Experiments and Modelling of the Cyclic Behaviour of Haynes 282

In this contribution the mechanical behaviour of the Ni-based superalloy Haynes 282, developed for high-temperature applications in aero and land based gas turbine engines, is studied. Experiments for cyclic loading have been performed at room temperature and elevated temperature. To capture the cyclic hardening/softening of the material at the different temperatures, a plasticity model has been calibrated against experimental data. The robustness and the uniqueness of the identified material parameters are ensured by performing sensitivity and correlation analyses. A criterion based on the strain energy density, is used for LCF life predictions of Haynes 282. The criterion has been tuned to fit test data for the different temperatures and it has been evaluated with respect to both cyclic experimental data and with respect to model response. The influence of uncertainties in experimental data on identified material parameters, fatigue life predictions and finite element predictions has been investigated

Multiscale Modeling of the Mechanical Behaviour of Pearlitic Steel

Pearlitic steel is a two-phase material with cementite lamellae embedded in a ferrite matrix. In this contribution a representative microscale model, capturing the behavior of the cementite and the ferrite and also the interaction between these phases, is proposed. The response from the micromodel is coupled by means of computational homogenization to a representative mesomodel containing grains, or colonies, of pearlite. The material parameters of the ferrite and the cementite are identified by calibrating the model to experimental data for the pearlitic steel R260. Different types of prolongation conditions, i.e. how to couple the mesoscale kinematics to the microscale kinematics, are investigated and their results are compared. Finally, the influence of the number of cementite directions and the number of crystallographic orientations on the macroscopic stress response is studied. Thereby, a sufficient mesomodel size is estimated

Computational modeling of gradient hardening in polycrystals

A gradient hardening crystal plasticity model for polycrystals is introduced in Ekh et al. (2007). It is formulated in a thermodynamically consistent fashion and is capable of modeling a grain-size-dependent stress-strain response. In this contribution we extend that model to also include cross-hardening. A free energy is stated which includes contributions from the gradient of hardening along each slip direction. This leads to hardening stresses depending on the second derivative of the plastic slip. The governing equations for a nonlinear coupled system of equations is solved numerically with the help of a dual-mixed finite element method. The numerical results show that the macroscopic strength increases with decreasing grain size as a result of gradient hardening: Moreover, cross-hardening further enhances the strengthening gradient effect

Flexoelectricity and the polarity of complex ferroelastic twin patterns

We study, by means of an atomistic toy model, the interplay of ferroelastic twin patterns and electrical polarization. Our molecular dynamics simulations reproduce polarity in straight twin walls as observed experimentally. We show, by making contact with continuum theory, that the effect is governed by linear flexoelectricity. Complex twin patterns, with very high densities of kinks and/or junctions, produce winding structures in the dipolar field, which are reminiscent of polarization vortices. By means of a "cold shearing" technique, we produce patches with high vortex densities; these unexpectedly show a net macroscopic polarization even if neither the original sample nor the applied mechanical perturbation breaks inversion symmetry by itself. These results may explain some puzzling experimental observations of "parasitic" polarity in the paraelectric phase of BaTiO3 and LaAlO3.EKHS is grateful to EPSRC for financial support (EP/K009702/1). SL and PG appreciate the support by Helmholtz Programme Science and Technology of Nanosystems (STN) (Vorhabensnumber 43.22.01). MS acknowledges support by MINECO-Spain through Grants No. FIS2013-48668-C2-2-P and No. SEV-2015-0496, and by Generalitat de Catalunya (2014 SGR301). X.D. appreciates the support of NSFC (51171140, 51231008, 51320105014, 51321003), the 973 Programs of China (2012CB619402), and 111 project (B06025)

A computationally efficient coupled multi-scale model for short fiber reinforced composites

A coupled multi-scale (macro–micro) model is developed to predict non-linear elasto-plastic behavior of short fiber reinforced composites. At the microscopic level, a recently proposed micro-mechanical model, developed based on a two-step orientation averaging approach, is used. A wide range of micro-structural parameters, including matrix and fiber constitutive parameters, fiber volume fraction and fiber aspect ratio, can be accommodated in the model. Different interactions including Voigt, Reuss and a self-consistent assumption are considered in the model. This micro-mechanical model is then incorporated in a Finite Element model of the macro-scale problem, enabling coupled macro–micro simulations of real-life structures/specimens. Numerical examples and comparisons with experimental data, taken from literature, show that the model gives good predictions. Besides, several strategies and techniques are employed to improve the computational efficiency of the model. These techniques include replacing originally utilized trapezoidal integration (for fiber orientations and calculation of the Eshelby tensor) with more efficient integration schemes, and using a more efficient method for data storage. Comparisons of the computational efforts shows that these improvements substantially decreased the computational cost of the model

Modelling avalanches in martensites

Solids subject to continuous changes of temperature or mechanical load often exhibit discontinuous avalanche-like responses. For instance, avalanche dynamics have been observed during plastic deformation, fracture, domain switching in ferroic materials or martensitic transformations. The statistical analysis of avalanches reveals a very complex scenario with a distinctive lack of characteristic scales. Much effort has been devoted in the last decades to understand the origin and ubiquity of scale-free behaviour in solids and many other systems. This chapter reviews some efforts to understand the characteristics of avalanches in martensites through mathematical modelling.Comment: Chapter in the book "Avalanches in Functional Materials and Geophysics", edited by E. K. H. Salje, A. Saxena, and A. Planes. The final publication is available at Springer via http://dx.doi.org/10.1007/978-3-319-45612-6_

Direct evidence of polar ferroelastic domain boundaries in semiconductor BiVO4

Ferroelastic domain boundaries in semiconductor bismuth vanadate, BiVO4, are examined using second harmonic generation microscopy. Although the bulk is centrosymmetric, domain boundaries produce homogeneous second harmonic (SH) signals. The polarization dependences of SH intensities exhibit strong anisotropy compatible with the polar symmetry m. The present results are compared with the experimental results of other ferroelastics we have observed so far. Unlike other ferroelastic materials, the directions of the SH maxima are in the same direction for all domain boundaries

Interaction of low-energy electrons with surface polarity near ferroelastic domain boundaries

We derive surface polarity at and near ferroelastic domain boundaries from molecular dynamics simulations based on an ionic spring model. Interatomic gradient forces lead to flexoelectricity which, in turn, generates polarity at the surface and in twin boundaries. We then derive generic properties of electron scattering spectra equivalent to those observed in low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) experiments. Negatively (positively) charged surfaces reflect (attract) incident electrons with low kinetic energy. The electron images reveal the valley and ridge surface structures near the intersection of the twin boundary and the surface. Polarity in surface layers is predicted to be visible in LEEM and MEM spectra at neutral surfaces, but much less when surfaces are charged. Inward polarity reflects electrons similar to negative surface charges, and outward polarity backscatters electrons like positive surface charges. Both the polarity in the twin boundary and the physical topography scatter electrons, consistent with experimental LEEM and MEM experiments on CaTi O 3 with (001) and (111) surface terminations.EPSR

Control of surface potential at polar domain walls in a nonpolar oxide

Ferroic domain walls could play an important role in microelectronics, given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate (CaTiO3). Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential