Ultrasonic Backscattering Measurements of Grain Size in Metal Alloys

Abstract

Grain size is a key property that is carefully controlled during processing to create specific mechanical properties and reliability in final components. Achieving specific grain sizes is especially important in aircraft engines where temperatures can range from 260°C at the bore to 704°C at the rim, creating fatigue cracking in the bore and creep damage in the rim region. These competing failure mechanisms led to nickel alloy disks possessing fine-grains, ~5 microns in the bore and coarse-grains, ~80 micron in the rim. We created an automated ultrasonic measurement tool which calculates grain size from ultrasonic backscattering measurement. The tool unifies the entire process including controlling the ultrasonic scanner, data acquisition, experimental and theoretical backscattering calculations, numerical optimization to obtain grain size, and visualization. We tested our tool on IN718, Waspaloy, Rene 88 and Alloy 10. Our automated “Ultrasonic Metallography Tool” produced a map of the grain size where the color scale is the grain size. The authors believe this is the first time such a map has been generated automatically from ultrasonic backscattering measurements. Our results compare favorably with traditional metallography agreeing to within 2 microns for some alloys.