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Laser ultrasonic investigations of recrystallization and grain growth in cubic metals Keyvani, Mahsa


This study evaluates the applicability of laser ultrasonics for metallurgy (LUMet) as a non-destructive tool in measurement of grain size evolution and quantifying recrystallization in FCC polycrystalline metallic materials. A systematic investigation has been conducted to correlate the ultrasonic attenuation parameter with the effective average grain size determined by metallography in two cobalt-based superalloys and pure copper. To correlate the ultrasonic attenuation parameter with the metallographic grain size, a series of thermo-mechanical treatments were carefully designed to generate samples with different average grain sizes. Equivalent area diameter (EQAD) and area weighted grain diameter (AWGD) including twin boundaries were selected as the measures of average grain size. The developed correlation for cobalt superalloys was used to monitor the recrystallization kinetics of cold-rolled and hot deformed specimens in real time through the refinement of the mean grain size as well as the grain growth kinetics. Furthermore, it was found that when a substantial tail exists in the microstructure, the AWGD-based correlation provides a better estimation of the average grain size than the EQAD-based correlation since the former changes according to the changes in grain size distribution. Moreover, the finite element modelling of wave propagation revealed that twin boundaries have similar scattering behavior as other high-angle grain boundaries. This suggests that the LUMet technique cannot be used to extract the fraction of twins. A versatile method was then introduced to harmonize all the existing empirical equations to evaluate the grain size change in FCC metals. It was observed that the amount of grain scattering is controlled by the single crystal elastic constants which should be known apriori. The harmonized equation can be used to measure the grain size evolution in other metals without the need to develop a new calibration or at least reduces the number of experiments and labor-intensive ex-situ characterizations required for the design of a new calibration.

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