Influence of microstructure on mechanical properties of tempered martensite
Ferritic steels with a tempered lath martensite microstructure are used in many applications of technological and economic importance, for example power generation, chemical plant and aerospace. The microstructure is hierarchical, each prior austenite grain containing many fine subgrains grouped into elongated units known as blocks, within which the crystallographic orientation of the subgrains is approximately uniform. Blocks with parallel long axes are further grouped into assemblages known as packets. The orientation relationship between austenite and martensite leads to a misorientation of around 60° between adjacent blocks in a packet.
A discrete representation of such a hierarchical microstructure represents an enormous challenge for micromechanical modelling. However such micromechanical modelling holds the chance to predict the mechanical response of different microstructures on external loads and thus to understand the relation between microstructure and mechanical properties. Within this project it is planned to apply the Fast Fourier Transform Method  to investigate the mechanical behaviour of material points containing very complicated microstructures. This method has recently been implemented at ICAMS and was successfully used to model the mechanical behaviour of ultrafine grained polycrystals with nanotwin lamellae. It is planned to study the constitutive behaviour of individual blocks first. These modelling results will be compared to experimental nanoindentation observations. In a second step, we will investigate the mechanical properties of a packet and at last model the stress strain curves of representative volume elements (RVEs) containing packets with different misorientations.
The results of these RVE calculations can be verified against uniaxial tensile tests on miniature specimens. This validation will consist firstly of the determination, using simulation and experiment, of a suitable material state (composition and heat-treatment route) to produce a microstructure resembling the model as closely as possible, i.e. with large, straight-sided martensite blocks. Previous work suggests that this can be most effectively achieved by minimising the carbon content and promoting austenite grain growth [2-4]. The microstructures of the proposed material states will be characterised using electron backscatter diffraction (EBSD) and the most suitable material state selected. Miniaturised tensile test specimens fabricated from this material state will be subjected to room-temperature tensile and high-temperature creep testing. The microstructure of the gauge length will be imaged using EBSD before and after testing to follow changes in block shapes, spatial and crystallographic orientations and misorientation angles and to characterise the locations of any cracks and voids. The results of the model and of the experimental testing will be compared. R.A. Lebensohn, N-site modeling of a 3d viscoplastic polycrystal using fast fourier transform, Acta Materialia 49: 2723, 2001.
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