The tensile deformation of a martensitic dual-phase steel

A single 0.12 wt.% C alloy has been heat treated to produce dual-phase ferrite-martensite composites of a variety of volume fractions and grain sizes of both phases. Two separate models have been used to describe the 0.2% proof stress of these composites, one based on the topological model of Gurland and the other based on the yield propagation model of Petch. Using the topological model, the 0.2% proof stress in all dual-phase steels could be represented by σ_0.2% (MPa) = 280 + 17.4d_I^{- 1 2} (1 - g′) + 576g′ where g′ is a parameter related to the volume fractions and contiguities of the two phases and d_I is the size of type I grains. This model had some inconsistencies and in particular did not show good extrapolations to either 100% martensite or 100% ferrite. Using the yield propagation model, the 0.2% proof stress could be written as σ_0.2% (MPa) = 122 + {16.4 + 432 (V_IId_II) ^{1 2}}d_c^{- 1 2} where d_c is an average between the ferrite grain size and the martensite islet size, d_II is the size of type II grains and V_II is the volume fraction of the secondary phase. This equation showed excellent extrapolation to 100% ferrite but some change in slope was observed towards 75% martensite. The reasons for the inadequacies of representation have been attributed mainly to autotempering in the martensite and to the shortcomings in the measurement of the ferrite and martensite continuities.