Microstructure Control And Fracture Behavior Of Wrought Magnesium Alloy Contained With Rare Earth

With the recognition of energy crisis and the awareness of environmental protection, magnesium alloys have been paid great attention in many kinds of fields such as aviation industry, aerospace industry, automobile industry, electronic products etc., because of their low density, high specific strength, high thermal and electrical conductivity, easily recycle and so on. In recent years, the studies have focused on the high performance wrought magnesium alloys contained with rare earth. However, the mechanical properties of wrought magnesium alloys contained with rare earth still have not met the requirements in practical applications. Therefore, new methods for improving the properties of wrought magnesium alloys have to be explored. The as-extruded alloys of GW103and ZK60were employed for the present research. The microstructures of the alloys were modified by several means, and effects of microstructure characteristics on fracture behavior were studied, in order to provide the theory for optimal processes and safe application dates as well as developing high performance magnesium alloys.The microstructure characteristics of GW103alloy under different aging conditions and the effects of them on the fracture behavior were investigated. The results show that there is a great amount of fine precipitations in the alloy aged at473K. Microcracks nucleate at grain boundaries because of the grain boundary precipitations (GBPs), and then propagate trans granular ly along the preferred orientation, which causes the transgranular fracture. For the alloy aged at523K, the size of GBPs enlarges. Stress concentration only emerges at large GBPs, resulting in the nucleating of the microvoids by interface debonding between GBPs and the matrix. Subsequently, the coalescence and growth of microvoids lead to the final fracture. Otherwise, when the twins intersect with GBPs, the microracks nucleate at the interface between GBPs and the matrix, and then propagate along the interface between the twin and the matrix. With the aging temperature increasing, the amount of twins and precipitations decreases and the slip deformation intensifies, thus the ductility of the alloy is improved. Therefore, the suitable aging temperature for GW103alloy which can achieve a good combination of strength and ductility is523K.The effects of aging treatment on low cycle fatigue (LCF) behavior of GW103alloy were studied; the influences of the precipitations on both crack initiation and propagation were discussed. The results show that at the total strain amplitude of±0.20%, the cracks initiate at the interface between deformation twins and the matrix, and then propagate transgranularly since the deformation twins coordinate with the dislocation slipping. Since the initiation and propagation of cracks are hindered by the present of β’ phases, the LCF lives increase. At the total strain amplitude of±0.40%, the slip deformation intensifies and the GBPs promote the initiating and propagating of cracks at grain boundaries, inducing the decrease of the LCF lives.It has been found that the crystal defects in GW103alloy caused by the plastic deformation can provide more nucleating cores for aging precipitation, the increase of the amount of P’phases, and the formation of precipitations at grain boundaries and interfaces between the twins and matrix. Because of the increase of precipitations, the dislocation slipping during deformation process is effectively hindered and the matrix is strengthened, especially for the2%deformed alloy which can achieve a good combination of strength and ductility. When plastic deformation increases, the microcracks nucleate at the interface between GBPs and matrix, and then propagate intergranularly. When combined with the formation of facets on the fracture surface, the tensile properties decrease.Elastic deformation has the inducing effect on precipitation behavior during isothermal aging process of GW103alloy, and the changes of precipitation morphology can directly affect the fracture behavior. With the application of elastic deformation during aging process, the precipitation of β’phases can be induced by increasing the defect density in the matrix, and the formation of GBPs and precipitate-free zones (PFZs) is promoted by the diffusion of vacancy to the grain boundary. During the deformation, the deformation resistance of alloy is improved by the increases of β’numbers, and the alloy is strengthened. But the degree of stress concentration on the grain boundary is intensified with the priority slipping of dislocation in the PFZs, so microcracks nucleate at grain boundaries and then propagate intergranularly, which causes the final fracture. The control of elastic deformation can promote the coalescence of GBPs, and the space between GBPs appears, which can relief the stress concentration at the grain boundaries, and the ductility is improved as well as the strength.With the adding of both Y and Nd, the grain size of as-extruded ZK60alloy is refined. At the same time, the ultimate strength and yield strength increase about8.4%and31.2%, respectively. The improvement of strength is attributed to the increase of solid solution content and the second-phases precipitation. Meanwhile, the probability of second phases to be broken and divorced from the matrix during the deformation is reduced because the second phases in the matrix are refined with the additions of both Y and Nd. Just for this reason, the formation of microvoids which causes the final fracture is hindered. And a good combination of strength and ductility can be achieved.