Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Patent
1989-10-31
1992-06-30
Zimmerman, John J.
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
428680, B32B 1500
Patent
active
051262134
DESCRIPTION:
BRIEF SUMMARY
This invention relates to protective coatings for articles of near -.alpha. titanium based alloys. It is particularly concerned with coatings for gas turbine engine compressor stage components formed in near -.alpha. titanium based alloys suitable to convey at least high temperature oxidation resistance and preferably also high temperature corrosion resistance to these components.
Pure titanium undergoes an allotropic transformation at 882.degree. C. from the lower temperature phase, designated the .alpha. phase, which has a hexagonal close-packed (hcp) structure to a higher temperature phase, designated the .beta. phase, which is stable up to the melting temperature and has a body centre cubic structure. Titanium alloys are conventionally categorised as .alpha.-type alloys, .beta.-type alloys or .alpha.+.beta. alloys by virtue of the nature and level of the alloying additions they contain. Near -.alpha. titanium alloys are titanium based with additions of, amongst other things, .alpha.-stabilising elements such as aluminium and tin that promote the hexagonal close packed structure of the .alpha.-phase. The .alpha.-phase has very good high temperature creep properties and in near -.alpha. titanium alloys these good creep properties are achieved while still maintaining adequate low temperature strength and forgeability.
This excellent balance of strength, ductility, microstructural stability and oxidation/corrosion resistance as compared with competitive materials such as steels or nickel based superalloys has resulted in near -.alpha. titanium alloys becoming increasingly important as constructional material for compressor components in advanced gas turbine engines.
A wide variety of these high strength near -.alpha. titanium alloys have been developed commercially to tolerate operating conditions involving prolonged exposure to air at temperatures up to 500.degree. C. The following commercial alloys of IMI Titanium Ltd (identified by their commercial designation and nominal composition in weight percent) are typical of the current generation of titanium alloys suitable for gas turbine compressor applications. These are: IMI 550 (Ti-4Al-2Sn-4Mo-0.5Si); IMI 679 (Ti-11Sn-2.25Al-5Zr-1Mo-0.25Si); IMI 685 (Ti-6Al-5Zr-0.5Mo-0.25Si) IMI 829 (Ti-5.5Al-3.5Sn-3Zr-0.25M-0.3Si) and IMI 834 (Ti-5.8Al-3.5Zr-0.7Nb-0.5Mo-0.06C).
There is continuing commercial interest in increasing the temperature tolerance of near -.alpha. titanium alloys in order that gas turbine engine cycle temperatures can be raised to increase engine efficiency without recourse to other materials. There is also the possibility that an increase in temperature tolerance might permit these titanium alloys to be used for components currently made of say nickel-based superalloys with consequent reduction in component weight. Some of the conventional near -.alpha. titanium alloys exhibit excellent creep resistance and good structural stability at temperatures significantly above the 500.degree. C. mentioned earlier--say to 650.degree. C. However, when such alloys are exposed to air at temperatures approaching 600.degree. C. they are subject to significant and detrimental surface modification. The principal factor involved in this surface modification is the uptake of oxygen into solid solution by the titanium. At these temperatures the reaction kinetics ensure rapid diffusion of oxygen into the region adjacent the exposed surface. Approximately 30 atomic percent may be dissolved in the titanium. The dissolved oxygen creates in the affected region a hard brittle zone. The affected zone is called the alpha case and its formation can substantially degrade the structural integrity of the affected article by loss of tensile ductility and of fatigue resistance even though the interior of the article is not subject to structural modification. Nitrogen plays some part in the degradation process which occurs but the predominant factors in alpha case formation are the presence of oxygen, the exposure time and the temperature. The terms "oxidation", "oxidation resis
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The Secretary of State for Defence in Her Britannic Majesty's Go
Zimmerman John J.
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