Monash University, Australia, announces that its researchers have developed a cold-rolling technology in which unalloyed magnesium “becomes super-formable at room temperature.” The team, led by ASM member and Materials Science Prof. Nick Birbilis, showed that despite possessing a strong texture, pure magnesium can be cold-rolled to a strain at least eight times that possible with conventional processing. The resultant sheet can be further formed without cracking due to grain size reduction to the order of one micron, and the dominance of inter-granular mechanisms over the usual slip and twinning. These findings provide a pathway for developing highly formable products from magnesium and other hexagonal metals that are traditionally difficult to form at room temperature.
Experiments showed that polycrystalline pure magnesium becomes super-formable at room temperature after it is extruded at or below 80°C (176°F). Specimens extruded in the temperature range 150 to 400°C (212 to 752°F) have poor formability at room temperature. They fracture when compressed by 20 to 30% reduction in height, consistent with previous studies.
However, specimens extruded at or below 80°C (176°F) do not fracture during compression at room temperature and a strain rate of 10−3 per second. In contrast to the high work hardening of the specimens extruded at higher temperatures, these super-formable specimens reportedly exhibit no work hardening after yielding. Instead, true stress decreases gradually with strain, implying minimal twinning and dislocation slip during compression.
Such behavior is associated with grain boundary sliding and/or dynamic recrystallization in magnesium alloys tested at elevated temperature. However, while the 400°C (752°F) extruded specimen fractures after ~20% height reduction, the 80°C (176°F) extruded specimen can be compressed from 10 to 1.5 mm without fracture. Further height reduction from 1.5 mm is possible if the compression test continues.
Read the Nature article here: https://www.nature.com/articles/s41467-017-01330-9
