Microstructure And Mechanical Properties Of GZ151K Alloy With Ultrasonic Surface Rolling Process

In recent years,many studies indicated that the gradient nanostructural magnesium alloy exhibited excellent property due to its unique microstructure.By comparing many surface strengthening technologies,we found that the comprehensive performances of materials which was processed by ultrasonic surface rolling process(USRP)was improved.The surface morphology of sample was improved after USRP,and the gradient nanostructure on the cross-sectional plane of that was formed,while the dimension of samples would not changed.Therefore,Mg-15Gd-1Zn-0.4Zr(GZ151K)alloy was processed by USRP in this study.The microstructure,mechanical properties and corrosion property of GZ151K alloy with USRP were investigated by some test facilities and instruments.The evolution of microstructure,mechanical characteristic,corrosion property and the fracture behavior of GZ151K alloy after USRP was detailed discussed.In addition,the GZ151K alloy,which was processed by the optimum USRP technology parameter,was aged.The microstructural and mechanical characteristics of specimens was presented to investigate the influence of USRP technology on the precipitation behaviour of GZ151K alloy during aging process.The primary results are listed in follows:1.The gradient structure on the cross-sectional plane of G1Z51K alloy was produced by USRP with different set pressure,and a mount of twins were distributed near the treated surface to form the severely deformed(SD)layer.With increasing the set pressure of USRP,the grain refinement and the formation of SD layer were promoted,the nano-twin gradient structure with different volume fraction was produced in GZ151K alloy.Comparing with the average surface roughness of untreated sample,it was clearly that the average surface roughness of USRP processed samples with different set pressure was sharply decreased.The surface broken were appeared due to excessive pressing when the set pressure of USRP reached 0.4 MPa.Moreover,the corrosion property was significantly improved after USRP due to the decreasing of surface roughness,but that was degraded when the surface was broken in the sample with 0.4 MPa-USRP.2.A gradient microhardness was formed along thickness in the cross-section plane of USRP processed samples,and the average microhardness of USRP processed samples were enhanced with the increasing of set pressure.The yield strength(YS)and ultimate strength(UTS)of specimens after USRP were significantly improved with the compromise of elongation.Comparing with the YS and UTS of original specimen,them of the USRP processed sample with 0.4 MPa reached 278.3 MPa and 325.6 MPa,respectively,while the elongation values of them was reduced from 12.3%to 4.2%.The reason why the sharply decreasing elongation of GZ151K samples after USRP process were about the nucleation changed of microcrack.The nucleation of microcrack in SD layer was changed from the junctions between the secondary phase and grain boundary to the junctions between twins,secondary phase and grain boundaries,and the microcracks extended in the twins.In addition,the USRP processed samples show a quite different strain hardening behavior comparing with the untreated sample.In the hardening stage ?,the USRP processed samples show a higher strain hardening rate and the strain hardening rate enhances with the enhancement of pressure.However,the USRP processed sample exhibited a lower strain hardening rate and the strain hardening rate decreases with the enhancement of pressure in the hardening stage ?.3.The original and 0.3 MPa-USRP deformed GZ151K alloy were processed by artificial aging treatments with different aging time on 150?,200? and 250?,respectively.The best parameter of the aging process was 200?/100 h.After peak aging treatment,the microhardness gradient tendency of USRP processed specimens along the thickness direction was similar,the phases composition and distribution of them were not changed.However,many twins were introduced into the specimens,Gd and Zn atoms was aggregated into twin boundary due to USRP.After peak aging,the precipitation of(Mg,Zn)3Gd phase was promoted in the twin region which improved by the precipitation strengthening of USRP processed GZ151K alloy.