Materials Science Forum Vols. 567-568

Abstract: In order to model the elasto-viscoplastic behaviour of an austenitic-ferritic stainless steel, the model initially developed by Cailletaud-Pilvin [1] [2] and used for modeling single-phase polycrystalline steel is extended in order to take into account the bi-phased character of a duplex steel. Two concentration laws and two local constitutive laws, based on the crystallographic slips and the dislocation densities, are thus simultaneously considered. The model parameters are identified by an inverse method. Simple tests among which tension test at constant strain rate and at different strain rates and uniaxial tension-compression test are used during the identification step. The predictive capabilities of the polycrystalline model are tested for non-proportional loading paths. It is shown that the model reproduces the over-hardening experimentally observed for this kind of loading paths. Then, yield surfaces are simulated during a uniaxial tension-compression test: it is shown that the distortion (i.e. plastic anisotropy induced by loading path) is correctly described.

Abstract: The micromechanical modelling encounters a problem that is different from basic assumptions of continuum mechanics. The material is not uniform on the microscale level and the material within an element has its own complex microstructure. Therefore the concept of a representative volume element (RVE) has been introduced. The general advantage, compared to conventional fracture mechanics, is that, in principle, the parameters of the respective models depend only on the material and not on the geometry. These concepts guarantee transferability from specimen to components over a wide range of dimensions and geometries. The prediction of crack propagation through interface elements based on the fracture mechanics approach (damage) and cohesive zone model is presented. The cohesive model for crack propagation analysis is incorporated into finite element package by interface elements which separations are controlled by the traction-separation law.

Abstract: This numerical study focuses on the recent observations of Man et al. [4] showing welloriented grains presenting no Persistent Slip Marking even if PSMs are observed in 86% of the surface grains in 316L austenitic stainless steel cycled at room temperature up to 60% of fatigue life. Scanning Electron Microscopy (SEM) permits us to build Finite Element (FE) meshes of the observed aggregates and to assign to the modelled grains the crystallographic orientations measured by Electron Back Scattering Diffraction (EBSD). Then, 3D FE computations using crystalline elasticity allow the evaluation of mean grain stress tensors and resolved shear stresses. The results could explain qualitatively the anomalous behaviour of the studied well-oriented grains which is partly due to the particular orientations and shapes of the neighbour grains. This study highlights the influence of crystalline elasticity and neighbour grains in microplasticity and crack nucleation.

Abstract: The distance functions such as G(r), K(r), H(r), g(r) and systematic scanning method connected with variance analysis have been used for quantitative characterization of functionally gradient materials structures. The methods have been tested on computer generated 3D model polycrystalline structures with diverse type of gradient function. Usefulness of the presented methods for quantitative description of FGM structures has been evaluated.

Abstract: The point sampled intercept method for estimation of the volume weighted mean volume of grains has been used. The results obtained for computer generated isotropic and anisotropic (with different type and level of anisotropy) 3D polycrystalline model structures have been analysed. On the basis of this analysis methodological recommendations have been formulated.

Abstract: The peculiarities of diffusion process and changes of long and short order parameters at an elastic and plastic deformation of compression and tension near grain boundaries with disorientation axis <111> are studied by the method of molecular dynamics. Whirl displacements of atoms destroying the order near grain boundary superdislocations are noticed at uniaxial deformation of compression. The fracture of superstructural and structural order at uniaxial deformation is accompanied by the deformation of amorphous area. Interdiffusion mechanism is changed in the dependence on deformation, temperature and boundary types.

Abstract: The formation of Frenckel pairs - vacancies and interstitial atoms takes place during external high energetical influence. The accumulation and aggregatization of point defects is noticed only at definite conditions. Vacancies form volume vacancy tetrahedrons and interstitial atoms form segregations. The conditions of the formation of point defects complexes on the basis of Frenckel pairs were studied by the methods of molecular dynamics in the dependence on temperature and intensity of external influence. Their possible role in the process of deformation and fracture of materials under study were evaluated. Comparison analysis of their stability was made for clusters of interstitial atoms. When the cluster had about 130 interstitial atoms, crowdion complexes were energetically profitable.

Abstract: Surface analysis of the aluminum thin plates plastically deformed under constrained cyclic deformation was performed by Fourier, morphology and fractal techniques. The plates are used for estimation of deformation damage and fracture in places with a priori known homogeneous and heterogeneous strain distribution. Advantages and disadvantages of Fourier, morphology and fractal techniques for characterization of the intersections and projections are discussed in the context of its applicability to investigation of plastic deformation and fracture (persistent strain planes and directions, strain localization angles and places, etc.).

Abstract: The relation of the internal stress and the parameters of the heterogeneous dislocation structure was suggested in the form of the classical Taylor formula relating the internal stress to the total dislocation density stored in the subgrain interior and in the subgrain boundaries. The other formula combines linearly the stress contribution generated by network dislocations and the stress contribution of the subgrain structure semiempirically related to the subgrain size. The formulas can evaluate the ratio of internal stress components due to sub-boundaries and free dislocations.

Abstract: Bending and contact strength of a carbon derived in-situ reinforced Si3N4-SiC micro/nanocomposite have been investigated. Four-point bending strength was measured using specimens with different effective volume and Weibull statistical analysis has been used for characterization of the strength values in the form of characteristic strength and Weibull modulus. The characteristic strength values of the investigated composite were σ0 = 675 MPa and σ0 = 832 MPa with the Weibull modulus of 6,4 and 8,6 for the specimens with effective volume 15,9 mm3 and 1,28 mm3 respectively. Contact strength of the investigated material was carried out using sphere on sphere and roller on roller methods and the obtained results are σ0S = 1997 MPa, σ0R = 1242 MPa, and mS = 17,1 mR = 6. Fracture origins in the specimens with effective volume of 15.9 mm3 tested in four-point bending were surface and volume located technological defects with dimensions from 10 μm to 180 μm, mainly in the form of clusters of pores and large SiC grains. Fracture mirror sizes were measured and the mirror constant was calculated. Fracture origins in the specimens with effective volume of 1.28 mm3 tested in four-point bending were surface (subsurface) defects with the same type as for the specimens with a higher effective volume. The fracture during the contact strength test „roller/roller“ has been initiated under the surface of the specimens and during the contact strength test „sphere/sphere“ by creation and growth of the cone cracks to critical size.