Verification of Nondestructive Creep Damage Evaluation of Ni-Base Superalloys of Controlled Solidification by the Laue Method and Effect of TBC on Materials
Authors: Yokoyama Ryoichi , Omote Kazuhiko , Kobayashi Daisuke , Iwasaki Shinya , Tateishi Toshikatsu 
Published: August 2024

Technologies for evaluating the creep damage of Ni-base superalloys of Directionally Solidified (DS) materials or single crystal materials, used as gas turbine blades, have been recently developed. One notable inspection technique is the electron back-scatter diffraction (EBSD) method which uses the misorientation analysis within grains of the Ni-base superalloy. However, this is a destructive technique. Another notable inspection technique is the rocking curve method, which uses the relationship between the full width at half maximum (FWHM) of a rocking curve by X-ray diffractometry and the creep damage of the Ni-base superalloy. It requires rotating the sample. Hence, these techniques have problems in practical applications.

We have proposed an exclusive technique by applying the transmission Laue method to Ni-base superalloys, to address the problems with the above techniques. All of these techniques utilize a master curve, made by evaluating test pieces damaged at several stages and used as a standard for sample evaluation, however how to evaluate the creep damage of sample by each method is different as described above.

There are four advantages to the new technique (patent pending), as follows;

Creep damage of a sample is evaluated

1. without moving the sample.

2. non-destructively.

3. regardless of the thickness.

4. in a conventional laboratory.

The first advantage uses the Laue method with polychromatic X-rays, which allows the sample to be measured without moving it. Second, the high energy X-ray microbeam probes the sample below any surface coatings by transmission. Thrird, our measurement technique probes the grains within the sample crystal regardless of their depth. Fourth, our new apparatus for generating high energy X-rays allows the samples measured in a conventional laboratory. We are calling this technique “the crystallite orientation deviation method”.

In this study, the new technique is primally verified through the theoretical examination for applying the Laue method to the DS or the single crystal material by considering elastic and inelastic crystallite orientation deviations.

Then, results of creep tests for test pieces of a Ni-base superalloy of DS material were compared with experimental results obtained from making two master curves by COD using the same test pieces cut out into two sets, respectively one set without thermal barrier coating (TBC) and the other set with TBC. As results, it was found that tendances between the results of COD and the results of creep tests had a good agreement with each other. That is, it validates that COD can successfully evaluate the creep damage of the Ni-base superalloy. Furthermore, it was also found that the results of test pieces without TBC had a good agreement with those with TBC. It shows that the TBC coating on the surface of Ni-base superalloy was correctly penetrated by the polychromatic X-rays based on COD.

We expect that this technique COD also can be applied to polycrystalline materials in addition to the single crystals, especially the materials having coarse grains. It suggests that many aircraft parts may be evaluated by COD.