The Deformation Modes And Cracking Modes Of Pure Mg And Mg-3Gd Alloy During Asymmetric Low-Cycle Fatigue

The poor plasticity,tensile strength and fatigue properties of pure magnesium(Mg)greatly limit its wide application in industry.It is found that adding rare earth elements to pure Mg can weaken the strong basal plane texture and improve its mechanical properties.In this study,pure Mg and Mg-3Gd alloys are used as research objects,uniaxial tension and compression experiment and quasi-in-situ fatigue experiment are used to explore the mechanical properties of Mg and Mg-3Gd alloys.Scanning electron microscopy(SEM),electron backscattering diffraction(EBSD)and slip trace analysis were used to characterize and analyze the microstructure evolution,deformation mechanism and fatigue microcracks of Mg and Mg-3Gd alloys,and the following results were obtained:(1)Rare earth element Gd greatly weakens the strong base plane texture of pure Mg,and the extreme density point of texture of Mg-3Gd alloy is deflected towards the extrusion direction(ED).Therefore,the texture of pure Mg and Mg-3Gd alloys is different,and the plastic deformation mechanism of them is very different under different loading modes.During the tensile-tension strain fatigue condition,the main deformation mechanism of pure Mg is slip during the whole fatigue cycle,but some twinning deformation occurs at the late fatigue stage.The main deformation mechanism of Mg-3Gd alloy during the whole tensile-tensile strain fatigue process is slip.During compressive-compressive strain fatigue condition,the main deformation mechanism of pure Mg is twinning and that of Mg-3Gd alloy is slip during the whole fatigue cycle.(2)During the tensile-tensile strain fatigue deformation,the number of slip traces in pure Mg grains(23 pairs)is more than that in Mg-3Gd alloy(15 pairs),because the average geometrical coordination factor(m’)of pure Mg is 0.85,which is higher than that of Mg-3Gd alloy(0.61).It is worth noting that the number of basal-basal slip trace transfers in Mg-3Gd alloy(10 pairs)is more than that in pure Mg(5 pairs),because the m_k(taking into account the Schmidt Factor and m’value)of Mg-3Gd alloy is 0.045,which is higher than that in pure Mg(0.037).During compressive-compressive strain fatigue,the average m’of pure Mg is 0.81,which is higher than that of Mg-3Gd alloy(0.62).However,due to the difference of deformation mechanism between the two alloys,the number of grains transferred by slip trace is more in Mg-3Gd alloy.(3)The fatigue microcrack propagation mechanisms of pure Mg and Mg-3Gd alloys are quite different,which is due to the different plastic deformation mechanisms under different loading modes.The proportion of intergranular cracks after fatigue failure of pure Mg alloy and Mg-3Gd alloy during tensile-tensile strain fatigue is～72%,while that of Mg-3Gd alloy is～66%.The intergranular microcracks of pure Mg are more easily connected to each other,while the intergranular microcracks of Mg-3Gd alloy are smaller and more dispersed and difficult to be connected to each other.The main crack types in pure Mg and Mg-3Gd alloys during compressive-compressive strain fatigue are transgranular crack and intergranular crack,respectively.