Incipient deformation twinning in dynamically sheared bcc tantalum

Mechanical twinning has been introduced into a body-centered cubic metal, tantalum, through shear-dominant dynamic loading at a high shear strain rate (up to 3 × 104/s) at 77 K. Direct measurement of shear stress combined with transmission electron microscopy (TEM) confirmed the occurrence of deformation twinning at applied global shear stresses of 520 MPa and above. The TEM characterization was focused on nanometer-sized individual twins that are interspersed and aligned in association with slip bands. These small twins belong to the {1 1 2}〈111¯〉 twinning system but show unusual morphologies and growth characteristics, and are believed to be incipient twins at the early stages of their development. TEM analysis revealed a multifaceted growth characteristic of the incipient twins following at least two {1 1 2} planes in the 〈111¯〉 zone. While the formation of a macroscopic twin is a result of growth and coalescence of an array of small twins, the early stage growth of an incipient twin is rationalized through a slip-assisted double-cross-slip growth mechanism.