Abstract: The butterfly martensite nuclei in an Fe-Ni-V-C alloy were observed under TEM,they con- sist of enveloping dislocations with Burger's vector(1/2)[]_(fcc) and arrays of dislocations with Burger's vector(1/2)[10]_(fcc) piled up within the former.The enveloping dislocations and the arrays of dislocations provide the required shear when the planes(11)_(fcc)move with each other to alter their stacking order and change into the planes(110)_(bcc) respectively.The necessary adjustment of the spacings is provided by the dislocations in martensite.The crystallographic relationship after double-shear is K-S one.

Key words: butterfly martensite, nuclei, enveloping dislocation, piling dislocation