Xiang Peng,Xinli Liang,Wencai Liu,Guohua Wu,Hao Ji,Xin Tong,Liang Zhang,Wenjiang Ding

National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite,School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai 200240,China

Abstract In this study,the tensile and high-cycle fatigue properties of as-cast,solid solution treated and as-extruded Mg-8Li-3Al-2Zn-0.5Y alloy are investigated.The results show that the yield strength and ultimate tensile strength of as-cast alloy is 198.1MPa and 222.5MPa,which are improved to 274.7MPa and 321.7MPa,282.4MPa and 319.3MPa after solid solution and extrusion treatment,respectively.The high-cycle fatigue strength(at 107 cycles,R=-1)of as-cast studied alloy is 65MPa,which is improved to 90MPa and 105MPa after solid solution and extrusion treatment,respectively.The improvement of fatigue property of the solid solution treated alloy is mainly due to the lattice distortion caused by solid solution hindering the crack propagation.However,the improvement of fatigue property of the as-extruded alloy is mainly due to that the refine grains,stacked dislocations and dispersed secondary particles impede the crack propagation.? 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Mg-Li alloys;Microstructure;Tensile property;S-N curve;Fatigue property.

1.Introduction

Mg-Li alloys are the lightest metal engineering materials with low density(1.35g/cm3-1.65g/cm3),high specifi strength and damping capacity,they have been widely applied in aerospace,military and 3C products[1-3].The two-phase(α+β)Mg-Li alloy combines the strength ofα-Mg(hcp)with the plasticity ofβ-Li(bcc).The addition of alloying elements such as Al,Zn,Y[4-8]will further improve the comprehensive mechanical properties of Mg-Li alloys.However,the safety and reliability under cyclic loading limits their further application,it is necessary to investigate the fatigue behavior of Mg-Li alloy to expand its application.

High-cycle fatigue fracture is one of the most common failure modes for engineering materials.According to statistics,in the case of failure of industrial materials,fatigue failure accounts for more than 50% of all failure causes[9].There are many factors affecting the fatigue properties of Mg alloys,including alloy composition,grain size and external environment,etc.At present,the research on the high-cycle fatigue behavior of Mg alloys mainly focuses on the nucleation and propagation mechanism of fatigue cracks,primarily including sliding deformation theory and twinning theory[10,11].The propagation path and rate after crack initiation mainly depend on the microstructure and the external environment.The fatigue cracks in as-cast Mg alloy are primarily initiated from free surface of the defect-free casting or pores,oxides,slip zones in the subsurface of the defective casting[12-14].The fatigue cracks of the solid solution treated Mg alloy are often initiated from oxide inclusions or pores in the surface or subsurface[15].Compared with the as-cast Mg alloy,the initiation and propagation rates of the fatigue crack is reduced in the deformed state due to its fine grains as well as the less inclusions and voids[14,16,17].

Researchers have made many attempts to improve the fatigue strength and reliability of Mg alloy[18-20],including the addition of rare earth elements,heat treatment,plastic processing and surface treatment,etc.However,until now,there have been few reports on fatigue property of Mg-Li alloy.In this paper,based on previous research,the Mg-8Li-3Al-2Zn-0.5Y(named as LAZ832-0.5Y)alloy with excellent comprehensive mechanical properties is selected as the research object[6,7,21-23].The tensile and fatigue properties of the LAZ832-0.5Y alloy with three different states are investigated.And the mechanism of fatigue crack initiation and propagation in the studied alloy is also investigated.

2.Experimental procedure

2.1.Materials and sample preparation

In this study,commercial pure Mg,Li,Al,Zn and Mg-20wt.%Y master alloy are used to prepare LAZ832-0.5Y alloy.These raw materials are melted in a vacuum induction melting furnace under an atmosphere of 0.3 atm of argon as a shielding atmosphere.Then,mechanical stirring is carried out for 3 min when the melt temperature reached 700°C,and then held for 10 min.When the melt temperature reached 710°C,the melt is poured into a steel mold.The actual chemical composition of the studied alloy is determined by ICP-AES(Inductively Coupled Plasma-Atomic Emission Spectrometry)as listed in Table 1.Some as-cast specimens are solid solution treated at 350°C for four hours.Extrusion is performed on the 20 MN maximum load compression testing machine.Before extruding,the ingots are firs homogenized at 350°C for four hours in an electric furnace,and then hot extruded to a cylindrical bar of 15mm in diameter at the extrusion ratio of 16:1.

Table 1Actual chemical composition of the studied alloy.

Table 2Fatigue properties and σf/σb ratio of the LAZ832-0.5Y alloy.

2.2.Tensile testing

The tensile specimens are prepared into dog-bone tensile specimens with gage size of 15mm×3mm×2mm.Tensile properties of the tested samples are performed at a strain rate of 1mm/min on Zwick/Roell Z020 tensile machine(A.S.T.GmbH,Dresden,Germany)at room temperature.

2.3.High-cycle fatigue testing

Hour-glass shaped round specimens are prepared for highcycle fatigue testing with a gage diameter of 5.8mm as shown in Fig.1.And the specimens are polished to avoid the influenc of surface defects induced by machining.High-cycle fatigue test is performed on the rotating bending fatigue machine(stress ratioR=?1).The stress-controlled fatigue test is conducted to assess the fatigue strength at a given number(107cycles)of cycle-to-failure.The formula for calculating fatigue load is as follows:

Fig.1.Shape and size of hour-glass-shaped round specimen for fatigue test.

wheremfor mass of sample,gfor the acceleration of gravity,Lfor the force arm designed to be 400mm,andDfor diameter of sample.In this study,the stress is increased by 5MPa each time,and the sample is considered to‘run out’if it had undergone 107cycles without breaking.

The S-N fittin curve is obtained by the point-by-point tracing method according to the results of fatigue test.Ordinate is stress(σ)and abscissa is lgNf(whereNffor fatigue life of alloy).Then the difference in fatigue failure behavior of alloy under different states could be compared according to the S-N curves.

2.4.Microstructural and fractographic analyses

The microstructure of the LAZ832-0.5Y alloy in different states is analyzed by optical microscope(OM;ZEISS),the microstructure and fracture surface of tensile and fatigue specimens is investigated by scanning electron microscopy(SEM;Phenom XL)equipped with EDS(Energy Dispersive Spectroscopy).The SEM analysis should be performed as soon as possible after the fatigue property test to avoid oxidation and contamination of the fracture surface,and appropriate fracture surface protection measures should be taken.Phase analysis is characterized by X-ray Diffraction(XRD;Ultima IV)with a resource of Cu-Kαradiation at a scan rate of 5°/min and a step size of 0.02°.The transmission electron microscopy observation is undertaken by TEM(JEOL2100)apparatus operated at 200kV equipped with selected area electron diffraction(SAED)and EDS to further investigate the structure of secondary phases.The specimens for TEM observation are mechanically thinned to～65mm and then polished by argon ion milling(Gatan 691).

3.Results

3.1.Microstructures

The optical micrographs of the LAZ832-0.5Y alloy with different states are shown in Fig.2.It can be obviously seen from the optical micrograph that the metallographic structure of the LAZ832-0.5Y alloy varies a lot in different states.The microstructure of the as-cast LAZ832-0.5Y alloy is primarily composed ofα-Mg,β-Li and some secondary particles mainly distributed inβ-Li[24,25].α-Mg is basically stripelike with sharp edges and different sizes.It can be seen from Fig.2(b)that the precipitates are significantl reduced after solid solution treatment at 350°C for four hours compared to the as-cast alloy.Theα-Mg grains are gradually spheroidized and the boundaries of theα-Mg grains become smooth after solid solution treatment.It can be seen from Fig.2(c)that the matrix of the studied alloy is elongated along the tensile direction after extrusion.Fig.2(d)shows the microstructure of the vertical extrusion direction,it can be seen that the Mg grains are crushed and refined

Fig.2.Optical micrographs of the LAZ832-0.5Y alloy under different states:(a)as-cast;(b)solid solution treated;(c)parallel to the extrusion direction;(d)vertical to the extrusion direction.

Fig.3 presents the SEM micrographs and EDS analysis results of the investigated alloy.It should be noted that the EDS device cannot detect Li element because the spectrum is cut off for all elements below boron.Both MgLiAl2phase and AlLi phase have FCC structures and their lattice parameters are also similar.AlLi precipitates can coexist with the MgLiAl2phase[23].Therefore,in this paper,Al-Li phase is used as the general term for AlLi phase and MgLiAl2phase.Al-Li phase is detected as shown in Fig.3(b),and the detected Zn element may be derived from the matrix.In addition,the Al2Y phase is detected after the solid solution heat treatment,and its atomic ratio is 2:1 as shown in Fig.3(d).Recrystallization occurs during the extrusion process[26,27],and a large amount of fin and dispersed secondary particles are distributed in the matrix.According to the EDS analysis results in Fig.3(f),the Al-Li phase precipitates can be observed after extrusion process.Combined with Fig.3(a-f),it can be seen the size of the Al-Li phase is smaller while that of the Al2Y phase is larger.After solution treatment,the Al-Li phase decomposes into the solid solution and the Al2Y phase remains.

Besides,the TEM analysis in Fig.4 is performed to further investigate the structure of the secondary particles.Al-Li phase and Al2Y phase are all detected and their diffraction patterns as well as the EDS analyses are also exhibited in the TEM micrographs.XRD analysis is performed to further identify the phase composition in the alloy.Fig.5 shows the XRD patterns of the different states of LAZ832-0.5Y alloy.The result shows that the microstructure of the alloy is mainly composed ofα-Mg,β-Li,Al-Li and Al2Y.After the solid solution treatment and the extrusion,it can be seen from the XRD pattern that the intensity of the peak of the Al-Li phase is gradually decreased(24.4°),the intensity of the peak of the Al-Li phase is greatly decreased or even disappeared(38.3°),and the intensity of the peak of the Al2Y phase is gradually decreased(32.5° and 57.6°).

Fig.3.SEM images with EDS point analysis of the LAZ832-0.5Y alloy under different states:(a)as-cast;(b)Al-Li phase in(a);(c)solid solution treated;(d)Al2Y phase in(b);(e)as-extruded;(f)Al-Li phase in(e).

3.2.Tensile properties and S-N curves of the LAZ832-0.5Y alloy

Fig.6(a)and(b)shows the stress-strain curves and tensile properties of the as-cast,solid solution treated and asextruded LAZ832-0.5Y alloy,respectively.According to the stress-strain curves,YS and UTS of the LAZ832-0.5Y alloy after solid solution treatment and extrusion treatment are greatly improved,but the plasticity after solid solution treatment is drastically reduced,while the plasticity of the studied alloy is significantl improved after extrusion.It can be seen from Fig.6(b)that the YS and UTS of the as-cast LAZ832-0.5Y alloy are 198.1±3.9MPa and 222.5±4.7MPa,respectively,and the elongation(EL)is 12.3±1.6%.The YS and UTS of the solution-treated LAZ832-0.5Y alloy increases to 274.7±3.8MPa and 321.7±5.9MPa,respectively,while the EL decreased from 12.3% to 4.9±1.1%.The YS and UTS of the as-extruded alloy are 282.4±5.2MPa and 319.3±4.6MPa,and the EL increased from 12.3% to 19.3±1.7%,which are 42.3%,43.5% and 7% higher than those of the as-cast state alloy,respectively.

Fig.7 shows the stress amplitude as a function of the number of cycles to failure(fatigue life Nf)for different states of the LAZ832-0.5Y alloy.Each point represents a fatigue fracture experiment under a corresponding stress load.It is obvious that the high-cycle fatigue life of the solid solution treated and as-extruded LAZ832-0.5Y alloy are much greater than that of the as-cast LAZ832-0.5Y alloy at all stress amplitudes.The fatigue limit of the as-cast alloy is 65MPa,and the fatigue limit of the solid solution treated and as-extruded alloy is 90MPa and 105MPa,respectively.This is much higher than the fatigue strength of Mg-Li alloy reported in the existing literature[28].It is worth noting that the UTS of the solid solution treated and the as-extruded LAZ832-0.5Y alloy are similar but their fatigue limit is significantl different.Moreover,it can be seen from the fittin curve that the fatigue life gap between the three state alloys increases as the number of cycles increases.

Fig.4.High resolution TEM micrographs of secondary particles corresponding to EDS result in the as-cast LAZ832-0.5Y alloy:(a)Al-Li phase;(b)Al2Y phase.

Fig.5.XRD patterns of the LAZ832-0.5Y alloy under different states.

Fig.6.(a)Stress-strain curves and(b)tensile properties of the LAZ832-0.5Y alloy under different states.

To correlate the fatigue strength(σf,at 107cycles)with the UTS(σb),theσf/σbratios of the investigated alloys are listed in Table 2.As shown in the Table 2,theσf/σbratios of the three studied alloys are all around 0.3,which is consistent with theσf/σbratio of 0.25-0.5 reported in some previous literatures[13,29,30],and theσf/σbratio after solid solution and extrusion treatment is higher than that of the as-cast alloy.Although the ultimate tensile strength of the asextruded alloy is close to that of the solution-treated alloy,the elongation of the as-extruded alloy is much larger than that of the solid solution treated alloy,and the fatigue strength is also significantl higher than that of the solid solution treated alloy.It can be seen that the fatigue strength of Mg-Li alloy is not only related to the ultimate tensile strength but also to its elongation.

Fig.7.S-N curves of the LAZ832-0.5Y alloy under different states.

3.3.Fatigue fracture analysis

Fig.8 shows the SEM images of the full views of the fatigue fracture surface of different states studied alloy under the same stress condition(120MPa).According to the characteristics of the fatigue fracture surface,it can be divided into three regions,namely fatigue crack initiation region A,fatigue crack propagation region B and fatigue collapse fracture region C.As shown in Fig.8(a),the fatigue crack of the as-cast LAZ832-0.5Y alloy mainly initiates from the oxides and micropores located at the subsurface of the sample,because the oxides and micropores produce a greater stress concentration than the sample surface[29].Region B is the crack propagation region with a river pattern.Fig.8(b)is the fracture surface of the solid solution treated alloy with rugged surface,the fatigue crack is initiated on the subsurface of the sample and the secondary particles.It can be seen from Fig.8(c)that the fracture surface of the as-extruded studied alloy is relatively fla and its crack propagation region is relatively large.

Fig.9 is magnifie SEM macrographs of three fatigue crack regions of the studied alloy under the three states.It can be seen from Fig.9(a-c)that there are oxides and micropores in the crack initiation region as indicated by the arrows in region A,they are stress concentration areas where the fatigue cracks are nucleated.In region B,the crack mainly propagates in a river pattern.In region C,some intergranular cracks appear and the fracture morphology of the collapse fracture region is similar to that of the static tensile fracture surface.The fatigue fracture surface of the solid solution treated alloy is similar to that of the as-cast alloy,but cleavage steps and grain boundaries appear in the crack propagation region as shown in Fig.9(e).However,there are more secondary cracks in region B of the as-extruded alloy as shown in Fig.9(h).

4.Discussion

4.1.Effect of microstructure on tensile and fatigue properties

According to the Mg-Li binary phase diagram,the investigated Mg-8Li-3Al-2Zn-0.5Y alloy is a dual-phase matrix structure(hcp+bcc),in which theα-Mg(hcp)is blocklike with sharp boundary and distributed in the Li matrix as shown in Fig.2(a).In the as-cast alloy,the secondary particles are mainly Al-Li and Al2Y,the melting point of the secondary phase Al-Li is relatively low and they would dissolve into the matrix during the solid solution treatment at 350°C for four hours.However,the melting point(1485°C)of the Al2Y phase is higher[6,21,22],it remains after the solid solution treatment as shown in Fig.2(b),and theα-Mg phase gradually spheroidizes and the boundary becomes smooth in the solid solution treated alloy.Similarly,since the homogenization treatment(equal to the solid solution treatment)is performed before the extrusion,the number of the secondary particles decreases in the as-extruded alloy,and theα-Mg grains are elongated along the extrusion direction.

It can be clearly seen from the magnifie SEM image that the fin and dispersed secondary particles disappear after the solid solution treatment,and the remaining precipitate is mainly the Al2Y particles due to its high decomposition temperature.It is because that the Al-Li phase decompose when the temperature exceeds 300°C and dissolve into the matrix during the solid solution treatment[7,23,29,31].It is worth noting that after the extrusion,the grains undergo dynamic recrystallization(DRX)and refinement and some large-sized secondary particles are crushed and dispersed,so that the number of secondary particles increases.Mg alloy(hcp)is more prone to DRX than bcc metal.This is because Mg alloy is a hexagonal close-packed structure with a very limited number of slip systems.Mg and Mg alloys have lower stacking fault energy and higher grain boundary diffusion speed,so that dislocations accumulated on the subgrain boundary can be absorbed by these grain boundaries,thereby accelerating the process of DRX.

The microstructures of the studied alloy in three different states are significantl different,resulting in differences in tensile mechanical behavior as shown in Fig.6.The main reason for the significan increase in strength of the alloy after solid solution treatment is that the secondary particles Al-Li dissolve into the matrix to produce solid solution strengthening[6,7,21,22].However,supersaturated solution causes lattice distortion and hinders the movement of dislocations and grain boundaries resulting in a decrease in plasticity[32].Compared with the as-cast alloy,the UTS and EL of the asextruded alloy are greatly improved,mainly because the extrusion refine the grains as shown in Fig.3(c),and the grain refinemen activates more slip system,which greatly improves the plasticity.

Fig.8.SEM images of the full views of the fatigue fracture surface:(a)as-cast(120MPa,6.7×104);(b)solid solution treated(120MPa,1.6×105);(c)as-extruded(120MPa,4.1×105).

It can be seen from Fig.7 that the fatigue strength of the as-extruded alloy is greater than that of the solid solution treated alloy,although their tensile strength is almost the same.As discussed in many literatures[11,15,32],the fatigue strength of Mg alloy is determined by the threshold stress of basal slip,which is induced by the solid solution and precipitation effects.At room temperature,the active dislocation slip system in magnesium is the{0001}〈110〉dislocation slip system[33,34].The microstructure observation by OM and SEM shows that precipitates increase after the extrusion process(see Figs.2 and 3).The precipitates interact with dislocations and induce threshold stresses for dislocation slip.At room temperature,the threshold stress in the Mg alloy cannot be overcome by thermal activation of dislocation motion.Therefore,the threshold stress may hinder the{0001}〈110〉basal slip,resulting in threshold stress amplitude for fatigue crack initiation.That is,the precipitates will pin dislocations,resulting in a further increase of the threshold stress of basal slip.Additionally,it is believed that the density of dislocations and twins are higher in the as-extruded alloy,and they acted as the precipitation nuclei during the extrusion process.As a result,there are much denser precipitates and much refine grains of the as-extruded alloy than the solid solution treated alloy indicating that the as-extruded LAZ832-0.5Y alloy exhibits higher crack initiation resistance,namely higher crack initiation duration and fatigue strength.

It should be noted that there exists a fatigue life gap between 106and 107cycles.The similar phenomenon is reported by He et al.[35]who explained this phenomenon with grain refinemen method.Both factors contribute to this phenomenon in this study.On one hand,the fatigue strength is the maximum stress that microcracks just does not propagate,rather than the maximum cyclic stress leading to cyclic slip in many studies[11,36].It is supposed that the fatigue cracks nucleate but do not propagate below the fatigue strength.On the other hand,once cyclic stress is up to the critical propagation stress,crack propagation rate become fast resulting in short fatigue life.Besides,when the cyclic stress is the same,such as 110MPa,the fatigue life of the as-extruded alloy is an order of magnitude higher than that of the solid solution treated alloy,due to its better plasticity.This is mainly because the microstructure of the alloy is refine after extrusion,and the crack propagation is slowed down,so the fatigue life is improved.

Fig.9.SEM images of crack initiation region,crack propagation region and fracture region of the LAZ832-0.5Y alloy under different states:(a-c)as-cast;(d-f)solid solution treated;(g-i)as-extruded.

4.2.Cyclic deformation mechanism

Fatigue fracture is a partial failure of a sample resulting from cyclic deformation.It is the result of a cumulative process that includes crack initiation,crack propagation and fi nal fracture[4,31].From the macroscopic point of view,the initiation of fatigue cracks always originates from the stress concentration zone,the high strain zone,the weakest grain,and the structural corners.From the microscopic point,it can be divided into three mechanisms:slip zone cracking,grain boundary cracking,the interface between non-metallic inclusion(or secondary particles)and matrix cracking[11].The micro-fractographic microscopic examination revealed that a few porosities and non-metallic inclusions are observed in the crack initiation region of the as-cast alloy.As shown in Fig.8(a),the fatigue crack mainly originates on the free surface of the sample mainly because the free surfaces experience the maximum tensile stress during rotating bend load fatigue.However,oxides and inclusions are found in the crack propagation region of the solid solution treated alloy and the as-extruded alloy,and they become strain concentration region under the cyclic load,and fatigue cracks are originated here[22,29].In the crack propagation region of the three different states LAZ832-0.5Y alloys,some parallel fatigue striations in the shape of beach lines can be observed.Among them,the fatigue striations of the solid solution treated alloy is tortuous.This is mainly because the solid solubility of the Al and Zn elements in theα-Mg matrix is large,and the formation of the supersaturated solid solution after solid solution causes lattice distortion which hinders the propagation of the fatigue crack[6,9,25].Moreover,after solid solution treatment,the grains grow slightly,and typical intergranular fractures occur in the fracture region,and the fracture section is rough.It is worth noting that many secondary microcracks appear in the as-extruded LAZ832-0.5Y alloy as shown in Fig.9(h).The presence of many secondary cracks is beneficia to absorb a large amount of energy,to withstand greater stress and strain,thereby the fatigue life of the alloy has been improved.In addition,it is believed that dispersed precipitates improve the strength of the grain and slow the propagation of the secondary cracks.However,the fracture section of the as-extruded alloy is fla and the fatigue striations is dense,mainly because the grain of the alloy is refine after extrusion,and the propagation of the fatigue crack is perpendicular to the extrusion direction,so that the resistance is large when the crack passes through grain and the grain boundaries.This results in a slower rate of crack propagation expansion,so the as-extruded alloy has the longest fatigue life.

Compared with the ordinary Mg alloys,the deformation mechanism of dual-phase Mg-Li alloy under cyclic loading exhibits some special features.Crack sources usually locate at slip belts,twins,and phase boundaries in dual-phase Mg-Li alloy[28].The fracture morphology of the cracks is also different due to the different crystal structures betweenα-Mg andβ-Li.It can be seen from the SEM images of crack initiation region and crack propagation region that some places are smooth and some have fatigue striations.It can be inferred that the platform fracture is theα-Mg phase and the fatigue striations region is theβ-Li phase.β-Li phase is an hcp structure with few slip systems.Many deformation twins appear in theα-Mg of the hcp structure under the cyclic loading,cracks propagate along the grain boundary ofα-Mg to produce smooth fracture sections[37].There are no deformation twins in theβ-Li phase because it is bcc structure with multiple slip systems.Dislocation slip and DRX occur under the cyclic loading,and much strain energy is absorbed.In the fatigue experiment,the LAZ832-0.5Y alloy showed obvious cyclic hardening[38].This is because plastic deformation occurred under the cyclic loading,increasing the dislocation density in theβ-Li phase.As the dislocation density continues to increase,the stacked dislocations caused the alloy to harden,leading to an increase in the fatigue life of studied alloy.

5.Conclusions

Tensile and fatigue properties of the as-cast,solid solution treated and as-extruded LAZ832-0.5Y alloy have been investigated.The following conclusions can be drawn based on the experimental results:

(1)The microstructure of the LAZ832-0.5Y alloy is composed ofα-Mg andβ-Li matrix with eutectic compounds Al-Li and Al2Y.After solid solution treatment,the Al-Li particles decompose and dissolve into the matrix,while Al2Y remains.After extrusion,the Mg grains are refine and the secondary particles Al-Li reprecipitates.

(2)Compared with the as-cast LAZ832-0.5Y alloy,the YS,UTS,and EL of the solution-treated LAZ832-0.5Y alloy increases to 274.7±3.8MPa,321.7±5.9MPa,and 4.9±2.1%,respectively.After the extrusion process,the YS and UTS of the as-extruded alloy increases to 282.4±5.2MPa and 319.3±4.6MPa,and the EL increased to 19.3±2.3%,respectively.

(3)Fatigue strength of the as-cast,solid solution treated and as-extruded LAZ832-0.5Y alloy atR=?1 and 107cycles are 65MPa,90MPa and 105MPa,respectively.The improvement of fatigue strength of the solid solution treated alloy is mainly related to solid solution strengthening.The improvement of fatigue strength of the as-extruded alloy is mainly related to grain refine ment and precipitation strengthening.

(4)The different fatigue properties of the LAZ832-0.5Y alloy under three different states involve different mechanisms of crack propagation.The improvement of fatigue property of the solid solution treated alloy is mainly due to the lattice distortion caused by solid solution hindering the crack propagation.The improvement of fatigue property of the as-extruded alloy is mainly due to the refine grains,stacked dislocations and dispersed secondary particles impede the crack propagation,thereby increasing the fatigue life.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgments

This work is supported by National Natural Science Foundation of China(Nos.51771115,51775334 and 51821001),Joint Fund for Space Science and Technology(Nos.6141B06310106 and 6141B06300401)and National Defense Science and Technology Innovation Special Zone Project(No.002-002-01).