Papers by Keyword: Heterogeneous Microstructure

Abstract: The increasing demand on thermo-mechanical strength, lightweight and formability in engineering applications require metallic materials with high sophisticated properties. Such functional alloys consist of heterogeneous composite-like microstructures, which are responsible for their stability in demanding service conditions (high temperature strength, low fatigue and creep resistance). External loads are distributed in between the phases of the alloys introducing high micro stress gradients, responsible for elastic and plastic deformation at the interfaces and micro crack initiation. Thus, the properties of such materials depend mainly on their phase shapes and 3D architectures leading to high stress gradients and elasto-plastic deformation under service conditions.This manuscript describes experimental studies on phase strain distribution for different heat treatment conditions in an AlMg4Si10 alloy. Neutron diffraction was used for strain measurement at an angle dispersive strain scanner with in-situ tensile test setup. Strain evolution under load and after unloading was measured to show elasto-plastic deformation behaviour in between the ductile α-Al matrix and stiff reinforcing Mg₂Si and Si phases. The degree of plastification, its effect on micro stress gradients and its influence on crack initiation could be discussed and comparisons to other composite materials could be drawn.

Abstract: The phase field approach to fracture modelling is based on a variational principle of the energy minimization as an extension of the Griffith’s brittle fracture theory. It introduces a scalar damage field, to differentiate between the fractured and intact material state. That way, it regularizes the sharp crack discontinuities and eliminates the need for the explicit tracking of the fracture surfaces. Moreover, the numerical implementation complexity is thus vastly reduced. In this contribution, the staggered phase field algorithm for the modelling of brittle fracture is implemented within the finite element program Abaqus. A common issue of the existing Abaqus implementations of the staggered phase field schemes is the computationally demanding fine incrementation of the loading applied, required to obtain an accurate solution. The computational time is reduced by imposing an appropriate convergence control paired with the Abaqus automatic time incrementation. Therefore, by taking advantage of the Abaqus computational efficiency, an accurate solution can be obtained for a moderate time step. The proposed model is verified on the symmetrically double notched tensile benchmark test. Compared to the existing implementations, it demonstrates an improvement in accuracy and the computational performance. Furthermore, a heterogeneous steel microstructure is analyzed displaying the model’s ability to solve crack nucleation and curvilinear crack paths.

Abstract: Tensile stress-strain and dynamic acoustic resonance tests were performed on Fe-C-Ni- Cu-Mo high-strength steels, characterized by a heterogeneous matrix microstructure and the prevalence of open porosity. All materials display the first yielding phenomenon and, successively, a continuous yielding behavior. This flow behavior can be described by the Ludwigson equation and developes through three stages: the onset of localized plastic deformation at the pore edges; the evolution of plastic deformation at the pore necks (where the austenitic Ni-rich phase is predominant); the spreading of plastic deformation in the interior of the matrix. The analytical modeling of the strain hardening behavior made it possible to obtain the boundaries between the different deformation stages.

Abstract: A stochastic approach has been presented for superplastic deformation of Ti-6Al-4V alloy, and probability functions are used to model the heterogeneous phase distributions. Experimentally observed spatial correlation functions are developed, and microstructural evolutions together with superplastic deformation behavior have been investigated by means of the probability functions. The strain-rate dependent failure strain can be correctly predicted by the model. As shown by the results the probability varies approximately linearly with separation distance, and significant deformation enhanced probability changes occur during the process. Since an initial microstructure is the most crucial factor that determines the properties of final microstructure, Monte Carlo simulation has been used coupled with the probability functions for the reconstruction of microstructures. By imposing the precisely optimized distributions of phase on the test specimens, therefore finite element implementation shows better agreement with experimental data of the failure strain.