Grain Boundary Facets Research Articles

Accelerated fracture due to tritium and helium in 21-6-9 stainless steel

The effects of internal tritium and helium on the room-temperature tensile properties of a nitrogen-strengthened stainless steel, forged 21Cr-6Ni-9Mn (NITRONIC 40), were investigated by thermally charging tritium into tensile specimens and aging for selected times. The precipitation of helium as bubbles on dislocations greatly increased the yield strength, and as a consequence of dislocation pinning, the deformation mode changed from long-range dislocation motion to deformation twinning. The tensile specimens exhibited a 90 pct decrease in tensile ductility at 1438 appm 3 He, accompanied by a severe change in fracture mode from ductile rupture to a dominantly intergranular fracture mode. Grain-boundary facets showed multiple striations where deformation twins had intersected the boundaries. Twinning began immediately upon yielding, and at small strains, the microstructure evolved into a fully hardened state, normally observed at 40 pct or greater strain in unexposed or hydrogen-charged 21-6-9. Fracture occurs at low strains in tritium-charged and aged 21-6-9, in part, because helium bubble precipitation causes the deformed microstructure to evolve to a heavily deformation-twinned state at a lower strain. Helium bubble precipitation on the grain boundaries may have caused a further loss in ductility. Fracture appeared to nucleate at the intersection of deformation twins with the grain boundary.

Crack nucleation due to elastic anisotropy in polycrystalline ice

This paper presents a theoretical analysis of crack nucleation in isotropic polycrystalline ice due to the elastic anisotropy of the constituent crystals. The singularity of the associated stress concentrations near a grain-boundary facet junction provides the mechanism for inducing microcrack precursors, if similar nuclei do not already exist. The first-order microstructural stress field created by the elastic anisotropy mechanism is linearly superposed on the applied stress field. This total stress field causes the precursors to nucleate into microcracks. The analysis of the nucleation stress is based on a solution to the general problem of an extending precursor in a combined stress field including the effects of Coulombic frictional resistance. The local material resistance is characterized in terms of a critical value for the maximum principal tensile stress which can be determined from the surface free energy of either the grain boundary or the solid-vapor interface. Model predictions show that: (a) the stress required to nucleate cracks in compression is about 2.5 times that in tension, unlike other microstructural models which predict them to be equal; (b) elastic anisotropy rather than dislocation pile-up as proposed by others may govern crack nucleation in tension over a wide range of strain rates; (c) the stress required to nucleate a crack in compression is strongly dependent on crystal orientation and, as a consequence of the random orientation of crystals in isotropic polycrystalline ice, there can be a distinct beginning and end to the microcrack nucleation phase when stress is increased and if failure does not occur prematurely; (d) the grain size effect due to the elastic anisotropy mechanism is similar to that due to the dislocation pile-up mechanism over the typical range of grain sizes encountered in nature; and (e) a generalization of the limiting tensile strain criterion ∗ ∗ Proposed by Shyam Sunder and Ting (1985). which accounts for the anisotropy of the constituent crystals is an excellent phenomenological approximation of the nucleation surface under multiaxial states of stress.

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as α2 phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (α2) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich α2 phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles andβ-Co matrix, such that (0 1 1)α2 ¶ (1 1 1)β-Co and [1\(\overline 1 \) 1]α2 ¶ [1\(\overline 1 \) 0]β-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.

Electron diffraction study of the faceting of tilt grain boundaries in NiO

Abstract The diffraction effects expected from a periodically faceted boundary containing a periodic array of dislocations in the long facet have been analysed. The characteristic manifestation in reciprocal space of periodic faceting was identified as the occurrence of bundles of reciprocal lattice rods; within each bundle the rods are displaced with respect to each other parallel to their length in a manner related to the facet geometry. Electron diffraction and microscopy were used to study the facet structure of tilt boundaries in NiO, and the boundary structure deduced from the observed diffraction effects was in good agreement with the imaging observations. In those cases where there was only one type of dislocation present in the long facet, it was possible to determine the average boundary plane, the dislocation spacing in the long facet, the facet period and the facet height from electron diffraction observations. This technique is especially useful for large-angle boundaries having high-index ro...

Grain Boundary Characterization in Ni3Al

Grain boundaries in both pure and boron doped Ni/sub 3/Al have been studied using a variety of electron microscopy techniques. Small angle boundary structures were examined in both bicrystal and polycrystalline specimens. Dislocaions with Burgers vector a2 are observed in the presence of boron, while dislocations with Burger vector a are observed in the absence of boron. A possible explanation for this behavior is the presence of a disordered layer at the interface in boron-doped Ni/sub 3/Al. The addition of boron to Ni/sub 3/Al was seen to induce faceting of grain boundaries, and to afford the boundaries some protection from etching. Using electron backscatter diffraction patterns, the frequency of occurrence of grain boundary types was found to be unchanged by the addition of boron.

The role of boundary plane: HREM investigation of the atomic structure of grain boundaries in gold

Computer simulations of large-angle grain boundaries (GBs) have indicated the importance of local atomic relaxation, rigid body translations, and of the boundary plane in determining GB energy. Experimental observations of GB structure often find GB faceting, which is an indication that some GB planes are preferred over others. Investigations of the atomic structure of large-angle >001> tilt GBs in NiO have shown, along with symmetrical GB con-figuations, the presence of asymmetric GBs of the type which has at least one of the crystals terminated at the boundary by a low index plane. Such boundaries are thought to be of low energy. For studying the role of the GB plane in a metallic GB we chose in the present work two <011> tilt boundaries in Au. The misorientation angles (θ) were selected such that one bicrystal orientation (θ=39°) was close to the Σ=9 reciprocal coincident site density, while the other (θ=55°) was near the misorientation for which (111) and (100) planes are parallel to each other (θ=54.74°). The latter misorientation is also close to Σ=41 (θ=55.88°).

The Accommodation of a Change in Grain Boundary Plane in Molybdenum

The accommodation of deformation at a grain boundary (GB) influences the mechanical and thermodynamical properties of the boundary. This paper focuses on the accommodation of a change in GB plane such as a GB step, GB faceting or GB curvature producing an effective change in the overall boundary plane. The stresses produced affect mechanical properties such as the continuation of plastic flow and the accommodation of an intergranular crack. Bcc materials are of interest because of the technological importance and because of the paucity of observations of bcc GB structure. Molybdenum is of particular interest because it shows an intrinsic GB brittleness.

Sulphur segregation and intergranular microcracking in ferritic steel weld metal

Abstract Metallographic and fractographic studies were undertaken to investigate weld metal microcracking in a 2·25Cr–1 M o steel fabrication. Numerous small microcracks were found in single-pass TIG welds in the as-welded condition. An unusual form of brittle fracture on prior austenite grain boundaries was observed, associated with a dense array of intergranular submicrometre sulphide inclusions. The observations indicated that sulphide precipitation had occurred by a solidstate mechanism as the weld metal cooled through the austenite range, while elemental sulphur segregation promoted grain-boundary faceting and embrittlement. The microcracks appeared to have formed on the previously faceted and embrittled grain boundaries by a low-temperature quench cracking mechanism. The factors responsible for the severe sulphur segregation are reviewed, and the implications are discussed in the context of recent interest in the role of grain-boundary sulphides in weld stress relief cracking.

Grain Boundaries, Fracture, and Heat Treatment of Commercial Partially Stabilized Zirconia

Studies of commercial PSZ show that fractography is quite feasible. Although the fracture was predominately transgranular, it commonly originated from grain‐boundary facets. This behavior indicates that strengths are controlled by energies less than those for polycrystalline fracture and are limited by grain boundaries. Appropriate heat treatment under reducing conditions gave strengths up to ≅ 600 MPa; Stoichiometry appears to be important.

Striation and faceting of grain boundaries in nickel due to sulfur and other elements

As is the case for free surfaces, the grain boundaries of metals may show striation and faceting as a result of the effect of certain elements. This is true for nickel containing sulfur, tellurium and bismuth, and for copper containing bismuth. In all of the cases, the phenomenon occurs only when the grain boundary concentration of the element concerned approaches saturation, and material transport is taking place towards or away from the grain boundaries. Thus, intergranular striation and faceting are telltale signs of a non-equilibrium segregation.

Anisotopy of migration and faceting of large-angle grain boundaries in ice bicrystals

Abstract Migration of large-angle grain boundaries in ice bicrystals has been investigated by the method of capillary force developed by Sun and Bauer (1970 a, b). Strong anisotropy of the migration rate was observed between two cases when the boundaries moved parallel and perpendicular to the direction of rotation axis ω. X-ray diffraction topographic observations of grain boundaries (〈1010>/34〉) during migration by the capillary force revealed that they had a strong tendency to be parallel to low-energy facets and that the boundaries that were not parallel to any low-energy facets moved faster than those that were parallel. From observations in three specific directions of the normals of these facets, it was concluded that the facets were the high-density planes of the coincidence site lattice composed of two adjacent crystals. The anisotropy of the migration rate was interpreted in terms of the difference in the density of steps among facets: the non-faceted boundary with many steps moves faster than t...

Faceting of high-angle grain boundaries in the coincidence lattice

Abstract Faceting of high-angle grain boundaries in Σ=3 and Σ=5 coincidence lattices of the f.c.c. structure was studied using thin-film bicrystal gold specimens of controlled geometry. A number of relatively high-index boundaries in the coincidence lattices (containing relatively low planar densities of coincidence sites) was found to break up into low-energy facets corresponding to low-index planes of the confidence lattices (containing high densities of coincidence lattice sites). These results are consistent with our general expectation that the grain boundary energy decreases as the planar density of coincidence sites increases, i.e., the two-dimensional periodicity of the boundary becomes shorter, and long-range distortions are reduced.

The Creep Fracture of Inconel X-750 at 700° C

The creep-fracture behaviour of Inconel alloy X-750 at 700° C has been studied using four grain sizes ranging from 9 to 200 μm over a range of stress from 207 to 414 N/mm2. Cavity nucleation was continuous through the tests, starting during primary creep. Interlinkage of cavities to form cracks of a grain-boundary facet in length was completed towards the end of the secondary stage of creep and catastrophic failure resulted when these cracks interlinked to form a crack of critical size. From the angular distribution of cavities, it was deduced that grain-boundary sliding played a dominant role during the earlier stages of the growth of cavities, whereas vacancy condensation and/or mechanical tearing were the dominant growth processes during the later stages of the growth of cavities. Ductility increased significantly as the grain size was decreased. This can be related to the extended tertiary creep stage observed in the small grain sizes as a result of the presence of a larger number of triple junctions that limit crack propagation. Variation of rupture life with grain size was an inverse plot of the variation of minimum creep rate with grain size. However, the product of the two (έ s × t r ) was not a constant. It was observed that the stress-sensitivity parameter n(έ ∝ σn) increased with increasing grain size. The fracture criterion applicable fo creep in this alloy at 700° C is related to the ease of linkage and final propagation of cracks of a grain facet in length. A quantitative criterion of failure based on the product of stress and the square root of the average crack length = K f , the creep-fracture toughness parameter, is reached.

Formation of annealing twins, faceting, and grain boundary pinning in copper bicrystals

In the course of a recent investigation of the kinetics of grain boundary migration in copper bicrystals, formation of annealing twins, faceting and grain boundary pinning have been observed. Stability and frequency of formation of annealing twins are related to boundary misorientation and temperature of anneal. Tendency for grainboundary faceting decreases with increasing temperature indicating that anisotropy of grain boundary energy is more pronounced at lower temperatures. In general, orientations of faceted boundaries corresponded to higher-order twin planes with respect to shrinking grains. At relatively high temperatures (950°C), specimen-thickness-dependent pinning effects also are observed. These observations are described and analyzed in terms of grain boundary structure, energy, and migration behavior.