CHARACTERIZATION AND MODELLING OF MAGNETO-AUXETICITY, THE MAGNETICALLY INDUCED AUXETIC BEHAVIOR, IN GALFENOL

Abstract

Iron-Gallium alloy (Galfenol) is a magnetostrictive smart material (λsat ~400 ppm) with potential for robust transduction owing to good magneto-mechanical coupling and useful mechanical properties. In addition, Galfenol exhibits a highly negative Poisson’s ratio (denoted by ν) along the <110> crystallographic directions on {100} planes with ν values of as low as -0.7 under tensile loads. Consequently, their samples become wider when elongated and narrower when compressed (aka auxeticity). This is an anisotropic, in-plane and volume conserving phenomenon with compensating contractions and expansions in the third (out of plane) direction.

Since there is good magneto-elastic coupling in Galfenol, a negative Poisson’s ratio is expected to be observed under application of magnetic fields even under zero stress conditions. This work deals with systematically studying the magneto-elastic contributions in Galfenol samples between 12 and 33 atomic percent Ga as a non-synthetic (no artificial linkages, unlike foams) ‘structural auxetic’ material, capable of bearing loads. This investigation addresses the profound gap in understanding this atypical behavior using empirical data supported by analytical modeling from first principles to predict the Poisson’s ratio at magnetic saturation, multi-physics finite element simulations to determine the trends in the strains along the <110> {100} directions and magnetic domain imaging to explain the mechanical response from a magnetic domain perspective.

The outcome of this effort will help comprehend the association between anisotropic magnetic and mechanical energies and hence the magnetic contributions to the atomic level interactions that are the origins of this magneto-auxetic characteristic. Also, it is well established that a number of mechanical properties such as shear resistance and toughness depend on the value of Poisson’s ratio. There is a slight increase in these mechanical properties with non-zero ν values, but as we enter the highly auxetic regime (n<-0.5), these values increase by magnitudes. Hence, the possibility of n values approaching -1.0 under applied magnetic fields at zero stress is extremely intriguing, as these properties can be much larger than is possible in conventional materials. This has potential for several novel applications where the value of Poisson’s ratio can be magnetically tuned to keep it near -1 under applied stresses.