Simulation of the interaction between an edge dislocation and ⟨111⟩ interstitial dislocation loops in α-iron

ABSTRACT

The dependence of the interactions of intermediate-size ½<111> self-interstitial atom (SIA) loops with an edge dislocation on strain rate and temperature was investigated by molecular dynamics (MD) simulations for the interatomic potential derived by Ackland et al. (A97). For low temperatures (T?=?1?K), the mechanisms of the interactions were in agreement with recent literature. It was shown that a second passing of the dislocation through the loop led to a different mechanism than the one that occurred upon first passing. Since these mechanisms are associated with different SIA loop sizes, and since the loop lost a number of SIAs upon first interaction, it was deduced that the dividing threshold between large and small loops (rendering them strong or weak obstacles, respectively) is at the vicinity of the loop size studied (169 SIAs). For higher temperatures (T?=?300?K), the strain rate dependence proved strong: for low strain rates, the dislocation absorbed the loop as a double super-jog almost immediately and continued its glide unimpeded. For a high strain rate, the dislocation was initially pinned due to the formation of an almost sessile segment leading to high critical stress.     

Internal friction in Al bicrystals with ⟨111⟩ tilt and twist grain boundaries

Internal friction measurements were performed on various ?111? tilt and twist grain boundaries in high-purity Al bicrystals. The temperature dependence of the grain boundary internal friction peak was determined, and the activation parameters of grain boundary relaxation were obtained. These parameters were found to change in a wide range depending on boundary geometry. The activation enthalpy of boundary relaxation and the pre-exponential factor of the relaxation time are related according to the compensation effect. The results are discussed in terms of the model of correlated relaxations. Bicrystals with vicinal Σ3 boundaries were observed to behave like single crystals, i.e. an internal friction peak did not appear. This evidences that both coherent and incoherent (60° ?111? tilt) twins possess a high mechanical resistance.     

Atomistic simulations of effect of hydrogen atoms on mechanical behaviour of an α-Fe with symmetric tilt grain boundaries

The effects of the hydrogen concentration, crystal orientation and grain size on the mechanical properties of an α-Fe bicrystal with symmetric tilt grain boundaries under tensile loading are investigated by molecular dynamics simulation. The results indicate that regardless of crystal orientation, the yield strength of bicrystal α-Fe decreases with the increase of hydrogen concentration. Hydrogen atoms have no influence on the primary dislocation (or twin) nucleation mechanism, but rather influence their multiplication process. The results also show that the degree of hydrogen embrittlement is obviously dependent on the misorientation angle, but it is almost independent of the grain size.     

Activation energy of self-diffusion along symmetric tilt grain boundaries 〈111〉 in the Ni3Al intermetallic compound

Activation energy of self-diffusion along symmetric tilt grain boundaries 〈111〉 in the Ni₃Al inter-metallic compound has been calculated depending on the temperature and misorientation angle. For comparison, two types of potentials of interatomic interaction have been used: pair Morse potentials and multi-particle Cleri-Rosato potentials. It has been shown that the activation energy of grain-boundary diffusion increases with temperature on applying the additional diffusion mechanisms. Three temperature ranges with various activation energies have been found.     

In situ and tomographic analysis of dislocation/grain boundary interactions in α-titanium

In situ straining in the transmission electron microscope and diffraction-contrast electron tomography have been applied to the investigation of dislocation/grain boundary and dislocation/twin boundary interactions in α-Ti. It was found that, similar to FCC materials, the transfer of dislocations across grain boundaries is governed primarily by the minimization of the magnitude of the Burgers vector of the residual grain boundary dislocation. That is, grain boundary strain energy density minimization determines the selection of the emitted slip system.     

Atomic structure and electronic properties of the ∑37(610) and ∑29(520) [001] tilt grain boundaries in Ge

400 kV high resolution electron microscopy (HREM), deep level transient spectroscopy (DLTS) and steady state electrical measurements have been applied to 37(610) and 29(520) [001] tilt grain boundaries (GBs) in germanium bicrystals. The atomic boundary structures were revealed by experimental HREM images taken under different defocus conditions. Later, structure models were refined by means of a trial-and-error method applying alternatively the image simulation and the molecular static calculation of relaxed structures. The structures were shown to be consistent with the modified structural unit model. Although the structures are different for the two GBs studied, DLTS data and steady state measurements were found to be quite similar for both GBs. Thus, the results point to the extrinsic origin of localized deep states at the GBs. The analysis of DLTS spectra indicates the impurity segregation at the boundary, e.g., the formation of vacancy-type oxygen complexes of a donor-like state at E _c-0.21 eV, which results in the fluctuation of the potential barrier. Defects in the GBs—like facets, atomic steps and secondary grain boundary dislocations—which are characteristic of both boundaries can act as nuclei to the impurity segregation.Presented at the Workshop on High-Voltage and High-Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

Observations of glide and decomposition of a⟨101⟩ dislocations at high temperatures in Ni-Al single crystals deformed along the hard orientation

Ni-44 at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard d001 ¢orientation. The dislocation processes and deformation behaviour were studied as a function of temperature, strain and strain rate. A slip transition in NiAl occurs from a?111? slip to non-a?111? slip at intermediate temperatures. In Ni-50 at.% Al single crystals, only a?010? dislocations are observed above the slip transition temperature. In contrast, a a?101?{101} glide has been observed to control deformation beyond the slip transition temperature in Ni-44 at.% Al. a?101? dislocations are observed primarily along both ?111? directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the a?101? dislocations along these directions is decomposed into two a?010? dislocations, separated by a distance of approximately 2 nm. The temperature window of stability for these a?101? dislocations depends upon the strain rate. At a strain rate of 1.4 210^?4 s^?1, a?101? dislocations are observed between 800 and 1000 K. Complete decomposition of a?101? dislocations into a?010? dislocations occurs beyond 1000 K, leading to a?010? climb as the deformation mode at higher temperatures. At lower strain rates, decomposition of a?101? dislocations has been observed to occur along the edge orientation at temperatures below 1000 K. Embedded-atom method calculations and experimental results indicate that a?101? dislocations have a large Peierls stress at low temperatures. Based on the present microstructural observations and a survey of the literature with respect to vacancy content and diffusion in NiAl, a model is proposed for a?101?{101} glide in Ni-44 at.% Al, and for the observed yield strength versus temperature behaviour of Ni-Al alloys at intermediate and high temperatures.     

Temperature dependence of the structure and shear response of a Σ11 asymmetric tilt grain boundary in copper from molecular-dynamics

We investigate the effect of temperature on the structure and shear response of a Σ11 asymmetric tilt grain boundary in a classical embedded-atom model of elemental copper using molecular dynamics simulations. As the temperature is increased the structure of the boundary disorders considerably, but with a boundary width that remains finite at the melting point. The disordering of the boundary structure becomes significant for homologous temperatures above 0.83 (1100?K). As temperature increases above this point the boundary width and roughness increases monotonically. Near the temperature where the boundary starts to disorder we observe a change in the temperature dependence of the ideal shear strength of the boundary, as well as the value of the coupling parameter β, defined as the ratio of the velocity of relative translation of the grains parallel to the boundary plane to that corresponding to the motion of the boundary normal to its plane.     

Observation of compensation effects during the thermal dissolution of aluminum oxide layers on tungsten and molybdenum 〈111〉 and on tungsten {110} in the presence of electric fields

Aluminum oxide layer dissolution was studied between 700 and 1200 K in the substrate areas of W〈111〉, Mo〈111〉, and on W{110} by means of FEM. Varying the electric field strength, F, between +45 and $\text{+105}\text{MV}\text{cm}$, two types of dissolution could be observed: dissolution by surface diffusion (low F's) and dissolution by ion desorption (high F's). It is assumed that aluminum suboxides — preferentially AlO — are involved in the dissolution processes. The preexponential factors, A_F, of an Arrhenius-Frenkel type equation were measured as a function of F. The field dependence of A_F is determined by the dissolution mechanism: (a) dissolution by diffusion: $\text{log}\text{A}{}^0{}_{\mathrm{F}}\text{=}\text{log}\text{A}{}^0{}_0\text{?}\text{ΔμF}\text{2.3k}{}^1\text{T}$ (μ  molecular dipole moment, ¹T ≡ isokinetic for W〈111〉, log A⁰₀ = ? 6.0 and ${}^1\text{T = 940}\text{K}$; for Mo〈111〉, log A⁰₀ = ? 3.1 and ${}^1\text{T = 860}\text{K}$; and (b) dissolution by ion desorption: $\text{log}\text{A}{}^{\mathrm{+}}{}_{\mathrm{F}}\text{=}\text{log}\text{A}{}^{\mathrm{+}}{}_0\text{+}\text{n}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{e}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{F}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{2.3k}{}^1\text{T}$; for A⁺₀ = ? 22 and ${}^1\text{T = 1200}\text{K}$; for W〈111〉, log A⁺₀ = ? 21 and ${}^1\text{T = 1200}\text{K}$. Using earlier proposed safeguards, isokinetic relationships (compensation effects) could be established for each of the two dissolution processes. The coordinates of the isokinetic points have the following average values: $\text{log}{}^1\text{A}{}^0{}_0\text{= 2.5}$ and ${}^1\text{T = 920}\text{K}$ for diffusion; $\text{log}{}^1\text{A}{}^{\mathrm{+}}{}_0\text{= ? 1}$ and ${}^1\text{T = 1240}\text{K}$ for ion desorption. The entropy changes (at $\text{T =}{}^1\text{T}$, zero field strength, and unit pressure) for the phase changes: solid layer → diffusion layer and solid layer → ion gas, are of the order of $\text{30}\text{cal}\text{K}$ · mol and $\text{90}\text{cal}\text{K}$ · mol, respectively. The two dissolution mechanisms can be described by the following Arrhenius-Frenkel type equations:

$\text{τ}{}^0{}_{\mathrm{F}}\text{=}{}^1\text{A}{}^0{}_0\text{exp}\text{[}\text{? (E}{}^0{}_0\text{+ ΔμF)}\text{k}{}^1\text{T}\text{]}\text{exp}\text{[(}\text{E}{}^0{}_0\text{+ ΔμF)}\text{kT}\text{]}$

for diffusion and

$\text{τ}{}^{\mathrm{+}}{}_{\mathrm{F}}\text{=}{}^1\text{A}{}^{\mathrm{+}}{}_0\text{exp}\text{[}\text{? (E}{}^{\mathrm{+}}{}_0\text{? n}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{e}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{F}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}\text{k}{}^1\text{T}\text{]}\text{exp}\text{[(}\text{E}{}^{\mathrm{+}}{}_0\text{? n}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{e}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{F}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}\text{kT}\text{]}$

for ion desorption.     

Monte Carlo simulation of the kinetics of decomposition and the formation of precipitates at grain boundaries of the general type in dilute BCC Fe–Cu alloys

The kinetics of decomposition of a polycrystalline Fe–Cu alloy and the formation of precipitates at the grain boundaries of the material have been investigated theoretically using the atomistic simulation on different time scales by (i) the Monte Carlo method implementing the diffusion redistribution of Cu atoms and (ii) the molecular dynamics method providing the atomic relaxation of the crystal lattice. It has been shown that, for a small grain size (D ~ 10 nm), the decomposition in the bulk of the grain is suppressed, whereas the copper-enriched precipitates coherently bound to the matrix are predominantly formed at the grain boundaries of the material. The size and composition of the precipitates depend significantly on the type of grain boundaries: small precipitates (1.2–1.4 nm) have the average composition of Fe–40 at % Cu and arise in the vicinity of low-angle grain boundaries, while larger precipitates that have sizes of up to 4 nm and the average composition of Fe–60 at % Cu are formed near grain boundaries of the general type and triple junctions.     

Raman study of the 90° grain boundaries in YBa2Cu3O7−δ thin films

YBa₂Cu₃O_7−δ (YBCO) films containing two different types of 90° grain boundaries were fabricated on the same substrates. Raman spectra from several parts of the basal-plane-faced tilt (TL) grain boundaries were measured and compared with those from the twist (TW) grain boundaries. The Raman results show that I(500)/I(340), the relative intensity of the A_1g mode near 500 cm⁻¹ with respect to that of the B_1g mode near 340 cm⁻¹, is suppressed in the TL boundaries, relative to the TW boundaries. The results can be interpreted to mean that the stress is stronger in the TL boundaries than in the TW boundaries. This may offer an alternative explanation for the weak-link behavior of the step-edge Josephson junctions.     

Molecular dynamic studies of the interaction of a/6⟨112⟩ Shockley dislocations with stacking fault tetrahedra in copper. Part II: Intersection of stacking fault tetrahedra by moving twin boundaries

The interactions of moving twin boundaries with stacking fault tetrahedra (SFTs) have been studied by molecular dynamics. The results reveal a spectrum of processes occurring during these interactions. In general, they lead to damage of the parent SFT and formation of new defects in the twin lattice. The character of these defects depends on the nature of the twinning front, the size of the SFT and its orientation with respect to incoming dislocations. Typical structures that may be produced in the twin include product-SFTs, free vacancies, planar stacking faults bounded by partial dislocations, mutually linked stacking faults on non-coplanar {111} _T planes, small {111} _T tetrahedra and their partial forms. Dislocation mechanisms involved in the formation of these defects are being analyzed.     

Exotic interactions between solitons of the （2＋1）-dimensional asymmetric Nizhnik-Novikov-Veselov system

Starting from the extended tanh-function method (ETM) based on the mapping method, the variable separation solutions of the (2+1)-dimensional asymmetric Nizhnik--Novikov--Veselov (ANNV) system are derived. By further study, we find that these variable separation solutions are seemingly independent of but actually dependent on each other. Based on the variable separation solution and by choosing appropriate functions, some novel and interesting interactions between special solitons, such as bell-like compacton, peakon-like compacton and compacton-like semi-foldon, are investigated.