The rotating components in the hot sections of land-based gas turbine are exposed to severe environment during several ten thousand hours at above 1100 oC operating temperature. The failure mechanism of the hot gas components would be accompanied by material degradation in the properties of high temperature and creep rupture strength. Many hot gas components in gas turbine are made of Ni-based superalloy because of their high temperature performance. In this work, we surveyed the time and temperature dependent degradation of Ni-based superalloy. We prepared the specimens from GTD111 that are exposed at 871 oC and 982 oC in 1,000 ~ 10,000 hours. We carried out the mechanical test and microstructural observation.
Many investigations about superalloys and coatings have been done in the laboratory, but evaluating the degradation condition of hot section components during service is still important not only for repair and reuse but also for outage prevention. Time dependent degradation for 1st stage blades of gas turbine was investigated. The degradation analysis for used blades was divided into microstructure changes by position in the blade and mechanical tests. In the microstructure analysis, the rafting and coarsening of γ', MC decomposition and TCP(Topologically closed packed) phase formation occurred and progressed with increasing service time, and especially at the leading and trailing edge of top layer which should be a check points for used blades. Stress-rupture test results of 25,000 and 52,000 EOH(Equivalent Operating Hours) for used blades were also compared with serviced time and position in each blade.
We investigated the effects of SiNx interlayers on the structural and electrical properties of nonpolar a-plane (11-20) GaN grown on r-plane (1-102) sapphire substrates by metal–organic chemical vapor deposition (MOCVD). The Nomarski optical microscope images showed that the deposition conditions of the SiNx layer could strongly affect the a-plane GaN surface morphology due to the different SiNx coverage. Basal-plane stacking faults (BSFs) and threading dislocation (TD) densities were reduced in the a-plane GaN samples with high SiNx coverage and multiple SiNx-treated GaN interlayers. These results indicate that TD reduction is associated with an increase in the 3D growth step and with the blocking of TD propagation. From on-axis (11-20) X-ray rocking curve (XRC) measurements, the anisotropy of full width at half maximum (FWHM) can be attributed to the crystal mosaicity due to insertion of different SiNx interlayers. The anisotropy of sheet resistance between the c-and m-axis was also clearly seen in a-plane GaN samples with a high density of defects, which was attributed to the BSFs as scattering centers.
We report on the anisotropic carrier transport properties of semipolar (112¯2) GaN films with low defect density. We utilized the asymmetric lateral epitaxy to obtain various semipolar (112¯2) GaN films having significantly reduced partial dislocations and basal-plane stacking faults (BPSFs). The directionally dependent carrier transport was observed with the lower sheet resistances (Rsh) along the [11¯00] direction. The Rsh ratios of semipolar (112¯2) GaN films were found to be relatively smaller than those of nonpolar a-plane GaN films, possibly due to low BPSF density and the reduced in-plane electric field induced by BPSF along the [112¯3] direction at wurtzite domain boundaries.
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