Deformation behavior at cryogenic temperature in extruded Mg–Al–Zn alloy

The effect of grain size on tensile deformation behaviors at room- and cryogenic-temperatures was examined using extruded Mg–Al–Zn (AZ31) alloys with different average grain sizes of 3.7, 14.0, and 38.7 μm. The mechanical properties were influenced by the grain size as well as the testing temperature. Strength increased with a finer grained structure and a lower testing temperature. Regarding the elongation-to-failure in tension, all the alloys tested at cryogenic-temperature exhibited about half the values of those at room-temperature. Deformed microstructural observations revealed that deformation twins formed in the alloys with coarse- and meso-grained structures, but decreased with grain refinement. The reduction of twin formation at both room- and cryogenic-temperatures is due to enhancement of grain boundary plasticity. Contrary to our expectation, it is interesting to note that grain boundary compatibility contributes to activation of non-basal dislocation slips, irrespective of testing temperatures. In addition, dislocation slips lead to occurrence of strain hardening after yielding even at 77 K.