# Anisotropy Corrected FMC/TFM Based Phased Array Ultrasonic Imaging in an Austenitic Buttering Layer
## 5 Conclusions  
The effect of anisotropic ultrasonic velocity on FMC/TFM image reconstruction is studied in the buttering layer of an austenitic nickel base superalloy (alloy 617). The orientations of the columnar grains with \(<\)100\(>\) crystallographic direction are observed to be in the buttering thickness direction (perpendicular to the buttering/10 Cr steel interface), leading to the minimum Young's modulus and, thus, the minimum ultrasonic velocity in this direction. The angle dependent ultrasonic velocity measured using a through transmission FMC/TFM approach indicated a variation of about 600 m/s in ultrasonic velocity, with the minimum in the buttering thickness direction (0\({}^{\circ}\)) and the maximum at about 50\({}^{\circ}\). The variation in ultrasonic velocity with angles in the range of \(\pm\)68\({}^{\circ}\) from the buttering thickness axis (0\({}^{\circ}\)) could be approximated with a simple sine curve.  
Splitting of defect indications was observed when the TFM images were generated, considering the buttering layer as an isotropic material having any single value of ultrasonic velocity in the range observed in the buttering layer. This led to reduced sensitivity of defect  
Figure 8: Variations in calculated time of flight values for 64 elements configurations working in pulse-echo mode.  
detection and resolution. The angle dependent ultrasonic velocity could be effectively used for obtaining TFM images with high sensitivity and good resolution. Further, an alternate approach is proposed and demonstrated for obtaining enhanced FMC-TFM imaging in the anisotropic buttering material by using a suitable isotropic velocity much higher than those observed in the material in any direction. The basis of this methodology is also described through suitable simulated conditions. It was found that there is a significant improvement in the SNR of angle-dependent velocity corrected TFM images, and the SNR in the alternate approach is almost close to the angle-dependent velocity corrected TFM images. The alternate approach can be directly implemented in any commercial equipment for improved inspection in an anisotropic material with a symmetrical anisotropy about the thickness axis in the inspection plane.  
Conceptualization, A.K.; methodology, S.P.; software, A.K. and S.P.; validation, S.P.; formal analysis, S.P., A.K. and K.V.R.; data curation, S.P.; writing--original draft preparation, S.P.; writing--review and editing, A.K. and K.V.R.; visualization, S.P.; supervision, A.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to restrictions.  
Authors are thankful to [PERSON], Head, Advanced Materials Fabrication Section for providing the sample with the buttering layer and [PERSON], Head, Fatigue Studies Section for the EBSD studies. Authors are also thankful to [PERSON], Materials Characterization and Engineering Group, and [PERSON], Metallurgy and Materials Group for their encouragement and support.  
The authors declare no conflict of interest.