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Abstract

For structural applications, tempered martensitic steels are an important material due to their good engineering properties. The lath martensitic microstructure is one of the mostcomplex ones, its hierarchical structure is built of sub-units of lath, blocks and packets, whichappear in prior austenite grains (PAGs) during quenching. However, the micromechanical processes and their relation to alloying compositions and heat treatments is not well understood and their numerical modelling attracts large research interest. In the last decades, several crystal plasticity models are discussed based on a local formulation, which neglects the influence of strain gradients. To incorporate this influence and to capture the indentation size effect, advanced nonlocal constitutive models, which are derived on the concept of the geometrically necessary dislocations (GNDs) have been introduced [Ma2014]. In the present study a model is proposed that considers the influence of complex microstructural features on the mechanical response of the material. This model needs a large number of material parameters, which are necessary to describe the specific and unique material behavior. The objective of this work is to develope a parameterization technique for nonlocal crystal plasticity models by means of an accessible and reproducible workflow.

To achieve this goal, we combine numerical simulations with the nanoindentation technique. In the first step, series of nanoindentation tests using a sphero-conical indenter tip are performed in single martensite packets in order to investigate the influence of microstructure on pile-up behavior. Afterwards, the resulting surface topologies are characterized. In the second step, simulations of the indentation process with approximated parameters are performed and the experimental and numerical surface topologies are compared. In an iterative procedure, the crystal plasticity parameters are adjusted such that an optimal match of both results is achieved at the end. Thus, a fully parameterized micromechanical model for tempered martensite can be built, which will support the understanding of microstructural features in mechanical behavior.

[Ma2014] A. Ma and A. Hartmaier, ”On the influence of isotropic and kinematic hardening caused by strain gradients on the deformation behaviour of polycrystals”, Philosophical Magazine, Vol94, 125-140 (2014).