On the ratchetting limit of forged duplex steels under combined thermal and mechanical loads

Critical loading conditions for the onset of plastic deformation of two-phase materials under combined thermo-mechanical loading are determined not only by the temperature dependence of mechanical properties but also and even more importantly by the evolution of internal thermal-stresses caused by the mismatch of the coefficients of thermal expansion of the phases. In uniaxially elongated microstructures (e.g. forged duplex steels or unidirectionally reinforced metal-matrix composites) thermally induced stresses can lead to local yielding of the phases and subsequently to irreversible macroscopic deformations under cyclic thermal loading - a phenomenon known as thermally induced ratchetting - even if the applied mechanical load is far from surpassing the elastic limit of the material. In this work the ratchetting limit of forged duplex steels with a uniaxially elongated microstructure is derived analytically for a two-bar model representing an idealized two-phase microstructure. Characteristic loading regimes are classified with respect to the macroscopic deformation caused by constant tensile load and cyclic thermal load. Additionally, deformational responses are calculated for micromechanical models with interwoven phase domains and deformation maps similar to Bree diagrams are developed. The derived ratchetting limits are compared with experimental results obtained for nitrogen alloyed duplex stainless steels.