Influence of Average Diameter for Ferrite Crystallites on Ultrasonic Attenuation in Armco Iron

Abstract

It is known that the ultrasonic attenuation changes by microstructure and grain size of polycrystalline metals.Generally, it is considered that the mechanisms that cause loss in metals are mainly elastic hysteresis, forced motion of dislocations and scattering by the grains.Several authors have reported that ultrasonic attenuation of polycrystalline metals is fit by the equationα=A1f+A2f2+A4f4Here α is ultrasonic attenuation, f is ultrasonic frequency, A1, A2 and A4 are the coefficients. The component proportional to the fequency shows the presence of elastic hysteresis (including motion of magnetic domain wall in ferromagnetic metals). The second term proportional to the second power of frequency is indicative of the motion of dislocation (in this study, the term is negligibly small). The last term proportional to the fourth power of frequency is Rayleigh scattering loss.The theory of Mason and McSkimin gives the follwing form for A4 on longitudinal wave when α is neper/cm:A4=2π3T/v4<(ΔK/K)2>Av.where T=volume of grain, v=velocity of the ultrasonic wave, K=average effective elastic modulus over all directions for propagating the ultrasonic wave in a given direction in a crystallite, ΔK=difference of the effective elastic modulus from its average as function of direction and <(ΔK/K)2>Av=average of the fraction over all directions in the crystallite (ΔK/K)2.Lifshits, Parkhomovskii and other authors also give the same form as above for A4 which is proportional to the third power of grain diameter D.Therefore, we prepared five samples of armco iron with different ferrite grain size.Transducer was X-cut quartz disk 2cm in diameter for longitudinal wave measurement. The attenuation was mesured at the 1, 3 and 5 megacycles per second.Theoretical and experimental results were compared with respect to the attenuation of Rayleigh scatter in specimens.