Room-Temperature Superplasticity in an Ultrafine-Grained Magnesium Alloy

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 T m) 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.

Metallurgical Alchemy by Ultra-Severe Plastic Deformation via High-Pressure Torsion Process

“…74,75) Moreover, Ultra-SPD enhanced the segregation of Li and Zn in the Mg/Mg and Al/Al grain boundaries, respectively. 74,75) Such microstructural modifications resulted in achieving room-temperature superplasticity in the MgLi and AlZn alloys, as shown in Figs. 11(a) and (b), respectively.…”

“…73) We showed recently that ultra-SPD can be employed to design new superplastic materials by engineering the chemistry and diffusivity of grain boundaries. 74,75) The application of ultra-SPD by 200 HPT turns to MgLi and AlZn alloys resulted in increasing the fraction of Mg/Li and Al/Zn interphase boundaries with high grain boundary diffusion and mobility. 74,75) Moreover, Ultra-SPD enhanced the segregation of Li and Zn in the Mg/Mg and Al/Al grain boundaries, respectively.…”

“…74,75) The application of ultra-SPD by 200 HPT turns to MgLi and AlZn alloys resulted in increasing the fraction of Mg/Li and Al/Zn interphase boundaries with high grain boundary diffusion and mobility. 74,75) Moreover, Ultra-SPD enhanced the segregation of Li and Zn in the Mg/Mg and Al/Al grain boundaries, respectively. 74,75) Such microstructural modifications resulted in achieving room-temperature superplasticity in the MgLi and AlZn alloys, as shown in Figs.…”

Superplastic Behavior at Lower Temperatures of High‐Strength Ultrafine‐Grained Al Alloy 7475

“…Recent studies on model alloys of the Al–Zn and Mg–Li systems showed that formation of the UFG structure—with grain boundaries that contain layers formed by segregations of alloying elements—could ensure signs of SP in these alloys, even at room temperature (RT) . In ref., it is reported that a Al–30Zn alloy in the UFG state demonstrated elongations of 235% at RT and 265% at 100 °C, at a strain rate of 10 −4 s −1 .…”

Effect of long-term natural aging on microstructure and room temperature superplastic behavior of UFG / FG Zn-Al alloys processed by ECAP

“…Furthermore, recent studies have shown that some materials exhibit superplastic behavior even at room temperature (RT) if grain sizes are decreased down to sub-micron levels [3 -19]. Although RT superplasticity was reported in some Sn-Bi [3], Pb-Tl [4], Pb-Sn [5] and Mg alloys [6], binary Zn-Al alloy system is one of the well-known and most commonly studied one for this purpose [7 -19]. In this alloy system, RT superplasticity was achieved in dilute Zn-Al alloys [7 -9], eutectic Zn-5Al [10] alloy and eutectoid Zn-22Al [11 -19] alloy.…”