Microstructure And Mechanical Properties Of AZ31 Mg Alloy With Laminate And Gradient Structure

A gradient nanostructure(GNS)layer will form in the AZ31 magnesium alloy sheet by surface mechanical attrition treatment(SMAT).It can significantly improve the surface microhardness,strength and wear resistance of Mg alloy with,but it will lose plasticity to some extent.In response to this problem,we propose to continue the accumulative roll bonding(ARB)in the SMATed sample to improve the plasticity of alloy.In this paper,we investigated the influence of SMAT on the microstructure of the commercial rolled AZ31 Mg alloy by optical microscopy,X-ray Powder diffractometer and scanning electron microscope.The mechanical properties of AZ31 magnesium alloy after SMAT was tested and analyzed by micro-hardness tester and universal tensile testing machine and the optimal processing time was obtained.In addition,we also conducted further study of the texture and grain orientation of the AZ31 Mg alloy after SMAT treatment using the EBSD system.Subsequently,the AZ31 Mg alloy sheets with SMAT for 4 min was preheated and immediately subjected to ARB.Two passes of rolling were performed successively to prepare a laminate and gradient structured(LGS)Mg alloy sheet.The microstructure and mechanical properties of LGS sample were also tested and analyzed.The main conclusions are as follows:1.An obviously gradient nanostructure was formed in the commercial rolled AZ31 Mg alloy sheet after SMAT with different duration,in which the grain size with nanoscale in the surface gradually increases to the micron grain in the matrix.After SMAT for 4 min and 6 min,the thickness of the gradient nanostructured surface layer reaches 99.2 μm and 136.5 μm,respectively.The grain size is correspondingly refined to 51.7 nm and 46.5 nm.It indicated that it formed the gradient nanostructured Mg alloys with different grain sizes after SMAT for different duration.The microhardness gradually decreases along the depth from the surface to the matrix in the SMATed sample.The yield strength and tensile strength of the AZ31 Mg alloy were significantly improved after SMAT,but it will lose the plasticity of the alloy to some extent.Compared with the as-received sample,the yield strength and tensile strength of the sample with SMAT for 4 min were 243.6 MPa and 299.7 MPa,respectively,which were improved by 30.1% and 8.4%,respectively.However,the fracture elongation decreased to 13.3%.The surface layer of the fracture of the SMATed sample is typically brittle fracture,while the coarse grain region of the matrix is still a typical ductile fracture.The coarse-grained structure of the as-received sample and SMATed sample exhibited a strong(0001)basal texture.The orientation of grains in the gradient nanostructured layer was random,which indicated that the basal texture of the alloy was weakened.2.After SMAT for 4 min,the thickness of the gradient nanostructured surface layer were 96.1 μm,98.2 and 101.4 μm in the AZ31 Mg alloy sheet with thickness of 1 mm,2 mm and 3 mm,respectively,which accounted for 19.2%,9.82% and 6.76 of the total thickness,respectively.It indicates that the gradient nanostructured Mg alloys with different volume fractions were formed.The microhardness gradually decreases along the depth from the surface to the matrix in all the SMATed sample with different volume fractions.In the case of a certain SMAT processing time,the thicker the sample thickness,the smaller the volume fraction of the gradient nanostructure layer.It caused the less improvement in strength,and it also has the less damage to plasticity.The SEM morphology observation of the tensile fractures of SMATed sample with different thicknesses indicated that the fracture pattern of the sample transitioned from brittle fracture in surface layer to ductile fracture in matrix.3.The ARB was performed on the sample with for 4 min and a laminate and gradient structured Mg alloy with alternating distribution of grain size was formed.The grain sizes at the interface of 400 °C and 450 °C pretreated samples are 3.24 μm and 3.41 μm,respectively,while the grain sizes at the matrix are 7.83 μm and 8.71 μm,respectively,and a large number of parallel twins are produced at the matrix.The microhardness at the interface of sample is the highest,followed by the surface layer,and the microhardness of the matrix is the lowest.After the treatment of SMAT and ARB,the microstructure of the AZ31 Mg alloy tends to be more uniform.Moreover,the tensile properties of the samples with two different pretreatment temperatures are not significantly different.The tensile fractures of the two samples indicated that the interface between the plates was uniformly bonded.Compared with the sample pretreated at 400°C,the interface of the sample with 450°C pretreatment temperatures was more closely combined with better bond quality.In addition,the dimple size of matrix of the 400°C pretreated sample is about 9.1μm,while it is slightly larger with dimple size of 9.8μm in 450°C pretreated sample,which is consistent with its plasticity.4.After two passes of rolling,the interface bonding between the plates is better and no obvious interface is observed.The fine-grain sizes of one and two pass samples were 3.24 μm and 3.05 μm,respectively,and the coarse-grain sizes were 8.71 μm and 7.95 μm,respectively.The number of twins in the two-pass sample was significantly reduced.Since the two pass samples were subjected to two intermediate heat treatments.Moreover,more strain caused the recrystallization softening,which resulted in a decrease in the microhardness of the second pass sample.Compared with one pass sample,the strength and ductility of the sample have been greatly improved after two passes of rolling,while the overall performance has been significantly enhanced.The interface strength increased from 5.3 MPa in one passed sample to 12.6 MPa in two passed sample with improvement of 138%.The interfacial bonding effect was significantly improved.The average size of the dimples in the fractures of the second passed sample was 13.1 μm,which was significantly larger than that of one passed sample,and the fracture is more sufficient.The gradient nanostructured Mg alloy with the best comprehensive mechanical properties has a GNS layer volume fraction in the range of 13~ 16% and a grain size gradient in the range of 31 nm ~3 μm.