: In situ observation of atomic scale twinning Process in HCP Crystals

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

NON-TECHNICAL SUMMARY Plastic deformation plays a crucial role in mechanical behaviors of crystals. Particularly, where the atoms are arranged in the pattern of hexagons, called hexagonal close packed metals and alloys such as magnesium or titanium-based alloys, twinning (two separate crystals having the same structure in a symmetrical manner) is an important type of plastic deformation, which critically influences the mechanical behaviors such as ductility, strength, work hardening, and fracture. As such, twinning has to be understood and controlled for designing and processing the hexagonal close packed metals and alloys. However, this has been impeded by the elusive understanding of atomic scaled mechanisms of twinning processes in the metals. Despite tremendous research efforts, for decades, how atom movements influence the mechanism of twinning remains poorly understood. The proposed research will employ high resolution transmission electron microscopy to investigate atomic-scale twinning processes in the materials, providing in-depth understanding on the role of atom movement in twinning of complex crystal structures. The project will provide important guidance for twinning-based alloy design and processing for achieving superior mechanical properties. Thereby, it will advance the application of light metal-based structures. The program will integrate research and education through training graduate/undergraduate students with diverse demographic backgrounds (particularly, female and minority) and their participation in national laboratories as well as outreach to elementary school through Pittsburgh Carnegie Science Museum. TECHNICAL SUMMARY Twinning plays a crucial role in mechanical behaviors of crystals. Particularly, in hexagonal close packed (HCP) metals and alloys, twinning, in addition to dislocation slip, can be profusely activated and critically influences their ductility, strength, work hardening, texture formation and fracture, primarily because twinning can carry deformation along the <c> axis of the HCP crystal where dislocation plasticity is limited. As such, twinning has to be controlled for designing and processing HCP alloys with improved mechanical properties. However, this has been impeded by the elusive understanding of atomic scaled mechanisms of twinning nucleation and growth in HCP crystals. In twinning, a part of the parent lattice is reoriented and the product lattice is mirrored by the parent about the twinning plane. Classically, such a lattice reorientation is achieved by a homogeneous simple shear which carries all or a fraction of the lattice points to the twin. The shear is mediated by coordinated movement of twinning dislocations on the twinning plane. The classical description of deformation twinning has been validated extensively in cubic structures. A significant difference in twinning of double-lattice structures, such as HCP, is that a twinning shear cannot carry all the parent lattice points to the twin positions. As a result, additional atomic movements, called shuffles, are required to accomplish twinning. Despite tremendous research efforts, for decades, how atom shuffles influence the mechanism of twinning remains poorly understood. Atomically-resolved direct experimental investigation are necessary for exploring the actual atomic shuffle and shear during twinning nucleation and growth, and hence obtaining a fundamental understanding on twinning mechanisms in HCP crystals. The proposed research will employ state-of-the-art in situ high resolution transmission electron microscopy (HRTEM) to investigate atomic-scale twinning processes in HCP crystals, such as twinning nucleation, growth and pertinent transformations as well as the orientation-dependent competition between dislocation plasticity and twinning at atomic resolution. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.