Effects of high-temperature ageing on the creep-rupture properties of high-tungsten cobalt-base superalloys

The effects of high-temperature ageing on creep-rupture properties were studied using cobalt-base superalloys containing about 14–20 wt% tungsten (W) at 1089 K (816 °C) and 1 311 K (1038 °C) in air. A high-temperature ageing for 1080 ks at 1273 K after solution treatment caused grain-boundary and matrix precipitates of W solid solution and carbide phases in these alloys, and grain boundaries were serrated especially in the alloys with higher W content. The high-temperature ageing largely improved the rupture life in the alloys with higher W content, particularly under lower stresses at 1089 K, whereas it caused the creep ductility to decrease a little in the alloy containing 20% W. The high-temperature ageing also improved the rupture life without decreasing creep ductility in these alloys under higher stresses at 1311 K. Under the same ageing conditions of 1080 ks at 1273 K, the initiation of grain-boundary cracks was retarded in the solution-treated and aged specimens, as well as in the aged specimens with serrated grain boundaries, for the alloys with higher W content at both 1089 and 1311 K. A large amount of grain-boundary serration also occurred in the non-aged specimens of the alloys with higher W content during creep at 1311 K, and contributed to the strengthening of the alloys. The solution-treated and aged specimen had almost the same rupture strength as the aged specimens with serrated grain boundaries in these cobalt-base alloys. The rupture strength of the solution-treated and aged specimens largely increased with increasing W content under the lower stresses at 1089 K and under the higher stresses at 1311 K. A ductile grain-boundary fracture surface, which was composed of dimples and grain-boundary ledges associated with grain-boundary precipitates, was observed in the solution-treated and aged specimens, as well as in the aged specimens with serrated grain boundaries at both 1089 and 1311 K. The fracture surface of the non-aged specimens was a brittle grain-boundary facet at 1089 K, but it became a ductile grain-boundary fracture surface, as serrated grain boundaries were formed owing to grain-boundary precipitates occurring during creep at 1311 K.     

Grain-boundary migration in single-phase and particle-containing materials

An understanding of the detailed mechanisms which control the migration of grain boundaries through multiphase materials is limited. In this article, the forces which may cause grain-boundary migration (such as those due to recrystallization, grain growth and a redistribution of chemical species) together with effects which oppose this motion (such as solute drag and particle pinning) are reviewed. Theories of grain-boundary migration are presented and, in particular, the influence of grain-boundary structure on these models is discussed. It is shown that experimental studies of grain-boundary migration in bicrystals offer the best opportunity of relating migration mechanisms to the grain-boundary structure. In the light of this an attempt is made to develop an experimental approach by which the interaction of a migrating boundary with a dispersion of particles might be studied.     

Grain-boundary slide-hardening in zinc bicrystals

Zinc bicrystals containing high-angle tilt boundaries, and others containing twist boundaries were subjected to constant shear stresses parallel to the boundary at temperatures in the range 20 to 300° C. At stresses both above and below that necessary to cause macroscopic slip in the constituent crystals, the grain-boundary sliding rate decreased with time. This slide-hardening at the grain-boundary was identified with the formation of asperities in the grain-boundary which were generated first near the intersection of boundary and free surface. Removal of the layer containing these asperities restored the boundary behaviour to that of a virgin bicrystal in certain cases. This leads to the conclusion that when slide-hardening occurs, the rate-limiting process for grain-boundary sliding is that associated with the deformation of the crystal regions which have been introduced into the original boundary plane by grain-boundary migration.     

Grain-boundary hardening and elastic recovery of micro-indentations

Variation in micro-hardness across a grain boundary, a well-documented effect found in many materials, is here confirmed for zone-refined iron containing 170 at. ppm of tin. Elastic recovery of the impression-diagonal was practically negligible but considerable recovery occurred in the direction of the indentation penetration depth. The latter is different for impressions located at grain boundaries compared with those located at grain interiors, when small loads are applied; however, this difference diminishes with increasing load. Excess hardening at grain boundaries also decreases as the load increases. The results are discussed in terms of the elastic-plastic properties of the material sampled by the impression, and evaluations of some of the parameters are presented. The correlation between grain-boundary hardening and elastic recovery of the impressions is also indicated.     

Grain boundary strength in non-cubic ceramic polycrystals with misfitting intragranular inclusions (nanocomposites)

The residual stress distribution in polycrystalline ceramics with thermal expansion anisotropy and misfitting intragranular dispersion is studied through micromechanical simulation. The effective grain boundary strength under remote tension is derived from the stability of a grain-boundary microcrack with thermal elastic residual stresses. This result is then applied to the strength and fracture properties of a two-phase nanocomposite (5 vol% SiC-Al₂O₃). The residual stresses from misfitting dispersion increase the effective grain boundary strength of the nanocomposite from 1.5 to 5 times more than that of the single-phase polycrystal, depending on the grain size of the matrix phase. The residual stresses reduce the instability range of microcrack precursors at grain junctions and increase the initial level of driving force for critical microcrack extension. Predicted strengthening of grain boundaries leads, in turn, to the superior inert strength of unnotched nanocomposite.     

Improvement of creep-rupture properties of wrought cobalt-based HS-21 alloys at high temperatures

The improvement of creep-rupture properties by serrated grain boundaries is investigated using wrought cobalt-based HS-21 alloys in the temperature range 816 to 1038° C (1500 to 1900°F). Serrated grain-boundaries are produced in the early stage of the grain-boundary reaction (GBR) by a heat treatment. Specimens with serrated grain boundaries have superior creep-rupture properties compared with those with normal straight grain boundaries. The rupture lives of specimens with serrated grain boundaries are more than twice as long as those of specimens with straight grain boundaries. The rupture elongation is considerably improved by serrated grain boundaries especially at lower temperatures. A ductile grain-boundary fracture is observed in specimens with serrated grain boundaries, while brittle grain boundary facets prevail in specimens with straight grain boundaries.     

Creep-rupture properties and grain-boundary sliding in wrought cobalt-base HS-21 alloys at high temperatures

The effects of serrated grain boundaries on the creep-rupture properties of wrought cobaltbase HS-21 alloys were investigated at 1311 and 1422 K. The amount of grain-boundary sliding and the initiation and growth of grain-boundary cracks were also examined during creep at 1311 K. Specimens with serrated grain boundaries exhibited longer rupture life and larger rupture ductility than those with straight grain boundaries, but these specimens had almost the same rupture life and rupture ductility under lower stresses at 1422 K, because serrated grain boundaries were also formed in specimens with originally straight grain boundaries. The average amount of grain-boundary sliding during creep at 1311 K increased with time (or with creep strain), but was almost the same in both specimens with serrated grain boundaries and those with straight grain boundaries at the same creep strain. Grain-boundary cracks or voids initiated in the early stage of creep in those specimens at 1311 K. Therefore, the strengthening by serrated grain boundaries at high temperatures above about 1311 K was attributed to the retardation of growth and linkage of grain-boundary cracks and voids.     

Wedge crack nucleation in Type 316 stainless steel

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.     

Retardation of grain boundary reactions in Ag-Pd-Cu alloys by additions of small amounts of other elements

The grain-boundary reactions in Ag-25 mass% Pd-10 mass% Cu alloys with 1 mass% aluminium, cobalt, tin, chromium or indium, were investigated by optical microscopy, electrical resistivity, and hardness tests. Addition of tin and chromium retarded grain-boundary reactions. The growth rate of nodules with tin addition was 1/18 of the alloy without additons. Grain-interior reactions are accelerated with chromium, tin and indium addition, with chromium the most effective. The activation energies for the grain-interior and grain-boundary reactions are 192 to 196 and 144 to 208 kJ mol^–1, respectively. Additions retard the growth but not the nucleation at the grain boundary. It is concluded that elements which form stable precipitates in the grains retard the grain-boundary reaction.     

An evaluation of the grain-boundary sliding contribution to creep deformation in polycrystalline alumina

The occurrence of grain-boundary sliding during creep in fine grained alumina was examined by inscribing marker lines on the tensile surfaces of specimens, prior to testing in four-point bending mode. There was considerable microstructural evidence for the occurrence of grainboundary sliding and grain rotation during creep deformation. Experimental measurements of the offsets in the marker lines at grain boundaries reveal that the grain-boundary sliding contribution to the total strain during creep deformation is 70 ± 6.2%. The extensive grain boundary sliding observed, together with the other mechanical properties, suggests that polycrystalline alumina exhibits superplastic characteristics. Several possible rate controlling mechanisms are examined critically in light of the present results and it is concluded that creep occurs either by an independent grain-boundary sliding mechanism or by an interface controlled diffusion mechanism.     

Interaction of Hydrogen Atoms with Grain Boundaries in Palladium Bicrystals

Hydrogen diffusion in palladium bicrystals containing a small-angle twist or tilt boundary or a large-angle boundary similar to a special boundary is investigated using molecular dynamics simulation. We assess the effect of grain boundaries on the hydrogen diffusion process. The types of grain boundaries considered here are shown to differ in their absorption activity for hydrogen. The temporal grain-boundary segregation of hydrogen atoms can be accounted for in terms of their coordination, which differs significantly from that in the grain bulk.     

Effects of grain boundary- and triple junction-character on intergranular fatigue crack nucleation in polycrystalline aluminum

The effects of grain boundary- and triple junction-character on intergranular fatigue crack nucleation were studied in coarse-grained polycrystalline aluminum specimens whose grain boundary microstructures were analyzed by SEM-EBSD/OIM technique. Fatigue crack nucleation occurred mainly along grain boundaries and depended strongly on both the grain boundary character and grain boundary configuration with respect to the persistent slip bands. However, it was little dependent on the geometrical arrangements between the grain boundary plane and the stress axis. Particularly, random boundaries become preferential sites for fatigue crack nucleation. The fatigue cracks were also observed at CSL boundaries when the grain-boundary trace on the specimen surface was parallel to persistent slip bands. On the other hand, no intergranular fatigue cracks were observed at low-angle boundaries. The fatigue cracks were observed at triple junctions as well as grain boundaries. Their nucleation considerably occurred at triple junctions where random boundaries were interconnected. The grain boundary engineering for improvement in fatigue property was discussed on the basis of the results of the structure-dependent intergranular and triple junction fatigue crack nucleation.     

Stress relaxation behavior of Cu/Sn/Cu micro-connect after electrical current

The effect of electromigration on stress relaxation behavior of pure tin solder joints was investigated. It was found that the stress relaxation rate was accelerated significantly after the sample was subjected to current stressing. The accelerating effect increased with the current stressing time. Measurements of the activation energy and stress exponent suggested that the dominant mechanism of the stress relaxation of pure tin solder joint went from dislocation climb to grain boundary diffusion after electromigration. As a result of grain boundary diffusion and sliding, grain boundary grooves were observed on the surface of the tin solder joints after electromigration. The groove was associated with the divergence of vacancy concentration at the grain boundaries. The vacancy concentration at the grain boundaries, which increased with the current stressing time, promoted the atomic diffusion along the grain boundaries, resulting in a higher stress relaxation rate.     

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.     

Non-equilibrium segregation of solutes to grain boundary

Mechanisms for the non-equilibrium segregation of solutes to static grain boundary during cooling (quenching-induced segregation) and to moving grain boundary during recrystallization (moving-induced segregation) are proposed. For quenching-induced segregation, in consideration of the local equilibrium among vacancies, solute atoms and vacancy-solute atom complexes, as well as the influence of equilibrium grain-boundary segregation, the theoretical dynamic formulae for this non-equilibrium segregation have been derived on the basis of the vacancy-dragging mechanism. Theoretical calculations have been carried out for the non-equilibrium segregation of boron to austenitic grain boundaries during isothermal holding and continuous cooling after heating at high temperature; the results agree well with those obtained from experiments. The model has also successfully explained the different behaviours of boron segregation during cooling in -Fe and in -Fe. For moving-induced segregation, based on the interaction between dislocations and the moving boundaries during recrystallization, a dislocation relaxation and widening grain-boundary mechanism of solute segregation on moving boundaries is proposed. Applying this model, we have calculated the boron segregation on moving boundaries during recrystallization in Fe-3% Si alloy; the results of these calculations agree with experimental results.     

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys were investigated at 1089 and 1311 K in air. The specimens with serrated grain boundaries and those with normal straight grain boundaries were aged for 1080ksec at 1273 or 1323 K to cause the matrix precipitates of tungsten-rich b c c phase and M₆C carbide. The creep-rupture strength of both specimens were improved by the high-temperature ageing. The rupture strength at 1311 K was the highest in the specimens with serrated grain boundaries aged at 1273 K, while the specimens with straight grain boundaries aged at 1273 K of the highest matrix hardness had the highest rupture strength at 1089 K. The high-temperature ageing did not decrease the rupture ductility of specimens. The ruptured specimens with serrated grain boundaries exhibited a ductile grain-boundary fracture surface which consisted of dimple patterns and steps, regardless of whether high-temperature ageing was carried out. The fracture mode of the specimens with straight grain boundaries was changed from the brittle grainboundary fracture to the ductile one similar to that of the specimens with serrated grain boundaries by high-temperature ageing, since large grain-boundary precipitates which gave nucleation sites of dimples were formed during the ageing. The grain-boundary cracks initiated in the early stage of creep (transient creep regime) in both non-aged and aged specimens of L-605 alloys in creep at 1089 and 1311 K, although the time to crack initiation is shorter in the specimens with straight grain boundaries than in those with serrated grain boundaries. Thus, the period of crack growth and linkage occupied most of the rupture life. The strengthening mechanisms of the aged specimens were also discussed.     

The fractal dimension of grain-boundary fracture in high-temperature creep of heat-resistant alloys

The fractal dimension of the grain-boundary fracture in high-temperature creep was estimated by the vertical section method on several creep-ruptured specimens of the cobalt-nickel- and iron-based heat-resistant alloys. Grain-boundary microcracks linked to the fracture surface were also taken into account in the present analysis by the box-counting method. In the specimens containing many grain-boundary microcracks linked to the fracture surface, the fractal dimension of the grain-boundary fracture was larger in the scale range of more than about one grain-boundary length than in the scale range less than this length. Thus, there was a cross-over in the fractal dimension of the grain-boundary fracture at about one grain-boundary length in these specimens. In the specimens containing much fewer microcracks, there was no clear cross-over in the fractal dimension of the grain-boundary fracture with regard to the scale of the analysis, irrespective of creep-ductility and grain-boundary configuration of the specimens. The fractal dimension of the grain-boundary fracture was generally larger in specimens with serrated grain boundaries than in specimens with straight grain boundaries in these heat-resistant alloys, because the fractal dimension of the grain boundary and the number of the grain-boundary microcracks were larger in the former specimen. The fractal dimension of the grain-boundary fracture did not tend to converge to unity when the scale of the analysis approached the specimen size. The inclusion of near-specimen size data with regard to the scale of the analysis did not affect the fractal dimension of the grain-boundary fracture in these alloys. Thus, the grain-boundary fracture in the creep-ruptured specimens exhibited a fractal nature, at least in the scale range below specimen size, although there was a cross-over in the fractal dimension of the grain-boundary fracture in specimens containing a large number of grain-boundary microcracks.     

Grain-boundary segregation in an Al-8 wt % Mg alloy

The grain-boundary segregation of Mg atoms in a high-purity Al-8wt% Mg alloy, water quenched from the solution heat-treatment temperature, has been investigated by energy dispersive X-ray microanalysis in a TEM/STEM electron microscope. Many grain boundaries showed a segregation of Mg atoms to a level 2 to 3 times higher than the alloy composition. In the as-quenched state, Mg was uniformly distributed along the grain boundaries, but formed clusters during ageing at room temperature. Hydrogen bubbles were often associated with these Mg-rich regions. Mg-depleted zones were observed adjacent to most boundaries, but in most cases on only one side of the boundary.     

Grain-boundary scattering in semiconductor films

Alternative technique for studying grain-boundary scattering phenomena in high-resistivity semiconducting films has been indicated. The combined effect of the presence of electric field and mechanical stress at the grain boundaries was considered. It is shown that the grain boundary potential, density of trap states, and carrier concentration of the films can be obtained by measuring reflectances of the films deposited on non-absorbing substrates.     

Electrical properties of the grain boundaries of oxygen ion conductors: Acceptor-doped zirconia and ceria

This article reviews the current understanding of the electrical properties of the grain boundaries of acceptor-doped zirconia and ceria, however, with an emphasis on the grain-boundary defect structure. From an electrical point of view, a grain boundary consists of a grain-boundary core and two adjacent space-charge layers. The grain-boundary cores of acceptor-doped zirconia and ceria are positively charged, probably owing to the oxygen vacancy enrichment there. Oxygen vacancies are therefore depleted in the space-charge layer. The grain-boundary conductivities of acceptor-doped zirconia and ceria are at least two orders of magnitude lower than the corresponding bulk values, depending on temperature and dopant level. Such a phenomenon is due to the facts: (1) that oxygen vacancies are severely depleted in the space-charge layer, and (2) that the grain-boundary impurity phase blocks the ionic transport across the grain boundaries by decreasing the conduction path width and constricting current lines. In materials of high purity, the effect of the space-charge depletion layer is dominant; however, in materials of normal purity, the effect of the grain-boundary impurity phase is dominant. A Schottky barrier model satisfactorily explains all the phenomenological observations of the grain-boundary electrical properties of materials of high purity, and experimental evidence soundly supports the model. Various factors (alumina addition and grain size) influencing the grain-boundary electrical properties are discussed, and some special aspects of nanocrystalline materials are highlighted.