Nondestructive Creep Damage Evaluation for Ni-Base Superalloys of Controlled Solidification by Laue Method and Effect of Sample Thickness

Ni-base superalloys exhibit high deformation resistance characteristics, even at elevated temperatures. However, it is difficult to determine the state of damage from the appearance and observation of the microstructure. Although it is possible to quantitatively estimate plastic strain and creep strain using electron backscatter diffraction (EBSD), it is a destructive method. On the other hand, X-ray diffraction using the Laue method has the potential to non-destructively evaluate material damage. In this study, we investigated the effect of sample thickness on the evaluation using the Laue method. We also compared the creep damage evaluation results of solidification-controlled Nibase superalloys using the EBSD and Laue methods and examined the possibility of non-destructive evaluation of creep damage using the Laue method. The specimens used to investigate the effect of sample thickness were single-crystal blocks of Ni-base superalloys. Samples were prepared with five different thicknesses. Next, we compared the full width at half maximum (FWHM) of Ni-base superalloys in single-crystal blocks of different thicknesses. The measured FWHM were almost constant, independent of the thickness. This may be because the combination of subgrains and X-ray microbeams results in a sparser distribution of detected diffraction spots and a decrease in the number of high-intensity diffraction lines. The specimens for comparison of the EBSD and transmission Laue methods were Ni-base directionally solidified (DS) superalloys, and damaged pieces were fabricated at several stages of creep testing up to rupture and evaluated using each method. As a result of comparing the EBSD and transmission Laue methods, the technique is capable of detecting microstructural changes associated with the later stages of creep deformation. These results indicate the transmission Laue method is a promising non-destructive evaluation method because the FWHM does not depend on the sample thickness and can perform evaluations like the EBSD method.