig. 5(a) the boundary between the broad faces of two GBAs indicated by a hollow arrow is obvious. Moreover, the GBA con®guration of ETE sympathetic nucleation indicated by a solid arrow is also observed in Fig. 5(a). The angle between the broad face and the edge of the GBA indicated by A in Fig. 6(a) is ca. 1358. The habit plane of GBA is the same as that of WidmanstaÈtten ferrite, i.e. {111}g, and the orientation relationship between the GBA and austenite matrix is that of K±S. Therefore, it is possible for the angle between one habit plane and its conjugate habit plane to be 1208. GBA can be formed by ETE and FTF sympathetic nucleation simultaneously. Many experimental results have exhibited the ledges on the broad face of GBA [15, 27, 35] as shown in Fig. 6. All these results indicate that GBA grows by the lateral migration of ledges. Due to the enrichment of solute atoms at the growth front of the ledge [18], or the obstruction of barriers (such as inclusions, other primary precipitates [39], secondary precipitates [40], etc), or the interaction between the concentration ®elds of the ledges [41], etc, the ledge stops migration, and a GBA stops growth. After an incubation, a new GBA nucleates ETE or FTF sympathetically. As we pointed out [18], sympathetic nucleation competes and alterations with ledgewise growth throughout the whole growth process. In a summary, it is sympathetic nucleation-ledgewise growth mechanism that is operative during the formation of GBA. 5. CONCLUSIONS

Surface relief accompanying the formation of grain boundary allotriomorph in steels has been ®rst studied by scanning tunneling microscopy. The following conclusions were reached. (1) Accompanying the formation of GBA in steels, the surface relief is produced. It is only associated with the GBA, not both GBA and the matrix in which accommodation slip takes place. (2) The GBAs which exhibit the surface relief e€ect only develop into one austenite grain. No surface relief is produced accompanying the formation of GBA which develops into the austenite grain adjacent to the one, in which the surface relief is produced associated with the precipitation of the GBA originating from the same grain boundary. (3) The formation of surface relief caused by GBA precipitation only developed into one austenite grain indicates that the interface between the

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GBA and the austenite matrix at one side of grain boundary is partially coherent, and the interface between the GBA and the adjacent austenite matrix at another side of the same grain boundary may be disordered. (4) The maximum shape deformation of the GBA relief is ca. 0.39, which is consistent with that of WidmanstaÈtten ferrite relief. The height is ca. 160± 350 nm. The aspect ratio of GBAs is one third on average. (5) Macroscopic shape change caused by the formation of sawteeth occurs. It is actually made up of reliefs associated with several ferrite plates. (6) The con®gurations of ETE and FTF sympathetic nucleation are observed simultaneously among the surface reliefs associated with GBAs precipitation. The super ledges (approximately 300 nm high) on the broad face of GBA are observed by STM. (7) The sympathetic nucleation-ledgewise growth mechanism is suggested to explain the formation of GBA.

REFERENCES 1. DubeÂ, C. A., Aaronson, H. I. and Mehl, R. F., Rev. Met., 1958, 55, 201. 2. H. I. Aaronson, Decomposition of Austenite by Di€usional Processes, ed. V. F. Zackey and H. I. Aaronson, Interscience Publishers, New York, 1962, p. 387. 3. Aaronson, H. I., Laird, C. and Kinsman, K. R., Phase Transformations, ASM, Metals Park, OH, 1970, p. 313. 4. Kinsman, K. R., Eichen, E. and Aaronson, H. I., Metall. Trans., 1975, 6A, 303. 5. Hall, M. G. and Aaronson, H. I., Metall. Mater. Trans., 1994, 25A, 1923. 6. Bo, X. Z., Fang, H. S., Wang, J. J. and Wang, Z. H., Scr. mater., 1997, submitted. 7. Watson, J. D. and McDougall, P. G., Acta metall., 1973, 21, 961. 8. Christian, J. W., Decomposition of Austenite by Di€usional Proscesses, ed. V. F. Zackey and H. I. Aaronson, Interscience, New York, NY, 1962, p. 371. 9. Christian, J. W., Metall. Mater. Trans., 1994, 25A, 1821. 10. Kurdjumow, G. V. and Sachs, G., Z. Phys., 1939, 64, 325. 11. Aaronson, H. I., Furuhara, T., Rigsbee, J. M., Reynolds, W. T. Jr. and Howe, J. M., Metall. Trans., 1990, 21A, 2369. 12. Furuhara, T. and Maki, T., Mater. Trans. JIM, 1992, 33, 734. 13. Aaronson, H. I. and Russell, K. C., Proc. Int. Conf. on Solid-to-Solid Phase Transformations, ed. H. I. Aaronson, D. E. Laughlin, R. F. Sekerka and C. M. Wayman, TMS-AIME, Warrendale, PA, 1994, p. 371. 14. Clark, H. M. and Wayman, C. M., Metall. Trans., 1977, 8A, 206. 15. Furuhara, T. and Aaronson, H. I., Acta metall. mater., 1991, 39, 2887. 16. Furuhara, T., Ogawa, T. and Maki, T., Scr. mater., 1996, 34, 381. 17. Yamamoto, M., Fujisawa, T., Saburi, T., Kurumizawa, T. and Kusao, K., Surf. Sci., 1992, 266, 289.

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18. Fang, H. S., Wang, J. J., Yang, Z. G., Li, C. M., Zheng, Y. K. and Li, C. X., Metall. Mater. Trans., 1996, 27A, 1535. 19. Wang, J. J., Fang, H. S., Zheng, Y. K. and Yang, Z. G., ISIJ Int., 1995, 35, 992. 20. Yang, Z. G., Fang, H. S., Wang, J. J., Li, C. M. and Zheng, Y. K., Phys. Rev. B, 1995, 52, 7879. 21. Swallow, E. and Bhadeshia, H. K. D. H., Mater. Sci. Technol., 1996, 12, 121. 22. Bo, X. Z., Fang, H. S. and Wang, J. J., Scr. mater., 1997, 37, 555. 23. Binnig, G., Rohrer, H., Gerber, Ch. and Weibel, E., Phys. Rev. Let., 1982, 49, 57. 24. Binnig, G. and Rohrer, H., Surf. Sci., 1983, 126, 236. 25. Aaronson, H. I. and Wells, C., Trans. AIME, 1956, 206, 1216. 26. Aaronson, H. I., Spanos, G., Masamura, R. A., Vardiman, T. G., Moon, D. W., Menon, E. S. K. and Hall, M. G., Mater. Sci. Eng. B, 1995, 32, 107. 27. Bradley, J. R., Rigsbee, J. M. and Aaronson, H. I., Metall. Trans., 1977, 8A, 323. 28. Howe, J. M., Metall. Mater. Trans., 1994, 25A, 1917. 29. Clark, H. M. and Wayman, C. M., Phase Transformations, ASM, Metals Park, OH, 1970, p. 59.

30. Smith, R. and Bowles, J. S., Acta metall., 1960, 8, 405. 31. Laird, C. and Aaronson, H. I., Acta metall., 1969, 17, 505. 32. Liu, Y. C. and Aaronson, H. I., Acta metall., 1970, 18, 845. 33. Speich, G. R., Decomposihon of Austenite by Di€usional Processes, ed. V. F. Zackey and H. I. Aaronson, Interscience, New York, 1962, p. 371. 34. Lee, H. J. and Aaronson, H. I., Acta metall., 1988, 36, 787. 35. Furnhara, T., Lee, H. J., Menon, E. S. K. and Aaronson, H. I., Metall. Trans., 1990, 21A, 1627. 36. Hall, M. G., Aaronson, H. I. and Kinsman, K. R., Surf. Sci., 1972, 31, 257. 37. Russell, K. C., Hall, M. G., Kinsman, K. R. and Aaronson, H. I., Metall. Trans., 1974, 5, 1503. 38. Menon, E. S. K. and Aaronson, H. I., Acta metall., 1987, 35, 549. 39. Fang, H. S. and Li, C. M., Metall. Mater. Trans., 1994, 25A, 2615. 40. Fang, H. S., Wang, J. J. and Zheng, Y. K., Metall. Mater. Trans., 1994, 25A, 2001. 41. Enomoto, M., Acta metall., 1987, 35, 947.