Showing papers on "Grain boundary strengthening published in 1977"

Theory and Applicability of a Recrystallized Grain Size Paleopiezometer

Abstract: An approximate theoretical relation is derived which relates stress during steady state creep to both subgrain size and dynamically recrystallized grain size. The relation results from equating the dislocation strain energy in the grain boundary to that in the enclosed volume. Available data on metals and silicates are in excellent agreement with the theory. For paleopiezometry, the recrystallized grain size must be preserved by quenching, by cooling under stress, or by inhibition of grain growth by intimate mixture of two or more phases. In general, stress may be underestimated using rocks in which grain size has been reduced by dynamic recrystallization, especially if the grain size is very small. Stress may be overestimated using coarse grained rocks in which the grain size has increased toward the steady state value. Quantitative limits remain to be established. The theoretical relation can in principle be applied to any metal or mineral if only the effective isotropic elastic moduli and the Burgers vector are known. When used as a paleopiezometer, the technique indicates that high stresses on the order of 100 MPa are not infrequently associated with mantle diapirism and with large scale thrust faulting. Consideration of the Mt. Albert ultramafic body suggests that texturally inferred stresses from peridotite massifs and from ultramafic xenoliths in alkali olivine basalts might reflect either horizontal variations in stress across a rising diapir or else a vertical variation in stress as defined by the pyroxene geobarometer (Mercier et al. 1977). In either case the stresses are probably characteristic of local diapirism. Stresses characteristic of global upper mantle flow might be inferred from xenoliths originating from above kimberlite-producing diapirs.

On the absorption of dislocations by grain boundaries

Abstract: A lattice dislocation may lower its elastic energy by dissociation in a high angle grain boundary. The absorption process involves the separation of the product grain boundary dislocations at a rate limited in general by climb and their interaction with any pre-existing network. This description of the absorption process is extended to random boundaries with the help of a new model for their structure in which the existence of an irregular arrangement of certain low energy groups of atoms is postulated. Experimental results are analysed in support of the model for the absorption process.

Metallurgical factors affecting the crack growth resistance of a superalloy

Abstract: During creep loading of IN-792, grain boundary morphology in conjunction with grain size strongly affected crack propagation. Compositional variations and fabrication techniques showed no significant effect. A primary requirement for materials to be used in gas turbine engine discs is satisfactory resistance to crack growth resistance in the 650 to 760°C range. Both conventional smooth and machine notched stress-rupture samples and dead weight loaded fatigue precracked fracture toughness specimens were evaluated in this study. Creep fractures took place by grain boundary cracking followed by rapid transgranular fractures. Composition variations had only very slight effects on crack propagation. Materials hot worked from castings had the same properties as those made by powder metallurgy techniques. The primary factors influencing the crack growth behavior were the grain size and grain shape. Increasing grain size markedly improved the toughness. By slow cooling through the gamma prime solvus a serrated grain boundary structure was developed that also improved the cracking resistance. Earlier creep fracture toughness studies have shown that the slow crack growth behavior can be described by a critical strain model in which the crack propagation is controlled by the yield strength, grain size, and a critical strain parameter. The present results are consistent with this model, with serrated grain boundaries introducing a four-fold increase in the critical strain parameter over that of smooth grained material.

Superplastic deformation of Ti-6Al-4V alloy

Abstract: The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium. Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight line when plotted as log (ekTd* 2/DgbGb3) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.

Periodic grain boundary structures in aluminium. II. A geometrical method for analysing periodic grain boundary structure and some related transmission electron microscope observations

Abstract: A crystallographic treatment is developed which clarifies the relation between the structure of a grain boundary and its location between relatively translated crystals. Characterization of line defects which can exist in grain boundaries is also facilitated by using this treatment, and the following topics are considered: (1) the computed structures in part I of this work; (2) steps at the cores of perfect grain boundary dislocations; (3) boundary structures related by c.s.l. symmetry; (4) partial grain boundary dislocations. Transmission electron microscope observations of topic 4 are presented.

A dislocation model of grain boundary electrical resistivity

Abstract: Grain boundary electrical resistivities are calculated on the assumption that the scattering is due primarily to the dislocations which constitute these boundaries. Dislocation densities in grain boundaries have been estimated using Frank's formula as modified to take account of lattice symmetry. Dislocation resistivities are calculated ab initio assuming resonance scattering at the Fermi surface, and measured dislocation resistivities are also used where these are available. The calculated grain boundary resistivities are in satisfactory agreement with experimental data for thirteen of the fourteen metals from seven subgroups of the periodic table, for which data is available.

The effect of relative crystal orientation on the liquid metal induced grain boundary fracture of aluminum bicrystals

Abstract: The effects of relative crystal orientation on liquid metal induced intergranular fracture were studied to: 1) verify the existence of the theoretically and experimentally predicted orientation dependence of this process; and 2) determine which crystallographically related property produced such dependence. A base metal of 99.993 pct pure aluminum and a liquid metal of Hg-3 at. pct Ga were chosen as the embrittlement couple. Bicrystals of aluminum were grown with symmetric tilt boundaries so that: 1) individual boundaries could be tested; and 2) the resulting embrittlement susceptibility of each boundary could be correlated with a controlled crystallographic variable, the tilt angle. A fracture mechanics testing method was developed which enabled the crack propagation resistance of each grain boundary in the Hg-3 at. pct Ga atmosphere to be determined by yielding the crack extension force of a propagating crack. The variation in crack extension force with symmetric tilt angle was analyzed to determine what crystallographically related property produced the observed variations. Consideration of the crystallographically related properties of: 1) slip compatibility between adjacent grains; 2) grain boundary atomic density; and 3) grain boundary energy led to the conclusion that the observed variations were caused by variations in grain boundary energy, and a mathematical relationship was developed that expressed the dependency.

Grain boundary diffusion creep in magnesium

Abstract: The creep of pure polycrystalline magnesium has been investigated at stresses beluw 6 MPa and temperatures from 150 to 325°C using both spring and tensile specimens. Examination of the grain size and temperature dependence of the creep rate revealed that, at the lower stresses, deformation occurred by the grain boundary diffusion creep mechanism of Coble, although the rate depended upon stress in a Bingham rather than a Newtonian manner, i.e. there was a threshold stress below which no diffusion creep occurred. At the lowest temperatures, threshold stresses ∼ 1 MPa were found in fine-grained material, decreasing to ∼0.1 MPa as the grain size and temperature were increased. Existing theories were unable to account for the magnitude of the threshold stress. Calculation of magnesium grain boundary self-diffusivity from the creep data gives a value of wDgb = 8 × 10−10 exp -(l05 × 103/RT) m3 S−1 where R is in J mol−1 K−l and w is the grain boundary width.

Stereological characterization of the interaction between interfaces and its application to the sintering process

Abstract: The degree or extent to which two different types of interfaces,e.g. grain boundaries and the void-solid interface in a sinter structure or grain boundaries and the particle-matrix interface in a dispersion hardened alloy, intersect, is an important factor in many materials. A method is presented here which allows the degree of this interaction to be quantitatively established from simple metallographic measurements made on a planar section through the structure. The results of the application of this technique to a number of copper sinter structures are presented. From these results some insight into the degree of interaction between the grain boundaries and the void-solid interface and its variation during the sintering process is obtained.

The nature of dislocations in high-angle grain boundaries

Abstract: The width of dislocations in grain boundaries is calculated by minimizing the interfacial energy of the boundary and the strain field energy of the dislocations. The calculations indicate that models, assuming equilibrium grain boundaries to consist of characteristic low energy structures with, if necessary, superimposed' arrays of localized misfit dislocations are of physical significance only for those boundaries whose energy depends strongly on the orientation relationship. For all other boundaries, dislocation models, although geometrically correct, seem to be of limited physical significance. Localized misfit dislocations are found to be unstable in these boundaries in the sense that their cores spread over the entire boundary area. The same applies to extrinsic grain boundary dislocations. They are found to be localized only in boundaries whose energy depends strongly on the mis-orientation. Measurements of the widths of misfit dislocations and observations on the width of extrinsic grain b...

Wedge crack nucleation in Type 316 stainless steel

Abstract: Type 316 austenitic steel has been heat-treated to produce a range of grain sizes and then creep-tested at 625° C at various stresses so as to examine the nucleation and the factors which effect the nucleation of grain-boundary triple point or “wedge” cracks. An internal marker technique was used to evaluate the extent of the grain-boundary sliding in relation to the total creep strain. Triple point crack nucleation occurred over the entire range of grain sizes and stresses examined when the product of the stress and grain-boundary displacement reached a critical value; the effective surface energy for grain boundary fracture, estimated using an expression derived by Stroh, was in approximate agreement with the surface free energy value indicating that only limited relaxation occurred by plastic deformation. The first cracks were observed to form along grain boundary facets perpendicular to the applied stress direction and with the sliding grain boundaries at high angles (60 to 80°) to the crack growth direction. Subsequent cracking occurred under conditions which deviated slightly from this initial condition, and the increase in crack density with strain was expressed in terms of geometrical factors which take account of the orientation effects.

Grain-boundary sliding controlled creep: its relevance to grain rolling and superplasticity

Abstract: A model of a polycrystal is developed which describes creep controlled by the rate of sliding on grain boundaries. Normal boundary stresses are assumed to relax rapidly by a mechanism which transfers material between boundaries to accommodate the sliding displacements. A self-consistent treatment of the shear stresses on the boundaries and the shears between grain centres only arises when the grains are allowed to roll or rotate. The calculated rates of rotation agree with rates observed during Stage II superplastic creep. Many features of the model tie in with the geometrical behaviour of grains deforming superplastically.

Mechanism of superplastic deformation in a magnesium alloy. II. The role of grain boundaries

Abstract: The change in the grain-boundary structure during superplastic deformation of a magnesium alloy (Mg + 1.5% Mn + 0.3% Ce) is investigated by diffraction electron microscopy. The presence of linear defects, extrinsic grain-boundary dislocations (EGBD), is established, whose density depends weakly upon the degree of extension but sharply increases with increasing strain rate. The scheme of the development of sliding according to which the interaction of lattice dislocations with grain boundaries increases their defectiveness, is proposed based on an analysis of the nature and behaviour of EGBD. In such non-equilibrium boundaries kinetic processes activate and grain-boundary sliding is facilitated. [Russian Text Ignored].

Beta brass bicrystal and tricrystal stress strain behavior

Abstract: Beta brass single crystal, bicrystal and tricrystal stress strain behavior were determined for a series of isoaxial specimens in which the bicrystals and tricrystals were incompatible in shear along the grain boundary. Volume fractions of grain boundary deformation were also determined to ascertain the effect of increased constraint on grain boundary strengthening. It was found that bicrystal flow stress was considerably higher than single crystal flow stress and that tricrystal strain hardening was only slightly larger than that of the bicrystal. For the tricrystals the grain boundary deformation zone is larger than that of corresponding bicrystals because of both configurational and constraint effects. This larger zone indicates that secondary stresses extend further from the boundary in tricrystals than in bicrystals. It was concluded that despite increased constraint, $$\bar \sigma _g b$$ , the average grain boundary stress for the tricrystals, is smaller than that for corresponding bicrystals and that because of the increased extent of the secondary stress,σ b i, the stress in the center component away from the grain boundary, is higher than that of the single crystal at corresponding strains. The relationship of these observations to corresponding stresses in polycrystalline material is examined.

The influence of grain size on the high cycle fatigue crack initiation of a metastable beta Ti alloy

Abstract: A study is made of crack initiation under high cycle fatigue loading for the metastable beta alloy Ti-8 Mo-8V-2Fe-3Al, focusing on the influence of beta grain size. It was shown that cracks initiate and grow in the beta grain boundary by a void formation and coalescence process. Comparison of S-N fatigue curves for materials of different reduction and grain size suggests that beta grain size is very influential in determining fatigue strength of metastable beta Ti alloys. (SDF)

Changes in grain boundary structure during the initial stages of recrystallization

Abstract: This paper shows that, during the earliest stages of a recrystallizing anneal, the extrinsic dislocation content of boundaries between deformed grains in an austenitic stainless steel falls to very low levels. This accommodation of extrinsic grain boundary dislocations into a high angle grain boundary during the initial stages of annealing is considered to be a process of boundary recrystallization. Further, it is suggested that a link exists between boundary recrystallization and the formation of nuclei for general recrystallization.

The influence of segregation on grain boundary cohesion

Abstract: Based upon a detailed thermodynamic analysis where a grain gives up one or several dilute solutes to the grain boundary, we have devised two models which adequately describe the energetics of grain boundary segregation induced intergranular fracture In the first model the cohesive energy of the precipitate is much smaller than the original material giving rise to easy separation of the boundary region In the second model a highly cohesive directionally bonded glassy segregate forms at the boundary The ternary glassy segregate bonds well with itself but not with the surrounding lattice giving rise to a weak interface between the grain boundary segregate and the lattice

On the formation of the diamond grain configuration during high temperature creep and fatigue

Abstract: A study has been made of the influence of test variables on the formation of the diamond grain configuration during high temperature creep and fatigue deformation of a wide variety of metals. The proposed mechanisms for the formation of this interesting grain morphology are reviewed. It is concluded that the diamond grain configuration arises from a balance between grain-boundary sliding, grain-boundary mobility, intragranular deformation and defect imbalance across the grain boundaries and that it tends to be stabilized by intergranular cavitation. While the phenomenon occurs during high temperature fatigue in a variety of metals irrespective of their crystal structure, during creep it has been observed only in to h c p metals. It is surmised that the occurrence of the diamond array of grain boundaries during creep deformation in h c p metals is aided by the limited number of slip systems which leads to high defect imbalances in adjacent grains and consequently high driving forces for grain-boundary migration. On the basis of quantitative metallography involving measurements of the number of edges per grain section, the number of grains meeting at vertices, angular distribution histograms and grain-boundary lengths in different angular orientations with respect to the stress axis in "annealed" and "diamond" microstructures, it is concluded that the shape of the "diamond" grain is essentially the same as that of the "annealed" grain but in a distorted form.