Evaluation of the Crystallographic Orientation Relationships between FCC and BCC Phases in TRIP Steels

Abstract

The crystallographic orientation relationships that are active during the transformation of austenite to bainite are studied for two TRIP steels by means of Electron BackScatter Diffraction (EBSD). A detailed evaluation of about 360 retained austenite grains and their BCC neighbours was performed. Three relationships were considered, namely Kurdjumov-Sachs, Nishiyama-Wassermann and Pitsch. It was found that the majority of the austenite grains had at least one neighbour that could be related with one of the three orientation relationships. The Kurdjumov-Sachs relationship appeared to be dominant and no strong indication for variant selection could be retrieved from the studied data. It was, however, also demonstrated that some precautions need to be made since a clear distinction between the evaluation of a small region of the microstructure and conclusions made for the complete material is necessary.KEY WORDS: TRIP steels; EBSD; phase transformation; crystallographic orientation relationships. 1601© 2009 ISIJ KS variants. The number of variants for the other orientation relationships can be deduced in a similar way. A g-a orientation relationship actually represents a misorientation between two crystallographic orientations. Therefore, this misorientation is most frequently described by means of an axis/angle pair ͗d͘w. Consequently, the last column of Table 1 gives the axis/angle representation with the minimum misorientation angle of the different orientation relationships.The Bain orientation relationship is the simplest orientation relationship, but it is never observed in steels. Therefore, this orientation relationship serves as a first approximation or a reference point when the transformation of austenite into a BCC phase is studied. Mostly, the Kurdjumov-Sachs and the Nishiyama-Wassermann relationships are used when the orientation relations between FCC and BCC phases are studied. Although there has been a lot of recent research [10][11][12][13][14][15][16][17][18][19] concerning which of these orientation relationships prevails, there is still considerable debate on the relative importance of these orientation relationships because the angular differences between the different orientation relationships is small which makes the experimental verification difficult. Recently, also other orientation relationships 5,6) were taken into account during the study of the crystallographic aspects of the FCC-BCC phase transformation. 10,20) An overview of some features of the different rational orientation relations is given in Fig. 2. Figure 2 illustrates how the different variants of the different orientation relationships would appear around one Bain variant on a {100} BCC pole figure. When the three Bain variants are shown on a pole figure, each of them is surrounded by eight KS and four NW variants, respectively. The crystallographic misorientation between two neighbouring KS variants is 10.53°, whilst the misorientation between Bain and a KS variant is 11.06°and the misorientation between Bain and a NW vari...

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