Room-Temperature Superplasticity in an Ultrafine-Grained Magnesium Alloy

Kaveh Edalati,Zenji Horita,Makoto Arita,Mitsuaki Furui,Takahiro Masuda,Ruslan Z Valiev,Xavier Sauvage

doi:10.1038/s41598-017-02846-2

Abstract

Superplasticity, a phenomenon of high tensile elongation in polycrystalline materials, is highly effective in fabrication of complex parts by metal forming without any machining. Superplasticity typically occurs only at elevated homologous temperatures, where thermally-activated deformation mechanisms dominate. Here, we report the first observation of room-temperature superplasticity in a magnesium alloy, which challenges the commonly-held view of the poor room-temperature plasticity of magnesium alloys. An ultrafine-grained magnesium-lithium (Mg-8 wt.%Li) alloy produced by severe plastic deformation demonstrated 440% elongation at room temperature (0.35 Tm) with a strain-rate sensitivity of 0.37. These unique properties were associated with enhanced grain-boundary sliding, which was approximately 60% of the total elongation. This enhancement originates from fast grain-boundary diffusion caused by the Li segregation along the grain boundaries and the formation of Li-rich interphases. This discovery introduces a new approach for controlling the room-temperature superplasticity by engineering grain-boundary composition and diffusion, which is of importance in metal forming technology without heating.

Highlights

Superplasticity, or ultrahigh tensile elongation prior to failure, is defined as the capacity of a material to be plastically deformed by over 400% under tension[1]
After extrusion as well as after 5 and 200 severe plastic deformation (SPD) cycles, the Mg-Li alloy exhibited a crystal structure containing α phase with a lattice parameter of 0.352 nm and β phases with the lattice parameters of a = 320 nm and c = 0.514 nm but with different microstructures
The fraction of α/α and β/β grain boundaries increases from 0.45 μm2/μm[3] in the extruded sample to 2.19 μm2/μm[3] and 3.37 μm2/μm[3] after 5 and 200 SPD cycles, respectively

Summary

Introduction

Superplasticity, or ultrahigh tensile elongation prior to failure, is defined as the capacity of a material to be plastically deformed by over 400% under tension[1]. We hypothesized that increasing diffusion at the grain boundaries in an ultrafine-grained Mg-Li alloy via SPD would lead to superplasticity of the alloy at room temperature.

Results

Conclusion