Correlating Ultrasonic Attenuation and Microtexture in a Near-Alpha Titanium Alloy

Abstract

Near-a titanium alloys are an integral part of aeroengines; however, since the 1970s, it has been recognized that laboratory and field components fail in a reduced number of cycles when a dwell at the peak stress is imposed. Research over the last few decades has shown that one of the primary reasons for the debit in fatigue life is related to the presence of microtexture in these alloys. Many aeroengine components were forged before the concept of microtexture, and its deleterious effects, had been realized. Thus, because of the increased potential for early failure of these components, a need exists for a nondestructive method to assess the degree of microtexture present in legacy hardware in order to separate those which are prone to dwell fatigue failure from those that are not. Hardware with a high degree of microtexture can be scheduled for more frequent inspections to reduce the risk of in-flight failure. The present work describes a methodology by which this can be achieved using ultrasonic attenuation measurements of the component in pulse-echo imaging mode. The results indicate nearly linear dependence of ultrasonic attenuation on microtextured region size in the d/k = 0.1 to 1.0 range, where d and k are the effective microtexture region size in the direction of wave propagation and the ultrasonic wavelength, respectively.

Root-cause estimation of ultrasonic scattering signatures within a complex textured titanium

“…For titanium alloys, the presence of coarse grain structures with preferentially-oriented 'macro-grains' can occur during the initial forging and processing of a component [13,14]. When this is the case, the backscatter and attenuation of a pulse-echo ultrasound measurement typically shows significant spatial variability due to phase and amplitude modulation as described by Thompson [7] and Yu [9].…”

“…The electron micrograph depicts a typical titanium alloy microstructure with a mixture of primary alpha particles and secondary alpha platelets, where individual grain sizes were 5-20 microns in diameter. The pole figures depict preferred crystallographic orientation states in each of the three regions, where the presence of microtexture is indicated by a local clustering of crystallite orientations in the pole figure image [13,14]. Of particular note is the pole figure of region 7, which shows a highly oriented clustering of microtexture in the [0001] direction with additional microstructure orientations reduced.…”

“…The interaction of ultrasound energy fields within polycrystalline materials can be quite complex and has been a challenging and active research area for more than 50 years [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The original classical work of Mason [1,2], Papadakis [3][4][5], and Stanke-Kino [6] led to the development of key relationships for the frequency-dependent signal response of ultrasonic waves propagating and scattering in polycrystalline materials, where three scattering regimes have been identified for long wavelength (Rayleigh >>D), similar wavelength (stochastic ~D), and short wavelength (geometric <<D) grain interactions relative to the mean grain diameter, D. For many decades these relationships have provided a means for estimating statistically defined material properties such as mean grain size, where ultrasound velocity, attenuation, and backscatter measurements have provided material property estimates within ~10% of actual values [9][10][11][12][13][14].…”

“…As described by Thompson [7], significant deviations away from the predicted mean-field response can occur when any of these four assumptions are not met. Modern titanium alloys, for example, often contain microtexture, which manifests as regions of microstructure in which crystallites share a common crystallographic orientation over an extended length scale [12][13][14]. In such instances, the frequency-dependent, mean-field response breaks down, and complex, discrete scattering events can occur between macro-grain boundaries involving many different length scales.…”

A spherical harmonic approach for the determination of HCP texture from ultrasound: A solution to the inverse problem

“…Exemplary efforts in this regard include [15][16][17]. Relations have been found in titanium alloys between ultrasonic scattering and the presence of macrozones (or micro-textured regions, MTRs) [18,19], which are colonies or regions with a large number of grains configured with similar crystallographic orientations and affect the fatigue lives of titanium alloy components [20]. This has triggered much related research [19,[21][22][23].…”