Impurity-dislocation Interaction Research Articles

Constitutive modeling of dynamic strain aging in commercially pure bcc metals

This paper explores the challenge of accurately modeling Dynamic Strain Aging (DSA) in metals using numerical models. The proposed modification to the Voyiadjis–Abed (VA) model addresses the limitations of current models by introducing an additional microstructures-based parameter to capture the effect of DSA. The modified VA model considers the impurity-dislocation interactions based on the concentration and diffusion kinetics of impurity atoms. Results show successful DSA capture in commercially pure Niobium and Vanadium using physical and microstructural parameters. This study provides a promising approach for accurately modeling DSA activation in bcc metals, enabling future research.

Dislocation mechanisms of low-temperature plasticity of nanocrystalline titanium: The role of impurity and grain boundary strengthening

The temperature dependences of the yield strength, the strain rate sensitivity of the deforming stress and the activation volume of the process of plastic deformation of nanocrystalline titanium VT1-0 under quasi-static tension in the temperature range 4.2–395 K are found. The nanoscale grain size was obtained using the method of cryomechanical grain fragmentation. A detailed thermal activation analysis of the experimental results was carried out and it was shown that the process of plastic deformation under conditions of ambient and low temperatures is determined by the overcoming of local impurity barriers by dislocations, and grain boundaries are a source of internal stresses. Empirical estimates are obtained for the parameters of the dislocation-impurity interaction. The unambiguous relationship between internal stresses and grain size established for monomodal nanocrystalline titanium made it possible to separate the effects of impurity and grain boundary hardening. The result obtained is an indirect basis for the conclusion about the impossibility of accumulation of dislocations in nanograins during plastic deformation. The decrease in the activation volume for a nanoscale grain at a constant impurity concentration is considered because of the manifestation of the dependence of the diameter of the dislocation loop generated by a grain boundary source on the grain size (confinement effect).

Kinetics of low-temperature plasticity of nanocrystalline titanium

The temperature dependences of the offset yield strength, strain-rate sensitivity of deformation stress, and the activation volume of plastic deformation in VT1-0 nanocrystalline titanium, have been determined under quasistatic tension in the temperature range 4.2–395 K. Thermal activation analysis of experimental data has been performed. The low-temperature plasticity of nanocrystalline titanium is shown to be controlled by the thermally activated passage of dislocations through local barriers formed by interstitial impurity atoms. The empirical values of the theoretical parameters of the dislocation–impurity interaction have been determined.

The gradual disappearance of yield plateau in Zr–Sn–Nb–Fe–Mo alloy by the trace addition of Cr and V

Trace chromium (Cr, 0.1 wt. %) and vanadium (V, 0.05 wt. %) were doped to the Zr–Sn–Nb–Fe–Mo alloy (denoted as Zir) to investigate the tensile properties at room temperature. A clear yield plateau was observed in Zir alloy, while the yield plateau was gradually disappeared by the addition of Cr to Zir alloy (denoted as Zir-0.1Cr) and by the combined addition of Cr and V to Zir alloy (denoted as Zir-0.1Cr-0.05 V). Microscopic characterization results revealed that doping of Cr and V retarded the re-crystallization process in Zir alloy. Firstly, the mean grain size of matrix was reduced from 1.9 μm to 1.7 μm and 1.5 μm by addition of 0.1% Cr and 0.1% Cr + 0.05% V to Zir alloy, respectively. The fraction of low angle grain boundaries (LAGBs) increased from 50.2% to 53.9% and 60.8% in Zir-0.1Cr and Zir-0.1Cr-0.05 V, respectively. Secondly, the volume fraction of second phase particles (SPPs) increased from 7.2% to 11.1% and 11.8% in Cr-containing and Cr + V-containing alloys, respectively. The mechanism of yield plateau in our alloy was explained by the impurity-dislocation interaction and dislocation multiplication mechanism, and the gradual disappearance of yield plateau behavior at the early stage of yielding could be attributed to the gradual increment of mobile dislocation density in Zir-0.1Cr and Zir-0.1Cr-0.05 V alloys. The tensile tests with different pre-strains proved that the gradual disappearance of yield plateau was caused by the increase of mobile dislocations with increasing pre-strain.

Thermally activated dislocation plasticity in body-centered cubic chromium studied by high-temperature nanoindentation

The deformation behavior of Chromium (Cr) was investigated with the goal to understand and quantify the thermally-activated dislocation plasticity and the transition to the temperature-/rate-independent regime in body-centered cubic (bcc) metals. High-temperature nanoindentation experiments were utilized to characterize the deformation behavior of bcc Cr from room temperature to 673 K. To validate the indentation method itself at elevated temperatures, we systematically studied the temperature-dependent indentation elastic modulus, which clearly shows a discontinuity at the magnetic phase transition at 308 K, which is quantitatively consistent with literature data. We characterized the kinetics of dislocation plasticity by analyzing the strain-rate sensitive behavior of the hardness at different temperatures. The observed signatures of the plastic relaxation mechanisms are discussed in the context of screw dislocation mobility governed by thermally-activated kink-pair nucleation, kink-drift or dislocation-impurity interaction.

Direct Observation of Impurity Segregation at Dislocation Cores in an Ionic Crystal.

Dislocations, one-dimensional lattice defects, are known to strongly interact with impurity atoms in a crystal. This interaction is generally explained on the basis of the long-range strain field of the dislocation. In ionic crystals, the impurity-dislocation interactions must be influenced by the electrostatic effect in addition to the strain effect. However, such interactions have not been verified yet. Here, we show a direct evidence of the electrostatic impurity-dislocation interaction in α-Al2O3 by visualizing the dopant atom distributions at dislocation cores using atomic-resolution scanning transmission electron microscopy (STEM). It was found that the dopant segregation behaviors strongly depend on the kind of elements, and their valence states are considered to be a critical factor. The observed segregation behaviors cannot be explained by the elastic interactions only, but can be successfully understood if the electrostatic interactions are taken into account. The present findings will lead to the precise and quantitative understanding of impurity induced dislocation properties in many materials and devices.

Contributions of phase and structural transformations in multicomponent Al-Mg alloys to the linear and nonlinear mechanisms of anelasticity

The effects of the processes of severe plastic deformation (SPD), recrystallization, and precipitation of the β phase in multicomponent alloys of the Al-5Mg-Mn-Cr and Al-(4–5%)Mg-Mn-Zn-Sc systems on the mechanisms of grain-boundary relaxation and dislocation-induced microplasticity have been studied in some detail. To stabilize the ultrafine-grained structure and prevent grain growth, dispersed Al-transition-metal particles, such as Al3Zr, Al6Mn, Al7Cr, Al6(Mn,Cr), Al18Cr2Mg3 have been used. We have special interest in alloys with additions of scandium, which forms compounds of the Al3Sc type and favors the precipitation of finer particles compared to the aluminides of other transition metals. After SPD, Al-(4–5%)Mg-Mn-Zr-Sc alloys exhibit an enhanced recrystallization temperature. The general features of the dislocation and grain-boundary anelasticity that have been established for the binary Al-Mg alloys are retained; i.e., (1) the decrease in the dislocation density in the process of recrystallization of cold-worked alloys leads to the formation of a pseudo-peak in the curves of the temperature dependences of internal friction (TDIF) and to a decrease in the critical amplitude of deformation corresponding to the onset of dislocation motion in a stress field; (2) the precipitation of the β phase suppresses the grain-boundary relaxation; (3) the dissolution of the β phase, the passage of the magnesium atoms into the solid solution, and the precipitation of the β’ phase upon heating hinder the motion of dislocations; (4) the coarsening of the highly dispersed particles containing Zr and Sc increases the dislocation mobility. The grain-boundary relaxation and dislocation-impurity interaction and their temperature dependences, as well as processes of the additional alloying of the binary alloys by Mn, Cr, Zr, and Sc, have been estimated quantitatively.

Contribution of phase and structural transformations to linear and nonlinear mechanisms of anelasticity in binary Al-Mg alloys

Effects of the processes of the recrystallization and precipitation of the β phase in Al-(0.3–12%)% Mg alloys on the mechanisms of grain-boundary relaxation and dislocation-induced microplasticity have been studied in some detail. The decrease in the dislocation density due to the process of recrystallization of cold-worked alloys leads to the formation of a pseudopeak in the curves of the temperature dependence of internal friction and to a decrease in the critical amplitude of deformation necessary to initiate dislocation motion in a stress field. The precipitation of the β phase in the structure suppresses the mechanism of grain-boundary relaxation; the dissolution of the β phase, which leads to the formation of impurity atmospheres, then to the passage of magnesium atoms into the solid solution, impedes dislocation motion. Depending on the total content of Mg in the alloy, the dislocation mobility upon the measurements of amplitude dependences of internal friction can be described in terms of either breakaway or friction models. The characteristics of grain-boundary relaxation and dislocation-impurity interaction and of their temperature dependences have been estimated quantitatively.

The Site Preference of Alloying Element Zr in NiAl Dislocation Core and its Effects on Bond Characters

NiAl is one kind of high-temperature alloys with broad potential applications in aerospace industry. Its mechanical properties are believed to be largely related to the dislocation behavior and impurity-dislocation interaction. In the paper we report first principles study of the alloying effect of Zr in the [10(010) edge dislocation core of NiAl. The binding energy of doping system decreases 3.77 eV when a Zr atom substituted for an Al, only decreases 1.06 eV with substitution for a Ni atom. The result of the binding energy shows that a Zr atom prefers to occupy an Al site in the dislocation core of NiAl. The analyses of the charge distribution, the interatomic energy and the partial density of states suggest that Zr will greatly enhance the interaction between Zr atom and neighboring host atoms, as well as that between host atoms. These results show that the alloying element Zr induced pinning effect on the edge dislocation motion is predicted, and could be helpful for understanding microscopic mechanisms of alloying-induce hardening in NiAl alloy.

Electronic origin of the anomalous solid solution hardening of Y and Gd in Mg: A first-principles study

Y and Gd demonstrate anomalous solid solution hardening efficiency, which cannot be understood using the elastic impurity-dislocation interaction theory. We performed first-principles calculations to investigate the effect of different alloying elements such as Al, Zn, Y, and Gd on the chemical bonding of Mg solid solutions. The present calculations clearly show that the anomalous solid solution hardening of Y and Gd in Mg may be understood based on the increased bonding strength of both Mg-Y (Gd) and Mg-Mg.

Static strain aging and dislocation–impurity interactions in irradiated mild steel

Interactions between dislocations and interstitial impurity atoms lead to strain aging phenomenon in ferritic steels that are affected by the defects produced during neutron radiation exposure. We present here results on static strain aging in a silicon-killed mild steel before and after neutron irradiation. It is noted that the degree of strain aging (as measured by the yield point following restraining) decreased with increasing neutron dose resulting in essentially non-aging type at the highest dose (∼10 19 n/cm 2). The strain aging kinetics were investigated using data at various aging temperatures and were found to be unaffected by the neutron radiation exposure. These experimental results are compared to those observed in dry hydrogen treated (partially denitrided) samples and are correlated with models on Cottrell locking.

Influence of high-magnetic-field on dislocation–oxygen interaction in silicon

Influence of high-magnetic-field treatments on dislocation vs. oxygen impurity interaction in Si was investigated. The locking strength of oxygen impurities against dislocations decreased with increase in exposed magnetic field up to 10 T at room temperature and 700 °C. Such phenomena could not be detected in high-purity float-zone-grown silicon crystals. These results suggest a spin-dependent solid-state reaction in impurity–dislocation interaction, which seems a possibility of modification of atomic configuration and displacement of crystalline defects in semiconductors.

Electronic states related to dislocations in silicon

AbstractDislocations and impurities in silicon, even though studied since many years, are now subject of a renewed interest. Moreover, many question related to dislocation‐ related electronic states remain still unsolved. The present contribution reviews several results, obtained by the authors, on dislocation impurity interactions and their effects on the electronic properties of defect states in silicon. Dislocations introduced by plastic deformation and oxygen precipitation in p‐type Czochralski (Cz) silicon have been investigated by junction spectroscopy methods. A deep hole trap, named T1, has been associated to dislocation‐related impurity centers, while additional deep traps have been related to contamination by grown‐in transition metals and to clusters involving oxygen atoms. Moreover, experimental results obtained by junction spectroscopy assessed the existence of dislocation related shallow states. These were found to be located at 70 and 60 meV from the valence and conduction band edge, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Possible nature of oscillations in the time dependences of the internal friction and effective shear modulus in Al-Cu alloys

The conditions of the appearance of in-phase oscillations in the time dependences of the low-frequency internal friction and effective shear modulus G eff in Al-0.01 wt% Cu and Al-0.001 wt % Cu alloys are studied. The in-phase oscillations are shown to appear in the alloys only when their dislocation-impurity structure is disturbed from equilibrium and the impurity concentration near a dislocation is optimum. This fact suggests that the in-phase oscillations are due to a collective character of dislocation-impurity interactions, which leads to a transfer of energy of translational dislocation motion to transverse dislocation vibrations in the main slip plane.

Effect of high-magnetic-field on dislocation-oxygen impurity interaction in Si

This paper reports effects of magnetic-treatments on an interaction between dislocations and oxygen impurity in Si. Though oxygen impurity in Si has a strong effect of immobilizing dislocations due to preferential segregation and formation of precipitates along dislocations at elevated temperatures, the capability of immobilization of the oxide precipitates against dislocations decreases under an application of magnetic field higher than 5 T and loses almost completely at 10T. These results suggest a spin-dependent solid-state reaction in oxygen impurity-dislocation interaction in Si.

On the Interaction of Dislocations with Impurities in Silicon

Dislocations and impurities in silicon have been widely investigated since many years, nevertheless many questions on this subject remain still unsolved. As an example, theory, models and experimental phenomena provide evidence of the existence of shallow bands in silicon induced by the dislocation strain field. Nevertheless, only deep bands, likely associated with contamination at dislocations, have been detected up to now by junction spectroscopy. The present contribution reviews several results, obtained by the authors, on dislocation impurity interactions and their effects on the electronic properties of defect states in silicon. Point and extended defects introduced in p-type Cz Si by oxygen precipitation and plastic deformation have been investigated with electrical methods. Different materials (oxygen precipitated and deformed Cz Si and Fz Si) were examined in order to separate the role of oxygen precipitation, plastic deformation and metallic contamination on non-radiative electronic transitions at defect centers. A deep hole trap, named T1, has been associated to dislocation-related impurity centers, while additional deep traps have been related to contamination by grown-in transition metals and to clusters involving oxygen atoms. Moreover, experimental results obtained by junction spectroscopy assessed the existence of dislocation related shallow states. These were found to be located at 70 and 60 meV from the valence and conduction band edge, respectively.

Dislocation–impurity interaction in Si

Dynamic dislocation–impurity interactions in CZ–Si doped with light impurity (N), acceptor (B), donor (P, As, Sb) and neutral (Ge) impurities were investigated in comparison with those in undoped CZ–Si. Dislocation generation was effectively suppressed in B-, P- and As-doped Si when the concentration was higher than 10 19 cm −3, while Ge impurity did not strongly suppress dislocation generation. Dislocations were immobilized by the stable complexes formed through the impurity segregation and reaction. It was found that B and N impurities promptly form strong locking agents, while P and As impurities form highly dense locking agents along dislocations. Dislocation velocity in Si doped with electrically active impurities increased with increasing concentrations of not only the donor (P, As, Sb) but also the acceptor (B) impurities in the temperature range of 650–950 °C. N and Ge impurities had no or little effect on the velocity of dislocations in motion. Co-doping of Si with Ge and B was effective for suppression of dislocation generation and retardation of dislocation velocity at low temperatures.

Interaction of doping impurities with the 30° partial dislocations in SiC: Anab initioinvestigation

In this work we investigated, by means of state-of-the-art first principle techniques, the effect of impurity segregation on the energetics, the structure, and the electronic properties of dislocation cores. We find that for the species we considered, Al, P, B, and N, i.e., the most common dopants in SiC, the impurity-dislocation interaction is attractive and that segregation energies are comparable or larger than dislocations reconstruction energies. As to the doping properties of the impurities they are deactivated by the interaction with the dislocation core environment, with the noticeable exception of P. We rationalized our results in terms of the effect of the dopants on the dislocation core band structure.

Impurity-kink interaction in the two-dimensional Frenkel-Kontorova model

The dislocation gliding in crystals with impurities is investigated by computer simulations for the two­ dimensional Frenkel-Kontorova model. It is shown that the impurity-dislocation interaction can result in both the stimulation of the double-kink formation (which increases the dislocation mobility) and the trapping of moving kinks by the impurities with the following kink-antikink recombination (which slows down the dislo­ cation motion). The relative importance of these competing mechanisms depend mostly on the temperature as well as the type of impurity. Here we demonstrate that the presence of impurities can increase the dislocation velocity at low temperatures which leads to the solid solution softening. With increasing temperature, a transition from the solid solution softening to the solid solution hardening regime takes place. The dislocation mobility in crystal lattice periodic poten­ tial (the Peierls relief1) is one o f the key problems in solid state physics w hich still attracts considerable interest. The Peierls m echanism determines the dislocation mobility and, consequently, the mechanical behavior in covalent crystals (Si, Ge), bcc metals, and some intermetallic compounds. D e ­ spite considerable theoretical and experimental efforts2,3 the m icroscopic processes w hich are responsible for the disloca­ tion mobility in the Peierls relief are still a matter o f inten­ sive discussions.4-9 It is com m only accepted that the dislocation m otion in materials w ith high Peierls relief is accom plished by the propagation o f kinks along the dislocation line. It has been assumed that the kink density (and, consequently, the dislo­ cation mobility) is determined by the processes o f thermal fluctuation kink nucleation and o f the kink-antikink pair re­ combination. However, this very transparent but simplified concept is insufficient to describe the mechanical properties o f real materials2,3 where the impurity-kink interactions play a crucial role in the kink dynamics. Standard m odels consid­ ering the impurities just as centers o f kink-antikink recombi­ nation processes2 seem to be inadequate and do not explain accumulated experimental data.3,8,10,11 The contradictions b e­ tween the theoretically predicted equilibrium density o f the kinks and that estimated from experimental data initiates a hypothesis regarding the existence o f so-called weak ob­ stacles controlling the equilibrium density o f kinks, how ­ ever, the nature o f these obstacles has not yet been clarified.3 The effect o f impurities on the mechanical properties o f the crystals is rather com plex and cannot be reduced only to the deceleration o f kinks or to the initiation o f their annihi­ lation as it w as supposed in the earlier m odels.2 In particular, besides the w ell-know n effect o f the solid solution hardening (SSH) w hich is clearly connected with the pinning action o f the impurities on the dislocations, in som e cases the alloying

Observation of dislocations in diffused 4H–SiC p-i-n diodes by electron-beam induced current

The electron-beam induced current (EBIC) method was employed to investigate the electrical activity of dislocations in silicon-carbide-diffused p-n diodes. It was observed that EBIC contrast depends on the type of defect (superscrew, screw, and edge dislocation). This dependence was attributed to spatial inhomogeneities in the electrical properties of the material around the dislocations due to different impurity-dislocation interactions during high-temperature (∼1900°C) diffusion. Chemical etching of the sample was used to define the nature of the defects observed by EBIC imaging. It was found that electrical breakdown of the diodes occurs at the location of superscrew dislocations.