Aluminum-silicon interfaces and nanocomposites: A molecular dynamics study

Molecular dynamics is used to examine the deformation and failure of Al-Si interfaces and of nanocomposites consisting of Al and Si nanograins loaded in tension. The modified embedded atom method, is used to describe the forces and energies in the system and accounts for the directional dependence of Si bonding. Various Al/Si interfaces with a common [011] axis parallel to the interface and polycrystals of eight hexagonal grains of 5 nm diameter with [011]-oriented columnar grains with zero, one, or two Si particles are examined to understand the effect of Si additions on the deformation and failure of the material. Three main features are demonstrated. First, imperfectly-matched Al/Si interfaces have relatively high tensile strengths and fracture energies that are larger than the ideal Griffith fracture energies. Second, different mechanisms dominate the early stages of non-linear deformation in the Al polycrystal and Al-Si nanocomposites: the Al polycrystal shows a mix of grain boundary deformation and dislocation activity while the Al-Si polycrystals show significant grain boundary sliding/shearing only at the Al-Si interfaces. The grain boundary sliding/shearing in the Al-Si nanocomposite is also reduced with increasing Si content, leading to higher yield stress. Third, the failure modes differ between the polycrystal and the nanocomposite: failure in the Al polycrystal initiates at a grain boundary and propagates transgranularly whereas failure in the Al-Si composites occurs by void damage accumulation at an Al/Si interface. The composite failure stresses are lower than the strengths of the Al-Si bimaterial interfaces due to stress concentrations in the composites and non-ideality of the as-fabricated composite interfaces but the normal stresses at the particle interfaces do reach values comparable to those of the bimaterial interfaces. These results suggest that Al-Si nanocomposites can be engineered for high hardness strength. (c) 2005 Elsevier Ltd. All rights reserved.

Time-Dependent Response of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) under Low to High Tensile Stresses

Agnieszka Ewa Switek

Ultra High Performance Fibre Reinforced Concretes (UHPFRC) are characterised by high mechanical performance, extremely low permeability and tensile strain hardening. For structural applications, in composite structures they are very well adapted for reinforcement and protection of existing concrete members. Cast in-situ, they undergo under restraint development of tensile eigenstresses at early age, which can lead in some specific cases of applications to localised macrocracking of a new layer and loss of protective function. On the other hand their tensile viscoelastic potential helps mitigate these stresses. The objective of this study was to investigate the tensile viscoelastic properties of UHPFRC at early ages and long-term in creep and relaxation tests. A large experimental campaign with tensile creep and relaxation tests was performed, with load levels ranging from 30 to 90% of the tensile strength, including creep and relaxation tests during the strain hardening phase, at two different loading ages of 3 and 7 days. Three main testing setups were used: TSTM (Temperature Stress Testing Machine), creep rigs and closed-loop servo-hydraulic testing machine with additional Acoustic Emission detection. The influence of early age evolution in mechanical response and underlying mechanisms governing this response were discussed. Linear viscoelastic models were used to analyse and to highlight on the basis of the experimental results the non-linear behavior depending on solicitation level and loading sequence. The non-linear threshold for viscoelastic response was estimated for the material loaded at 3 and 7 days. The early age mechanical response at 3 days was proportional to the load level in the instantaneous part for three stress levels tested 30, 60 and 90% of the tensile strength, and showed non-linear creep from 60% on. It was observed that for a very low stress level of 13% the creep deformation was decreasing. Relaxation tests with compensation of simultaneously monitored shrinkage deformation were performed, and showed a viscoelastic potential similar to the one observed for creep for the low load level of 30%. Incremental creep and relaxation tests were successfully performed for 3 days age specimens. For higher stress levels, the viscoelastic response from creep tests was more pronounced than in relaxation tests, which was highlighted by analysis with linear viscoelastic models. The creep response at 7 days showed that the threshold for non-linear viscoelasticity is close to 9 MPa (81% of the tensile strength) load level. The Acoustic Emission non-destructive testing method was used to get insight on the physical mechanisms involved in viscoelastic deformations. A strong correlation between AE number of events and the deformation was found, also the amplitude and energy of AE events was found to be of the same order during the instantaneous and delayed deformations. UHPFRC loaded at 7 days age in incremental test showed linear behavior until very high stress level corresponding to the hardening domain. The non-linear and tertiary creep was obtained during the last loading step, for 12 and 11 MPa respectively, which occurred in the hardening domain. The Acoustic Emission monitoring confirmed very close correlation between non-linear creep and AE cumulative number of events induced by microcracking. In order to precisely detect non-linear phenomena in incremental test, analysis with linear viscoelastic models was performed. Comparison between creep and relaxation test results showed that non-linear viscoelastic phenomena were more actively involved in creep tests. The outcomes of this research, bridging the material science and structural engineering, provide useful data to foster applications in new and existing structures, making the best use of the tensile viscoelastic potential of UHPFRC.

EPFL2011

Mechanical behavior of aluminum-silicon nanocomposites: A molecular dynamics study

William Curtin

The mechanisms of deformation and failure in Al-Si nanocomposites, formed by adding Si particles to Al nanocrystalline materials, are investigated using molecular dynamics. The deformation and fracture mechanisms are found to be different in the Al-Si composites as compared to single-phase all-Al materials. The plastic deformation of all-Al polycrystals is associated with a mix of grain boundary deformation and dislocation activity while the deformation in the Al-Si nanocomposites is associated with predominantly grain boundary sliding/shearing at the Al/Si interfaces and little deformation elsewhere. The Al-Si nanocomposites also have a higher yield stress than the all-Al nanocrystals, consistent with recent experimental data. The failure of the Al polycrystals occurs by crack initiation at triple junctions and grain boundaries accompanied by localized shearing, leading to either trans- or intergranular cracks, while failure in the Al-Si nanocomposites occurs by void damage accumulation, culminating in crack formation, at an Al/Si interface. The tensile strength in the Al-Si nanocomposites correlates well with the fundamental Al/Si interface strength when the interface stress concentration caused by the modulus mismatch is considered. In spite of very different failure modes, the tensile strengths of the Al-Si and all-Al materials are similar. These results show that Al/Si interfaces control the mechanical behavior in the nanocomposites, reducing the role of bulk metal deformation modes, indicating that Al-Si nanocomposites can be engineered for enhanced hardness over all-Al nanocrystals of the same grain size. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

2006

A granular model of solidification as applied to hot tearing

Stéphane Vernède

Hot cracking is a spontaneous failure of an alloy during solidification. It is a severe problem for casting industry as it reduces the productivity of cast houses and limits the range of alloys compositions that can be industrially produced. Hot tears occurs at the end of solidification, when solid grains are separated by thin liquid films. At this stage of solidification, fluid flow between the grains is difficult and the solid network is not continuous enough to transmit stresses. The material is extremely brittle. Moreover, the deformation induced by the thermal shrinkage of the material tends to localize in hot spots, i.e., in zones which solidify at last. These zones become in tension and the concomitant effect of the tensile stress with the intrinsic brittleness of the material result in a hot crack. Even though the general mechanisms of hot tearing are understood, the importance of each physical parameter remains ill defined. The present work is mainly focused on the derivation of a granular model of mushy zone (the zone where solid and liquid coexist) for aluminium alloys, i.e., a model which explicitly considers the behaviour of each grain while being sufficiently simple to allow the computation of large mushy zones. First, a solidification model based on the Voronoi diagram of a random set of nuclei is derived. This model computes the solidification in each polyhedron considering back-diffusion and coalescence. Second, a pressure drop calculation is performed in the network of connected liquid channels assuming the centre of solid grains to be fixed. This model considers a Poiseuille flow in each channel, Kirchhoff's conservation of flow at nodal points and flow Losses compensating solidification (KPL model). Finally, the displacement of the solid grain centres is considered and the mechanical behaviour of the mushy zone is computed assuming perfectly rigid solid grains. This model shows the progressive formation of grain clusters and the localization of fluid flow at high solid fraction. Therefore, it allows to define transitions in the behaviour of the mushy zone. These transitions are essential for hot cracking models, but should be introduced as parameters in continuum approaches. Finally, the mechanical model shows a strong localization of deformation in liquid channels normal to the stress direction. Such channels create ideal sites for the nucleation and the propagation of hot cracks.