TY - BOOK

T1 - Crystallography-microstructure-relations of hierarchical lath martensite from a micromechanical point of view

AU - Petersmann, Manuel

N1 - no embargo

PY - 2018

Y1 - 2018

N2 - Highly dislocated lath martensite is an essential microstructure component of many multi-phase advanced high-strength steels (AHSS) such as dual-phase-, transformation / twinning induced plasticity and precipitation hardened steels. In the last decade novel experimental microstructure characterisation methods based on lattice diffraction phenomena enabled to obtain a clearer picture of the overall microstructural state of lath martensite, revealing that under certain circumstances it forms a hierarchical microstructural arrangement where the smallest units (laths) group to definite blocks that again assemble definite packets. Beside this general trend, the exact microstructure formation during transformation is highly sensitive on the materials processing history as well as temperature rates and external loadings during transformation. Modelling of the transformation necessitates a multi-scale description and a multitude of experimental data for the model verification. Since transformation induced plasticity results from accommodation processes of the highly anisotropic transformation strains at the microscale, the morphological aspects, i.e. the crystallographic variants related to the lattice change of the transformation must be taken into account. This work is motivated by experimental data obtained from electron backscattering diffraction measurements necessary to calibrate stress sensitive constitutive relations formulated at the microscale for their use in finite element models. In order to be able to accomplish such a goal (i) there must be a definite link between the experimental data and variables of the model and (ii) the model must comprise microstructurally and micromechanics motivated relations. However, for none of these two problems a generally accepted strategy exists up to date. Based on the requirements for the microstructure of a thermally cycled and mechanically loaded maraging steel forming a lath martensitic microstructure, first a unification of crystal plasticity and the crystallographic theory of martensite formation is proposed for point (i). For point (ii) phenomenological scaling relations for non-local effects as well as constitutive laws for the stress dependence of the transformation, dislocation plasticity, nucleation and coupling effects fitting this framework are advised.

AB - Highly dislocated lath martensite is an essential microstructure component of many multi-phase advanced high-strength steels (AHSS) such as dual-phase-, transformation / twinning induced plasticity and precipitation hardened steels. In the last decade novel experimental microstructure characterisation methods based on lattice diffraction phenomena enabled to obtain a clearer picture of the overall microstructural state of lath martensite, revealing that under certain circumstances it forms a hierarchical microstructural arrangement where the smallest units (laths) group to definite blocks that again assemble definite packets. Beside this general trend, the exact microstructure formation during transformation is highly sensitive on the materials processing history as well as temperature rates and external loadings during transformation. Modelling of the transformation necessitates a multi-scale description and a multitude of experimental data for the model verification. Since transformation induced plasticity results from accommodation processes of the highly anisotropic transformation strains at the microscale, the morphological aspects, i.e. the crystallographic variants related to the lattice change of the transformation must be taken into account. This work is motivated by experimental data obtained from electron backscattering diffraction measurements necessary to calibrate stress sensitive constitutive relations formulated at the microscale for their use in finite element models. In order to be able to accomplish such a goal (i) there must be a definite link between the experimental data and variables of the model and (ii) the model must comprise microstructurally and micromechanics motivated relations. However, for none of these two problems a generally accepted strategy exists up to date. Based on the requirements for the microstructure of a thermally cycled and mechanically loaded maraging steel forming a lath martensitic microstructure, first a unification of crystal plasticity and the crystallographic theory of martensite formation is proposed for point (i). For point (ii) phenomenological scaling relations for non-local effects as well as constitutive laws for the stress dependence of the transformation, dislocation plasticity, nucleation and coupling effects fitting this framework are advised.

KW - martensite

KW - finite element method

KW - modelling

KW - constitutive law

KW - crystallography

KW - micromechanics

KW - Martensit

KW - Kristallographie

KW - Finite Elemente Methode

KW - Modellierung

KW - Konstitutivgesetz

M3 - Doctoral Thesis

ER -