https://material.karlov.mff.cuni.cz/people/janecek/studenti/FyzMetNano/Estrin_AM61_2013_782.pdf

Extreme grain refinement by severe plastic deformation: A wealth of challenging science

A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past five years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a review of the relations between microstructure and mechanical properties is presented focusing on plasticity mechanisms, strain-hardening, yield stress, texture, fracture and fatigue. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest and fundamental importance. Finally, this publication indicates some of the main avenues for future investigations required in order to sustain the quality and the dynamism in this field.

High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships

A model is proposed for the evaluation of the stacking fault energy (SFE) in Fe–Mn–C austenitic alloys, at different temperatures. It accounts for the variation of the Gibbs energy of each element during the austenite to e martensite transformation, plus their interactions. The Gibbs energy due to the antiferromagnetic to paramagnetic transition is also taken into account. The required data have been obtained from the literature. The result shows a decrease of the SFE with temperature, with a saturation below the austenite Neel temperature. The result agrees with the mechanical and thermal martensitic transformation limits proposed by Schumann. The plasticity mechanisms depend on the SFE. The mechanical martensitic transformation occurs below 18 mJ/m 2 , and twinning between 12 and 35 mJ/m 2 , in agreement with the tensile tests and the deformation microstructures observed in an Fe–22 wt.% Mn–0.6 wt.% C alloy at 77, 293 and 693 K.

Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe–Mn–C alloys

Twinning-induced plasticity (TWIP) steels

Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy

Effect of grain and twin boundaries on the hardening mechanisms of twinning-induced plasticity steels

We present a thermochemical model of the stacking-fault energy (SFE) in the Fe–Mn–C system with few percent of Cu, Cr, Al and Si in addition. Aluminium strongly increases the SFE, contrary to chromium, while the effect of silicon is more complex. Copper also increases the SFE, but strongly decreases the Neel temperature. The SFE is the relevant parameter that controls mechanical twinning, which is known to be at the origin of the excellent mechanical properties of these steels. Using this model, copper containing Fe–Mn–C grades were elaborated with SFE below 18 mJ/m 2 , in the range where ɛ-martensite platelets form instead of microtwins during plastic deformation. This substitution of deformation modes, confirmed by X-ray diffraction, does not significantly damage the mechanical properties, as long as the SFE is greater than 12 mJ/m 2 , which avoids the formation of α′-martensite.

Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe–Mn–C steel

The steel Fe–22 wt.% Mn–0.6 wt.% C exhibits a low stacking fault energy (SFE) at room temperature. This rather low value promotes mechanical twinning along with strain which is in competition with dislocation gliding, the so called twinning-induced plasticity effect. The proposed modeling of the mechanical behavior introduces the formation of mechanical microtwins in a viscoplasticity framework based on dislocation glide at the mesoscopic scale in the case of a simple tensile test. The important parameter is the mean free path of the dislocations between twins, whose reduction explains the high hardening rate (by a dynamical Hall–Petch-like effect). It takes into account the typical organization of microtwins observed in electron microscopy (geometrical organization by using a twin-slip intersection matrix). To take into account the polycrystalline disorder, the macroscopic flow stress is calculated by assuming that the deformation work is equal in each grain for each strain step. This model gives an intermediate rule between Taylor and Sachs approximations and is simpler to compute than self-consistent methods. The parameters for gliding are first fitted on results at intermediate temperatures (without twinning), and the whole modeling is then correlated at room temperature. The simulated results (microstructure and mechanical properties) are in good agreement with experience.

Sébastien Allain

Papers

High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships

Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe–Mn–C alloys

Effect of grain and twin boundaries on the hardening mechanisms of twinning-induced plasticity steels

Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe–Mn–C steel

A physical model of the twinning-induced plasticity effect in a high manganese austenitic steel