The influence of the microstructure on the effective properties of duplex-type materials under purely thermal loading

Abstract
An analysis is carried out to study the influence of the microstructure on the effective thermomechanical properties of cylindrical specimens of ferritic-austenitic duplex steels under the conditions of purely thermal cyclic loading in the temperature interval ranging from 20 °C to 900 °C. Three different representations for the microstructure of duplex steels with extreme matrix-inclusion topology are realised: a two-zone model and models with a random distribution of the inclusion phase at two different characteristic length scales. The first approach allows to gain some analytical estimates for the coefficients of thermal expansion and for the critical temperature at which the behaviour departs from a thermoelastic one. An additional theoretical analysis based on the Eshelby approach deals with the influence of the morphology of grains/clusters of the inclusions phase on the effective properties. Using an additional topological parameter (number of clusters) together with other parameters characterising the phase distribution makes possible the comparison of results obtained from the different geometries. Conditions for the microstructure-related transition between different types of response of specimens to the thermal cyclic loading, namely ratchetting and plastic shakedown, are discussed.