Dirk Helm

Twinning and detwinning during compression–tension loading measured by quasi in situ electron backscatter diffraction tracing in Mg–3Al–Zn rolled sheet

Twinning and detwinning are the important deformation modes in magnesium alloys during cyclic loading at room temperature. To analyze these two deformation mechanism, cyclic compression–tension experiments were performed on Mg–3Al–1Zn rolled sheet along the rolling direction. In these tests, the microstructure evolutions of a series of grains during deformation were traced by using quasi in situ electron backscatter diffraction (EBSD). Important quantities like the Schmid factors of twinning system, the fraction of twinning during compression, and the fraction of twinning after reverse loading were calculated on the basis of measured quantities. The influence of Schmid factor of twinning variants on detwinning upon reverse loading was analyzed. Detwinning would prefer to proceed during reverse loading if the Schmid factor of twinning in the twinning area before reverse loading is sufficiently large.

Modelling grain growth in the framework of Rational Extended Thermodynamics

Grain growth is a significant phenomenon for the thermomechanical processing of metals. Since the mobility of the grain boundaries is thermally activated and energy stored in the grain boundaries is released during their motion, a mutual interaction with the process conditions occurs. To model such phenomena, a thermodynamic framework for the representation of thermomechanical coupling phenomena in metals including a microstructure description is required. For this purpose, Rational Extended Thermodynamics appears to be a useful tool. We apply an entropy principle to derive a thermodynamically consistent model for grain coarsening due to the growth and shrinkage of individual grains. Despite the rather different approaches applied, we obtain a grain growth model which is similar to existing ones and can be regarded as a thermodynamic extension of that by Hillert (1965) to more general systems. To demonstrate the applicability of the model, we compare our simulation results to grain growth experiments in pure copper by different authors, which we are able to reproduce very accurately. Finally, we study the implications of the energy release due to grain growth on the energy balance. The present unified approach combining a microstructure description and continuum mechanics is ready to be further used to develop more elaborate material models for complex thermo-chemo-mechanical coupling phenomena.

Implementation of a Constitutive Model for the Mechanical Behavior of TWIP Steels and Validation Simulations

High manganese content TWinning Induced Plasticity (TWIP) steels are promising for the production of lightweight components due to their high strength combined with extreme ductility, see [1]. This paper deals with the implementation of a constitutive model for the macroscopic deformation behavior of TWIP steels under mechanical loading with the aim of simulating metal forming processes and representing the behavior of TWIP-steel components – for example under crash loading - with the Finite Element code LS-DYNA® and refers to our recently published papers: [2],[4],[5]. Within the present paper we focus on the implementation of the model formulated in [2] and its extension to stress dependent twinning effects.

Experimental investigations and multiscale modeling of the microstructure evolution and the mechanical properties of a ferritic steel grade during the production process

The process chain for sheet metals after casting to produce components made of semi-finished products is complex and the resulting mechanical properties of the produced material depend strongly on the evolution of the microstructure. After casting, a typical process chain consists of hot rolling, cold rolling, annealing, skin pass rolling, and sheet metal forming. In order to represent the microstructure evolution in an adequate way, a multiscale modeling concept is applied for the process steps cold rolling, annealing, and sheet metal forming. In this Integrated Computational Materials Engineering (ICME) concept, the strong microstructure evolution during the production of semi-finished products is modeled by using crystal plasticity for the representation of the cold rolling process and a cellular automaton is incorporated to model the annealing procedure. In both cases, only the microstructure in an adequate unit cell is considered. For sheet metal forming, the whole component has to be simulated toget...