Scatter in nonlinear ultrasonic measurements due to crystallographic orientation change induced anisotropy in harmonics generation

Abstract

Present study endeavors to establish the physical basis of an unprecedented trend in scatter, observed in nonlinear ultrasonic (NLU) parameter, associated with varying degree of crystallographic orientation change across crystallites in a polycrystalline material. It is shown that this scatter arises due to anisotropy in harmonics generation as a result of orientation change of slip systems in polycrystals with respect to the wave propagation direction. A near α titanium alloy has been taken as a model alloy to demonstrate this effect of crystallographic orientation change vis-à-vis change in the orientation of slip systems. Scale of crystal orientation change is shown to have a strong correlation with the degree of scatter in NLU measurements. Further, the study establishes the dominating effect of the scale of crystalline orientation change on harmonics generation as compared to variation in other microstructural parameters such as dislocation density, interface structure etc. Frequency distribution analysis of the scatter indicates that the distribution depends on the colony size which exhibits a linear correlation with standard deviation value. The dislocation string vibration model has been extended for harmonics generation in polycrystalline aggregates to explain the trend in the scatter during measurement of NLU parameter in the material.