Deformation behavior of nitrogenated FeTiMn and FeTi single crystals

Abstract

The orientation dependence of the plastic-flow properties of nitrogenated Fe0.19Ti0.39Mn single crystals was investigated at temperatures between 77 and 473 K. Three orientations were chosen so that slip could be examined on {110} planes (orientation D), or on {112} planes in both the twinning (orientation A), and antitwinning (orientation B) directions. The plastic-flow properties of nitrogenated Fe0.16Ti single crystals of orientation D were investigated to evaluate the influence of manganese on ductility. The results showed that the primary effects of the coherent TiN particles on the nitrogenated crystals were to increase the critical resolved shear stress (CRSS) by about 250 MPa, to promote Lüders extension, and to decrease ductility as compared with non-nitrogenated crystals. Nitrogenated crystals of orientation B exhibited plastic instability after the upper yield point at all the temperatures investigated. Nitrogenated crystals slipped on the same slip system as non-nitrogenated crystals but showed a greater tendency for twinning below 295 K. At 295 K and below the nitrogenated FeTiMn crystals of orientation D showed greater elongation and more ductile fracture behavior than the nitrogenated FeTi crystals of this orientation. There was no apparent orientation dependence of the CRSS of the nitrogenated FeTiMn crystals. The nitrogenated crystals that showed necking prior to fracture exhibited pronounced shear bands within the necked region. These localized shear bands formed soon after necking began and subsequent deformation consisted of shearing within a band; final separation occurred by either ductile or brittle fracture prior to a complete shearing off of the two halves of the crystal. The onset of the localized shear bands and their orientation with respect to the primary slip system and the tensile axis appeared to be in accord with a recent model of shear localization in single crystals.