Micromechanical modeling and simulations of transformation-induced plasticity in multiphase carbon steels

Aerospace Engineerin

Crystallographically based model for transformation-induced plasticity in multiphase carbon steels

The microstructure of multiphase steels assisted by transformation-induced plasticity consists of grains of retained austenite embedded in a ferrite-based matrix. Upon mechanical loading, retained austenite may transform into martensite, as a result of which plastic deformations are induced in the surrounding phases, i.e., the ferrite-based matrix and the untransformed austenite. In the present work, a crystallographically based model is developed to describe the elastoplastic transformation process in the austenitic region. The model is formulated within a large-deformation framework where the transformation kinematics is connected to the crystallographic theory of martensitic transformations. The effective elastic stiffness accounts for anisotropy arising from crystallographic orientations as well as for dilation effects due to the transformation. The transformation model is coupled to a single-crystal plasticity model for a face-centered cubic lattice to quantify the plastic deformations in the untransformed austenite. The driving forces for transformation and plasticity are derived from thermodynamical principles and include lower-length-scale contributions from surface and defect energies associated to, respectively, habit planes and dislocations. In order to demonstrate the essential features of the model, simulations are carried out for austenitic single crystals subjected to basic loading modes. To describe the elastoplastic response of the ferritic matrix in a multiphase steel, a crystal plasticity model for a body-centered cubic lattice is adopted. This model includes the effect of nonglide stresses in order to reproduce the asymmetry of slips in the twinning and antitwinning directions that characterizes the behavior of this type of lattices. The models for austenite and ferrite are combined to simulate the microstructural behavior of a multiphase steel. The results of the simulations show the relevance of including plastic deformations in the austenite in order to predict a more realistic evolution of the transformation process

Crystallographically based model for transformation-induced plasticity in multiphase carbon steels

The microstructure of multiphase steels assisted by transformation-induced plasticity consists of grains of retained austenite embedded in a ferrite-based matrix. Upon mechanical loading, retained austenite may transform into martensite, as a result of which plastic deformations are induced in the surrounding phases, i.e., the ferrite-based matrix and the untransformed austenite. In the present work, a crystallographically based model is developed to describe the elastoplastic transformation process in the austenitic region. The model is formulated within a large-deformation framework where the transformation kinematics is connected to the crystallographic theory of martensitic transformations. The effective elastic stiffness accounts for anisotropy arising from crystallographic orientations as well as for dilation effects due to the transformation. The transformation model is coupled to a single-crystal plasticity model for a face-centered cubic lattice to quantify the plastic deformations in the untransformed austenite. The driving forces for transformation and plasticity are derived from thermodynamical principles and include lower-length-scale contributions from surface and defect energies associated to, respectively, habit planes and dislocations. In order to demonstrate the essential features of the model, simulations are carried out for austenitic single crystals subjected to basic loading modes. To describe the elastoplastic response of the ferritic matrix in a multiphase steel, a crystal plasticity model for a body-centered cubic lattice is adopted. This model includes the effect of nonglide stresses in order to reproduce the asymmetry of slips in the twinning and antitwinning directions that characterizes the behavior of this type of lattices. The models for austenite and ferrite are combined to simulate the microstructural behavior of a multiphase steel. The results of the simulations show the relevance of including plastic deformations in the austenite in order to predict a more realistic evolution of the transformation process.Mechanics, Aerospace Structures and MaterialsAerospace Engineerin

Parametric study of multiphase TRIP steels undergoing cyclic loading

The behavior of multiphase steels assisted by transformation-induced plasticity (TRIP steels) undergoing low cycle, fully-reversed strain-controlled deformations is studied by means of numerical simulations based on micromechanical models. The ferritic phase is simulated using a single-crystal elasto-plasticity model for BCC crystals whereas the austenitic phase, which may transform into martensite, is simulated by a crystallographic phase transformation model coupled to a single-crystal elasto-plasticity model for FCC crystals. The influence of the TRIP mechanism on the overall behavior of the steel is investigated for selected variations of microstructural properties such as phase morphology, local carbon concentration in the austenite and austenitic grain size. The results of the simulations show a strong initial hardening associated to the martensitic transformation in accordance with experimental results. The simulations indicate an asymmetric hardening behavior under extension and contraction, particularly for large austenitic volume fractions and lower carbon concentrations. © 2010 Elsevier B.V

Modelling of the effects of grain orientation on transformation-induced plasticity in multiphase carbon steels

The effects of grain orientation on transformation-induced plasticity in multiphase steels are studied through three-dimensional finite element simulations. The boundary value problems analysed concern a uniaxially-loaded sample consisting of a grain of retained austenite surrounded by multiple grains of ferrite. For the ferritic phase, a rate-dependent crystal plasticity model is used that describes the elasto-plastic behaviour of body-centred cubic crystalline structures under large deformations. In this model, the critical-resolved shear stress for plastic slip consists of an evolving slip resistance and a stress-dependent term that corresponds to the projection of the stress tensor on a non-glide plane (i.e. a non-Schmid stress). For the austenitic phase, the transformation model developed by Turteltaub and Suiker (2006 Int. J. Solids Struct. at press, 2005 J. Mech. Phys. Solids 53 1747-88) is employed. This model simulates the displacive phase transformation of a face-centred cubic austenite into a body-centred tetragonal martensite under external mechanical loading. The effective transformation kinematics and the effective anisotropic elastic stiffness components in the model are derived from lower-scale information that follows from the crystallographic theory of martensitic transformations. In the boundary value problems studied, the mutual interaction between the transforming austenitic grain and the plastically deforming ferritic matrix is computed for several grain orientations. From the simulation results, specific combinations of austenitic and ferritic crystalline orientations are identified that either increase or decrease the effective strength of the material. This information is useful to further improve the mechanical properties of multiphase carbon steels. In order to quantify the anisotropic aspects of the crystal plasticity model, the simulation results for the uniaxially-loaded sample are compared with those obtained with an isotropic plasticity model for the ferritic grains. © 2006 IOP Publishing Ltd

Transformation-induced plasticity in multiphase steels subjected to thermomechanical loading

The behaviour of transformation-induced plasticity steels subjected to combined thermomechanical loading is studied at the microscale by means of numerical simulations. The microstructure is composed of an austenitic phase that may deform plastically and/or transform into martensite, and a ferritic phase that may deform plastically. The micromechanical models capturing these effects are derived from a thermodynamical framework, which has been extended from previous work in order to adequately account for the thermal contributions to the kinematics and the Helmholtz energy. The models are used in numerical simulations on a polycrystalline sample composed of an aggregate of multiple austenitic and ferritic grains of various orientations. The thermomechanical response of the sample is studied under (i) isothermal straining at different temperatures above the martensitic start temperature, and (ii) under different paths of straining and cooling to temperatures below the martensitic start temperature. The first type of analysis shows that at lower temperatures the transformation mechanism is more dominant than the plasticity mechanism, whereas the converse occurs at higher temperatures. The second type of analysis illustrates that, in comparison to a benchmark initially stress-free sample at room temperature, the transformation rate under straining is higher when performed on a pre-cooled sample, but the transformation rate under cooling is lower when carried out on a pre-strained sample. The results of this analysis indicate that, for optimizing the formability of this class of steels, it is recommended to make a judicious choice regarding the thermomechanical loading parameters during manufacturing processes

A micromechanical study of the deformation behavior of TRIP-assisted multiphase steels as a function of the microstructural parameters of the retained austenite

A study was conducted to investigate the mechanical behavior of a discrete aggregate of ferritic and austenitic grains representing a microstructure of transformation-induced plasticity (TRIP)-assisted steels. The main objective of the study was to investigate the specific effect of each microstructural parameter on the effective mechanical behavior. The constitutive model was presented to investigate the influence of potential plastic deformation in austenite before transformation was considered. The type of multiphase steels considered in the analysis consisted of isolated grains of retained austenite embedded in a ferritic matrix. The behavior of the bainitic phase was combined with the effective behavior of the matrix, while the constitutive model was applied to simulate the elastic, plastic, and transformation mechanisms in the austentitic grains and the elasto-plasto response of the matrix

Comparison of texture evolution in fcc metals predicted by various grain cluster homogenization schemes

We introduce a new material point homogenization scheme - the 'Relaxed Grain Cluster' (RGC) - based on a cluster of 2 x 2 x 2 grains and formulated in the framework of finite deformations. Two variants of this scheme, which allow for different degrees of relaxation, are compared to two variants derived from the infinitesimal-strain grain interaction model regarding the evolution of texture predicted for plane-strain compression of a commercial aluminum alloy. The RGC schemes give the closest match to experimental reference on both the alpha-fiber and the beta-skeleton line. The intensity of the brass texture component is found to be rather sensitive to the homogenization scheme. However, the observed decrease in texture intensity as a function of, the homogenization scheme for the Cu and S component on the beta-skeleton line can be correlated to the number of degrees of freedom in the cluster which are left unconstrained by the respective scheme. This is in line with the significant dependence of the Cu and S component intensity on boundary conditions reported in earlier studies

Modelling of the effects of grain orientation on transformation-induced plasticity in multiphase carbon steels

Aerospace Engineerin