Showing papers on "Grain boundary strengthening published in 1994"

A unified approach to grain boundary sliding in creep and superplasticity

Abstract: Rachinger grain boundary sliding is a characteristic of high temperature deformation in both creep when the grain size is large ( d > λ ) and superplasticity when the grain size is small ( d λ ), where d and λ are the grain size and the subgrain size, respectively. An analytical procedure is used to determine the rate equation for Rachinger sliding when d > λ . Data for superplastic metals are examined to give the rate equation for Rachinger sliding when d λ . A unified model is developed for Rachinger sliding at all grain sizes, where the rate of sliding is controlled by the rate of accomodation through intragranular slip. It is demonstrated that the predictions of this model are in good agreement with experimental data under both creep and superplastic conditions.

Mechanical properties of nanophase metals

Abstract: Nanophase metals have grain-size dependent mechanical properties that are significantly different than those of their coarse-grained counterparts. Pure metals are much stronger and apparently less ductile than conventional ones; intermetallics are also strengthened, but they tend toward increased ductility at the smallest grain sizes. These property changes are primarily related to grain size limitations, but they are also affected by the large percentage of atoms in grain boundaries and other microstructural features. Strengthening appears to result from a limitation of dislocation activity, while increased ductility probably relates to grain boundary sliding. A brief overview of our present understanding of the mechanical properties of nanophase metals is presented.

An evaluation of the strain contributed by grain boundary sliding in superplasticity

Abstract: Grain boundary sliding is an important mode of deformation in superplasticity. Measurements of the contribution of sliding to the total strain generally give values of about 50–70% so that there is an apparent “missing strain” of about 30–50%. It is demonstrated that the experimental method used to measure the sliding contribution will lead to estimates in the range of about 45–90% even when all the deformation is by grain boundary sliding and the associated accommodation process. Since the problem of accommodation is less severe at the specimen surface, it is shown that estimates of the sliding contribution from surface marker lines will tend to lie at the lower end of this predicted range. It is concluded that grain boundary sliding accounts for essentially all the deformation under optimum superplastic conditions and there is no “missing strain” of about 30–50%.

The impact of grain boundary character distribution on fracture in polycrystals

Abstract: The importance of structural effects on intergranular fracture is discussed in order to understand, predict and control fracture in polycrystals. The heterogeneity of fracture in a polycrystal has been found to be due to the difference in structure-dependent propensity to intergranular fracture among grain boundaries. The grain boundary character distribution (GBCD) which describes the type and frequency of grain boundaries is shown to be one of important microstructural factors affecting fracture processes and characteristics. A recent prediction of GBCD-controlled toughness and brittle-ductile transition is introduced. The importance of the connectivity of grain boundaries, the so-called grain boundary correlation number, is also discussed. Recent successful achievement of the control of intergranular brittleness is presented as an application of the result of basic research on fracture to the control of intergranular brittleness by grain boundary design and control in advanced materials.

The study of grain size dependence of yield stress of copper for a wide grain size range

Abstract: Room temperature yield strength of copper has been measured by means of miniaturized disk-bend test as a function of grain size ranging from about 30 nm to 180 μm. It has been established that grain size dependence of strength does not obey the Hall-Petch relation in the entire range of grain sizes studied. The results obtained suggest that a gradual change of deformation mechanism takes place with decreasing grain size. Nanostructured samples appear to be rather ductile.

Effect of Pre-cold-working on Diffusional Reversion of Deformation Induced Martensite in Metastable Austenitic Stainless Steel

Abstract: Metastable austenitic stainless steels undergo deformation induced transformation to bcc martensitic structure during cold-working. The martensite induced reverts to austenite at a relatively low temperature and this leads to the formation of ultra fine austenite grains of less than 1 μm in diameter. In this paper, the effect of pre-cold-working on the morphology of reversed austenite was investigated by means of transmission electron microscopy and tensile test. The alloy used is an Fe-18.08%Cr-8.65%Ni alloy. Since this alloy has metastable austenitic structure at room temperature, it almost transforms to lath-martensite by 50% cold-rolling. Further cold-rolling above 50% deforms transformed martensite itself, and results in the formation of dislocation-cell structure instead of lath-martensitic structure. The diffusional reversion of deformation induced martensite takes place at around 900 K and precold-working to lath-martensite not only promotes the reversion but also gives a large effect on the microstructure of reversed austenite: Reversed arstenite is characterized, in a specimen with 50% pre-cold-working, by the stratum structure of austenite laths and blocks, which looks like a lath-martensitic structure, while in a specimen with heavy pre-cold-working, by the structure of fine equiaxed grains. On the discussion of grain boundary strengthening in the former case, 0.2% proof stress depends not on the lath size but on the block size of reversed austenite.

Grain Growth Kinetics in Alumina–Zirconia (CeZTA) Composites

Abstract: Grain growth kinetics in alumina-rich, zirconia-alumina composites have been examined as a function of volume fraction of zirconia. Both the microstructural scale and character alter during grain growth. Grain growth exponents between n = 3 and 4 were observed and have been rationalized in terms of current models for grain growth and particle coarsening. Grain growth constants were measured and compared with available data. Coupled grain growth was observed in that the ratio of the alumina-to-zirconia grain size was constant, for a given zirconia content. This behavior is discussed in terms of the topological constraint present during growth and the observed microstructural evolution. The measured activation energies were consistent with boundary-diffusion-controlled grain growth.

Calculation of electrical resistivity produced by dislocations and grain boundaries in metals

Abstract: The partial-wave method is used for the calculation of the residual electrical resistivity caused by dislocations and grain boundaries in various metals. The dislocation core is regarded as a resonance scattering line defect containing surplus free volume. Scattering by elastic distortions of the lattice is neglected. An estimation of the effective carrier concentration in transition metals is performed on the basis of Mott's theory. The proposed model for the evolution of the grain-boundary structure with misorientation angle regards the approach and confluence of the line defects forming the boundary, and results in a decrease of the resonance part of the total scattering cross section. The high-angle grain boundaries are assumed to consist of cylindrical voids. Taking into account the values of the steady Burgers' vectors, quite satisfactory agreement with the available experimental data is obtained.

Grain boundary structure, energy and strength in molybdenum

Abstract: Molybdenum has many attractive properties for high-temperature structural applications. However, its usefulness as a structural material is impaired by brittleness at and below ambient temperature, resulting from its intrinsically weak grain boundary strength. According to our previous study, the fracture strength in molybdenum depends markedly on the grain boundary character and, in particular, on the orientation relationship between the two adjacent crystals and the orientation of the grain boundary plane. However, our present knowledge is still far from a complete understanding. In this study, the relationship between the strength, grain boundary structure and grain boundary energy has been investigated for purified biccrystals of molybdenum with various symmetric tilt boundaries, by means of transmission electron microscopy and optical interferometry. The main results obtained are as follows. (1) There is a relatively good correlation between the fracture strength and the grain boundary energy. The energy cusps, for instance, are observed for (112) and (111) Σ3 coincidence boundaries, which are high in fracture strength, while the energy is higher for near (114) and (122) Σ9 boundaries, which are low in strength. (2) Σ1 small angle and near (112) Σ3 coincidence boundaries have a good coherence, which agrees well with the result of the boundary energy measurement. Grain boundary dislocations are observed and they can be described by the boundary dislocation model. (3) On the (114) Σ9 coincidence boundary with low fracture strength, the structure is poor in coherence compared with Σ1 and Σ3 boundaries. However, the grain boundary structure can also be described by the grain boundary dislocation (displacement shift complete (DSC) dislocation) model. (4) It is considered that the grain boundary structure in molybdenum with a high covalency in bonding is not greatly different from that in normal metals.

Cavitation and cavity-induced fracture during superplastic deformation

Abstract: The characteristics of fracture by cavitation in superplastic materials are reviewed. Particular attention is paid to the theoretical developmental aspects of cavity nucleation, cavity growth and cavity interlinkage. Various factors, including grain boundary sliding, impurity atoms or particles, phase proportion, deformation temperature, strain rate, strain and grain size, are discussed. Finally, methods for controlling cavitation during superplastic deformation are summarized, and problems which require further work are also presented.

Yttria-ceria stabilized tetragonal zirconia polycrystals: Sintering, grain growth and grain boundary segregation

Abstract: An analysis is presented of grain growth and densification of yttria-ceria stabilized tetragonal zirconia polycrystals (Y, Ce-TZPs) using both isothermal and non-isothermal techniques. The characteristics of Y, Ce-TZPs are compared to those of Y-TZP and Ce-TZP and the effect of increasing ceria concentration at constant yttria content is evaluated. During non-isothermal sintering two regimes are distinguished: below 900–1000°C the neck area increases strongly by surface diffusion accompanied by only very little densification and grain growth, in the temperature interval 900–1000°C to 1200°C the materials densify to 95% of the theoretical density via a grain boundary diffusion mechanism and grain growth accelerates. Dense materials with grain sizes of 0·15–0·20 μm can be prepared by isothermal sintering at 1100–1150°C. In Y, Ce-TZP it is yttrium that segregates to the grain boundaries at 1150–1400°C. The yttrium content of the grain boundaries in Y, Ce-TZP is independent of temperature and ceria-concentration under the investigated experimental conditions. Grain growth in dense TZP is controlled by a solute drag mechanism at elevated temperatures (>1200°C); this drag is highest for Y-TZP, absent for Ce-TZP and moderate for Y, Ce-TZP.

On the grain boundary statistics in metals and alloys susceptible to annealing twinning

Abstract: Grain boundary distribution which includes grain boundary character distribution (i.e. distribution of boundaries by the reciprocal density of coincidence sites Σ) as well as distributions of boundaries by misorientation angles and axes is an important parameter describing polycrystalline microstructure. Numerous experimental data on grain boundary distributions in low stacking fault energy f.c.c. materials that are susceptible to annealing twinning have been analyzed and it has been established that there is a certain stable grain boundary distribution characterized by the dominance of Σ3n boundaries in all statically recrystallized materials of this class. computer modelling based on the assumption that multiple twinning is the main process controlling structure formation has confirmed this conclusion. It has been also found that distribution of lengths of different types of grain boundaries is more sensitive to the stacking fault energy and treatment of the material. Relation between grain boundary distributions and grain orientation distribution has been studied both experimentally and by computer modelling. It has been established that the grain boundary distribution is not completely determined by texture but is only influenced by it, because the grain boundary spectrum is primarily dependent on the orientation correlations which may exist between various crystallites of a polycrystal. Control of grain boundary distributions by means of various treatments has been demonstrated and possibilities of grain boundary design for improving the bulk properties of polycrystalline materials are discussed.

On the microstructural aspects of the nonhomogeneity of superplastic deformation at the level of grain groups

Abstract: The surface relief of a superplasticity deformed magensium alloy (Mg-1.5% Mn-0.3%Ce) was studied. Zones/bands of localized deformation were detected by means of vacuum etching. Between the localized deformation bands were less-deformed regions. After 20% elongation in vacuum (1.33 × 10−3 Pa), zones of intensive vacuum etching were observed as two intersecting bands of localized deformation oriented at 35–60° to the tensile axis. Spacing between the localized deformation bands is 6–8 grain diameters after a tensile elongation of 20% and 3–4 grain diameters after an elongation of 160%. The observed bands of strain localization are explained from the viewpoint of cooperative grain boundary sliding, i.e. shifting of grain groups as a whole unit along grain boundary surfaces oriented close to the maximum shear stress direction. It is suggested that the cooperative nature of GBS be taken into account when evaluating the real local strain rate and the real strain-rate sensitivity of grain boundary sliding process.

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

Abstract: The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°–1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.

The effect of grain size and Fe:Co ratio on the room temperature yielding of FeCo

Abstract: Compression tests have been used to determine the Hall-Petch parameter and the ductility has been measured under tension in three FeCo alloys, Fe30Co, Fe50Co and Fe70Co (compositions given in atomic percent throughout) in both their ordered and disordered states. It was that disorder, either constitutional or thermal, decreased the Hall-Petch parameter and increased the ductility, results explained by the increased ease of slip transmission on disordering. Surface slip trace observations showed that all the alloys exhibited wavy slip in their disordered states whilst planar slip occurred in the ordered states. The compression tests also showed that Fe50Co had a higher yield strength in the disordered state than in the ordered state, irrespective of grain size, as noted by others. In contrast, irrespective of grain size, the yield strength of Fe70Co was much lower in the disordered state than in the ordered state. On the other hand, Fe30Co was stronger in the ordered state at fine grain sizes but weaker at coarse grain sizes. This behaviour was interpreted in terms of the short range order upon quenching from 800°C.

The double effect of grain size on the work hardening behaviour of polycrystalline copper

Abstract: Following the approach by Ashby, one can consider that strain compatibility between adjacent grains of a polycrystal generates geometrical dislocations. These dislocations participated in the strengthening mechanism in conjunction with statistically stored dislocations which are related to the single-crystal behavior. The dislocations of either species are indistinguishable and, as a whole, they may contribute to cell formation. The dislocation structure formed in a polycrystal is then a function of the major or minor intergranular accommodation complexity. At intermediate strain values the accommodation is distributed over the cells leading to a linear relationship between the tensile stress and the inverse of the cell size, whatever the grain size of the tested samples. The aim of the present work is to check that the presence of statistical and geometrical dislocations in the grains, as well as the fact that at the very early stage of plastic deformation the mean free path of dislocations is of the order of the grain size, leads to a double effect of the grain size on the work hardening behavior of polycrystalline copper. Moreover, careful analysis of the mechanical behavior of polycrystalline copper, including the microstructural aspects of plastic deformation, is performed, allowing the understanding of themore » relationship between the work hardening ratio and the grain size.« less

Densification and creep in the final stage of sintering

Abstract: Densification and creep by grain boundary diffusion is modelled for the late stage of sintering. The grain structure is described as a regular array of tetrakaidekahedra with pores at each grain boundary between next-nearest neighbors. The diffusion problem on the surface of the tetrakaidekahedron is solved numerically using the heat-conduction option of the finite element code ANSYS. From the calculated normal stresses on the boundary facets one assembles the macroscopic constitute behavior. Since the assumed grain shape is the Wigner-Seitz cell of the body-centered cubic lattice, the resulting viscosity tensor has cubic symmetry. Isotropic bulk and shear viscosities are obtained by applying the procedures developed for the elasticity theory of polycrystals. The resulting bulk viscosity is well approximated by a closed-form solution developed previously. Due to the pronounced cubic anisotropy of the model, the isotropic shear viscosity cannot be determined unambiguously. The model includes the effect of viscous grain boundary sliding. The influence of surface diffusion on the sintering rate is also explored.

The effect of grain boundary precipitates on the creep behavior of Inconel 718

Abstract: Inconel 718 has been heat treated to produce materials with microstructures that have the same distribution of γ″-strengthening precipitates within the grains but with grain boundaries that are either clean or have a nearly uniform distribution of 5 μ δ-Ni3Nb precipitates Creep tests on specimens with these two types of microstructures have been conducted in the temperature range of 600–650 °C at constant applied stresses in the range of 745–820 MPa It is observed that the presence of precipitates at grain boundaries increases the creep rate and its stress exponent It is suggested that the presence of precipitates on grain boundaries changes the creep mechanism from volume-diffusion controlled to grain-boundary controlled It is likely that the “constant structure” creep model proposed by Robinsonet al is applicable to the material with precipitates on the grain boundaries

Microstructural Mechanics Model of Anisotropic-Thermal-Expansion-Induced Microcracking

Abstract: Thermal-expansion-induced microcracking in single-phase ceramics has been simulated using a simple mechanics model based upon a regular lattice of brittle, elastic springs. Microcracks preferentially form at grain boundaries and propagate either into the bulk or along grain boundaries, depending on the toughness of the boundaries relative to the grain interiors. The present results show that anisotropic-thermal-expansion-induced microcracking can be more severe for either large or small grain size samples depending on the damage measure employed. At very small misfit strains, the large grain microstructure develops microcracks before the small grain microstructure. However, over most of the misfit strain regime examined, the total length/area of all cracks in a sample is larger when the grain size is small. This is manifested in a larger decrement of the elastic modulus in small grain size samples as compared with large grain size samples at the same misfit (AT). However, large grain sizes are more detrimental with regard to fracture properties. This is because the fracture stress scales as inversely with the crack length and large grain samples exhibit larger microcracks than small grain samples. Unlike in the unconstrained samples, when a sample is constrained during a temperature excursion, the stress created by the overall thermal expansion can directly lead to fracture of the entire sample. I. Introduction ANY ceramic materials are known to undergo spontaneM ous cracking when cooled from high processing temperatures. The presence of microcracks modifies several physical properties including thermal diffusivity, dielectric constant, acoustic transparency, and elastic moduli.' Microcracking can also lead to an increase in fracture toughness: presumably associated with the microcracking-induced dilatation and also partly due to the formation of a process zone ahead of a propagating rack.^,^ However, the contribution of microcracking to toughening is usually minor.' The tendency to form these cracks is known to increase with increasing temperature excursions and increasing grain size6 and is often attributable to residual stresses that develop from either thermal contraction anisotropy or non-shape-preserving phase transformations.' In multiphase

Effect of Boron on the Resistance to Secondary Working Embrittlement in Extra-low-C Cold-rolled Steel Sheet

Abstract: The mechanism for improving the resistance to the secondary working embrittlement by adding B to extra-low-C, Ti and Nb co-added interstitial-atom free steels with various P contents was studied by investigating the brittle-ductile transition temperature of drawn cups under various drawing ratios, and by evaluating the grain boundary segregation behavior of B and P. The transition temperature was lowered by adding B under all the conditions studied, while it was raised by employing a higher drawing ratio and with increasing P content. The effect of P content on the transition temperature was not affected by B content. By increasing the holding time at 1123 K, the transition temperature was significantly raised irrespective of the B content. It was proved by Auger electron spectroscopy that P atoms segregate to the grain boundary with or without B addition. The results obtained in this study support the mechanism that grain boundary strengthening by segregated B itself was predominant for improving the resistance to the secondary working embrittlement by B addition. However, it was also observed that P segregation was decreased with increasing the B addition. Therefore, it can be concluded that the improvement by B addition to the secondary working embrittlement was predominantly due to grain boundary strengthening by segregated B itself, and additionally by the site competition between B and P at the grain boundary.

TEM study of microstructures of longitudinal cross-section of Ag-clad Bi-based (2223) tapes

Abstract: TEM observation of the microstructure of longitudinal cross-sections of Ag-sheathed (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10+ y tapes has revealed that the grains adjacent to the Ag-sheath are nearly single 2223 phase with their c -axis perpendicular to the Ag-sheath, but in the interior of the superconductor core an intergrowth of the 2223 and 2212 polytypoids within the grain becomes evident. Small-angle grain boundaries and twist grain boundaries are predominant in the a−b direction, while the large-angle colony boundaries are formed by the intersection of several colonies. These high-angle boundaries are very clean and the lattices are very well matched. A thin amorphous layer is commonly found at the colony boundaries, which is expected to have a detrimental effect on critical current density if the transport current flows in the c -direction. The amorphous structures are also found at the triple colony junction. High density of stacking faults and twist grain boundaries is a characteristic structure on the longitudinal cross-section of the tape. [001] edge-type dislocations on the (001) planes and a small amount of mixed type of dislocation have been observed which may act as effective pinning centres. But linear and planar defects parallel to the c -axis were hardly found in the c -direction of the tapes.

Cobalt intergranular segregation in WC-Co composites

Abstract: Interfaces between carbide grains in the tungsten carbide-cobalt composite have been considered. Different techniques, such as transmission electron microscopy and energy dispersive X-ray analysis have been used to characterize the orientation relationship, the nature of the planes and the chemical composition of the grain boundaries.The cobalt concentration at WC-WC grain boundaries was determined by X-ray energy selective analysis in the TEM. Cobalt profiles were performed across low-angle grain boundaries, coincidence and general grain boundaries. Cobalt segregation was found whenever dislocations were imaged in the grain-boundary plane of a low-energy grain boundary. The segregation value was compared with the segregation ratio measured in special grain boundaries characterized by a coincidence site lattice.

A method to improve grain boundary current-carrying capability in melt-textured YBa2Cu3O7- delta

Abstract: A major impediment for many applications of bulk high-Tc superconductors is the extremely low current-carrying capability due to grain boundary weak links. A method has been developed to eliminate secondary phases and reduce microcracks at the grain boundaries in YBCO superconductor by removing a fraction of the liquid phase formed during melt texturing. Samples fabricated using this method consist of large domains containing several grains of oriented platelets with grain boundaries free of secondary phases and microcracks. The processing conditions have been tailored to obtain domains extending over a length of 15 to 20 mm, a width of 8 to 10 mm and a thickness of up to 5 mm. The grain boundary misorientation is characterized by optical microscopy as well as X-ray pole figure analysis. These grain boundaries, though having large misorientation angles up to 27.80 degrees , are found to carry high currents at 77 K, both in self-field and in an applied magnetic field of 1.5 T. These results give promise of the manufacture of polycrystalline YBCO superconductors with high current-carrying capability.