Research On Mechanism Of Shear Band Formation Of AZ31Mg Alloy

Cold rolling, hot rolling at400℃and accumulative roll bonding at400℃of AZ31Mg alloys were carried out. The thickness reduction per pass was controlled not to exceed5%during the cold rolling process. The cracks on the surface were obvious when total reduction reached25%. The thickness reduction per pass was controlled not to exceed10%during the hot rolling process. Failure occurred when total reduction reached80%. ARB was carried out for2cycles with50%reduction each cycle. OM, SEM and EBSD were conducted for characterization of the samples.During cold rolling process of the AZ31Mg alloy, twins and shear bands appears as the cold rolling reduction increases. There are twins within the shear bands. Once initiation, shear bands are observed to break through grain boundary and penetrate several grains. Cracking and shear localization occurs within the shear bands.During hot rolling process of the AZ31Mg alloy, twins and shear bands appears as the cold rolling reduction increases. There are twins within the shear bands. Shear bands are observed to propagate in several grains. EBSD results showed that the major types of twin within shear bands are double twin<11-20>38°and contraction twin<11-20>56°.The results of cold rolling and hot rolling show that the formation of the shear bands is closely related to double twins and contraction twins. The occurrence of double twins and contraction twins, which contributes to the favorable orientation for slip, eventually leads to the formation of shear bands.The reason for shear band crack is that the stress concentration of the pile-up dislocations leads to the formation of voids at the advance of pile-up dislocations, which results the initiation of crack.Grain size distribution of ARBed AZ31Mg alloy is not homogenous and a large number of DRX grains emerge. Compared with deformed grains, the orientation of DRX grains rotate to some extensive degree. There are a large number of shear bands and twins in the sample. Plenty of DRX grains could also be observed in the twins and shear bands.The results of ARB indicated that the mechanism of dynamic recrystallization was jointly controlled by rotation dynamic recrystallization (RRX), continued dynamic recrystallization (CDRX) mechanism and discontinuous dynamic recrystallization (DDRX). A rational reason for the suppression of shear band cracks should be the relaxation of strain localization during ARB process.We propose a model of shear band formation. Shear bands can break through grain boundary and penetrate several grains after initiation. This model reveals the shear band formation process during cold rolling, hot rolling at400℃and accumulative roll bonding at400℃, respectively.