Micromechanical Modeling of the Banded Structure of Martensite in Small Inclusions

The stability of austenite against strain induced martensitic transformation (SIMT) strongly depends on the grain size of the austenite. This is observed in austenitic steels with grain sizes from several pm up to several hundred pm and for much smaller austenitic grains (three orders of magnitude) that are embedded in a non austenitic matrix. fcc Fe-precipitates may be formed in dilute Cu-Fe alloys by a certain heat treatment. These Fe-particles transform only by plastic deformation of the alloy. The SIMT leads to a banded structure with parallel layers of martensite. We investigate the influence of the external stress state on the microstructure (number of twin bands, relative thickness of the bands) with the help of a micromechanical model. The grain size effect on the austenite's stability against SIMT and on the microstructure may be explained by a thermodynamic transformation criterion and the local mechanical driving force.