Low cycle fatigue of a polycrystalline Ni-based superalloy: Deformation substructure analysis

The deformation behavior of the polycrystalline superalloy R104 has been characterized via scanning transmission electron microscopy (STEM). Specimens tested under low cycle fatigue (LCF) conditions were examined based on testing temperature, total strain range, and/or cycle number. Extensive electron microscopy characterization has revealed non-planar deformation to be a dominant mode in polycrystalline R104, resulting from ample cross-slipping processes between {1 1 1} and {1 0 0} planes. Somewhat unexpectedly, this mechanism appears at both low and high temperatures, although the macroscopic material response is quite different.     

Thermal fatigue in polycrystalline alumina

Damage accumulation in polycrystalline alumina subjected to cyclic thermal loading was studied via non-destructive elastic modulus and internal friction measurements. These nondestructive techniques were sensitive to cracks formed by thermal loading. Thermal shock damage was observed to saturate as a function of an increasing cumulative number of thermal shock cycles. The observed power law relationship between the damage saturation and thermal shock difference implies a fatigue-like power law relation in stress. The exponent has a value of approximately 12 for the range of T included in this study. Thermal shock damage induced changes in internal friction were found to be a function of a crack damage parameter. These thermal fatigue results of polycrystalline (unreinforced) alumina are also compared to thermal fatigue results for SiC whisker-alumina composites.     

Crack-void interaction in polycrystalline alumina

Strength of alumina-void samples have been measured in a four-point bend test. Knoop microhardness indentor was used to introduce controlled surface cracks in sintered alumina containing up to 8 vol % spherical voids for the bend test. It was found that as the volume per cent of voids are increased, the strength remained unchanged as the indented crack interacts with the voids. When strong inclusions are considered, an analysis showed that with increase in volume fraction and decrease in inclusion size, the crack interaction resistance for alumina can be improved significantly.     

Compressive strength and microplasticity in polycrystalline alumina

The compressive strength of polycrystalline alumina at 23° C is found to be strain-rate sensitive, but insensitive to environment. Scanning electron microscopy of specimens loaded to near failure indicates the origin of the strength-strain-rate dependence to be localized plasticity in the form of twinning and, possibly, slip. The interaction of these deformation bands with grain boundaries causes the initiation of microcracks. Higher stresses produce still more twin/slip-nucleated microcracks, which finally coalesce at failure. It is suggested that twinning also may be related to the tensile failure of alumina.     

Observations and mechanisms of fracture in polycrystalline alumina

The fracture mechanics of polycrystalline alumina have been investigated using the double cantilever beam cleavage method. The fracture resistance of alumina was measured at temperatures up to 1500°C. The fracture resistance of alumina increased with increasing grain size for all temperatures studied. Electron fractography studies of the fractured surface indicate that the large grain alumina contains more transgranular fracture than the small grain material. This preponderance of transgranular fracture is shown to accompany a higher fracture resistance in the material. It is thus concluded that one mechanism of variation of strength in these materials is through variation in the relative amounts of transgranular and intergranular fracture. The lower softening point of SiO₂ causes alumina with this material as a binder to lose its strength at lower temperatures than does MgO bonded material. The mechanism of fracture in these polycrystalline ceramics leads to a suggested method for improved mechanical strength.     

Static and dynamic fatigue of polycrystalline alumina

The fatigue failure of polycrystalline alumina was measured in a moist air environment at 30° C as a function of constant applied tensile stress and stressing rate. The good correlation found between the fatigue test data and fracture mechanics theory indicates that fatigue is controlled by the slow crack growth of pre-existing flaws and that static and dynamic fatigue test techniques adequately define the fatigue parameters needed for failure predictions. Comparisons of proof-test predictions with experiment indicate that the proof test can be effective in eliminating weak samples from the population and in assuring against the delayed failure of polycrystalline alumina in a moist environment.     

Sub-critical crack extension and crack resistance in polycrystalline alumina

Sub-critical crack extension can readily be observed in controlled fracture tests in fourpoint bending. A natural crack of any desired lengthc which exceeds the notch depthc ₀ by the amount c =c –c ₀ can be introduced into bend specimens by stable crack propagation. The stress intensity factor to achieve c increases considerably with increasing c. In pre-cracked specimens the stress intensity factorK _I0 to start the crack and the critical valueK _IC strongly depend on the natural crack length c whereasK _I0 andK _IC are independent ofc ₀ in solely notched specimens. From a quasi-continuous evaluation of the load-deflection curve recorded during controlled fracture, the differential work of fracture can be obtained as a function of the achieved crack length. It may be regarded as the crack extension resistanceR of the material because the balance between the energy release rateg ₁ andR is maintained throughout the experiment. By that, a formal analogy to theR-curve concept of fracture mechanics is given. The steady increase ofR is explained by multiple crack formation and by the interference of the fracture surfaces due to the angular development of the crack front.     

Nanoindentation investigation of micro-fracture wear mechanisms in polycrystalline alumina

The initiation of surface damage under point loading has been investigated in polycrystalline alumina materials using low load continuous depth-sensing indentation equipment (nanoindentation). Pure alumina and liquid phase sintered materials containing 10% by weight of magnesium or calcium monosilicate have been examined and data obtained from plots of displacement as a function of load assessed in relation to erosive wear rates. In the pure alumina material, discontinuities (pop-ins) in the load-displacement trace appear to be associated with the induction of radial cracking around a plastic impression. The pop-ins provide information on the indentation load at which fracture was initiated, and an estimate of the energy associated with crack formation. SEM imaging of the indentations before and after etching allowed crack paths to be related to microstructural features.     

Deformation and recrystallization of polycrystalline silicon

A systematic study of recrystallization of hot-deformed single and polycrystalline silicon has been carried out. New grains were formed by nucleation and growth at temperatures between 1280 and 1410° C and the final grain size was a function only of the degree of deformation. For polycrystalline chemical vapour deposition (CVD)-grown silicon hot deformed at temperatures between 1 100 and 1310° C it was established that the largest grains (of the order of a few millimetres) can be grown after about 5% deformation. At this low strain the rates of nucleation and growth were low and consequently the time needed to complete recrystallization was rather long (20 h). Below 4% deformation no recrystallization occurred (critical degree of deformation is 4%). For both single and polycrystalline silicon grain growth after primary recrystallization was limited. This was due to the low mobility of high-angle grain boundaries in covalently bonded materials.     

The existence of non-labile sodium in polycrystalline sodium beta alumina

Experiments were carried out to measure the rate and extent to which silver and potassium ions exchange with the sodium ions in Monofrax sodium beta alumina (1.32Na₂O·11Al₂O₃) and high soda polycrystalline sodium beta alumina (nominal stoichiometry of 1.80Na₂O·11Al₂O₃). Ion exchange in molten nitrate melts at 350°C is complete within about 10 hours for 1–2 mm sized Monofrax beta alumina crystals. For the polycrystalline samples, however, exchanges with both silver and potassium reach limiting values of about 87% for silver and 84% for potassium after 50–80 hours. This is interpreted on the basis of second phases present in the polycrystalline samples and the apparent existence of non-labile sodium within the beta alumina grains.     

Analysis of static and dynamic fatigue of polycrystalline alumina

The fatigue failure of polycrystalline alumina in a moist air environment at 30° C has been analysed in terms of a modified Weibull distribution function using fracture mechanics theory. The good correlation obtained between the fatigue test data and fracture mechanics theory indicates that fatigue is controlled by the slow crack growth of pre-existing flaws. The distribution of these pre-existing flaws can be represented by the modified Weibull distribution which provides an upper and a lower limit strength and thus is more realistic for the physical phenomena it represents. Comparison of proof-test predictions with experiment indicate that the proof test can be effective in eliminating weak samples from the population.     

Synthesis of polycrystalline alumina fibre with aluminium chelate precursor

Polycrystalline alumina fibre was successfully synthesized by pyrolysis of preceramic fibre formed from aluminium chelate compound. Ethyl 3-oxobutanoatodiisopropoxyaluminium (EOPA) was reacted with glacial acetic acid yielding a polymeric product. The absorption bands ascribed to Al-O from 630–705 cm^–1 changed from a sharp to a broad band on treatment with acetic acid. The¹³CNMR spectrum of EOPA changed from sharp singlets to multiplets after the reaction with acetic acid. The viscosity of the polymeric product increased in intensity with increasing amount of acetic acid. The viscosity of the polymeric product formed from EOPA-30 mol % acetic acid was 450 Pa s at 30 °C, and decreased to 5.4 Pa s with increasing measurement temperature from 30–70 °C. The²⁷Al resonance at 35 p.p.m. increased in intensity with increasing viscosity of the polymeric precursor. The molecular weight of the precursor was distributed from 400–800. The polymeric precursor pyrolysed at 500 °C in air was amorphous to X-rays, and crystallized in -alumina at 840 °C. The precursor fibres were pyrolysed, to yield fine-grained fibres of -alumina, at 1300 °C for 1 h.     

Elevated-temperature crack growth in polycrystalline alumina under static and cyclic loads

An experimental investigation has been conducted to study the crack growth characteristics of a 90% pure aluminium oxide in 1050 °C air under static and cyclic loads. It is shown that the application of both sustained and fluctuating tensile loads to the ceramic, tested in a precracked four-point bend specimen configuration, results in appreciable subcritical crack growth. The crack velocities under cyclic loading conditions are up to two orders of magnitude slower than those measured in static loading under the same maximum stress intensity factor. Cyclic crack growth rates are markedly affected by the loading frequency, with a decrease in test frequency causing an increase in the rate of crack advance. Detailed optical and electron microscopy observations have been made in an attempt to study the mechanisms of stable crack growth and the mechanistic differences between static fatigue fracture. Under both static and cyclic loads, the predominant mode of fracture is intergranular separation. The presence of a glass phase along the grain boundaries appears to have a strong effect on the mechanisms of crack growth. Apparent differences in the crack velocities between static and cyclic fatigue in alumina arise from crack-wake contact effects as well as from the rate-sensitivity of deformation of the glass phase. Our results also indicate that the cyclic fatigue crack growth rates cannot be predicted solely on the basis of sustained load fracture data. White stable crack growth occurs in the 90% pure alumina over a range of stress intensity factor spanning 1.5 to 5 MPa m^1/2, such subcritical fracture is essentially suppressed in a 99.9% pure alumina, ostensibly due to the paucity of a critical amount of glass phase. Both static and cyclic fracture characteristics of the 90% pure alumina are qualitatively similar to those found in an Al₂O₃-SiC composite wherein situ formation of glass phases, due to the oxidation of SiC in high-temperature air, is known to be an important factor in the fracture process.     

The wetting behavior of NiAl and NiPtAl on polycrystalline alumina

In order to understand the beneficial effect of Pt on the adherence of thermally grown alumina scales, sessile drop experiments were performed to study the wetting of poly-crystalline alumina by nickel–aluminum alloys with or without platinum addition ranging from 2.4 to 10 at%. Subsequent interfacial morphology was examined using atomic force microscopy. Platinum addition enhances the wettability of NiAl alloys on alumina, reduces the oxide/alloy interface energy and increases the interfacial mass transport rates.     

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.