Phase transformation induced transitional twin boundary in body-centered cubic metals

Body-centred cubic (BCC) metals possess unstable intrinsic stacking fault and high resistance for deformation twinning, which often induce complex twinning dynamics that cannot be fully understood in the frame of classic twinning theory. Here, we reveal the intrinsic coupling between phase transformation and deformation twinning in pure niobium (Nb) subjected to extreme deformations. Shuffle-mediated polymorphic transformations between BCC (β), omega (ω), orthorhombic martensite (α") phases induce three different twinning paths, without the involvement of classic twinning shear. Residual interfacial phases (mostly ω phases) on TBs can help to reduce the transformation barrier and act as important precursors for the discrete twin thickening, well inteperating several uncommon twinning dynamics experimentally-observed in BCC metals and alloys in literatures. These findings not only advance our understanding of the origin of uncommon twinning behaviors in a broad class of BCC metals and alloys, but also benefit the development of high performance BCC materials.