Grain refinement of AZ31 and ZK60 Mg alloys - Towards superplasticity studies

Low temperature superplastic (SP) behavior (mechanical and deformation mechanisms) of two commercial Mg-based alloys (AZ31 and ZK60) was characterized. The two alloys were tested in the as extruded condition with initial grain size of 15 um (AZ31) and fine (2 um) and coarse (25 um) grains mixed randomly for the ZK60. Strain rate was activated in the range 10^-5-1 s^-1 at 450 K (0.49T_m) in order to determine the deformation capacity curves (elongation to failure vs. strain rate), and to evaluate the strain rate sensitivity coefficient, m, from the stress versus strain rate curves. Optical, scanning and transmission electron microscopy observations (SEM and TEM) were performed to elaborate on the dynamic recrystallization (DRX) grain growth, fracture modes and deformation mechanisms at the SP mode. In addition, X-ray diffraction was utilized to track for microstructural classification. Although low temperature was applied, the ZK60 exhibited superplastic-like behavior and the maximum peak of elongation (220%) was detected at 1 x 10^-5 s^-1 with m equal to 0.2. In AZ31 SP behavior was suppressed due to grain growth, while for ZK60, DRX was detected. However, for the latter alloy, it was observed that the coarse/fine grain interface was the trigger for microcracking initiation. Actually, this phenomenon reduces the SP capacity of the ZK60 alloy. Surface observations and TEM findings indicate that grain boundary sliding with homogeneous character is the controlling SP deformation mode. Typical dislocation features have supported this deformation mode, mainly by grain boundary dislocation pile-ups. More sophisticated extrusion processes as equal angular channel extrusion (EACE) is likely to be considered in the future as a mean to improve grain homogeneity and produce ultra fine grain microstructure.