Showing papers on "Grain boundary strengthening published in 1990"

Role of grain size in the strength and R-curve properties of alumina

Abstract: An investigation of the interrelationships between strength, crack-resistance (R-curve) characteristics, and grain size for alumina ceramics has been carried out. Results of identation-strength measurements on high-density aluminas with uniform grain structures in the size range 2 to 80 μm are presented. A theoretical fit to the data, obtained by adjusting parameters of a constitutive frictional-pullout relation in a grain-bridging model, allows determination of the critical microstructural parameters controlling the R-curve behavior of these aluminas. The primary role of grain size in the toughness characteristic is to determine the scale of grain pullout at the bridged interface. It is shown that the strength properties are a complex function of the bridged microstructure, governed at all but the finest grain sizes by the stabilizing effect of the R-curve. The analysis confirms the usual negative dependence of strength on grain size for natural flaws that are small relative to the grain size, but the dependence does not conform exactly to the −1/2 power predicted on the basis of classical “Griffith-Orowan” flaws. The analysis provides a self-consistent account of the well-documented transition from “Orowan” to “Petch” behavior.

An In Situ Transmission Electron Microscope Deformation Study of the Slip Transfer Mechanisms in Metals

Abstract: The slip transfer mechanisms across grain boundaries in 310 stainless steel, high-purity aluminum, and a Ni-S alloy have been studied by using thein situ transmission electron microscope (TEM) deformation technique. Several interactions between mobile lattice dislocations and grain boundaries have been observed, including the transfer and generation of dislocations at grain boundaries and the nucleation and propagation of a grain boundary crack. Quantitative conditions have been established to correctly predict the slip transfer mechanism.

TEM in situ deformation study of the interaction of lattice dislocations with grain boundaries in metals

Abstract: The passage of dislocations across grain boundaries in metals has been studied by using the in situ TEM deformation technique. A detailed analysis of the interaction of glissile matrix dislocations with grain-boundary dislocations has been performed. The results show that the dislocations piled-up at the grain boundary can: (1) be transferred directly through the grain boundary into the adjoining grain; (2) be absorbed and transformed into extrinsic grain-boundary dislocations; (3) be accommodated in the grain boundary, followed by the emission from the grain boundary of a matrix dislocation; and (4) be ejected back into their original grain. To predict which slip system is favourable for slip transfer, three criteria have been considered, namely: (1) the angle between the lines of intersection of the incoming and outgoing slip planes with the grain boundary, this should be as small as possible; (2) the resolved shear stress acting on the possible slip systems in the adjoining grain, this should ...

Weak-link-free behaviour of high-angle YBa 2 Cu 3 O 7–δ grain boundaries in high magnetic fields

Abstract: A CRUCIAL issue in both the physics and the application of high-temperature superconductors is the low transport critical current density (Jct) of polycrystalline materials. Much thinking about this issue has been defined by the thin-film YBa2Cu3O7–δ bicrystal experiments of Dimos et al.1,2, which clearly showed greatly reduced values of Jct at high-angle grain boundaries (mis-orientation angle ≳ 10°). The reproducibility of this result for a wide range of misorientation relationships led them to conclude that all high-angle grain boundaries act intrinsically as Josephson junctions. By contrast, we describe here two high-angle grain boundaries with superconducting properties that clearly lack the weak-link signatures characteristic of Josephson junctions. Our bulk-scale bicrystal results provide direct evidence that not all high-angle grain boundaries are alike in their superconducting properties, and that at least some high-angle boundaries can carry significant supercurrents at 77 K in high magnetic fields.

Effect of grain size on brittle and semibrittle strength: Implications for micromechanical modelling of failure in compression

Abstract: Triaxial experiments were performed at room temperature and confining pressures up to 450 MPa on four pure, dense calcite rocks whose average grain sizes range over four orders of magnitude. Volumetric strain was measured during some of the experiments and microstructural studies were conducted to identify the active deformation mechanisms. The brittle fracture strength and macroscopic initial “plastic” yield stress in the semibrittle field follow empirical Hall-Petch relations. The confining pressure at the brittle-ductile transition depends inversely on grain size, but the stress ratio σ3/σ1 at the transition is nearly the same for the different rocks. We assume that the initial flaw size scales with grain size and compare our experimental data to the fracture mechanics models of Ashby and Hallam (1986) for brittle fracture and Horii and Nemat-Nasser (1986) for the brittle-plastic transition in compression. The first model predicts that small confining pressures are sufficient to inhibit work softening behavior; however, our data indicate that localization occurs for significantly higher values of confining pressure than predicted. Furthermore, we find that localization is inhibited with increased confining pressure because of the increased activity of plastic flow mechanisms, rather than because of the increased difficulty of crack propagation alone. With certain assumptions, the model predicts the experimentally determined slope of the Hall-Petch relation in the brittle field, although it underestimates the compressive strength of the rocks. The second model predicts that the stress ratio σ3/σ1 at the brittle-plastic transition scales with the, square root of the grain size; however, the experimental data do not corroborate the model unless the square of the ratio of the mode I fracture toughness to the plastic yield stress in shear scales with the grain size. The stress ratio at the brittle-ductile transition is apparently a constant for many different rock types; we suggest that the physical basis for this relationship is that the ductility of most mineral aggregates falls within a small range.

Nanoindentation study of the mechanical properties of copper‐nickel multilayered thin films

Abstract: The mechanical properties of multilayered Cu‐Ni thin films with bilayer thicknesses of 1.6–12 nm were investigated by a nanoindentation technique. Force‐displacement curves generated during loading and unloading of a diamond tip indenter were used to determine the hardness and elastic properties of the films. No enhancement in the elastic properties (supermodulus effect) was seen, but an enhancement in the hardness was observed. It is suggested that the enhancement, which displayed a Hall–Petch‐type behavior, can be understood as owing to dislocation pinning at the interfaces analogous to the mechanism of grain boundary hardening.

Superelastic behaviour of a fine-grained, yttria-stabilized, tetragonal zirconia polycrystal (Y-TZP)

Abstract: The microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated. Both hot indentation and tensile tests were carried out at temperatures between 1273 and 1923K over the strain rate range from 2.7 × 10−5 to 2 × 10−3 s−1. It was found that the material exhibited extensive plasticity at temperatures higher than 1473K; a maximum tensile elongation of over 800% was recorded. Microstructural examination did not indicate the presence of a glassy phase at grain boundaries. Yttrium, however, was found to segregate to the grain boundaries. The microstructure of the Y-TZP was thermally unstable and appreciable grain growth was observed at emperattures higher than 1723 K; the grain growth was enhanced by external stresses, i.e. dynamic grain growth was observed. Grain growth at elevated temperatures resulted in apparent strain rate sensitivity exponents of approximately 0.33 at 1723K. This value decreased with increasing temperature. The grain size-compensated strain rate, however, was found to depend approximately on the square of the flow stress, i.e. to exhibit a true strain sensitivity value of 0.5, which suggests a grain boundary sliding mechanism. Microstructures from samples that were deformed superelastically indicated that grains remained equiaxed; this observation is consistent with a grain boundary sliding mechanism. The activation energy for superplasticity, under the conditions of constant structure, in Y-TZP was calculated to be 720 kJ/mol.

Investigation of the effects of boron on Ni3Al grain boundaries by atomistic simulations

Abstract: A series of simulations has been performed on grain boundaries in Ni and Ni3Al with and without boron doping using embedded atom-style potentials. A new procedure of obtaining “reference” data for boron related properties from electronic band structure calculations has been employed. Good agreement with existing experimental structural and energetic determinations was obtained. Boron is found to segregate more strongly to grain boundaries than to free surfaces. Adding boron to grain boundaries in Ni and Ni3Al increases their cohesive strength and the work required to pull apart the boundary. This effect is much more dramatic for Ni-rich boundaries than for stoichiometric or Al-rich boundaries. In some Ni-rich cases, adding boron increases the cohesive strength of the boundary to such an extent that the boundaries become stronger than the bulk. Bulk Ni3Al samples that are Ni-rich produce Ni-rich grain boundaries. The best cohesive properties of Ni3Al grain boundaries are obtained when the boundary is Ni saturated and also with boron present. Boron and nickel are found to cosegregate to the grain boundaries.

Slip transfer across grain and phase boundaries

Abstract: A simple geometrical model is presented with the aim to connect measured distributions of orientation relationships between adjacent grains in single and dualphase brasses and the efficiency of grain and phase boundaries as dislocation obstacles. From the orientation relationships a slip transfer number is evaluated from the angle between the slip plane normals and the angle between the slip directions of the grain neighbors. Taking into account the different stacking fault energy and the different hardness of α-phase brass and β-phase brass and using reasonable limiting conditions for the above angles the result of the calculations is the same as obtained from the Hall-Petch analysis of the yield stress: phase boundaries are stronger dislocation obstacles than grain boundaries.

On the chemistry of grain boundary segregation and grain boundary fracture

Abstract: This paper considers the problem of impurity segregation in metals and the effect of these impurities on grain boundary cohesion. The primary goal of this paper is to provide a physical model that will allow us to think about these two processes. We describe both of them in chemical terms. Segregation is treated as a distribution of a solute between two phases. In this way, it is a typical example of heterogeneous equilibrium. We also consider the various driving forces for solute segregation and find that the correlation between decreased solubility and increased segregation, first proposed by Hondros and Seah,[9] is still an adequate one. We introduce the discussion of grain boundary fracture by pointing out that as the impurity enters the boundary, it establishes chemical bonds with the structural units of the boundary. The segregated boundary can then be thought of as a string of molecular units with bonds of different types. Some of these bonds will be weaker than others, and they will be the ones that eventually fracture when a stress is applied. We consider the cause of these weak bonds and suggest that the primary reason for them is the transfer of electronic charge from the metal atoms to the impurity, as proposed in previous work.[3] However, some of the ideas in the earlier models should be amended based on new results obtained from the quantum mechanical analysis of bonding in metals presented by McAdon and Goddard.[10,11] We also suggest that intergranular brittleness of intermetallic compounds such as Ni3Al, which occurs in the absence of impurity segregation, can be explained by the charge distribution present at the grain boundary. Finally, we provide a critique of other models that have been used to describe grain boundary fracture and segregation.

Application of the Hall‐Petch Relation to Microhardness Measurements on Al, Cu, Al‐MD 105, and Al‐Cu Alloys

Abstract: The hardness of materials, H, is dependent on grain diameter, d, in a similar way as the flow stress in the Hall-Petch relation: H = Ho + KHd−1/2, where, Ho and KH are constants. The microhardness of 2S-Al (99.5% Al), pure Cu, Al-MD 105 (Al-1% Al2O3), and Duralumin (Al-3.94% Cu) is found to vary with grain size according to the Hall-Petch equation with reasonable accuracy. The grain boundary hardening KH of Al-MD 105 is found to be the highest although this material recrystallizes to larger grain sizes than those for the other materials; this is attributed to the resistance of boundaries to deformation arising mainly from the presence of hard, second phase alumina particles. However, the contribution of solution, precipitation, and dispersion hardening may be added to grain boundary hardening according to Hansen and Lilholt.

Fracture toughness and hardness of zinc sulphide as a function of grain size

Abstract: The erosion properties of brittle materials depend upon plastic deformation and crack generation at an impact or indented site. Vickers indentations have been used to investigate the plastic processes and crack systems in chemical vapour deposited zinc sulphide of different grain sizes. The hardness,H, and the “local” fracture toughnessKc, are dependent upon the grain size of the material. For small grain size material (<50 Μm) the Vickers hardness was found to increase with decreasing grain size in accord with the Petch mechanism, i.e.H=H0 +kd−1/2 wherek andH0 are constants andd is the grain diameter. A maximum hardness of ca. 4 GPa has been observed for material with an average 0.5 Μm grain diameter. In large grain size material, hardness anisotropy within the grains causes significant experimental scatter in the hardness measurements because the plastic impression formed by the indenter (load 10 N and 100 N) is smaller than the grain diameter. The values ofKc obtained using an indentation technique show that for grain sizes less than 8 ΜmKc decreases with decreasing grain size. For materials with a grain size in the range 500 Μm to 8 Μm, well developed median cracks were not observed, however, the radius of the fracture zone was measured in order to estimate an “effective”Kc. The “effective”Kc was found to increase approximately linearly with the reciprocal root of the grain size. Consideration of the models for elastic/plastic impact and micromechanics of crack nucleation in conjuction with the variation ofKc andH, indicate that zinc sulphide with a mean grain size of 8 Μm will give the optimum solid particle and rain erosion resistance.

Structure of Alumina Grain Boundaries Prepared with and without a Thin Amorphous Intergranular Film

Abstract: The presence of a thin amorphous intergranular film along grain boundaries in alumina is expected to affect the properties of the interface and hence those of the material. In the present study, two types of grain boundaries have been formed in hot-pressed alumina bicrystals. In one case, the surfaces of the sintered crystals were kept as clean as possible, while in the other a thin layer of SiO2 was intentionally deposited onto the surface of one crystal. The distribution of SiO2 along the resulting grain boundary was then monitored by transmission electron microscopy and compared with the morphological features of the interface. In the special cases chosen here, the glass receded into large pores which grew into the alumina itself. However, the presence of the glassy phase during the early stages of sintering clearly did influence the characteristics of the resulting grain boundaries.

Special grain boundaries in high-Tc Bi2Sr2CaCu2O8+δ

Abstract: The structure of grain boundaries of highly dense Bi 2 Sr 2 CaCu 2 O 8+δ was investigated by high-resolution transmission electron microscopy. Special grain boundaries were observed for which the c -axes of adjacent grains were parallel to each other. Grains which are rotated with respect to each other by approximately 45° around the c -axis form coherent (001) grain boundaries. For (001) twin boundaries the rotation angle is 90° around the c -axis. For coherent (001) grain boundaries and twin boundaries, the interfaces lie between two adjacent (001) BiO layers. This indicates that the formation of (001) interfaces between these two layers is energetically most favourable due to the large bond lengths resulting in weak chemical bonds. Polytypoids, i.e. lamellae of Bi 2 Sr 2 Ca n −1 Cu n O 4+2 n +δ phases with ≠2, are frequently observed at twin boundaries and also occur at small angle tilt boundaries to accommodate for the mismatch. This yields compositional variations along the boundary. The dislocations in the boundary introduce either additional perovskite layers or Bi-O double layers. The latter dislocations occur less frequently and convert one lamella with n =3 or n =4 into two lamellae of n =1 phase. Incoherent grain boundaries show no evidence of intergranular phases. Thus, the weak-link behaviour of grain boundaries, which is observed in YBa 2 Cu 3 O 7− x might not occur in Bi 2 Sr 2 CaCu 2 O 8+δ ceramics.

Dynamic recrystallization and grain size

Abstract: The idea that information about the stress history of a geological sample can be extracted from measurements made of the microstructure in the laboratory is attractive. While not attempting to be a review of palaeopiezometers, this chapter will present evidence for a ‘universal relation’ between deformation stress and the steady-state recrystallized grain size resulting from stress-induced boundary migration. This law is, of course, derived from measurements from laboratory experiments. Any application to naturally deformed rocks and minerals should only be carried out if all possible sources of confusion and error are fully understood. In particular, problems could arise from the effects of stress changing with time, and from subsequent deformation in other stress and temperature regimes leading to static recrystallization. More lengthy discussions of these problems are presented elsewhere (Mercier et al. 1977, Poirier 1985).

Modeling matrix grain growth in the presence of growing second phase particles in two phase alloys

Abstract: In a number of two phase systems such as in α-β titanium alloys, second phase particles as well as the matrix grains grow simultaneously. A systematic study has been undertaken to understand matrix grain growth behavior of such materials in the presence of growing second-phase particles. Using the classical expressions of the driving force for grain growth and a modified form of the Zener retardation to account for preferential location of particles at triple junctions, grain growth of α and β matrices in the presence of growing second phase particles have been modeled by first order nonlinear differential equations. The contributions of grain boundary curvature, nonspherical nature of particles and nonorthogonal nature of particle grain boundary contact angles have also been incorporated into these models. Matrix grain sizes predicted by these methods were found to correlate well with the experimental results.

The strength and ductility of Ni3Si

Abstract: Experiments at room temperature on polycrystalline Ni 3 Si (with 23 at.% Si) have established that the addition of a small amount of boron (0.19 at.%) not only raises the ductility, as reported earlier by Oliver and White [ Mater. Res. Soc. Symp. Proc. 81 , 241 (1987)] and by Taub and Briant [ Acta metall. 35 , 1597 (1987)], but also lowers the effectiveness with which grain boundaries impede slip. Auger electron spectroscopy revealed that nickel cosegregates with boron to grain boundaries. The two mechanical effects are attributed to a boron-induced increase in the ease with which grain boundaries transmit slip. This increase, in turn, is attributed to constitutional disordering within the vicinity of the grain boundaries. In addition, the experiments suggest that the yield strenght, α y , is related to the grain size, d , through the expression: α y = α 0 + kd − n , where α 0 and k are constants and where 0.5 ⪕ n ⪕ 1.0 . This relationship is explained in terms of a dislocation interaction model of the yield strength.

Effect of grain size on superplastic behavior of Y-TZP

Abstract: This paper reports that grain size strongly influences the properties of fine-grained superplastic metal alloys. In principle, fine grain sizes enhance grain boundary sliding which is the dominant superplastic deformation mechanism. Recently, some fine-grained ceramics have been shown to be superplastic in tension. A maximum tensile elongation of about 800% has been recorded in a yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). During superplastic deformation, concurrent grain growth, and in particular dynamic grain growth, was observed in this material. It has been demonstrated that this concurrent grain growth strongly affects the superplastic flow stress in the material, and also confuses the accurate determination of the stress dependency of the superplastic strain rate. It is important, therefore, to understand the precise grain size dependence of superplastic flow in ceramics. It has been shown that the superplastic strain rate of Y-TZP is inversely proportional to the grain size raised to a 1.8 power. The effect of concurrent grain growth, however, was not considered in their analyses.

Chemically induced grain boundary migration and recrystallization in Al2O3

Abstract: The chemically induced grain boundary migration in Al2O3 has been observed during depletion of Fe2O3 from Al2O3Fe2O3 solid solution as well as additon of Fe2O3 in Al2O3. Many migrating boundaries are faceted. In some receding grains, the faceted planes of different grain boundaries advancing into the same grain are parallel to each other. The faceting is attributed to the anisotropy of driving force in thin diffusion layer of the receding grain; this result provides an experimental support for coherency strain energy as the major driving force for the migration. When the supply of the solute is excessive during alloying, recrystallization occurs in Al2O3 grains.

Transport-critical-current behaviour and grain boundary microstructure in YBa2Cu3Ox

Abstract: The microstructures of polycrystalline YBa2Cu3Ox samples were varied through different sintering mechanisms to study transport-current characteristics in magnetic fields. TEM experimental results indicate that the transport critical current in polycrystalline YBa2Cu3Ox is closely correlated with grain boundary microstructures. Specifically, the Jc(H) is severely suppressed as the grain boundaries are contaminated with the second phases. The field required for the Jc(H) values to decrease to zero is also strongly affected by the grain boundary microstructure. The critical-current behaviour and closely related microstructural changes are explained with Josephson weak-link and flux-flow models.

Dislocation Analysis of the Coupling of Grain Boundary Sliding and Migration during the Deformation of Zn Bicrystals

Abstract: The coupling of grain boundary sliding (GBS) and boundary migration (BM) is investigated during the deformation of Zn bicrystals with 30° 〈1010〉 symmetric tilt boundaries which are close to the special boundaries with ∑ = 15 An intensive and regular BM is observed during the development of GBS in Zn bicrystals with the 30° 〈1010〉 boundary, and BM is very weak in the case of deformation of Zn bicrystals with 86° 〈1010〉 boundaries Selected bicrystals have different directions of the crystallographic grain rotations relative to each other, which results in differences in the interaction between lattice dislocations (LDs) and grain boundaries (GBs) The nature of the coupled GBS–BM development is considered on the basis of their dislocation mechanism and the crystallography of the LD–GB interaction [Russian Text Ignored]

Strengthening of grain boundaries by segregated interstitials in iron

Abstract: The energy levels of the various physicochemical states of a hydrogen or carbon atom in iron are compared. Both types of atom segregate strongly to grain boundaries in a iron. The embedded atom the...

Critical current density and atomic structure of grain boundaries in high-Tcsuperconductors

Abstract: Geometrical modelling of grain boundaries is applied to determine the changes in their atomic structure as a result of atomic relaxation and reconstruction induced by deformation and diffusion processes. The energy associated with the primary and secondary coincidence tilt boundaries in the superconducting Y-Ba-Cu-O oxide is evaluated. The relaxed grain boundary configuration, wherein the surfaces coalesce to maintain continuity of the atomic planes, is obtained by minimization of the total energy of the boundary. The critical current density associated with the grain boundary is determined from the tunnelling behaviour of the superconducting pairs (electrons or holes) across internal boundaries. A quantitative model is presented to express the critical current density associated with the boundary as a function of the misorientation angle and these results are found to be in good agreement with available experimental data.