Isao Matsui

Microstructures of electrodeposited Ni were studied from the perspective of growth modes during electrodeposition. The electrodeposited Ni had a heterogeneous microstructure composed of nanocrystalline- and microcrystalline-grains. Electron backscatter diffraction analyses showed that nanocrystalline- and microcrystalline-grains were preferentially oriented to specific planes. Secondary ion mass spectrometry also revealed that coarse-grained regions had higher S content than that of finer-grained regions. Hence, microstructural heterogeneity in electrodeposited Ni is reflected by the overlap of inhibited and free growth modes. Our discussion surrounding microstructural heterogeneity also provides insight into other electrodeposited nanocrystalline systems.

In this study, electromagnetic stirring (EMS) was applied to the 7150 aluminum alloy during the solidification process to refine the microstructure of rectangular sectional sample. EMS was applied as clockwise and counterclockwise alternatively with reverse imposition time T. Then the effects of T and the cross-sectional dimension of the sample, that is the aspect ratio AR and corner radius R were evaluated. The average grain size at the center part was smallest at T=1 s and increased with increasing T. The grain size at the corner had the same tendency at the round type mold, the corner radius of which is half of the mold thickness. But it was smallest at T=2 s at non-round type mold. In the rectangular sectional sample with small R, the flow was not reached enough to the corner at T=1 s. But in T=2 s, the flow was reached to the corner because the flow became disturbed. In the round type mold, the flow was also reached to the corner even in T=1 s. This is considered to be a reason for the difference in tendency of grain size at the corner. Moreover, even in AR=5, the microstructure could be refined by applying EMS.