Showing papers on "Grain boundary strengthening published in 1995"

Fundamentals of grain and interphase boundary diffusion

Abstract: Analytical Models of Grain Boundary Diffusion. Diffusion Along Dislocations and Small-Angle Grain Boundaries. Grain Boundary Diffusion in Thin Films. Diffusion Along Migrating Grain Boundaries. Structural Effects on and Mechanisms of Grain Boundary Diffusion. Experimental Methods for Determination of Grain Boundary Diffusion Data. Index.

Effect of grain size on mechanical properties of nanocrystalline materials

Abstract: The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

Experimental study of the influence of stress, temperature, and strain on the dynamic recrystallization of Carrara marble

Abstract: Carrara marble has been deformed experimentally at temperatures ranging between 500° and 1000°C, at confining pressures of 200 and 300 MPa, and up to very large strains in extension and compression, in order to study the microstructural and mechanical property changes associated with dynamic recrystallization. Microstructural studies were made by optical microscopy on ultrathin sections, and automatic grain size measurement techniques were used. When the temperature is sufficiently high (≈ 600°C) and the stresses are high enough for deformation twinning to occur, twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size. At stress levels too low to activate twinning, recrystallization occurs in two stages: the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions, followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume. The recrystallization process requires larger prestrains at the lower temperatures. The size to which migration-recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms. Within the range of experimental observations, dynamically recrystallized grain size does not depend on strain rate or temperature. Separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry. No unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found. However, extrapolation of the experimental data to low, natural strain rates suggests that weakening following recrystallization may occur in nature.

On the Hall-Petch relationship and substructural evolution in type 316L stainless steel

Abstract: Tensile specimens of Type 316L stainless steel having grain sizes in the range 3.1–86.7 μm were deformed to 34% strain at temperatures 24, 400 and 700°C and strain rate 1 × 10−4s−1 to investigate the Hall-Petch (H-P) relationship, the nature of stress-strain curves and the substructure development. Upto ∼5% strain the H-P relationship exhibits bi-linearity whereas the single Hall-Petch relation is exhibited at larger strains. The presence of bi-linearity is explained by the back stress associated with the difference in the dislocation densities in the vicinity of grain boundary and in the grain interior. The log stress (σ)-log strain (e) plots depict three regimes and follow the relationship σ = Ken in each regime, but with varying magnitudes of the strength coefficient (K) and strain-hardening exponent (n).

Effects of microstructural variables on the deformation behaviour of dual-phase steel

Abstract: An expression for the stress of martensite in dual-phase steel was developed, which shows the interdependence of the stress of martensite and strain hardening in the ferrite matrix and the contribution of microstructural variables (the volume fraction of martensite fm, ferrite grain size df, and martensite particle size dm). The onset of plastic deformation of martensite in dual-phase steel was predicted to depend on its yield strength and the microstructural variables, and this was verified by the modified Crussard-Jaoul analysis. It was found that for this dual-phase steel, refining the grain size and increasing fm increase the flow stress and raise the strain hardening rate at low strains, but little affect the strain hardening rate at high strains. The effect of the ferrite grain size on the flow stress of this dual-phase steel was found to obey the Hall-Petch relation, i.e. σ = σ 0 e + K e d f − 1 2 , where the Hall-Petch intersection σ0e and slope Ke are functions of strain, fm and dm. The effects of the plastic deformation of martensite and the microstructural variables on the strain hardening rate and the Hall-Petch behaviour were analysed in terms of the densities of statistically stored dislocations and geometrically necessary dislocations using the previously developed theoretical model.

On the enhanced grain growth in ultrafine grained metals

Abstract: The static grain growth has been investigated in ultrafine grained copper with an initial grain size of 160 nm. It has been revealed that its kinetics follows to normal grain growth behaviour, but a grain growth starts at a relatively low temperature (0.32 T m ). Good fits with experimental data for several ultrafine grained metals have been obtained if the activation energy for grain boundary diffusion is assumed to be lower than for coarse grained materials, but increases during grain growth. It is suggested that this unusual behaviour of the activation energy is caused by the presence of non-equilibrium grain boundaries in ultrafine grained materials and their recovery during heating.

Effect of SiC Submicrometer Particle Size and Content on Fracture Toughness of Alumina–SiC “Nanocomposites”

Abstract: A simple model was developed which describes the effect of SiC submicrometer particles on the fracture toughness of alumina–SiC “nanocomposites.” This effect was attributed to the change in the fracture mode observed in alumina on adding SiC submicrometer particles, which was suggested to be a result of both matrix weakening and grain boundary strengthening. The model suggests that the increase in fracture toughness should be obtained only for small additions (less than 5 wt%) of SiC.

Microfracture and material removal in scratching of alumina

Abstract: A bonded-interface sectioning technique is used to examine subsurface damage modes and to identify mechanisms of material removal in repeated single-point scratching of alumina as a function of grain size, load, and number of passes. In the fine grain alumina, the lateral and median crack system is observed, together with intergranular microcracks and intragrain twin/slip bands distributed within the plastic zone. The distributed form of damage, namely twin/slip bands and intergranular microcracks, are also observed in the coarse grain alumina; but no evidence is found for well-defined median and lateral cracks in this material. The mechanism of material removal in alumina is identified as grain dislodgement resulting from grain boundary microcracking, irrespective of the grain size. Extension of lateral cracks is found to contribute to the material removal process only in the fine grain alumina scratched under a large load and after several passes. A model for the microfracture-controlled material removal process is proposed that relates the volume of material removed to the applied load and material properties including grain size, elastic modulus, hardness, and short-crack toughness. Removal rate is shown to be proportional to grain sizeI1/2 and to loadP2. The model and the experimental results obtained in scratching are used to describe the action of an individual abrasive grit in grinding and other abrasive machining processes.

The Nature and behavior of grain boundaries

Abstract: Structure of Grain Boundaries.- Structure of Grain Boundaries - Theoretical Determination and Experimental Observations.- On Grain Boundary Dislocation Contrast in the Electron Microscope.- Some Properties of the Disclination Structure of Grain Boundaries.- Coincidence and Near-Coincidence Grain Boundaries in HCP Metals.- Computer Simulation of Asymmetric Grain Boundaries and their Interaction with Vacancies and Carbon Impurity Atoms.- Energetics of Grain Boundaries.- Grain Boundary Phase Transformations.- Behavior of Grain Boundaries Near the Melting Point.- The Interaction of Migrating Liquid Inclusions With Grain Boundaries in Solids.- An Electron Microscope Study of Configurational Equilibrium at Twin-Grain Boundary Intersections in Fcc Metals.- Grain Boundary Curvatures in Annealed Beta Brass.- Grain Boundary Motion and Related Phenomena.- On the Theory of Grain Boundary Motion.- The Behavior of Grain Boundaries During Recrystallization of Dilute Aluminum-Gold Alloys.- Mechanisms of Electromigration Damage in Metallic Thin Films.- Solute Effects on Grain Boundary Electro-Migration and Diffusion.- Growth Selection in High-Purity Cadmium.- A Crystallographic Alternative to the Coincidence Relationships in Copper.- Influence of Solutes on the Mobility of Tilt Boundaries.

Effect of the Grain Boundary Thermal Expansion Coefficient on the Fracture Toughness in Silicon Nitride

Abstract: The effect of thermal expansion mismatch stress between silicon nitride and different grain boundary phases on the fracture toughness of silicon nitride was investigated. Different sintering aids in the Y-Mg-Si-Al-O-N system produced silicon nitride specimens with very similar microstructures but different grain boundary phase compositions and different values of fracture toughness. The fracture toughness of the silicon nitride increased as the thermal expansion coefficient of the grain boundary phase increased. The presence of tensile residual stress at the grain boundary caused by thermal expansion mismatch between the silicon nitride and the grain boundary phase enhanced crack deflection and grain bridging.

Anisotropic grain growth in TiO2-doped alumina

Abstract: Grain growth in TiO 2 -doped alumina was studied in a high density, ultrafine matrix (0.4 μm. Normal grain growth, anisotropic grain growth and abnormal grain growth were observed. With 0.15-0.4 wt.% TiO 2 , samples initially undergo normal grain growth until a crystal microstructure is attained and anisotropic grain growth in nucleated. Large anisotropic, platelet-shaped grains grow rapidly by a step growth process until impingement of the large grains essentially stops further growth. The volume fraction of anisotropic grains ranged from 20 to 100 vol.% suggesting that physical properties dependent on grain shape and volume fraction can be tailored. Critical requirements are proposed for the in situ growth of anisotropic grains.

Size-dependent solute segregation and total solubility in ultrafine polycrystals: Ca in TiO2

Abstract: Grain boundary segregation at ultrafine grain sizes has been studied. Using a STEM microanalysis technique to quantify the grain boundary coverage of calcium (0.34 mol%) in TiO2 ranging in grain size from 50 to 750 nm, it is found that below grain sizes of 150–350 nm, segregation deviates from conventional isotherms, exhibiting a clear size dependence. In this size regime the interfacial area to volume ratio is as important as temperature and composition in determining grain boundary coverage. In the present system, grain boundaries become saturated with calcium when the coverage reaches approximately one half of an equivalent monolayer. The experimental results can be modeled by a statistical thermodynamical treatment of segregation which takes into account the large density of grain boundary sites in this size range. We also find direct evidence of enhanced total solubility at very fine grain sizes due to grain boundary segregation.

Reliability of alumina ceramics: Effect of grain size

Abstract: The grain size dependence of fracture strength and Weibull modulus of alumina using a high purity, commercial starting powder was investigated. In the regime of an average grain size between 1.7 and 11 μm, fracture strength increases with decreasing grain size. This behaviour could be explained quantitatively by modelling the failure-causing defect as either a spherical pore or a hemispherical surface pit with a circumferential crack proportional to the average grain size. No dependence of Weibull modulus on average grain size and hence on R-curve behaviour could be observed. Theoretical considerations show that the closure stresses in alumina responsible for the R-curve behaviour are too low to affect the Weibull modulus.

Cooperative phenomena at grain boundaries during superplastic flow

Abstract: In situ observations in a scanning electron microscope (SEM) performed on different microstructural scales in Pb—62%Sn specimens, superplastically deformed in single shear and in simple tension, showed sliding of grain groups [cooperative grain boundary sliding (CGBS)], rotation of grain groups (cooperative grain rotation) and cooperative grain boundary migration (correlated migration of sliding grain boundaries). The observed macroscopic pattern of the CGBS surfaces is consistent with predictions of slip-lines field theory. The progress of the sliding of grain blocks at the scale of grain groups can be modeled in terms of cellular dislocations. The micromechanism for such sliding and the migration of sliding grain boundaries at the scale of the individual interface might be interpreted from the viewpoint of grain boundary dislocations.

Grain boundary structure and intergranular segregation in Al2O3

Abstract: The relationship between intergranular segregation and grain boundary (GB) crystallography was investigated by TEM/EDX on a Mg and Ti doped commercial-grade alumina. A large enrichment of titanium and impurities such as calcium and silicon occurs at nearly all grain boundaries. The well ordered twin boundaries do not contain segregation nor some grain boundaries whose plane is parallel to a dense plane of alumina for both grains. In grain boundaries with a high segregation level the nature and content of the major element strongly vary with GB crystallographic parameters, depending on the orientation of the grain boundary plane. HREM studies of grain boundary structure do not reveal any intergranular glassy phase in the material.

Acceleration of grain boundary motion in Al by small additions of Ga

Abstract: The temperature dependence of 〈111〉 tilt grain boundaries with angles of misorientation of 38·2° and 40·5° was investigated in bicrystals of both pure (99·999%) Al and the same Al doped with 10ppmGa in the temperature regime between 400 and 580°C. The grain boundary mobility for the investigated grain boundaries over the entire investigated temperature range is enhanced by minor additions of Ga. Both the activation enthalpy and the pre-exponential factor are affected by Ga doping. The orientation dependence of grain boundary mobility is strongly reduced but not completely removed. The drastic rise in grain boundary mobility by the addition of Ga to pure Al is interpreted as a consequence of a change in the boundary structure and the mechanism of boundary migration owing to a pre-wetting phase transition and formation of a liquid (or quasiliquid) Ga-rich layer on the grain boundary.

Microstructural Changes during Annealing of Work-Hardened Mechanically Milled Metallic Powders (Overview)

Abstract: The behavior of work-hardening which occurs during mechanical milling (MM) treatment in metallic powders, and the process of recovery and recrystallization which occurs during annealing in the MM powders were over-viewed showing the results obtained by the authors using an industrial pure iron powder. Through the MM treatment, metallic powders stores extremely large strain energy, and this results in the marked work-hardening and the formation of a fine structure with nanocrystalline grains. In the case of iron, the hardness of powder can be increased to DPH1024 in practice, and the crystalline grain size is to be reduced to the limiting value of 3.4 nm in principle. The polycrystallization of dislocation cells and subgrains also proceeds on the grain refining process. When the MM powders are annealed, the powders undergo different microstructural changes depending on the degree of work-hardening subjected by the prior MM treatment. In the case that powders are not work-hardened so much, usual recovery and recrystallization occur with raising annealing temperature. However, when powders are extremely work-hardened to the level where crystalline grain size nearly reaches the limiting value, only the process of normal grain growth occurs during annealing and this results in the softening of MM powders. Under the usual milling conditions, the degree of work-hardening of powders is in the middle stage, so that both of the above two processes are possible with overlapping each other. It was confirmed in both of MM iron powders and annealed iron powders that the relation between hardness and polycrystalline grain size gives a good fit to the Hall-Petch relationship in a wide grain size range up to 6 nm. In addition, some examples are introduced at the end, in order to excellent properties of materials produced from MM powders.

Wetting transition on grain boundaries in Al contacting with a Sn-rich melt

Abstract: The contact angle θ at the intersection of a grain boundary in Al bicrystals with the solid Al/liquid Al−Sn interphase boundary has been measured for two symmetric tilt {001} grain boundaries with tilt angles ϕ of 32° and 38.5°. The temperature dependencies θ(T) present the evidence of the grain boundary wetting phase transition at Tw. The observed hysteresis is consistent with the assumption that the wetting transition is of first order. The determined discontinuity in the temperature derivative of the grain boundary energy is−5.6 μJ/m2K (Tw1=617°C) for the boundary with a low energy (ϕ=38.5°) and −17 μJ/m2K (Tw2=604°C) for the grain boundary with a high energy (ϕ=32°).

Effect of grain size on the martensitic transformation in NiTi alloy

Abstract: The effect of grain size on the martensitic transformation in Ni42Ti shape memory alloy has been studied. The kinetics of grain growth has been evaluated and the influence of different grain sizes on the transformation temperatures and the thermodynamic magnitudes has been reported. Image analysis and flow calorimetry techniques have been used. The study shows that grain boundaries favour the martensitic transformation and at the same time obstruct retransformation. Enthalpy and entropy variations are independent of grain size, but elastic energy decreases with the grain size.

Ball milling of ductile metals

Abstract: Pure copper powder was employed to study the effects of ball milling on the development of the structure and properties of ductile metals. The results indicate that larger spheres with diameters of about 2-2.5 mm are created after 20 h of ball milling. The formation of such spheres is mainly due to sphere-to-flake or sphere-to-sphere welding. This welding is not complete, leaving large pores and curved voids in the spheres. The average grain size of such spheres is 10-100 nm. The increase in lattice strain is about 0.2%. The microhardness increases from 45 MPa (unmilled) to 220 MPa (milled for 20 h). High-resolution transmission electron microscopy (HRTEM) investigations show the following: (a) the deformation of ball-milled copper proceeds by [112](11 $$($) over bar 1) twinning or high-order twinning; (b) the [112](11 $$($) over bar 1) twins are thickened by passage of (a/6)[112] twinning partial dislocations; (c) subgrains lend to form in the twins. In addition to twinning, dislocation slip plays an important role in the deformation process; the mobility of 60 degrees dislocations and their pile-up in the crystals can lead to the formation of subgrains. Crystal refinement leads to an increase in the number of grain boundaries; both low-angle and high-angle grain boundaries with local strain and a high density of dislocations are observed. The estimated mean dislocation density is more than 10(14) m(-2), which is hardly ever reached in plastically deformed metals. The different kinds of structural defects which exist in the grain boundaries and within the crystals may result in increased strength and microhardness, increased free energy and changes in other properties of ball-milled materials.

Molecular dynamics simulations of densification processes in nanocrystalline materials

Abstract: Molecular dynamics computer simulations were employed to investigate the dynamic processes of diffusion, grain growth, sintering, and consolidation in nanocrystalline copper (n-Cu). At room temperature, fully dense n-Cu was found to be stable. At 1100 K, a large fraction of atoms in the n-Cu became amorphous which provides a new mechanism of grain growth and recrystallization. Atomic mobility in dense n-Cu decreased with time as the grain boundaries relaxed. The initial configurations of the grains were shown to have a strong influence on pressureless sintering. Both hydrostatic pressure and uniaxial stress loading accelerated the process of densification on porous n-Cu. The latter, however, was found more efficient due to grain boundary sliding.

The grain size dependence of ductile fracture toughness of polycrystalline metals and alloys

Abstract: A systematic assessment of the grain size dependence of ductile fracture toughness has been made using available experimental data from various metals and alloys. Based on this assessment the following semiempirical equation has been proposed: K IC =K IC °+k F d −1 where KICo and kF are experimental constants and d is the average grain diameter. The above equation has been rationalized by dislocation theory. It is proposed that after deformation at large plastic strain a polycrystalline material can be treated as a composite consisting of the grain interior and grain boundary zone. The fracture toughness KIC of such a composite can be expressed as K IC =K IC GI + 2t (KC IC GBZ −K IC GI )d −1 where KICGBZ and KICGI are the fracture toughness of the grain boundary zone and the grain interior, and t is the grain boundary zone thickness. The grain size dependence of ductile fracture toughness has also been discussed in terms of the influence of yield strength and strain to fracture as well as of the effect of deformation homogeneity across the grain. It is found that the effect of grain boundaries on KIC is complex; they can either toughen the polycrystal by enhancing the grain boundary deformation or cause embrittlement by promoting microvoid nucleation in the grain boundary zone.