The relationship between microstructure and fracture resistance of TRIP-assisted multiphase steels has been investigated by processing and characterizing eight specifically designed microstructures differing in terms of the volume fractions of the constitutive phases, i.e. ferrite, bainite, retained austenite and martensite, by the stability of the retained austenite, and by the connectivity of the phases. Fracture resistance has been quantified by the fracture strain in uniaxial tension, ε¯f, the fracture toughness at cracking initiation, JIc, and by the essential work of fracture, we. The fracture mechanisms were characterized by microscopic observations and by profilometric measurements of the crack tip necking process typical in thin sheet fracture. The fracture toughness at cracking initiation of TRIP-aided steels corresponds to an equivalent Dual Phase steel in which martensite replaces retained austenite and is thus not affected by the retained austenite stability. The ductile tearing resistance of TRIP-aided steels, quantified by we, increases significantly with the retained austenite stability. This improved tearing resistance is explained by the effect of the additional work-hardening induced by the late transformation of retained austenite within the crack tip necking zone.

The mechanical behaviour of transformation-induced plasticity (TRIP)-assisted multiphase steels is addressed based on three different microstructures generated from the same steel grade. The mechanisms responsible for the work-hardening capacity and the resulting balance between strength and resistance to plastic localization are investigated at different length scales. The macroscopic mechanical response is determined by simple shear, uniaxial tension, Marciniak and equibiaxial tension supplemented by earlier tensile tests on notched and cracked specimens. It is shown that the transformation rate reaches a maximum for stress states intermediate between uniaxial tension and equibiaxial tension. At an intermediate length scale, the true in situ flow properties of the individual ferrite–bainite and retained austenite phases are determined by combining neutron diffraction and digital image correlation. This combined analysis elucidates the partitioning of stress and strain between the different constitutive phases. Based on these results, supplemented by transmission electron microscopy and electron backscattered diffraction observations, a general overview of the hardening behaviour of TRIP-assisted multiphase steels is depicted.

The stress and strain partitioning between the different phases of transformation-induced plasticity (TRIP)-aided multiphase steels is evaluated using a mean field homogenization approach. The change of the austenite volume fraction under straining is predicted using a micromechanics-based criterion for the martensitic transformation adapted to the case of small, isolated, transforming austenite grains. The parameters of the model are identified from the mechanical response and transformation kinetics measured under uniaxial tension for two steels differing essentially by the austenite stability. The model is validated by comparing the predictions with tests performed under different loading conditions: pure shear, intermediate biaxial and equibiaxial. An analysis of the effect of the austenite stability on strength and ductility provides guidelines for optimizing properties according to the stress state.

The flow behaviour of the constitutive phases in multiphase steels, possibly exhibiting a mechanically-induced phase transformation (TRIP effect), is investigated using neutron diffraction conducted during uniaxial tensile loading. The BCC and FCC lattice strains of several specimens containing different amounts ferrite, bainite, martensite and metastable retained austenite are measured along elastic and plastic deformation. The validity of the measurements, as well as the strengthening resulting from the TRIP effect, are evaluated on the basis of overall mechanical equilibrium.

Abstract The strength and formability properties of high performance steels have been recently improved thanks to the combination of several strengthening mechanisms such as dislocation strengthening and mechanically-induced martensitic transformation within complex microstructures. These finely grained metastable microstructures are generated during controlled multistage thermomechanical treatments involving several phase transformations. However, the way the different active mechanisms combine in order to improve the properties of these steels is not yet clearly established. The present study proposes an overview of an experimental program devoted to the micromechanical characterisation of Dual Phase and TRIP-assisted multiphase steels. Thanks to different techniques such as X-Ray and neutron diffraction, strain mapping and transmission electron microscopy (TEM), it was possible to characterize the flow behaviour of the different constitutive phases and to measure the critical parameters of the microstructures responsible for the work hardening capabilities of the TRIP-assisted multiphase steels. Comparison of the experimental stress and strain partitioning within the heterogeneous microstructures with the predictions of different modelling assumptions was also carried out. La résistance mécanique et les propriétés de formabilité des aciers à haut performance ont été améliorées récemment grâce à la combinaison de plusieurs mécanismes de renforcement comme le renforcement par dislocations et la transformation martensitique induite mécaniquement dans des microstructures complexes. Ces microstructures métastables à grains fins sont engendrées lors de traitements thermomécaniques contrôlés, à plusieurs étapes, impliquant plusieurs transformations de phase. Cependant, la manière dont les différents mécanismes actifs se combinent pour améliorer les propriétés de ces aciers n’est pas encore clairement établie. Cette présente étude propose une revue du programme expérimental dévoué à la caractérisation micromécanique des aciers biphasés et des aciers multiphasés à effet TRIP. Grâce à différentes techniques comme la diffraction des rayons X et des neutrons, la cartographie de la déformation et le TEM, il a été possible de caractériser les propriétés d’écoulement des différentes phases constitutives et de mesurer les paramètres microstructuraux critiques responsables de la capacité d’écrouissage des aciers multiphasés à effet TRIP. On effectue également une comparaison du partitionnement expérimental de la contrainte et de la déformation à l’intérieur des microstructures hétérogènes avec les prédictions de différentes hypothèses de modélisation.

TRIP-assisted multiphase steels exhibit excellent combinations of the properties required in forming operations. These properties result from a composite effect – the presence of hard phases (bainite, austenite, martensite) dispersed in a ductile matrix (ferrite) – and the straininduced martensitic transformation (TRIP effect). However, in some circumstances, the appearance of the hard and brittle martensite drastically reduces the crack resistance and impairs some forming processes. The cracking initiation and tearing resistance of different steel grades has been measured in order to better understand the relationship between microstructure and fracture resistance. The influence of microstructural parameters are analysed by in-situ testing and characterised by SEM and OIM. The fracture toughness is shown to decrease significantly with increasing volume fraction of retained austenite.

The tranformation-induced plasticity (TRIP) effect, i.e. the mechanically activated martensitic transformation of metastable austenite, has been proven for some years to contribute very effectively to the deformation process in a large variety of iron-based alloys. In order to enlighten the influence of the stress triaxiality on the kinetics of the mechanically-induced martensitic transformation in TRIP-assisted multiphase steels. several specimens presenting austenite with different mechanical stabilities were strained under different stress states (pure uniaxial tension, uniaxial tension of notched and DENT specimens and stretching). It is shown that the stress triaxiality has a large effect on the mechanical stability of austenite dispersed in a multiphase microstructure and consequently on the mechanical properties of the investigated steels.

A physically based model for TRIP carbon steels is developed suitable to predict the macroscopic behaviour of multi-constituent aggregates. It includes the effects of phase composition and morphology on flow stress and strain hardening. In a first part, a detailed description of the stress-assisted and strain-induced martensitic transformation kinetics is given based on a generalised form of the Olson–Cohen model. The appearance of the much harder martensitic phase during plastic straining gives rise to a strong hardening of the retained austenite islands. The matrix behaviour is described using a model previously developed for ferritic–martensitic steels. A quite simple but accurate homogenisation approach is used to determine the TRIP steel behaviour. The predicted evolution of strain-induced martensite volume fraction, flow stress and incremental work hardening is in good agreement with experimental data and illustrates the critical importance of the retained austenite stability on the formability of TRIP steels.