Electron beam melting (EBM) is an established powder bed-based additive manufacturing process for the fabrication of complex-shaped metallic components. For metastable austenitic Cr-Mn-Ni TRIP steel, the formation of a homogeneous fine-grained microstructure and outstanding damage tolerance have been reported. However, depending on the process parameters, a certain fraction of Mn evaporates. This can have a significant impact on deformation mechanisms as well as kinetics, as was previously shown for as-cast material. Production of chemically graded and, thus, mechanically tailored parts can allow for further advances in terms of freedom of design. The current study presents results on the characterization of the deformation and strain-hardening behavior of chemically tailored Cr-Mn-Ni TRIP steel processed by EBM. Specimens were manufactured with distinct scan strategies, resulting in varying Mn contents, and subsequently tensile tested. Microstructure evolution has been thoroughly examined. Starting from one initial powder, an appropriate scan strategy can be applied to purposefully evaporate Mn and, therefore, adjust strain hardening as well as martensite formation kinetics and ultimate tensile strength.
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.
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