A crystallographic extension to the Olson-Cohen model for predicting strain path dependence of martensitic transformation

A modification to the empirical Olson-Cohen strain-induced austenite to martensite transformation kinetic model is proposed. The proposed kinetic model accounts for the stress state at the grain level and the crystallography of the transformation mechanism. Two transformation mechanisms sensitive to the local stress state are incorporated in the model. First, the resolved shear stress on a slip plane in the direction perpendicular to the Burgers vector determines the stacking fault width (SFW) which in turn determines the potential nucleation sites. Second, the stress triaxiality governs the probability of the structural alpha'-martensite formation at a nucleation site. The kinetic model is implemented in the elastoplastic self-consistent (EPSC) crystal plasticity model to study the stress state and texture dependence of the strain-induced alpha'-martensite transformation and the mechanical response of metastable austenitic steels. The simulations are compared with experimental mechanical and phase fraction data from different austenitic steels subjected to simple tension, plane strain tension, equibiaxial tension, simple compression, and torsion. It is demonstrated that the appropriate modeling of alpha'-martensite phase fractions allows capturing the experimentally measured mechanical response. The implementation and insights from these predictions, including the role of texture evolution on martensite transformation, are discussed in this paper. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effects of irradiation and testing temperature on tensile behaviour of stainless steels

Nadine Baluc

Irradiated 304 and 316 stainless steel samples were investigated. The steels were neutron irradiated to 1.5 and 7.5 dpa at 550 K. Similar types of steels were irradiated at 550 K with 590 MeV protons in the PIREX facility at PSI, Switzerland. The doses reached in this case were 0.15 and 0.3 dpa. The stress-strain relationships at different temperatures were measured. The irradiation and deformation microstructures were investigated using transmission electron microscopy (TEM). Two modes of deformation were found, twinning and channelling, depending on the testing temperature. These results are discussed in terms of deformation mechanisms at different temperatures correlated with radiation hardening. Finally, possible correlations between deformation modes and previous irradiation assisted stress corrosion cracking (IASCC) studies are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

2000

Deformation-induced martensitic transformation under multiaxial loading

Wei-Neng Hsu

The deformation-induced martensitic transformation observed in specific materials can enhance their mechanical properties or result in functional properties. For example, the transformation induced plasticity (TRIP) effect results in a good combination of ductility and strength in some stainless steels, and the superelasticity in NiTi alloys enables the materials to experience large deformation yet recover to the original shape. These unique properties have attracted significant technological interests for various structural and functional applications. Since the multiaxial deformation is expected during manufacturing and service of these components, it is crucial to understand the transformation behavior under complex loading conditions in addition to uniaxial loading. This study aims to establish the link between microstructural evolutions and the martensitic transformation induced by multiaxial deformation, by combining cruciform multiaxial mechanical tests with in situ X-ray and neutron diffraction, high-resolution digital image correlation (HRDIC) and electron microscopy characterizations. Several alloys exhibiting martensitic transformations are studied: a nanostructured superelastic NiTi alloy, a coarse-grained NiTi, and metastable austenitic stainless steels 201 and 304. In the nanostructured superelastic NiTi, the distinctive transformation characteristics of nanoscaled subgrains under different loading directions are revealed, showing the path dependency of the martensitic variant selection. Moreover, it is found that multiaxial mechanical cycling leads to faster degradation in the superelasticity than the uniaxial cycling. The mechanisms for the material degradation, including the dislocation accumulation and the retained martensite, are discussed in terms of the crystallographic orientation and the load path. In the coarse-grained NiTi, HRDIC captures the activation of high-Schmid-factor martensite variants and the contribution of each variant to strain accommodation under uniaxial loading. In addition to the Schmid law, it is observed that shear transmission across grain boundaries influence the martensite variant selection. A post-mortem transmission electron microscopy (TEM) investigation reveals deformation twinning occurring within the martensite as an additional strain accommodation mechanism. In austenitic stainless steel 201, uniaxial loading is found to facilitate the martensitic transformation comparing to equibiaxial loading. However, 304 stainless steel exhibits the opposite trend: equibiaxial loading yields more martensite than uniaxial loading. The discrepancy is rationalized by the distinct deformation mechanisms depending on the evolving deformation textures under different load paths as well as the stacking fault energies of the materials. Furthermore, the HRDIC study on 304 steel shows the dependency of the slip activity on the loading direction at the grain level, supporting the nucleation mechanism of martensite proposed in literature.

EPFL2019

Comportement mécanique des milieux granulaires sous sollicitations cycliques

Frédéric Mayoraz

The following work deals with the characterisation of the behaviour of road bases granular material, made of tunnel excavation material and loaded by passing vehicle axles. This granular media, don't correspond generally to the standards demanded for the construction of road pavements. The thesis is a part of the project COST 337 ("Unbound granular materials for road pavements") whose aim is to develop a guideline for the design of road pavements in Europe (material production, laboratory tests, modelling of mechanical behaviour). At the beginning, several loadings of road pavements are described (mechanical, thermal, hydric), concentrating on mechanical loading. Full-scale tests and finite element simulations led to the knowledge of the order of magnitude of stresses and deformations as well as to the understanding of the rotation of the principal stresses, due to the passing of a vehicle axle. This theoretical part is followed by laboratory cyclic tests with sand to estimate the influence of the principal stress rotation on the deformability of granular media and the importance to take this phenomenon into account for the modelling. Further laboratory tests were carried out on high diameter samples (160 and 250 mm) made of tunnel excavation material. Those tests were made by performing a large number of cycles (80'000 to 250'000 cycles) and various stress paths. The aim of the tests was to measure the evolution of elastic (stiffening of material) and plastic (accumulation of permanent deformations) material characteristics during the cycles. Tools have been developed in the framework of image analysis to assess the eventual change of morphology of this non-standard granular media. Finally the modelling of the accumulation of plastic deformations is discussed in accordance to the theory of elasto-plasticity and visco-plasticity. To remove the limits of elasto-plasticity to simulate a large number of cycles, a visco-plastic approach is developed. The elegant approach, replacing the time by the number of cycles, allows to approximate the permanent deformations of a road pavement in dependence on the stress path. It provides a tool to estimate the rutting in road pavements.