Crystal plasticity simulation of the effect of grain size on the fatigue behavior of polycrystalline Inconel 718

A microstructure-based model that accounts for the effect of grain size has been developed to study the effect of grain size on the fatigue life of Inconel 718 alloys. The fatigue behavior of two alloys with different grain size was determined by means of uniaxial cyclic deformation tests under fully-reversed deformation ($R_\varepsilon$ = -1) at 400$^\circ$C in the low cycle fatigue regime. The model was based in the determination of the fatigue indicator parameter (based on the local crystallographic strain energy dissipated per cycle) by means of computational homogenization of a representative volume element of the microstructure. The mechanical response of the single crystal within the polycrystal was modelled through a phenomenological crystal plasticity model which was modified to account for the effect of grain size on the monotonic and cyclic hardening/softening mechanisms. The microstructure-based crack initiation model parameters were calibrated from the experimental tests of the material with fine grain size. The results of the fatigue simulations were in good agreement with the experimental results in terms of the cyclic stress-strain curves and of the number of cycles for fatigue crack initiation. The model did not show any grain size effect on the fatigue life for the largest cyclic strain ranges while the predicted fatigue life predicted was considerably longer in the case of the microstructure with fine grain size for the lowest strain ranges, in quantitative agreement with experimental data. These differences were attributed to changes in the deformation modes between homogeneous plastic deformation at large cyclic strain ranges and localized plasticity in a few grains at low cyclic strain ranges

Discrete dislocation dynamics simulations of dislocation-θ′\theta' precipitate interaction in Al-Cu alloys

The mechanisms of dislocation/precipitate interaction were studied by means of discrete dislocation dynamics within a multiscale approach. Simulations were carried out using the discrete continuous method in combination with a fast Fourier transform solver to compute the mechanical fields. The original simulation strategy was modified to include straight dislocation segments by means of the field dislocation mechanics method and was applied to simulate the interaction of an edge dislocation with a $\theta'$ precipitate in an Al-Cu alloy. It was found that the elastic mismatch has a negligible influence on the dislocation/precipitate interaction in the Al-Cu system. Moreover, the influence of the precipitate aspect ratio and orientation was reasonably well captured by the simple Orowan model in the absence of the stress-free transformation strain. Nevertheless, the introduction of the stress-free transformation strain led to dramatic changes in the dislocation/precipitate interaction and in the critical resolved shear stress to overcome the precipitate, particularly in the case of precipitates with small aspect ratio. The new multiscale approach to study the dislocation/precipitate interactions opens the possibility to obtain quantitative estimations of the strengthening provided by precipitates in metallic alloys taking into account the microstructural details

Three dimensional (3D) microstructure-based modeling of interfacial decohesion in particle reinforced metal matrix composites

Modeling and prediction of the overall elastic–plastic response and local damage mechanisms in heterogeneous materials, in particular particle reinforced composites, is a very complex problem. Microstructural complexities such as the inhomogeneous spatial distribution of particles, irregular morphology of the particles, and anisotropy in particle orientation after secondary processing, such as extrusion, significantly affect deformation behavior. We have studied the effect of particle/matrix interface debonding in SiC particle reinforced Al alloy matrix composites with (a) actual microstructure consisting of angular SiC particles and (b) idealized ellipsoidal SiC particles. Tensile deformation in SiC particle reinforced Al matrix composites was modeled using actual microstructures reconstructed from serial sectioning approach. Interfacial debonding was modeled using user-defined cohesive zone elements. Modeling with the actual microstructure (versus idealized ellipsoids) has a significant influence on: (a) localized stresses and strains in particle and matrix, and (b) far-field strain at which localized debonding takes place. The angular particles exhibited higher degree of load transfer and are more sensitive to interfacial debonding. Larger decreases in stress are observed in the angular particles, because of the flat surfaces, normal to the loading axis, which bear load. Furthermore, simplification of particle morphology may lead to erroneous results

Proposed sets of critical exponents for randomly branched polymers, using a known string theory model

The critical exponent for randomly branched polymers with dimensionality d equal to 3, is known exactly as 1/2. Here, we invoke an already available string theory model to predict the remaining static critical exponents. Utilizing results of Hsu et al. (Comput Phys Commun. 2005;169:114-116), results are added for d = 8. Experiment plus simulation would now be important to confirm, or if necessary to refine, the proposed values.N.H. March wishes to acknowledge that his contribution to this study was brought to fruition during a visit to DIPC in 2015.Peer Reviewe