EXPERIMENTAL INVESTIGATION OF THE DYNAMIC ELASTIC-MODULUS AND VIBRATION DAMPING IN MOLYBDENUM AND ALUMINA-COATED TI-25AL-IONB-3V-IMO ALUMINIDES CYCLED AT ELEVATED-TEMPERATURES

Titanium aluminides such as Ti-25Al-10Nb-3V-1Mo have become the focus of high temperature materials research due to their ability to maintai n their strength up to 1,000 degrees C. However, at temperatures in ex cess of 650 degrees C, Ti-25Al-10Nb-3V-1Mo undergoes severe oxidation which, in turn, can degrade its mechanical properties. To address this problem, plasma sprayed coatings were applied to the substrate to pro vide protection in the high temperature environment. In this study 0.1 -0.125 mm thick alumina and molybdenum coatings were plasma sprayed on to a Ti-25Al-10Nb-3V-1Mo substrate. Changes in dynamic elastic modulus and vibration damping due to thermal cycling at elevated temperatures were measured. The modulus and damping were measured using the piezoe lectric ultrasonic composite oscillator technique (PUCOT), The behavio r of the Al2O3 and Mo coated specimens was compared to that of the unc oated samples. Thermal cycling of the coated samples at 600 degrees C led to an increase in the modulus and decrease in the damping. Most of the changes in properties occurred after the first thermal cycle. The rmal cycling of the samples at 750 degrees C and 850 degrees C led to complete failure of the coatings. A specimen with a 1.0 mm alumina coa ting exhibited cracks even when cycled at 600 degrees C.