Atomic-level Observation of Grain and Grain Boundary Structure of mechanically milled, nanocrystalline Fe powder

Project Summary

High-resolution transmission electron microscopy (HREM) was used to image both grain and grain boundary structures of the mechanically milled fine grained iron powder. Direct HREM observation of as milled powder revealed that grain boundaries of the powder appeared to be wavy and irregular, but no amorphous and/or other characteristic defect structure was observed. Further, detailed image analysis indicated that the fragmentation and rotation process of nano-meter size grains had occurred during severe plastic deformation. A mechanism for the formation of the fine grained microstructure is proposed based on the reorientation process of individual grains.

Figure 1 shows a selected area diffraction pattern of a mechanically milled pure iron (MM-Fe) powder calculated from [111]Fe HREM image. It should be noted that the entire image is divided into 14 areas on the image contrast basis to examine the local rotation of individual crystalline grain. In Grain 7, three sets of {110}Fe planes (d=0.203nm) are clearly visible (one of its [111] direction is almost parallel to the incident electron beam). The relative misorientation angles between adjacent regions are measuredand the result shows that grains can be grouped according to their orientation then each groups and each of those are separated by high angle boundaries. For example, area 4, 5, 6 and 10 have similar orientation. Similarly, grains 7, 8 and 11 are separated by only low angle boundaries. This trend shows good agreement with the subgrain structure observed in BF image. It should be noted that FFT image taken from area 4 and 5 shows two sets of [111] reflections at an angle 30. This indicates that the grain boundary in between these two grains is inclined from the electron beam direction, so these two grains are partly overlapped .

Fig.2: FFT

Figure 2 showsa HRTEM image of the mechanically milled, nanocrystalline Fe powder. The image is approximately 20.5 nm wide. The grain that occupies most of the figure is in a <111> orientation. The hexagonal arrangement of columns of Fe atoms in this orientation are visible as white spots, as in the simulated image. White lines were drawn periodically along the three sets of edge-on {110} planes in the grain, and these are shown superimposed on the HRTEM image. Two sets of {110} planes running vertically in the figure are straight (or nearly so), but the set that is approximately horizontal in the figure, bends considerably. Dark lines were drawn on all of the nearly horizontal, bent {110} planes in the figure to accurately indicate their positions. The arrows in figure mark two wedge-shaped regions that together form a partial disclination dipole. The wedge-shaped regions are approximately 3.5 nm apart. Each of the wedge-shaped regions contains a number of terminating {110} planes, which are individual dislocations with a Burgers vector, which is the displacement vector that describes the magnitude and direction of their strain field, of b = a/2<111>. The partial dislocation dipoles appear to be wedge disclinations with a Frank vector w, which is the rotational vector that describes the distortional power of the disclination, that is parallel to the defect line (in this case, the <111> viewing direction. Unfortunately, it is not possible to exactly determine the wedge and twist components of the partial disclinations in Fig. 2 because a HRTEM image only reveals atomic displacement perpendicular to the electron beam direction and there may be displacements parallel to the beam that are not visible. However, this image demonstrates that it is possible to directly observe the individual dislocations that constitute partial disclination dipoles in metals at the atomic level, even in mechanically milled powders that have undergone severe plastic deformation.

Fig.1: HREM

Related Publications
[1] M. Murayama1, J.M. Howe1, H. Hidaka2 and S. Takaki2
Science, in press.
1 Department of Materials Science and Engineering, University of Virginia
2 Department of Materials Science and Engineering, University of Kyushu, Fukuoka Japan

[2] M. Murayama1, J.M. Howe1, H. Hidaka2 and S. Takaki2
to be published in ISIJ International (2002).
1 Department of Materials Science and Engineering, University of Virginia
2 Department of Materials Science and Engineering, University of Kyushu, Fukuoka Japan