Diffusion-Accommodated Grain Boundary Sliding and Dislocation Glide in the Creep of Sintered Alpha Silicon Carbide

Constant compressive stress creep experiments in the temperature and stress ranges of 1770 to 2020 K and 35 to 373 MPa have been performed on polycrystalline alpha SiC. Specimens having average grain sizes of 3.5, 4.9, and 7.5 μm have been studied in order to determine the grain size dependence of steady-state creep in this material and to distinguish the relative contributions of diffusion-accommodated grain boundary sliding and dislocation glide. Analysis of steady-state creep rate data revealed different kinetics of creep below and above 1920 K as follows: activation energies of 387–541 and 838–877 kJ/mol, stress exponents of 1.0–1.4 and 1.4–2.5, and inverse grain size exponents of 2.5 and 4. The m value of 2.5 indicated Coble-controlled creep with a contribution of Nabarro–Herring creep in the low-temperature range. The m value was less reliable for the high-temperature range because of the large differences in the microstructures of the three materials. The large number of high aspect ratio grains with increasing average grain size resulted in different contributions of diffusion-accommodated grain boundary sliding and dislocation glide between thsee materials.     

Determination of Pipe Diffusion Coefficients in Undoped and Magnesia-Doped Sapphire (α-Al2O3): A Study Based on Annihilation of Dislocation Dipoles

The breakup of dislocation dipoles in plastically deformed samples of undoped and 30-ppm-MgO-doped sapphire (α-Al₂O₃) was monitored using conventional TEM techniques. Dislocation dipoles break up into prismatic dislocation loops in a sequential process during annealing; i.e., dislocation loops are pinched off at the end of a dislocation dipole. This pinch-off process is primarily controlled by pipe diffusion, and pipe diffusion coefficients at temperatures between 1300° and 1500°C were estimated by monitoring the kinetics of the dipole breakup process. We determined D _P^U= 8.1(_–4.3^+9.1) × 10^–3 exp [–(4.5 ± 1.3 eV )/ kT )] m²/s for the undoped material. The pipe diffusion kinetics for the MgO-doped crystal was determined at 1250° and 1300°C and was about 6 times higher than for undoped sapphire. Finally, climb dissociation of the dislocations constituting the perfect dipoles in sapphire is common; annihilation of one set of partials can result in the formation of faulted dipoles, which can pinch off to form faulted dislocation loops. D _P^U for faulted dipoles in the undoped material was determined at 1300° and 1350°C, and was about 4–10 times higher than for perfect dipoles.     

The evolution of machining-induced surface of single-crystal FCC copper via nanoindentation

The physical properties of the machining-induced new surface depend on the performance of the initial defect surface and deformed layer in the subsurface of the bulk material. In this paper, three-dimensional molecular dynamics simulations of nanoindentation are preformed on the single-point diamond turning surface of single-crystal copper comparing with that of pristine single-crystal face-centered cubic copper. The simulation results indicate that the nucleation of dislocations in the nanoindentation test on the machining-induced surface and pristine single-crystal copper is different. The dislocation embryos are gradually developed from the sites of homogeneous random nucleation around the indenter in the pristine single-crystal specimen, while the dislocation embryos derived from the vacancy-related defects are distributed in the damage layer of the subsurface beneath the machining-induced surface. The results show that the hardness of the machining-induced surface is softer than that of pristine single-crystal copper. Then, the nanocutting simulations are performed along different crystal orientations on the same crystal surface. It is shown that the crystal orientation directly influences the dislocation formation and distribution of the machining-induced surface. The crystal orientation of nanocutting is further verified to affect both residual defect generations and their propagation directions which are important in assessing the change of mechanical properties, such as hardness and Young''s modulus, after nanocutting process.     

Modeling of complex interfaces: Gadolinium‐doped ceria in contact with yttria‐stabilized zirconia

Gadolinium‐doped ceria (GDC) and yttria‐stabilized zirconia (YSZ) are well‐known electrolyte materials in solid oxide fuel cells (SOFCs). Although they can be used independently, it is common to find them in combination in SOFCs, where they are used as protective layers against the formation of secondary phases or electron conduction blockers. Despite their different optimum operating temperatures, it appears that oxygen conduction is not affected by their interface. However, the intrinsic mechanisms of oxygen diffusion at these interfaces still remain unclear. One of the main difficulties when modeling the contact between different materials, or indeed different particles of the same material, is caused by the structural complexity of these systems. If we wish to evaluate the properties of the materials, we first need to obtain a model that includes the main features of the GDC/YSZ interface, such as large‐scale defects or cation interdiffusion in the contiguous phase. Since the generation of such a mixed system is complicated, we show here how the “amorphization and recrystallization” strategy can help us to obtain realistic systems. In this, the first of our papers on the structure and properties of layered GDC/YSZ materials, we discuss the structural features of the grain boundary between GDC and YSZ obtained by molecular dynamics simulations.     

Can misfit dislocations be located above the interface of InAs/GaAs (001) epitaxial quantum dots?

ABSTRACT: InAs/GaAs(001) quantum dots grown by droplet epitaxy were investigated using electron microscopy. Misfit dislocations in relaxed InAs/GaAs(001) islands were found to be located approximately 2 nm above the crystalline sample surface, which provides an impression that the misfit dislocations did not form at the island/substrate interface. However, detailed microscopy data analysis indicates that the observation is in fact an artefact caused by the surface oxidation of the material that resulted in substrate surface moving down about 2 nm. As such, caution is needed in explaining the observed interfacial structure.     

低温等径角挤压制备1050铝合金在磁退火后的均匀延伸率和屈服下降现象

采用低温等径角挤压（cryoECAP）制备了超细晶（UFG）1050铝合金。采用拉伸试验、透射电子显微镜和电子背散射衍射等方法,研究了UFG 1050铝合金在90~210 ℃、无磁场和12 T强磁场下退火4 h后的拉伸行为和显微组织。1050铝合金经cryoECAP退火后,晶粒尺寸为0.70~1.28 μm,极限抗拉伸强度与屈服强度之比小于1.24,均匀延伸率小于2.3%。随着退火温度从90 ℃上升到210 ℃,屈服下降现象变得明显,这是因为在拉伸变形过程中,为了维持所施加的应变速率,可动位错有所减少。均匀延伸率从1.55%下降到0.55%,位错密度从5.6×10¹⁴ m^-2下降到4.2×10¹³ m^-2,大角度晶界含量从63.8%增加到70.8%,使得位错湮灭速率提升,从而导致了应变硬化能力的降低。在90~210 ℃的强磁场退火条件下,低含量的大角度晶界（61.7%~66.2%）可以提供一个较慢的位错湮灭速率,从而导致较高的均匀延伸率（0.64%~1.60%）和更慢的屈服点后的流变应力下降。     

A kinetics formulation for low-temperature plasticity

The mechanism of low-temperature plastic deformation is controlled by thermally activated dislocation movements. An evolutionary constitutive law based on the principles of deformation kinetics is described in this article. The constitutive law is expressed with a sinh function designed for computational efficiency. It is derived from rigorously defined kinetics principles. The approximation involved in the sinh function is defined so that in applications an exact evaluation can be made of the validity limits. The system of the constitutive law and the external constraints lead to the operational equations. Applications are developed for constant strain-rate loading, constant stress-rate loading, stress relaxation, creep, and ratchetting processes. The analysis provides a unified treatment for low-temperature plastic deformation.     

Effect of Cu-Rich Phase Precipitation on the Microstructure and Mechanical Properties of CoCrNiCux Medium-Entropy Alloys Prepared via Laser Directed Energy Deposition

CoCrNiCux (x=0.16,0.33,0.75,and 1) without macro-segregation medium-entropy alloys (MEAs) was prepared using laser directed energy deposition (LDED).The microstructure and mechanical properties of CoCrNiCux alloys with increas-ing Cu content were investigated.The results indicate that a single matrix phase changes into a dual-phase structure and the tensile fracture behaviors convert from brittle to plastic pattern with increasing Cu content in CoCrNiCux alloys.In addi-tion,the tensile strength of CoCrNiCux alloys increased from 148 to 820 MPa,and the ductility increased from 1 to 11％with increasing Cu content.The nano-precipitated particles had a mean size of approximately 20 nm in the Cu-rich phase area,and a large number of neatly arranged misfit dislocations were observed at the interface between the two phases due to Cu-rich phase precipitation in the CoCrNiCu alloy.These misfit dislocations hinder the movement of dislocations during tensile deformation,as observed through transmission electron microscopy.This allows the CoCrNiCu alloy to reach the largest tensile strength and plasticity,and a new strengthening mechanism was achieved for the CoCrNiCu alloy.Moreover,twins were observed in the matrix phase after tensile fracture.Simultaneously,the dual-phase structure with different elastic moduli coordinated with each other during the deformation process,significantly improving the plasticity and strength of the CoCrNiCu alloy.     

Determining the Burgers vectors and elastic strain energies of interface dislocation arrays using anisotropic elasticity theory

A formalism for describing interface dislocation arrays linking the Frank–Bilby equation and anisotropic elasticity theory under the condition of vanishing far-field stresses is developed. The present approach enables the determination of a unique reference state for interface misfit dislocations, within which the Burgers vectors of individual dislocations are defined and allows for the unequal partitioning of elastic fields between neighboring crystals. The elastic strain energies of interface dislocation arrays are computed using solutions for short-range elastic fields. Examples of applications to simple interfaces are given, namely symmetric tilt and twist grain boundaries, as well as a pure misfit heterophase interface.     

Structural Analysis of Highly Relaxed GaSb Grown on GaAs Substrates with Periodic Interfacial Array of 90° Misfit Dislocations

We report structural analysis of completely relaxed GaSb epitaxial layers deposited monolithically on GaAs substrates using interfacial misfit (IMF) array growth mode. Unlike the traditional tetragonal distortion approach, strain due to the lattice mismatch is spontaneously relieved at the heterointerface in this growth. The complete and instantaneous strain relief at the GaSb/GaAs interface is achieved by the formation of a two-dimensional Lomer dislocation network comprising of pure-edge (90°) dislocations along both [110] and [1-10]. In the present analysis, structural properties of GaSb deposited using both IMF and non-IMF growths are compared. Moiré fringe patterns along with X-ray diffraction measure the long-range uniformity and strain relaxation of the IMF samples. The proof for the existence of the IMF array and low threading dislocation density is provided with the help of transmission electron micrographs for the GaSb epitaxial layer. Our results indicate that the IMF-grown GaSb is completely (98.5%) relaxed with very low density of threading dislocations (10⁵ cm⁻²), while GaSb deposited using non-IMF growth is compressively strained and has a higher average density of threading dislocations (>10⁹ cm⁻²).