Guilhem Martin

Automatic processing of an orientation map into a finite element mesh that conforms to grain boundaries

A new procedure for microstructure-based finite element modeling of polycrystalline aggregates is presented. The proposed method relies (i) on an efficient graph-based community detection algorithm for crystallographic data segmentation and feature contour extraction and (ii) on the generation of selectively refined meshes conforming to grain boundaries. It constitutes a versatile and close to automatic environment for meshing complex microstructures. The procedure is illustrated with polycrystal microstructures characterized by orientation imaging microscopy. Hot deformation of a Duplex stainless steel is investigated based on ex-situ EBSD measurements performed on the same region of interest before and after deformation. A finite element mesh representing the initial microstructure is generated and then used in a crystal plasticity simulation of the plane strain compression. Simulation results and experiments are in relatively good agreement, confirming a large potential for such directly coupled experimental and modeling analyses, which is facilitated by the present image-based meshing procedure.

Influence of the Martensitic Transformation on the Microscale Plastic Strain Heterogeneities in a Duplex Stainless Steel

The influence of the martensitic transformation on microscale plastic strain heterogeneity of a duplex stainless steel has been investigated. Microscale strain heterogeneities were measured by digital image correlation during an in situ tensile test within the SEM. The martensitic transformation was monitored in situ during tensile testing by high-energy synchrotron X-ray diffraction. A clear correlation is shown between the plasticity-induced transformation of austenite to martensite and the development of plastic strain heterogeneities at the phase level.

A strategy to improve the work-hardening behavior of Ti–6Al–4V parts produced by additive manufacturing

ABSTRACT To improve the mechanical properties of additively manufactured parts, specific heat treatments must be developed. Annealing of electron beam-melted Ti–6Al–4V was performed at sub-transus temperatures and followed by water quenching. Such treatments generate an α + α′ dual-phase microstructure. Microstructural and mechanical characterizations revealed that the heat-treated specimens show a broad range of tensile properties, depending on the fraction of martensite. The specimens treated between 850°C and 920°C exhibit an increase in strength and ductility, which is related to a remarkable hardening behavior. Work-hardening is attributed to kinematic hardening arising from the mechanical contrast between the α and α′ phases. GRAPHICAL ABSTRACT IMPACT STATEMENT Innovative heat treatments leading to α + α′ dual-phase microstructures are developed on Ti–6Al–4V parts produced by additive manufacturing. They lead to unprecedented work-hardening capabilities for this alloy.

Factors contributing to plastic strain amplification in slip dominated deformation of magnesium alloys

While plastic strains are never distributed uniformly in polycrystals, it has recently been shown experimentally that the distribution can be extremely heterogeneous in magnesium polycrystals even when the deformation is dominated by slip. Here, we attempt to provide insight into the (macroscopic) factors that contribute to this strain amplification and to explain, from a local perspective, the origins of this strain amplification. To do this, full field VPFFT crystal plasticity simulations have been performed under the simplifying assumption that twinning is inoperative. It is shown that the experimentally observed heterogeneity can be reproduced when a sufficiently high anisotropy in slip system strength is assumed. This can be further accentuated by a weakening of the texture.

Combined Use of DIC, EBSD and Simulation to Understand the Microscale Plastic Strain Distribution in Mg Alloys

One of the concerns in developing magnesium sheet with room temperature formability, is the complex deformation behavior it exhibits compared to other structural alloys. Owing to its strong plastic anisotropy, crystallographically informed models are required for the prediction of plasticity and failure. Significant effort has been made to develop and use crystal plasticity models to predict the macroscopic stress-strain response of magnesium alloys both under simple monotonic loading paths (e.g. tension and compression along different loading directions), e.g. [1], and more complex loading paths (e.g. various biaxial loading paths), e.g. [2]. Our work in this area has relied heavily on the use of mean-field selfconsistent crystal plasticity simulations (i.e. the Los Alamos Visoplastic Self Consistent Model [3]). Recognizing, however, the microscale plastic strain heterogeneity found in magnesium alloys, however, raises questions about the limits of such mean-field assumptions.