Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM) in a Bainitic Steel Containing Retained Austenite

Provided by: Professor John J. Jonas, Department of Metallurgy, McGill University, Montreal, Canada

When an austenite grain transforms to ferrite, bainite or martensite, the Kurdjumov-Sachs orientation relationship calls for grains of 24 different orientations to be formed. However, variant selection, which is related to the slip systems active during austenite deformation, results in the appearance of only a small fraction of these orientations.

The phenomenon of variant selection during phase transformation was investigated in a bainitic steel containing retained austenite. The steel was hot-rolled below the Tnr (minimum recrystallization temperature) to a true strain of e = 0.8 in the austenite temperature range. The orientation relationships between the sheaves of bainite and the parent austenite grain were determined using OIM and expressed in Rodrigues space. It was shown in this work [1,2] that a deformation of 0.8 leads to marked variant selection.

Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM™) in a Bainitic Steel Containing Retained Austenite

Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM™) in a Bainitic Steel Containing Retained Austenite

Figure 1 - ND (left) and RD (right) inverse pole figure depictions of the orientation of the prior austenite grain

Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM™) in a Bainitic Steel Containing Retained Austenite

Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM™) in a Bainitic Steel Containing Retained Austenite

Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM™) in a Bainitic Steel Containing Retained Austenite

Figure 2 - ND (left) and RD (right) inverse pole figure depictions of the orientation of the bainite sheaves resulting from the transformation of the austenite grain of Figure 1

Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM™) in a Bainitic Steel Containing Retained Austenite
Figure 3 - The locations of the Kurdjumov-Sachs minimum angle rotation axes displayed in Rodrigues-Frank (RF) space. Here the crystal of the parent austenite is taken as the reference frame.The variants are labeled 1 to 24, going from aI to -dIII in the Bishop and Hill notation [2]

Microtextural Characterization of Variant Selection of Orientation Imaging Microscopy (OIM™) in a Bainitic Steel Containing Retained Austenite