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 Mg_(89)Zn_(4)Y_(7)alloy or softening in Mg_(99.2)Zn_(0.2)Y_(0.6)and Mg_(97)Zn_(1)Y_(2)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 unrecrystallizedα-Mg grains due to a higher dynamic recrystallization activity during the extrusion process.

Mechanical properties and failure mechanisms of Mg-Zn-Y alloys with different extrusion ratio and LPSO volume fraction

In long period stacking ordered(LPSO)phase containing Mg-Zn-Y alloys,high elastic modulus and deformation kinks of LPSO phase considerably enhance the tensile yield strength,with slight detriment of or benefit to the ductility depending on its volume fraction.In present work,uniaxial tensile tests and fracture toughness tests are carried out using Mg99.2Zn0.2Y0.6,Mg97Zn1Y2,Mg89Zn4Y7 and Mg85Zn6Y9(at%)materials with different extrusion ratios.Extrusion processing enhances both strength and ductility due to the recrystallization of Mg grains.Variable plastic deformation mechanisms are activated depending on volume fraction of Mg and LPSO phase as well as their relative size during bending.{1012¯}<101¯1¯>tensile twins in Mg grains and deformation kinks in LPSO phase are observed,which dissipate large amount of deformation energy favoring for toughness.However,inherently brittle LPSO phase is detrimental to toughness.Microstructure-motivated empirical models for yield strength and fracture toughness prediction based on rule of mixtures are calibrated by experimental data.Energy release rates of individual mechanisms are estimated,which quantitatively indicate strong Mg/LPSO interaction.