The microstructural break down: the effect of initial grain size

Christian Rupert Rehrl

Research output: ThesisDoctoral Thesis

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Abstract

Production of high quality materials is an important target of the metal forming industry. A low quantity of inclusions and a well defined microstructure are essential if good mechanical and physical properties are to be obtained. Remelting processes are widely used to remove impurities and lead to a more homogeneous microstructure compared to the cast structured state. Mechanical properties and microstructure are improvable. In metals without a solid state phase transformation (Ni, Cu, Al, gamma-iron), structural break down is the unique process to refine and homogenize the heterogeneous state. Hence, it is necessary to understand deformation mechanisms and the acting softening phenomena; recovery and recrystallization. The present thesis is devoted to the fundamental understanding of the effects of initial grain size and forming parameters on the microstructural refinement and homogenization. Experiments were performed using the model material pure nickel and a technically relevant austenitic stainless steel (Böhler A220) at different forming conditions. Different initial grain size states in the µm- to mm range were used. Special attention has been devoted to the investigation of the deformation microstructure and the potential of crystal plasticity-FEM models to predict the crystal fragmentation processes. For pure nickel and the austenitic stainless steel, the softening mechanisms acting during the deformation at elevated forming temperatures are dynamic recovery and dynamic recrystallization, nevertheless, in both materials the initial microstructure has a significant effect on the structural evolution during hot forming. A coarsening of the starting microstructure retards and slows down the dynamic recrystallization kinetics. The increase in grain size directly reduces the density of potential nucleation sites and decreases the stored energy, which is a well known driving force for microstructural instability, at these positions. New grains are formed by a discontinuous dynamic recrystallization process, where initial grain boundaries act as potential nucleation sites. The nucleation process can be characterized by extensive grain-boundary motion, bulging and annealing twinning, whereas the alloying content of the austenitic steel clearly reduces the grain boundary mobility. For the break down process in coarser grained structures, intragranular inhomogeneities (deformation bands or subgrain boundaries) serve as nucleation sites. For the first time it could be clearly shown for pure nickel that the dynamically recrystallized grain size depends significantly on the initial microstructure. With crystal plasticity models based on dislocation slip, the crystal orientation evolution - which is strongly dependent on the initial grain orientation, sample geometry and boundary conditions - and the evolution of strain gradients and rigid body rotation can be captured. It has been shown that such models can predict quite well the generated inhomogeneities from grain to grain interaction or the sample geometry, however they cannot explain the dislocation structure governed fragmentation. A slight modification of the first model used, by deactivation of slip systems and the implementation of a structural length scale, delivers the experimentally observed substructures. The problem of refining coarse grained materials is clearly based on a weak tendency to recrystallize. For an efficient industrial forming process, a double hit forming strategy which uses static- and dynamic recrystallization is recommended.
Translated title of the contributionKornfeinung bei der Warmumformung: Der Einfluss der Korngröße
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Clemens, Helmut, Assessor B (internal)
  • Pippan, Reinhard, Assessor A (internal)
Publication statusPublished - 2011

Bibliographical note

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Keywords

  • Pure nickel
  • Austenitic stainless steel
  • Electron backscattering diffraction (EBSD)
  • Finite element analysis
  • Recrystallization
  • Nucleation
  • Grain refinement

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