Near Alloys

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Near- titanium alloys are the classic high-temperature alloys. This alloy class isideal for high temperatures since it combines the excellent creep behavior of alloyswith the high strength of + alloys. Today their upper operating temperatureis limited to about 500 to 550C.

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Ti-8-1-1 was the first titanium alloy particularly developed for high temperatures.The high Al content led, however, to stress corrosion problems; therefore,all conventional titanium alloys in use today are limited in aluminum content to amaximum of 6%. The follow-up alloy, Ti-6-2-4-2, developed by Timet, also has thislower Al content. In the 1970s investigations by RMI were of central importanceto the further success of elevated temperature titanium alloys: research discoveredthat small additions of only up to 0.1 wt. % Si would substantially improve thecreep behavior of Ti-6-2-4-2. The alloy was named Ti-6-2-4-2-S.

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The first commercial Si-containing titanium alloy was developed by IMI in theUnited Kingdom: IMI 679. The follow-up alloy, IMI 685, was the first alloy to beused in the -annealed condition due to the superior creep performance of thismicrostructure. One of the most advanced conventional high temperature titaniumalloys today is the American TIMETAL 834, originally developed in the UK.

+ Alloys

Ti-6Al-4VMost popular titanium alloy.More than 50% of all alloys are of this composition.Developed in1950s at the Illinois Institute of Technology.One of the very first titanium alloys to be made.

Reasons for the success:1.- Good balance of its properties.2.- Most intensively developedand tested titanium alloy, which is a major advantage.

Especially in the aerospace industry, the largest user of Ti-6Al-4V.

Other alloysTi-6-6-2 and IMI 550: primarily developed for high strength..High strength and high toughness is realized with Ti-6-2-4-6.6-2-2-2-2, Ti-55-24-S or Ti-17 were primarily developed for elevated temperature applications in gas turbine engines up to about 400C.

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Metastable AlloysOver the last few decades the importance of metastable alloys has steadily increased. These alloys can be hardened to extremely high strenght levels of more than 1400MPa. The complex Microstructure enables the designer to optimize for both high strenght and high toughness. This is true for TIMETAL and beta C.

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Other Metastable alloys like TIMETAL 21S was specially developed as an oxidation resistant foil alloy to be used as the matrix for long fiber reunforced titanium alloys. TIMETAL LCB is a price-sensitive alloy (LCB= low- cost beta) particularly aimed at applications beyond the aerospace sector e.g for te automative industry.

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Beta CEZ was developed in France for application at moderate temperature in gas turbine engines. Due to its very fine grained bimodal microstructure, the japanese alloy SP 700 C. Wide spread application of alloys is, however, limited bye its relatively high specific weight, modest weldability, poor oxidation behavior, and complex microstructure.

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