Joint twinning- and transformation-induced plasticity

Introduction to high manganese austenitic steels 

High manganese austenitic steels with 15-30 wt% Mn are attractive for structural applications in the automotive industry because of their outstanding mechanical properties such as high strength and high ductility. 
Under applied loading, the hardening mechanism in this class of steels is due to Transformation Induced Plasticity (TRIP) and/or Twinning Induced Plasticity (TWIP).


Mechanical twinning and/or athermal phase transformation 

Austenitic steel systems which exhibit TRIP or TWIP are, for instance, Fe-Mn-C or Fe-Mn-Al-Si. Related alloys where both phenomena can occur concurrently are Fe-Cr-Ni stainless steels which are used in the fields of energy conversion, household and cryogenic applications as well as chemical industries.
Depending on the chemical composition and the deformation temperature, additional plastic deformation mechanisms such as mechanical twinning and/or athermal phase transformation phenomena can occur besides dislocation slip in such steels. 


How do TRIP and TWIP depend on the stacking fault energy?

The activation barriers for these partially competing mechanisms are strongly dependent on the stacking fault energy (SFE). With decreasing SFE, the plasticity mechanisms change from (i) dislocation glide to (ii) dislocation glide in conjunction with mechanical twinning to (iii) dislocation glide in conjunction with martensitic phase transformation. In general, martensitic transformation is observed in very low SFE steels (below 20 mJ/m2) while twinning is observed in medium SFE steels (20-40 mJ/m2). When the SFE exceeds 45 mJ/m2, dislocation glide becomes the predominant mode of plastic deformation. In this class of steels, the gamma-austenite phase is a metastable fcc phase, which can transform into ε-martensite (hcp) or a'- martensite (bcc/bct). Two different transformation paths have been reported. The transformation path is influenced by the Mn content, where the g / ε transformation typically occurs in high Mn steels (15-30 wt% Mn) while the g / ε / a' transformation path typically occurs in medium Mn steels (5-12 wt% Mn).


How can competing TWIP and TRIP effects be modelled?

The objective of this project is to develop a physically-based crystal plasticity model for high Mn steels that can capture the activation of different plastic deformation mechanisms, in particular the TRIP and TWIP effects, and their interaction with dislocation plasticity and their respective dependence on the substructure, based on the SFE of the material. The model is implemented within an existing crystal plasticity computational framework, the Düsseldorf Advanced Material Simulation Kit (DAMASK).

A crystal plasticity model for twinning- and transformation-induced plasticity
Acta Materialia 118 (2016) 140-151
In this paper a dislocation density-based crystal plasticity model incorporating both transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) is presented. The approach is a physically-based model which reflects microstructure investigations of ε-martensite, twins and dislocation structures in high manganese steels. Validation of the model was conducted using experimental data for a TRIP/TWIP Fe-22Mn-0.6C steel. The model is able to predict, based on the difference in the stacking fault energies, the activation of TRIP and/or TWIP deformation mechanisms at different temperatures.
crystal plasticity twinning and TRIP Act[...]
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Acta Mat. 2011, 59, p. 364