Twinning-induced large tensile strains in directionally-solidified Fe(1-x)Ga(x) oligocrystals

Abstract

Fe1-xGax alloys require highly textured samples for optimal magnetostrictive performance. This is accomplished through directional solidification techniques, yielding very large grains (≈5 mm) and few grain boundaries in a sample. These oligocrystals have a drastically different microstructure than traditional materials, and render classical strain measurement techniques impractical. In this study, we measure the mechanical properties of commercial Fe81.6Ga18.4 oligocrystals using sub-sized tensile specimens and digital image correlation (DIC). While many samples fractured around 360 MPa (the literature yield/tensile stress), most had discontinuous yielding as low as 186 MPa. Many of the samples also exhibited large tensile strains, up to 5 %, which is uncharacteristic for a generally brittle material, with large specimen-to-specimen variations in the total elongation. In all specimens, DIC showed high strain bands that develop at very defined angles (≈60° from the tensile axis), although fracture in these high- strain regions was surprisingly rare. Electron back-scattered diffraction was performed on specimens to help correlate the observed strain field with the underlying crystallography, and revealed significant deformation twinning.