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

Abstract

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 ϵ‐martensite − into α’‐martensite, resulting in the well‐known transformation induced plasticity (TRIP) effect. The hardness of individual microstructural constituents (austenite, ϵ‐martensite, α’‐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 ϵ‐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 ϵ‐martensite are analyzed using transmission electron microscopy (TEM). α’‐martensite is found underneath such indents, and is formed during the indentation process.

The role of grain size in static and cyclic deformation behaviour of a laser reversion annealed metastable austenitic steel

“…After discussing the effect of grain size on the α 0 -martensite fractions and the microstructure after cyclic deformation, the reasons for the less pronounced cyclic hardening in case of small austenitic grain size should be considered. Besides the reduced α 0 -martensite formation regarded as the major factor, a further contribution is attributed to the different mechanisms responsible for the α 0 -martensite-related hardening, namely (i) slightly higher indentation hardness of the α 0 -martensite (see nanoindentation experiments 61,63 ), (ii) fine grain size of α 0 -martensite in comparison to that of the austenite and (iii) reduced mean free path for dislocations in the austenite due to the α 0 -martensite nuclei. 3,61 Whereas mechanisms (i) and (iii) are reported to be independent on grain size for the present grain size ranges 3 (contrary to the UFG alloy, cf.…”