Grain refining during heavy deformation in Fe-C alloys with (α+θ) two-phase structure

Mechanical milling using high energy ball mill was applied to Fe-(0-3. 8)mass%C alloy powders with (ferrite+cementite) two-phase structures to give an ultimate large strain to the powders. Relation between strength and niicrostructure was investigated in the mechanically milled Fe-C alloys, by means of X-ray diffractometry, transmission electron microscopy (TEM) and hardness testing. After milling for 360ks, the niicrostructure consists of nanocrystalline ferrite and grain boundary amorphous layer in the alloys with less than 2mass%C, while it is composed of nanocrystalline ferrite, nanocrystalline cementite and grain boundary amorphous layer in the alloys with more than 2mass%C. Below lmass%C ferrite grains become smaller with increasing carbon content, and the grain size remains constant to be lOnm above lmass%C. Hardness of the Fe-C alloy powders increases with grain refining, but it is independent of the volume fraction of undissolved cementite. TEM observation indicates that these alloy powders have equiaxed grains, which is characterized by the substructure having few dislocations within the grains. These findings suggest the outset of deformation in grain boundary amorphous layer, i. e. , grain boundary sliding, in the nano-sizecl grain materials.