Aiming at the plastic deformation behavior and intrinsic mechanism of magnesium, the team led by Professor Shan Zhiwei from Xi'an Jiaotong University (XJTU) has carried out in-depth research.

They found out that for submicron-sized Mg single crystals, when compressing along the <c> axis, plastic deformation dominated by cone dislocation slip occurs first. Surprisingly, as work hardening progressed, the samples, which were thought to have exhausted their plasticity, did not fail.

Deformation process of submicron magnesium single crystal column under <c> axial compression

(a) Initial sample (b) Formation and movement of dislocations (c) New grains formed in the lower right corner of the sample (white arrows) (d) Dislocations generated in the new grains (white arrows) (e) The sample was compressed flat (f) Electron diffraction collected on a flat sample (g) The stress-strain curve shows three stages of deformation: elastic deformation, plastic deformation-work hardening stage, plastic deformation-strain jump stage

When the flow stress increased to the level of 1 GPa, the sample was suddenly flattened without cracks. In addition, the flattened sample is no longer a single crystal, but consists of multiple small grains with a coaxial <a> axis orientation relationship. Moreover, large numbers of basal and non-basal dislocations are inside small grains.

Through systematic crystallographic analysis, microscopic analysis, atomic-scale characterization, combined with molecular dynamics simulations, the team proposed that the new grains are formed through a cone-basal transition.

After the formation of new grains, the plasticity that has been exhausted is regenerated, and the sample can continue to undergo large plastic deformation when loading is continued.

The research refers to this deformation-induced formation of new grains in the matrix grains as "deformation graining". This process does not have to rely on diffusion, and can occur rapidly at room temperature, and the new grains formed have a specific crystallographic orientation correspondence with the matrix grains.

The new grains grow up during loading, shrink during unloading, and grow again during secondary loading, reflecting the high mobility of grain boundaries.

In the newly formed grains, the plastic deformation coordinated by dislocations and twinning can continue to occur, so that the sample regains the plastic deformation ability.

This research has enriched the understanding of the plastic deformation mechanism and provides new inspiration for the deformation processing of magnesium: processing under high stress or high strain rate can induce a new deformation mechanism by high stress, thereby improving the deformation processing ability of magnesium.

New grains and their grain boundary structures

The work was published in Nature Communications under the title Rejuvenation of plasticity via deformation graining in magnesium. (Link to the paper: https://www.nature.com/articles/s41467-022-28688-9)

XJTU Professor Liu Boyu is the first author, and Shan is the first corresponding author. Professor Zhang Zhen from Hefei University of Technology is the co-first author and corresponding author. XJTU Professor Ma En and Professor Li Ju from the Massachusetts Institute of Technology are the co-corresponding authors.

Magnesium is the lightest metal structural material and has broad application prospects in aerospace, transportation, electronics and medical fields.