Narrow Results

This work presents a computational approach for the determination of equilibrium strain and dislocation density in nitride-based heterostructures. Verification of this approach is demonstrated in the calculation of critical layer thickness for In_xGa_1-xN/GaN (0001) as compared to other work. The present modeling approach can also be applied to heterostructures of varying compositional profiles and structure. To demonstrate this, we studied the equilibrium strain and misfit dislocation density profiles for metamorphic In_xGa_1-xN/GaN (0001) heterostructures containing uniform layers on top of linearly-graded buffers. These structures were also compared to similar arsenide-based heterostructures involving In_xGa_1-xAs/GaAs (001) with the zinc blende crystal structure. The generalized energy minimization modeling tool presented here can be readily extended to arbitrarily graded nitride heterostructures with polar, semi-polar, or non-polar orientations.

Positron annihilation spectroscopy (PAS) is one of the nuclear techniques used in material science. The present measurements are used to study the behavior of defect concentration in one of the most important materials — aluminum alloy — which is a 7075 alloy. It has been shown that positrons can become trapped in imperfect locations in solids and their mean lifetime can be influenced by changes in the concentration of such defects. No changes have been observed in the mean lifetime values after the saturation of defect concentration. The mean lifetime and trapping rates were studied for samples deformed up to 58.3%. The concentration of defect range varies (from 10¹⁵ to 10¹⁸ cm^-3) at the thickness reduction, (from 2.3 to 58.3%). The range of the dislocation density varies (from 10⁸ to 10¹¹ cm/cm³).

An experimental investigation is undertaken to examine the possibility of producing ultra-fine grained bulk material through high-speed impact compression followed by annealing. A gas gun was employed to impose high-rate deformation on oxygen-free high-conductivity copper specimens to 90% strain. Samples were also quasi-statically compressed to identical final strains and similar heat treatment. Results show that after impact compression, grain boundaries widen and become less sharply defined, and many narrow twins are formed. For dynamic loading, grain boundary slip appears to accompany dislocation movement. Two dislocation characteristics were identified and the dislocation density was lower than that in samples compressed quasi-statically. Small dislocation loops were also observed. Portions of grains in specimens subjected to impact were mechanically broken into sizes less than 1 μm before annealing. The microhardness of impacted and statically compressed samples increased respectively by HV50 and HV60. After annealing at 190°C for 1 hour, ultra-fine grains with grain sizes ranging from 40∼200 nanometers were observed in impacted samples. This study highlights the potential of utilizing impact compression to produce bulk material with ultra-fine grains.

The technique of severe plastic deformation (SPD) enables one to produce metals and alloys with an ultrafine grain size of about 100 nm and less. As the mechanical properties of such ultrafine grained materials are governed by the plastic deformation during the SPD process, the understanding of the stress and strain development in a workpiece is very important for optimizing the SPD process design and for microstructural control. The objectives of this work is to present a constitutive model based on the dislocation density and dislocation cell evolution for large plastic strains as applied to equal channel angular pressing (ECAP). This paper briefly introduces the constitutive model and presents the results obtained with this model for ECAP by the finite element method.

Dislocation densities threading semiconductor crystals are a problem for device developers. Among the issues presented by the defect density is the appearance of the so-called shallow levels. In this work, we introduce a geometric model to explain the origin of the observed shallow levels. We show that a uniform distribution of screw dislocations acts as an effective uniform magnetic field which yields electronic bound states even in the presence of a repulsive Coulomb-like potential. This introduces energy levels within the band gap, increasing the carrier concentration in the region threaded by the dislocation density and adding additional recombination paths other than the near band-edge recombination. Our results suggest that one might use a magnetic field to destroy the dislocation density bound states and therefore minimize its effects on the charge carriers.

In this paper, AlGaN/GaN HEMTs with an AlN buffer layer were fabricated. Analyses on the crystal quality of the GaN epitaxial layer by Raman spectroscopy have been purposed. By introducing an AlN layer on sapphire substrate, the maximum drain current of the HEMT increased from 481 mA/mm to 522 mA/mm at $V_G=6$ V. Subthreshold slope was reduced from 638.3 mV/decade to 240.9 mV/decade and the electron mobility increased from 1109 cm² V${}^{-1}$s${}^{-1}$ to 1781 cm² V${}^{-1}$s${}^{-1}$. These results showed that using an AlN buffer layer can improve the crystal quality of the GaN epitaxial layer, thus optimize the device performances of the GaN-based HEMTs.

Dislocations in a material will, when present in enough numbers, change the speed of propagation of elastic waves. Consequently, two material samples, differing only in dislocation density, will have different elastic constants, a quantity that can be measured using Resonant Ultrasound Spectroscopy. Measurements of this effect on aluminum samples are reported. They compare well with the predictions of the theory.

Positron annihilation spectroscopy (PAS) is one of the nuclear techniques used in material science. The present measurements are used to study the behavior of defect concentration in one of the most important materials aluminum alloys which is the 7075 alloy. It has been shown that positrons can become trapped at imperfect locations in solids and their mean lifetime can be influenced by changes in the concentration of such defects. No changes have been observed in the mean lifetime values after the saturation of defect concentration. The mean lifetime and trapping rates are studied for samples deformed up to 58.3%. The concentration of defect range vary from 10¹⁵ to 10¹⁸cm^-3 at the thickness reduction from 2.3 to 58.3%. The dislocation density varies from 10⁸ to 10¹¹cm/cm³.

Positron annihilation lifetime spectroscopy (PALS) is a nuclear technique used in material science. Positron annihilation lifetime technique (PALT) measurements are used to study the behavior of defect concentration and dislocation density in a set of 2024 aluminum alloy. It has been shown that positrons can become trapped at imperfect locations in solids and their mean lifetime can be influenced by changes in the concentrations of such defects. No changes were observed in the mean lifetime after defect concentration became saturated. The mean lifetime and trapping rates for the samples deformed up to 36.4 percent. The concentration of defects range from 1.133 × 10¹⁶ to 2.061 × 10¹⁸ cm^-3 at strains from 1.7 to 22.7%.

To describe the viscoplastic behavior of materials under cyclic loading, a dislocation density-based constitutive model is developed based on the unified constitutive theory in which both the creep and plastic strain are integrated into an inelastic strain tensor. The stress evolution during cyclic deformation is caused by the mutual competition and interaction between hardening and recovery. To incorporate the physical mechanisms of cyclic deformation, the change of mobile dislocation density is associated with inelastic stain in the proposed model. The evolution of immobile dislocation density induced by strain hardening, dynamic recovery, static recovery and strain-induced recovery are simulated separately. The deterioration of yield strength following the hardening in tension (or compression) and subsequently in compression (or tension) is described by the Bauschinger effect and reduction of immobile dislocation density, the latter is induced by static- and strain-induced recovery. A kinematic hardening law based on dislocation density is proposed, both isotropic hardening and softening are described by determining the evolution of hardening parameters. The experimental data of P91 steel under different strain rates and temperatures are adopted to verify the proposed model. In general, the numerical predictions agree well with the experimental results. It is demonstrated that the developed model can accurately describe the hardening rate change, the yield strength deterioration and the softening under cyclic loading.

In recent years, Fe–Mn–Al–C steel has been widely regarded as a promising lightweight material due to its excellent specific strength and good balance of strength and toughness. However, the performance of welded joints is crucial for the application of Fe–Mn–Al–C low-density steel. This paper aims to study the effect of different solution treatment times on the mechanical properties and microstructure of tungsten inert gas (TIG) welding of Fe-29Mn-9Al-0.9C low-density steel. After solution treatment, the phase types of the weld change and the granular ferrite transforms into austenite. Electron backscattered diffraction (EBSD) results show that with the increase of solution treatment time, dislocation density decreases, recrystallized grains grow, and the texture changes from uniform distribution to cubic texture again. It is concluded that in the early stage of recrystallization, dislocation walls are formed due to dislocation slip, which impedes dislocation movement in the process of stretching and thus improves the tensile strength. By controlling the time of solution treatment, the maximum tensile strength can be increased to 845.05MPa and the maximum elongation can be increased to 38.21%, which are all higher than for the welded joints without solution treatment.

Dislocation density based multiple-slip constitutive formulations and specialized computational schemes are introduced to account for large-strain ductile deformation modes in polycrystalline aggregates. Furthermore, new kinematically based interfacial grain boundary regions and formulations are introduced to account for dislocation density transmission, absorption, and pile-ups that may occur due to grain boundary misorientations and properties.

The fine-grained as-rolled Mg-7.83Li alloy demonstrated the elongations to failure ranging from 500 to 600% at temperatures of 473-623 K at an initial strain rate of 1.67×10^-3 s^-1. The sample for temperature-strain-rate-changing tension exhibited a taper appearance and induced considerable dynamic grain growth. Observation by scanning electron microscope revealed that there are many dimples on the fracture surface and the fracture mode is intergranular fracture. The models of number of dislocations and dislocation density inside the grain and at the grain boundary were incorporated into the calculation of constitutive equations and four modified Ruano-Wadsworth-Sherby deformation mechanism maps (DMMs) were constructed. The new DMMs gave the distribution and the variation of dislocation parameters. It was shown that grain boundary sliding accommodated by slip controlled by lattice diffusion is the dominant superplastic mechanism of the present alloy at 573 K.

Here in this paper, a multiscale framework based on crystal plasticity is proposed coupling with thermal activation mechanism, as well as the continuum damage mechanism. The microscopic hardening phenomenon is revealed by a dislocation density evolution model, which is constructed according to the temperature. The physical-based exponential function of shear strain rate is posed to describe the thermal material behavior replacing the general phenomenological power-law equation. A 3D spatial distribution of stress, strain and damage is presented in the finite element method, parameters of which are previously determined by a RVE calculation and fitting test compared to the experimental data. Finally, some discussions of stress heterogeneity and texture evolution are proposed and conclusions are made.