Deformation And Heat Treatment Of GW83K Alloy

Among the most commonly used strengthening approaches,deformation and heat-treatment can improve the mechanical properties of magnesium alloy significantly via deformation strengthening and precipitation strengthening,hence promote the application of Mg alloy which is characterized of lightweight,high strength rate and high modulus rate.In this current research,high strength was obtained in Mg-8Gd-3Y-0.4Zr(GW83K)alloy through the combination of forging,extrusion and subsequent heat-treatment.Furthermore,effect of forging and extrusion parameters on the formability,microstructure and mechanical properties were investigated,and the optimal processing parameters were figured out.Ageing treatment was employed to further strengthen the alloy in different condition(Solid solution treated,Forged,Forged + Extruded),and the effect of ageing treatment was analyzed.For alloy with the best mechanical properties,its strengthening mechanism was studied and its unique bimodal structure was supposed to account for the excellent combination of strength and ductility.For forgeability,the suitable temperature is above 380?,and a total deformation up to 45% is acceptable at 430?.With forging temperature increasing from 380? to 430?,considerable dynamic recrystalization(DRX)took place at the expense of twinning,resulting in the improvement of mechanical properties.But at 480?,complete DRX grains fully consumed the deformation twin and coarsened,leading to the decrease of tensile properties.Holding the forging temperature at 430? and raising total deformation,DRX and texture with c-axis of most grains normal to elongation direction(ED)were intensified evidently,accounting for the increasing mechanical properties along ED and the texture also enhanced the anisotropy of tensile properties between ED and forging direction(FD).In terms of the extrusion deformation,the microstructure of GW83K transferred from bimodal structure with coarse grains embedded in the ultrafine DRX grains to fully-DRXed morphology as the extrusion temperature increase from 325? to 450?,and accordingly,its strength increased at first and declined after the peak point was reached.The best mechanical properties was obtained with extrusion temperature of 375? and extrusion ratio of 4: yield strength(YS)337MPa,ultimate strength(UTS)392MPa and elongation(EL)15.4%.The initial hardness of GW83K alloy was improved significantly via the plastic deformation(forging and extrusion),with that the initial hardness of forged GW83K alloy 5HV more than that of the solid solution treated alloy and the forged+extruded GW83K alloy with bimodal microstructure 10 HV harder than the forged alloy.With that the peak ageing time and the peak hardness decrease with the increase of ageing temperature in all the samples,the ageing curves for GW83K alloy with different conditions and under different ageing temperatures were similar to each other.At the first stage of ageing,there exists a incubation period during which the hardness was only increased slightly,after the incubation period,the hardness increased sharply until the peak hardness was reached.Besides,the forged+extruded can reach a higher peak-aged hardness than alloys with other conditions.Excellent mechanical properties with YS 408 MPa,UTS 512 MPa and EL 5.5% was obtained when aged at 175? for 512 h.The exceptional mechanical properties of GW83K with bimodal structure were attributed to the effect of anisotropy resulting from the texture and the strengthening effect of its unique bimodal microstructure.When tensile-tested at ambient temperature,the tension twinning was suppressed greatly,which helped to enhance the strength of the alloy.It should be noted that,the coarse grains in the bimodal structure were mostly the distorted grains of the forged microstructure while the fine grains were DRX grains which shows weakened texture.Moreover,as the fraction of coarse grains increase,the strength of GW83K alloy decreases while the ductility increases,as the coarse grains were capable of coordinating the local stress concentration and inhibiting the propagation of cracks and hence improve the ductility.