The “Solidification” of Grain Boundaries with Increasing Temperature

1994 ◽  
Vol 357 ◽  
Author(s):  
Witold Lojkowski ◽  
Bogdan Palosz
Keyword(s):  
Solid State ◽  
Melting Point ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
High Temperatures ◽  
Low Temperatures ◽  
Melting Behavior ◽  
Wetting Transitions ◽  
Increasing Temperature ◽  
Segregation Of Impurities

AbstractThe aim of the paper is to explain the recently observed de-wetting grain boundary transition with increasing temperature. On the example of a bicrystal from the Fe-6at.%Si alloy, it was found recently that as temperature is increased, the following GB transitions take place: “solid” (or regular) GB-→“premelted” GB →“solid” GB. At the same time the wetting/de-wetting transitions have taken place. Another example of such GB behavior was discovered during sintering of alumina. The inverse melting behavior is explained as follows: low melting point impurities cause GB premelting at low temperatures, However de-segregation of impurities at high temperatures causes return of the GB structure to its regular “solid” state.

scholarly journals Increasing contribution of grain boundary compliance to polycrystalline ice elasticity as temperature increases

2018 ◽  
Vol 64 (246) ◽  
pp. 669-674
Author(s):  
COLIN M. SAYERS
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Orientation Distribution ◽  
Resonant Ultrasound Spectroscopy ◽  
Wave Velocities ◽  
Polycrystalline Ice ◽  
Resonant Ultrasound ◽  
Increasing Temperature ◽  
Shear Compliance

ABSTRACTMeasured elastic stiffnesses of ice polycrystals decrease with increasing temperature due to a decrease in grain boundary stiffness with increasing temperature. In this paper, we represent grain boundaries as imperfectly bonded interfaces, across which traction is continuous, but displacement may be discontinuous. We express the additional compliance due to grain boundaries in terms of a second-rank and a fourth-rank tensor, which quantify the effect on elastic wave velocities of the orientation distribution as well as the normal and shear compliances of the grain boundaries. Measurement of the elastic stiffnesses allows determination of the components of these tensors. Application of the method to resonant ultrasound spectroscopy measurements made on ice polycrystals enables determination of the ratio BN/BS of the normal to shear compliance of the grain boundaries, which are found to be more compliant in shear than in compression. The ratio BN/BS is small at low temperatures, but increases as temperature increases, implying that the normal compliance increases relative to the shear compliance as temperature increases.

Crystal defects responsible for mechanical behaviors of a WC–Co composite at room and high temperatures – a simulation study

2019 ◽  
Vol 75 (2) ◽  
pp. 134-142 ◽  
Author(s):  
Jing Fang ◽  
Xuemei Liu ◽  
Hao Lu ◽  
Xingwei Liu ◽  
Xiaoyan Song
Keyword(s):  
Plastic Deformation ◽  
Grain Boundaries ◽  
High Temperatures ◽  
Boundary Migration ◽  
Strain Curve ◽  
Crystal Defects ◽  
Effect Of Temperature ◽  
Triple Junctions ◽  
Deformation Processes ◽  
Increasing Temperature

The microstructure evolution and changes in the structures of crystal defects of the nanocrystalline WC–Co composite in the process of uniaxial compression were studied by simulations at both room and high temperatures. The deformation processes were demonstrated as a function of stress and temperature for the stages prior to and after yielding of the composite. The Peierls stresses were evaluated for Co and WC dislocations with increasing temperature. The deformation mechanisms for each stage of the stress–strain curve were disclosed, in which the effect of temperature was clarified. It was found that with the increase of stress, from elastic deformation to plastic deformation then to yielding of the composite, the dominant mechanisms are grain boundary migration, formation and motion of dislocations in Co, concurrent motion and reaction of dislocations in Co and WC, and then rotation of WC grains in combination with motion of Co and WC dislocations. At the yielding stage, sliding of WC grain boundaries plays an increasingly important role in the contribution to plastic deformation at high temperatures. With strain the proportion of mobile dislocations decreases, and dislocations pile up at triple junctions of WC grains, WC/WC grain boundaries and WC/Co phase boundaries in priority order, leading to the nucleation and propagation of microcracks in these regions.

Effects of Impurities And Alloying Elements on Iron Grain Boundary Cohesion

1997 ◽  
Vol 475 ◽  
Author(s):  
D.E. Ellis ◽  
X. Chen ◽  
G.B. Olson
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Physical And Mechanical Properties ◽  
Alloying Elements ◽  
Metallic Materials ◽  
Crucial Importance ◽  
Intergranular Embrittlement ◽  
Long Time ◽  
Impurity Doped ◽  
Segregation Of Impurities

In metallic materials, where grain boundaries(GB) are of crucial importance, impurities and alloying elements play an important role in determining their physical and mechanical properties because the behavior of a grain boundary may change drastically with the presence of impurities and alloying elements. For example, in iron and its alloys, including industrially important steels, the intergranular embrittlement is usually associated with segregation of impurities, like P and S, toward the GBs. On the other hand, alloying elements, like Mo and Pd, are helpful for intergranular cohesion in iron, due to either direct cohesion effect or effect upon embrittling potency of other impurities. Understanding the mechanisms of impurity-promoted embrittlement and the consequent cohesion(decohesion) effects is becoming more and more important and remains as a challenge for materials scientists. There have been intensive investigations on these mechanisms for a long time and with the progress in computing techniques in recent years, calculations on more realistic representations of impurity-doped grain boundaries have become possible[1–4].

Solid-state wetting transitions at grain boundaries

2008 ◽  
Vol 495 (1-2) ◽  
pp. 119-125 ◽  
Author(s):  
Paul Wynblatt ◽  
Dominique Chatain
Keyword(s):  
Solid State ◽  
Grain Boundaries ◽  
Wetting Transitions

STEM Investigation of Segregation and Local Structure at Grain Boundary in Tetragonal Zirconia

1997 ◽  
Vol 3 (S2) ◽  
pp. 549-550
Author(s):  
H. Gu ◽  
F. Wakai
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Solid Electrolytes ◽  
Tetragonal Zirconia ◽  
Boundary Structure ◽  
Contrast Imaging ◽  
Amorphous Films ◽  
Grain Boundary Structure ◽  
Z Contrast ◽  
Segregation Of Impurities

Y or Ca stabilized tetragonal ZrO2 (TZP) exhibits superplasticity at high temperature, and can also be used as solid electrolytes. Those properties are dictated by structure and chemistry of grain boundaries, which can be controlled by segregation of impurities or additives. The grain boundaries were found either covered by amorphous films or free of the film. Co-segragation of additives and stabilizers has also been observed. To fully understand the correlation between segregation and grain boundary structure, a dedicated STEM (VG HB601) capable of EDX/EELS analysis and phase/Z-contrast imaging is employed to study 3Y-TZP doped with 0.3 and 0.9 mol% SiO2.Although Y-L lines arc dominated by overlapping Zr-L lines in EDX, Y excess at grain boundaries can still be measured by “spatial difference” which removes Zr signal with a spectrum from the bulk. The co-segregation of Si and Y is also observed (Fig. 1) at many boundaries. Their average excesses arc 5±2 nm−2and 25±10 run−2 respectively, close to 1 monolayer each of SiO2 and Y2O3.

scholarly journals The Effect of Non-Hydrostatic Stress on Intergranular Water Veins and Lenses in Ice

1972 ◽  
Vol 11 (61) ◽  
pp. 81-101 ◽  
Author(s):  
J. F. Nye ◽  
S. Mae
Keyword(s):  
Melting Point ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Hydrostatic Stress ◽  
Pure Shear ◽  
Triple Junctions ◽  
Polycrystalline Ice ◽  
Glacier Ice ◽  
Set Up ◽  
Ice Water

AbstractPolycrystalline ice at the melting point has been observed in the laboratory to contain veins of water at the places where three grains meet. Under uniaxial compression lenticular water inclusions appeared at grain boundaries perpendicular to the stress, while the nearby vein began to freeze. A similar effect occurred in tension on grain boundaries parallel to the stress. When the stress on the plane of the boundary was a pure shear stress, no effect was observed. The water lenses produced by stress increased in size and decreased in number after the stress was removed. The effect under compression is explained quantitatively by the combined effects of curvature and pressure on the melting point of an ice–water interface. The rate of formation of the lenses and of their coarsening is greatly reduced by the internal pressures set up in the lenses as a result of expansion on freezing and contraction on melting; transient creep to accommodate volume changes is an essential part of the process. The effect in a grain boundary under tension may arise from pressure caused by sliding on other grain boundaries; it was absent in a bicrystal.It is concluded that internal melting and freezing at grain boundaries and veins will occur in temperate glacier ice, with some effect, not discussed here, on its permeability to water. Any pure solid at its melting point which has a dihedral angle for the liquid phase in contact with a grain boundary between 0° and 60° should show similar behaviour, in that non-hydrostatic stress should cause liquid to move away from triple junctions between grains and into grain boundaries. There may be implications for the Frank theory of the upwelling of melt fluid in the Earth’s upper mantle.

Exafs Studies of Grain Boundary Diffusion and Segregation

1986 ◽  
Vol 77 ◽  
Author(s):  
S. M. Heald ◽  
H. Chen ◽  
J. M. Tranquada
Keyword(s):  
Thin Film ◽  
Solid State ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Diffusion ◽  
Structural Information ◽  
Solid State Diffusion ◽  
Interface Reactions ◽  
Tin Bronze ◽  
State Diffusion

ABSTRACTThe dominant pathway for thin film interdiffusion and interface reactions is often via grain boundaries. We have made EXAFS measurements of grain boundary constituents for two systems: interdiffusion in Ag-Au bilay-ers and solid state reaction of Nb with a copper-tin bronze to form Nb3Sn. The Ag-Au results indicate that Au in saturated Ag grain boundaries has an environment similar to a dilute Au in Ag solution with reduced coordination. For the Nb-bronze reaction, the results for the Cu environment indicate distinct changes in the grain boundary environment when small amounts of Ti, Hf, Zr, and Ta are added to the starting Nb. Both results demonstrate the ability of EXAFS to probe grain boundary environments, and to provide important structural information in understanding solid state diffusion and interdiffusion in thin film systems.

Amorphization Of Zr-Al Under Mechanical Alloying At Different Temperatures: A Re-Entrant Melting Phenomenon

1997 ◽  
Vol 481 ◽  
Author(s):  
H. W. Sheng ◽  
K. Lu ◽  
E. Ma
Keyword(s):  
Thermodynamic Properties ◽  
Solid Solutions ◽  
Low Temperatures ◽  
Melting Behavior ◽  
Milling Process ◽  
Dynamic Effects ◽  
Coexistence Region ◽  
Melting Phenomenon ◽  
Different Temperatures ◽  
Increasing Temperature

ABSTRACTZr100−xAlx powder blends have been subjected to ball milling at different temperatures to investigate the amorphization process. At low temperatures the Zr-Al solid solutions amorphized under the polymorphous constraints, whereas at higher temperatures there was an obvious twophase coexistence region. The Al concentration for the complete amorphization of Zr-Al increased with increasing temperature, suggesting a re-entrant melting behavior. Both of the temperature- and composition-dependent amorphization mechanisms are analyzed in terms of the thermodynamic properties of the phases involved, as well as the dynamic effects brought in by the non-equilibrium milling process.

Wetting Transitions of Grain Boundaries

1999 ◽  
Vol 586 ◽  
Author(s):  
John Blendell
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Abrupt Change ◽  
Equilibrium Shape ◽  
Second Phase ◽  
Curved Surfaces ◽  
Wetting Layer ◽  
Wetting Transitions ◽  
Microscopic Morphology ◽  
Morphology Changes

ABSTRACTIt has been shown that the concepts used to determine the equilibrium shape of crystals can be extended to determine the conditions under which grain boundaries will be fully wetted, partially wetted, or not wetted by a second phase. Recent experimental observations on the equilibrated morphologies of solid or fluid wetting phases along grain boundaries, reveal features that are predicted, and in some cases required, by this construction. Theory distinguishes between cases where surfaces are smoothly curved or where there are facets, edges and corners. In the latter case the conventional comparison of the energy of the original grain boundary with the sum of the surface energy of the two surfaces of the wetting layer leads to erroneous predictions. The correct predictions are obtained by comparing the Wulff shape of the grain boundary (the interfacial energy minimizing shape for a fixed volume of material) with a carefully defined sum of Wulff shapes of the surfaces of the wetting layer. Where orientations that are wetted join with those that are not, there is almost always an abrupt change of orientation. Faceting on two hierarchical levels can occur. Microscopic morphology changes along macroscopically curved surfaces follow well defined rules predicted by theory. The analogy between the thermodynamics of interface faceting and phase transformations allows the well known concepts of phase equilibria to be used to understand the predicted structures. The predictions of the model will be used to identify the nature of the faceting observed in alumina in the presence of a second phase.

Investigations of the Bonding Changes Associated with Grain Boundary Embrittlement

1996 ◽  
Vol 458 ◽  
Author(s):  
V. J. Keast ◽  
J. Bruley ◽  
D. B. Williams
Keyword(s):  
Electronic Structure ◽  
Grain Boundary ◽  
Energy Loss ◽  
Grain Boundaries ◽  
Edge Structure ◽  
Grain Boundary Embrittlement ◽  
Impurity Elements ◽  
Electron Energy Loss ◽  
Segregation Of Impurities ◽  
Boundary Embrittlement

ABSTRACTThe embrittlement of materials through the segregation of impurities to the grain boundaries is a common and industrially important problem. Despite considerable investigation, the mechanism by which the impurity elements cause embrittlement is not well understood. A change in the electron energy loss near edge structure (ELNES) has been observed at Cu grain boundaries containing Bi. This result provides experimental evidence that a change in the electronic structure at the grain boundary is responsible for embritdement.

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