Atomic-Scale In Situ TEM Investigation of Deformation Twinning in HCP Crystals

[摘要] Twinning plays critical role in the deformation of hexagonal close-packed (HCP) metals. Developing HCP alloys with improved properties requires a fundamental knowledge on the twinning mechanisms which, however, remain largely theoretical contentions. Here, by using state-of-the-art in situ transmission electron microscopy, the atomic mechanisms of twinning in HCP metals were unprecedentedly revealed. Classically, the most prevalent {1 0 -1 2} twinning proceeds by shuffle-gliding of twinning dislocations (TDs). However, this mechanism has been challenged by extensive discrepancies in experiments. Here, it was found that the twinning nucleation and growth were primarily mediated by direct transformation of matrix prismatic planes into twin basal planes, which established the same lattice correspondence as an ideal {1 0 -1 2} twin. In detwinning, the twin boundaries retreated from incoherent basal│prismatic and prismatic│basal interfaces to coherent twin boundaries; thereafter, the classical TD mechanism dominated in the detwinning. The findings demonstrate that the twinning formation is not necessarily a shear-dominated process on the twinning plane. Moreover, a new mechanisms of HCP→FCC transformations was discovered, which proceeded by {1 -1 0 0}hcp→{1 1 1}fcc transformation. The FCC phase assisted the {1 0 -1 2} twinning of the HCP matrix as it transformed back to HCP structure through {1 1 1}fcc→(0 0 0 1)hcp transformation.It is generally believed that {1 1 -2 1} twinning is mediated by TDs. Here, direct evidences for the TD mechanism and theoretical twinning elements were unambiguously revealed. In stark contrast to the general consensus, atomic shuffles happened along 1/6<1 -1 0 0> on alterative basal planes. Classically, {1 1 -2 2} twinning is a contraction twinning mode. Here, a {1 1 -2 2} tension twinning mode was discovered, wherein basal plane was K2. The finding was further corroborated by the large twinning shear and dominant elementary TDs.This dissertation resolves long-standing controversies in HCP twinning, reveals prolific new transformation mechanisms pertinent to twinning, and provides crucial information for the twinning-based design and processing of HCP alloys with desired mechanical properties.

[发布机构] the University of Pittsburgh