The macro- and micromechanics of TRIP-assisted multiphase steels, experiments and modeling

TRIP-assisted multiphase steels exhibit enhanced strength and ductility properties. These properties result from the unique combination of various work-hardening and damage mechanisms taking place in a multiphase microstructure consisting of an intercritical ferrite matrix with a dispersion of bainite + martensite + metastable retained austenite grains. Martensite plays a crucial role in these mechanisms for the improvement of plastic properties (through the TRIP effect, i.e. the mechanically-induced martensitic transformation), and also in the damaging process. This study aims at establishing how martensitic transformation influences the mechanical properties (plasticity and fracture) when occurring in a multiphase microstructure. On the one side, macro- and micromechanical tests, SEM and TEM, Mössbauer spectroscopy and x-ray diffraction are used to characterise the mechanisms of deformation, transformation, and fracture at the various relevant scales. On the other side, computational unit cell models are employed for assisting (i) the development of micromechanically-based constitutive models, (ii) the interpretation of experimental results

On the role of martensitic transformation on damage and cracking resistance in trip-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J(R) curves and crack tip opening displacement (CTOD) measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved

Multiscale mechanics of TRIP-assisted multiphase steels: I. Characterization and mechanical testing

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. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

On the measurement of the nanohardness of the constitutive phases of TRIP-assisted multiphase steels

The nanohardness of the phases present in the microstructure of two TRIP (for TRansformation Induced Plasticity)-assisted multiphase steels differing by their silicon content was measured by nanoindentation in an atomic force microscope. It is observed that the softest phase in both steels is the ferritic matrix, followed by bainite, austenite and martensite. It is also shown that the silicon content of the steel grades is responsible for an increase of the hardness of the ferritic matrix due to solid solution strengthening. Finally, the influence of the preparation mode of the surface prior to the nanoindentation measurements has been investigated. An electropolishing stage after mechanical polishing is acceptable to allow valuable nanohardness measurements. (C) 2002 Elsevier Science B.V. All rights reserved

Experimental investigation of the influence of the stress state on the mechanical stability of austenite in multiphase steels

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

Multiscale mechanics of TRIP-assisted multiphase steels: II. Micromechanical modelling

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. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Influence of the mechanical stability of retained austenite on the damaging micromechanisms in TRIP-assisted multiphase steels

Deals with the damaging of transformation induced plasticity (TRIP)-assisted multiphase steels. These steels present a multiphase microstructure containing bainite and retained austenite grains dispersed in a ferrite matrix. During straining, the retained austenite (metastable at room temperature) transforms to martensite and thus brings about the well-known TRIP effect. Our aim was to better understand the damage properties of these steels. Two steels with different chemical compositions and heat treatment conditions were studied as regard to damaging. We have established that the behaviour of these steels regarding damaging processes depends more on the proportion of the non-intercritical phases in their microstructure than on the mechanical stability of austenite.Anglai

The role and significance of martensite on the mechanical properties of TRIP-assisted multiphase steels

TRIP-assisted multiphase steels exhibit enhanced strength and ductility properties. However, such high-strength formable steels do not seem adequate for applications in which good toughness properties are required. These properties result from the unique combination of various work-hardening and damage mechanisms taking place in a multiphase microstructure consisting of an intercritical ferrite matrix with a dispersion of bainite+martensite+metastable retained austenite grains. Martensite plays a crucial role in these mechanisms, particularly in the TRIP effect (i.e. the mechanically-induced martensitic transformation) for the improvement of plastic properties, but also in the damaging process. This study aims at establishing how martensitic transformation influences the mechanical properties (plasticity and fracture) when occurring in a multiphase microstructure. On the one hand, the dislocation density generated within ferrite by the martensitic transformation is shown to lead to a high work-hardening rate. On the other hand, it is shown how martensite strongly influences the appearance of microvoids in these steels and deteriorates the resulting fracture toughness.Anglai