Ultrasonic Signal Attenuation in Engine Titanium Alloys

The titanium alloys used in rotating jet engine components present interesting UT inspection challenges. They have complicated, duplex, anisotropic structures which vary on several dimensional scales [1,2]. Individual metal crystallites with typical dimensions on the order of several microns comprise the fine scale structure (microstructure). Colonies of aligned crystallites, which develop from prior beta grains during cooling, can have dimensions of several millimeters and comprise the large scale structure (macrostructure). Cylindrical billets destined for use in rotating engine components are typically inspected using 5 MHz broadband transducers, with higher frequency inspections planned. Because macrostructure dimensions often exceed the sonic wavelength, sound beams can be distorted during propagation, leading to modifications of ultrasonic signals. These modifications are of two basic types: (1) identical reflectors at the same depth but located at different sites on a specimen produce different sonic echoes due to the influence of the local macrostructure (signal fluctuation); and (2) the average signal strength is different from the value it would have in a similar fine-grained material (signal attenuation)