Microstructure And Mechanical Properties Of Multi-layer Al Prepared By ARB

Recently,materials with heterogeneous lamella structure are characterized by an excellent combination of high strength and good ductility.In this paper,a pure aluminum with a purity of 99.2%(2N-Al)was used as starting material.Heterogeneous lamella structural Al/Al multilayer composite with same material was successfully fabricated by accumulative rolling bonding(ARB)process and followed by various annealing treatment.The as-ARB materials were annealed at different temperatures for different time.The microstructure evolution of Al/Al multilayer material was studied by optical microscope(OM),scanning electron microscope(SEM)and electron backscattering diffraction(EBSD).The mechanical properties of the deformed materials and annealed materials were evaluated by Vickers hardness tester and multi-functional tensile test.The excellent combination of strength and ductility was obtained by proper heat treatment process.By studying the relationship between microstructure and properties under different treatment,the following results were obtained:(1)In the as-ARB prepared specimens,the distributions of misorientation angle in ARB2 layers(a strain of 1.6)and ARB6 layers(a strain of 4.8)are different.The average grain size is 0.46?m in the ARB2 layer and 0.22?m in the ARB6 layer,respectively.The ARB2 layers are dominated by low angle grain boundaries,and the fraction of high angle grain boundaries is only 12.4%,while the ARB6 layers are dominated by high angle grain boundaries,with a faction of up to 61.6%.After ARB,the strength of Al/Al multilayer sample significantly increases,reaching more than 4 times of the starting specimen(annealed 1mm sample),however the ductility decreases significantly and the total elongation is only about 10%.(2)During the annealing process,the grain size of ARB2 layer and ARB6 layer grow significantly with the increase of annealing temperature,but the growth rate of ARB6 layer is higher than that of ARB2 layer.Because higher stored energy in ARB6layer was released during the recovery/recrystallization process,leading to fast growth of the grains.On the other hand,the thermal stability of the Al/Al multilayer sample is considerably high.For example,after annealing at 225 ? for 30min,some elongated grains can be observed in the ARB2 layers,while the grains in ARB6 layers are all recrystallizated equiaxed grains.With the increase of annealing temperature,the faction of high angle grain boundaries in ARB2 layers increases,ranging from 12.4% to 23.1%.While the faction of high angle grain boundary in ARB6 layers decreases,ranging from 61.6% to 44.2%,and finally reached a stable state.(3)The strength of the annealed sample gradually decrease with increasing annealing temperature while the elongation gradually increase.When the annealing temperature is lower than 225 ?,yield drop phenomenon is observed while great increase in elongation can be found in 225 ? 30min annealed sample.(4)By comparing the grain size in ARB2 layer and ARB6 layer of the Al/Al multilayer sample after annealing for 5 min(at 250 ? and 275 ?)and for 30 min(225? and 250 ?),it is found that the grain size of ARB2 layer changes less than ARB6layers.Therefore,the ARB6 layer has a greater effect on mechanical properties than ARB2 layer.