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Formation of twins

We coupled atomic-scale simulation, dislocation theory, and in-situ TEM to innovate a new nanopillar design. With MD, we studied the influence of nanopillar geometry with the idea of altering the stress fields generated under compression to induce intersecting slip paths. This figure shows the non-conventional, discovered shape that successfully isolated a slip-to-twin transition event under different stress states.

Grain-scale effects on twinning

With powder metallurgy, we fabricated bulk Mg samples with equiaxed and microlaminated grain structure. Results show that a reversed yield anisotropy was observed in the microlaminated sample, which is related to the low twinning activity, limited twinning growth and the anisotropy induced by the microlaminated grain structure based on our EBSD and TEM investigations.

Disconnection-mediated twin embryo growth in Mg

As extensive studies exist for the growth mechanisms of mature twins, a comprehensive understanding on twin embryo growth is still lacking. In this work, we explored different processes occurred during the growth of a twin embryo in single Mg crystals, from the nucleation and propagation of twinning disconnections, to the motion of twin boundaries and twin tips. Atomistic simulations are used to reveal the atomic-scale growth mechanisms. Twinning disconnections nucleated at the intersections between basal-prismatic/prismatic-basal interfaces and twin boundaries, and propagate at constant velocities for a given applied shear stress. Accumulation of twinning disconnections on twin boundaries occurred, suggesting that disconnection propagation is the rate-limiting process for twin embryo growth. These results pave the way for developing the phenomenological model describing twin embryo growth at larger length-scale. A logarithmic relation between twin length and twin thickness was developed and the ratio of disconnection velocity to twin tip velocity determines twin embryo shape. The data point extracted from the work of Yu et al. follows the trend predicted by MD data, which validates our simulation and proves the robustness of the phenomenological model. Our work fills an important gap in the understanding of twinning by providing a clear and consistent description of the early stages of twin embryo growth. The time evolution of twin length and thickness are studied together for the first time, with kinetic parameters such as twin tip velocity and twinning disconnection velocity obtained.

Twin Nucleation from a Boundary in HCP Mg

Extensive experimental and numerical studies have been devoted to probe the origins of deformation twinning in metals over the decades. However, fundamental insight into the origins of twinning has been precluded by both the complexity of the atomic displacements, as well as the ultrafast kinetics that govern twin nucleation. Here, we demonstrate an in-situ straining approach to confine and capture twinning events at an internal nanoscale boundary in HCP Mg. With increasing applied strain, nucleation of a twin embryo at the internal boundary and its subsequent propagation into the crystal is observed. A stable twin embryo consisting of several atomic layers is seen to form under stress and remain in the boundary after unloading. As the twin transitions from an embryo to a lamella, an intermediate stage involving the coalescence of two embryos into a larger one in the boundary occurred prior to propagation into the crystal. Both the coalescence and propagation process are reversible upon unloading. Calculations by a multiscale, crystal plasticity model predict that twin type and variant selection of the stable embryo are governed by the internal stress state prevailing at the boundary. Our results shed light into the fundamental role played by dislocations and local stress at the boundary in controlling the nucleation of twin embryos.

Summary

The class of hexagonal close packed (HCP) metals, such as Mg and Ti, can provide a potential solution to the ever-increasing portfolio of structural applications that require advanced materials with unprecedented combinations of mechanical properties. Successful incorporation of HCP metals into engineering designs is, however, hindered by their limited plasticity. Recent results have introduced the exciting potential of "twin mesh engineering" to overcome the long-standing problem of simultaneous strengthening and toughening in HCP metals.

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Formation of twins

Grain-scale effects on twinning

Twin Mesh Module

Disconnection-mediated twin embryo growth in Mg

Twin Nucleation from a Boundary in HCP Mg

Summary