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Register a new userDirectional Anisotropy of Crack Propagation Along $Sigma$3 Grain Boundary in BCC Fe
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Crack growth behaviour along the coherent twin boundary (CTB), i.e., $Sigma$3{112} of BCC Fe is investigated using molecular dynamics (MD) simulations. The growth of an atomistically sharp crack with {112}$<$110$>$ orientation has been examined along the two opposite $<$111$>$ directions of CTB under mode-I loading at a constant strain rate. Separate MD simulations were carried out with crack inserted in the left side, right side and middle of the specimen model system. The results indicate that the crack grows differently along the two opposite $<$111$>$ directions. In case of a crack inserted in the left side, the crack grows in ductile manner, while it propagates in semi-brittle manner in the case of crack inserted in the right side. The directional dependence of crack growth along the CTB is also confirmed by the stress-strain behaviour. This anisotropy in crack growth behaviour has been attributed to the twinning-antitwinning asymmetry of 1/6$<$111$>$ partial dislocations on {112} planes.
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Molecular dynamics simulations revealed significant difference in deformation behaviour of $<$100$>$ BCC Fe nanowires with and without twist boundary. The plastic deformation in perfect $<$100$>$ BCC Fe nanowire was dominated by twinning and reorientation to $<$110$>$ followed by further deformation by slip mode. On the contrary, $<$100$>$ BCC Fe nanowire with a twist boundary deformed by slip at low plastic strains followed by twinning at high strains and absence of full reorientation. The results suggest that the deformation in $<$100$>$ BCC Fe nanowire by dislocation slip is preferred over twinning in the presence of initial dislocations or dislocation networks. The results also explain the absence of extensive twinning in bulk materials, which inherently contains large number of dislocations.
Mg grain boundary (GB) segregation and GB diffusion can impact the processing and properties of Al-Mg alloys. Yet, Mg GB diffusion in Al has not been measured experimentally or predicted by simulations. We apply atomistic computer simulations to predict the amount and the free energy of Mg GB segregation, and the impact of segregation on GB diffusion of both alloy components. At low temperatures, Mg atoms segregated to a tilt GB form clusters with highly anisotropic shapes. Mg diffuses in Al GBs slower than Al itself, and both components diffuse slowly in comparison with Al GB self-diffusion. Thus, Mg segregation significantly reduces the rate of mass transport along GBs in Al-Mg alloys. The reduced atomic mobility can be responsible for the improved stability of the microstructure at elevated temperatures.
Moessbauer transmission spectra for the 14.41-keV resonant line in 57Fe have been collected at room temperature by using 57Co(Rh) commercial source and alpha-Fe strain-free single crystal as an absorber. The absorber was magnetized to saturation in the absorber plane perpendicular to the gamma-ray beam axis applying small external magnetic field. Spectra were collected for various orientations of the magnetizing field, the latter lying close to the [110] crystal plane. A positive electric quadrupole coupling constant was found practically independent on the field orientation. One obtains the following value Vzz=+1.61(4)x10^19 V/m^2 for the (average) principal component of the electric field gradient (EFG) tensor under assumption that the EFG tensor is axially symmetric and the principal axis is aligned with the magnetic hyperfine field acting on the 57Fe nucleus. The nuclear spectroscopic electric quadrupole moment for the first excited state of the 57Fe nucleus was adopted as +0.17 b. Similar measurement was performed at room temperature using as-rolled polycrystalline alpha-Fe foil of high purity in the zero external field. Corresponding value for the principal component of the EFG was found as Vzz=+1.92(4)x10^19 V/m^2. Hence, it seems that the origin of the EFG is primarily due to the local (atomic) electronic wave function distortion caused by the spin-orbit interaction between effective electronic spin S and incompletely quenched electronic angular momentum L. It seems as well that the lowest order term proportional to the product L.LAMBDA.S dominates, as no direction dependence of the EFG principal component is seen. The lowest order term is isotropic for a cubic symmetry as one has LAMBDA=lambda.1 for cubic systems with the symbol 1 denoting unit operator and lambda being the coupling parameter.
While it is known that alloy components can segregate to grain boundaries (GBs), and that the atomic mobility in GBs greatly exceeds the atomic mobility in the lattice, little is known about the effect of GB segregation on GB diffusion. Atomistic computer simulations offer a means of gaining insights into the segregation-diffusion relationship by computing the GB diffusion coefficients of the alloy components as a function of their segregated amounts. In such simulations, thermodynamically equilibrium GB segregation is prepared by a semi-grand canonical Monte Carlo method, followed by calculation of the diffusion coefficients of all alloy components by molecular dynamics. As a demonstration, the proposed methodology is applied to a GB is the Cu-Ag system. The GB diffusivities obtained exhibit non-trivial composition dependencies that can be explained by site blocking, site competition, and the onset of GB disordering due to the premelting effect.
A detailed theoretical and numerical investigation of the infinitesimal single-crystal gradient plasticity and grain-boundary theory of Gurtin (2008) A theory of grain boundaries that accounts automatically for grain misorientation and grain-boundary orientation. Journal of the Mechanics and Physics of Solids 56 (2), 640-662, is performed. The governing equations and flow laws are recast in variational form. The associated incremental problem is formulated in minimization form and provides the basis for the subsequent finite element formulation. Various choices of the kinematic measure used to characterize the ability of the grain boundary to impede the flow of dislocations are compared. An alternative measure is also suggested. A series of three-dimensional numerical examples serve to elucidate the theory.
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