Inverse Hall–Petch relationship in the nanostructured TiO2: Skin-depth energy pinning versus surface preferential melting
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The functional dependence of stress, elastic modulus, melting point, and their interdependence on the identities (bond order, nature, length, and strength) of a representative bond of the specimen has been established for deeper insight into the transition from the conventional Hall–Petch relationship (HPR) to the inverse HPR (IHPR) for nanostructured TiO2. Theoretical reproduction of the observed inverse HPR suggests that the intrinsic competition between the energy-density gain (elastic modulus enhancement) and the cohesive-energy remnant (melting point depression) in the grain boundaries originates and the extrinsic competition between the activation and the inhibition of atomic dislocations activates the IHPR.Keywords:
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The effect of a serrated grain boundary on stress corrosion cracking (SCC) of Alloy 600 was investigated in terms of improvement of SCC resistance. Serrated grain boundaries and straight grain boundaries were obtained by controlled heat treatment. SCC cracks preferentially initiated and grew at grain boundaries normal to the tensile loading axis. Resolved tensile stress normal to the grain boundary was lower in serrated grain boundaries compared to straight grain boundaries. The specimen with serrated grain boundaries showed higher SCC resistance than that with straight grain boundaries due to a lower resolved tensile stress normal to the grain boundary.
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Mo enriched thick GBs in nanocrystalline Ni alloy are stronger barriers than conventional GBs to the transmission of dislocations.
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The phase transformations can proceed not only between bulk phases but also in free surfaces, grain boundaries and interphase boundaries. In this review we consider the grain boundary phase transformations in Cu- and Al-based alloys. In particular, among those transformations are the transitions between compete and incomplete grain boundary wetting. The wetting phase can be either liquid or solid. If the wetting phase is solid, the portion of wetted grain boundaries can increase also with decreasing temperature. The transition itself can be discontinuous (of the first order) or continous (of the second order). The thin layers of grain boundary phases (called also the grain boundary complexions) can occur in the conditions (temperature, pressure and concentration) where only one volume phase can exist. The phenomenon of the pseudo-incomplete (or pseudo-partial) grain boundary wetting is also discussed. In this case the non-zero grain boundary contact angle coexists with thin layer of grain boundary phase. The new lines of respective grain boundary phase transformations appear in the conventional phase diagrams for three-dimensional phases. The grain boundary phase transitions can strongly influence the properties of grain boundaries themselves and those of polycrystals as a whole. For example, the presence of grain boundary layers can increase the plasticity (if the phase is ductile) or decrease it (of the grain boundary phase is brittle). The influence of grain boundary phase transitions increases with decreasing grain size and becomes critical in nanograined materials.
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From the Contents: Part 1: From intergranular order to disorder.- Introduction: brief history of the intergranular order concept.- Geometrical order.- Mechanical stress order.- Atomic order.- Order or disorder at high temperature.- Grain boundary order and energy.- Grain boundary order or disorder: what conclusion?.- Part 2: From the ideal grain boundary to the real grain boundary.- Defects in the grain boundary structure.- Intergranular segregation.- Precipitation at grain boundaries.- Interactions between dislocations and grain boundaries.- Relaxation of the intergranular stresses.- Part 3: From the free grain boundary to the constrained grain boundary.- The triple junction.- Grain boundary network - grain boundary texture.
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Localized deformation near grain boundaries in an Al-0.996 mass%Mg2Si-0.395 mass%Si alloy was investigated by interference microscopy and scanning tunneling microscopy.Both the displacement of scratch lines and the steps at the grain boundaries were observed. The number of the grain boundaries with displacement of scratch lines had a maximum at the grain boundaries making angles of about 45 degrees to the tensile axis. While a large numbers of the steps were observed at the grain boundaries making angles of near 90 degrees to the tensile axis. In both types of grain boundaries, the direction of the maximum resolved shear stress (corresponding to Fmax) on a grain boundary plane is close to the moving direction of the grain boundary.When the step was formed simultaneously at two neighboring grain boundaries, fold was not observed. In such a case, the direction of Fmax on the grain boundary plane was slightly deviated from the moving direction. The values of Fobs (corresponding to the resolved shear stress on the grain boundary plane in the moving direction) tended to increase with the amounts of steps and displacements of scratch lines at the grain boundaries.
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