摘要:

It has long been known that the addition of so-called reactive elements, such as Y, to alloys forming chromia scales improves the protectiveness of those scales by reducing the rate of scale growth and by reducing the tendency of the scales to spall during thermal cycling. It has been suggested that reduced spallation might arise from better scale adherence or from improved resistance to fracture.In this study Ni-30Cr and Ni-30Cr-0.5Y alloy samples were oxidized at 1000°C in pure O2for various times, then were either furnace cooled to room temperature, or thermally cycled between 1000°C and different lower temperatures. Scale fracture events, which are detected by acoustic emission, were collected throughout the experiment. Anin situX-ray diffraction technique was used to measure the elastic strains in the oxide scales at the isothermal scale growth temperature and at several temperatures during cooling.These measurements found higher elastic strains in the Cr2O3scales, during both isothermal oxidation and cooling, but for fewer cracking events, on the yttrium-containing alloy than on Ni-30Cr. We infer that the addition of yttrium increased the adherence of the scale to the substrate, since the scale was able to withstand higher elastic stresses.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689514

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

The use of the scratch test to assess the adhesion of a range of hard coatings is well established. The technique can provide a semi-quantitative measure of adhesion provided that a well-defined adhesion-related failure mode is used as a basis for the assessment. In this study the testing has been extended to thermally grown oxides and it has been determined that both buckle and wedging failure modes can be identified in the scratch test depending on the thickness of the oxide and the mechanical properties of the substrate. The critical normal load at which these failure modes are first observed as a function of thickness can be used to determine those compressive and shear loading regimes where the oxide scale is stable with respect to spallation. This approach can also be applied to hard coatings, such as TiN deposited by physical vapour deposition. The experimental observations are discussed in light of theoretical models for scale spallation.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689515

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

The oxidation and sulfate induced corrosion behavior of an oxide dispersion strengthened nickel base superalloy, MA 6000, has been studied at 900°C and 1000°C. The experimental program involved thermogravimetry. X-ray diffraction and scanning electron microscopy. The kinetics of high temperature oxidation in air increased with increasing temperature. The experimental activation energy for oxidation in air indicated that a chromia-rich scale formed the protective oxide layer. In the case of sulfate induced corrosion, however, increasing temperature decreased the scale growth rate. This result has been explained by considering the relative stability of nickel sulfide at 900°and 1000°C. Metallographic studies of the scales are also presented.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689516

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689517

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

There is now a shift towards combined cycle for large scale power generation. In these plants, the main spallation issue will be that of combustor can alloys. Units using reheat will add a new dimension to this problem. Spallation of highly cooled turbine blading as a possible issue should not be neglected, however.Combined cycle will be linked in the longer term to coal gasification processes. Where the gasifiers are of the entrained or fluidised bed type, HC1 induced spallation of downstream heat exchangers is a neglected aspect of much experimental work. This form of attack is likely to occur over a wide range of temperatures.In the UK and Northern Europe CHP (Combined Heat and Power), using natural gas as a fuel, will take an increasing share of the market. The most advanced concepts will use recuperative gas turbines, as these have many advantages at low to medium outputs. Again, scale spallation will be an important design consideration.Household or Domestic CHP could well be a new entry into the field of environmentally beneficial, localised power generation. Many of the prime movers under consideration for this sector will require a high temperature air preheater. Here the need to avoid excessive oxidations and scale spallation will be paramount.The paper calls for more efforts on alloys other than chromia formers; alumina formers offer much better high temperature behaviour, apart from their spallation behaviour. There is also a strong economic case for work on alloys reliant on silica. Although these are normally viewed as having poor spallation resistance, they are used in quantity by the Automotive Industry in cyclic applications.A brief review is given of spallation and related phenomena on alumina forming ODS alloys intended for advanced indirect fired gas turbine cycles. Emphasis is given to the statistically variable nature of spallation, and to the fact that, in practice, spallation failures involve more than one mechanism.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689518

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689519

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor