Observable Second Order Ferromagnetic to Paramagnetic Phase Transformation in Magneto-Mechanically Coupled Galfenol under Shock Load

The strain and subsequent volumetric change resulting from magnetostriction has been shown to result in a second order ferromagnetic-to-paramagnetic phase transformation, but the effect has had negligible influence on the mechanical response of ferromagnetic materials placing the effect in the realm of interesting but impractical phenomena to exploit for strong shock mitigation. That is, until the recent development of highly magnetostrictive materials such as Galfenol. This work explores the shock and spall load response of Galfenol up to peak stresses of 24 GPa. The HEL and spall behavior measured by plate impact experimentation are reported; however, the most significant finding is the observation of a peak stress dependent cusp between the HEL and peak state. The cusp is observed to be dependent on a low applied magnetization suggesting it to be a second order ferromagnetic-to-paramagnetic phase transformation which, by way of strong magneto-mechanical coupling, results in a 3-wave structure within the velocity-time profiles. Interestingly, the Hugoniot elastic limit and spall strength do not seem noticeably influenced by the applied magnetization despite the never before seen magnetic response of the cusp.