Showing papers on "Grain boundary strengthening published in 1993"

Hall-Petch strengthening in nanocrystalline metals

Abstract: The tensile behavior and microhardness of nanocrystalline Cu and Pd were measured as a function of grain size In the case of Cu, an increase in strength with grain refinement continues down to the finest-grain material tested, whereas Pd shows little dependence of strength on grain size in the nanocrystalline regime Both nanocrystalline Cu and Pd are significantly stronger than conventional grain size material A literature search and experiments show that negative Hall-Petch slopes at ultrafine grain sizes observed in some studies are not associated with room temperature creep but rather with the use of a single sample annealed repeatedly to change the grain size

Grain boundary design and control for high temperature materials

Abstract: Recent experimental work has been reviewed concerning the effects of grain boundary structure on grain boundary sliding and migration, and other boundary phenomena involved in high temperature plasticity. A basic knowledge of the structure-dependent intergranular phenomena is important for a full understanding of the heterogeneity and characteristic features of high temperature deformation and fracture, because the structural effects of grain boundaries enhance the boundary-induced heterogeneity of deformation and fracture in polycrystals. The grain boundary character distribution (GBCD) has been shown to be important and useful in explaining and controlling high temperature plasticity, superplasticity and fragility in polycrystals. The potential use of grain boundary design and control for high temperature materials has been discussed on the basis of recent work concerning GBCD, texture and other microstructural factors. The recent achievement of grain boundary design and control for high temperature materials has been briefly introduced.

A new method for tensile testing of thin films

Abstract: A new method for tensile testing of thin films is presented. The strain is measured directly on the unsupported thin film from the displacement of laser spots diffracted from a thin grating applied to its surface by photolithography. The diffraction grating is two-dimensional, allowing strain measurement both along and transverse to the tensile direction. In principle, Young’s modulus, Poisson’s ratio, and the yield stress of a thin film can be determined. Cu, Ag, and Ni thin films with strong ⟨111⟩ texture were tested. The measured Young moduli agreed with those measured on bulk crystals, but the measured Poisson ratios were consistently low, most likely due to slight transverse folding of the film that developed during the test. The yield stresses of the evaporated Cu and Ag thin films agreed well with an extrapolation of the Hall-Petch relation found for bulk materials. Ni thin films are known to deviate from a bulk Ni Hall–Petch relation for submicron grain sizes, and sputtered Ni films show much higher yield stresses than electrodeposited or vapor-deposited films of similar grain size. Our sputtered Ni films had higher yield stresses than other sputtered films from the literature.

Model for the prediction of the mechanical behaviour of nanocrystalline materials

Abstract: A model is proposed in the present paper to account for the mechanical behaviour of nanocrystalline materials. In this model, the distribution of the grain size in nanocrystals is simulated with a logarithmic normal distribution, and one dislocation per grain is assumed. The plastic yielding for nanocrystalline materials is considered to be controlled by the stress required to attain dislocation loops (the Frank-Read source) in a set of larger grains with their critical semicircle configuration. The dislocations in the rest of the smaller grains are considered to be in the subcritical configuration, which produces a reversible deformation and only contributes to an inelastic deformation. The model presents a very good agreement with the σyvs. Dav−1/2 relationships for five nanocrystalline materials; of these, three metals exhibit a negative Hall-Petch slope and two a positive Hall-Petch slope. The model also predicts a decrease in Young's modulus with diminishing grain size, which is in agreement with experimental results for nanocrystalline copper and palladium.

Three-dimensional probabilistic simulation of solidification grain structures: Application to superalloy precision castings

Abstract: A two-dimensional (2-D) probabilistic model, previously developed for the prediction of microstructure formation in solidification processes, is applied to thin section superalloy precision castings. Based upon an assumption of uniform temperature across the section of the plate, the model takes into account the heterogeneous nucleation which might occur at the mold wall and in the bulk of the liquid. The location and crystallographic orientation of newly nucleated grains are chosen randomly among a large number of sites and equiprobable orientation classes, respectively. The growth of the dendritic grains is modeled by using a cellular automaton technique and by considering the growth kinetics of the dendrite tips. The computed 2-D grain structures are compared with micrographie cross sections of specimens of various thicknesses. It is shown that the 2-D approach is able to predict the transition from columnar to equiaxed grains. However, in a transverse section, the grain morphology within the columnar zone differs from that of the experimental micrographs. For this reason, a three-dimensional (3-D) extension of this model is proposed, in which the modeling of the grain growth is simplified. It assumes that each dendritic grain is an octaedron whose half-diagonals, corresponding to the crystallographic orientations of the grain, are simply given by the integral, from the time of nucleation to that of observation, of the velocity of the dendrite tips. All the liquid cells falling within a given octaedron solidify with the same crystallographic orientation of the parent nucleus. It is shown that the grain structures computed with this 3-D model are much closer to the experimental micrographie cross sections.

The dendritic growth of γ' precipitates and grain

Abstract: The growth pattern of γ precipitates in the grains and at the grain boundaries has been investigated in a Ni-24Co-4Al-4Ti-5Cr-5Mo (weight percent) alloy of very small lattice misfit between the precipitate and the matrix phases under varying heat-treatment conditions. When aged at temperatures lower than the solvus temperature (T s = 1150 °C) by more than 30 °C after direct cooling from the solution-treatment temperature, the nucleation density is high. In this condition, the supersaturation is quickly removed because of the overlapping diffusion fields and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres in the grains and truncated spheres at nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains, and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150 deg) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shapes. When aged at temperatures just belowT s (above 1140 °C), the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just belowT s.

Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3‐mol%‐Yttria‐Stabilized Tetragonal Zirconia Polycrystals (3Y‐TZP)

Abstract: Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y-TZP in order to determine their influence on the superplastic behavior between 1,200 and 1,300C and on the room-temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y-TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y-TZP with 5 wt% glass, but the room-temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y-TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y[sub 2]O[sub 3] and ZrO[sub 2], but the strain rate did not scale inversely with the viscosity of the silicae phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y-TZP with and without glassy grain boundary phases.

Theory of grain boundary motion in the presence of mobile particles

Abstract: A theory of grain boundary motion in the presence of mobile particles is put forward. It is shown that the boundary-particle-interaction leads to a hysteresis in the velocity-driving relationship. The extent of the hysteresis depends on particle mobility, which is very sensitive to particle size. The effect of particles is discussed for planar and curved boundaries as well as volume particle distributions. The theory accounts for a smaller limiting grain size during grain growth than predicted by Zener drag. The concept can be generalized to include all kinds of mobile obstacles for boundary migration. In such cases not the distribution of obstacle spacing rather the distribution of obstacle mobilities will control microstructure evolution.

Microstructures and hardness of ultrafine-grained Ni3Al

Abstract: The microstructural evolution of the ultrafine-grained intermetallic compound Ni 3 Al is studied as a function of annealing at different temperatures. The ultrafine microstructure is produced by a high plastic torsional straining. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to characterize the microstructural evolution and microhardness is used to determine mechanical behaviour. The as-deformed microstructure exhibits an almost fully disordered crystalline structure with coherent domain size of about 18 nm, a strong torsional texture and high internal elastic strains. On annealing the as-deformed samples at different temperatures, the recrystallization of the material into a granular type structure containing non-equilibrium grain boundaries is first observed. This is followed by the transformation from non-equilibrium grain boundaries with simultaneous grain growth. This transformation is correlated with an increase of hardness. A new concept of non-equilibrium grain boundaries transparency is presented to interpret this singular behaviour. The results are compared to those obtained on an ultrafine-grained Al-1.5% Mg alloy produced by the same technique and which exhibits the same mechanical behaviour.

An explanation to the abnormal hall-petch relation in nanocrystalline materials

Abstract: According to the experimental results of the structural characteristics and the energetic state of the interfaces in the nanocrystalline (NC) materials that a reduction of grain size in nm regime would result in a decrease of interfacial excess volume and interfacial excess energy, a qualitative model explaining the abnormal Hall-Petch relationship in NC samples is presented. It can be deduced from this model that a thermal annealing to the as-prepared NC sample will relax the NC interfaces, and consequently increase the normal-abnormal H-P transition grain size, which is confirmed by the experimental observations in the literature. From this analysis one can see that the densification of grain boundary in the NC materials, which plays an important role in the mechanical behaviors, should be considered in any attempt to improve the properties of NC materials.

Effect of grain size on yttrium grain boundary segregation in fine-grained alumina

Abstract: In this work, the effects of the mean grain size and of the total yttrium content on the distribution of this element in fine-grained alumina are studied. The yttrium distribution is determined in the TEM using energy dispersive X-ray analysis by measuring the mean grain boundary composition as well as the mean size and number of precipitates per unit volume. Two different types of microstructure are observed, depending on both the grain size and the total yttrium content: in the first, only grain boundary segregation is observed, whereas in the second, the grain boundaries are saturated with yttrium, resulting in the intergranular precipitation of an yttrium-rich second phase. For a given composition, the microstructure changes from the first kind of microstructure to the second kind during grain growth. Predictions using a simple geometrical model based on the mass conservation of yttrium are in good agreement with the experimental results for both types of microstructure. It is shown that the kinetics of grain growth are not significantly affected by the yttrium when it is used along with magnesia in alumina.

The role of grain boundaries in high temperature deformation

Abstract: The grain boundaries in polycrystalline materials often play a very significant role in the flow and fracture characteristics at elevated temperatures. These characteristics are reviewed by defining four distinct grain size ranges, termed macroscopic, mesoscopic, microscopic and nanoscopic respectively. The role of grain boundary sliding in the mesoscopic grain size range is examined and it is shown that sliding measurements may be analyzed to give a rate equation for the sliding process. Grain boundary sliding accounts for essentially all of the strain in metals with microscopic grain sizes in the superplastic region of flow, but in ceramic materials with microscopic grain sizes the presence of glassy phases at the boundaries is also important. Nanoscopic grain sizes provide the potential for attaining high superplastic ductilities but there are difficulties associated with achieving fully dense materials with grain sizes of the order of approximately 0.01 μm.

A theoretical study of grain growth in porous solids during sintering

Abstract: The processes of grain boundary migration, pore drag and pore/boundary separation are described on the basis of the phenomenological equations for boundary migration and surface diffusion. Cylindrical pores on triple grain junctions are assumed to represent the open porosity during intermediate-stage sintering. It is found that cylindrical pores can hardly detach from migrating boundaries. Three-dimensional closed pores, however, which predominate during final stage sintering, can separate from migrating grain junctions. The separation process is modelled numerically and the conditions for separation are formulated. Analytical approximations for the pore mobility are shown to describe the numerical results well. They serve to establish effective mobilities of grain boundaries bearing pores in various configurations. Classical theories of grain coarsening are modified by using these effective mobilities. Mechanical constitutive models of sintering contain the grain size as an internal variable. The present analysis leads to an evolution equation for the average grain size, which depends on the volume fraction of the pores and on their configuration.

On the yield stress of nanocrystals

Abstract: A generalization of the Hall-Petch relationship is proposed. The generalized relationship takes account of the contributions from intergrain sliding, generation of lattice dislocations, and influence of disclination-like defects. From this approach, a critical size corresponding to a maximum of the Hall-Petch size dependence is obtained. The value of the critical size essentially depends on the state of boundaries and it explains contradictory results for the microhardness of nanocrystals (NCs), since grain-boundary sliding is facilitated in unrelaxed nanocrystals and constrained in aged ones.

Processing of nano-grained materials

Abstract: Sintering and deformation were studied in nano-grained (n-)TiO 2 and n-TiAl as part of a program to develop materials for near-net shaping and superplasticity applications. An important concern for processing nano-grained materials is the control of grain growth during both densification and deformation. In this study, the effectiveness of doping n-TiO 2 with yttrium for controlling grain growth during isothermal annealing was examined. In addition, an empirical constitutive law for the densification of n-TiO 2 was determined. Comparison of sinter-forming in n-TiO 2 with larger grained oxide ceramics shows many similar features. The studies on TiAl examined the hardness as a function of grain size, indentation time and temperature. At large grain sizes, the hardness obeys the Hall-Petch relation, but below a critical grain size, approximately 30 nm, the hardness decreases with decreasing grain size. Finally, the potential for synthesizing metallic glasses with nanoscale amorphous particles is discussed.

Mobility control of ceramic grain boundaries and interfaces

Abstract: Grain boundary mobility and grain-liquid boundary mobility in ceramics vary vastly from material to material. Their characteristics are sensitive to the crystal structure, the nature of bonding, orientation, stoichiometry and composition. More directly, mobility can be lowered by decreasing the interfacial energy and anisotropy or increasing solute drag, liquid viscosity, particle pinning and grain interlocking. Judicious doping and scavenging emerge as especially effective methods for mobility control. Static and dynamic grain growth data of zirconia, alumina, and silicon nitride are cited to support the above proposition.

Extension of the Hall-Petch relation to two-ductile-phase alloys

Abstract: The Hall-Petch relation, developed originally for single-phase alloys, has been extended to alloys containing two ductile phases. The extended Hall-Petch relation can separate the contributions fro...

The effect of prior austenite grain size on the transformation behaviour of C-Mn-Ni weld metal

Abstract: The influence of prior austenite grain size on the transformation behaviour and microstructural development of C-Mn-Ni weld metals was investigated. It was found that increasing the grain size depressed the start temperature of grain-boundary ferrite and slightly increased the acicular ferrite start temperature. The microstructural products also changed from a boundary-dominated effect in the small grain sizes, to an intragranular-dominated effect in the large grain sizes, and at the same time, the morphology of the acicular ferrite was seen to change to a larger aspect ratio.

Grain Size-Microcracking Relation for NaZr2(PO4)3 Family Ceramics

Abstract: The grain size-microcracking relation was examined for low thermal expansion NaZr 2 (PO 4 ) 3 family ceramics. By measurements of the strength, Young's modulus, thermal expansion, and grain size of polycrystalline ceramics sintered at appropriate conditions, the critical grain size for microcracking was determined. The critical grain size was proportional to the inverse square of the maximum thermal expansion difference.

Evolution of pore microstructures during healing of grain boundaries in synthetic calcite rocks

Abstract: The morphologies of calcite grain boundaries were analyzed to provide insight into the evolution of pore networks in unfractured rock. Two synthetic calcite rocks were fabricated by hot isostatically pressing (HIP-ing) dried analytical-grade powders of pure CaCO3 and CaCO3 plus 5% Al2O3 at 600° C and 200 MPa confining pressure for 3 hours (HIP-1). Some samples were HIPed a second time at different temperatures and pressures to investigate the stability of the structures (HIP-2a-c). SEM and TEM were used to image both grain faces and grain boundary cross-sections. Structures on grain faces vary from open shallow basins with peripheral rims, to labyrinths of irregular ridges and channels, to isolated circular depressions. All of these structures are mirrored across the plane between grain faces. The grain size in both the single and two-phase samples increased markedly during HIP-1. Migrating boundaries either dragged pores along or broke away leaving grain interiors dotted with small voids. The structures present after HIP-1 were not stable but evolved considerably in a way dependent on the conditions of the HIP-2. Confining pressure had the most pronounced effect. With low confining pressure, the grain-boundary porosity evolved into isolated circular depressions but the total pore volume did not noticeably decrease. With high confining pressure, the pore volume virtually disappeared. The structures present after HIP-1 are strikingly similar to those that develop in intragranular cracks during healing. We infer that grain boundaries and intragranular cracks heal by similar processes. Decomposition, localized melting, impurities, and anisotropic surface energies played no evident role in forming the grain-boundary structures. The timing of the formation of the porosity and of the subsequent healing processes is more difficult to ascertain. Some structures appear to have evolved gradually throughout the constant, high temperature stage of HIPing. The most obvious structures, however, appear to have evolved on grain boundary cracks that opened during cooling.

Work softening and Hall-Petch hardening in extruded mechanically alloyed alloys

Abstract: Mechanically alloyed and extruded aluminum alloys developed for service at temperatures up to 500°C in aerospace applications exhibit work softening, as is typical for alloys with grain or subgrain sizes of about 1 μm and below. Experimental evidence is presented and compared with a recent theory of work softening based on the low energy dislocation structure (LEDS) concept. It is concluded that the observed work softening stems from a reduction in the τ 0 part of the flow stress, i.e. that part which does not depend on the density of trapped dislocations. In the present alloys, τ 0 is dominated by Hall-Petch grain boundary strengthening. Transmission electron microscopy evidence suggests that the softening arises, because plastic strain weakens what appears to be a continuous layer of “grain boundary substance” (perhaps carbon, Al 4 C 3 , or otherwise carbon-enriched substance) at which the boundaries are strongly anchored. With the boundaries thus immobilized, the relative lattice rotations among neighboring grains caused by the strain are accomodated by “geometrically necessary” dislocation rotation boundaries formed from reaction products of glide dislocations directly at the film surfaces. This is a new form of LEDS never previously reported. It contributes to, although does not necessarily dominate, the high recrystallization and service temperatures of the alloys.

Grain control in mechanically alloyed oxide dispersion strengthened MA 957 steel

Abstract: Mechanically alloyed oxide dispersion strengthened stainless steels tend to recrystallise into columnar grains, a microstructure ideal for certain creep applications. In other circumstances, equiaxed grain structures are desired. In this paper, two methods are described which have been developed to ensure the reproducible development of equiaxed or refined grain microstructures in an alloy, MA957, which has previously not been amenable to control. Grain refinement has been achieved by controlling the stored energy, so that grain boundary velocities are reduced to a level which allows nucleation to develop at many sites, and by inducing a phase transformation from ferrite to austenite.MST/1779

Theory of Coble creep for irregular grain structures

Abstract: Coble creep occurs by the diffusion of matter along grain boundaries. Under the action of an externally applied stress, matter diffuses from grain boundaries in compression to those in tension. Individual grains elongate and macroscopic creep ensues. To date, Coble creep has been derived only for simple periodic grain structures. In this work we show how to solve the Coble creep problem for irregular, two-dimensional grain structures consisting of straight grain boundary segments connected by triple points. Since the normal stress distributions at the grain boundaries are represented by cubic polynomials, 4ngb constants have to be determined for a two-dimensional grain structure consisting of ngb grain boundary segments. We show how the corresponding system of 4ngb equations can be set up for freely chosen boundary conditions at the grain boundary-surface intersections. Several calculated examples with clusters consisting of 25 grains illustrate the power of the technique.

Influence of granularity on the critical current density in melt-cast processed Bi2Sr2CaCu2Ox

Abstract: The microstructural analysis of melt-cast processed Bi2Sr2CaCu2Ox reveals the existence of grain boundaries on two different length scales. Plate-shaped crystal grains of approximately 10 mu m length are textured along the c direction on a scale of 200-300 mu m, forming bundles of arbitrary orientation. To investigate the impact of the grain boundaries within and between bundles on the critical current density the authors have analysed the variation of the magnetic hysteresis with the sample size using different powder fractions and bulk material. It could be shown that the boundaries between the platelets within a bundle limit the overall transport current density in bulk material, whereas the boundaries between bundles do not further reduce its value. The results are in agreement with a TEM analysis of the grain boundaries.

Nucleation fields and grain boundaries in hard magnetic materials

Abstract: Numerical micromagnetic calculations of the nucleation field of two-dimensional magnetic microstructures reveal the effects of interparticle interactions on the magnetization reversal mechanism. The results show that the magnetic behavior of interacting particles drastically changes with the grain size. For an average grain diameter D>100 nm, the particles of magnetic microstructures are strongly coupled by long range magnetostatic interactions. Therefore, the reduced nucleation field of misoriented grains determines the coercive field of large grained particle ensembles irrespective of the grain boundary type. In smaller grains long-range demagnetizing fields become less effective. Nonmagnetic boundary phases prevent the expansion of a reversed domain nucleus into the neighboring grains. If the average grain diameter approaches the domain wall width, short-range exchange interactions between neighboring grains increase the nucleation field. >