| As the demand for high thrust gas turbine engines increases, critical components such as high pressure turbine discs are normally exposed to ever extreme conditions of high temperature and stress. Consequently a thorough understanding of material degradation mechanisms during service is indispensable. The precipitation kinetics of the detrimental sigma phase in the disc alloy Udimet 720Li have been characterized and quantified. The present investigation includes the exposure of the alloy to temperatures between 700℃ and 775℃ and times up to 5000 hours. Stress effect on the sigma precipitation has also been examined by applying stresses up to 250㎫. Minor phases such as sigma, carbide and boride were extracted from the exposed samples by the electro-chemical extraction method, and subsequently the extracted residues were quantitatively analysed using the X-ray diffraction technique combined with the Rietveld method. Variations of minor phases contents have been investigated with exposure time, temperature and applied stress. It is shown that applied stress considerably accelerates the precipitation rate of sigma phase. For example, after 2000 hours exposure at 725℃. 0.4wt.% sigma phase is found. If a stress of 250㎫ is applied. the amount of sigma phase rises up to 2.5wt.%. From these quantitative examinations, time-temperature-transformation diagram is presented far sigma phase formation in Udimet 720Li. Such observations have been correlated with microstructural characterization carried out using optical microscopy. |
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