Hipeng Zheng,Ruizhi Wu,b,*,Legn Hou,Jinghui Zhng,Milin Zhng,b

a Key Laboratory of Superlight Materials and Surface Technology,Ministry of Education,Harbin Engineering University,No.145 Nantong Street Nangang District,Harbin 150001,China

b College of Science,Heihe University,Heihe 164300,PR China

Abstract Based on the traditional two-layer accumulative roll bonding(TARB),the geometrical variations and mathematical relationship during the four-layer accumulative roll bonding(FARB)were derived and summarized.Furthermore,the multi-layer accumulative roll bonding(MARB)technology was proposed and the geometrical variations and mathematical relationship of MARB were simultaneously derived and summarized.Experimentally,Mg-14Li-3Al-2Gd(LAGd1432)sheets were fabricated by TARB and FARB,respectively.Compared with the TARB,the FARB has a higher accumulative efficien y in terms of accumulative layers,total number of interfaces,interface spacing,total deformation and equivalent strain.Therefore,the FARB-processed sheets in lower cycles have the similar microstructure and mechanical properties of the TARB-processed sheets in higher cycles.In addition,FARB process can further break through the deformation limit of TARB process in a single cycle through adopting two-step rolling in one cycle with 50% deformation in one pass and 75% accumulative deformation in one cycle,which can effectively solve the problem of poor interface bonding of the latest interface brought by the last cycle,and thus significantl improve the phenomenon of unstable performance of the ARB-processed sheets.? 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:ARB;Mathematical relationship;Equivalent strain;Strengthening mechanism.

1.Introduction

Mg-Li alloy possesses the smallest density among engineering metallic materials,which is an ideal engineering material for energy saving and environmental protection in future[1-4].Recently,the development of Mg-Li alloys in composition design,strengthening mechanism,potential performance and new severe plastic deformation(SPD)were summarized in detail by Wu et al.[5-9].However,due to the drawback of low strength,wide application of Mg-Li alloys is restricted.Therefore,the improvement of the mechanical properties of Mg-Li alloy has become an urgent problem to be solved.Severe plastic deformation(SPD)is regarded as one of ideal technologies to address the poor mechanical properties of Mg-Li alloy[8].Furui et al.studied the superplastic properties of a two-phase Mg-8Li alloy processed by equal channel angular pressing(ECAP)where grains are significantl refine and the Mg-8Li alloy exhibited excellent superplastic properties with the deformation temperature of 473K and the initial strain rate of 1.0×10?4s?1[10].Hou et al.studied the microstructure and mechanical properties of ARBprocessed Mg-5Li-1Al sheets(α-phase)where nanocrystals were achieved and the strengthening mechanisms were strain hardening and grain refinement which significantl improved the mechanical properties[8].Su et al.studied the microstructural evolution and mechanical properties of Mg-9Li-1Zn alloy processed by high-pressure torsion(HPT)where significan grain refinemen was achieved after HPT processing with an average grain size reducing from 30μm to approximately 230nm through 10 turns and the alloy exhibited excellent mechanical properties[11].

Among SPD processes,ARB breaks the limitation of low reduction of traditional rolling and can continuously fabricate ultra-fin grain metal materials of thin sheets.Therefore,ARB is considered to be the promising deformation process to fabricate bulk ultra-fin grain metal material[11-14].However,although ARB process is widely used in the preparation of metal sheet,the research and application of ARB process in Mg-Li alloy is still rarely involved.The research about Mg-Li alloy processed by ARB in recent years is summarized in detail by Wu et al.[8,15-17].At present,the application of ARB in Mg-Li alloy is mainly concentrated inαphase andα+βphase Mg-Li alloy,while the research and application of ARB inβphase Mg-Li alloy with high Li content are rarely reported.At the same time,the current ARB process applied to Mg-Li alloy is almost based on the conventional two-layer accumulative roll bonding(TARB),rarely to optimize the traditional ARB process to improve the microstructure and properties of Mg-Li alloy.The traditional ARB process adopts double-layer and one-step rolling in one cycle[18],which has excessive workload and low efficien y of accumulated strain,and the most critical problem is the poor interface bonding of the latest interface brought by the last cycle,which makes the strengthening effect of the ARB process is limited.In addition,although there are some researches on the three/four-layer accumulative roll bonding at present,they are mainly focused on the composite rolling of dissimilar metal.The main research direction is the change of the microstructure and properties of different metals after composite compared with single metal,but the influenc of different superimposed layers and different accumulative effi ciency under different ARB processes on the microstructure and properties of the alloy in the ARB process are rarely involved.

In this paper,the ARB process was systematically optimized.Firstly,the conventional two-layer accumulative roll bonding(TARB)was extended to four-layer accumulative roll bonding(FARB)and multi-layer accumulative roll bonding(MARB)by optimizing the conventional accumulative roll bonding process.Based on the disadvantages of the conventional two-layer accumulative roll bonding(TARB),we adopt the four-layer accumulative roll bonding(FARB)process with two-step rolling in one cycle to optimize the traditional ARB process,and then multi-layer accumulative roll bonding(MARB)technology is proposed.At the same time,the relevant mathematical relations were obtained by the derivation from the change rule of geometric size and strain rate in the process of FARB and MARB.Secondly,based on the theoretic analysis,the effects of different ARB processes on the microstructure and mechanical properties of the Mg-14Li-3Al-2Gd alloy(βphase Mg-Li alloy)were compared and analyzed.Finally,the relationship between the accumulative efficien y and the evolution of microstructure and properties were established.

2.Geometrical changes and mathematical analysis for TARB,FARB and MARB

At present,the traditional accumulative roll bonding is mainly two-layer accumulative roll bonding(TARB)with double-layer and one-step rolling in one cycle.The schematic illustration of TARB and FARB is shown in Fig.1.The surface treatment process and TARB process were described in detail by Wang et al.[16].In the process of FARB,after surface treatment,four equal-sized sheets are stacked together.Before rolling,the stacked sheets are heated at 473K for 15min.Then,the stacked sheets are rolled with a reduction of 50%.After annealing at 473K for 20min,the sheets are further rolled with a reduction of 50%.Then,the sheets are water-cooled.Finally,the roll-bonded sheets are cut into four equal-sized sheets for the next rolling cycle.

The geometrical variations and mathematical relationship of TARB are summarized in Table 1[18,19].The total layer number,total interface number,and thickness of individual layer of TARB after n cycles can be calculated from Eqs.(1)-(3),respectively.

wheref(l)T,f(i)T,andf(t)Trepresent the total layer number,total interface number,and fina thickness of individual layer of TARB afterncycles,respectively.h0andnrepresent the initial thickness of individual sheet and the number of cycles,respectively.Therefore,the total reductionf(r)Tafterncycles can be expressed by the Eq.(4)as follow:

Assuming von Mises yield criterion and plane strain condition,the equivalent strain of TARB afterncycles is expressed by the Eq.(5)as follow[18]:

whereεTandnrepresent the equivalent strain and the number of cycles,respectively.

The FARB adopts the four-layer and two-step rolling in one cycle and the geometrical variations and mathematical relationship are summarized in Table 2.

Table 1Geometrical changes and mathematical relationship of the TARB at different cycles.

Table 2Geometrical changes and mathematical relationship of the FARB at different cycles.

The total layer number,total interface number,thickness of individual layer,total reduction,and equivalent strain of FARB after n cycles can be expressed by the Eqs.(6)-(10):

wheref(l)F,f(i)F,f(t)F,f(r)F,andεFrepresent the total layer number,total interface number,thickness of individual layer,total reduction,and equivalent strain of FARB afterncycles,respectively.h0and n represent the initial thickness of individual sheet and the number of cycles,respectively.

Fig.1.Schematic illustration of TARB and FARB process:(a)TARB,(b)FARB.

Comparing the Eqs.(1)-(5)and Eqs.(6)-(10),it can be concluded that the accumulative efficien y of FARB is significantl higher than that of TARB in terms of equivalent strain.If the superimposed number of sheets ism,that is to say multi-layer accumulative roll bonding(MARB),the geometrical changes and mathematical relationship can be derived

Fig.2.The accumulative layers and equivalent strain after n cycles under different superimposed layers in MARB process:(a)superimposed layers-cycleaccumulative layers,(b)superimposed layers-cycle-equivalent strain.

wheref(l)M,f(i)M,f(t)M,andf(r)Mrepresent the total layer number,total interface number,thickness of individual layer,and total reduction of MARB sheets afterncycles,respectively.h0,m,andnrepresent the initial thickness of individual sheet,the number of superimposed sheets,and the number of cycles,respectively.

Assuming plane strain condition and von Mises yield criterion,the equivalent strain of superimposed m-layers after n cycles is expressed by the Eq.(15):

In order to intuitively depict the variation in the number of layers and the equivalent strain of MARB with the different number of superimposed sheets(m)after n cycles,the calculated curves are shown in Fig.2.Fig.2(a)shows the accumulative layers of the MARB with the different number of superimposed sheets(m)after n cycles.Fig.2(b)is the three-dimensional relationship diagram of the superimposed sheets(m)-the number of cycles(n)-the equivalent strain(ε).

With the increase of cycles,the accumulative number of layers increases sharply,which is especially noticeable in high cycles.With the increase of superimposed sheets(m),the accumulative efficien y of total layer number has a significan improvement,which results in a sharp decrease in the layer interval and a significan increase in the alternating rate of interfaces generation and disappearance.Theoretically,the number of interfaces can be calculated by Eq.(12).

In Fig.2(b),the equivalent strain of MARB by increasing m and n is visually shown.In the same number of rolling cycles(n),with the increase of superimposed sheets(m),the accumulative efficien y of total equivalent strain increases sharply,which leads to a significan increase in strain strengthening for the mechanical properties of the alloy.

3.Experimental

To manifest the advantages of MARB,we choose Mg-Li-Al-Gd alloy and FARB processing for experiments.TARB is also conducted for comparison.

The raw materials used in this paper include commercial pure(CP)Mg ingot(99.9 wt%),CP Li ingot(purity 99.9 wt%),CP Al ingot(99.9 wt%)and the master alloy of Mg-20%Gd.Mg-14Li-3Al-2Gd alloy is designed for alloy composition.They were melted in a graphite crucible in a vacuum induction furnace at argon atmosphere.The melt was poured into a permanent mold to obtain as-cast ingot.In order to eliminate component segregation and obtain uniform microstructure and properties,the as-cast ingot was homogenized at 473K for 10h.Then,the as-received ingot was cut into sheets with dimensions of 60mm×40mm×2mm,which were subsequently used for TARB and FARB processing.

Phases of the alloys were measured with Rigaku TTRIII X-ray diffraction(XRD).LEICA DM IRM optical microscope(OM)and JSM-6480 scanning electron microscope(SEM)equipped with EDS(JED 2200)were utilized for the microstructural investigations.In order to obtain optical micrographs,2%(vol.)and 4%(vol.)nitric acid alcohol solution were used for the as-cast alloy and the ARB-processed alloy with the etching time of 3-5s and 5-10s,respectively.Rolling Direction-Normal Direction(RD-ND)plane was selected as the observation plane of the ARB-processed Mg-14Li-3Al-2Gd sheets.The tensile tests were carried out on Instron 5500R electro-universal tester machine with a strain rate of 1×10?3s?1at room temperature.The axis of the tensile test was selected parallel to RD.The gauge length was 16mm.The Vickers microhardness(HV)was measured under a loading of 50 gf and a holding time of 15s.The interface bonding strength was tested by the method in reference[8,15].

Fig.3.Microstructure,XRD pattern and EDS of the as-cast Mg-14Li-3Al-2Gd alloy(a)optical micrographs,(b)SEM morphology of the second phase at grain boundary,(c)the EDS of the marked spots in(b),and(d)XRD pattern.

4.Experimental results and discussion

4.1.Microstructure and phase analysis of the as-cast alloy

The microstructure,XRD pattern,and the EDS of the ascast Mg-14Li-3Al-2Gd alloy are shown in Fig.3.The matrix of the Mg-14Li-3Al-2Gd alloy isβ-Li phase with coarse grains.The average grain size of as-cast Mg-14Li-3Al-2Gd alloy is 245.8μm,which is measured and calculated by intercept method.There are some secondary phases existing at grain boundary.The morphology and size of these second phases are shown in Fig.3(b).From the results of EDS and XRD pattern,it can be concluded that it is Al2Gd phase.

4.2.Microstructure of the TARB-processed and FARB-processed Mg-14Li-3Al-2Gd sheets

Figs.4 and 5 show the optical micrographs of rolling direction-transverse direction(RD-TD)plane of the TARB-processed and FARB-processed sheets.As shown in Figs.4 and 5,after ARB,the grains of Mg-14Li-3Al-2Gd sheets present irregular multilateral morphology,but there is no strong rolling microstructure,which indicates that the severe plastic deformation causes strong dynamic recrystallization of Mg-14Li-3Al-2Gd alloy.At the same time,ARB process can effectively refin the grain of Mg-14Li-3Al-2Gd alloy.TARB6 and FARB4 sheets have the smallest grain sizes in their respective processes with the average grain size are about 14.5μm and 5.3μm,respectively.

It is difficul to observe the effect of the interface on the number,size and morphology of the grains because of the deformed morphology of the rolled sheets where grain boundary and interface are mutually doped.Therefore,we characterize and describe this by the morphology of annealed sheets.Figs.6 and 7 show the optical micrographs of rolling direction-normal direction(RD-ND)plane of the TARBprocessed and FARB-processed sheets annealed at 300°C for 30min.With the increase of cycles,the grain size of the annealed TARB and FARB sheets is continuously refined The TARB6 and FARB4 sheets possess the smallest grain size,about 15～30μm and 6～9μm,respectively.

Combined with the Figs.6 and 7,microstructure has different evolutions between low and high cycles.In low cycles,the thickness of the single layer is relatively large with the evolution of the microstructure only existing in the individual layer.Recrystallization occurs sufficientl in the individual layer and the grains hardly grow across the interface,as shown in Figs.6(a),(b),(c)and 7(a),(b).At this stage,the grains in the individual layer are still large.Due to the higher accumulative efficien y of FARB,in the same cycle,the specimens under FARB have fewer and smaller grains in the individual layer compared to TARB.In high cycles,the thickness of the individual layer is small,which has a great influenc on the size and morphology of the grains.At this stage,with the decrease of the thickness of individual layer,the number of grains in the individual layer decreases rapidly and the morphology of the grains also gradually changes from equiaxed grains to similar rectangles elongated along the RD,as shown in Figs.6(d),(e),(f)and 7(c),(d).The recrystallization occurs across the layer and the grains grow across the interface,which leads to the disappearance of the previous interface.The disappearance of the interface is the key factor for ARB-processed material to achieve excellent quality,and the higher accumulative efficien y of FARB greatly accelerates the occurrence of this behavior,which is a great improvement of FARB relative to TARB.

Fig.4.Optical micrographs of the TARB-processed Mg-14Li-3Al-2Gd sheets(RD-TD plane)(a)TARB1,(b)TARB2,(c)TARB3,(d)TARB4,(e)TARB5,and(f)TARB6.

Based on the above analysis,the layer interval(individual layer spacing)determines the fina grain refinemen and morphology of the grain,and the refinemen rate is controlled by accumulative efficien y,as shown in Fig.8.In the same way,the interface should have the similar effect on the unannealed microstructure.

4.3.Mechanical properties

4.3.1.Strength and elongation

Fig.9 shows the strength,elongation and hardness of the as-cast,TARB-processed and FARB-processed Mg-14Li-3Al-2Gd sheets.The as-cast alloy only possesses the yield strength(YS),ultimate tensile strength(UTS),elongation(EL),and hardness value of 96.2±4.6MPa,128.6±4.8MPa,36.2%,and 40.9±0.8 HV,respectively.Compared to the as-cast Mg-14Li-3Al-2Gd sheets,the YS and UTS of the ARB-processed Mg-14Li-3Al-2Gd sheets are significantl improved.The TARB6 and FARB4 sheets possess the YS of 201.6±4.7MPa and 231.6±3.8MPa,209.6% and 240.7%higher than that of the as-cast Mg-14Li-3Al-2Gd alloy,respectively.As shown in Fig.9(a),the strength improvement efficien y of FARB is significantl higher than that of TARB.Additionally,the elongation values of TARB and FARB have the same trend,firs decreasing and then increasing,which is mainly governed by the strengthening effect and the interface bonding quality.Compared to TARB,the plasticity of FARB is better maintained,as shown in Fig.9(b).Hardness test plane in Fig.9(b)was rolling direction-transverse direction(RD-TD)plane.With the increase of cycles,the hardness values of TARB-processed and FARB-processed Mg-14Li-3Al-2Gd sheets continue to increase.The TARB6 and FARB4 sheets possess the hardness of 73.2±0.9 HV0.05and 76.6±0.7 HV0.05,171.63% and 187.29% higher than that of the as-cast Mg-14Li-3Al-2Gd alloy,respectively.In the same number of cycles,compared to TARB,the higher accumulative efficien y of FARB makes the samples always possess relatively better mechanical properties.

Fig.5.Optical micrographs of the FARB-processed Mg-14Li-3Al-2Gd sheets(RD-TD plane)(a)FARB1,(b)FARB2,(c)FARB3,and(d)FARB4.

The strengthening mechanism of the Mg-Li alloy during traditional accumulative roll bonding processes is mainly classifie into strain hardening and grain refinemen strengthening[20,21].Strain strengthening plays a significan role in the strengthening mechanism of severe plastic deformation(SPD.)Wu and Zhou[22]studied the strain-hardening behavior of Mg-6Li-1Zn alloy thin sheets at elevated temperatures where the principle of the strain-hardening mainly include dislocation multiplication,dislocation motion and mutual dislocation interaction.

The strength comparison between TARB-processed and FARB-processed Mg-14Li-3Al-2Gd sheets at different cycles is shown in Fig.10(a).According to Eqs.(5)and(10),the cycle-equivalent strain curves of TARB and FARB are shown in Fig.10(b).With the increase of cycle,strength and equivalent strain both increase.In the same cycle,the strength of FARB is higher than TARB.The equivalent strain of FARB is larger than TARB,and the difference becomes larger and larger with the increase of cycles.

From Figs.4(a)and 5(a),compared with TARB,after the firs cycle,the grain size of FARB1 is more significantl refined which causes fin grain strengthening playing a significan role in the low cycle of FARB.Meanwhile,the better interface bonding with two-step rolling in one cycle and higher equivalent strain also cause the strengthening effect of FARB1 being higher than TARB1.In the high cycle,the grain refinemen of TARB and FARB both become no longer significant The better interface bonding with two-step rolling in one cycle and higher equivalent strain become the key factor in the improvement of mechanical properties in TARB and FARB-processed Mg-14Li-3Al-2Gd sheets.

4.3.2.Hardness fluctuatio

From Fig.9(b),ARB can effectively improve the hardness of Mg-14Li-3Al-2Gd alloy.However,in the ARB process,the formation of interface will lead to the non-uniformity of hardness along the normal direction(ND),so the stability of hardness distribution along the ND is also an important index to measure the mechanical properties of ARB sheets.Fig.11 shows the hardness fluctuation along the ND of TARB and FARB sheets(RD-ND plane).The hardness fluc tuation of the as-cast alloy is relatively small.Although the hardness of TARB1 and FARB1 sheets have been significantl improved,the hardness values are quite unstable and fluctu ate greatly,which is mainly caused by the existence of the interface and the uneven deformation between single sheets.With the increase of cycles,on the one hand,the interface gradually breaks up and disappears,and the bonding between layers is realized;on the other hand,the interface spacing decreases rapidly,which greatly alleviates the deformation unevenness between layers.These two aspects jointly affect the hardness fluctuation Compared with TARB process,FARB process has technology advantage of four-layer and two-step rolling in one cycle,which not only improves the interface bonding through two-step rolling in one cycle,but also to further decreases the hardness fluctuatio by promoting the sharp reduction of interface spacing,which results in the more uniform and stable hardness performance of FARB sheets under the same cycle.

Fig.6.Optical micrographs of the TARB-processed Mg-14Li-3Al-2Gd sheets annealed at 300°C for 30min(RD-ND plane):(a)TARB1,(b)TARB2,(c)TARB3,(d)TARB4,(e)TARB5,and(f)TARB6.

4.3.3.Interface bonding performance

For composite sheets,the interface bonding performance is an important index of its mechanical properties.To some extent,the interface bonding properties determine the mechanical properties of composite sheets.The interface bonding properties of TARB and FARB sheets are shown in Table 3.Compared with the interface bonding strength of the two ARB processes,FARB sheets have higher interface bonding strength.TARB1 and FARB1 have interface bonding strength of 24.4MPa and 36.3MPa,respectively.With the increase of cycles,the interface bonding strength increases.TARB6 and FARB4 have interface bonding strength of 29.5MPa and 39.8MPa,respectively.By comparing the relevant literature[4,8,15-17,23-28],the interface performance of TARB sheets and FARB sheets are excellent,especially FARB sheets possess much higher interface bonding strength.

Table 3Interface shear strength of the ARB-processed Mg-14Li-3Al-2Gd sheets.

Fig.7.Optical micrographs of the FARB-processed Mg-14Li-3Al-2Gd sheets annealed at 300°C for 30min(RD-ND plane):(a)FARB1,(b)FARB2,(c)FARB3,and(d)FARB4.

Fig.8.Grain size and layer thickness(adjacent interface spacing)of the annealed ARB-processed Mg-14Li-3Al-2Gd sheets in different cycles.

Fig.9.Comparison mechanical properties of the TARB-processed and FARB-processed Mg-14Li-3Al-2Gd sheets:(a)Yield Strength(YS)and Ultimate Tensile Strength(UTS),(b)Elongation(EL)and Hardness(RD-TD plane).

Fig.10.The strength comparison and cycle?equivalent strain curve of the TARB-processed and FARB-processed Mg-14Li-3Al-2Gd sheets:(a)the strength comparison,(b)cycle-equivalent strain curve.

Fig.11.The hardness fluctuation of the ARB-processed Mg-14Li-3Al-2Gd sheets at different distances from surface along ND(RD-ND plane):(a)TARB,(b)FARB.

With the increase of cycles,the accumulative deformation increases gradually.The increasing positive strain of RD-ND plane and shear strain of RD-TD plane promote better bonding between the interfaces.Comparing the interface bonding performance of TARB and FARB sheets in the same cycle,FARB sheets have better interface bonding performance.In TARB process,a single pass in a single cycle possesses the deformation limitation.If the deformation reduction is too low,the interface cannot be well bonded,while if it is too high,the sheets are easy to crack.In FARB process,this drawback can be avoided through adopting two-step rolling in one cycle with 50% deformation in one pass and 75% accumulative deformation in one cycle,thus significantl improving the interface bonding performance.

In the ARB process,the interface bonding state directly affects the interface bonding.The interface bonding of the ARB-processed Mg-14Li-3Al-2Gd alloy is carried out in four stages.I stage:Physical contact,that is,surface layer fractures and nascent metal surface forms;II stage:Chemical action,that is,the deformation energy overcomes the energy barrier of metallurgical bonding,and activates the nascent metal surface and realizes the bonding of metallic bond;III stage:Dislocation behavior in the interface region,that is,the nascent metal surface evolves into the dislocation wall,then the grain boundary-interface structure is formed through the dynamic recrystallization behavior of the interface[16].VI stage:Dynamic recrystallization homogenization,that is,dynamic recrystallization occurs in the whole area of the combined sheet,the straight grain boundary-interface structure is replaced by the secondary dynamic recrystallization structure,and finall the homogeneous structure with the grain across the original interface is formed.Pressure bonding and metallurgical bonding provide physical basis and energy conditions for the fina dynamic recrystallization bonding[8,16].

5.Conclusion

1.Mg-14Li-3Al-2Gd sheets were fabricated by TARB and FARB,respectively.Compared with the TARB,the FARB has a higher accumulative efficien y in terms of accumulative layers,total number of interfaces,interface spacing,total deformation and equivalent strain,which leads to the excellent performance of FARB under low cycles.

2.The ARB process efficientl refine the grains of Mg-14Li-3Al-2Gd alloy.TARB6 and FARB4 sheets have the smallest grain sizes in their respective processes with the average grain size are about 14.5μm and 5.3μm,respectively.The number of accumulated layers and interfaces have a significan effect on grain size and morphology,which is especially noticeable in high cycles.Meanwhile,the layer interval(single layer thickness)determines the fina grain size and the refinemen rate is controlled by accumulative efficien y.

3.With the increase of cycles,the strength and hardness of TARB-processed and FARB-processed Mg-14Li-3Al-2Gd sheets continue to increase.The TARB6 and FARB4 sheets possess the YS of 201.6MPa and 231.6MPa,respectively,which are 209.6% and 240.7% higher than the as-cast alloy.The hardness of TARB6 and FARB4 sheets are 73.2 HV and 76.6 HV,respectively,which are 171.63% and 187.29% higher than the as-cast alloy.At the same time,compared with the TARB sheets,the FARB sheets present better hardness stability.

4.With the increase of cycles,the interface shear strength gradually increases,and the interfacial bonding ability is significantl enhanced.The subsequent cycles are helpful to further improve the bonding effect of the previous interface.Compared with the TARB sheets,the FARB sheets have higher interface bonding strength,which significantl improves the performance stability of the composite sheets.

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 paper was supported by Natural Science Foundation of China(51771060,51871068,51971071,52011530025),Domain Foundation of Equipment Advance Research of 13th Five-year Plan(61409220118),the Fundamental Research Funds for the Central Universities(3072020CFT1006).