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[...]
PDF-Dokument [1.5 MB]

What are the effects of strain rate on the mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel ?

In one project the effects of quasi-static and low-dynamic strain rate (ε̇= 10−4 /s to ε̇= 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21±3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited
a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain.

Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel
Materials Science & Engineering A 711 (2018) 78–92
Benzing et al 2018 MSE-A mechanical prop[...]
PDF-Dokument [3.5 MB]

 

Acta Mat. 2011, 59, p. 364