Research on Phase Transformations and Microstructure Control with Emphasis of Grain Boundary Character of the Matrix

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The following two experimental studies have been carried out to examine the effects of grain boundary character of the matrix on phase transformations.[1] Effect of grain boundary plane on the variant selection of grain boundary precipitates.Variant selection of BCC precipitates forming on grain boundaries was investigated in a Ni-43%Cr alloy. When the grain boundary planes of the FCC matrix is near {111} wthin 20 degrees, the variants of the precipites are highy limitted and the dominant variant is observed. On the other hand the grain boundary planes are far from {111} over 30 degrees, the dominant variant is not observed. These results can be explained from the viewpoint of activation energy for nucleation of the variants forming on the grain boundaries.[2] Character of grain boundaries acted as martensitic nucleation site in an Fe-Ni-Mn alloy.Character of grain boundaries acted as martensitic nucleation site was investigated in the initial stage of transformation where the volume fraction of martensite less than 0.01. The number of the grain boundaries with martensite was fifiteen and much fewer compared with the total baoundaries, 2504. This indicates that the grain boundaries as high potent nucleation sites is highly limited. It was found by ECP analysis that most of the grain boundaries acted as nucleation sites were low-angle boundaries with the misorientation angle near 12 degrees and the boundary planes of those were near {110}.