ON THE THERMOMECHANICAL DEFORMATION-BEHAVIOR OF DUPLEX-TYPE MATERIALS

Two-phase duplex-type materials possess microstructures containing rou ghly the same amounts of the constituent phases whose grains form inte rwoven networks. Duplex stainless steels are typical representatives o f this material group. In these steels the constituent phases austenit e and Ferrite have different coefficients of thermal expansion. On pur e thermal loading or thermomechanical loading the yield strength of th e phases can be exceeded. Specimens of a forged duplex steel with a un iaxially anisotropic microstructure deform irreversibly even under pur e thermal cycling conditions with a monotonic accumulation of strain. The results of a systematic finite element based micromechanical analy sis of the thermomechanical deformation behavior of duplex steels are presented and discussed. The analysis is based on a quantitative chara cterization of both the real and model microstructures. Additionally, an extended constitutive material law for the thermomechanical loading of the duplex steel is proposed. For dual-phase materials this descri ption incorporates an additional thermomechanical strain increment as a very important contribution to the total strain increment. Both the micromechanical model and the analytical model are used to analyse the experimental findings from dilatometer tests. The micromechanical app roach allows the evolution of the irreversible strains in the two phas es generated in a thermal cycle to be modeled. It is shown that the ma trix-phase is always more deformed than the inclusion-phase, irrespect ive of which of the two phases (austenite or Ferrite) forms the matrix . This prediction is confirmed by electron microscopic observations of a thermally cycled duplex steel. Based on these results a mechanism d riving the ratchet effect is proposed.