Type of presentation: Poster

MS-9-P-2417 Martensite Formation in Austenite Related to Deformation Twins in Adjacent Ferrite Grains in Duplex Stainless Steel

Kildahl P.¹, Karlsen M.^{1 2}, Aursand M.², Hjelen J.¹

¹Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim, Norway, ²Statoil Research Centre, Rotvoll, Trondheim, Norway

jarle.hjelen@material.ntnu.no

Charpy impact testing has been performed in order to provoke twinning in a UNS S31803 duplex stainless steel. Bright field optical microscopy of samples deformed at temperatures below -40°C revealed deformation twins which appear to continue into adjacent austenite grains. Samples deformed at -46°C and -70°C have been examined by EBSD. To be able to examine the same region of interest by optical microscopy and EBSD, the final sample preparation was polishing with OP-U suspension.

Figure 1 shows a bright field optical image of a sample deformed at -70°C. Two deformation twins in the ferrite phase are apparently penetrating the subjacent austenite grain. In the EBSD orientation map shown in Figure 2, a change in orientation (color change) for the elongated features relative to the twins is observed at the phase boundary. This misorientation is 31.5°. Both of the deformation twins have a {112} twinning plane and <111> twinning direction. This well-known bcc twinning mode, can be presented as a rotation of ±60° about a common <111> axis.

Figure 3 is showing a phase map were bcc and fcc structures are red and green, respectively. It is observed that the elongated features inside the austenite, and the deformation twins in the ferrite, all have a bcc structure. It should also be noted that the image quality map in Figure 4, is darker for the austenite phase, while bright for the ferrite phase, including the twins. This indicates a higher degree of deformation in the austenite lattice, especially for the elongated features.

The orientation relationship, between the austenite grain (fcc) and the elongated feature (bcc), can be presented as a rotation of 90° about a common <112> axis. This is in accordance with the orientation relationship as proposed by Kurdjomov-Sachs (K-S) for the crystallographic relation that connects the parent and the product orientation during the diffusionless γ (fcc) to α_M (bcc) transformation. This indicates that the elongated features observed in connection with the deformation twins are deformation-induced martensite. An equal relationship also applies for the sample deformed at -46°C. Rotated data sets, with (001)[100] austenite orientation, for samples deformed at -46°C and -70°C has revealed two martensite variants in accordance to theoretical K-S variants; type II.6 and III.3, respectively [1].

In samples deformed at temperatures below -40°C, elongated features with martensitic structure are observed in the austenite. These features are in connection with the deformation twins in the ferrite phase. A general increase in deformation twin density was observed for decreasing temperatures, with a transition temperature at -40°C.

Reference:

[1] M. Karlsen et al. Metallurgical and Material Transactions A, 40A:310-320, 2009

The authors wish to acknowlegde Senior Engineer, PhD Yingda Yu for providing technical support and guidance during the EBSD analysis.

Fig. 1: Bright field micrograph of specimen polished with OP-Ususpension. The lighter grains are austenite phase and the darker matrix is the ferrite phase, with deformation twins visible as dark bands. Inside the marked area, two parallell bands are crossing the phase boundary.	Fig. 2: EBSD orientation map of the delimited region in Figure 1. The two parallell bands change color (orientation) when crossing the phase boundary. The misorientation is 31.5°. The angle axis pair between the yellow austenite grain and the pink band is a rotation of 90° about a common <112> axis.
Fig. 3: Phase map showing the ferrite phase (bcc) in red and austenite phase (fcc) in green. It is observed that the elongated features in connection with the deformation twins have bcc-structure.	Fig. 4: Image quality map shows light twins inside the bright ferrite phase. In contrast, the austenite grain is darker than the ferrite grain. The elongated features appear even darker. Areas with dark colors indicate a higher degree of deformation in the lattice.