The effect of the 18R-LPSO phase on the fatigue behavior of extruded Mg/LPSO two-phase alloy through a comparative experimental-numerical study

The fatigue behavior of four extruded Mg-Y-Zn alloys containing different volume fractions of long-period stacking ordered (LPSO) grains was investigated through a comparative study combining experiments and crystal plasticity finite element simulations. Strain controlled low-cycle fatigue experiments were conducted at different strain amplitudes and revealed a limited cyclic hardening in Mg₈₉Zn₄Y₇ alloy or softening in Mg_99.2Zn_0.2Y_0.6 and Mg₉₇Zn₁Y₂ alloys. A decrease in the fatigue life against the plastic strain with the increase in LPSO phase volume fraction was observed and was related the limited ductility of extruded LPSO grains. Stress-strain hysteresis curves were used to calibrate and validate a crystal plasticity model taking into account twinning and detwinning. The interaction of the different phases on the distribution of local micro-mechanical fields at the grain scale was then analyzed on synthetic microstructures under strain-controlled conditions. Deformation twinning activity was predicted in coarse unrecrystallized grains and tended to disappear with the increase in the LPSO phase volume fraction. Cleavage-like facets observed in LPSO grains were related to high tensile stress, especially at the Mg/LPSO interface, due to the limited number of deformation mechanisms in LPSO crystal to accommodate out-of-basal plane strain. The increase of the fatigue limit with the increase in LPSO phase volume fraction was finally associated with the decreasing presence of coarse unrecrystallized alpha-Mg grains due to a higher dynamic recrystallization activity during the extrusion process.