Microstructure Evolution And Mechanical Properties Of Pure Mg And Mg/Al Laminated Composites Fabricated By Accumulative Roll Bonding

In order to improve the strength, elastic modulus and corrosion resisitance of Mg sheet, accumulative roll bonding (ARB) was utilized to fabricate ultra-fine grained pure Mg sheets, pure Mg/AA5052 and pure Mg/AA1050 laminated composites. Optical microscope, scanning electronic microscope (SEM) and transmission electronic microscope (TEM) were employed to observe microstructure and interface structure of ARBed sheets. The global texture evolution and local texture of the ARBed sheets were measured by neutron diffraction and EBSD. Then, the relationship between microstructure and mechanical properties of these ARBed sheets was investigated.For the ARBed pure Mg sheets, the microstructure was refined due to the large strain imposed on the sheet after 1 ARB cycle. During the following ARB cycles, the homogeneity of the microstructure was improved while the average grain size didn't change obviously. The unnoticeable refining effect during the subsequent ARB processing can be attributed to the interval reheating and dynamic recrystallization. ARBed Mg sheet exhibited a typical rolling texture whose intensity almost kept stable during the whole ARB processing. The EBSD measurement indicated that ARB was not an effective method in grain refinement and texture modification for the pure Mg sheet. Moreover, the following rolling was still required to improve the bonding quality of the ARBed pure Mg sheet.The pure Mg/AA5052 laminated composite was processed by ARB at 400?C up to 3 cycles successfully. Necking and rupture of Al layers took place at the final cycle because of the difference of the flow properties between the Mg layer and Al layer, and then introduced a waviness structure into the Mg/AA5052 laminated composite Mg grains were not refined during ARB process due to high deformation temperature and interval annealing while the grain size of Al layer was decreased with increasing of ARB cycle. The texture type of Mg layer was typical rolling texture and the dominant texture of Al layer wasβfiber rolling texture with scatter around the ideal orientations. The significant waviness structure caused by the shear band reduced both the intensity and the sharpness of the rolling texture of both Mg and Al layer. Strengths of the Mg/AA5052 laminated composite were enhanced significantly after sandwich preparation and then increased slightly with the increasing of the ARB cycles till the second ARB cycle. However, strengths along the rolling direction decreased dramatically after the third ARB cycle. Obvious cracking of the coarse Mg/Al intermetallic compound and rupture of the Al layers led to the dramatic decreasing of yield strength along the rolling direction after the final cycle.According to the results above, it was necessary to reduce ARB temperature and avoid formation of the Mg/Al intermetallic compounds in the Mg/Al laminated composite. Thus, pure Mg/AA1050 laminated composite was processed by ARB at ambient temperature. The two constituent layers kept continous during ARB and no waviness structure could be observed in the Mg/AA1050 laminated composite. As similar as the Mg/AA5052 laminated sheet, the Mg grains in the Mg/AA1050 laminated composite were not refined during ARB process due to the dynamic recrystallization while the grain size of Al layer decreased with increasing of ARB cycle. Mg17Al12 phase with thickness of 150nm formed at Mg/Al interface after 3 cycles. There existed a definite orientation relationship between Mg17Al12 and Mg, that is [(1|-)11]_Mg17Al12//[01(1|-)0]_Mg(110)//(1|-)11]_Mg17Al12, with good lattice matching between the two phases at the interface. The texture type of Mg layer was typical rolling texture. The major components of the Al layer were characterized as a combination texture type including evidentβfiber texture and Rotated Cube shear component. Strengths of the Mg/AA1050 laminated composite increased gradually with the increasing of the ARB cycles. After 2 cycles, the laminated composite behaved like other single phase materials.