EDDY CURRENT EVALUATION OF ELECTRICAL ANISOTROPY IN POLYCRYSTALLINE TI-6AL-4V

摘要

To conclude, some interesting parallels can be observed between the reported electromagnetic approach and conventional ultrasonic evaluation methods. Ultrasonic techniques can be used to exploit the fact that in polycrystalline materials, grain to grain differences in crystallographic orientation and the presence of grain boundaries provide source for scattering of ultrasonic energy. The presence of texture and additional phases of material also play an important role in the ultrasonic response of the material and the scatter provides valuable data which can be used to characterize the microstructural features. In ultrasonic flaw detection, the acoustic grain noise is clearly detrimental due to reduced detection threshold. Likewise, electromagnetic inspection techniques benefit from the fact that noncubic systems exhibit electrically anisotropic properties, allowing for microstructural characterization, and suffer from the fact that the electrical scatter originating from varying local resistivity raises the noise floor, thereby reducing flaw detectability. The electrical anisotropy observed with eddy currents in noncubic metals is therefore analogous to the elastic anisotropy observed with ultrasonic techniques and has strong implications for the nondestructive evaluation of polycrystalline titanium alloys. In the case of titanium alloys of primarily hexagonal symmetry, arguments favor the use of both eddy current and ultrasonic techniques for materials characterization with both providing useful information about microstructure. Although the electrical anisotropy of noncubic crystals is a well known physical fact, to the best of our knowledge, the significant role played by the microscopic electrical anisotropy of individual grains in the macroscopic eddy current response of the polycrystalline material has only recently been pointed out and investigated in any depth.