Microstructure-ultrasonic inspectability relationships in Ti6242: Signal-to-noise in fine-grain-processed Ti6242
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I. INTRODUCTION
NONDESTRUCTIVE evaluation by ultrasonic testing requires that detectable flaws have different acoustic behaviors from the bulk material under inspection. Materials with large, elastically anisotropic grains, such as cast ingots of steels, titanium alloys, and nickel alloys, present significant challenges to ultrasonic testing, because the sound waves reflected from grains represent a background “noise,” which can mask flaws.[1] Inspection techniques have been developed that use focused ultrasonic beams to maximize the fraction of a flaw within any instantaneously insonified volume and identify indications based both on maximum signal and signal-to-noise ratio.[2,3] The scattering of sound (attenuation of a propagating sound wave) in a polycrystalline metallic body can be described as a function of grain dimensions, intrinsic material characteristics, and ultrasound frequency.[4,5] Depending on the size of the scattering acoustic entity relative to wavelength, there are three different functional relationships among scattering, frequency, and grain dimensions:
over 40-fold among microstructures. Additionally, the scattering mechanism would change between the case of randomly oriented aTi particles and that of aTi particles contained within crystallographically oriented colonies. The speed of sound in aTi is about 6 mm/ms. At an ultrasonic frequency of 5 MHz, the wavelength is about 1.2 mm. Colony sizes greater than about 200 mm could change the scattering character from Rayleigh toward stochastic (phase), as described by the preceding equations. Experimentally, the size of prior b Ti grains and the nature of colony structures were indicated to be important variables affecting ultrasonic noise in single-phase and two-phase titanium alloys.[9] Thermomechanical processing techniques have been developed that use dynamic recrystallization in the a 1 b temperature range to achieve uniform-fine-grain (UFG) aTi particles and prevent colony formation.[10] Since the dynamic recrystallization generally results in a random structure, an absence of crystallographic texture would be expected. A study was defined, the subject of this report, to quantify the effect of UFG processing on the ultrasonic inspectability of Ti6242.
for l . 2pD, a 5 Tn 4U, termed “Rayleigh” scattering;[6] for l , 2pD or l > D, a 5 Dn 2S, termed “stochastic” or “phase” scattering;[7] and for l ¿ D, a } 1/D, termed “diffusion” scattering.
[8]
Here, a is attenuation, l is the wavelength of the ultrasound, n is frequency of the ultrasound, D is average grain diameter, T is scattering volume of the grains, and U and S are scattering factors based on elastic properties of the material. Thus, the scattering of sound in titanium can be particularly sensitive to microstructure, because when aTi particles are arranged into “colonies” with common crystallographic (and elastic) orientation, they can behave as extremely large grains. An individual aTi particle might be 5 mm in diameter, but a colony of aTi particles could be greater than 200 mm in d
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