Experimental studies of deformation structures

The aim of Karin Yvell´s PhD project is to investigate and characterize the microstructure evolution during plastic deformation. The project focuses mainly on the study of austenitic stainless steels. Involved in the project are AB Sandvik Materials Technology in Sandviken, Dalarna University in Borlänge and KTH (Royal Institute of Technology) in Stockholm.

The microstructure is studied by using electron backscatter diffraction (EBSD), a scanning electron microscope (SEM) based technique. The technique uses colors to visualize the microstructure in various ways, to enhance special structures and properties, which makes EBSD a powerful tool for microstructure characterization. Typical EBSD measurements included in the project are grain and subgrain size determination, boundary characterization, phase identification and texture analysis.

The microstructure, of an austenitic stainless steel (316L) deformed at a high strain rate, shown in the figure is visualized using different colors for different grain orientations in the sample. Thick black lines are used to mark the grain boundaries and red lines surrounds annealing and deformation twins. The thin black lines shows low angle boundaries formed during the deformation. The orientation of the grains and all the boundaries contributes in their own way to the mechanical properties of this sample. 

The PhD project, in brief, includes studies of:

  • The phase distribution, before and after roll forming, in a partially heated high strength stainless steel. The heating was found to cause a phase transformation which improved the ductility. This phase transformation was reversed by the roll forming and resulted in restored strength.
  • The microstructure evolution during wire rod rolling in a wire rod block. The grain and sub grain size and the recrystallization fraction was determined after each roll pass. After the last roll pass the reduction in grain size had leveled out and the recrystallization was almost complete.
  • The effect of deformation, by compression at high strain rates at room temperature and at 500 °C, on the microstructure and the size distribution. No changes due to the strain rate, at constant temperature, were observed in the microstructure. Only changes in the strain gave significant differences. A bimodal lognormal size distribution was found to describe the size distribution for both grains and subgrains.
  • The effect of different temperatures during compression deformation at high strain rates on the microstructure and on the deformation mechanisms. Deformation by twinning was found to decrease with increasing temperature.
  • Interrupted in-situ tensile tests in the SEM which enables the study of the evolution of the deformation structure in individual grains. By studying alloys with different chemical compositions knowledge about the relationship between composition, microstructure and mechanical properties was gained.

The goal for the PhD project is to contribute to an increased understanding of the connection between a materials chemical composition, microstructure and mechanical properties, which improves the process of designing new steels with unique properties.