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.     

Application of a model for grain boundary sliding to high temperature flow of carrara marble

It has been demonstrated that grain boundary sliding may contribute up to 50 percent of the total strain during experimental, high temperature deformation of Carrara Marble (Schmid, Paterson and Boland, 1980), yet the creep behavior was characterized by a high stress exponent and an apparent thermal dependence related to volume diffusion of carbon in calcite. By adopting the model of Gifkins (1976, 1977) for dislocation accommodated grain boundary sliding, incorporating Nabarro's model of creep by climbing edge dislocations (Weertman, 1975) and using the experimentally determined relationship between stress and subgrain (recrystallized grain) size, a model is developed which fits the high temperature creep data very well. In effect, the model assumes that deformation occurs by a combination of climb of edge dislocations and dislocation accommodated grain boundary sliding. It is shown that the model can be easily and reasonably extended to include creep by climb-controlled dislocation glide.     

Quantitative evaluation of high temperature deformation mechanisms: a specific microgrid extensometry technique coupled with EBSD analysis

A microgrid extensometry method has been developed and used to obtain information about intragranular and intergranular creep mechanisms. An oxide grid was deposited on a creep specimen using an electron lithography technique. This oxide grid offers high backscattered electron contrast and can withstand long duration creep tests under vacuum in the 700–850 °C range without degradation. Specific methods were used to measure in-plane displacements at the grid nodes or at the grain boundaries using correlation of grid images taken before and after the creep test. The local strain and grain boundary sliding (GBS) data were then calculated. Combined information about grain boundary crystallography and GBS has been obtained by superimposing the electron backscattered diffraction (EBSD) map on the deformation maps. To illustrate the potential of this set of processes, two examples of application on a nickel-base disc superalloy are presented. The first one concerns the influence of the creep temperature on the local strain and the GBS. The second application quantitatively shows the influence of grain boundary character on GBS of this material.     

A technique for the measurement of diffusion creep from marker line displacements

A procedure is presented which allows the determination of the relative displacement of grains across a common grain boundary during diffusional creep deformation. This relative displacement comprises the strain produced by accretion of material at the grain boundary and by grain-boundary sliding. The only measurements necessary are of marker line displacements across the grain boundary.     

Effect of dopants on alumina grain boundary sliding: implications for creep inhibition

We investigate by means of periodic density functional theory the mechanism of grain boundary sliding along the α-alumina Σ11 tilt grain boundary. We identify minimum and maximum energy structures along a preferential sliding pathway for the pure grain boundary, as well as for grain boundaries doped with a series of early transition metals, as well as barium, gadolinium, and neodymium. We predict that the segregation of those dopants results in a considerable increase in the grain boundary sliding barrier. Grain boundary sliding occurs by a series of bond breaking and forming across the grain boundary. Our results suggest that the presence of large cations inhibits the regeneration of bonds during sliding, which results in a decrease in total number of bonds across the grain boundary interface, thereby raising the barrier to sliding. Trends in predicted grain boundary sliding energies are in good agreement with recently measured creep activation energies in polycrystalline alumina, lending further credence to the notion that grain boundary sliding plays a dominant role in alumina creep.     

Deformation mechanisms in an austenitic stainless steel (25Cr-20Ni) at elevated temperature

The creep behaviour at elevated temperature of an austenitic stainless steel (25Cr-20Ni), both with and without antimony additions, has been reanalysed. Formerly, the creep behaviour was interpreted by considering creep mechanisms based on diffusional (Coble) creep and threshold stresses. In the present paper, it is proposed that an alternative mechanism of grain boundary sliding, accommodated by slip in grain boundary mantle regions, can in fact be used to describe more accurately the creep behaviour. Quantitative predictions, based on phenomenological equations for describing creep controlled by grain boundary sliding, are made of the influences of grain size, stress and antimony addition on creep rates, and of the influence of grain size on the activation energy for creep of 25Cr-20Ni stainless steel. Comparison of these predictions with those based on creep models incorporating only diffusional flow are made. Furthermore, the existence of a threshold stress in creep of single-phase, massive materials is strongly questioned.     

Grain boundary sliding revisited: Developments in sliding over four decades

It is now recognized that grain boundary sliding (GBS) is often an important mode of deformation in polycrystalline materials. This paper reviews the developments in GBS over the last four decades including the procedures available for estimating the strain contributed by sliding to the total strain, ξ, and the division into Rachinger GBS in conventional creep and Lifshitz GBS in diffusion creep. It is shown that Rachinger GBS occurs under two distinct conditions in conventional creep depending upon whether the grain size, d, is larger or smaller than the equilibrium subgrain size, λ. A unified model is presented leading to separate rate equations for Rachinger GBS in power-law creep and superplasticity. It is demonstrated that these two equations are in excellent agreement with experimental observations. There are additional recent predictions, not fully resolved at the present time, concerning the role of GBS in nanostructured materials.     

The activation areas for grain boundary sliding

The activation areas for grain boundary sliding in Al, Pb, Sn, Zn, and Cu are compared with those for creep in the same materials. It is found that the activation area-stress relation for grain boundary sliding is similar to that for creep. This observation is consistent with a dislocation or ledge mechanism of grain boundary sliding.     

The role of grain boundary sliding and reinforcement morphology on the creep deformation behaviour of discontinuously reinforced composites

In this study, the role of grain boundary sliding behaviour on the creep deformation characteristics of discontinuously reinforced composites is investigated numerically together with the other influencing parameters: reinforcement aspect ratio, grain size and interfacial behaviour between the reinforcement and the matrix. The results obtained for the composites are compared with results obtained for a polycrystalline matrix material having identical grain size and morphology. The results indicate that, with sliding grain boundaries, the stress enhancement factor for the composites is much higher than the one observed for the matrix material and its value increases with increasing reinforcement aspect ratio, reduction in the matrix grain size and sliding interfacial behaviour between the reinforcement and the matrix. In the composites, the contribution of the grain boundary sliding to overall steady state creep rates occurs in a larger stress range in comparison to the matrix material. Experimentally observed higher creep exponent values or stress dependent creep exponent values for the composites could not be explained solely by the mechanism of grain boundary sliding. However, experimentally observed large scale triple point grain boundary cavitation in the composites could occur due to large grain rotations resulting from grain boundary sliding.     

Transient Creep Associated with Grain Boundary Sliding in Fine-Grained Single-Phase Al2O3

Transient creep data for high-purity polycrystalline alumina are examined at the testing temperature of 1150–1250 °C. The data are analysed in terms of the effect of stress and temperature on the extent of transient time and strain.In order to explain the observed transient creep, a time function of creep strain is proposed from a two-dimensional model based on grain boundary sliding. The grain boundary sliding is assumed to take place by the glide of grain boundary dislocations accommodated by dislocation climb in the neighboring grain boundaries. The time function for a creep strain obtained from the model is given in a form

The change of grain boundary internal friction peak during high temperature deformation at different modes

The change of grain boundary internal friction peak during high temperature deformation at different modes (i.e., static tensile creep, cyclic tensile creep, and cyclic reverse torsion) in high purity aluminum was studied in conjunction with microstructure examinations. It was observed that the internal friction peak decreased with increasing plastic strain for all the modes, indicating that grain boundary sliding was degraded by the deformation. Nevertheless, at the same strain the decrement of internal friction was different for different modes, in particular smallest for reverse torsion. The origin of the decrease of internal friction and the difference among different modes is interpreted in the light of microstructure observations.     

Continuous creep cavity nucleation by stochastic grain-boundary sliding

Creep cavitation in metals and ceramics is generally considered to occur by the nucleation, growth, and coalescence of grain-boundary cavities. By considering grain-boundary slidings as the process driving force, a stochastic model is proposed for continuous cavity nucleation in metals and ceramics subjected to creep loading. The nucleation rate is shown to be directly proportional to the number of grain-boundary sliding events. The dependence of the number of cavities on grain boundary sliding displacement, creep strain, and time are established and compared with available experimental data of alumina, copper, and copper alloys. This comparison supports the contention that creep cavity nucleation in metals and ceramics does originate from stochastic grain-boundary sliding.     

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.     

On Grain Size Dependence of Minimum Creep Rate

The available experimental results have beensummarized concerning the effect of grain size onminimum creep rate.There are two types of creeprate-grain size relations.One is that there is a criti-cal grain size above which creep rate is independentof grain size,below which creep rate increases withthe decrease of grain size.The other is that there isan intermediate grain size at which creep resistanceis optimum.The first relation usually occurs athigher temperatures(>0.5 T_m),and intermediatestress ranges,while the second relation at interme-diate temperature ranges(0.4-0.5 T_m)and higherstresses.For the two types of creep rate-grain sizerelations,the increase of the creep rates with the de-crease of grain size for small grain sizes is all due tograin boundary sliding.For large grain sizes,a dis-location climb mechanism is dominant in creepdeformation for the first relation,while aHall-Perch grain boundary strengthening effect isbelieved to play an important role by dislocationglide mechanism for the second relation.     

The effect of the diffusion creep behavior on the TSV-Cu protrusion morphology during annealing

As through-silicon vias (TSVs) are key structural elements of 3D integration and packaging, creep deformation, which causes TSV-Cu protrusion, is critical for TSV reliability. Here, the effect of the diffusion creep behavior on the TSV-Cu protrusion morphology is analyzed using experiment and simulation. The protrusion morphology of TSV-Cu after annealing treatment is examined using a white light interferometer. The diffusion creep mechanism of TSV-Cu is determined by observation of the TSV-Cu microstructure using a scanning electron microscopy and a focused ion beams. The TSV-Cu grain size is measured using an electron backscatter diffraction system. The diffusion creep rate model of TSV-Cu is deduced based on the energy balance theory and is introduced into the finite element model to clarify the influence of diffusion creep on TSV-Cu protrusion. It is determined that the diffusion creep of TSV-Cu is mainly caused by grain boundary diffusion and grain boundary sliding. The diffusion creep strain rate is positively correlated with the ambient temperature and the external load but negatively correlated with the grain size. The amount of TSV-Cu protrusion increases with decreasing grain size. The simulation results show that the “donut”-shaped protrusion morphology is more likely to occur in TSV-Cu with smaller grain sizes near the sidewall region of the via.     

Creep in pure and two phase nickel-doped alumina

Creep in pure and two phase nickel-doped alumina has been investigated in the stress range 0.70 to 4.57 kgf mm^–2 (1000 to 6500 psi), and temperature range 1450 to 1800° C, for grain sizes from 15 to 45 m (pure alumina) and 15 to 30 um, (nickel-doped alumina). The effect of stress, grain size and temperature on the creep rate suggests that diffusion controlled grain-boundary sliding is the predominant creep mechanism at low stresses and small grain sizes. However, the stress exponents show that some non-viscous boundary sliding occurs even at the lowest stresses investigated. This mechanism is confirmed by metallographic evidence, which shows considerable boundary corrugation in the deformed aluminas. At higher stresses and larger grain sizes the localized propagation of microcracks leads to high stress exponents in the creep rate equation. The nickel dopant, which introduces an evenly distributed spinel second phase into the alumina matrix, increases the creep rate and enhances boundary sliding and localized crack propagation. The weakening effect of the second phase increases with grain size, and tertiary creep occurs at strains of 0.5% and below in large grained material.     

Creep in polycrystalline aluminium

Abstract

The present paper reports a study of creep in large grained polycrystalline aluminium. Two techniques were employed to understand this process in more detail. The first was stereoimaging to measure the local strain in the specimen. This measurement was made by photographing the specimen at various times during the creep test, which was carried out in a scanning electron microscope, and using these images to calculate the strain. The second technique was analysis of electron backscattering diffraction patterns, as generated in a scanning electron microscope. This technique allowed examination of changes in crystallography that accompanied the creep process. The results of the experiments showed that strain built up in different grains at different rates. There was never a discontinuity in strain across the grain boundary, and strain relaxation was observed in different grains at different times during the test. Recrystallisation was also observed to occur. In some cases, an existing grain migrated into another grain, presumably through strain induced grain boundary migration. In other cases, there appeared to be nucleation of a new grain with a different orientation.     

超塑性Y-TZP的压缩塑性形变

通过恒定横梁速度和恒定载荷压缩试验,对超塑性3mol％Y₂O₃稳定四方ZrO₂多晶体的压缩塑性形变进行了研究．测定了平均晶粒尺寸从0．30～1．33μm的3Y－TZP材料的塑性流动应力,应力指数和蠕交活化能;用扫描和透射电镜观察了试样的显微结构．结果表明,3Y－TZP材料塑性形变的机理为扩散适应的晶界滑移．随着晶粒尺寸由0．30μm增大至1.33μm,应力指数从3．2减小至1．4,活化能从580kJ／mol减小至500kJ／mol．形变机理随晶粒大小发生变化．对于晶粒较粗的3Y－TZP材料,当应变速率较高时,形变过程中在材料内产生晶间孔穴．     

A creep rupture model accounting for cavitation at sliding grain boundaries

An axisymmetric cell model analysis is used to study creep failure by grain boundary cavitation at facets normal to the maximum principal tensile stress, taking into account the influence of cavitation and sliding at adjacent inclined grain boundaries. It is found that the interaction between the failure processes on these two types of adjacent facets reduces the failure time significantly when cavitation is creep constrained. In all cases the time to cavity coalescence on transverse facets appears to be a useful lower bound measure of the material life-time. Sliding at the boundaries of the central grain of the cell model is accurately represented; but in some computations a stress enhancement factor is used to incorporate also the effect of sliding between surrounding grains. The influence of grain boundary viscosity is included in the model and it is found that even in the absence of sliding, cavitation on inclined boundaries may significantly reduce the failure time.     

Effects of the creep deformation on the fractal dimension of the grain boundaries in an austenite steel

The change in the fractal dimension of the grain boundaries during creep was investigated using an austenitic SUS304 steel at 973 K. The fractal dimension of the grain-boundary surface profile (the fractal dimension of the grain boundaries, D, 1 < D < 2) in the plane parallel to the tensile direction (in the parallel direction) and in the transverse direction, was examined on specimens deformed up to rupture (about 0.30 creep strain). Grain boundaries became serrated and the fractal dimension of the grain boundaries increased with increasing creep strain, because the density of slip lines which formed ledges and steps on grain boundaries increased as the creep strain increased. The increase in the fractal dimension due to creep deformation was slightly larger under the higher stress (118 MPa) than under the lower stress (98 MPa), while the increase of the fractal dimension with strain was a little larger in the specimens tensile-strained at room temperature (293 K) than in the crept specimens. These results were explained by the grain-boundary sliding and the diffusional recovery near grain boundaries, which lowered the increase of the fractal dimension with the creep strain. The fractal dimension of the grain boundaries in the parallel direction was slightly larger than that in the transverse direction in both creep at 973 K and tensile deformation at room temperature, especially at the large strains. This could be correlated with the shape change of the grains by creep or plastic deformation. Grain-boundary cracks were principally initiated at grain-boundary triple junctions in creep, but ledges, steps and carbide precipitates on serrated grain boundaries were not preferential nucleation sites for the cracks.