Effect Of Heat Treatment And Ultrasonic Nano-surface Modification On Low Cycle Fatigue Properties Of LZ91 Mg-Li Alloy

At present,dual-phase Mg-Li alloys have broad application prospects in the aero-space and transportation industries.However,the disadvantages of low strength and poor fatigue properties of Mg-Li alloys limit their wide use in industry.In this study,LZ91 dual-phase Mg-Li alloy was used as the research object.The heat treatment and ultrasonic nano-surface modification(UNSM)technology were used to treat the LZ91Mg-Li alloy.In addition,the microstructure,tensile properties and low cycle fatigue properties of LZ91 Mg-Li alloy before and after treatment were investigated.The fol-lowing conclusions were drawn:(1)The microstructure,tensile properties and low cycle fatigue properties of the Base state LZ91 Mg-Li alloy were studied.The LZ91 Mg-Li alloy is a typical two-phase Mg-Li alloy.Its microstructure consists of?phase(HCP structure)and?phase(BCC structure).In addition to?phase and?phase,there are some second phase par-ticles such as Mg Li₂Zn,Mg₂Zn₃ and Mg Li Zn.The yield strength and ultimate tensile strength of the Base state of LZ91 Mg-Li alloy is 105 and 136 MPa.The low-cycle fatigue test of the Base state LZ91 Mg-Li alloy shows that the fatigue life decreases with the increase of the strain amplitude.From the fracture surface and the microscopic morphology of the specimen surface,fatigue cracks origin in the?phase of the surface,and the crack propagation mode is transgranular propagation.(2)After solid solution and artificial aging treatment,the T6 state LZ91 Mg-Li alloy was obtained.The microstructure of the T6 LZ91 Mg-Li alloy was still composed of?phase and?phase.Due to the precipitation of supersaturated solid solution,the needle-like?phase was formed at the grain boundary of?phase.After the heat treat-ment,the metastable phases such as Mg Li₂Zn and Mg₂Zn₃ were transformed,and only the Mg Li Zn phase was detected.The needle-like?phase hinderd the movement of dislocations and improved the ultimate tensile strength of the T6 LZ91 Mg-Li alloy.After heat treatment,the plasticity of the LZ91 Mg-Li alloy was significantly improved,and enhanced the low-cycle fatigue life at the allowable strain amplitude.It was found that fatigue cracks origin in the?phase on the surface of the specimens.The?phase hinders the growth of fatigue cracks,delays the crack growth rate,and improves the fatigue life.(3)The Base state and T6 state of LZ91 Mg-Li alloy were processed using the UNSM.UNSM treatment changed the shape of the surface grains and improved the surface roughness.The depth of the hardened layer of the UNSM-Base state of LZ91Mg-Li alloy is about 425?m,and that of the UNSM-T6 state is about 375?m.After UNSM treatment,the yield strength,tensile strength,and the elongation at break of the UNSM-Base state is 128 MPa,153 MPa,decrease of 52%,respectively;the yield strength,tensile strength,and the elongation at break of the UNSM-T6 state is 112 MPa,147 MPa,decrease of 42%.UNSM technology improves the low-cycle fatigue life of the UNSM-Base state under high strain amplitude.However,UNSM technology does not improve the fatigue life of T6 specimens.From the fracture and surface microstruc-ture of the specimens,it was found that UNSM technology changed the crack propa-gation path of the LZ91 Mg-Li alloy in the UNSM-Bsae state and the UNSM-T6 state,thereby affecting the low-cycle fatigue life.Heat treatment and UNSM method can effectively improve the tensile properties and low-cycle fatigue properties of LZ91 Mg-Li alloy.After heat treatment,the plas-ticity and strength of the LZ91 Mg-Li alloy is significantly improved,and the low-cycle fatigue life is improved under the allowable strain amplitude;UNSM treatment improved the yield strength and ultimate tensile strength of LZ91 Mg-Li alloy,but the plasticity is reduced.UNSM treatment can also increase the low-cycle fatigue life of LZ91 Mg-Li alloy under high strain amplitude,reduces its low strain amplitude low-cycle fatigue life.However,the tensile properties and fatigue properties of LZ91 Mg-Li alloy in UNSM-T6 state were not significantly improved.