The formation of deformation kinks in body centred cubic crystals

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Abstract
Single crystal specimens of niobium with axes near 〈111〉 and 〈011〉 have been deformed at 295 K in either tension or compression in order to investigate the formation of kink bands in body centred cubic structures. Slip occurred in the 〈111〉 direction on both {1 0} and {11 } planes and the relative values of the critical resolved shear stresses were deduced. Kinks were observed, as broad bands, only in the 〈111〉 orientation specimens when deformed in tension. These had interfaces near the variant of the {111} plane which was perpendicular to the active slip direction. Their formation involved an effective macroscopic shear in a direction close to 〈1 0〉 which was perpendicular to the slip plane. The kinks were found to contain small plates on a different variant of {111} and these have been interpreted as secondary kinks. These results confirm the general hypothesis that kinking of single crystals occurs only when slip results in an increase of the Schmid factor. Further it has been demonstrated that kinks can be nucleated by slip on a mirror plane. These kinks are the only example of this type of mode, since those observed in other crystal structures are nucleated by slip in a direction parallel to a two-fold axis (or perpendicular to a mirror plane). No kink bands were observed in niobium when the active slip system was {11 }〈111〉, as this satisfies neither of these criteria.