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The morphologies of couplings of thin-plate martensites in an Fe-31%Ni-0.28%C alloy were investigated by means of transmission electron microscopy and they were discussed in terms of the phenomenological martensite theory. The results obtained are as follows.(1) The morphologies of couplings such as spear, wedge, kink, diamond, composite spear and zig-zag types were observed. These couplings consisted of variants in the same plate group and some of wedge and zig-zag type couplings were found to be composed of variants in different plate groups.(3) From the average form of the shape strain matrices of the variants, it is considered that the spear, wedge, diamond and zig-zag type couplings are self-accommodating, whereas the kink type is not.(4) The diamond and composite spear type couplings were the composite of some of the three fundamental types, spear, wedge and kink ones.(5) The junction planes of couplings of variants in the same plate group could be derived from the planes of intersection of the strain ellipsoids of the variants and the sense of the shear directions of the shape strains of the variants at the junction planes.

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