Corrosion protection of metals by water-based silane mixtures of bis-[trimethoxysilylpropyl]amine and vinyltriacetoxysilane

It is proposed that the difficulties encountered with the meaning of subcritical crack growth arose from a misunderstanding of the Griffith equation. This equation is G=2γ for an equilibrium crack (stable or unstable) where γ is the intrinsic surface energy. When G>2γ the crack has a velocity v depending on the crack extension force G−2γ, even in a vacuum, and the following equation, well verified for adherence of elastomers, G−2γ=2γφT(v) where φT(v) is related to viscoelastic losses or internal friction at the crack tip, is generalized to other materials. At a critical speed vc, dφ/dv becomes negative; as a negative branch cannot be observed the velocity jumps to high values on a second positive branch, so that G=Gc is a criterion for crack speed discontinuity, not the Griffith criterion. The multiplicative factor 2γ on the right-hand side accounts for the shift of the v-K curves with environment. No stress corrosion is needed to explain subcritical crack growth. Subcritical crack growth in glasses and ceramics and velocity jump in brittle polymers are shown to agree with this proposal. This model can also explain stick-slip motion when a mean velocity is imposed in the negative branch. Occurrence of velocity jump or stick-slip depends on the geometry tested and the stiffness of the apparatus. A second kind of stick-slip associated with cavitation in liquid-filled cracks is discussed. When the surrounding medium can reach the crack tip and reduce the surface energy, even at the critical speed vc, the critical strain energy release rate Gc is reduced in the same proportion as γ, and a loading which would have given subcritical growth will give a catastrophic failure. Reduction of surface energy in the Rehbinder effect and in embrittlement by segregation is discussed. Finally, the evolution of ideas concerning the Irwin-Orowan formula and fracture toughness is examined.

Subcritical crack growth, surface energy, fracture toughness, stick-slip and embrittlement

Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials.

New opportunities for 3D materials science of polycrystalline materials at the micrometre lengthscale by combined use of X-ray diffraction and X-ray imaging

Any fracture process ultimately involves the rupture of atomic bonds. Processes at the atomic scale therefore critically influence the toughness and overall fracture behavior of materials. Atomistic simulation methods including large-scale molecular dynamics simulations with classical potentials, density functional theory calculations and advanced concurrent multiscale methods have led to new insights e.g. on the role of bond trapping, dynamic effects, crack-microstructure interactions and chemical aspects on the fracture toughness and crack propagation patterns in metals and ceramics. This review focuses on atomistic aspects of fracture in crystalline materials where significant advances have been achieved over the last ten years and provides an outlook on future perspectives for atomistic modelling of fracture.

/pdf/atomistic-aspects-of-fracture-25ru8cr80a.pdf

Atomistic aspects of fracture

The penetration of liquid metals into the grain boundaries of polycrystalline solid metals is a precursor process triggering liquid metal embrittlement for some specific metal couples. Synchrotron radiation X-ray microtomography is used to investigate the penetration of liquid gallium into the grain boundaries of aluminium. The large difference in absorption coefficients between Ga and Al allows micrometer thick liquid Ga films to be observed as they are wetting grain boundaries of Al. The effects of annealing are presented and discussed. This technique opens the way for in-situ measurements of the kinetics of liquid metal penetration in materials. It might also be used to visualise and analyse the three-dimensional grain structure of a metallic polycrystal.

Penetration of liquid gallium into the grain boundaries of aluminium: a synchrotron radiation microtomographic investigation

Liquid metal embrittlement is the reduction in the elongation to failure that can be produced when normally ductile solid metals are stressed while in contact with a liquid metal This review describes its principal characteristics and the several models which have been advanced in attempts to explain the occurrence and different features of the process Comparison between theory and experiment indicates that many, but not all, of its aspects are consistent with a mechanism which operates by reducing the fracture surface energy of the solid metal Literature reports show that liquid metal embrittlement can occur with a very wide range of material combinations, and while most of the data refer to laboratory studies, it is clear that the phenomenon is also of technological significance as a potential cause of plant damage

M. G. Nicholas

Papers

Liquid metal embrittlement