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.     

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.     

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.     

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.     

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.     

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

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.     

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.     

Creep of polycrystalline alumina,pure and doped with transition metal impurities

Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities (Cr, Fe). Divalent iron in solid solution was found to enhance both aluminium lattice and oxygen grain-boundary diffusion. Large concentrations of divalent iron led to viscous Coble creep which was rate-limited entirely by oxygen grain-boundary diffusion. Nabarro-Herring creep which was rate-limited by aluminium lattice diffusion was observed in pure and chromium-doped material. Chromium additions had no effect on diffusional creep rates but significantly depressed non-viscous creep modes of deformation. Creep deformation maps were constructed at various iron dopant concentrations to illustrate the relative contributions of aluminium grain boundary, aluminium lattice, and oxygen grain-boundary diffusion to the diffusional creep of polycrystalline alumina.     

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.     

Creep in non-ductile ceramics

The effect of a transition in creep behaviour in non-ductile ceramics (i.e. those with limited slip systems available) from diffusion controlled creep, to a mechanism involving non-viscous grain-boundary sliding and localized crack propagation is examined. Localized crack propagation is considered as a transition between diffusional creep and instantaneous fracture, and the fracture strength is used as a guide to predict the conditions necessary for the onset of this high strain-rate creep mechanism. In this way the variation in stress, temperature and grain size dependencies of creep rate reported in the literature for these materials may be explained and experimental evidence in support of the present hypothesis is presented.     

On the initiation of wedge-type cracks at grain-boundary triple junctions during high-temperature creep

The typical grain boundary cracks are often formed at the grain-boundary triple junction as a result of blocking of grain-boundary sliding. However, a theoretical discussion has not fully been made on the nucleation of grain corner cracks at high temperatures where diffusional recovery occurs. In this study, a continuum mechanics model which incorporated the recovery effect by diffusion of atoms has been developed to explain the initiation of wedge-type cracking during high-temperature creep. A good agreement was found between the result of calculation based on this model and experimental results in austenite steels. It was considered that there is a critical creep rate for wedge-type cracking. The model was also applied to the prediction of the rupture life in creep.     

Transient cavity growth in ceramics under compression

The transient cavity growth behaviour of liquid phase-sintered ceramics subject to compressive loads is examined. Three possible sources of transient behaviour are suggested, and their ranges of applicability evaluated. By considering the values of the characteristic time for individual transient modes, it has been determined that transient cavity growth in ceramics probably originates from transient grain-boundary sliding. Assuming that the creep-induced cavities nucleate and grow on grain boundaries that are parallel to the loading axis, a transient cavity growth model is developed on the basis that the local stress which drives cavity growth is induced by transient sliding of adjacent grain boundaries. Results of the proposed model are compared with small-angle neutron scattering measurements of a hot-pressed silicon carbide and a liquid phase-sintered alumina, both of which contain a continuous, amorphous grain-boundary phase. The different cavity growth behaviours observed in these ceramics are discussed in conjunction with transient grain-boundary sliding.     

Grain-boundary sliding measurements in Al2O3 by machine vision photogrammetry

The nucleation, growth and coalescence of grain-boundary cavities is the primary damage mechanism observed during creep of structural ceramics. Furthermore, grain-boundary sliding (GBS) has been identified as the driving force process. Although the creep characteristics of structural ceramics have been extensively studied, very little is known about the details of GBS during creep and how GBS relates to cavitation kinetics. This paper presents the results of a study using a machine vision system to measure Mode II GBS displacements in a Lucalox Al₂O₃. Specifically, sliding displacements as large as 0.4 m were measured. The measured displacements indicate that some grain boundaries experienced shear strains and strain rates of 4200% and 2.3×10^–2 s^–1, respectively. The techniques utilized for these measurements are described in detail, and data gathered during a 2 1/2 h compressive creep test under a stress of 138 MPa at 1600 °C are presented and discussed.     

The influence of stress distribution on the deformation and fracture behaviour of ceramic materials under compression creep conditions

The stress distribution developed in test pieces during compression creep has been determine using the finite element method. The analyses are shown to account precisely for the inhomogeneous distribution of grain-boundary cracks developed during creep of polycrystalline magnesia and indicate that the accommodation of grain-boundary sliding by cavity formation is the rate-controlling process during high temperature creep of reaction-bonded silicon nitride.     

Grain-boundary sliding during diffusional creep

Grain-boundary sliding and diffusional changes at grain boundaries were monitored on the surface and in the interior of a magnesium alloy Magnox ZR55 tested under diffusional creep conditions. The behaviour is compared and contrasted to that observed under recovery creep conditions. It was found (i) that diffusional and recovery creep exhibit distinctively different angular dependencies of grain-boundary sliding, (ii) that the surface and interior grains exhibit the same sliding and diffusional changes (in the plane of the surface) under diffusional creep conditions, (iii) that a previously presented method for the measurement of diffusional creep [1], when modified as described here, allows the determination of diffusional and sliding components for samples with either ascut or annealed surface conditions and (iv) that under diffusional creep conditions the value ofγ is 0.5.     

Microstructure and mechanical characteristics of alpha-alumina-based fibres

The high-temperature mechanical behaviour of alumina-based ceramic fibres has been investigated by the comparison of a dense pure alumina fibre, a porous pure alumina fibre and a zirconia-reinforced dense fibre. Tensile and creep tests have been conducted up to 1300°C in air in parallel with microstructural investigations on the as-received and tested fibres. Room-temperature behaviour of the fibres is close to that of bulk materials having the same microstructure, but the fibre form allows higher failure stresses to be attained. High-temperature deformation of the three fibres is achieved by grain-boundary sliding ( ), and is accompanied by isotropic grain growth. The specific microstructures of each fibre induce differences in the creep threshold levels as a function of temperature and stress and also in creep rates and resistance to damage. Despite better resistance to creep and damage of the zirconia-reinforced fibre, alumina-based fibres are limited to applications below 1100°C. Grain boundaries are the principal cause of mechanical degradation at high temperature with these fibres.     

Effect of dynamic recrystallization on the importance of grain-boundary sliding during creep

During creep of polycrystalline materials at elevated temperatures, a certain amount of the strain is accommodated by grain-boundary sliding (GBS). The relative importance of GBS depends on the stress and grain size and sometimes temperature. During high-strain deformation, dynamic recrystallization often occurs with the resultant grain size only related to the stress. In this situation the importance of GBS is then dependent only upon stress and sometimes temperature. In dynamically recrystallized Magnox Al80 deformed atT>0.8T _m, 16 to 23% of the imposed strain is accommodated by GBS. A comparison has been made between the experimental results and some theoretical models for the importance of GBS during creep, modified to take account of recrystallization. The best fit to the data is obtained with the modified form of Langdons model. Deformation mechanism maps constructed with this model suggest that dynamic recrystallization can cause a switch of mechanism from dislocation creep to dominant GBS at intermediate temperature (T<673 K) and low stress. Deformation mechanism maps have also been constructed for calcite based on the data of Schmidet al. These suggest that GBS is an important mechanism in calcite deformed under geological conditions.     

Creep of polycrystalline sodium chloride containing a dispersion of alumina

The creep of polycrystalline NaCl contaning a fine dispersion of Al₂O₃ particles is analysed in terms of dependence on stress, temperature, volume fraction and size of dispersion, and grain size of samples. Compressive creep experiments around 0.8 Tm show that the dispersion inhibits diffusive creep. The creep is characterized by a threshold stress above which the creep rate increased linearly with applied stress. The threshold stress decreases with increasing temperature and is proportional to the volume fraction of the dispersion in agreement with a model proposed by Burton. The activation energy corrected for the temperature dependence of the threshold stress falls within a narrow range consistent with grain-boundary diffusion of chlorine in sodium chloride. The grain-size dependence is not consistent with a modified diffusive creep model but it is suggested that it may be controlled by inhibited grain-boundary sliding according to a new model.