Spatio-temporal characteristics of propagative plastic instabilities in a rare earth containing magnesium alloy

An experimental investigation of the spatio-temporal characteristics of propagative plastic instabilities in quasi-static, room-temperature tensile tests of fully annealed Mg ZEK100, a rare earth containing Mg alloy (0.2 wt.% Nd), is presented with the aid of stereo digital image correlation. Results from specimens aligned with the sheet transverse direction revealed the nucleation of a Lüders band at the fixed specimen end, followed by continuous propagation to, and ultimate termination at, the moving end during yield point elongation (YPE). Serrations in tensile flow curves, observed both during and after YPE, were attributed to propagating deformation bands, similar to the Type A Portevin-Le Châtelier (PLC) bands. Both strain and strain-rate contours enabled exploration of band nucleation and quantification of band kinematics. We also used TEM, EBSD, and electron diffraction to examine microstructural evolution during tensile deformation. Results were used to determine which, if any of the commonly cited microstructural mechanisms, such as diffusing solute pinning of mobile dislocations, precipitate shearing of dislocations, or solute locking of dislocations by a rare earth addition, is responsible for the observed Lüdering and PLC behavior. We found evidence that Lüdering is due to twinning. Although we could not definitively demonstrate a microstructural mechanism that is responsible for PLC effect, pinning and depinning of dislocations via diffusing Zn solute clouds could not be ruled out as the underlying cause. Implications of the experimental results for theoretical model development are discussed.