Particle-level simulations are employed to investigate the mechanism by which nonmagnetizable particles enhance the field-induced shear stress in magnetorheological fluids. Large amplitude oscillatory shear reveals that nonmagnetizable spheres increase the suspension stiffness; the transition to nonlinear deformation remains unaffected suggesting that the nonmagnetizable spheres do not alter the stability of the clusters of magnetizable spheres. Snapshots reveal that nonmagnetizable spheres participate in stress transfer via repulsive-force clusters in a mechanism similar to jamming in hard-sphere suspensions. Partial stresses, number of repulsive-force clusters, and transient rheological behavior further support that nonmagnetizable spheres directly enhance the stress via repulsive-force clusters. The repulsive-force clusters contain both magnetizable and nonmagnetizable spheres, which likely explains the observation that nonmagnetizable spheres enhance the magnetic field-induced stress, even though they are not magnetizable.

Materials modelling at the atomistic scale provides a useful way of investigating the widely debated fundamental mechanisms of hydrogen embrittlement in materials like aluminium alloys. Density functional theory based tensile tests of grain boundaries (GBs) can be used to understand the hydrogen enhanced decohesion mechanism (HEDE). The cohesive zone model was employed to understand intergranular fracture from energies obtained in electronic structure calculations at small separation increments during ab initio tensile tests of an aluminium Σ11 GB supercell with variable coverages of H. The standard rigid grain shift test and a quasistatic sequential test, which aims to be faster and more realistic than the rigid grain shift method, were implemented. Both methods demonstrated the effects of H on the cohesive strength of the interface. The sequential method showed discrete structural changes during decohesion, along with significant deformation in general compared to the standard rigid approach. H was found to considerably weaken the GB, where increasing H content led to enhanced embrittlement such that, for the highest coverages of H, GB strength was reduced to approximately 20% of the strength of a pure Al GB - it is proposed that these results simulate HEDE. The possibility of finding H coverages required to induce this effect in real alloy systems is discussed in context by using calculations of the heat of segregation of H.

The effects of H segregation to a Σ11 symmetric tilt Al grain boundary are investigated using atomistic simulations, as part of a wider study on cracking in 7xxx series alloys. Density functional theory based simulations of uniaxial straining of grain boundaries containing 11 different concentrations of H were performed under the cohesive zone fracture mechanics framework. The theoretical strength of grain boundaries is shown to be supressed by H segregation, and the cause of this is attributed to the prevention of the formation of Al ligaments across grain boundaries. Segregated concentrations of relevant alloying elements (Zn, Mg, and Cu) show minimal impact on the H embrittlement process investigated, namely H enhanced decohesion (HEDE). Further modelling, of H transport and grain boundary precipitates, is required to confirm the validity of the HEDE mechanism in the case of 7xxx alloys.

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