| 1. |
- Das, D. K., et al.
(författare)
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Microstructural degradation of plain and platinum aluminide coatings on superalloy CM247 during isothermal oxidation
- 1999
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Ingår i: Materials Science and Technology. ; 15:10, s. 1199-1208
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Tidskriftsartikel (refereegranskat)abstract
- Isothermal oxidation at 1100°C of a high activity plain aluminide coating and a platinum aluminide coating, developed by the pack cementation technique, on cast nickel base superalloy CM247 has been carried out with the primary objective of systematically understanding the coating degradation process during oxidation. While the weight gains during oxidation for both plain aluminide and platinum aluminide coatings follow parabolic kinetics from the very beginning of oxidation exposure, the bare alloy was seen to exhibit a considerably long initial transient oxidation period (∼20 h), beyond which the parabolic law was followed. The parabolic rate constant for the platinum aluminide coating was found to be nearly two orders of magnitude lower than that for the plain aluminide coating. Alumina was identified as the only oxide phase that formed on both plain aluminide and platinum aluminide coatings during most of the oxidation exposure, although NiAl2O4 was also found in the case of the plain aluminide coating beyond ∼200 h. The oxide layer on the bare alloy, however, was found to consist of Al2O3, Cr2O3, and NiAl2O4. The microstructural degradation of both the plain aluminide and platinum aluminide coatings during oxidation was seen to occur in three distinct stages which, however, differed for each coating. This stagewise degradation, which involves final obliteration of the interdiffusion layer in each case, is discussed in detail. © 1999 IoM Communications Ltd.
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| 2. |
- Das, O. K., et al.
(författare)
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Evolution of aluminide coating microstructure on nickel-base cast superalloy CM-247 in a single-step high-activity aluminizing process
- 1998
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. ; 29:8, s. 2173-2188
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Tidskriftsartikel (refereegranskat)abstract
- This study deals with the aluminizing of a directionally cast Ni-base superalloy, namely CM-247, by a single-step process using a high-activity pack. It is observed that significant incorporation of Al into the substrate surface during aluminizing continues over a period of about 1 hour and is not restricted merely to the first few minutes, as reported in the literature. Based on the microstructural details of the coatings formed at various stages of aluminizing, it is concluded that the coating growth in the above process takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion. In the final stages, the outward diffusion of Ni dominates the coating formation process. The above mechanism of coating formation is different from the one that prevails in the conventional two-step high-activity coating process in that the reaction front for the formation of NiAl remains spatially stationary despite the outward diffusion of nickel during the intermediate stage. It is also shown in the present study that the content of the Al source in the pack affects the coating structure significantly. It is further demonstrated that the microstructure of the aluminide coatings depends not only on the amount of Al incorporated in the sample during aluminizing but also on the time over which the uptake of this Al takes place.
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| 3. |
- Krishna, G. R., et al.
(författare)
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Role of Pt content in the microstructural development and oxidation performance of Pt-aluminide coatings produced using a high-activity aluminizing process
- 1998
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Ingår i: Materials Science and Engineering A. ; 251:1-2, s. 40-47
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Tidskriftsartikel (refereegranskat)abstract
- The present study highlights the effect of Pt content on the microstructure of Pt-aluminide coatings produced using a single-step high-activity aluminizing process. The amount of Pt in the coating was varied by changing the thickness of the initial electroplated Pt layer between 1 and 15 μm. The aluminium uptake from the pack was found to be almost the same for all the coatings produced using a Pt layer of thickness 2.5 μm and above, with a somewhat lower uptake for the coating corresponding to a 1 μm thick Pt layer. The coating microstructure, which consisted of an outer two-phase (PtAl2 in a matrix of NiAl) layer, an intermediate NiAl layer and an interdiffusion layer, was also found to be independent of the Pt layer thickness when it was in the range 2.5-10 μm. In the case of the 1 μm Pt layer, however, the whole of the Pt remained in solid solution in the NiAl phase. For a Pt layer thickness exceeding 10 μm, on the other hand, a continuous surface layer of PtAl2 phase was observed. The above mentioned influence of the thickness of the Pt plated layer on the microstructure of the Pt-aluminide coatings observed in the present investigation could be explained in terms of the Pt concentration in the diffusion layer resulting from the interdiffusion between the Pt layer and the superalloy substrate during the pre-aluminizing diffusion treatment. Cyclic oxidation tests on these Pt-aluminide coatings reveal that the presence of Pt in aluminide coatings, in general, enhances oxidation resistance. However, in order to fully realize the beneficial effects of Pt on oxidation behaviour, a certain minimum Pt content in the coating was found to be necessary.
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| 4. |
- Bendz, D., et al.
(författare)
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Kinematic wave model for water movement in municipal solid waste
- 1998
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Ingår i: Water Resources Research. - 0043-1397. ; 34:11, s. 2963-2970
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Tidskriftsartikel (refereegranskat)abstract
- The movement of water in a large (3.5 m(3)) undisturbed sample of 22-year-old municipal solid waste has been modeled using a kinematic wave approximation for unsaturated infiltration and internal drainage. The model employs a two-parameter power expression as macroscopic flux law. The model parameters were determined and interpreted in terms of the internal geometry of the waste medium by fitting the model to one set of infiltration and drainage data. The model was validated using another set of data from a sequence of water input events. The results of the validation show that the model performs satisfactorily, but further development of the model to incorporate spatial variability would increase its capability.
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| 5. |
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| 6. |
- Bobba, A. G., et al.
(författare)
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Application of a watershed runoff model to north-east pond river, Newfoundland: To study water balance and hydrological characteristics owing to atmospheric change
- 1997
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Ingår i: Hydrological Processes. - 1099-1085. ; 11:12, s. 1573-1593
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Tidskriftsartikel (refereegranskat)abstract
- The hydrological sensitivities to long-term climate change of a watershed in Eastern Canada were analysed using a deterministic watershed runoff model developed to simulate watershed acidification. This model was modified to study atmospheric change effects in the watershed. Water balance modelling techniques, modified for assessing climate effects, were developed and tested for a watershed using atmospheric change scenarios from both state of the art general circulation models and a series of hypothetical scenarios. The model computed daily surface, inter- and groundwater hows from the watershed. The moisture, infiltration and recharge rate are also computed in the soil reservoirs. The thirty years of simulated data can be used to evaluate the effects of climatic change on soil moisture, recharge rate and surface and subsurface flow systems. The interaction between surface and subsurface water is discussed in relation to climate change. These hydrological results raise the possibility of major environmental and socioeconomic difficulties and have significant implications for future water resource planning and management. (C) 1997 John Wiley & Sons, Ltd.
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| 7. |
- Bobba, A. G., et al.
(författare)
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Application of first-order and Monte Carlo analysis in watershed water quality models
- 1996
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Ingår i: Water Resources Management. - 0920-4741. ; 10:3, s. 219-240
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Tidskriftsartikel (refereegranskat)abstract
- Functional analysis and Monte Carlo simulation were used to quantify uncertainties in model simulation of pollution behaviour and effects. The first-order part of the functional analysis method provides a measure of uncertainties in dependent variables in terms of uncertainties in independent variables. The procedure is based on first-order terms in the Taylor series expansion of the dependent variable about its mean value with respect to one or more independent variables. The major assumption is that all independent and dependent variables are the second moment variables (SMV), which means that the behaviour of any SMV is completely described by its mean and standard deviation. The mathematical simplicity of the procedure allows application by simple input-output models. Consequently, it has been applied to many environmental simulators, e.g. hydrological models, stream water quality models, lake water quality models and ground water pollution models. The Monte Carlo simulation method uses a large number of repeated trials or simulations with the values for stochastic inputs or uncertain variables selected at random from their assumed parent probability distributions to establish an expected range of model uncertainty.
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| 8. |
- Bobba, A. G., et al.
(författare)
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Computer program (shock) to predict acid shocks in watersheds using stochastic analysis
- 1996
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Ingår i: Computers & Geosciences. - : Elsevier BV. - 1873-7803 .- 0098-3004. ; 22:4, s. 399-408
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Tidskriftsartikel (refereegranskat)abstract
- High hydrogen ion concentrations of runoff events were analyzed in terms of the probability distributions of frequency, duration, magnitude, and time of occurrence. Simple stochastic models were applied to the probability distributions of the annual frequency of high hydrogen ion concentration flows and their magnitudes. A consideration of the statistical properties of the given stochastic variables led to the development of a technique with which higher hydrogen flow events exceeding any higher level of interest may be investigated without resorting to reanalysis of the historical data. The proposed methodology was applied to the daily hydrogen ion concentration flow records of Mersey River Watershed, N.S., Canada, which is affected by acid precipitation. Copyright (C) 1996 Elsevier Science Ltd
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| 9. |
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| 10. |
- Singh, V.P., et al.
(författare)
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Kinematic wave modelling of saturated basal flow in a snowpack
- 1997
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Ingår i: Hydrological Processes. - 0885-6087 .- 1099-1085. ; 11:2, s. 177-187
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Tidskriftsartikel (refereegranskat)abstract
- Movement of snowmelt water through a thin saturated layer at the infiltrating base of a snowpack is modelled by applying the kinematic wave theory. Analytical solutions are obtained for flow depth, velocity and discharge assuming that the rate of input to the saturated layer due to vertical percolation is constant. This assumption results in a linear rise and recession of the snowmelt hydrograph. The solutions are extended to the case of time-varying input. An explicit consideration of infiltration leads to a free boundary problem.
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