Showing papers on "Grain boundary strengthening published in 1991"

The dependence of grain size on stress during dynamic recrystallisation

Abstract: The steady state grain size during dynamic recrystallisation by grain boundary migration is shown to be simply related to deformation stress for a number of metals and minerals. The data plotted on a scale of grain size (D) divided by Burger's vector (b) against stress (σ) divided by shear modulus (μ) fills in a remarkably narrow range bounded by loci of form σ μ ( D b 2/3 = K with K = 1 and 10. Models of the dynamic recrystallisation process are developed to show that such a relation can be predicted by considering a dynamic balance between the rate of formation of the deformation substructure and the mean velocity of recrystallising grain boundaries. Hence providing a physical basis for the empirical relation derived from the normalised plot of experimental results.

Grain-size strengthening in terms of dislocation density measured by resistivity

Abstract: The effect of grain size on flow stress has been investigated in terms of dislocation density. The measurement of dislocation density was made for nickel having a high stacking fault energy, by means of electrical resistivity with which the dislocation density can be measured up to larger strains compared with transmission electron microscopy. It was found that the dislocation density for a given strain in specimens deformed in tension at 77 and 295 K increases in a linear manner with the reciprocal of grain size. It was also ascertained that the flow stress is proportional to the square root of dislocation density, irrespective of grain size, deformation temperature and the amount of plastic strain (ϵ). From the above two relationships, an equation between flow stress and grain size was obtained in a general form, which gives the Hall-Petch relation as the limited case at yield point or at small strains.

Transport processes and noise in YBa2Cu3O7−δ grain boundary junctions

Abstract: In the high temperature superconductors the weak link nature of extended defects as grain boundaries is responsible for the deterioration of the superconducting transport properties. From the study of individual grain boundaries in thin-film bicrystals of YBa2Cu3O7−δ general relations describing the superconducting transport and noise characteristics of grain boundaries in the high temperature superconductors were found. Firstly, the critical current density Jgbc decreases as the misorientation angle between the adjoining grains increases. Secondly, the Jgbc ϱN products, where ϱN is the normal resistance times unit area of the grain boundaries, scale proportional to about (1/ϱN)q with q ranging between 1 and 1.5. Thirdly, all grain boundaries show large amounts of low frequency 1/f noise. The transport and noise characteristics can be explained by a junction model which is based on an insulating layer at the grain boundary interface containing a large number of localized defect states.

Compositional changes of minerals associated with dynamic recrystallizatin

Abstract: The rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously. The enhanced exchange rate may result from a reduction in grain size (shorter distance for volume diffusion), dissolution and growth of grains by diffusion creep (pressure solution), or the movement of high-angle grain boundaries through strained grains during recrystallization in the dislocation creep regime. The migration of high-angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization. The operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases (An1 and An79) at 900°C, 1 GPa confining pressure, and a strain rate of ∼2x10-6s-1. The polygonal, recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition. At the conditions of these experiments, the volume interdiffusion rate of NaSi/CaAl is too slow to produce any observable chemical change, and microstructural-chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine-grained (2–10 μm) aggregates. These results indicate that strain-induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars, pyroxenes, and amphiboles.

Microstructure and strength of nanocrystalline copper alloy prepared by mechanical alloying

Abstract: Polycrystalline materials having an ultrafine grain size may be prepared by mechanical alloying. Such a material has been prepared here with a copper matrix and a uniform dispersion of particles which stabilises the fine microstructure. It is shown that the grain size of the copper matrix may be explained in terms of the conventional models of boundary pinning by particles, even for grain sizes below 40 nm. For grain sizes larger than about 100 nm, material strength may be explained by dislocation-particle interactions as illustrated by TEM observations. For grain sizes below this limit, however, strengthening is not as great as dislocation theory would predict based on the distribution of particles in the material; in addition TEM observations show no indication of the presence of dislocations. A different deformation mechanism seems to control strengthening for these materials of nano-scale grain size.

Analysis of texture evolution in channel die compression. I, Effects of grain interaction

Abstract: The effect of grain interactions on deformation patterns and texture evolution is examined by the finite element method for a particular set of grains deformed in channel die compression. The material behavior within each element is determined from a slip based constitutive formulation that fully accounts for finite deformations and lattice rotations with deformation. Compatibility and equilibrium are enforced across the grain boundaries, and grain boundary sliding is not permitted. In order to make the analyses numerically tractable, an idealized two-dimensional geometric model is used to model the deformation in the longitudinal-transverse plane; and the strains are assumed to be uniform through the compression direction. Boundary conditions simulating homogeneous plane-strain compression are applied to the model region. The results reveal complex deformation patterns arising from grain interactions. The analyses also show that the crystallographic texture and the spread of orientations within a grain depend not only on the orientations of the neighboring grains, but also on the constraints provided by grains located several grains away.

The effect of grain size on the yield strength of FeAl and NiAl

Abstract: The yield strength, σy, has been measured as a function of grain size, d, for a number of FeAl and NiAl alloys of different aluminum contents and the data fitted to the Hall-Petch relationship, σy = σ0 + kd−12, where σ0 is the lattice resistance and k the Hall-Petch slope. For NiAl, both σ0 and k are a minimum at the stoichiometric composition and increase with decreasing aluminum content. For FeAl, σ0 and k are a maximum at the stoichiometric composition but show a minimum around the iron-rich composition Fe−45 at.% Al. The data do not indicate any relationship between k and the degree of (constitutional) disorder but suggest that k is simply proportional to σ0.

Experimental studies of grain boundary diffusion of hydrogen in metals

Abstract: Electrochemical permeation tests and silver decoration tests were performed in polycrystalline nickle with different grain sizes. Comparison of experimental results with theoretical predictions of a newly proposed grain boundary diffusion model indicates that grain boundaries are not high diffusivity paths for hydrogen. The sominant role played by grain boundaries during mass transport of hydrogen is retardation due to the trapping effect. The grain boundary diffusivity of hydrogen is concentration dependent. When the hydrogen concentration is extremely low, the grain boundary diffusion of hydrogen is virtually stopped. It is postulated that the low energy sites within grain boundaries are responsible for such retardation.

Grain growth enhancement in alumina during hot isostatic pressing

Abstract: HIP experiments carried out on pure alumina show that grain growth is enhanced both during and after the densification process. These results are compared with those published in the literature on several superplastic ceramics and metals, and a general phenomenological model for grain growth enhancement is proposed. This model is used to simulate grain size gradients generated during the HIP densification of a complex part. TEM examination of the samples shows that point defects and dislocation loops are responsible for grain growth enhancement.

Penetration of tin and zinc along tilt grain boundaries 43° [100] in Fe-5 at.% Si alloy: Premelting phase transition?

Abstract: Tin and zinc penetration along the tilt grain boundary 43° [100] in b.c.c. Fe-5 at.% Si alloy is studied in the temperature range from 652 to 975°C. Wetting transition of grain boundary by the tin-rich melt at Tw = 810 ± 5°C is observed. About Tw there is a thin wetting film at grain boundary. With zinc penetration along the grain boundary a wetting film has been observed at all temperatures studied. Behind that film there is a region with an unusually high diffusivity of zinc, and below that region there is a region of “ordinary” grain boundary diffusivity. Such a phenomenon may be explained in terms of the phase transition “grain boundary-thin wetting film on the boundary”, which is commonly known as a premelting phase transition. A model is proposed which explains the form of the temperature dependence of the concentration cBt, at which such transition occurs, and, in particular, the influence of the “paramagnet-ferromagnet” transition in the bulk on the premelting transition. The influence of the temperature dependence of the volume solubility limit, c0, on the cBt(T) dependence is also discussed. In critical region below Curie point Tc critical exponents d of magnetic part of activation free energy of bulk and grain boundary diffusion are calculated. Critical index d for grain boundary diffusion by premelting layer, as well as activation energy in paramagnetic region, lies in the interval between bulk values of d and estimation of d for truly two-dimensional grain boundary diffusion.

On the coarsening of precipitates located on grain boundaries and dislocations

Abstract: The Lifschitz-Slyozov-Wagner theory of particle coarsening is applied to the case in which all the precipitates lie on grain boundaries. Earlier studies of the grain boundary precipitate coarsening problem have assumed that all mass transfer is limited to the grain boundary region and have shown that in the long time limit the average particle size increases with time as t 1 4 . The present investigation considers diffusion of solute to occur both through the bulk material and along the grain boundary. Employing an asymptotic analysis due to Marqusee and Ross, one can show that the time dependent average particle size varies as t 1 3 and the coarsening rate constant depends on both the bulk and grain boundary diffusivities. The case of coarsening of precipitates lying on dislocations is also discussed and again cubic growth kinetics are found.

Ductile fracture toughness of polycrystalline armco iron of varying grain size

Abstract: The influence of polycrystalline grain size on the ductile fracture toughness (Jic) of Armco iron has been studied over the grain size range of 38–1050 μm. Experimental evidence for the various stages of ductile fracture during fracture toughness testing was obtained through scanning electron metallography. Jic decreases with coarsening of grain size and follows a parabolic relation with d−1/2. The critical stretch zone width in fracture mechanics specimens decreases with increasing grain size; at 1050 μm grain size, the stretch zone is not detectable and the material fails by cleavage fracture. The cleavage fracture behaviour at room temperature has been explained in terms of a stress concentration ahead of the crack tip that reaches the level of the cleavage fracture stress. The variation of fracture toughness Jic with grain size was studied in terms of the plastic zone size. Indirect methods based on Kjc and Pj fail to estimate the plastic zone size correctly. Microhardness measurements were found to be quite suitable for estimation of the plastic zone size. At finer grain sizes, the plastic zone size encompasses a substantial number of grains while at the coarsest grain size (1050 μm it is confined to a single grain, a feature that corresponds with the occurrence of cleavage fracture. The present measurements on Armco iron are shown to be consistent with the Jic data of Klasen et al. [Mater. Sci. Engng80, 25 (1986)] on microalloyed steels once one has accounted for inclusions and the higher carbon content.

Interaction between lattice dislocations and grain boundaries in f.c.c. and ordered compounds: A computer simulation

Abstract: The interaction of ½ screw and 60° dislocations with symmetric [110] tilt boundaries was investigated by atomistic simulations using many-body potentials representing a pure f.c.c. metal and ordered intermetallic compounds. The calculations were performed with and without an applied shear stress. The observations were: absorption into the grain boundary, attraction of a lattice Shockley partial dislocation towards the grain boundary and transmission through the grain boundary under the influence of a shear stress. It was found that the structural unit model may help to predict the interaction mechanism for long period boundaries and that the interaction in ordered compounds shows similarities to the interaction in f.c.c. metals. Some comparisons with experimental observations have been made.

Wetting and premelting phase transitions in 38° [100] tilt grain boundary in (Fe-12 at.% Si)-Zn alloy in the vicinity of the A2-B2 bulk ordering in Fe-12 at.% Si alloy

Abstract: Zinc penetration along grain boundary tilt of 38° [100] in bicrystals of the Fe-12 at.% Si alloy is studied by means of an electron microprobe analysis. At Tord = 770°C the alloy undergoes an A2-B2 ordering transition. At Tw = 749°C a wetting transition occurs in grain boundaries of the alloy. At temperature above Tw, the diffusion permeability of the grain boundaries changes abruptly at Cbt. This change may be explained, assuming a premelting transition to occur in grain boundaries. The Cbt (T) line on the phase diagram terminates by critical point at Tcrit = 807 ± 2°C. A protrusion can be seen on the Cbt (T) line and on the solvus line near Tord. The maximum on the temperature dependence of the diffusion permeability of grain boundaries corresponds to approximately the same temperature. The hypothesis of a premelting transition on grain boundaries provides possibility of explaining all the observed phenomena, in particular, the disappearance of wetting and premelting below the ordering temperature in the bulk.

Stability, microstructural evolution, grain growth, and coarsening in a two-dimensional two-phase microstructure

Abstract: The stability of certain microstructural features and how this affects grain growth and coarsening in a two-dimensional polycrystalline two-phase material with isotropic energies for the two grain boundaries and the interphase interface is examined using classical concepts A catalog of stable microstructural features depends on surface-energy ratios; it is shown to include a four-grain junction that is stable for a range of surface-energy ratios, and, for a given set of ratios, is stable over a range of angles Three-grain junctions and grain boundaries had been known to have a limited range of stability with respect to being “wet” by the other phase Inferences about grain growth, coarsening, and evolving morphology are obtained from the integrated curvature of the bounding surface of a grain, which depends on the number and phase of neighboring grains, as well as the order of their arrangement Under some conditions, a dispersion of second phase grains grain boundaries will tend to a uniform size; this second phase resists coarsening

Instability of Grain Boundary Grooves due to Equilibrium Grain Boundary Diffusion

Abstract: All previous theoretical investigations on grain boundary grooving are based on the assumption, that the grain boundary does not participate in the material transport. In the present paper it is shown theoretically and experimentally for the first time, that by removing this restriction one obtains completely different groove profiles in comparison to those predicted by the classical grooving theory. In couples of an Al-bicrystal contacting an In-A-melt and Cu-bicrystals in contact with a Bi Cu-melt instabilities of the grain boundary groove are observed. Deep, channel-like grooves at the grain boundary intersection with the solid-liquid interface appear after isothermal annealing. Diffusion of In into Al and Bi into Cu grain boundaries was detected. With a mathematical model it is shown, that the observed instabilities are result of the superposition of two processes: classical grooving due to curvature dependent morphological rearrangement of the grain boundary intersecting the solid-liquid interface and grain boundary diffusion of solute (In or Bi) occurring simultaneously. Profiles of the instabilities and the kinetics of the process are calculated.

Microwave heating of grain boundaries in ceramics

Abstract: It has been suggested that enhanced sintering of ceramics by microwave heating may be the result of accelerated grainboundary diffusion caused by the grain boundaries being heated to temperatures significantly greater than the bulkspecimen temperature. Heat flow calculations show that the temperature difference between the grain boundary and grain interior will be a small fraction of a degree, even under extreme conditions.

Effect of boron additions on the ductility and fracture behavior of Ni3Al single crystals

Abstract: The effect of B additions on the ductility and fracture behavior of Ni3Al single crystals has been investigated. Tensile tests at low and high temperatures were carried out in air at constant strain rate for two selected orientations. The results show a positive effect of B additions on both the ductility and the fracture stress at room temperature, but not at high temperatures. The largest improvement in ductility over that found for the pure material was found for additions of 0.2 at.% B. Scanning electron microscopy analysis of the fracture surfaces shows a combination of massive slip on {111} planes, some regions with cleavage-like appearance, and heavily dimpled areas. The improvement in ductility at room temperature has been explained in terms of the increased interfacial strength between the metal matrix and Ca-rich non-metallic inclusions on which the microvoids nucleate. The observations presented show that B additions improve the ductility of both polycrystalline and single crystalline Ni3Al. It is suggested that a “bulk effect” should be considered in addition to the grain boundary strengthening effect of B when explaining the improvement in ductility of polycrystalline Ni3Al due to B additions.

On the spreading of grain boundary dislocations and its effect on grain boundary properties

Abstract: The effect of dissociation of trapped lattice dislocations (TLDs) on the energy of grain boundaries (GBs) is calculated. Two possible effects are considered. The first is the change of energy of the GBs caused by the presence of TLDs cores. It was shown to be smaller than 5% of the grain boundary energy. The other, more important, is an increase of the energy of the strain fields connected with grain boundaries. Dissociation of TLDs causes an increase of incompatibility strains between plastically deformed grains. The energy of these strains is of the order the GB energy in equilibrium. The EGBDs strain fields provide a driving force for GB sliding and migration. In fact, EGBDs cannot leave the grain boundaries. Recovery of these dislocations can take place only by sliding and migration of grain boundaries.

Alternative length scales for polycrystalline materials. I, Microstructure evolution

Abstract: It is a well-documented experimental observation that properties of grain boundaries depend on the atomic structure of the boundary. Yet constitutive relations for properties of polycrystalline materials containing a variety of grain boundaries currently do not take account of this boundary-to-boundary variability. Instead, a single-length scale—the average grain diameter—is utilized with the underlying assumption that all grain boundaries are the same. In this (I) and the following paper (II), we extend the accepted viewpoint to encompass a binary classification of grain boundaries based on their misorientation angle. The resultant new length scale is that associated with clusters of grains linked by grain boundaries sharing misorientations in the same category. This first paper focuses on how a model polycrystal is generated, the energetics of the model, and its evolution under various external influences.

Modeling of grain boundary structures and properties in intermetallic compounds

Abstract: This paper reports that one of the main reasons for studies of grain boundaries in metallic materials is the intergranular brittleness. This has been a long standing problem in structural steels and other disordered alloys and it is now well established that this phenomenon is associated with segregation of certain embrittling elements to grain boundaries. Thus in pure metals and in disordered alloys grain boundaries are not intrinsically susceptible to brittle fracture. On the other hand, intermetallic compounds often fracture in an intergranular manner even when there is no appreciable segregation of impurities to the grain boundaries. The most prominent example is Ni{sub 3}Al, which crystallizes in the face-centered-cubic based L1{sub 2} structure. This material is fully ductile in the single crystal form although exhibiting an anomalous temperature dependence of the yield stress, but brittle in the polycrystalline form. The intergranular fracture occurs in this material without any appreciable segregation of impurities to the grain boundaries and, therefore, the grain boundary brittleness appears to be an intrinsic property of this compound. However, a number of ordered alloys with the same crystal structure, for example Cu{sub 3}Au, are ductile as polycrystals. Hence, the first outstanding question is what is the physicalmore » reason for the intrinsic brittleness of grain boundaries in the materials like Ni{sub 3}Al.« less

A creep rupture model accounting for cavitation at sliding grain boundaries

Abstract: An axisymmetric cell model analysis is used to study creep failure by grain boundary cavitation at facets normal to the maximum principal tensile stress, taking into account the influence of cavitation and sliding at adjacent inclined grain boundaries. It is found that the interaction between the failure processes on these two types of adjacent facets reduces the failure time significantly when cavitation is creep constrained. In all cases the time to cavity coalescence on transverse facets appears to be a useful lower bound measure of the material life-time. Sliding at the boundaries of the central grain of the cell model is accurately represented; but in some computations a stress enhancement factor is used to incorporate also the effect of sliding between surrounding grains. The influence of grain boundary viscosity is included in the model and it is found that even in the absence of sliding, cavitation on inclined boundaries may significantly reduce the failure time.

A study of grain boundary statistics in 304 and 316L stainless steels

Abstract: The distribution of grain boundaries by misorientation parameters in two austenitic stainless steels, AISI types 304 and 316L, having different stacking fault energies, have been studied by means of transmission electron microscopy. The results obtained show that grain boundary spectrum is characterized by the dominance of Σ3n boundaries and is quasi-stable in f.c.c. materials with low and medium stacking fault energy that are susceptible to annealing twinning. However, the distribution of the lengths of different grain boundary types in these materials is sensitive to the stacking fault energy. Only Σ3 boundaries have been found to be energetically favoured, while the observed preponderance of Σ9 and Σ27 is due to geometrical reasons: they are necessary for providing connection between Σ3 boundaries in the process of multiple twinning.

Computer simulation of the effect of grain size on the properties of polycrystalline specimens by finite element method

Abstract: On the basis of the data of single crystal deformation and the characteristics of polycrystal deformation, the deformation of a set of polycrystals with special orientations and special arrangement of grains were simulated in this paper to investigate the effect of grain size on the plastic deformation of grains and the stress distribution in polycrystals, particularly near grain boundary. The results obtained indicate that the relation of the true stress and grain size is according with the well known empirical relation σ = σ 0 (ϵ) + κ(ϵ)d − 1 2 and it is found that the results by computer are a complement of experimental data. The κ(ϵ) and σ0(ϵ) values, vary with the true strain. The polycrystal strengthening is mainly due to the contribution of the work hardening and the grain boundary rim (complex slip rim) which varies with gain size and true strain. The presence of grain boundary makes the stress near grain boundary markly up or down depending on orientation of grain and orientation difference between adjacent grains.