Precipitation-strengthened micromechanical behaviors of magnesium alloy under cyclic loading

The microscopic-deformation mechanisms of an extruded magnesium alloy with and without precipitates [Guinier-Preston (GP) zones] subjected to cyclic deformation were investigated by in-situ neutron-diffraction (ND) measurements and crystal-plasticity modeling. The relationship between the macroscopic-cyclic-deformation behavior and the microscopic responses (particularly twinning and detwinning) at the grain level was established. The general deformation-mechanism evolution in the solution-state (ST) sample was similar to that in the peak-aged-state (PA) sample over fatigue cycles. Both samples plastically deformed by extension twinning during compression, and by a sequential process of detwinning and dislocation motion under reverse tension. The main difference is that in the PA sample, the presence of precipitating particles constrains the twinning/detwinning behaviors, which leads to an increase in the participation of dislocation slip in the plastic deformation and then induces a strengthening effect during cyclic loading. Based on the combination of the previous in-situ ND results and crystal-plasticity model, our work provides a comprehensive analysis of the interaction between the precipitation strengthening and twinning/detwinning mechanism under the whole multi-cycle cyclic loading and their effect on the macro- and micro-mechanical behavior of the precipitate-strengthened magnesium alloys.