Atomistic simulations of diffusional creep in a nanocrystalline body-centered cubic material

Molecular dynamics (MD) simulations are used to study diffusion-accommodated creep deformation in nanocrystalline molybdenum, a body-centered cubic metal. In our simulations, the microstructures are subjected to constant-stress loading at levels below the dislocation nucleation threshold and at high temperatures (i.e., T > 0.75T_melt), thereby ensuring that the overall deformation is indeed attributable to atomic self-diffusion. The initial microstructures were designed to consist of hexagonally shaped columnar grains bounded by high-energy asymmetric tilt grain boundaries (GBs). Remarkably the creep rates, which exhibit a double-exponential dependence on temperature and a double power-law dependence on grain size, indicate that both GB diffusion in the form of Coble creep and lattice diffusion in the form of Nabarro–Herring creep contribute to the overall deformation. For the first time in an MD simulation, we observe the formation and emission of vacancies from high-angle GBs into the grain interiors, thus enabling bulk diffusion.     

X-ray diffraction line broadening analysis of nanostructured nickel powder

X-ray diffraction peak profile analysis is a powerful method for investigation of the microstructural characteristics of nanocrystalline materials produced by severe plastic deformation. Williamson–Hall method, its modification, and Fourier transform methods like Warren–Averbach and its modification are used to extract microstructural information based on integral breadth and Fourier coefficients of the peak profiles. In this work, pure nickel powders were milled for 2, 5, 20, and 40 h to clarify the microstructural variations for this metal with FCC atomic structure. By using the Williamson–Hall and Warren–Averbach methods and their modifications, crystallite size, microstrain, dislocation density, and character of dislocation were extracted. It was obtained that the fraction of edge-type dislocation decreased with milling time (above 50%), while after prolonged milling, the fraction of screw-type dislocations decreased, so that 50%-edge-to 50%-screw ratio was obtained. It was also found that the values of the crystallite size obtained from classical Warren–Averbach and modified Warren–Averbach (area-average) were closed.     

Non-monotonic lattice parameter variation with crystallite size in nanocrystalline solids

An anomalous dependence of the lattice parameter on the crystallite size of nanocrystalline ball-milled powders of metals was observed: lattice contraction followed by lattice expansion with decreasing crystallite size. These data were determined by application of detailed X-ray diffraction measurements. To this end the lattice parameters of the metals investigated – nickel, copper, iron and tungsten – were precisely determined by correcting for influences of stacking faults, in the face-centred cubic metals, as well as by correcting for instrument-related aberrations. The non-monotonic variation of the lattice constant was interpreted as the result of two competing mechanisms: interface-stress-induced contraction vs. expansion as a result of the stress field generated at the crystallite boundary due to the increased excess free volume in the crystallite boundary upon decreasing crystallite size.     

Atomistic simulations of grain boundary segregation in nanocrystalline yttria-stabilized zirconia and gadolinia-doped ceria solid oxide electrolytes

Hybrid Monte Carlo–molecular dynamics simulations are carried out to study defect distributions near Σ5(3 1 0)/[0 0 1] pure tilt grain boundaries (GBs) in nanocrystalline yttria-stabilized zirconia and gadolinia-doped ceria. The simulations predict equilibrium distributions of dopant cations and oxygen vacancies in the vicinity of the GBs where both materials display considerable amounts of dopant segregation. The predictions are in qualitative agreement with various experimental observations. Further analyses show that the degree of dopant segregation increases with the doping level and applied pressure in both materials. The equilibrium segregation profiles are also strongly influenced by the microscopic structure of the GBs. The high concentration of oxygen vacancies at the GB interface due to lower vacancy formation energies triggers the dopant segregation, and the final segregation profiles are largely determined by the dopant–vacancy interaction.     

Thermal stability data of noncrystalline elemental solids

Amorphous solids crystallize, on heating, at a temperature,T _x, ExperimentalT _x data are compiled for amorphous elemental solids.     

Grain boundary segregation,chemical ordering and stability of nanocrystalline alloys: Atomistic computer simulations in the Ni–W system

Atomistic computer simulations are used to investigate the equilibrium solute distribution and alloying energetics in nanocrystalline Ni–W. Composition and grain size-dependent trends in grain boundary segregation and chemical ordering behavior are evaluated and we find the equilibrium state to be significantly influenced by the nanostructure. The energetics of alloying are assessed through computation of the segregation, formation, and grain boundary energy, and these quantities are linked to previous thermodynamic models of nanostructure stability. With comparison to experiments, we conclude that nanocrystalline Ni–W alloys are synthesized in a metastable state. These findings have important consequences for theories of nanostructure control in general and particularly for the thermal stability of nanocrystalline Ni–W.     

Microstructure evolution during cold rolling in a nanocrystalline Ni–Fe alloy determined by synchrotron X-ray diffraction

Stress softening after cold rolling is observed in an electrodeposited nanocrystalline Ni–Fe alloy. The grain-size distribution becomes much broader after the cold rolling. Microstructure changes, though moderate, such as simultaneously decreased dislocation and twin densities with grain growth during cold rolling, are systematically proved by synchrotron high-energy X-ray diffraction, transmission electron microscopy and differential scanning calorimetry (DSC). The amorphous fractions in the form of grain boundaries are evidenced by the diffuse-background scatterings and large DSC values. Partial dislocation separation calculation, a dislocation mean free path and annihilation model, and texture development together reveal that the current nanocrystalline Ni–Fe alloy exhibits the combined behavior of perfect dislocation slip and grain-boundary mediated deformation.     

Crystal structure determination of CoGeTe from powder diffraction data

The crystal structure of cobalt germanium telluride CoGeTe has been determined by direct methods using integrate intensities of conventional X-ray powder diffraction data and subsequently refined with the Rietveld method. The title compound was prepared by heating of stoichiometric amount of Co, Ge and Te in silica glass tube at 670 °C.CoGeTe adopts orthorhombic symmetry, space group Pbca with unit cell parameters a = 6.1892(4) Å, b = 6.2285(4) Å, c = 11.1240(6) Å, V = 428.8(1) Å³ and Z = 8. Its crystal structure is formed by [CoGe₃Te₃] octahedra sharing both edges and corners. CoGeTe represents a ternary ordered variant of α-NiAs₂ type structure. An important feature present in CoGeTe is an occurrence of short Co–Co distance across the shared edge of [CoGe₃Te₃] octahedra. Differential thermal analysis (DTA) has revealed that CoGeTe melts incongruently at about 725 °C; CoGeTe decomposes into GeTe, CoGe and CoTe₂. Temperature dependence of the electrical conductivity and value of Seebeck coefficient at 300 K are also reported.     

纳米Co78Ni22合金组织演变的原位高温X射线分析

采用高温X射线原位分析法研究了喷射电沉积制备的晶粒尺寸5.1 nm的Co78Ni22合金连续加热及等温条件下组织转变.结果表明:在连续加热条件下,随着加热温度升高,晶粒尺寸经历由低温缓慢长大到中温异常长大再到高温正常长大过程,同时ε相向α相转变.在等温条件下,随等温时间的延长,ε相向α相转变且转变的温度降低,随等温温度的升高,这种转变加快,晶粒尺寸也会发生明显的长大.Co78Ni22合金加热过程中的异常晶粒长大与ε相向α相转变有关.     

Atomistic simulations of lattice defects in tungsten

The mechanical behavior of materials is ultimately determined by events occurring at the atomic scale. The onset of plastic yield corresponds to triggering of dislocation motion. Subsequent hardening is mainly controlled by interaction of gliding dislocations with other lattice defects such as forest dislocations, grain boundaries, interfaces and surfaces. Finally, material failure is influenced by processes at the tip of a crack propagating in a crystal lattice. In this work we review atomistic simulations of lattice defects in tungsten. We show that these studies are able to provide not only a detailed understanding of defect properties but also reveal how the fundamental processes at the atomic scale are linked to macroscopic material behavior.     

Atomistic simulation of tension deformation behavior in magnesium single crystal

The deformation behavior in magnesium single crystal under c-axis tension is investigated in a temperature range between 250 K and 570 K by molecular dynamics simulations. At a low temperature, twinning and shear bands are found to be the main deformation mechanisms. In particular, the {1012} tension twins with the reorientation angle of about 90° are observed in the simulations. The mechanisms of {1012} twinning are illustrated by the simulated motion of atoms. Moreover, grain nucleation and growth are found to be accompanied with the {1012} twinning. At temperatures above 450 K, the twin frequency decreases with increasing temperature. The {1012} extension twin almost disappears at the temperature of 570 K. The non-basal slip plays an important role on the tensile deformation in magnesium single crystal at high temperatures.     

Atomistic characterization of pseudoelasticity and shape memory in NiTi nanopillars

Molecular dynamics simulations are performed to study the atomistic mechanisms governing the pseudoelasticity and shape memory in nickel–titanium (NiTi) nanostructures. For a 〈1 1 0〉 – oriented nanopillar subjected to compressive loading–unloading, we observe either a pseudoelastic or shape memory response, depending on the applied strain and temperature that control the reversibility of phase transformation and deformation twinning. We show that irreversible twinning arises owing to the dislocation pinning of twin boundaries, while hierarchically twinned microstructures facilitate the reversible twinning. The nanoscale size effects are manifested as the load serration, stress plateau and large hysteresis loop in stress–strain curves that result from the high stresses required to drive the nucleation-controlled phase transformation and deformation twinning in nanosized volumes. Our results underscore the importance of atomistically resolved modeling for understanding the phase and deformation reversibilities that dictate the pseudoelasticity and shape memory behavior in nanostructured shape memory alloys.