Direct observation of deformation twinning under stress gradient in body-centered cubic metals

Abstract In body-centered cubic (BCC) metals, plastic deformation usually takes place by deformation twinning under high strain rate. Although numerous efforts have been conducted on deformation twinning, the twinning mechanism is still shrouded in mystery. Here, we report a direct observation of deformation twinning in BCC tantalum and niobium by the use of in situ and aberration-corrected electron microscopy. Twin lamellae grow up from coherent twin boundaries by repeatedly extruding and subsequently expanding of bulges on the twin boundaries. A substantial stress gradient is detected around twin lamellae along the twinning direction. The twin growth can be described by a self-thickening mechanism through dislocation reactions at steps on coherent twin boundaries thus absent continuous supply of dislocations. The dislocation reactions can be activated and deactivated by the transition between normal step dislocations and complementary zonal dislocations. These results provide insights to deformation twinning in bulk BCC metals.