Activation Energy For Dislocation Motion Research Articles

Термоактиваційний аналіз температурної залежності твердості квазікристалів системи Al—Cu—Fe

The results of thermoactivation analysis of the temperature dependence of hardness for icosahedral quasicrystals (QC) of the Al—Cu—Fe system obtained as a coating, massive compact and ingot are presented. QC as well as covalent crystals at room temperature are brittle without signs of macroplastic deformation at standard methods of mechanical testing and only the method of local indenter loading makes it possible to deform QC to significant degrees of deformation without fracture. In the studied temperature range 77—1073 K, the HV(T) hardness dependences have the same character, despite the state in which the QC was obtained. The HV(T) dependence consists from two sections: an athermal low-temperature section (77—600 K) and a section (>600 K) where the hardness decreases sharply with increasing temperature. The presence of a low-temperature athermal section on the HV(T) dependence is explained by the phase transition of the QC to a more plastic approximant phase. Phase transition of this type can be associated with a high density of phason defects, which are formed during the deformation of the QC that leads to violations of the atomic structure. Based on the experimental data of the temperature dependences of the Vickers hardness (HV) obtained by the authors and the literature data, the values of the activation energy of the dislocation motion U and the activation volume V of a number of icosahedral quasicrystals were calculated. It is shown that the value of U  0,97—1,83 eV, and V is (65—132)∙10-24cm3. Previously, the method of thermoactivation analysis of tempera¬ture dependence of a flow stress was applied to materials with different crystal structures (BCC, FCC metals, covalent crystals, refractory compounds, intermetallics, high entropy alloys). In comparison with crystalline materials, the values of thermal activation parameters of the deformation process for QC are close to refractory compounds (carbides, borides) which have a covalent component in the interatomic bond. Keywords: quasicrystals, activation energy of dislocation motion, activation volume, hardness, temperature.

EBSD study of the effect of electropulsing treatment on the microstructure evolution in a typical cold-deformed Ni-based superalloy

Evolution of grain structure and grain boundary character distribution in a cold-deformed Ni-based superalloy during the electropulsing treatment (EPT) was investigated by EBSD technique. Compared with the conventional heat treatments (CHT), the nucleation and growth of SRX grains were significantly accelerated by EPT, which was mainly attributed to the high mobility of dislocations and solute atoms, and the efficient energy transfer under the effect of EPT. Especially, with the introduction of electric current, the activation energy of dislocation motion could be reduced by generating additional energy near the dislocation line, which made the motion of dislocation easy to start. The dislocations would also be easy to move in the form of slipping and climbing, under the effect of electron wind force. On the other hand, most of Σ3 boundaries were newly formed through growth accidents in the form of coherent Σ3 boundaries during EPT, and the mutual interactions between different Σ3n (n = 1, 2, 3) boundaries were rather limited. In addition, EPT could indirectly promote the formation of Σ3 boundaries by accelerating the SRX process, but the formation of Σ9 and Σ27 boundaries was inhibited at the temperatures higher than 700 °C. Moreover, the connectivity of random boundaries in the cold-deformed alloy treated by EPT was also discussed.

Linking Nanoscales and Dislocation Shielding to the Ductile–Brittle Transition of Silicon

The ductile–brittle transition of nano/microscale silicon is explored at low-temperature, high stress conditions. A pathway to eventual mechanism maps describing this ductile–brittle transition behavior using sample size, strain rate, and temperature is outlined. First, a discussion of variables controlling the BDT in silicon is given and discussed in the context of development of eventual modeling that could simultaneously incorporate all their effects. For description of energy dissipation by dislocation nucleation from a crack tip, three critical input parameters are identified: the effective stress, activation volume, and activation energy for dislocation motion. These are discussed individually relating to the controlling variables for the BDT. Lastly, possibilities for measuring these parameters experimentally are also described.

Evaluation of Dislocation Mobility in Wurtzite Semiconductors

ABSTRACTThe indentation hardness and yield strength of various wurtzite-structured semiconductors, such as AlN, GaN, InN, and ZnO, were summarized together with those of 6H-SiC. From analysis of the data, the activation energy for motion of an individual dislocation was deduced to be 2–2.7 and 0.7–1.2 eV in GaN and ZnO, respectively, and the evaluated activation energy for dislocation motion showed a dependence on the dislocation energy in the minimum length. The results were evaluated in terms of homology and the basic mechanism of the dislocation process. Dislocation motion is thought to be primarily controlled by the atomic bonding character of the semiconductors.

OS0421 マグネシウム系における転位移動障壁に対する温度効果の原子レベル解析

In this study, the activation energy of dislocation motion in Mg at finite temperature is evaluated using the combination of nudged elastic band method and constraint molecular dynamics method. In tha case of edge dislocation, while the energy barrier at 0 K for prismatic slip is found to be twice larger than that for basal slip, their energy level is significantly small. On the other hand, energy barrier for screw dislocation glide on prismatic plane is quite large, and the obtained result suggests that the process of recombination and redissociation of partial dislocations is a key factor to understand the temperate dependence of activation energy for dislocation motion.

Dislocation motion in Sb‐doped SiGe on Si substrate

AbstractWe have successfully grown antimony‐doped Si1−xGex epi‐films with different doping level and a boron‐doped one on Si (001) substrate. Using these samples, we have investigated the effect of these impurities in Si1−xGex epifilm on dislocation velocity. Although dislocation velocities in the B‐doped SiGe film were comparable to those in the undoped one because of the low concentration of B, those in Sb‐doped samples are remarkably enhanced, which is qualitatively similar to the effects in bulk Si. The reduction of the activation energy of dislocation motion depended on the doping level of Sb and the amount of the reduction was 1.0 eV in the highest Sb‐concentration film. This value is significantly larger than those reported in bulk Si. The velocities in the lowest Sb‐concentration samples are also enhanced though the samples are thought to be electrically intrinsic at the temperatures where dislocation velocities were measured. Therefore, the large activation energy reduction is possibly attributed to multiple effects of the Fermi level change and other characteristic effects of the SiGe epifilm.

Dislocation Analysis in (112)B HgCdTe/CdTe/Si

High-quality (112)B HgCdTe/Si epitaxial films with a dislocation density of ∼9 × 105 cm−2 as determined by etch pit density (EPD) measurements have been obtained by thermal cyclic annealing (TCA). The reduction of the dislocation density by TCA has led to a simple rate-equation-based model to explain the relationship between dislocation density and TCA parameters (time, temperature, and number of anneals). In this model, dislocation density reduction is based on dislocation coalescence and annihilation, assumed to be caused by dislocation motion under thermal and misfit stress. An activation energy for dislocation motion in n-type (112)B HgCdTe/Si of 0.93 ± 0.1 eV was determined. This model with no adjustable parameters was used to predict recent TCA annealing results.

Dislocation dynamic modelling of the brittle–ductile transition in tungsten

Brittle–ductile transitions in metals, ceramics and semiconductors are closely connected with dislocation activity emanating near to crack-tips. We have simulated the evolution of crack-tip plasticity using a two-dimensional dislocation dynamics model which has been developed to include two symmetric slip planes intersecting the crack-tip, and applied to single-crystal tungsten. The dislocation mobility law used was physically based on double-kink nucleation on screw dislocations, with an activation energy reduced by the local stress. Even in the strong stress gradients near a crack-tip, the dislocations are found to self-organise so that the internal stress in the array is effectively constant with time and position over a wide range of strain rates and temperatures. The resultant net activation energy for dislocation motion is found to be constant and close to the activation energy experimentally measured for the brittle–ductile transition. Use of a fracture criterion based on the local crack-tip stress intensity factor, as modified by the stresses from the emitted dislocations, allows explicit prediction of the form and temperature of the brittle–ductile transition. Predictions are found to be in very close agreement with experiment.

High-temperature strength and dislocation mobility in the wide band-gap ZnO: Comparison with various semiconductors

The mechanical strength of bulk single crystal wurtzite ZnO was investigated at elevated temperatures by means of compressive deformation. The yield stress of ZnO in the temperature range of 650–850°C was found to be around 10–20MPa, i.e., extremely lower than that of GaN, a typical wide band-gap semiconductor. On the basis of the observed temperature dependence of yield stress, the activation energy for dislocation motion at elevated temperatures in ZnO is deduced to be 0.7–1.2eV, which follows the relation of activation energy for dislocation motion versus band-gap energy known in a variety of semiconductors.

Dynamics and characters of dislocations in ZnSe

The mechanical strength of single-crystal ZnSe is investigated at elevated temperatures by means of compressive deformation. The yield stress of ZnSe in the temperature range 150-300°C is around 7-13 MPa, easily deformed even at low temperature. From the temperature dependence of the yield stress an activation energy for dislocation motion in the ZnSe is deduced to be 0.5-0.7 eV. The stacking fault energy is estimated to be 7-9 mJ/m 2 from the dissociation width of dislocations into partials.

Yield strength and dislocation mobility in plastically deformed ZnSe

The strength of single- and polycrystalline ZnSe was investigated at elevated temperatures by means of compressive deformation. The yield stress of ZnSe was rather low around 13 MPa, easily deformed even at low temperatures 150 °C. The activation energy for dislocation motion in ZnSe was estimated to be 0.5–0.7 eV. The intrinsic stacking fault energy was measured 7–9 mJ/m 2 from the dissociation width ∼20 nm of dislocations into partials by weak-beam transmission electron microscopy. The recombination-enhanced dislocation motion was detected through the change of dislocation morphologies induced by the electron irradiation during weak-beam observation.

Hardness, Yield Strength, and Dislocation Velocity in Elemental and Compound Semiconductors

Current knowledge on macroscopic plasticity indications, i.e., hardness and yield strength, and on microscopic indication, i.e., velocity of individual dislocations, in elemental and IV-IV, III-V, and II-VI compound semiconductors including GaN and ZnSe are reported and discussed on their mutual correlations. The Vickers hardness of the semiconductors can provide conventional information on the material plasticity in a wide temperature range up to their inciting points over a wide range of size scales in various material forms. Hardness H y in diamond- and sphalerite-type semiconductors has a universal relationship on their temperature dependence similar to the yield strength t y with a relation H v = (70-100) τ y in the low temperature region. Yield strength obtained by normal tensile or compression tests are expressed by an experimental equation as a function of the strain rate and temperature. The velocities of various types of dislocations measured directly in several semiconductors are described with an empirical equation as a function of the stress and the temperature. Through the analysis of yield strength data in terms of the collective motif in in of dislocations during the plastic deformation, the dislocation motion. rate-controlling plastic deformation, are deduced. The activation energy for dislocation motion has a linear relation to the band gap energy, depending on the types of semiconductors, elemental. III-V compounds, and II-VI compounds.

High-temperature strength of III–V nitride crystals

The hardness of AlN and GaN bulk crystals was investigated in the temperaturerange 20–1400°C by the Vickers indentation technique. The hardnesses of GaNand AlN at room temperature are 10.8 and 17.7 GPa, respectively. Withincreasing temperature the hardness shows a gradual decrease and then a steepdecrease from around 1000°C, which indicates the beginning of macroscopicdislocation motion and plastic deformation. The yield strength of GaN bulkcrystals determined directly by means of compressive deformation is around100–200 MPa in the temperature range 900–1000°C, i.e. similar to that of α-SiCand much higher than those of Si, Ge, GaP, GaAs, InP and InAs. On the basisof the observed temperature dependence of yield stress, the activationenergy for dislocation motion at elevated temperatures in GaN is deduced.

Dislocation motion in a random solid solution

Abstract The thermally activated motion of a dislocation through an array of randomly distributed solute atoms is studied. The influence of fluctuations in the solute density on the activation energy for dislocation motion is investigated, and the stress dependences of the effective activation energy and the activation volume are determined. It is shown that, in creep deformation, fluctuations in the activation energies may lead to a creep strain which increases with time according to a power law, with an exponent that increases linearly with increasing temperature. Implications of the calculated activation energy spectrum for low-frequency internal friction are discussed.

Dislocation motion in a random solid solution

Abstract The thermally activated motion of a dislocation through an array of randomly distributed solute atoms is studied. The influence of fluctuations in the solute density on the activation energy for dislocation motion is investigated, and the stress dependences of the effective activation energy and the activation volume are determined. It is shown that, in creep deformation, fluctuations in the activation energies may lead to a creep strain which increases with time according to a power law, with an exponent that increases linearly with increasing temperature. Implications of the calculated activation energy spectrum for low-frequency internal friction are discussed.

Study of relaxation of residual internal stress in Mg composites by internal friction

When a metal matrix composite is subjected to temperature changes, thermal stresses arise at the interfaces between the matrix and the reinforcement as a result of the considerable mismatch between the thermal expansion coefficients of the matrix and the reinforcement. Even moderate temperature changes can produce thermal stresses that exceed the matrix yield stress. Consequently, new dislocations are generated at the interfaces causing thermal fatigue (microstructural changes, matrix plastic deformation and irreversible shape changes). The microstructural changes in the matrix can be detected by damping and stress relaxation measurements. The logarithmic decrement and modulus defect were measured after thermal treatment at temperatures between room temperature and temperatures up to 400°C. The strain dependence of the logarithmic decrement can be divided into two regions. The values of the logarithmic decrement are also influenced by heat treatment and foreign atoms in the matrix. The results can be explained assuming that straining and thermomechanical treatment produce changes in the microstructure of the composites. The stress relaxation curves were analysed and the modulus defect, relaxation strength and activation energy for dislocation motion were estimated. It is very difficult to draw conclusions on the mechanisms responsible for the stress relaxation but part of the relaxation strength is due to reversible dislocation movement.

Yield strength and dislocation mobility in plastically deformed bulk single-crystal GaN

The mechanical strength of bulk single-crystal wurtzite-GaN grown by the hydride vapor phase epitaxy technique is investigated at elevated temperatures by means of compressive deformation. The yield stress of GaN in the temperature range 900–1000 °C is around 100–200 MPa, i.e., similar to that of 6H-SiC and much higher than those of Si and GaAs. From the temperature dependence of the yield stress an activation energy for dislocation motion in the GaN is estimated to be 2–2.7 eV.

Strain-rate dependent mechanism of cooperative dislocation generation: application to the brittle–ductile transition

A strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature is described. The massive dislocation activity, which commences near the crack tip at the brittle-to-ductile transition temperature is modeled in terms of this mechanism. The strain-rate dependence of the critical temperature arises from the glide of both pre-existing dislocations and dislocations which are ‘thermally nucleated’ below the critical temperature by the cooperative process. Depending on their relative contributions, the apparent activation energy associated with the brittle-to-ductile transition temperature is either equal to or larger than the activation energy for dislocation motion. We compare the predictions of the model with observations in TiAl.

Atomistic Simulation of kinks for 1/2a<111> Screw Dislocation in Ta

We study the structure and formation energy of kinks in 1/2a<111> screw dislocation in metallic Ta Embedded Atom Model Force Field parameterized using quantum mechanical computations. We studied a/3<112> kinks using a simulation cell containing four dislocations in a quadrupole arrangement. We impose periodic boundary conditions in the directions perpendicular to [111] and fixed boundaries in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a<111> dislocation lead to 8 distinguishable single kinks and 16 kink pairs. The different mismatches of core configurations along [111] direction cause variations in kink formation energy. The lowest formation energy of a kink pair is determined to be 0.73 eV. The geometric features of such kink pair have been studied with the help of structural analysis of the atomistic model. We also compare the activation energy for dislocation motion via the double kink mechanism with the activation energy for a rigid dislocation motion from a dipole annihilation simulation. We find that the migration energy for dislocation motion via double kink formation is 0.016 eV/b, which is less than the quarter of the migration energy associated with the kink free motion of a straight dislocation, 0.073 eV/b.

Effect of ultrasonic treatment on the mobility of short dislocations in Si crystals

The effect of ultrasonic treatment on the mobility of short surface dislocations in Si crystals is investigated. It is found that ultrasonic treatment of Si crystals changes the velocity of dislocations under a permanent mechanical load. The nature of variation of dislocation velocity is determined by the sign of external stresses acting on the sample: compressive forces decrease while tensile forces increase the velocity of dislocations. After ultrasonic treatment of the samples, a decrease in the activation energy for dislocation motion and the enhancement of the electroplastic effect are observed. A possible mechanism of the observed effects is considered.