Ultrafine ferrite formation through isothermal static phase transformation

A novel thermomechanical route has been developed to produce an ultrafine-grained structure through warm deformation of meta-stable austenite followed by isothermal static transformation to ferrite. The thermomechanical parameters influenced the evolution of ferrite phase transformation (i.e. nucleation and growth), resulting in different levels of ferrite grain refinement. For the first time grains as fine as 200 nm were produced through a traditional diffusional transformation of austenite to ferrite. The use of a model Ni-30Fe austenitic alloy showed that the strain has a strong effect on the nature of the intragranular defects developed throughout the microstructure. At a low strain, microbands were the dominant intragranular features. There was a transition strain above which a complex cell/subgrain substructure with high misorientation angle appeared in the vicinity of prior austenite grain boundaries. This region was extended with strain and promoted significant ferrite nucleation sites at an early stage of phase transformation, resulting in the local formation of nanosized ferrite grains near the prior austenite grain boundaries with ultrafine grains towards the interior of the original austenite grain. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.