The morphology and crystallography of lath martensite in Fe-C alloys

doi:10.1016/S1359-6454(02)00577-3

Acta Materialia

Volume 51, Issue 6, 2 April 2003, Pages 1789-1799

https://doi.org/10.1016/S1359-6454(02)00577-3 Get rights and content

Abstract

The morphology and crystallography of lath martensite in Fe-C alloys containing various carbon contents from 0.0026 to 0.61% were studied by analyzing electron back scattered diffraction patterns in scanning electron microscopy and Kikuchi diffraction patterns in transmission electron microscopy. As carbon content increases, the sizes of both packet and block decrease. In low carbon steels (0.0026–0.38%C), a block which is observed as having different contrasts under optical microscopy contains two groups (sub-blocks) of laths which are of two K-S variants with a misorientation of about 10 degrees. On the other hand, in the high carbon alloy (0.61%C), a block consists of laths of a single K-S variant.

Introduction

Ferrous martensite exhibits a variety of morphologies [1], i.e. lath, butterfly, lenticular and thin-plate. Among them, lath martensite has overwhelming industrial significance because it appears in most heat-treatable commercial steels. The size of martensite lath is very small, and hence individual laths are not clearly observed in optical micrographs. However, since lath martensites have a tendency to align themselves parallel to one another in the large area of the parent grain, lath martensite exhibits a characteristic microstructure at the optical microscopic scale. Current views hold that an austenite grain is divided into packets (the group of laths with the same habit plane) and each packet is further subdivided into blocks (the group of laths of the same orientation (same variant))[2], [3], [4]. Since the strength and toughness of martensitic steels are strongly related to packet and block sizes (effective grain sizes in lath martensite structure) [5], [6], the characteristics of morphology and crystallography of lath martensite are of great importance.

There have been many studies conducted on the morphology and crystallography of lath martensite in Fe-alloys [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. However, the crystallographic feature of lath martensite structure has not been completely clarified yet because of discrepancy between various experimental results. For example, the inter-variant relationships between neighboring blocks in a given packet have been reported to be twin-related [5], [7] or non-twin relationship [3], [8], [9], [10]. It is also not clear whether a packet consists of laths with all possible six variants of Kurdjumov–Sachs (K–S) relationship [11] or only few specific variants among possible six ones [2], [3], [4], [5]. These inconsistencies in the previous studies might be caused by the inaccurate orientation measurement which was analyzed by the selected area electron diffraction pattern.

Recently, however, new and more accurate techniques for the orientation measurement with a margin of error less than 1 degree, such as the Kikuchi diffraction pattern taken from local area on transmission electron microscopy (TEM), and the electron back scattered diffraction pattern (EBSP) on scanning electron microscopy (SEM), have been developed and used to study the crystallography of lath martensite. For example, Kelly et al. [12] reported, by the analysis of Kikuchi diffraction pattern obtained with a spot size of about 7nm, that the orientation between martensite and austenite in low carbon alloy steels is essentially the Greninger–Troiano relationship which is close to the K–S relationship with a deviation of a few degrees. Gourgues et al. [13] showed by EBSP measurement that martensite block boundaries in Fe-0.2C alloy are mainly not twin boundaries, but those with the misorientation of [011]/60 degrees. However, there has been no systematic study on the crystallography of lath martensite in Fe-C alloys with various carbon contents.

The purpose of the present study is to re-examine the morphology and crystallography of lath martensite in Fe-C alloys containing various carbon contents from 0 to 0.6 mass% by Kikuchi diffraction pattern and EBSP analyses.

Section snippets

Experimental procedure

An ultra-low carbon steel (interstitial free steel) with 0.0026 mass% C and 0.14 mass% Mn (hereafter Fe-0.0026C) and Fe-0.18, 0.38 and 0.61mass% C alloys were used. Details of chemical compositions of these alloys are shown in Table 1. All specimens were austenitized at 1373K or 1573K and quenched in water or iced brine to obtain full lath martensite structure. Prior austenite grain size in the Fe-0.0026C alloy was 670 μm, and in other alloys about 200μm.

Microstructures were observed by optical

OM observation

Fig. 3 shows optical micrographs of lath martensite in Fe-C alloys with different carbon contents. In the Fe-0.0026C alloy (Fig. 3(a)), large parallel blocks are seen in a packet with strong contrast. The Fe-0.18C alloy, as shown in Fig. 3(b), also exhibits clear block structures, although they are slightly finer than the blocks in the Fe-0.0026C alloy. In the Fe-0.38C alloy (Fig. 3(c)), blocks and packets are finer than in the lower carbon alloys. Lath martensite in the Fe-0.61C alloy shows a

Discussion

In this study, the variation in the morphology and crystallography of lath martensite with carbon content was studied in detail. The present results are schematically summarized in Fig. 10. As carbon content increases from 0.0026% to 0.61%, block and packet sizes decrease. In low carbon alloys (0.0026%C–0.38%C, Fig. 10(a)), packets consist of well developed parallel blocks. There are three blocks with different orientations in a packet. Each block consists of laths of two specific K–S variant

Conclusions

The morphology and crystallography of lath martensite in Fe-C alloys with different carbon contents such as 0.0026, 0.18, 0.38 and 0.61 mass% were examined by means of OM, SEM and TEM. Main results obtained are as follows:

1.
As carbon content increases from 0.0026% to 0.61%, block and packet sizes decrease.
2.
The orientation relationship between austenite and martensite is near Kurdjumov-Sachs relationship and some laths seem to have nearly Nishiyama relationship.
3.
In low carbon alloys

Acknowledgements

The authors wish to thank Dr. S. Zaefferer at the Max Planck Institute for Iron Research (Düsseldorf, Germany) for allowing us to use the program for on/off-line crystallographic analysis on TEM. The financial supports of the Iron and Steel Institute of Japan and the Ministry of Education, Science and Culture through a Grand-in-Aid for Encouragement of Young Scientists (B) No.14750575 are gratefully acknowledged.

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¹: Nippon Steel Co., 1-1 Tobihatacho, Tobihata-ku, Kitakyushu, Fukuoka 804-8501, Japan.

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The morphology and crystallography of lath martensite in Fe-C alloys

Abstract

Introduction

Section snippets

Experimental procedure

OM observation

Discussion

Conclusions

Acknowledgements

Acta Metall

Scripta Metall

Metallography

Acta Metall. Mater

Acta Metall

Acta Metall

Mater. Sci. Forum

Trans. ASM

Trans. ASM

Trans. JIM

Metall. Trans. A

Metall. Trans

Phys. Met. Metall

Mater. Sci. and Technol