Spread of transformation and plate dimensions of isothermally formed martensite

The evolution of microstructure has been studied as a function of reaction temperature, grain size, superimposed elastic stress and prior plastic strain in the austenite in an Fe-23.2wt% Ni-2.8wt%Mn alloy that transforms isothermally at subzero temperatures. For a given reaction temperature, with increasing grain size the "spreading-out" process increases. For a given reaction temperature and grain size, an applied elastic stress enhances the spreading-out process and a prior plastic strain in the austenite reduces it. For a given grain size and martensite content, the spreading-out process increases with increasing driving force. These observations support the postulation that the propagation of the reaction is mainly caused by stimulation across the grain-twin boundaries of the austenite. It was found that the previous model did not describe the kinetics of propagation. However, starting with the Guimarães-Gomes approach, a simple analytical expression has been obtained which gives simultaneous consideration to both the spreading-out and the "filling-in" processes and thus describes the experimental data well. The dimensions of the martensite plates have also been measured as a function of grain size. For a given amount of martensite and test temperature, both the mean radius and the mean semithickness increase with increasing grain size. However, the mean semithickness-to-radius ramains independent of grain size.