4Welcome to MultiScalePhase
Thermomechanical and metallurgical analysis of heat treatments of multiphase steels with hierarchical multiscale numerical techniques
The 3-years project MultiScalePhase, started in the 1st May 2012, is a research project founded by the by the Portuguese Foundation for Science and Technology via project PTDC/EME-TME/122287/2010 and by FEDER via the “Programa Operacional Factores de Competitividade” of QREN with COMPETE reference: FCOMP-01-0124-FEDER-020517.
It is well known that multiphase (MP) steels developed in the past decades offer impressive mechanical properties. The advantages of MP-steels, such as dual phase (DP) steels, are high levels of ultimate strength, higher work hardening rate and elimination of yield point elongation with considerable increase in ductility and formability.
Subsequently, huge efforts have been made on exploring various aspects of MP-steels. However, their characterization, including their behaviour within heat treatments, is not yet well understood. Some MP-steels are produced by the intercritical heat treatment of low carbon steels. They possess a heterogeneous microstructure, which consists of any of martensite, bainite, pearlite or austenite in a matrix of a softer phase known as ferrite. Finding optimum phase combinations is a demanding challenge and accurate predictive models are necessary to minimize the costly conventional methods of development.
Different analytical techniques such as rule of mixtures have been used to characterize MP-steels but none of which could capture its real material behaviour in terms of the stress–strain trend, phase transformations and in terms of the mechanics of deformation which take place in the composite material due to complexity of deformation process involved in such materials.
The mechanical behaviour of MP-steels is attributable to their microstructure. Consequently, modelling the mechanical behaviour of the MP-steel materials has to be done based on microstructural levels, which are many, but only at the phase level the material can be considered as an isotropic continuum.
Additionally, thermal and phase transformation induced Residual stresses, which appear in heat treatment and manufacture processes, can only be accounted if macro and microscales are accounted for using, for instance, hierarchical homogenization models. Numerical models that consider microstructural transformations as analytical equations, crystal plasticity based and self-consistent models cannot reproduce these phenomena. Grain boundaries, phase distribution and occurrence of local heterogeneities cannot be represented in these models as well. Therefore, hierarchical multi-scale representations must be taken in account as FEM models.
During the manufacture and/or heat treatment process, phase transformation phenomena take place. In this project, within the microstructural representation as representative volume element (RVE), the material grains (particles) are explicitly discretized by finite elements. Therefore, each finite element will be considered as a fraction of a grain. Thermomechanical, metallurgical (including transformation kinetics, grain size, etc.) and diffusion equations will be developed and implemented at the microlevel in order to reproduce these phenomena. Even anisotropy levels can be analysed microstructurally and used macroscopically.
Discretizing each grain, the numerical capability will be also used to study non-uniform deformations within the individual crystals of a polycrystal. Such approach will be also used to study grain-scale heterogeneous deformations that lead to the formation of macroscopic shear bands in plane strain compression.