Strain Hardening of Phases in High-Alloy CrMnNi Steel as a Consequence of Pre-Deformation Studied by Nanoindentation

Metastable high-alloy CrMnNi steels exhibit a martensitic phase transformation from austenite via an intermediate hexagonal phase arranged in deformation bands - often referred to as epsilon-martensite - into alpha'-martensite, resulting in the well-known transformation induced plasticity (TRIP) effect. The hardness of individual microstructural constituents (austenite, epsilon-martensite, alpha'-martensite) is studied by nanoindentation in a scanning electron microscope. The indentation hardness of the austenite in both its undeformed state and after tensile deformation, with elongation of up to 14%, is measured in order to study the influence of plastic deformation as well as of the grain orientation of the austenite. The microstructure after deformation is characterized by means of electron backscatter diffraction. Load-displacement curves are recorded in both a pre-deformed specimen and in an undeformed specimen, with both having similar grain orientations relative to the axis of indentation. Pronounced pop-in events are observed for undeformed austenite and epsilon-martensite. Indentation hardness values are determined for all microstructural constituents and compared to the undeformed state in order to obtain information on the influence of the orientation of austenitic grains and the plastic deformation. A significant increase in indentation hardness is observed, while some tendencies for orientation dependence are found. In addition, pop-in events occurring in epsilon-martensite are analyzed using transmission electron microscopy (TEM). alpha'-martensite is found underneath such indents, and is formed during the indentation process.