J. Woirgard

High-Temperature Isothermal Internal Friction Measurements in NiCoCrAlY Alloys

Superalloys used in hot sections of gas turbines must be protected to prevent high-temperature corrosion. MCr Al Y alloys (M = Ni or Co or both) deposited by means of low pressure plasma spraying are known to provide efficient protection. However, the differential changes with temperature of the mechanical properties of the substrate and protection can alter the behavior of the whole. Tension tests performed in NiCoCrAIYTa alloy have shown a fast decrease in elastic modulus, above 870 K, accompanied by an increase in ductility. Internal friction measurements have been carried out in a variable frequency pendulum in the 10 - 4 -30 Hz frequency range, between 300 and 1150 K. The Q - 1 versus vibration frequency diagrams revealed two relaxation peaks superimposed onto an exponential low-frequency background. The elastic modulus drop has also been proved to be thermally activated. The parameters associated with the main relaxation peak (H p = 272 kJ/mole and ν 0 = 5 x 10 1 5 s - 1 ) indicate the presence of a diffusional process in the temperature range corresponding to the drop in elastic modulus and the increase in ductility. Measurements in β and γ phases, which are the two phases present in NiCoCrAIYTa, have also been carried out to determine precisely the exact diffusion mechanism. The general aspect of the damping spectra in these two phases is quite similar to that observed in the alloy. In the β phase, the relaxation peak obtained after removal of the low-frequency background is characterized by an energy of 250 kJ/mole and a frequency constant of 6 x 10 1 4 s - 1 . This peak is associated with relaxation of nickel atoms in the β phase. In the γ phase, we obtain a relaxation peak whose parameters change during thermal cyclings and finally reach the values obtained in the alloy. That peak is again due to the relaxation of nickel atoms associated with the progressive dissolution of γ precipitates. In conclusion, the relaxation effect responsible for the ductility is the diffusion of nickel in the γ phase, but the role of the interface and in particular of the interdiffusion has to be taken into account.

Internal friction in spinels

In the present paper, relating to MgO(Al 2 O 3 ) n , we describe results obtained by isothermal internal friction experiments in plasma-sprayed polycrystals and single crystals grown by the Verneuils method. In some cases, a very large internal friction peak has been observed, probably the highest ever found in crystalline materials. Measurements have been done between room temperature and 1100 K, indicating that an Arrhenius law was very well obeyed over more than five decades in frequency, corresponding to a single relaxation mechanism. The composition dependence of the relaxation strength was also investigated in both single crystals and polycrystals for n, the molar ratio of Al 2 O 3 and MgO, ranging between 1 to 10. Highest relaxation strengths were observed for molar ratios between 2 and 3, corresponding to compounds with large oxygen vacancy concentrations and thus the elastic dipole was identified as a Mg + + -O vacancy complex. The dipole has been found to have monoclinic symmetry from tests of single crystals with (100), (110), and (111) orientations, indicating an elastic distortion lying along a (110) direction.

Internal Friction in Silicon-Carbon/Glass Ceramic Composites

Composite materials composed of a glass-ceramic matrix and silicon-carbon (SiC) fibers have been recently developed for high-temperature applications. In the present work, damping properties and the dynamic modulus of unidirectional composites made with SiC-Nicalon fibers and aluminum silicate based matrix (glassy or partially recrystallized) have been studied in a variable frequency torsion pendulum over wide temperature and frequency ranges. A relaxation peak, characteristic of the vitreous phase, has been observed in specimens with different ratios of the recrystallized phase. The apparent activation energy (H p = 1.24 eV) and the limit relaxation time (τ 0 = 2.10 - 1 6 s) were found to be constant for all samples. On every specimen, a decrease of the peak width, accompanied by an increase in the relaxation strength, was observed for measurements at increasing temperatures. In glassy matrix specimens, the low-frequency damping background has been found to increase abruptly. In the partially crystallized composite specimens, a peak caused by the glassy transition superimposed on to the high-temperature background was also observed.