Abstract
Design and processing of advanced lightweight structural alloys based on magnesium and titanium rely critically on a control over twinning that remains elusive to date and is dependent on an explicit understanding on the twinning nucleation mechanism in hexagonal close-packed (HCP) crystals. Here, by using in-situ high resolution transmission electron microscopy, we directly show a dual-step twinning nucleation mechanism in HCP rhenium nanocrystals. We find that nucleation of the predominant {1 0 −1 2} twinning is initiated by disconnections on the Prismatic│Basal interfaces which establish the lattice correspondence of the twin with a minor deviation from the ideal orientation. Subsequently, the minor deviation is corrected by the formation of coherent twin boundaries through rearrangement of the disconnections on the Prismatic│Basal interface; thereafter, the coherent twin boundaries propagate by twinning dislocations. The findings provide high-resolution direct evidence of the twinning nucleation mechanism in HCP crystals.
Highlights
Design and processing of advanced lightweight structural alloys based on magnesium and titanium rely critically on a control over twinning that remains elusive to date and is dependent on an explicit understanding on the twinning nucleation mechanism in hexagonal close-packed (HCP) crystals
As more than one atom exists in the motif of the HCP crystal, shear alone cannot move all atoms to their correct positions in the twin; additional atomic adjustments are always required in the twinning process[17], which significantly affects the dynamic process of twinning[13,18,21,22] and poses tremendous challenges on direct atomic interrogation
By using advanced crystal manipulation techniques and in situ high-resolution transmission electron microscopy (HRTEM), twinning nucleation processes in HCP rhenium nanocrystals are directly captured at atomic resolution
Summary
Design and processing of advanced lightweight structural alloys based on magnesium and titanium rely critically on a control over twinning that remains elusive to date and is dependent on an explicit understanding on the twinning nucleation mechanism in hexagonal close-packed (HCP) crystals. By using in-situ high resolution transmission electron microscopy, we directly show a dual-step twinning nucleation mechanism in HCP rhenium nanocrystals. The findings provide high-resolution direct evidence of the twinning nucleation mechanism in HCP crystals. By using advanced crystal manipulation techniques and in situ high-resolution transmission electron microscopy (HRTEM), twinning nucleation processes in HCP rhenium nanocrystals are directly captured at atomic resolution. It is revealed that the {1 0 −1 2} twinning nucleates through a dual-step mechanism lead by transformations of parent prismatic (P) planes into the twin basal (B) planes; this process establishes the lattice correspondences of the twin plus a minor rotational deviation from the ideal parent–twin mutual-orientation; subsequently, ideal twin forms by the rearrangement of interfacial defects on the P│B interfaces and ensuing formation of CTB. The findings provide direct evidences to the twinning nucleation mechanism in HCP crystals
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