Study On Microstructures And Mechanical Behavior Of As-extruded Magnesium Alloys Under Various Thermomechanical Treatments

Twinning play an important role in plastic deformation of magnesium alloys.Currently twinning has attracted wide attention for the unique properties on strength andductility. But the underlying mechanisms of twinning in magnesium alloy are not wellunderstood, and still need further research.In this paper, based on the ZK21(Mg-1.5%Zn-0.6%Zr, wt%) as-exrudedmagnesium alloy, and modified with rare-earth2wt%Er. Heat treatment wereconducted for grow up the grain size. As-extruded alloys and heat treatment alloys wascompressed at room temperature, optical microscope, x-rays diffraction analysis (XRD),scanning electron microscopy (SEM), transmission elexton microscopy (TEM) andenergy dispersive spectroscopy (EDS) was used to study the microstructure ofas-extruded, heat-treatment state and compressed state alloys. The deformation behaviorof as-extruded alloys and heat-treatment alloys in compressive test was analyzed.The result showed that, the primary deformation mode of extruded ZK21-0Er alloyis {10-12} twin during the quasi-static compressive test, the basal slip is limited due tothe small orientation factor; but for ZK21-2Er alloy, the non-basal slip plays animportant role on compresive test because the zero orientation factor of basal slip, andgrain boundary sliding serves to relax the stress concentration. After400×40h heattreatment, the average grain size of ZK21-2Er alloy is same to extruded ZK21-0Er and400×1h ZK21-0Er alloys. The twinning was limited on quasi-static compressive testafter400×1h of ZK21-0Er alloy due to the solid solution of Zn, but the {10-12}twinning restart when the strain rate increased to1s-1for higher stress concentration.The twinning also was limited on ZK21-2Er alloy, even the strain rate increased to1s-1,and the activity of non-basal slip is enhanced by the interaction of RE elements withdislocation, which results in decreasing the CRSS values for non-basal slip.The magnesium alloys containing a high density of pre-exising twinning canmanufacture by quasi-static compression at ambient temperature. The twinningZK21-0Er and Mg-1.8Mn alloys were manufacture after5%and8%pre-compressionrespectively. The twinnig alloys has different vacancy concentration after differenttemperature quenching. The effect of quenching on the microstructures and mechanicalproperties of twinning magnesium alloy is analyzed.Results of the room temperature compression of twinning alloys showed that the yield strength and compressive strength decreased after quenching: yield strength ofZK21-0Er decreased from208MPa (as-deformation) to137MPa (350quenching),108MPa (400quenching) and103MPa (450quenching). The in-situmicrostructure observation was performed in compressive test of as-compressiveZK21-0Er and quenched ZK21-0Er alloy, which has the similar twinning feature. Theevident de-twinning was found in compressive test of quenched alloy. The highervacancy concentration may stimulate the de-twinning, and effect to compressivebehavior.The tensile yield strength and tensile strength decreased after quenching, and theelongation increased: yield stress decreased of ZK21-0Er from98MPa (as-deformation)to93MPa (350quenching),64MPa (400quenching),58MPa (450quenching).The in-situ microstructure observation was performed in tensile test of as-compressiveZK21-0Er and quenched ZK21-0Er alloy, which has the similar twinning feature. Theeffect of quenching was not outstanding in tensile behavior.The quenching can change the deformation behavior of twinning alloys on tensileand compressive tests, the tensile and compressive yield strength ratio of as-deformedZk21-0Er alloys is0.47, the ratio increased after quenching:0.68(350quenching),0.59(400quenching) and0.56(450quenching). The result show quenching canimprove the asymmetry in tension and compression of as-deformed magnesium alloys.