The role of grain-boundary sliding in the creep of polycrystals

A theoretical analysis is performed to describe the development of overall strain in a polycrystal when internal deformation and sliding of its constituent grains occur concurrently. It is demonstrated that, although sliding is not an independent deformation mode, its occurrence does lead to increased overall strain and the effect is quantified. The extra strain associated with sliding cannot be derived from measured boundary displacements simply by geometrical arguments which do not involve the physical properties of the material.     

Role of grain-boundary diffusion in the grain growth in nanocrystalline nickel

The grain growth in a nanocrystalline nickel during nonisothermal annealing is studied by differential scanning calorimetry (DSC) and transmission electron microscopy. Nanocrystalline nickel is prepared by electrodeposition at a pulsed voltage. Its nonisothermal annealing is accompanied by anomalous grain growth; at a temperature corresponding to a DSC peak, a bimodal grain structure forms. The processing of DSC signals in terms of the Avrami formalism permits the grain growth activation energy to be determined, which is found to be close to the activation energy of grain-boundary self-diffusion. The anomalous grain growth creates conditions such that grain-boundary diffusion is a controlling stage of the process.     

The migration of grain-boundary grooves at the solid-liquid interface

When a solid-liquid interface is intersected by a grain boundary, a grain-boundary groove forms at the interface. Experiments have been carried out using camphene (a transparent analogue of a metal in terms of solidification behaviour) which show that, if the grain boundary is not perpendicular to the interface, the groove will be dragged laterally across the interface. This effect has been modelled in a thin sandwich of silicone oil by mechanically dragging a cusp of oil across an otherwise undisturbed oil surface. The present work is relevant to the development of growth textures during the slow solidification of relatively pure metallic melts.     

Relations between the parameters of grain-boundary diffusion of zinc in aluminum

Experimental results on the grain-boundary diffusion of zinc in aluminum bicrystals containing 〈100〉 and 〈111〉 tilt boundaries at normal and high hydrostatic pressures have been considered. A comparison of the diffusion activation energies and the activation volumes for grain boundaries having the same crystallographic parameters has revealed a linear relation between these kinetic constants. The diffusion activation energy is found to increase with the activation volume. This relation is shown to correspond to the compensation effect well-known in diffusion theory.     

Lattice and grain-boundary diffusion of phosphorus in commercially-pure copper

The diffusion rates of phosphorus in 3 batches of commercially-pure copper with different impurity content have been determined in the temperature range 574 to 1046° using the tracer-sectioning technique. There is clear evidence of grain-boundary diffusion occurring below 720°. The three activation energies and preexponential factors obtained for volume diffusion areQ _l = 1.43 eV, 1.44 eV, 1.43 eV;Dqi = 7.02 × 10^-3 cmVs, 7.98 x 10^-3 cm²/s, 4.38 × 10^-3 cm²/s. For grain-boundary diffusion the respective average values are Q_b = 0.70 eV and D_obδ≈ 4.2× 10^-7 cm3/s.     

Measuring the five-parameter grain-boundary distribution from observations of planar sections

A stereological method is described for estimating the distribution of grain-boundary types in polycrystalline materials on the basis of observations from a single planar section. The grain-boundary distribution is expressed in terms of five macroscopically observable parameters that include: three parameters that describe the lattice misorientation across the boundary and two parameters that describe the orientation of the grain-boundary plane normal. The grain-boundary distribution is derived from measurements of grain orientations and the orientations of the lines formed where grain boundaries intersect the plane of observation. Tests of the method on simulated observations illustrate that the distribution of boundaries in a material with cubic symmetry can be reliably determined with about 10° of resolution from the analysis of 5×10⁴ or more line segments. Furthermore, grain-boundary distributions directly observed from serial sections of a SrTiO₃ polycrystal are compared to those resulting from the stereological analysis of a single plane. The comparison shows that the stereological method provides a reasonable estimate of the measured distribution. The differences between the directly observed grain-boundary distribution and that derived from the stereological analysis are consistent with the results from the simulation. This article is based on a presentation made at the symposium “Characterization and Representation of Material Microstructures in 3-D” held October 8–10, 2002, in Columbus, OH, under the auspices of ASM International’s Phase Transformations committee.     

Direction of grain-boundary migration in the weld metal of an austenitic stainless steel

The moving direction of the grain boundary (GB), after solidification in the weld metal of AISI310S stainless steel, was examined through a computer simulation technique using the vertex dynamics model and by observing the microstructure. The results are as follows. (1) The grain-growth exponent in the vertex model was fitted to describe the experimental data. (2) The vertex dynamics model can predict the moving direction of a grain boundary in the weld metal after solidification.     

Nonequilibrium grain-boundary cosegregation of nitrogen and chromium in NiCrMoV steel

It is concluded in this article that nonequilibrium grain-boundary cosegregation (NCGS) of nitrogen and chromium occurs in NiCrMoV steel. That conclusion is reached from experimental observations of the parallel segregation isotherms and the maximum coverage of Cr and N at grain boundaries during the isotherms. This means that the nonequilibrium segregation of Cr induces that of N, in NiCrMoV steel.     

Processing effects on the grain-boundary character distribution of the orthorhombic phase in Ti-Al-Nb alloys

The grain-boundary character distribution of the orthorhombic (O) phase in Ti₂AlNb intermetallic alloys was investigated. The alloys were thermomechanically processed either above or below the bcc transus temperature. Using electron backscattered diffraction, the twin-related O-phase variant interfacial planes were identified and quantified. For the subtransus-processed samples, the equiaxed-O/equiaxed-O grain boundaries tended to primarily prefer 65-deg misorientations and secondarily prefer 90-deg boundaries. Of the 65-deg misoriented boundaries, which were preferentially rotated about [001], ∼40 pct contained (110) twin-related interfacial planes. The observations were rationalized by the α ₂-to-O phase transformation. It is suggested that for subtransus processing within the α ₂-containing phase regimes, the resulting heat-treated O+bcc microstructures evolve such that the O/O boundaries tend to exhibit distinct twin-related variants with misorientations between 55 and 65 deg. For a supertransus-processed alloy, it was found that approximately equal distributions of the six resolvable O variants were formed from the dominant parent bcc orientation. The resulting O/O boundaries tended to cluster at near-90-deg misorientations, which can be explained by the bcc/O orientation relationship. It is suggested that whenever the O phase primarily transforms from the bcc structure, the resulting O+bcc microstructures evolve such that the O/O boundaries tend to exhibit misorientations near 90 deg.     

Effects of grain-boundary segregation and precipitation on the hydrogen susceptibility of nickel

The straining electrode tensile test was employed to study hydrogen embrittlement in three grades of nickel. Aging at intermediate temperatures (400 to 800 °C) after solution annealing caused sulfur segregation to grain boundaries in the materials. This sulfur segregation was found to influence intergranular hydrogen embrittlement, and the effect was most prominent when the surface supply of hydrogen was low and when the grain boundaries were free of graphite precipitates. Aging also induced graphite precipitation at the grain boundaries of one grade of nickel (Nickel 200). The presence of these grain-boundary particles reduced susceptibility to hydrogen embrittlement, but at the same time caused a form of aging embrittlement. The effect of the second-phase precipitate, when present, masked the effect of sulfur segregation on hydrogen susceptibility. Formerly with Bell-Northern Research, Ottawa, ON, Canada.     

Atomistic simulations of the effects of segregated elements on grain-boundary fracture in body-centered-cubic Fe

We studied the detailed fracture behavior of a Σ=5 symmetrical-tilt grain boundary at low temperatures in Fe, using empirical interatomic potentials. For loadings just above the Griffith value, the crack propagates along the boundary for a distance of about 5 nm and then deflects toward the grains. When the boundary is loaded well above the Griffith criterion in pure bcc Fe, the crack deflects and propagates in an intragranular manner. Lattice trapping effects were observed in the initial stages, as the crack propagates along the grain boundary in a brittle manner with a periodicity given by the structural unit of the grain boundary. The effects of impurities on crack propagation along the grain boundary were simulated with various amounts of substitutional (Cr and Ni) and interstitial (H and C) impurities. The H impurities result in a strong embrittlement of the grain boundary, and no deflection of the fracture to the inside of the grains is observed. The element C has the opposite effect, inducing the deflection of the fracture to the interior of the grains from the beginning of the simulation. For the substitutional Ni and Cr impurities, the effects on grain-boundary fracture behavior are less dramatic, with Cr decreasing the resistance to grain-boundary fracture, if present in high concentrations. These effects agree with expectations based on the relative energies of segregation of the impurities to the grain boundary and free surface.     

新型炼钢转炉煤气净化回收系统

分析了目前国内外炼钢转炉烟气除尘采用的两种方法,即LT法和OG法,这两种方法都没有对炼钢转炉800～1000℃以下余热进行回收.提出采用新型除尘器处理炼钢转炉烟气,可回收炼钢转炉烟气的余热,实现节能减排.     

The effect of strain on the trapping of hydrogen at grain-boundary carbides in Ni-Cr-Fe alloys

The present work quantifies the role of plastic deformation on the hydrogen-trapping behavior within the material and at the carbides. Isothermal desorption spectroscopy and thermal desorption spectroscopy (TDS) were performed on unstrained and strained alloy 600 (Ni-15Cr-8Fe) and alloy 690 (Ni-30Cr-8Fe), in order to quantify the effect of strain on the trapping energy. The results show that the M₂₃C₈ carbides in alloy 690 were stronger traps than the M₇C₃ carbides in alloy 600. It was further shown that cold work tended to increase the binding energy of hydrogen to the trap sites associated with grain-boundary carbides, although this effect was small.     

In situ observations of chemically induced grain-boundary migration and discontinuous precipitation in the aluminum-zinc system

In situ observations are reported for chemically induced grain-boundary migration (CIGM) and discontinuous precipitation (DP) in the Al-Zn system. In each case, it is found that the migration of the grain boundary is a steady continuous process, at least on the scale of the resolution of the electron optical techniques employed. However, there are significant differences; the grain-boundary velocities are such that a diffusion field must exist in the parent crystal for the case of CIGM but little or no volume diffusion penetration for steady DP. For CIGM, it is argued that the coherency strain energy in the diffusion field is sufficient for the development of a small residual chemical force; for steady DP, a large fraction of the total chemical force is available to act directly across the grain boundary. In the latter case, agreement is obtained between observed and calculated dynamic boundary shapes, using a method in which the chemical force (corrected for losses due to continuous precipitation in the parent phase) is balanced against capillary forces. Formerly with McMaster University     

Comparison of CapiFlow and frame by frame analysis for the assessment of capillary red blood cell velocity

The aim of this study was to describe the aerodynamics related to velopharyngeal function during speech in bilabial, dental, and velar articulatory positions in Swedish speakers. Repeated syllables including voiceless stop consonants and sentences including combinations of voiceless stops and nasal consonants were uttered by 11 normal adults. Oropharyngeal pressure was assessed with a miniature pressure transducer positioned transnasally, in combination with measurement of nasal pressure and nasal airflow. The corresponding velopharyngeal opening area was estimated. The results suggest that the velopharyngeal function during stop consonants is similar in different articulatory positions. Differences in oropharyngeal pressure and nasal airflow between bilabial versus dental and velar positions were found, presumably due to differences in volume and compliance of the vocal tract.