Two-Dimensional Observation of Grain Boundary Sliding of ODS Ferritic Steel in High Temperature Tension

Abstract

High-temperature tensile deformation was performed using ODS ferritic steel, which has grain structure largely elongated and aligned in one direction, in the direction perpendicular to the grain axis. In the superplastic region II, two-dimensional grain boundary sliding (GBS) was achieved, in which the material did not shrink in the grain-axis direction and grain-boundary steps appeared only in the surface perpendicular to the grain axis. In this condition, a classical grain switching event was observed. Using kernel average misorientation maps drawn with SEM/ EBSD, dominant deformation mechanisms and accommodation processes for GBS were examined in the different regions. Cooperative grain boundary sliding, in which only some of grain boundaries slide, was also observed.

Two-dimensional grain boundary sliding and mantle dislocation accommodation in ODS ferritic steel

“…Specifically, dislocation activities were limited within mantles in region II/III but also observed in cores in region III. Significant GBS was also detected in region III, where small amount of GBS had previously been identified [18].…”

“…Specifically, (i) the dislocations adopted a parallel alignment below region II instead of a complicated substructural evolution above region II/III, and (ii) dislocations below region II cannot be maintained without dispersion particles but those above region II/III are preserved alone after deformation. These differences between dislocation characteristics imply that another 18 dislocation accommodation operates in regions I and II. Xun and Mohamed concluded that the mechanism was the Ball-Hutchison type [3], which is accommodated by transgranular slippage and the pile-up of dislocations.…”

“…Shear tests were conducted to achieve more refined evaluation than ever. Compared with 6 conventional tensile tests that generated two GBS modes around 45° and 135° with respect to the tensile axis [18,19], shear tests produced only one GBS mode along the shear direction.…”

“…This idea is similar to previous attempts by Muto et al [16], who deformed a bundle of pencils acting as a two-dimensional model aggregate and evaluated their sliding behaviors. Based on this concept, two-dimensional GBS has been achieved and investigated in oxide dispersion strengthened (ODS) ferritic steel exhibiting an elongated and aligned grain structure [17][18][19][20].…”

“…In our former work [18], the deformed microstructures were evaluated by electron back-scattered diffraction (EBSD) in conjunction with surface microgrids drawn by focused ion beam (FIB) in regions II (optimum for GBS), II/III (border), and III (dislocation creep), respectively. Significant GBS was detected in regions II and II/III, where the density of geometrically necessary dislocations (GNDs) increased around the sliding boundaries.…”