AbstractTransmission electron microscopy studies have been carried out on the structures of chips formed during the high-speed machining of steel. The chip bodies, which reached temperatures of ~300°C during machining, had a pearlitic-ferritic structure with very fine, elongated, parallel-sided ferrite cells. The structure of the flow zone, which attained a maximum temperature of ~900°C, consisted, in all cases, of equiaxed ferritic cells, 0·5–1·5μm dia., with dislocation tangles in many cell boundaries, but few internal dislocations. No pearlite was observed in the flow zone, but initially pearlitic areas showed cementite networks at cell boundaries and very small cementite particles. Coarse cementite particles in spheroidized steel remained largely intact in the flow zone. Textures developed in the flow zone and in the chip body were ferrite shear textures. The flow zone is a thermoplastic instability, and this work suggests that the observed fine, equiaxed cell structures have the characteristic of permitting unlimited strain in thermoplastic instabilities in steel, and probably also in other metals and alloys.MST/105