Evidence for exceptional low temperature ductility in polycrystalline magnesium processed by severe plastic deformation

Abstract

An investigation was conducted to examine the mechanical behavior and microstructure evolution during deformation of ultrafine-grained pure magnesium at low temperatures within the temperature range of 296 -373 K. Discs were processed by high-pressure torsion until saturation in grain refinement. Dynamic hardness testing revealed a gradual increase in strain rate sensitivity up to m ≈ 0.2. High ductility was observed in the ultrafine-grained magnesium including an exceptional elongation of ~360% in tension at room temperature and stable deformation in micropillar compression. Grain coarsening and an increase in frequency of grain boundaries with misorientations in the range 15°  45° occurred during deformation in tension. The experimental evidence, when combined with an analysis of the deformation behavior, suggests that grain boundary sliding plays a key role in low strain rate deformation of pure magnesium when the grain sizes are at and below ~5 µm.

Effect of severe plastic deformation on the biocompatibility and corrosion rate of pure magnesium

“…The decrease in yield strength of magnesium at very fine grain sizes was reported earlier [39,40] and it was attributed to the onset of grain boundary sliding as a deformation mechanism. Recent reports have shown that fine-grained magnesium can exhibit improved ductility [15] and this is due to a combination of grain boundary sliding and basal slip propagation through grain boundaries having low misorientations [25]. Therefore, HPT processing may be used to produce ductile magnesium with moderate strength.…”

“…Therefore, HPT processing may be used to produce ductile magnesium with moderate strength. It is worth noting also that the strength of the HPT-processed material is strongly dependent on the strain rate and it may be increased by an appropriate annealing treatment [25].…”

“…Thus, a recent report demonstrated the fabrication of a biodegradable stent from a magnesium alloy processed by ECAP followed by extrusion and laser cutting [24]. In addition, HPT processing also shows the development of superplasticity and improved hardness in magnesium alloys and a recent report described the development of exceptional ductility in pure magnesium at room temperature after processing by HPT [25].…”

Microstructure and Hardness Evolution in Magnesium Processed by HPT

“…A Shimadzu DUH211s dynamic hardness tester with a Berkovich indenter was used for testing. The maximum applied load was 200 mN, the loading rate was ~70 mN/s and the dwell time was set to 0 s in order to avoid room temperature creep which has been shown to take place in magnesium processed by HPT 6 . The values of hardness were plotted as colour coded maps as a function of the indentation position.…”

“…Many papers have reported the formation of ultrafine grained structures in pure magnesium [3][4][5][6] and AZ31 [7][8][9][10][11] , AZ61 12 , AZ80 13 , AZ91 [14][15][16] , ZK60 [17][18][19] , Mg-Zn-Y 20,21 , Mg-Gd-Y-Zr 22,23 , Mg-Zn-Ca [24][25][26][27] and Mg-Dy-Al-Zn-Zr 28 alloys. The processed alloys exhibit improved strength but also some papers report superplastic behaviour 16,19,22,29,30 , improved hydrogen storage properties 3,[31][32][33][34][35] and improved corrosion resistance 24,27,36 .…”