Abstract
Lath martensite is a complex hierarchical compound structure that forms during rapid cooling of carbon steels from the austenitic phase. At the smallest, i.e., ‘single crystal’ scale, individual, elongated domains, form the elemental microstructural building blocks: the name-giving laths. Several laths of nearly identical crystallographic orientation are grouped together to blocks, in which–depending on the exact material characteristics–clearly distinguishable subblocks might be observed. Several blocks with the same habit plane together form a packet of which typically three to four together finally make up the former parent austenitic grain. Here, a fully parametrized approach is presented which converts an austenitic polycrystal representation into martensitic microstructures incorporating all these details. Two-dimensional (2D) and three-dimensional (3D) Representative Volume Elements (RVEs) are generated based on prior austenite microstructure reconstructed from a 2D experimental martensitic microstructure. The RVEs are used for high-resolution crystal plasticity simulations with a fast spectral method-based solver and a phenomenological constitutive description. The comparison of the results obtained from the 2D experimental microstructure and the 2D RVEs reveals a high quantitative agreement. The stress and strain distributions and their characteristics change significantly if 3D microstructures are used. Further simulations are conducted to systematically investigate the influence of microstructural parameters, such as lath aspect ratio, lath volume, subblock thickness, orientation scatter, and prior austenitic grain shape on the global and local mechanical behavior. These microstructural features happen to change the local mechanical behavior, whereas the average stress–strain response is not significantly altered. Correlations between the microstructure and the plastic behavior are established.
Highlights
Max-Planck-Institut für Eisenforschung, Max-Planck-Straße 1, 40237 Düsseldorf, Germany; Department of Materials Science and Engineering, Massachusetts Institute of Technology (MIT), Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, 3001 Leuven, Belgium
It is measured in units of volume (UV) which corresponds to the volume of a voxel, i.e., UL3
Here the h1 1 1i{1 1 0} slip systems are exclusively used and the initial hardening rate (h0 ), the initial resistance (g0 ), and the saturation resistance (g∞ ) are adjusted to reproduce the stress–strain curve up to the ultimate yield stress of a fully lath martensitic microstructure obtained from a commercial steel (Dillidur 450) by AG der Dillinger Hüttenwerke
Summary
The approach used in this study to generate martensitic microstructures allows to independently vary several of the characteristic features of lath martensite as outlined above in a systematic way and, can be used to derive holistic microstructure–. Subblock generation: For each packet, a different habit plane is selected that is parallel to a {1 1 1} plane of the austenitic grain. The 6 variants of the (111)γ habit plane occur in the following pairs: V1–V4, V2–V5, and V3–V6 This variant selection is considered when assigning the crystallographic orientations. The thickness of the subblocks in the direction normal to the habit plane, tsubblock It is measured in units of length (UL) which corresponds to the side length of a voxel. The average volume of the lath, Vlath , controlled via the number density of seeds used in the Voronoi tessellation It is measured in units of volume (UV) which corresponds to the volume of a voxel, i.e., UL3.
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