Non-destructive testing of Terfenol-D

Abstract

Chemical composition and grain structure have a direct effect on the magnetostrictive behaviour of the compound Terfenol-D. The rare-earth (RE):iron and Dy:Tb ratios affect not only the magnetostriction but also the magnetocrystalline anisotropy and its temperature dependence. Grain structure has a critical effect on the strain coefficient, d 33 (dλ/d H) and its dependence on applied uniaxial prestress as a function of applied field. For transducer applications these factors are significant because gradients in composition and grain misorientation, leading to a reduction in strain coefficient, will cause inefficiencies. In both cases, the full potential of Terfenol-D can only be realised by applying larger fields. However, this in turn leads to higher induction losses in the Terfenol. The present paper describes a method and non-destructive techniques for a qualitative investigation into compositional and grain structure variations along the length of a magnetostrictive rod. The results of scans along different samples show how gradients in chemical composition and grain misorientations can be identified without recourse to sectioning samples. The application of uniaxial stress and DC bias fields along the length of a rod enable the scanning response to optimised. Features of the scan can then be correlated with the main defects observed by sectioning the sample.